Sailor's Pines, Newyago County, Michigan   Ernie Ostuno
  Jul 24, 2004 01:31 PDT 

Sailor's Pines is located in Newyago County, between Grand Rapids and
Cadillac in the Lower Peninsula of Michigan. This is an even-aged stand
of about 10 acres of white pine. The trees are very homogenous in size
and height, and it's as close to being a "pure" stand of white pine as I
have seen. In all the 10 acres there are only two other individual trees
of other species; one hemlock and one maple. I don't know the origin of
the stand, but the even-aged homogeneity suggests field succession. I
visited this area in March, 2000 and estimated the height of most trees
were in the 100 foot range. A couple of the pines had been uprooted by
the severe downbursts of May 1998 and had been sawed up. I counted 
about 120 rings on both trees, which were cut about three feet up. So
apparently this stand originated about 1880, even though it is
advertised as "virgin pine" in some local publications:



Re: Sailor's Pines, Newyago County Michigan   Lee E. Frelich
  Jul 26, 2004 07:39 PDT 


The downburst you mentioned from May 1998 is probably the same storm that hit Minneapolis, where it did 100 million dollars in damage to cars from
hail, knocked out power to 500,000 homes for up to two weeks, damaged the
roofs of 100,000 homes, and blew down 20,000 trees in Minneapolis, so that
some streets were blocked for 10 days before all the trees could be
removed. I remember that the thunderstorm cell developed over Minneapolis
and persisted until it reached the Atlantic (technically a derecho since
the damage path was so widespread). I was particularly impressed by the
damage on one block in my neighborhood where five cars were parked along
the street in a row and each one had a 3 foot dbh elm tree blown down on
it. All five cars were crushed to a thickness of about 2 feet.

Some of the trees in the park next to my home were laying on the ground 100
feet away from the spot where they were rooted. Obviously these mature
trees became airborne after being pulled out of the ground.

During the storm I watched several linden and sugar maple trees about 15
inches dbh fold up their branches so that they all pointed the same
direction as the wind, and the whole tree presented a profile about 3-4
feet wide to the wind. These trees minimized their exposure and survived
without damage.

This was not the only time derechos originating in Minnesota did damage in
eastern states. The cell that caused major forest damage in the Adirondacks
in 1995 also got its start in northern MN, where it levelled 50,000 acres
of forest. The 1999 derecho that levelled 400,000 acres of forest in the
boundary waters of northern MN and adjacent Canada also did damage all the
way to the Atlantic, but it had a more northerly track through Ontario.

RE: Sailor's Pines, Newyago County Michigan   Ernie Ostuno
  Jul 26, 2004 15:40 PDT 

I think you are right about the origin of the 1998 derecho. I have
looked at radar data from Michigan and the line of convection really
intensified as it moved across Lake Michigan. I work at the National
Weather Service in Grand Rapids and people here still vivdly recall that
storm roaring through here around 5-6 am. We had measured wind speeds of
over 100 mph near Grand Haven. There were several large swaths of tree
damage at a few of the state parks along Lake Michigan. I was working in
State College, PA at the time and the squall line reached there in the
afternoon. It passed across northern PA, generally along PA Route 6 and
I had a chance to survey some of the damage in the following days. There
was a large blowdown along Bucktail State Park, where I estimated top
winds at 90-100 mph.

One of the projects I am working on is equating wind speeds with tree
damage. Of course the most difficult part is all the variables involved,
some examples of which are exposure, health of the tree, soil
condition/type, etc. If you (or anyone else) would like to review a
presentation I have put together on this subject you can download it

Feedback is welcome!

RE: Sailor's Pines, Newyago County Michigan   Lee Frelich
  Jul 26, 2004 16:49 PDT 


Your observation about time of arrival in MI makes sense, since the derecho
developed in Minneapolis at about 10:30 pm. It developed so fast that they
never issued a warning, since the damage was done before the weather
service realized anything had happened. The weather channel had predicted
extremely severe weather several hours earlier, by putting us in the 'white
zone', or zone of maximum chance of development on their severe weather
map. I was watching the doppler radar image and didn't see anything until
after the storm. Apparently the doppler image lags behind the actual by 10
or 20 minutes.

Regarding the presentation, after 20 years of wind disturbance research in
forests, I agree in general that whole stands of trees downed, including
healthy trees, indicates F2 damage. Sometimes stands of older trees with
rot at the base are leveled by upper F1 winds.

