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This week’s outlook: Pleasant start, unsettled middle and end

After plenty of rain on Friday, a cold front crossed the region, drying our weather and leading to a pleasant weekend. This trend is expected to continue through Tuesday, but conditions will start to deteriorate on Wednesday.

The two main features dominating our weather pattern currently are a surface high pressure just to our west, and an upper level cold pool over New England.

When the cold front crossed the region, a surface high pressure system built in, as is typical behind a cold front, leading to plenty of sunshine. This sunshine heats the ground quite efficiently. However, the passage of the cold front was also associated with a large upper level cold pool of air. The featured image above this post shows Monday’s NAM at 500mb valid for 2:00pm this afternoon shows a closed contour/trough of lower heights (cold air) centered directly over New England. This will help to generate cold temperatures in the middle and upper portions of the atmosphere, leading to instability (weather.cod.edu)

 

This cold pool does two things: it reinforces westerly and northwesterly winds behind it, which generates a downsloping flow, helping to warm our daytime temperatures to just below average, despite the chilly airmass and chilly nights behind the cold front. While this downsloping flow initially also helps to provide a lot of sunshine, the fact that we now have sun heating the ground beneath very cold upper level temperatures generates plenty of atmospheric instability, allowing air to rise quickly, generating clouds. This is called self-destructive sunshine — the sun heating the ground generates the warmth at the ground and the instability to generate clouds — this is typical when we are under the influence of an upper level cold pool, and explains why at times we had plenty of dark, cumulus clouds on Sunday afternoon, after an initially clear sky.

Monday’s weather: This cold pool will actually be a bit stronger than it was on Sunday, as the 500mb heights show a closed off contour, whereas on Sunday, it was still an open trough. Thus, although Monday’s weather should be very similar to Sunday’s in that the day will start off beautiful, but have an increase in cumulus clouds during the afternoon, the cumulus clouds may actually result in a few scattered showers this time around. The showers should be light and isolated in nature, however, so an umbrella should still not be needed.

Regardless, high temperatures should be in the upper 60s to lower 70s — even at area beaches since the downsloping westerly and northwesterly winds will block any seabreezes. Winds may pick up during the afternoon, however, being sustained at 10mph with some gusts to 20mph, because of strong atmospheric mixing that occurs when we have unstable conditions — the stronger winds from just above the ground can “mix” with the surface, generating stronger winds.

Moving forward to tonight and Tuesday: the upper level cold pool will slowly move out to sea, helping temperatures warm a tad. 

In the image above, notice how a ridge of higher 500mb heights begin to move in as the cold pool moves out to sea. This helps to generate a slightly warmer airmass. Thus, temperatures will not be able to drop tonight into Tuesday morning as much as they did last night, but low temperatures could still drop into the upper 40s to around 50. Additionally, Tuesday itself will be a few degrees warmer, and will not have the afternoon clouds and potential showers that today will have — temperatures should rise into the middle and upper 70s.

Tuesday night and Wednesday: Unfortunately, as the ridge of higher heights and warmer temperatures moves in, another trough will be entering the county from Manitoba and press through the Great Lakes, colliding with our ridge. This clashing of airmasses will help to generate a warm front, which will be approaching the area later on Tuesday night and into Wednesday. This will increase our chances of rain after midnight on Wednesday morning, and increase the amount of clouds and showers for Wednesday. The airmass will be relatively warm, so temperatures could still hit 70 degrees, despite the increase in clouds.

Wednesday night and Thursday: This is when the chance for heavy rain and thunderstorms will be at its highest. This is because in addition to the warm front, the trough from the Great Lakes will have turned into another upper level cold pool, and move southeastward towards our area. This further increases the amount of lift in the atmosphere, leading to the potential for heavy rain and thunderstorms on Wednesday night. Low temperatures should hold around 60 degrees.

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Last night’s GFS at 500mb valid for Thursday morning at 2:00am shows yet another closed trough/cold pool moving into our area (weather.cod.edu)

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The result of the new trough/upper level cold pool moving in on top of the warm front is to generate a surface low pressure in SE Canada, and an increase in the coverage of rain and thunderstorms. A secondary surface low pressure system will develop just off the coast on Thursday (weather.cod.edu).

High temperatures on Thursday are a bit uncertain due to the exact location of the warm front. The incoming trough may halter the warm front and weaken it, which may moderate the airmass somewhat. Despite this, we still feel that the warm front will make enough progress to push our temperatures into the mid 70s from the city and westward, and upper 60s to around 70 eastward. Although the temperatures may not be much warmer than they will be on Wednesday, being south of the warm front, yet still east of the cold front from the incoming trough could place us in the warm sector, leading to more instability. This leads to another chance of scattered thunderstorms on Thursday, especially if we can get a few peaks of sunshine to further destabilize the atmosphere.

Thursday night through the weekend: The cold front should cross on Thursday night. However, the cold pool will interact with the cold pool from today, as that one will not have moved much due to a blocking pattern. This will create an even larger cold pool just offshore. That being said, since the cold front will have crossed, our surface pressures will be higher, which will prevent widespread precipitation.

Regardless, areas of vorticity will rotate around this cold pool, and the same type of instability will result from this that we are having today — only to a larger extent. Overall, this means periods of clouds and sun on Friday and Saturday, with a few scattered showers and thunderstorms. The cold pool should move out by Sunday, leading to mostly sunny skies. High temperatures should be in the low to mid 70s.

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