The same storm system that gave Arkansas a powerful tornado last night, and will very likely be giving parts of the Mississippi Valley more powerful tornadoes, will be very slow to drift northeastward, thanks to an Omega blocking pattern. This will eventually usher in a multi-day period of rain, starting tomorrow afternoon, and lasting through Thursday.
What is currently a large, cutoff trough will cut off even more from the general jet stream flow over the next day or so, as it very slowly drifts northeastward. This allows it to remain to the west enough to be able to generate Gulf of Mexico moisture, but far enough to the northeast for it to reach our latitude. Had the storm been smaller in scale, the moisture would not have been as expansive in nature.
Notice the large storm system that will be in the Upper Midwest, and all of the closed, circular 500mb height contours wrapping around it. This indicates a large cutoff low that is slow to move, which is mainly triggered by the large Omega blocking to the north of the system. For more on Omega blocking, check out our article from Friday. The most efficient lifting for precipitation tends to occur on the east side of a trough, which is exactly where we will be. Also notice how to the east of the trough, the height contours are oriented from the Gulf of Mexico, straight northeastward towards our area. That will be the general track of the rain.
Let’s move forward with more details on the rain itself and its timing.
Initially, there will still be a large area of surface high pressure to our north and northeast, wedging towards our area. This will lead to rain struggling to reach our latitude and particularly longitude during a decent portion of the day tomorrow.
Today’s NAM shows this well — notice the blue lines of higher pressure pushing down towards our area, which help to lead to a bit more downward motion in the atmosphere. This is why most of the rain is to our west — closer to the low pressure center — and it dries out quickly as we head east. This leads to light rain for parts of New Jersey, but we would not be surprised if NYC, Long Island, and parts of SW CT are initially dry tomorrow morning and early tomorrow afternoon. Still, however, the chilly, marine flow from the high pressure’s easterly flow off the Atlantic will allow for the possibility of drizzle when it is not raining steadily. Regardless, although tomorrow looks cloudy and unsettled, it will certainly not be a washout, thanks to residual surface high pressure. The area that does stand the chance for some period of steadier rain will be in New Jersey — particularly western New Jersey — but even this rainfall will not be that heavy.
Moving forward to Wednesday is when the concerns for much heavier rain are elevated.
Moving forward to Wednesday morning, the wedging of higher pressure is finally shifting a bit further to the northeast. Notice how this is now replaced by blue isobars from the south — a much richer source of moisture. This leads to southeasterly flow at the surface which originated from much warmer water to the south — combine this with a large southwesterly flow from the Gulf of Mexico above the surface, which was illustrated in the previous images, and we have an incredibly moist atmospheric profile with plenty of lift. The lifting will also be aided by a developing warm front, as warm air pushes northward from the south, yet the cooler, marine air remains to the north. We will remain to the north of this warm front, which will lead to steady, heavy rain and temperatures in the low 50s throughout the day on Wednesday, while areas to the south will have more chances of severe weather. Still, with plenty of lift in the atmosphere from the warm front and large storm system, a rumble of thunder cannot be ruled out on Wednesday night, when heavier rain will continue to fall. Wednesday will definitely be a washout, so certainly make sure to have umbrellas with you. 1-3″ of rain is possible throughout the area on Wednesday, which will lead to some isolated flooding concerns. The heaviest rain of the day will be during the morning, and again during the late evening and the night. Wednesday afternoon’s rain may be a bit lighter, but we will keep you updated on this as time goes on.
The heaviest rain will last into Thursday morning, before the activity becomes a bit more scattered in nature on Thursday afternoon. The Omega block will finally start to weaken at this time, which may allow for the storm to move into Canada and weaken. This leads to the chance of sunshine for Friday and the weekend. But even with the weaker system in Canada, there is still a slight chance of a stray shower from time-to-time, but the weekend looks primarily dry with plenty of cumulus clouds — thanks to the lingering instability from the weaker cutoff low in Canada — and peaks of sun as well.