Our analysis of tree susceptibility after the Boundary waters blowdown of
1999, which is the biggest known blow down in North America, shows that for
most species of trees, chance of blowing down increases with trunk
diameter, reaching an asymptote near the largest size that the species
attains. If all other factors are equal, early successional species (aspen
and jack pine in our case) are more likely to blow down than late
successional species (white cedar, red maple, paper birch). The spire
formed conifers balsam fir and black spruce had different patterns than the
other species, with an S-shaped increase in susceptibility with size (small
trees are almost never toppled, and large one are extremely susceptible).
Graduate student Roy Rich and I will be publishing a couple of papers on
this topic soon.

RE: Sailor's Pines, Newyago County Michigan    Ernie Ostuno
   Jul 26, 2004 22:27 PDT 


Thanks very much for the input. I have gotten feedback from a few people
in the field of meteorology on what I put together, but what I really
need is feedback from the tree experts. I look forward to seeing the
papers when they are published.

One thing I would like to do is firm up the rough scale of wind
speeds/tree damage based on actual wind speed measurements in areas
where there were blowdowns. After nearly ten years of study, I only have
a handful of cases where there are reliable wind measurements close to
areas of tree damage. In fact, the 1998 derecho provided some of the
only examples of this, both in MI and PA. The problem is that even in
large, long-lived downburst-producing storms, there can be great
variability in the intensity of the winds. Why there are such variations
is not completely understood, but part of the reason may be the
formation of small vortices that can act as mini-tornadoes. We call
these vortices "non-supercell tornadoes" to differentiate them from the
stronger tornadoes associated with the rotating updrafts (mesocyclones)
of supercell thunderstorms.

The '98 storm produced a very locally intense swath of damage across
Muskegon County, MI. This may have been from some type of non-supercell
tornado rather than a microburst. It is interesting to see how white
pine has been released in this blowdown area, as many of the canopy oaks
were felled by the storm. What species were in the understory at the
Boundary Waters that have taken advantage of the demise of the big

You mentioned something that I have noticed over the years, i.e., the
increased likelihood of early successional species being felled compared
to later successional species. Here in Michigan it's the eastern
cottonwood (Populus deltoides) that is usually the first to snap, along
with jack pine.

If you want a good source of current radar pictures I recommend these
two web sites:

RE: Sailor's Pines, Newyago County Michigan   Lee Frelich
  Jul 27/28, 2004 


The big blow down in the boundary waters took out jack pine and aspen, and
left small black spruce, white cedar, balsam fir and paper birch, which are
the climax species group there. The blow down essentially advanced
succession 50-100 years.

We have copies of the doppler radar image from the boundary water blow
down, and it is the most impressive I have ever seen. The bow echo takes on
the shape of a V at the height of the storm.

You can view a radar image movie of the big blowdown at this website address:

It is interesting that just after the storm front passes Ely, it develops 
into two small V formations that are side by side (shown in red), and then 
the double V combines into a single large V as the storm exits the east 
side of the Boundary Waters in Cook County.

Some of the worst damage was done in the northern V of the double V 
formation, where winds were in the middle F2 strength. When the storm 
passed Ely, winds of 80 mph were recorded, and yet there was little tree 
damage near the Ely weather station. Just after the storm passes Ely, it 
was probably worse than it looks on the image because the radar beam going 
north from Duluth was traversing supercell thunderstorm for almost 100 
miles, and thus the V formation was probably hidden until the storms moves 
a little further east so that the radar beam is entering from the rear of 
the storm. We have had the same problem in Minneapolis, where a tornado 
was approaching the city, but the appendage containing the tornado on the 
southwest side of the supercell was not visible on doppler radar coming 
from the city. We had a very fuzzy image because the radar beam had to go 
all the way through the supercell to see the tornado and had to rely on 
weather spotters calling in reports of the approaching tornado.

You can see pictures of the boundary waters blowdown at:

In the aerial photos of heavy blowdown you can clearly see the strip of 
trees along the lakeshores that remained standing, as well as all the green 
stuff under the blown down trees, which were suppressed saplings and shrubs 
before the storm.


Kinzua Bridge wind damage photos   Dale J. Luthringer
  Jul 28, 2004 06:05 PDT 

Lee, Ernie,

Here are some shots of the wind damage at the Kinzua Bridge Trestle at
Kinzua State Park on 7/21/03. It blew the middle of the trestle out.
The state was in the process of conducting repairs when this happened.


