Discussion for April 25th-27th RainStorm

Okay, I have a lot to get through in this discussion, so let's get to it. First up is the WV imagery, so I can show you what's going on currently.

So, what are we looking at? Currently, the ULL is closed and spinning in the MW states. There is a sub-tropical jet feed right out of the Gulf of Mexico. A potent shortwave is tracking southeastward out of Central Canada, and another ULL is dropping south from Labrador. The MW ULL will slowly weaken and open as it slowly treks to the east, through the OH Valley and then eastward into New England. On the WV imagery, there is a clear -NAO pattern in place over the Northern Atlantic, which is important because it helps to slow the pattern down. However, the 12Z ECMWF is a little more progressive than the 12Z GFS in lifting everything out earlier.


Now, let's look at the current surface analysis.

This is from Intellicast, at 8PM Saturday. The warm front is slowly lifting through the OH Valley, there is a ripple of low pressure that developed on the warm front over Southwestern Virginia, and the ULL is back over the border of Iowa and Missouri. High pressure has since shifted offshore, but the ridge axis has yet to move off the coast.


There will be a persistent easterly to northeasterly fetch over the next several days. This is due to upper level confluence over Northern New England. This is important because it helps to maintain high pressure and CAD, or cold air damming to the east and northeast. The parent ULL is still well to the west. The ULL helps to lift the warm front northward overnight into Sunday morning. However, the warm front should stay south of Long Island, keeping the NYC Metro Area cool sectored. Climatology at this time of year also suggests the warm front won't move north of Long Island. An onshore moist marine flow can be expected into Tuesday, before the low level flow turns more north and northwesterly by Tuesday afternoon and evening.


Rain will enter the picture later on tonight. There could be a few hours of virga because the rain is entering a cool, dry airmass and the dry sub cloud layer. Insentropic lift is when warm, moist air overruns cool, dry air and the moisture becomes enhanced as the two air masses clash. It becomes apparent near the warm front. Omega increases at 700mb across the area. Also apparent is convergence in the 1000mb-500mb layer also near the approaching warm front. The warm front helps the lifting as moisture from the GOM overruns the warm front, resulting in a stratiform rain.


A round of rain will occur later on tonight into Sunday afternoon, before there is a break in the action. However, moist easterly flow will continue to keep in clouds, fog, and drizzle. The MW ULL could be far enough east at this point, so it provides its own lift. A second surface low, indicated by the NAM/GFS/GGEM will track from the Mid Atlantic coast eastward out to sea on Monday, resulting in another round of rain. The cold front, associated with the ULL will move through on Tuesday, bringing another round of showers as its asslociated anomalous cold pool (temperatures around -25c-30C) moves through as well.


One last thing to worry about in southern areas of the CWA is the potential for thunderstorms, in which a few could become severe. The warm front will help to destablize the atmosphere. However, positive LI indexes and very little CAPE values will help to hinder the threat. Not sure on how much sunshine we will see to help fire off convection. A few storms could become severe due to steep lapse rates and being near the warm front. Can't rule this out as dynamics should be pretty potent.


Now onto QPF values.


HPC Guidance

12Z GFS Guidance

12Z GGEM Guidance

12Z NAM Guidance

I don't foresee any flooding concerns. Some places could see over 2 inches of rainfall, but this will be spread out over the course of a few days. But expect a raw, rainy ugly couple of days!!