Unseasonably cool again Monday and Tuesday with a few scattered showers Monday afternoon; widespread light rain later Wednesday into Thursday before warmup of uncertain magnitude for several days and then potential temperature roller-coaster

Plain-language summary:

 

After a somewhat warmer weekend for most, it will turn unseasonably cool again Monday and Tuesday with a few scattered showers Monday afternoon along and northwest of the Appalachians, but it will otherwise be dry, with frost possible outside urban areas and away from large bodies of water Monday night. Widespread clouds and light rain with continued cool weather are likely later Wednesday into early Thursday before the weather pattern changes with a warmup through at least early the following week, but how much of a warmup is still uncertain. The following week could feature alternating periods of cool weather and summerlike warmth, with potentially stormy periods, though this is also rather uncertain.

 

Meteorological discussion:

Slightly warmer weekend for most before cooler again Monday and Tuesday with a few scattered showers Monday afternoon, but mostly dry otherwise

 

The cool, somewhat unsettled weather pattern will continue at least through Thursday and perhaps Friday, even though no heavy rainfall is expected. This weekend is somewhat warmer than earlier this week, with a temporary slightly milder southwesterly and then westerly flow with temperatures near or slightly below average ahead of gradual cold advection on the back side of low-pressure center over northern Quebec, despite there not really being any well-defined cold front. The closest feature to a true cold front is a surface trough over Maine that is triggering a line of convective showers this afternoon. Even though the high-latitude blocking is slowly easing (NAO trending to near neutral), and the AO is actually positive, the continued deep upper-level ridge in western North America (+PNA) is promoting a persistent upper-level trough in eastern North America, bringing yet another fresh chilly air mass (though again, not the coldest arctic air mass) into the region through Tuesday with a cool surface high-pressure approaching from central Canada. Given the high sun angle in May (it is already solar summer!), the sun will do its best to warm up the surface and low-levels, leading to steep low-level lapse rates and some instability. If there is enough residual low-mid level moisture, scattered showers will develop along and northwest of the Appalachians mostly Monday afternoon, where there is a subtle upper-level trigger, and when there is the most instability. Given the cold air mass, the stronger showers could produce small hail or graupel. Otherwise, it will be mostly dry, with also the chance for frost outside urban areas and large bodies of water Monday night if skies clear enough, which it likely will for many though some mid-level clouds may remain. Temperatures will be at least 10F (6C) below average again for this period.

 

Source: TropicalTidbits

 

Light rain and more cool weather later Wednesday into Thursday, then weather pattern changes and warms up through following Monday, but uncertain by how much

 

Later in the week, the North American weather pattern will finally shift east, with an upper-level trough pushing into the U.S. West Coast pushing the western North American ridge moving eastward into the central and eastern U.S. at least temporarily (PNA turning to neutral or slightly negative). But first, another disturbance/surface low-pressure system will move from the Midwest U.S. on Tuesday eastward into our region on Wednesday and will almost cut-off as the cold air to the north retreats. As the system almost becomes a cut-off low, it will slow down and produce showers and light rain across the region later Wednesday into Thursday before slowly weakening and moving to the east by Friday, though perhaps still with some lingering clouds. Given the lack of strengthening of the system or access to Gulf of Mexico moisture, heavy rainfall is not expected despite the widespread clouds and showers. This cut-off low will delay and mute the warmup a bit, though by how much is still a bit uncertain. There could be a relatively weak cold front to push cooler air again Saturday before a stronger low-pressure system ejecting from the Rockies next weekend pushes into central or northern Quebec around the following Monday, which would push a truly warm and perhaps more moist air mass into our region at that time, perhaps the warmest of the year so far. However, there is considerable model disagreement and run-to-run inconsistency on how the features will evolve, perhaps relating to how strong the leftover cut-off low and encroaching upper-level ridge are. The exact evolution, which is uncertain for now, will determine which days are the warmest, which days have rain, and how warm it will get. 

 

Source: TropicalTidbits

Source: TropicalTidbits

 

Alternating periods of cool weather and summerlike warmth possible for following week with warmth possibly winning for a change, also potentially stormy at times

 

The weather pattern change will likely persist into the following week, with a neutral or slightly negative PNA and neutral to slightly positive AO and NAO. Most indications are that our region will still be in the baroclinic zone, with the upper-level ridge in the eastern U.S. not pushing far enough north to fully encompass our region with sustained warmth. It appears to be a period of more changeable temperatures and variable cloudiness, with pushes of possibly summerlike warmth with deep southwesterly flow followed by much cooler surface high-pressure systems from central Canada. In the end, it could actually be slightly warmer than average for this period which would be quite a change, though there will likely be a stronger than usual temperature gradient with cool air hanging on to the north of our region and summerlike warmth becoming more dominant to the south of our region. This means that only a relatively small change in the upper-level ridge strength will determine if cooler air dominates, warmer air dominates, or a stormy battleground zone dominates our region. This kind of weather pattern also leads to frequent warm and cold fronts that could trigger periods of moderate to heavy rain, even thunderstorms, unlike the frequent light showers recently that do not bring a lot of total rainfall (except for last Wednesday). However, it is possible that the upper-level ridge axis just to the west of our region favors more synoptic-scale descent and westerly flow that does not favor the best moisture advection from the Gulf of Mexico, with the best moisture staying to the south of our region.

 

Source: TropicalTidbits

 

Source: Climate Prediction Center