Sunday, December 23, 2012

Forecast for Monday, December 24, 2012

A mostly clear and chilly night is expected with most areas away from Philadelphia and the Atlantic Ocean dipping down into the twenties for low temperatures. In Philadelphia and along the coast, temperatures will drop to around freezing.

We start off with some sunshine for Monday, followed by increasing clouds. Precipitation should develop around or after 5 PM on Christmas Eve ( a few models bring this in a bit earlier). This precipitation will be in response to an area of low pressure moving into the Ohio Valley and Central Appalachians. I am fairly confident it will snow north of the Pennsylvania Turnpike and north of Route 1 at some point during this event. There is still some uncertainty as to where the rain vs. snow line sets up and our current thinking is it will be north of or around Interstate 95 and Interstate 195. The models have been waffling with the location of this line run to run and have trended slightly cooler today. Precipitation intensity has also come up a smidge introducing the chance of evaporational cooling turning the rain to snow in the areas that are right on the cusp of rain vs. wet snow during times of heavier precipitation (but this appears to be short-lived with warmer air coming in aloft). I also think the models might be underestimating the strength of the high to our north to an extent and this would translate to the models being too warm at the surface (but it still looks a tad too warm when taking this into account). It is going to be hard to get it snow tomorrow outside of the northern areas; although it isn't impossible to get processes to work out perfectly in favor of snow as far south as Cherry Hill, Philadelphia, and Toms River during the first moments of precipitation. I don’t think this will be much more than a light intensity precipitation rate event (despite some hints of a bit more moisture to work with). We are starting this precipitation during  or right after the peak daytime heating without much time for the air temperature to cool off and we would have to wish for a heavy burst of precipitation and this is no guarantee and the odds are against this burst happening based on the expected system strength and organization. Also, being on the right side of a low pressure area, in a modestly cold air mass, doesn't help us get snow either with an unfavorable wind direction that usually moderates temperatures (and this is very possible, especially aloft) even if in this case it is nearly negligible. So in summary, areas to the north and west have a decent shot of getting some light snowfall to accumulate. Southeastern NJ, Central Delaware, and Southern Delaware have a decent shot at receiving just rain. In-between there, it could go either way…but the odds favor rain or a mix of rain and snow (with most mix during the initial hour or two of precipitation) vs. all snow. Also, the surface temperatures will probably be above freezing, meaning in this questionable area, it would have to take steady snow for it to pile up. Don’t get your hopes up for much. As this precipitation tapers off, drizzle is likely going to occur. In the Poconos and perhaps the Lehigh Valley into Central Pennsylvania, surface temperatures could drop back to 32 degrees allowing for patchy freezing drizzle. Again, at this time, temperatures appear too mild a snowy solution with the exception of northern areas. Those northern areas include Reading, Allentown, Quakertown, and Mount Pocono. If you're in the Poconos or the Central Pennsylvania mountains, you probably stand the best chances of getting accumulating snow out of this with 1 to 3 inches possible. Some localized totals of around an inch is possible immediately below Blue Mountain. South of there, Allentown might pick up a coating to an inch of snow. By the way, the temperature profile could favor some sleet mixing with the rain in the questionable area as well. 

There could be some patchy fog as well in the evening hours of Monday into Tuesday Morning. 

Next up is the Wednesday into Thursday storm that will spawn the Boxing Day rain/snow/sleet/ and freezing rain event for the Mid-Atlantic. This system is moving onshore to the West Coast as I write this post. This likely means we aren't getting a good sampling just yet from our weather balloons and it may take another 24 hours to ingest some decent data into the model outputs. Also, I tend to favor letting the first system get through the system before pinpointing the setup of the next as the exact track of the first system can greatly influence what happens with the energy behind it. The models have also been waffling with this system as well and we are not getting a complete picture at this moment of the track and strength of the secondary low that may form along the coast. Does this low hug the shoreline, move 50 miles offshore, or move right over the Delaware Valley? That is a question that doesn't have an answer just yet and the differences are there if you pick one of the choices to my question in terms of temperature and precipitation type. Right now, the best consensus and upper air pattern analysis tends to lend credibility toward narrowing down a solution of the system either hugging the coast or moving inland over the Delaware Valley. This isn’t a lock. The current consensus means that Philadelphia could see some brief sleet or freezing rain, followed by rain. Some models indicate a drenching rain. This track would also introduce strong wind gusts to the picture, especially along the coast and east of the low pressure center. How warm we end up getting varies based on the outcome of the storm and its track. The Poconos would see snow and sleet, transitioning over to freezing rain with the potential there of an ice storm. Colder air does rush in as the system departs on this consensus track, meaning that flurries and snow showers will return for Thursday with a gusty wind. Of course, there is probably going to be a tricky area in the Lehigh Valley with regards to ice vs. rain. I am not going to get into specifics as well  on how much falls where but still note that the Lehigh Valley, the Poconos, Central Pennsylvania stand a better chance of seeing at least some frozen precipitation from this than Philly, Millville, Wilmington, or the Shore will.

Dare do I say there is a third system potentially in the works to impact us next weekend! It is way too early to speculate, especially given that some models don’t even have much of anything happening. Let us get through the mid-week storm. 

Merry Christmas and Happy Holidays!