Kinzua Bridge Thread

RE: big blow down images   Ernie Ostuno
  Jul 29, 2004 02:10 PDT 

Lee (and others),

The Duluth NWS office has put together a web presentation of this storm:

RE: big blow down images   Lee E. Frelich
  Jul 29, 2004 06:09 PDT 


Thanks for the link.

Its amazing that the weather service never told those of us doing research
on the big blow down about this paper! At the meeting on blow down
research the Forest Service put on, we had meteorologists from offices far
away who knew very little about the storm.

There is one rather comical error in the paper. They state that the storm
persisted for 6000 nm (nm is the official international symbol for
nanometer, and 6000 nm is a small fraction of a millimeter). I guess they
intend nm to be nautical miles in this context.

I have seen the map of derecho frequency in the eastern U.S. before. It is
intertesting that the three biggest derechos in recent times (July 4, 1977,
July 12, 1995, July 4, 1999) are outside of the zone minimum frequency
shown on Johns and Hirt map. I wonder if there is a separate phenomenon
going on with these north woods derechos? They have extraordinary
windspeeds and persist over extraordinarily long paths.

derecho information   Paul Jost
  Jul 29, 2004 10:16 PDT 

Here's another good Derecho link:

It includes analysis of many recent derecho events. It is surprising
that NOAA doesn't consolidate this information more effectively.

Paul Jost
RE: big blow down images   Ernie Ostuno
  Jul 30, 2004 11:18 PDT 


This is probably not a paper that was submitted to a refereed
publication, so that would explain the lack of convention in the use of
nm for nautical mile. I am guessing that this study was a quick overview
that was put together for the website, and therefore may not have even
existed when the meeting you mentioned took place.

It is not unusual for strong derechos to form in the Upper Midwest in
mid-summer, usually along the northern periphery of a "heat ridge", a
mid level high pressure area centered over the central Plains. The area
is called the "Ring of Fire" by meteorologists and this synoptic setup
has produced some of the strongest, longest lived derechos. The July
1999 event is a classic, if extreme, example. I am wondering if the far
northern derechos are underrepresented in the climatology because of
lack of historical severe weather records in Canada, which would mean
that those that get going along the border don't have a long enough
path length to be counted before they cross the border.

I remember reading about the 1999 storm shortly after it happened,
mainly its impact on the big trees at the BWCA. There are some things in
this paper I was unaware of, such as the fact that the remnants of the
storm's circulation went on to produce severe weather all the way to the
Gulf Coast in following days. Mesoscale convective vortices can have
some interesting developments. The MCS that was responsible for the
devastating Johnstown, PA flood of 1977 moved into the Atlantic and
became a tropical storm once it got over the warm waters of the Gulf

I would like to see forestry research merge with meteorological research
on the topic of wind damage to trees. I mentioned before that one thing
that is sadly lacking for meteorologists is a detailed standard
reference based on empirical observations that equates tree damage to
wind speeds. We have a wind threshold of 50 knots for our definiton of a
"severe" thunderstorm that prompts a warning and we use tree damage as
part of the verification process for those warnings. What kind of tree
damage occurs at 50 knots versus that at 40 knots? I am concerned about
the scientific integrity of the criteria and verification procedures.
Right now our criteria states that 50 knot winds will bring down
healthy, large (6 inches or greater diameter) trees or limbs. In order
for a warning to verify, there must be two examples of this type of tree
damage occurring in the warned area, within 15 miles and 10 minutes of
eachother. The unfortunate thing is that we usually don't have the
resources to check to see if the trees were healthy.

RE: big blow down images   Lee E. Frelich
  Jul 30, 2004 12:25 PDT 


I doubt that a 50 knot wind would break off healthy trees or branches. All
of the damage I see after such marginal events is always breakage of rotten
branches, uprooting of trees that have had their roots restricted or cut by
construction, and breakage of trees that are hollow or that were planted
too deep.

Another problem with the 50 knot wind threshold is that damage done by this
wind speed probably varies from region to region. For example, a 50 knot
wind in a Minnesota bur oak forest would not do any damage, because such a forest is adapted to wind.

I have always advocated using the so-called high end threshold of 65 knots
for severe weather, because we get too many warnings with the 50 knot
threshold. Of course that may not be true in other parts of the country.

