Update Tue 8:25 AM — Last night’s models show a broad area of scattered showers and thunderstorms through the late morning far northwest of the city and then scattered activity throughout the afternoon in our area.

The HRRR shows the actual cold font moving though about 5-6 PM with more organized thunderstorm activity from 5 PM lasting as late as 7 PM -8 PM in the city.

Update Mon 9:45 PM — Tonight’s early models have some disturbances moving through Tuesday morning. Some showers possible. The forecast CAPE values have increased for Tuesday afternoon along with the thermal instability and upper air support. A broad area of showers and thunderstorms are forecast for much of Tuesday afternoon. Some of these storms could be very strong.

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A somewhat unchanging, blocked upper air pattern had given us the nice weather over the weekend.

A strong, almost stationary closed upper air low pressure system over Maine will rotate a cold front through our area Tuesday afternoon.

Update Mon 2PM: This morning’s models show showers and thunderstorms a bit earlier, starting as early as 1 PM. Showers and thunderstorms are likely between 2-5 PM on Tuesday. (CAPE values will be over 1000 J/Kg, making the possibility of some strong storms. PWAT is in the 1.7″ range )

Cool beautiful weather, Wednesday through possibly Saturday, expected.

The upper air pattern continues to be stationary and blocked with the development of what is an “Omega Block” pattern:

Here’s the ICON model upper air winds for Thursday showing the omega block—

The blocked pattern may break down over the weekend.

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 —

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.

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.

Another cold front moves through over the weekend.

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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.

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.