WEATHER EVENTS: November 2005 Archives

Threat of severe storms ends here, but major severe outbreak underway to Chicago's east; more than two dozen twisters already reported

The threat of severe thunderstorms is over in the Chicago area. Strong winds, falling temperatures and 3-6 hours of dry weather are immediately ahead--then snow showers arrive in Wednesday's pre-dawn hours. The precipitation-free weather soon to take hold and last half the night, is the result of the larger storm system's "dry slot", as mentioned in the previous update. This is a very windy but dry region of any largescale storm. But the feature migrates and snow showers are likely here as it exits the area before daybreak Wednesday.

While the severe weather threat has ended here, a major severe weather outbreak is underway across a good part of the nation's mid-section to the city's south and east. More than two dozen reports of twisters have already been tallied by NOAA's Storm Prediction Center this afternoon--and more are likely as we head through the evening and the night. Southern Indiana, extreme southern Illinois, western Kentucky and western Tennessee have been hardest hit through 5:30 pm. Now the focus is on a powerful squall line with cloud tops from 37,000 ft to 52,000 feet is sweeping northeastward through Indiana and southeast Michigan--and is headed for Ohio. It is part of a long line of powerful and severe thunderstorms which extend south to the Gulf Coast, where storms are crossing the Texas coastline into the Gulf of Mexico. We expect a number of additional tornadoes is likely to be reported from these areas. There are four active tornado watches in effect from Lousiana north to Detroit and a series of severe thunderstorm and tornado warnings are in effect as this update is filed.

Snow is sweeping across Iowa on winds which, in the western half of the state, are gusting 40+ mph. Temperatures have plunged into the low and mid 30s across all of Iowa. It's this chilly air which is to take up residence in the days ahead--part of a radically different pattern which may feature regular outbreaks of chilly air the remainder of November---a month which until now has been so mild it ranks 12th warmest of all November 1-15 periods on record here since 1871.

--Tom Skilling

4 P.M. TUESDAY
A period of strong thunderstorms or downpours--perhaps even a spell of
severe weather---threatens parts of the Chicago metro area early this
evening just ahead of this autumn's latest powerful storm's "dry
slot"--the narrow notch of dry air evident in the comma shaped cloud
mass visible on satellite imagery with storm's like today's--shifts
into region. If a weather watch is to be issued for any part of our
this area, it is likely to happen before 6 or 7pm--or not at all.
The latest computer model depictions of the intensifying storm
confirm a 60 mph band of southerly winds just 1,600 ft. above the
surface and advancing on the area from central and southern Illinois
and expected to converge with the 10-20 mph ESE winds currently in
place here between that time. Known as "speed and directional
convergence", the pile-up of air which results from this type of
situation, leads to vigorous upward motion through the atmosphere--in
today's case, into a 155 mph southwesterly jet stream. Thunderstorms
able to form in this type of situation are prolific rain producers,
capable of downpours and hailstones. But they move quickly in the fast
steering wind regime in place aloft. Any gusts they produce
effectively incorporate the storm's rapid forward movement with the
storm's outflowing winds which makes these gusts potentially extreme
and damaging. It should be noted, these gusts are best able to make it
to the ground when a cooler air isn't in place in the lower
atmosphere--as is the case at the moment as this update is filed at
mid-afternoon. So one key to the potential to high wind production
here will be whether a warm front can briefly pass, placing
Chicago--even if briefly--in the intensifying storm's "warm
sector"--which isn't likely to be very warm here at all, and certainly
not for very long since cold air is waiting in the wings
. 70-degree temperatures and mid 60-degree dewpoints (indicative of
humid air) are in place downstate and are helping fuel t-storm
development there. A major severe weather outbreak threatens a broad
area, especially to Chicago's south. The Storm Prediction Center has
an 8 state area designated as a "high" risk region---not an everyday
occurrenc---and a testament to the current storm's strength.
Meantime, snow and wind on the storm's backside sweeps into Chicago
after a 3-6 hour precip-free period in this storm's dry slot tonight.
Snow showers and flurries appear likely to sweep into Chicago in a
sharply colder and very windy weather regime by daybreak Wednesday and
through a good part of the day. Snow accumulations over sections of
northern Wisconsin and Upper Michigan threaten to exceed 6-8" and the
snow, though falling on warm ground and pavement, the speed at which
snow falls there should be impressive. Snowfall rates here and
temperatures just above freezing in Chicago shouldn't allow
significant accumulation.
Powerful winds and wind chills in the 20s are predicted
Wednesday---a day likely to be the first this autumn in which
temperatures aren't able to reach, let alone exceed 40-degrees. This
should be the coldest day in the nearly 8 months March 21.
Interestingly, the average first date of sub-40-degree occurrence each
fall is on November 9--putting tomorrow within a week of that time.
We'll have much more on this and what looks like a colder overall
pattern likely to last awhile tonight on the WGN 9 O'Clock News. All
the best!
---Tom Skilling

A northbound warm front ignited active thunderstorms earlier this evening responsible for lightning which struck a residence in west suburban Sugar Grove around 9pm. Firefighters were dispatched in response to the fire which resulted. Radar scans placed cloud tops as high as 36,000 ft. Other powerful thunderstorms have raked sections of Indiana and Michigan. Hailstones 3/4" to 0.88" in diameter peppered Hartford City and Marion, Indiana.