RE: Kinzua Bridge wind damage photos   Edward Frank
  Jul 30, 2004 20:33 PDT 


I understand that you aren't familiar witht he details of every tornado
in the country. I visited Kinzua Bridge shortly after the bridge was
blown down. The report at the link you said the weather team determined
it had been an f1 tornado at the site.

How do you determine whether something was a tornado, with rotating
winds versus a unidirectional downblast? Are there ground
characteristics that identify a tornado? Does a tornado leave a path?
Is there a certain pattern of tree fall that is evident? Apparently
according to the news reports no one actually saw it. Was rotation
detected in the radar images?

My perspective upon seeing the site was that all of the trees were lying
in the same direction, so it looked like a unidirectional wind. I
understand this impression may be erroneous, but how could I tell from
looking at a site?

Ed Frank
RE: Kinzua Bridge wind damage photos   Ernie Ostuno
  Jul 30, 2004 23:07 PDT 


The first thing we look at before a storm survey is the archived radar
images of the storm. This will give us a good idea of whether it was a
potential downburst aka "straight-line winds" or a tornado. There are
some complicating factors. Downbursts can occur near the path of
tornadoes, and small vortices (non-supercell tornadoes)can occur near
downburst damage and be superimposed on it. Supercell thunderstorms
(those with persistent rotating updrafts) are the ones that produce the
stronger tornadoes. Generally, what you look for in terms of tornado
damage is a very sharp edge to the damage and also evidence that light
debris was carried aloft for long distances. Tornadoes have a vertical
component to the wind, while downbursts are generally horizontal.
Therefore, debris is much more likely to be lofted and carried long
distances by tornadoes. An example...if you find tree branches sitting
in an open field hundreds of feet from the nearest trees, or fiberglass
insolation in the tops of trees a long way from the nearest building,
you are looking at evidence of a tornado rather than a downburst. Of
course, getting eyewitness accounts helps, too. A very important thing
to do is match up the radar images/conceptual storm model with the
actual damage in both space and time.

The second half of the slideshow I linked before goes into
differentiating between downburst and tornado damage:

The State College, PA NWS office just linked a survey of the July 27th
Lancaster County tornado:

Here's a video presentation describing the survey:


RE: big blow down images   Ernie Ostuno
  Jul 31, 2004 09:47 PDT 


My experience in both PA and MI is that once winds reach about 35-40
knots, large rotten limbs and trees start falling. A good example of
this was on July 4, 2003 when an outflow boundary from a dying line of
thunderstorms moved across Kent County (the area surrounding Grand
Rapids). The local law enforcement radio network passed on to us about a
dozen reports of large trees down, while observed winds were generally
in the range of 35-45 knots. The next day I went out and tracked down
every report. Eleven of the twelve trees were rotted. There was one
healthy tree of about 12 inches diameter that was snapped about 20 feet
from the base. It was odd since there were not even any twigs or
branches down from surounding trees.

I agree with you that the wind threshold should be raised and that we
should avoid issuing so many warnings for marginal storms. However, I am
facing an uphill battle since so many large rotted trees apparently
exist that even when 40 knot winds occur, the reports of downed trees
come pouring in.

RE: big blow down images   Lee Frelich
  Jul 31, 2004 16:30 PDT 


If you are getting limbs down in 40 knot winds, then you must have a lot
of silver maple in your area. Silver maple and a few other species do have
very brittle wood and usually have rot, since those species don't defend
their heartwood against fungal invasion.

Regarding the healthy tree downed in 40 knot winds, it may have had hidden
damage from old branches that were not pruned properly. Or, maybe it was an
unlucky tree and the wind just happened to swirl the right way in that
spot, or it may have one of the two problems mentioned in the following

Downed trees that are apparently healthy at relatively low wind speeds are
going to become quite common in the near future. The reason is that most
nurseries for the last 20 years have sold trees with the soil line way to
high, so that the trees are planted too low. They also have a very high
frequency of girdling roots. These problems can both cause the trunk to
suddenly separate from the roots in relatively low wind speeds, or even on
a calm day. They can also cause hidden rot higher up in the trunk and
cause the crown to break off. These problems develop 10-20 years after
planting. The public is pretty much unaware that few of the trees they
plant today will ever become mature.

We just had a tornado watch issued for Minneapolis. Maybe I will get to
see some trees go down later on.

