Wednesday 9 a.m.
The sunny days, clear nights and partly foggy dawns continue day after day and should do so until Saturday in Chicago and through the weekend in most of the Northeast. Yesterday, I showed the ECMWF operational forecast (a suite of forecasts is created with each model, but only one becomes the "operational" version) and it showed a threatening-looking storm seemingly headed toward New York City. The model had not shown that the day before, and it looks different again today. The video shows the newest idea. The storm has not formed yet, and so any or all of the formerly proposed tracks and locations are subject to more than the usual amount of error.
Two cloud-producing systems will evolve into systems that eventually change the pattern. The clouds in the Northwest are associated with a strong upper-air trough. Only a portion of that trough will kick eastward by the weekend, and the main trough will be reinforced by another Pacific storm that will arrive in southwestern Canada on Sunday or Monday. That system may have the strength to dislodge the Northeast ridge next week. Meanwhile, the loosely organized cloud areas in the Southeast should organize into a sizable storm over the western Atlantic this weekend. Once it actually takes shape, we should be able to tell where it will go and what it will do.
The cold front is accompanied and preceded by a band of showers, some heavy. As the front continues SE today, some thunderstorms should develop.
Yesterday, there was a sharp boundary between air that was cool and dry enough to suppress thunderstorms and air that was warm and humid enough to support them. This map shows the lightning strikes from 8 a.m. ET yesterday to (almost) the same time today.
As the second low pressure area develops off the East coast, it will work in concert with a high pressure area from Canada to orchestrate cooler-than-usual conditions with showers in the Middle and North Atlantic states Thursday.
Looking ahead to <strong>next</strong> weekend, the Mothers Day Weekend, we see quite a difference between the GFS model and European models on where cold Canadian air is heading at 2 a.m. Sunday.
Rain is spreading across the Middle Atlantic states today. Dampness will linger from southern New York state to Virginia tomorrow even as the main rain area moves offshore.
For the rest of the week and this weekend, the upper-air "steering winds" will be arranged in two separate streams. The northern branch will send air from central Canada toward New England.