The warm front's passage has placed Chicago in milder, more humid air—a sector still capable of severe weather production given many of the attributes of tongith's atmosphere already discussed. But of particular interest are powerful non-thunderstorm wind gusts sweeping eastward out of the Plains and due to take up residence here daytime Wednesday. These winds are raking across central Iowa and expected to reach the Chicago area toward morning. The convergence (pile-up) of air taking place ahead of these powerful winds may ignite another line of active, possibly severe thunderstorms—even a few isolated tornadoes—which could reach the Chicago metro area between 2 and 4 am. Once they pass, strong, non-thunderstorm winds are likely Wednesday with gusts to 40 m.p.h. not out of the question.

-- Tom Skilling

Issued 2PM TUESDAY -- I want to update you on the potentially potent severe weather situation--including the threat of powerful thunderstorms and tornadoes---which appears to be coming together for tonight.

There are never guarantees a given set-up will go on to produce violent weather, strong as it may appear. And, even when it does, such weather is highly selective--damaging often damaging one area and sparing another. We have no skill yet in meteorological science to tell you many hours in advance precisely which communities are at risk. But, the current situation has an ominous look to it and is worth flagging--even if it ends up passing quietly.

It must be said up front that our in-house review of storm records indicates only 2% of all the tornadoes which have occurred since 1950 in the 16 counties which include and surround Chicago have occurred in November. So, the probability of a tornado at this time of year is comparatively low historically and the prospects of such a thing happening at night would appear to further reduce the possibility of occurrence because of the absence of critical daytime heating. But, that doesn't mean it can't happen--and there appear to be compensating variables capable of encouraging storm development despite the late hour. We need only look at what happened this past week in southern Indana to remind ourselves nocturnal severe weather is entirely possible, even at this time of year. As mentioned in our vignette piece this morning (Tuesday) on the developing weather situation which ran here on our weather blog and on our Chicago Tribune weather page this morning, Dr. Joe Schaefer, Director of the Storm Prediction Center tells us nocturnal (nighttime) severe weather is often a feature of many late season storm outbreaks.

We've been monitoring this situation for several days and continue to be concerned at what we see. Severe thunderstorms and tornadoes are all to often notoriously fast-developing and the atmosphere continues to give us strong signals it's poised for a significant eruption later this evening and tonight. Our best thinking is that the period from 8 p.m. to 12 midnight warrants special vigilence for it's during this interval of time the meteorological elements of storm production come together over northern and central Illinois, southern Wisconsin and northern Indiana. The hours immediately surrounding 8 to 12 p.m. must be monitored as well. (Those farther east in Indiana, Michigan and Ohio are likely to face these storms even later tonight). If anything's going to happen, that appears to be the period during which rotating, supercell thunderstorms are most favored to erupt. A layer of cool, fairly stable air hugging the surface north of an incoming warm front is the one variable forecasters are monitoring most carefully. Such layers have been able to extinguish the low-level thunderstorm circulations necessary to spin up tornadoes, even while allowing thunderstorms to produce other severe weather attributes. The belief at this point is that other meteorological variables may prove strong enough to render the shallow layer less a factor in this case--but time will tell.

Here are the factors which make Tuesday night vulnerable to severe weather development. Unseasonably warm air to Chicago's south is loaded (by late season standards) with moisture which provides the fuel for thunderstorm development. Dewpoints in the low/mid 60s have advanced north to Quincy and Springfield as of the time of this release. (It's already 80-degrees downstate in St. Louis versus 58-degrees in Waukegan). At the same time, the the air is dry in central and western Kansas and Nebraska with dewpoints only in the 20s and 30s. Upon sunset, the humid air will remain warm while the dry air to the west will cool precipitously, strengthen the already strong southerly winds blowing over Missouri and Kansas. As these moist, strong SSW winds intersect a warm front, along which Chicago and northern Illinois' easterly winds converge with the strong SSW winds to the south, a powerful "lift" will occur, cooling the air and supporting thunderstorm growth. This convergence will enhance the upward flow of air into an unusually unusually powerful WNW jet stream aloft. SSW winds in the lower atmosphere and WNW winds aloft increase the tendency of air to rotate as it ascends. Shifting wind direction with height is called "vertical wind shear" and serves as a critical component of severe storm development. What's more, temperatures are falling more quickly than usual with height indicating an "unstable" atmosphere--one in which air is encouraged to rise and keep rising. As an intensifying low pressure approaches from the west with a cold front later Tuesday night, powerful WNW winds behind it increase the lift by converging with the south and east winds on the storm's east side. As all this happens, a pocket of powerful jet stream level winds races in from the west--with a tendency for these winds to diverge ("split") over this area.

Diverging high level winds signal an upwelling of air on a fairly large scale from below--not a good sign. Put simply, the situation appears explosive and must be monitored.

The ominous confluence of factors is to race east and out of the area well before sunrise. One of the cooler air masses of Fall, 2005 follows.

-- Tom Skilling