RE: big blow down images   Lee E. Frelich
  Aug 02, 2004 06:00 PDT 


The discussion from the last several e-mails brings up another cause of
frequent tree death which is common (as you point out) in natural
stands. Many second growth naturally regenerated stands are 60-120 years
old, and are undergoing self thinning. Since 100-200 saplings fill the same
canopy space as one 200 year old sugar maple, obviously most trees must
succumb to competition. Because they are starved for nutrients, water and
light by the better competitors, they are susceptible to a number of
pathogens. In a 100 year old stand, I would expect that most trees would be
in various stages of the process of dying. This is totally natural. Some of
these trees fall on ordinary windy days, and I have seen them fall several
times on calm days. Pathologists and silviculturists who don't know much
about stand development are always mistakenly raising the alarm about dying
forests. Even some of the so-called acid rain damage was normal stand

Regarding nurseries and planting, some nurseries in Minnesota do a very
good job (they will only plant things properly), and others thrive on the
return business from customers who buy new sets of trees every 5 or 10
years without any suspicion that they are being taken to the cleaners. The
Minneapolis City forestry department plants bare root trees (so that the
proper planting level can be seen and the roots can be spread properly) in
holes dug by hand (so that surrounding soil is not compacted), and provides
the correct amount of mulch and pruning for all trees on all streets in the
entire city.

On the topic of relative damage to buildings and trees, sometimes the trees
surrounding absorb the wind energy and protect the building. Also, it
depends on the species of tree and how the building was built. Houses that
merely have the roof setting on the walls and surrounded by bur oak trees,
could easily lose their roof with little or no damage to the oaks (common
in the suburbs of Minneapolis). In the inner city of Minneapolis, 100 year
old houses were apparently very well built, since the tornado we had in
1982 completely took down the urban forest canopy (mostly elm in that
area), and did little damage to houses.

There is some really interesting research being done and a lot of papers
published by Stephen Mitchell at the university of British Columbia,
Vancouver, and Heli Peltola and Seppo Kelomaki, at the University of
Joensuu, Finland, on the mechanical effect of wind on trees. It has limited
application to the Midwest and eastern U.S.

RE: big blow down images   Ernie Ostuno
  Aug 02, 2004 10:19 PDT 


Thanks for the input on the causes of mortality for mature forests. I
have seen many examples over the years of what you describe. In fact, I
think this may explain the majority of the downed large trees/limbs that
we get during high wind events in places like Lower Michigan and
especially PA, where there are large areas of mature second growth

I have seen many examples of mortality from windstorms as well in the
old growth hemlocks/hardwords sites I frequented in PA. This is
excluding such major events as the tornado/downburst damage at Tionesta.
At an old growth site like the Snyder-Middleswarth Natural Area (SMNA)
in PA, just about every time I visited I found at least one example of a
big hemlock that had fallen recently enough that the needles were still
green. One of the contributing factors here is the tendency for the old
hemlocks to have heartrot, but in some cases the recently fallen trees
looked healthy. If you walk through a place like the SMNA, you are
struck by the sheer volume of big hemlocks that have fallen and are in
various states of decay. I wonder how the frequency of mortality here
compares to the second growth forests you describe, i.e., what
percentage of the trees per unit area are dying per unit time?

As for building vs tree damage, one factor that may play a role is the
tendency for winds to decrease quickly in proximity to the ground due to
friction or poorly understood effects of fluid dynamics. I have always
wondered how strong the vertical gradient of wind speeds can be in the
lowest 100 feet above ground level. There was one case I remember from
1996, when an F2 tornado pased over a house that was surrounded by 60-80
foot tall Norway spruce. The trees were on each side of the house but
they did not shield the house from the direction of the strongest winds.
Every spruce tree was sheared off at roughly 30 feet above the ground,
as if a giant lawn mower had passed over them. Yet the house sustained
no damage, nor did anything else appear disturbed on the ground near the
house, including a pretty flimsy looking bird feeder. In this case,
winds flowing into the tornado at F1 or F2 level speeds were probably
elevated at tree top level, with winds remaining much lower near the
ground. can see photos of a recent F3 tornado including examples of
tree and building damage here:

Note how the tree in this photo is stripped of all limbs:

From the cases I have seen, this is typical for a tree that has been hit
by F3 and higher wind speeds, and trees are usually debarked by lots of
small flying debris at the F4 nd F5 level. However F2 or higher ratings
are almost never given based solely on damage to trees.

