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Showers and thunderstorms possible Tuesday through Thursday

Another gorgeous afternoon is on tap for today, as a ridge of high pressure remains in place. Temperatures will generally be in the upper 70s and lower 80s, though a developing sea breeze will lead to gusty winds and cooler temperatures along the shore.

Tuesday: Changes are in store for the rest of the week, however. A blocking pattern remains in-place to our northeast, which will cause a strong ocean low pressure system to retrograde to the west, towards our coast. This will weaken the ridge of high pressure, and allow a trough of low pressure to move through the Great Lakes. When a trough of low pressure interacts with a warm airmass and weakens a ridge, it is often a good recipe for shower and thunderstorm development.

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Today’s NAM model at 500mb (middle of the atmosphere) valid for tomorrow afternoon shows a large ocean system retrograding towards our coast, weakening the ridge of high pressure. At the same time, a trough will be moving into the Great Lakes. (weather.cod.edu)

One fortunate aspect is that the blocking pattern will make the trough slow to move. This means that we may still be able to salvage the entire morning and some of the afternoon tomorrow with sunshine, though clouds will be on the increase throughout the afternoon. Temperatures should rise into the upper 70s and lower 80s, with somewhat humid conditions. The threat for rain will start during the late afternoon and early evening from west to east.

As far as what we can expect with the thunderstorms, there is a decent amount of instability — particularly at the surface — and plenty of moisture throughout the entire atmosphere. This will allow for thunderstorms to develop, with the potential for very heavy rain. However, the wind shear is quite weak, and drying and cooling in the middle of the atmosphere that additionally helps for strong winds and hail will not be there, as we do not have a cold pool truly interacting with our warm airmass. This means that strong wind gusts are not expected, and the chance for hail — while not zero — will be small. The slow wind shear will not allow thunderstorms to maintain strong to severe levels for very long, but will mean that the storms move very slowly, which could further cause flash flooding. The biggest threat for more sustained rainfall will be on Tuesday night.

Moving forward to Wednesday, a cold front in association will never clear the region, so more scattered showers and thunderstorms are possible during the afternoon. However, although the moisture will be there, the instability will not be as impressive, so the heavier rain threat should be a bit lower. Temperatures should once again rise into the upper 70s and lower 80s, with humid conditions.

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Today’s NAM Model at 500mb valid for Thursday afternoon shows a potent system moving into our region, bringing the potential for severe thunderstorms. The GFS model does not agree, however. (weather.cod.edu)

Thursday: Where exactly the front stalls will be critical to the forecast, because if the front remains close enough to our area, this day has by far the most potential for severe weather. If the front sags a bit further south, then the severe weather would be relegated to the Mid-Atlantic. A disturbance moving into the Plains that may produce a major severe weather outbreak tomorrow will continue to trek eastward throughout the week. Thanks to the blocking pattern, the trough moving into the Great Lakes will be slow to move out, and the aforementioned disturbance will interact with that trough, creating a much larger trough.

This leads to the potential for plenty of lift and mid and upper level instability, as a cold and dry mid and upper level atmosphere will be present. Additionally, the mid-level winds will be quite strong — between 40 and 50 knots at 500mb — which allows for plenty of wind shear (change in speed and/or direction of winds with height), which allows severe thunderstorms to organize and maintain themselves.

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Today’s NAM Model valid for Thursday afternoon shows a favorable combination of wind shear and instability for severe thunderstorms. (weather.cod.edu)

If the front remains far to our south, then we will not see much in the way of surface instability and moisture to generate and maintain strong thunderstorms. But if the front is closer to us, then a strong line and complex of thunderstorms forming to our west will be moving towards our area, and may be able to maintain its strength — producing strong to severe thunderstorms with gusty winds and hail, and would easily be our biggest threat for severe weather this season to date. The NAM model shows a favorable environment for thunderstorms, while the GFS model does not.

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