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
In only 13 of the past 136 years have Chicagoans enjoyed warmer temperatures in the opening 7 days of November. Though cooler easterly winds have replaced Monday’s mild, gusty southwest flow, the first week of the month ranks as the warmest here since 1978 and the 14th warmest of the last 136 Novembers. The period’s 53.4° average temperature has soared 7.7° above the long term average dating back to 1870. Departures of that magnitude have a way of not lasting. One of Fall 2005’s coolest air masses to date arrives for a short stay later Wednesday into Friday morning. But warmth returns Saturday and may include Chicago’s third November 70°.
The current pattern may be showing the first signs of fatigue in the 1-2 week range. A chunk of frigid air off the bitter, sub-zero arctic air mass which has gripped northwest North America for more than a week, could be headed this way toward Thanksgiving.
Gusty southwest winds will boost temperatures back into the mid 60s this afternoon and again Tuesday before a cold front moves through from the northwest Tuesday night. After a couple days under the influence of cool high pressure (but still above-average temperatures for this time of the year), computer models agree on a big warm-up this coming weekend with record highs in jeopardy both Saturday and Sunday. It is rare for Chicago to experience 70° temperatures in mid November, but once a pattern is established, it is hard to break it down. With the jet stream mean position flow west-to-east along and north of the U.S.-Canadian border, bitterly cold air is trapped in northern and central Canada, and only brief incursions of cool air are allowed to penetrate this far south. Warm air holds much more moisture, and converging winds produce strong uplift and instability, so cold frontal passages will trigger thunderstorms and the potential for severe weather.
Overnight showers and thunderstorms associated with a passing cold front will move east this morning. Indications were that welcome rainfall totals in the 0.25 to 0.75" range occurred with greatest amounts on the north side of the metro area. During the week ahead, the jet stream should hold a west-east flow over the Great Lakes and with only minor diversions south, cold air should continue to hold well north of the U.S.-Canadian border. As a result, temperatures this week are expected to see-saw from 5 to 10° above normal to as much as 20° above normal. Triggering these fluctuations will be cold fronts again Wednesday and this coming weekend. Southerly flow in advance of the fronts will pump temperatures well into the 60s midweek and again next weekend. The +100 m.p.h. jet stream will continue to direct a parade of fronts off the ocean into the pacific Northwest, resulting in heavy snows in the mountains and rain or a rain/snow mix in the valleys.