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I probably also should make everyone aware of the fact that this is a lot different of a system from the January 15 Saskatchewan screamer, which produced 6-10 Inches of snow in the New York City area, and out on Long Island. The reasons why are talked about in the rest of the post. Usually a Saskatchewan screamer will produce 2-4 Inches (locally somewhat more or somewhat less) 50 to 100 miles north of the track of the vorticity maximum, and 4-8 Inches 100 to 200 miles north of the track of the vort. Of course, the vort must be stronger than +20 and the 500mb low has to close off before the s/w reaches 80 degrees west longitude. In cases where the vort is weaker than +20, or the system remains an open wave (no 500mb low forms), 1 to 2 inches of snow can be anticipated 50 to 100 miles north of the track of the vorticity maximum, and 2 to 4 inches 100 to 200 miles north of the track of the vort. Other factors could enhance the ability of the system to produce heavier totals or those factors may act against it, therefore, resulting in less accumulation (these being, ratios, relationship between moisture, omega and the snow growth layer, speed of the s/w, CSI/CI, etc). This though is just what's normal, as I want everyone to know the basics of how these systems work.
The current front running shortwave, (an Alberta Clipper) which could have provided the phasing needed to catch the subtropical jet energy, induce phasing and bring a major snowstorm to a lot of people in the Eastern part of the country, obviously will not, but at the same time, it won't go through the northern Mid Atlantic region harmlessly.
This Clipper isn't going to be that strong, so snowfall accumulations in most places will stay under 4 inches. Remember, an Alberta Clipper normally produces 2 to 4 inches 50 to 100 miles north of the track of the vort, and 4 to 8 inches 100 to 200 miles north of it, if the vorticity maximum is greater than +20, and the s/w closes off at the 500mb level (develops a 500mb low) before reaching 80 degrees west. This Clipper won't form a 500mb low, but the vorticity will be greater than +20. So if that’s right, then the system should give areas 50 to 100 miles north of the track of the vorticity maximum 1 to 2 inches of snow, and those 100-200 miles north of it, up to as much as 2 to 4 inches. But like always it seems, there’s some things complicating this situation. These factors mostly suggest less snowfall that what an average clipper of the same variety would produce
First, sun angle, which at this time of year is about the same as what it would be in Mid October, so more solar radiation makes it through the clouds and helps to warm the surface, and melt the snow on the roadways. This takes on a lot of importance in snowfall forecasting during the later half of the season, especially in March and Early April.
Second, 1000-500mb thicknesses are marginal for good snow to liquid ratios. The 0z RUC suggests 1000-500mb thicknesses of between 5340 and 5400 across the area where the heaviest QPF is being generated (Most of Pennsylvania and into New Jersey, decreasing east), this would suggest a close to 10:1 ratio.
The initially pretty decent looking jet structure, with much of Pennsylvania in the right front entrance region of the departing 140 Kt, 250mb jet eventually weakens through the afternoon.
In regard to Convective Instability, initially (Using UNV, (State College, PA) as an example) the ETA has good convective instability signal with Total Totals indices running near 40, and 500-700mb Lapse rates of greater than 6.00 C/km, Some weak CAPE is also present, as we can see from the 12 hour ETA sounding below valid at 12z Tuesday.
By 18z though, TT indices decrease, and lapse rates which initially were mostly dry adiabatic (above 6.00 C/km) are now around 5.70 C/km and moist adiabatic, which is more stable. Furthermore, with the exception of central Pennsylvania where there more negative EPV is forecasted, further east into Eastern Pennsylvania and New Jersey there is little CSI signal at all.
Maximum Isentropic ascent, and Theta-e advection seems to be across the Delmarva, where the GFS output suggests more QPF than does the previous runs of the ETA. The last complicating thing is the marginal boundary layer temperatures, which combined with the increasing sun angle (as just talked about) isn’t a great thing to see when you’re looking for accumulations on the roads, or other paved surfaces.
This probably will be an 8 to 12 hour moderate event across much of Pennsylvania, New Jersey and the Northern Delmarva. A general 2-4 inches of snow will fall during this period, with some 5 inch totals maybe across the Laurel Highlands and Central Mountains of Pennsylvania, in the favored upslope regions. I really can’t commit to anything more than that given the marginal ratios, boundary layer temperatures, and decreasing instability. As far as snow growth goes, the GFS is the only model which hinted at anything favorable, but it only bring the good saturation and omega into the -12 to -15 Celsius layer for a short time. This of course, also doesn’t do much to help an already complicated situation.
After the latest run of the ECMWF, I don’t think the Southern branch system will effect anyone across the Northern Mid Atlantic or Northeast.
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