NY NJ PA Weather Forecasts

Careful Thoughts

1:15 PM

The past few hours sure have been interesting in model guidance and when looking at the observations.

Precipitation is breaking out over the Delmarva, Maryland, and beginning to develop all over Virginia.  Pressure falls are still developing over the Southeast, but the primary low is still in the Great Lakes.  As a result, warm air advection is still dominating the forecast area.  Temperatures are above freezing over much of central/southern New Jersey and southeastern Pennsylvania.  However, notice that dew points are well below freezing.  Any precipitation that does eventually fall, will force temperatures to fall and evaporational cooling may be rather strong this afternoon and evening.

Now, let’s discuss models.  If you take them at face value, they’ve already busted.  Why?  Well, in terms of the supposed dry slot over the Mid Atlantic, there should be little if any moisture nor precipitation over southern New Jersey, Maryland, nor southern Pennsylvania.  This region at was suppose to have strong dry air advection producing RH values falling below 90% and 70%.  You don’t have virga when RH values are below 70% in Virginia.  Clearly, there is a lot of moisture to work with over the East coast and this is seen very well on the water vapor images. 

So what are the models say here?  The models are producing this low on Monday night due to all the PVA rotating through this trough.  The best area of weakness starting this afternoon is going to be just off the coastal plain, thus the precipitation and low development.  It’s time to stop taking model verbatum and look at the overall pattern that is developing here. 

1. We have plenty of moisture in the atmosphere.

2.  We have a developing thermal gradient from the surface to 850 MB with much of the forecast area ending up on the cold side of that thermal gradient.  The thermal gradient will not intensify until the cold front moves off the coast to “refresh” the cold air in place. 

3. The pattern is dicating that a strong low will not develop along the coast, but rather a series of weak waves.

What we will need to watch for is how each disturbance develops and forms off the coast this evening.  Each wave of precipitation will have the ability to produce light to moderate precipitation, likely snow, over the forecast area.  However, given the fast nature of the 500 MB pattern, a concentrated well developed low pressure system seems unlikely.  This pattern is very difficult to forecast for and will have to be NOW-CAST through the next 24 to 48 hours. 

I will continue to update through the afternoon and keep an eye on those pressure falls along the Southeast coast.  There’s a lot of moisture down there, but no well developed area of low pressure, which means that while the radar shows heavy precipitation move ENE, that doesn’t mean the show is over as there is plenty of mid and low level moisture around to kick up another area of moderate precipitation.

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