Wednesday, 11:30 a.m.
A cold front has passed through Chicago in the past couple of hours, and the high humidity of recent days has begun to drop. That same front blasted across the Dakotas into Minnesota over the past 24 hours, generating some strong to severe thunderstorms in the process. There will be more strong to severe thunderstorms this afternoon and early tonight ahead of the front from parts of southern and eastern Great Lakes and Ohio Valley into the mid-Mississippi Valley, but the front will not be stopped.
By tomorrow, the front will be crossing New England and the Appalachians into the mid-Atlantic states, though it will be losing some of its punch as it does. The humidity will drop across New England and even down into Maryland and Virginia for a while tomorrow night and Friday into Saturday. Given how long the humidity has been high east of the Appalachians and how much rain has fallen on a daily or nearly daily basis over the past two to three weeks, the break, now matter how little it may be in areas along and east of the I-95 corridor, will be welcome!
The break is going to be significant upstream of the Appalachians. Temperatures will actually be below average by several degrees for a couple of days across the Midwest and eastern Plains all the way to the central and southern Appalachians. Here's the 6 July 10 GFS ensemble forecast of temperature anomalies for tomorrow:
Now the same, but for Friday:
Lastly, for Saturday:
As the surface high that moves into the Great Lakes and Ohio tomorrow night and Friday begins to weaken a bit over the weekend, the upper levels of the atmosphere will begin warming. In response to that, the surface temperatures will warm Friday into the weekend, bringing anomalies back up. The surface dew points will respond a little more slowly, but they'll be creeping with time over the weekend as well.
The response will be a little slower in the Great Lakes and Ohio Valley to the return of warmth and humidity, perhaps not really occurring until sometime next week, but both will go up with time. In contrast, the humidity will return rather easily along the Eastern Seaboard. The surface high will stay up over northern New England into the weekend, which will allow the surface winds to gradually turn right back into the east and southeast over time. In turn, that will push the tropical air slowly but surely back inland.
At the same time, an upper-level low forming over the Northeast by Saturday will have the effect of helping pull that moisture back toward it, especially as the upper-level ridge offshore starts to strengthen toward the coast once again. Look at the 12z July 10 NAM 500mb forecast for Saturday afternoon:
This will keep a few scattered showers and thunderstorms as part of the forecast across eastern New York and New England into the weekend, and they could easily spread back into New Jersey and eastern Pennsylvania, if not even farther south and southwest by Sunday afternoon and Monday.
With this upper-level low likely to be pinched southwestward by the two ridges (one in the Atlantic, the other over the central Plains) connecting early next week, the humidity will be pulled even farther inland, giving rise to scattered convection. Look at the GFS 500mb forecast for next Tuesday evening:
As the ridges connect across the Midwest to the Northeast, temperatures will inevitably go up, and the temporary relief from the tropical humidity and heat will be over. So, if you get that relief, enjoy it! It won't last for long!
A dominant ridge will keep it hot from the Ohio Valley to the East into next week, while the disturbance north of Cuba is slow to develop as it approaches the southeastern Gulf of Mexico.
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.