The Week Ahead

MON 5/19: Good thing the May sun is strong. While we've had some mild days now-and-then, we remain in a cool pattern overall thanks to a persistent upper level trough over eastern North America. This trough is preventing any true, sustained mild weather from building north and eastward and is instead keeping all the heat bottled up over the southwest U.S. Though it hasn't been terribly cold, it's the absence of warmth that's more noticeable. The weather this week will really help to hammer home that point, as daytime readings will struggle to reach the low 60s most of the upcoming week, and in some cases could remain in the upper 50s. This late in the month, the average daytime high at Hyannis is around 64F but days in the 70s become far more common. Hopefully, by next weekend, just in time for Memorial Day weekend, the pattern will shift around and allow some warmer air to build into the region.

With the jet stream featuring a deep trough over the Northeast and an accompanying upper level low centered north of the Great Lakes, a pool of cold upper level air has moved into the region. On the weather charts, you can see the upper level storm well on the water vapor loop, as it spins in place and dominates the weather pattern. In terms of sensible weather, you can "see" the cold air aloft in the development of widespread afternoon cumulus clouds, forming in response to daytime heating. Across interior New England and over New York State, these clouds are building high enough to produce some showers. The upper storm is also spinning energy centers through the Great Lakes and Northeast, enhancing the cloud and shower development, much like we saw Sunday afternoon and evening.

During the course of the work week, the weather pattern will be stuck in its current state, with the large upper level vortex only very slowly drifting eastward across New England. This makes for a challenging weather forecast, as the atmosphere will remain in a rather unstable state through Friday thanks to: 1) the cold air aloft that is conducive to at least some clouds and showers from time to time and 2) the presence of fast-moving upper level energy centers which can help spark showers. But what makes the forecast challenging is not so much the possibility of clouds and showers but the fact that it won't be doing it all the time. This time of year, the absence of clouds and wet weather can mean a fairly nice day, despite cool temperatures. For instance, Sunday, while it clouded over and showered late, much of the day was sunny and pleasant - this despite the same unstable atmosphere that will be present much of the upcoming week and could ruin any given 6 hour stretch of weather. Timing is everything.

At the current time, there seems to be two windows most conducive to unsettled weather over the next 4 to 5 days. The first comes Tuesday afternoon and evening and the second comes later in the work week. On Tuesday we will track a piece of energy as it rotates underneath the upper level storm center and makes a b-line for the Mid-Atlantic. This small energy center will help to spawn a surface low pressure that will track south of New England Tuesday night and strengthen as it does so - sound familiar? This small storm will turn winds onshore for a time and spread a shield of rain onto the Cape Tuesday afternoon and evening. While heavy rain is not expected, up to 1/4 inch is possible around the area as we come just under the northern periphery of the storm.

After a partly sunny Wednesday (a day that will be somewhat similar to Monday though with a slightly higher risk of a pop-up shower) the next best chance of wet weather comes later Thursday into Friday as the actual upper level storm center rotates across New England, producing a more widespread pop-up shower and thunderstorm situation - though again, the actual probabilities of precipitation remain rather low.

Next weekend's forecast is quite interesting, as things could turn out milder around here than computer guidance is currently suggesting (which is low to mid 60s). While we are waiting for spring to arrive this week, folks over the west and eventually the central part of the nation will be heating up under a strong upper level ridge. This bubble of heat will migrate eastward during the course of the week, but will actually be blocked and eventually shunted somewhat northward into Canada as it runs into the cold upper level low over our region. As such, while we are holding near 60F across New England on Friday, places to our *northwest* in Canada could actually be quite a bit milder.

By the weekend, the upper level trough will begin to shift offshore and the mild ridge will work eastward toward New England. The question for us is timing and temperatures. It appears that on Saturday we may still be just barely influenced by the lingering trough to our east. So while sunshine seems a good bet, temperatures remain in question. Can it get milder? The answer will depend on whether we can tap into some of that mild air from our north. If so, we could be break out so-to-speak, with temps. spiking into the upper 60s Saturday as opposed to the computer generated 62F. Northwest winds may actually carry milder air southward across New England. Either way, on Sunday, things should turn milder as the core of the chilly air finally shifts east and readings perhaps make a run at 70F. Fingers crossed.