Please let me know when the research that you and others have worked on
is published and where I can find it.

RE: big blow down images   Lee E. Frelich
  Aug 02, 2004 10:55 PDT 


Your question about mortality rates in old growth has been well
studied. Basically 5-10% of the canopy turns over each decade in old
growth temperate forests. Since hemlock logs take 50-100 years to decay,
there are usually quite a few in various stages of decay on the
ground.   This regime of 5-10% turnover starts at about age 150-200 years
since the last stand levelling disturbance (F2 winds or fire) and continues
until the next one.

Canopy turnover in any one decade depends on the severity storms that hit
the stand, but also how long it has been since the last significant storm.
If it has been longer there will be a bigger crop of rotten trees ready to
die, and they may go down in lower wind speeds since they have had more
time to rot.

RE: Extreme damaging winds   Lee E. Frelich
  Aug 18, 2004 10:58 PDT 


Today I found a presentation on the web by Miller and Johns on Extreme
Damaging Winds that breaks derechos into two types with and without
supercell thunderstorms. They say the ones with supercell thunderstorms
can produce higher windspeeds for longer periods than those that are
strictly a bow echo.

The storm that hit Minneapolis on July 1, 1997, and the Boundary Waters on
July 4, 1999 are given as examples of supercell derechos, whereas the May
30, 1998 storm is shown as an example of a non-supercell derecho.

This apparently explains the differences I had noticed; the supercell
derechos have high winds for 20 minutes and very heavy rain, whereas the
others only have high winds for 5 minutes and little to a moderate amount
of rain. In either case the peak winds can reach 130 mph, but the longer
exposure to high winds combined with heavy rain would certainly cause more
tree damage during a supercell derecho.

The July 4th 1999 storm apparently had a supercell embedded within it
during the time the most forest damage was done, and then turned into a bow
echo as it progressed across Lake Superior. I don't know whether the storm
can be both types at once.

The 'Big Three' blowdowns in the Midwest (July 4, 1977, July 12 1995, and
July 4 1999) were apparently all supercell derechos. All three had
long-lasting winds and extremely heavy rain, and all three caused extensive
forest damage over hundreds of thousands of acres, and had a core zone
about 10 x 50 miles where the forest was totally levelled. I couldn't find
a classification of the 1995 storm, but I know that it hit a weather
station near Itasca State Park, MN, that had winds over 100 mph for 20
minutes, and 122 mph winds sustained for periods of 1 minute.

In contrast, storms of the bow echo type cause moderately severe tree
damage over a huge area, but they don't have a core zone of total damage.

Now my observations are finally starting to make sense. It is amazing that
we are just beginning to understand the phenomenon that probably causes the
most widespread wind damage in the U.S.

RE: Extreme damaging winds   Lee E. Frelich
  Aug 18, 2004 12:54 PDT 


I heard of an extensive blow down in high elevation forests in the the
mountains of Colorado a few years ago that some people claim was caused by
the jet stream hitting the ground, although I don't know if this is true.
It lasted for several hours and was not associated with a thunderstorm.

Cyclonic winds (the same ones in blizzards, known as extra-tropical lows,
or mid-latitude lows) can also get quite strong during spring and fall,
especially over the Great Lakes or the ocean, where there is little
friction. They can cause some small areas of heavy forest damage, and
cause scattered gaps to form over large areas of forest. For example, at
the end of November 1966, we had steady winds of 70 mph for 5 or 6 hours at
our place on the Door Peninsula, and several large white pines went down
gradually, creating a gap in the forest about 200 feet long.

I think the Mount Washington wind was a strong cyclonic wind, and the
elevation contributed to its high measured speed.

Higher wind speeds (around 300 mph) have been recorded indirectly in
tornadoes through analysis of flying debris in photographs and Doppler
radar estimates of rotation speed.


At 01:33 PM 8/18/2004, you wrote:

    In terms of extreme winds, I think we've discussed tornados,
hurricanes/typhoons, blizzards, and derechos. Are there any other
extreme wind phenomena that you know of? I wonder how the super wind on
Mount Washington back in 1927, or whenever it occurred, would now be
officially classified?

