We're going to be stuck in this weather up until the weekend. Reason for this: the surface warm front. The ULL, which affected us with the unseasonably cold airmass of this past weekend is now a 50/50 low and for now, is not going anywhere. This can be blamed to the west based -NAO which again has set up in the North Atlantic. So the pattern up there is somewhat blocky. The west based -NAO does now allow the surface warm front to move northward over the NYC metro area. Instead, the warm front will stall to the south of New Jersey.
Let's put up the 12Z NAM as an example.
What's fascinating about this? Looks at the temperature gradient. The 10M winds show a strong onshore flow (southeasterly low level winds) north of the warm front, while places SW of the warm front are in a southwesterly low level wind flow. This is as the surface low is still transferring east-southeast of Long Island and there is a lower level circulation just off the Jersey coastline.
The surface warm front is still well back in the OH Valley, but the 700mb and 850mb warm front has surged north of the surface warm front. Very dry, stable air is sitting north of the surface warm front in the cool sector, with dewpoints in the 20's and 30's. This is due to presence of high pressure nestled over New Jersey. But over Central PA where rain has fallen, temperatures have actually fell, into the upper 30's. So if the surface warm is still back over West Virginia, what is causing the rain to fall? Temperature difference in the mid levels to the surface. This is creating pretty decent instentropic lifting. There's your rain.
Clouds did increase, due to the presence of a weakening shortwave, being sheared by a PV in Eastern Canada, and a ridge in the southern states. This shortwave is treking NE in fast upper level, relatively zonal flow. The SE ridge heights were a little bit over-forecasted by the models, and because of this, the s/w is treking further southward as the height field bent southward a bit. At the surface, low pressure is slowly weakening as it treks eastward through the OH Valley, and will transfer to a weak area of low pressure off the Long Island coast. The surface warm front will attempt to approach overnight, and this combined with insentropic lift and ascent (southerly to southeasterly moist inflow) will provide for a period of light to moderate overrunning rainfall. Not expecting much in the way of QPF, about .50 inches or so. Easterly onshore flow should continue tomorrow with temperatures hovering in the upper 40's or so. The surface low pulls out tomorrow evening, with clearing skies. Should be somewhat of a temperature gradient with cool, moist marine air to the north of the warm front, as it rides along the nose of the 500mb ridge axis, and a more pronounced southerly flow to the south of the warm front.
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