Sunday, April 22, 2012

Forecast for Monday, April 23, 2012

Rain has overspread the area in advance of a Nor’easter beginning to make its way up the coastline. The rain should pick up intensity during the remainder of the afternoon hours. I expect torrential rainfall to occur between 6 p.m. and 11 p.m. and even some embedded thunderstorms. Rainfall totals of two to four inches are still expected when blending the GFS, EURO, and NAM together. The NAM late morning computer model guidance shows that localized rainfall amounts in excess of four inches are possible. Even though it has been dry, this much rainfall in a short period of time will certainly lead to flash flooding/flooding of roadways, fields, small streams, and urbanized areas. There is a chance that some of the larger creeks and small rivers see some flooding if four inches or more of rain is realized over a particular basin. The large rivers will see strong rises in response to the runoff, but I do believe that they will not flood due to the very dry antecedent conditions.

Strong winds remain a distinct possibility, especially across New Jersey and Delaware. The center of the storm will move close to or over Philadelphia. There is still a 25 to 50 mile window of variation amongst the guidance as to the exact track and it can make a difference with your wind and severe weather threat. The strongest winds will be east of the center. Sustained winds of 20 to 30 MPH across interior New Jersey and Delaware with wind gusts to 45 or 50 MPH expected. Along the coastal communities, wind gusts in excess of 50 MPH are possible. With trees nearing full canopy, a soaking rain to soften the ground, and these wind gusts…some downed trees and power lines are possible. Strong, coastal low pressure areas in the spring months have been known to produce a few rotating thunderstorms on the northeast quadrant. This quadrant could clip some of our eastern counties and result in isolated….brief…weak tornadoes (more like "gust"nadoes and downbursts from excessive winds in the lower atmosphere). The wind will also result in tidal and coastal flooding this evening around times of high tide. Widespread minor tidal flooding with some scattered moderate levels of coastal flooding is expected.

On the backside of this system, there will be substantial wraparound moisture impacting the Appalachian Plateau of Western Pennsylvania into Western New York. With the unseasonably warm spring even out there…leaves are also out on the trees in that area. Temperatures are expected to crash out there and become cold enough for up to a foot of wet, heavy snow in spots. This will be extremely destructive and power outages out there could last for up to two weeks. The National Weather Service Forecast Offices in Central and Western Pennsylvania have used some unusually strong language about the developing situation to warn residents in those areas.

On Monday and Tuesday, the low will be breaking down over the northeast while keeping us cool. Scattered showers will be likely on both days around here. While we will see rain showers, the air could be cold enough for some wet snowflakes in the Poconos. A second round of strong winds is likely as the departing low interacts with an incoming area of high pressure to tighten the pressure gradient. A weak disturbance will pass just south of the area Wednesday night and Thursday, bringing another chance for showers. Next weekend looks dry.