Overview: The deep low that brought the cold front through Thursday night will move off into Canada, leaving our area in an upper trough of cooler air.
This upper jet level trough will usher in a weak secondary cold front Saturday afternoon.
Behind the weak front, high pressure builds in for Sunday.
Average season high temperatures this time of year is ~72-73º
Upper Trough — GEFS jet level wind flow (250mb height) forecast for Saturday afternoon. (Click on image for a larger view.)
Saturday
The jet trough will usher in a weak secondary cold front Saturday afternoon causing some considerable high clouds (cirrus) and some considerable mid-level cloudiness in the far northwestern suburbs. (Basically, more cloudiness expected the further northwest you are.)
Sunny skies in the morning and early afternoon, some high to mid-level cloudiness in the late afternoon. More cloudiness in locations far north and west of the city. High temp 75.0º ± 1.1º NBM Model, Blue Bell. Light winds.
Sunday
High pressure builds in. Sunny skies. High temp 71.4º ± 1.1º NBM model, Blue Bell. Breezy at times.
Next Week
Next week, much of the country moves into a much warmer upper air ridge, although the northeast remains in a cooler upper northwesterly flow. Except for some impulses moving down, no significant storms.
GEFS upper wind forecast for next Tuesday. Northwesterly flow over our area (1) with upper ridge (2) in the middle of the country. Jet Flow very far north (3). Surface High pressure (H) over our area. (Click on image for a larger view.)
Thu 05:41 PM Forecast Review — The trend was for an earlier start and the heavy rain came through about 1.5 hours earlier than forecast.
Rain totals were in the 1-2.5″ range, according to the latest MRMS —Showers will linger this evening as weak low pressure formed near us.
Update Thur 10:45 AM— This morning’s high resolution models just becoming available. Front moves through 2-4 PM with heavy rain, some embedded thunderstorms and high wind gusts both prior to and after the frontal passage. Rain totals 1.5-2″ with locally higher amounts.
Here are some areas of potentially severe weather based overlap of higher CAPE values and higher helicity —
HRRR high helicity/CAPE overlapped areas in red box. 4 PM (Click on image for a larger view.)NAM-NEST helicity/CAPE overlap areas at 12 PM (Click on image for a larger view.)HIRESW-ARM-MEM2 high CAPE, High Helicity 2-4 PM
Update Wed 9:45 PM — Tonight’s models just coming in suggest an earlier frontal passage, about 3 PM. An earlier start for showers, about 10 AM. Heavy rain and high wind gusts expected earlier, about 2-3 PM. Rain will continue into the evening. About 1-2 inches of rain total.
Wed 7:20 PM — The timing of the the cold front passage for Thursday has been a difficult call for much of the week, as the models have all differed with this system. Things have come more together this afternoon, with the deep low pressure system over Ohio propelling the front through earlier than previously thought: about 9 PM Thursday.
Ahead of this front, strong dynamics, at all levels including the lowest levels (“lower level jet”) will bring gusty winds and abundant moisture.
GFS wind forecast at about 3000 feet (925 mb) tomorrow. Strong 50 mph winds will mix down to the surface. High precipitable water (PWAT) will also be in the mix. Main low pressure system will spawn low pressure over Delaware. (Click on image for a larger view.)
Some significant showers develop as early as 11 AM to noon over our area. The main dynamics move through during the afternoon. The time-frame to watch is around 4-5 PM, where high wind gusts and heavy rain will be moving through and continue until about 9 PM Thursday.
Currently, the forecast CAPE values are less than 600 (suggesting that any thunderstorms that form could be strong, but not severe) and while helicity values are high (>400), the helicity doesn’t overlap with the higher CAPE; tornadic activity is not likely.
The main issue will be heavy rain (latest NBM is forecasting over 2″ of rain) and high wind gusts. Stay tuned.
Tues 9:22 PM — A brief plunge in the jet will bring a cold front through about 1 AM Friday morning. A strong upper closed low will form over Michigan, but will lift into Canada on Friday.
A southeasterly flow of moisture ahead of this low will result in showers developing as early as Wednesday afternoon. With this frontal passage, rain with some embedded thunderstorms will develop on Thursday during the daytime and especially during the evening.
Fairly heavy rain (1-2 inches total) and increasing winds will be our weather over the 36 hour period from Wednesday afternoon until about midnight Thursday.
GFS Forecast midnight Thursday showing the backend of the rain with the cold front and the closed low over Lake Huron. (Click on image for larger view)
Another cold front moves through over the weekend.
Original post
Autumn 2021 begins on Wednesday, and just in time, the jet stream makes a deep plunge with colder air briefly pushing into the U.S..
As mentioned in Friday’s Weekend Forecast, some models have the “540 thickness line” (the generalized demarcation between rain and snow in winter) making its first appearance for the season on the weather map.
The GFS isn’t forecasting as cold a plunge in the jet flow as Friday’s ICON model, but it still shows a considerable plunge in the jet.
The cold front that is associated with this plunge will move through sometime late Wednesday into Thursday with some heavy rain possible.
Prior to the front, continued warm weather on Monday with increasing cloudiness on Tuesday. Some showers possible on Wednesdayahead of the actual front.
GFS 250 mb (jet stream level) wind flow for Thursday at 7 AM. (Click on image for larger view.)
This jet plunge will pinch off into a closed upper low that near Ohio, eventually opening up over Friday into the weekend. As a result, some of the details in timing with this week’s forecast are uncertain at this time.
