Summerlike heat and humidity Monday through Wednesday with some showers and thunderstorms; cooler and dry Thursday, warmer Friday; likely cooler with rain next weekend before possible prolonged mostly dry period with increasingly hot temperatures

Plain-language summary:

 

The long awaited end to the persistently cool weather pattern dominating the first half of May is upon us. This begins with a rather warm but not hot weekend, even with slight cooling in northern areas today, before a brief spurt of summerlike heat and humidity Monday through Wednesday. However, there will be varying degrees of cloudiness limiting high temperatures slightly for many areas, especially in northern areas, with scattered showers and thunderstorms on Tuesday becoming more widespread on Wednesday, though heavy rainfall totals are unlikely. It will be much cooler on Thursday with mostly dry and sunny conditions before a warmer Friday before likely turning cool and rainy temporarily some time next weekend. There could be a prolonged relatively dry and increasingly warm period through the end of May afterward, but this is a change from previous forecasts and is rather uncertain, with some cooler, rainy days also being possible.

Meteorological discussion:

 

Slightly cooler today, mostly in northern areas, before heat and humidity first half of week, especially Tuesday, with scattered to widespread showers and thunderstorms Tuesday and Wednesday 

 

The persistent cool pattern of the first half of May is over, with the long-awaited weather pattern change upon us now. A pronounced upper-level trough/ridge in western/eastern U.S. weather pattern (-PNA) is firmly in place today, and associated with multiple elongated low-pressure systems to eject from the central Rockies northeastward while a subtropical western Atlantic surface high-pressure system dominates. This is allowing for a deep and strong southerly to southwesterly push of hot, humid air from Texas and western Gulf of Mexico across most of the eastern U.S. A weak cold front yesterday evening brought slightly cooler air to northern areas today with brief low-level northerly flow, but it is still warmer than most of the month earlier, especially given that the sun will shine for most. 

 

Source: TropicalTidbits

 

Source: Climate Prediction Center

 

As the disturbances from the Rockies push northeastward deep into Quebec on Monday and Tuesday, the southwesterly flow will return and push the hot and humid air into our region. However, our region will be on the northern periphery of the upper-level ridge and subtropical air mass, promoting a "dirty" warmup with varying degrees of cloudiness, first on Monday in northern and northeastern areas as the warm and humid air makes a push into the antecedent cooler air. As the warm and humid air surges in, the highs will occur late in the day for most, above 80F (27C) for most from Montreal on southwestward, while areas to the northeast will not get above 68F (20C). Then, on Tuesday, the peak of the heat and humidity will arrive and cause scattered showers and thunderstorms that could limit daytime highs. Low-elevation areas, especially in southern areas, that avoid showers and thunderstorms could reach 90F (32C) on Tuesday. Wednesday is likely even cloudier and likely cooler for most though still humid as a strong cold front approaches (associated with the trough/ridge pattern moving east), with more widespread showers and thunderstorms. However, the cloudiness may also prevent the instability needed for stronger thunderstorms especially along and northwest of the Appalachians, and the dynamics may have also weakened or sheared out too much for severe thunderstorms. Although the thunderstorms could bring briefly heavy rainfall given the high humidity, they will move fast enough given the strong mid to upper-level flow to prevent heavy rainfall totals in any one location. On Wednesday, areas southeast of the Appalachians will likely have fewer clouds and therefore be the hottest, with some areas reaching 90F (32C). Areas north of a line from Ottawa, Ontario to Bangor, Maine may never get into the hot and humid air mass, instead with the warm front stalling there and producing clouds.

 

Source: TropicalTidbits

 

Sharp cooldown for Thursday, but still not particularly chilly, before warmer Friday before likely rainy and cooler temporarily next weekend

 

After Wednesday, the North American weather pattern will de-amplify, with a broad, cool Canadian surface high-pressure system likely dominating through Friday night shunting storminess and humidity well to the south of our region temporarily. Temperatures will be below average at first on Thursday but not as much as earlier in May given the dry and mostly sunny nature of the high-pressure system . Although the high-pressure center staying near or just north of our region is not favorable for true warm advection that would lead to a quicker warmup, the strong May sun angle will warm the air mass anyway with the cessation of cold advection, leading to Friday being warmer. A storm from the southwest will likely enter our region and bring rain and temporarily cooler weather next weekend, with it affecting our region either Saturday or Sunday depending on how fast it moves, though heavy rainfall appears unlikely given the lack of strong dynamics or deep Gulf of Mexico or Atlantic moisture. The cooler air is just from the clouds and rain evaporating into an initially dry air mass (i.e. dynamic cooling), not due to any true cold subarctic or arctic air mass. If the storm is a bit farther south, weaker, and more detached from the main polar jet stream to the north, it would move slower and arrive later and possibly not even bring the cool air and rain to northern areas.

 

Source: TropicalTidbits

 

Possibility of warming, drying trend through end of May, but this is far from certain 

 

After next weekend's storm moves off to the east, for the following week through the end of May, the blocking upper-level ridge could strengthen over the Great Lakes or even northern Ontario into central Quebec, close enough to keep most of our region, especially northern areas, in a prolonged mostly dry, relatively sunny period. This is supported by a persistent broad upper-level trough in Gulf of Alaska through western Canada (-PNA). The AI model runs especially have been showing this scenario. This is a shift from the blocking ridges we saw in western North America and Greenland earlier this month that promoted persistent upper-level troughs and slow-moving cut-off upper-level lows in eastern North America. This also promotes a weak or even reversed north-to-south temperature gradient, also a shift from predictions a week ago showing our region more in the potentially stormy battleground zone between hot and humid air masses to the south and cooler air masses to the north, as I mentioned in the last blog post. This would lead to weak flow or west to west-northwesterly flow across our region from central Canada, but without any tapping of colder subarctic and arctic air. In this case, the strong May sun will heat up the source region in central Canada, but with no access to Gulf of Mexico or Atlantic moisture to produce widespread clouds or rain. This would lead to dry, sunny, and increasingly hot weather across our region, especially in western areas, with some rather dry cold fronts coming from the north possible if the upper-level ridge sets up west of our region as most models show. The strong May sunshine and dry air would also quickly dry out the ground, making it easier to get hotter during the daytime as the sun's energy is no longer going into evaporating ground moisture. It would also rapidly increase the wildfire danger. That would actually be eerily similar to the end of May 2023, which featured a flash drought from the Great Lakes to eastern Canada with an unusually prolonged period of dry, sunny weather with very low humidity, culminating in record heat with still low humidity in the beginning of June that led to a big outbreak of wildfires in Quebec, and that was also a developing El Niño summer. 

 

Source: TropicalTidbits

 

However, some other model runs show the blocking ridge being far enough to the northwest or weak enough to allow storms to enter from the southwest, west, or even northwest that would bring a period of cooler and showery or rainy weather. There may also just be a more typical faster-moving weather pattern with a succession of warmups and cooldowns with periodic rains in between associated with warm and cold fronts.