Monday, 11:55 A.M.
Hopefully this past night was the last of the freezing temperatures and frost for the vast majority of the country away from the Rockies. Temperatures slipped to freezing or lower in parts of central and northern Pennsylvania underneath high pressure, as the chilly air mass that set records in so many places from the Plains on east moved into the Appalachian. With the air mass beginning to moderate, tonight won't be quite as chilly as last night, and there should be no new places with frost that managed to escape it the past couple of nights.
As the surface high migrates from the central Appalachians this morning to the Southeast coast by tomorrow evening, a southwest flow of much warmer air will develop from the southern Plains into the Ohio Valley, pulling warmer air across the Plains eastward. That's borne out not just in the noticeable uptick in surface temperatures, but just as much in the 850 mb temperatures, shown here for tomorrow evening:
This surge of warm and increasingly humid air won't just happen with no consequences! It will run into enough resistance to generate a lot of clouds along with some showers and thunderstorms. The initial push will be through the Midwest into the Great Lakes this afternoon and tonight as an upper level disturbance tracks across the region.
The second warm pulse, stronger than the first, will ignite more strong thunderstorms in parts of the Midwest and Ohio Valley tomorrow afternoon into tomorrow night. The warm air will have little difficulty getting into the Ohio Valley, but as it tries to move farther northeast, it will run into more and more resistance thanks in part to the northwesterly flow aloft. That's revealed when you look at the big picture aloft, the Wednesday afternoon GFS 500 mb forecast for Wednesday afternoon:
With that upper-level low only slowly retreating southeastward away from New England, the upper-level flow has to go around it in some fashion, and one of the ways that happens is to southeastward and under the upper-level low. And clearly over the next two days, a new upper-level low gradually forms and passes north of the Great Lakes. It won't be as far south as this one, and the source region isn't as cold, so the cooldown behind it won't be as strong. Nevertheless, Thursday and Friday will be cool by the second half of May standards across the Midwest into the Great Lakes and Ohio Valley, and Friday Thursday through Saturday in New England. The cooldown in the mid-Atlantic states won't be as strong, but the holiday weekend will be cooler than average to start.
Meanwhile, cooling is in store for the Southwest. After the heat of last week throughout the West was trimmed over the weekend to the Southwest, an upper-level low rolling into central California tonight and tomorrow will bring the cooling into the desert regions. Showers will get into the Cascades and parts of Nevada, but it's unlikely to rain in any meaningful way in the deserts. Still, it will be rather windy and cooler as this storm rolls into place. In the wake of the upper-level low, it will warm some in the Northwest and dry out. It won't warm as much as it did last week ahead of the upper-level trough swinging in over the weekend.
The nicest weather will be across the South, where an upper-level ridge of high pressure will build this week. Underneath it, sunshine will prevails most of the week into the start of the weekend. It will begin to feel like summer too, with temperatures getting into the 90s over the second half of the week and into the weekend, with humidity levels that will steadily increase to a point. The next realistic chance of rain may not come until later in the holiday weekend, when scattered thunderstorms are apt to pop up. Before then, it should be dry with an abundance of sunshine.
Two systems will delay the onset of warm weather in the Ohio Valley and the East over the next week or so, but then it should get warm all across the country heading into the Memorial Day weekend.
A turn to much colder air over the Northeast and mid-Atlantic states will set the stage for a rain and snow storm later this weekend before it turns much warmer later next week.
It's warm now, but will turn much colder this weekend, with a storm threat later Saturday into Sunday. Warmth will return by the second half of next week.
Though it is cold now east of the Mississippi, with a couple of opportunities for snow into the weekend, a blast of warmth is due for much of the country east of the Rockies next week.
Warm air will once again surge eastward from the Plains to the East Coast this weekend and early next week. A strong storm next Tuesday and Wednesday will then be followed by colder air later next week.
A storm in Southeast Texas will generate severe thunderstorms this afternoon and tonight, and some wet snow on its western flank as it heads into the Ohio Valley tomorrow.