RE: Extreme damaging winds   Ernie Ostuno
  Aug 19, 2004 02:11 PDT 

Lee and Bob,

Of course this is a topic that is near and dear to my heart, so I would
be interested if you can give me the link to the Miller and Johns paper.
I think this is the one, correct?:

Miller, D. J., and R. H. Johns, 2000: A detailed look at extreme wind
damage in derecho events, Preprints, 20th Conf. on Severe Local Storms,
Orlando, FL, Amer. Meteor. Soc., 52-55.

The best overall reference I have seen on convective downdrafts is this

Wakimoto, R.M., 2002: Convectively driven high wind events. Severe
Convective Storms, Meteor. Monogr., No. 50, Amer. Meteor. Soc., 255-298

Let me tell you that even though supercells and derechos have been
numerically simulated by computer models since at least the early 1980s,
there are still some very important processes that are not well known
that occur on rather small scales in time and space. Specifically, it's
been my observation that very small areas of high winds occur that
cannot be resolved by current weather radars or can be well simulated by
computer models. These are probably the result of small scall vortices
that are stretched by strong updrafts in the vicinity of the
thunderstorm gust front. One of the most important future developments
in storm forecasting will be better radar networks (there is talk of
putting small radars on cell phone towers in the future) that can
resolve these vortices.

Getting back the the Miller and Johns paper, a supercell is nothing more
than a thunderstorm with a persistent rotating updraft, which is called
a mesocyclone. Supercells can be discrete or part of a line, such as a
derecho. A mesocyclone can also occur at the northern "comma head" of a
bow echo, so technically a supercell can be part of a bow echo, but most
meteorologists usually do not make that distinction and will call the
mesocyclone in the bow echo the "rotating comma head" (don't ask me why,
I guess it's just tradition). To give you an idea of the names commonly
used, let's start small and get bigger: A supercell (typically 5-10km in
diameter) can be part of a bow echo (typically 10-100km in length) which
can be part of a line echo wave pattern or LEWP (25-200km) which can be
part of a derecho (50-500km), which can be part of a mesoscale
convective system or MCS (100-1000km). These systems are usually defined
based on their circulations and characteristic appearance on radar and
satellite imagery.

Why are supercells special? As you mentioned, the main reason is that
they typically last a long time and therefore they cover a lot of ground
with the opportunity to do a lot of damage. They also produce not only
very strong downbursts, but also the stronger tornadoes (greater than
F2). Bow echoes mainly produce damage through the descent to the ground
of a "rear inflow jet" which originates several thousand feet above the
surface due to a horizontal bouyancy gradient along the pool of
rain-cooled air that forms near the surface. The narrow ribbon of strong
winds can produce a long track of wind damage that may be enhanced by
the spin up of little vortices (gustnadoes) in the area of strong
turbulence along the updraft/downdraft interface.

I haven't heard of the Colorado event, but I know of a high wind event
in the Teton wilderness of Wyoming in July 1987. This was either a 2.5
mile wide tornado or the mesocyclone of the supercell thunderstorm
reaching the surface. The jury is still out on this one, but 15,000
acres of primarily mature lodgepole pine forest were flattened.

Mount Washington is kind of a special place as far as wind is concerned.
The elevation of about 6,300 feet isn't that impressive as far as
mountains go, but it happens to be in an area where there can be large
pressure gradients between deepening ocean storms ("noreasters") and
strong continental surface high pressure areas. So the air flowing from
high to low pressure meets this little orographic feature at higher
velocities relative to other places in the world with higher elevations
but weaker surface pressure gradients. Of course, this effect is totally
independent of thunderstorm downdrafts.


RE: Extreme damaging winds   Lee E. Frelich
  Aug 19, 2004 05:59 PDT 


Thanks for the references. I think what I found on the internet was Miller
and Johns Powerpoint presentation, based on the paper you cite below, which
appears to have a few updates made during 2001. It has a lot of nice
pictures and radar images. The paper can be found at:

I will have to look up the Wakimoto paper, since I would like to know more
about how supercell storms generate their downbursts. We had one in
southern MN during 1998 that generated 125 mph winds even though it was an
isolated cell and was moving very slowly. Ironically, it did little damage
because its downburst came down in the same area as a mile-wide tornado
with 175-200 mph winds a few months earlier, in St.Peter MN. St.Peter had
been my favorite small town to visit during October because it had many
historic buildings from the 1850s as well as streets lined with sugar
maples planted when the buildings were built. Both the buildings and maples
are now gone, and have been replaced with those ugly little-leaf lindens
(lollipop trees as I call them) and other assorted junk that nurseries sell
these days.

