We have quite the storm already developing across the central US, which you can see on the latest national radar stretches from eastern Nebraska, all the way to Ohio, and south to the Gulf coast. This low is currently centered over Missouri and will track east-northeast into the southwest most portion of Pennsylvania while gradually strengthening.
This generally east to west track is the result of blocking in the form of a double barrel, or banana high pressure to our north and northwest, a low pressure system over the Canadian maritimes (our recently departed clipper), and the Polar Vortex situated over Hudson Bay in Canada. Once the low reaches SW PA, it will jump to the coastal water of New Jersey. While this is going on, copious amounts of moisture will spread into the area from the west, with scattered snow showers developing later this afternoon, and more steady snow coming this evening during the superbowl. There are several factors at play here as to who gets what when it comes to snow, ice, or rain, or a combination of the three.
It’s already plenty cold here, with temperatures ranging from near 20 in the northern areas to near 30 in the Philadelphia region, not to mention we are just recovering from a rather brutal Saturday with very cold temperatures and wind chills thanks to a fresh Arctic air mass that invaded behind our last clipper. The ground is VERY cold, and even it some areas get slightly above freezing tomorrow. Additionally, there is a healthy snowpack from the Lehigh Valley on north, which will help to keep temperatures from rising that quickly. These are the reasons why I’m worried that ice will be a problem even in the face of above 32 degree temperatures.
Additionally, the strength of the high to our north, and the position of the low our the Canadian maritimes are of paramount importance in the evolution of this storm. Both prevent the low from gaining latitude, and any deviation in position or strength from the general model consensus will result in big differences in the snow/ice/rain forecast. Models tend to underestimate cold air, especially low level cold air, and given how cold its been, and a new area of high pressure sliding into our region as the storm does, these factors will be a problem when it comes to ice.
As warm air rises up and over the colder air at the surface, snow will initially break out, but then as warmer air continues to invade, the snow will change over to sleet and then freezing rain, from south to north. The questions become how far north does this complete changeover line get, and how long does the freezing rain hang on given the above factors.
Right now, I think the models are removing the cold air too quickly, as they often do, and even when the temperature rise to 32 or 33, the ground is cold, and ice will continue to accumulate, at least for a time. The image below is from the overnight European model run, and it shows you where the snow/ice/rain lines line up approximately during the height of the storm.
All this being said, I think the complete changeover line (from snow to ice and/or rain) gets somewhere within the region of I-78 in the Lehigh Valley. It may be a few miles north, or a few miles south of this area. And that’s not to say it will be all snow above this line either, as I believe there will be some mixing with sleet even in parts of the Poconos.
Philadelphia should go to rain after an inch or two of snow. This includes the immediate southern and eastern suburbs of the city, as well as western southern NJ and northern most Delaware, and those along the PA/MD border farther west. Just outside of the city in the northwest suburbs, as well as central NJ including Trenton, 2-4″ should fall, followed by the changeover to ice and eventually rain. Farther north than that, up into the southern Lehigh Valley, the Reading area, and more northern NJ from Phillipsburg to the NYC area, 3-6″ will be the rule. 6-10″ is quite possible in the northern Lehigh Valley, and 8-12″+ in the Poconos and NW New Jersey. Please note that the lower end of snowfall accumulations will occur if there’s an earlier changeover or more mixing, whereas the higher end will occur if it is indeed colder than modeled. It will be all snow north of that into NY state and New England. I am very concerned about icing in the northwest suburbs of Philadelphia, from the area of the east-west turnpike into the Lehigh Valley, particularly below I-78, and central NJ, where I do not think ground temperatures warm up enough to prevent at least moderate ice accumulations.
The 75th percentile map below from the WPC is probably a good estimation of snow amounts, although the southern and northern fridges are likely overdone.
The large majority of this storm will occur overnight tonight into tomorrow morning, with things tapering off around and after lunchtime. Be careful later tonight after the superbowl, because it’s very likely there will be snow and/or ice falling no matter where you are, with the exception of central and southern Delaware and far southern NJ. The commute tomorrow morning will be a nightmare for those north and west of the city, particularly in the Lehigh Valley and Poconos.
After the storm departs, the cold will hold into the foreseeable future, with several more chances of winter weather in the coming weeks.
More later if warranted.