Fog/stratus clouds difficult to dislodge without wind and a supply of dry air

 

Left to their own devices, fog and dense stratus clouds tend to linger this time of year. They block sunlight which, in turn, limits warming near earth’s surface. That has two effects. First, it strengthens the temperature inversion —which is the atmospheric set-up in which warm air sits above a layer of cool, moist air trapping cloudiness and fog.

 

Second, by slowing or preventing daytime warming, the air mass near the ground remains saturated with the small water droplets from which cloudiness and fog are formed. This prevents clearing.

 

About the only way to scour the thick overcast from the skies is through the introduction of stronger winds which produce “mixing” of drier air into the moisture-laden pool of air hugging the ground.

 

 

Such winds are to begin blowing here later today.  Clearing doesn’t always follow immediately. But,  the hope is these winds will start the process by which skies are to begin brightening later today. Any improvement in the weather will be welcome after a morning and midday dominated by a thick, sun-obscuring overcast.

 

 

The strongest winds are predicted to reach north and northwest sections of the Chicago area first. It’s these areas which may first see some holes punched in the overcast—a development which would permit at least some mixed sun.

 

Fog coverage extends into 7 states; visibility improvement to come only slowly

The area covered by clouds and fog is quite extensive, reaching across all or part of 7 states. This portends potentially serious flight delays for holiday air travelers since fog slows air traffic greatly.  Worse timing for this flight-delaying form of weather couldn’t be found.

 

 Strong SSW winds to ward off fog and allow strong warming Thanksgiving Day (Thursday)

Once stronger winds take hold, enough mixing is expected that widespread dense fog may not form Wednesday night (Thanksgiving Eve) — at least not with the coverage which has been observed in the just completed overnight period. And, with luck, the powerful wind regime predicted to take hold Thursday, will be able to deliver unseasonably mild temperatures to the Chicago area for the popular holiday.

 

Warmest Thanksgiving temp in 46 years possible and first Thanksgiving Day 60 in 14 years expected 

 

If these breaks in the overcast accompany the arrival of wind Thursday, mid 60s become a real possibility for Thanksgiving—temperatures rare at this time of year. Such abnormally mild readings might well be the warmest observed here on a Thanksgiving since a 69-degree high in 1966—this area’s highest Thanksgiving reading over its 142 year observational record.

 

Fall season’s coldest air yet follows Friday and Saturday

 

The warmth breaks sharply as a cold front passes Thursday night sending area temperatures into free-fall. Most of Friday will be spent in the 30s and Saturday’s 35-degree official forecast high here would qualify as Chicago’s coldest daytime temperature in the more than 8 months since March 5 last spring.

 

Disturbance expected to arrive from Plains in the Monday/Tuesday time frame could bring area its first wintry cocktail of mixed precip

 

The low pressure expected to emerge from the Rockies and move into the Plains, next heading for central Illinois and northern Indiana next Monday and Tuesday, is the first system this season which may deliver sections of the Chicago area a wintry cocktail of precipitation.

 

Lake waters are still warm and the easterly winds expected to flow off them may limit precipitation in Chicago to a chilly rain later Monday and/or Monday night—possibly mixed with some pellets of ice. But inland areas could see a mix of sleet and snow Monday night/Tuesday making this a disturbance to monitor closely.

 

Re-development of Greenland blocking signals cold close to November/chilly December open

 

Blocking in the form of an abnormally warm dome of air predicted near Greenland next week sets the stage for a chilly end to November and a cold start to December here in the Midwest.