The town where I grew up (Janesville, WI) had a downburst from an isolated
supercell a few years ago, and as the storm approached, there were the
fragments of a forest from the west side of the city suspended in the sky
(it literally looked like a forest that had been through a blender), and of
course this debris was slammed down onto the ground when the downburst arrived.


RE: Extreme damaging winds   Robert Leverett
  Aug 19, 2004 06:09 PDT 

Ernie and Lee:

   I've always been interested in weather phenomena and general climate.
As a statistics nut, I'm forever fascinated with the extremes of
weather. My first assignment in the U.S.A.F. in 1964 was at Ellsworth
AFB, a SAC base in western South Dakota. The extremes of the Great
Plains came as sudden, unexpected shocks. One weather extreme I never
dreamed I'd ever witness or even thought possible was a sudden change in
temperature of almost 44 degrees in the unbelieably short time span of a
minute and a half. I didn't think such radical temperature changes were

    A meteorologist at the Base told me that one such sudden change
occured at Spearfish, S.D. and another at Great Falls, Montana. The one
at Ellsworth AFB was only slightly under the all time record. What on
Earth allows such a sudden change in temperature? Are there spots on the
planet where such gyrations occur more frequently?

     Another record that seems utter bizarre is the second lowest
temperature recorded in the CONUS at Peter's Sink, Utah. Why Utah when
there are many areas of similar physical structure - at least I would
imagine? The temperature at Peter's Sink (or Peter Sinks - can't
remember which) was about -69.7 as originally recorded. I saw an entire
study done on the dynamics of teh site. It was way over my head, so I
didn't really understand what created such unusual conditions at the
site, unless that one ultra low temperature was a fluke. I think the
next lowest recorded there was around -50.


RE: Extreme damaging winds   Ernie Ostuno
  Aug 20, 2004 03:45 PDT 

Peter Sinks, UT is an extreme example of what we popularly refer to as a
"frost hollow", an area where local temperatures are significantly
colder than the surrounding area. Here's a link with more info:

The Utah temperature extremes at Peter Sinks are replicated in a less
spectacular fashion in the eastern U.S. where similar special
circumstances occur. One of the places is close to State College, PA and
many Penn State meteorology students have experinced the night time cold
of a place locally known as "the barrens":

Similar to the Utah site, the geography and soil characteristics allow
optimum conditions for radiational cooling and a lack of mixing which
result in localized temperatures that are much colder than the
surrounding area. As the link above explains, repeated cutting of the
forest (and probably the resulting fires) many years ago for charcoal
have resulted in a lack of vegetation and sandy soils, which help
contribute to the favorable radiational cooling conditions.

The South Dakota temperature antics at Spearfish result when a dense
arctic airmass "sloshes around" in a valley next to air that's been
warmed by compression as it descends from the mountains to the west.
This is the famous "chinook" or "snow eater" wind. The world record
fluctuations are described here:

Go down to section 3.1.4

Of course, there are also cases of sharp drops behind arctic cold
fronts. Having grown up in the Northeast U.S. where cold fronts usually
are heralded with thunderstorms or snow squalls, I was quite surprised
to see how quick the temperature would rise and fall when I lived in the
Plains States and how these great temperature swings would occur with
little or no precipitation. Usually there was nothing more than a wind
shift. I do recall the spectacular frontal passage in Oklahoma during
the big arctic outbreak of January 1985. The wind howled up to 30-50 mph
when the front came through, accompanied by a very impressive dust
storm. The temperature dropped from around 60 degrees that afternoon to
near zero the next morning, and all of this was without any
precipitation falling.


RE: Extreme damaging winds   Don Bragg
  Aug 20, 2004 07:11 PDT 

Having gone to college at Utah State University in Logan, which is about 40 miles from this area, I can attest to how pronounced these frost pockets can be.  There are a series of them in this area, and as my family and I drove through them this February in our minivan (which has a fairly accurate outdoor thermometer), it was not unusual to see a drop of 20 degrees or more in these pockets.

The big Colorado blowdown of 1997 happened largely on the Routt National Forest.  I was in the general area last fall for a silviculture tour, and the USFS staff from that area was bracing for a major spruce beetle outbreak in the area from the timber killed by this storm.  Unfortunately, it appears that a very large area of old-growth Engelmann spruce and subalpine fir has been lost.  Here are a couple links related to this event: