Messy Winter Storm Coming!

Good evening everyone. We have a very interesting weather situation unfolding during the course of the day tomorrow through Wednesday for the State College area (and rest of Northeast). And of course, this winter storm system will not be clear-cut at all (just like every other one this season). Yet again, Central PA will not see JUST snow with this event as a change to sleet/frz rn for some portions of the area seems likely. So, let's take a look at what is currently going on:
Above is the current surface weather map for the US (courtesy of the HPC division of NOAA). We have a nice dome of 1036mb High pressure moving over the plains currently. This will move through the northern part of the country tonight and tomorrow bringing fresh cold air with it. To the south, in 4 corners region, lies the storm that will move NE tomorrow and bring snow with it.

The problem with the forecast (as always) is how much warm air gets brought into the upper levels of the atmosphere. We will have fresh cold air at the surface but the High pressure system will be moving out into the Atlantic during the day on Wednesday. This is not a great set-up for a major snowstorm. However, there is still a lot of potential here and certainly the possibility of a satisfying storm for the State College area.

The models have finally trended together and are close on a solution. They are not in total agreement but they are a whole lot closer than a day or 2 days ago. We'll take a look at the GFS, NAM/WRF, and the CMC (using the 12z runs for continuity...and the fact that the 18z runs are not the greatest).

12z GFS (for KUNV): Moderate snow develops after midnight on Tuesday night as the LOW moves through TN. Thicknesses are 539 dm which is barely cold enough to support snow BUT the 850 mb temps are nicely below freezing at -4.9 degrees C. As the snow intensifies toward daybreak on Wednesday, warm air is making an intrusion in the upper levels and thicknesses go up to 540 dm which is right on the border for snow (850 mb temps are borderline but still below freezing). The LOW is now over Norther VA and exits over Cape May during the afternoon on Wed. We get about .7o inches of QPF from this run which would be just over 7" of snow if it remained all snow. What concerns me is the placement of the 850 mb LOW. It goes over Northwestern PA and you don't want to be on the S side of the 850 mb LOW if you want to get the best snows. This tells me that the GFS says we will go over to sleet for a bit even though thicknesses remain marginal. Verbatim, the GFS gives us 4-8" (probably more on the 4/5" range with sleet mixing in).

850mb Heights 48 HR Forecast (HPC/NOAA)

12z NAM/WRF (for KUNV): The NAM is a bit slower than the GFS and brings precip in around daybreak (or a bit before) on Wednesday. The overall idea of the NAM is very similar to the GFS except the LOW tracks a bit further North which ends up giving us a little more warm air to deal with. Thicknesses climb to 541 at one point during the storm and the NAM even shows the surface warming to 32.1 degrees F. I find this hard to believe giving the good cold air damming situation and the fresh cold air we will be getting tomorrow. I'm confident that we will never get above freezing at the surface. Basically, the NAM wants to through a mostly sleet storm at us with about 1-3" of snow on the front end. As usual, the NAM is wetter with .90 inches of QPF. I think this is a bit overdone (although we do have a nice SW flow).

6 Hr Precip/850mb Temps 54hr Prog NAM (HPC/NOAA)

12z CMC (for KUNV): The CMC is the furthest north of the 3 models and tracks the LOW through KY, WV, over Allentown PA, and over Boston. As such, we REALLY warm the mid-levels of the atmosphere and go to sleet and most likely freezing rain for a lot of the storm. Storm QPF totals look about the same as the GFS.

My thinking: I'm going to go with a blend of the GFS and NAM and lean a bit more toward the GFS at this point. I think the NAM is too overdone with the QPF (it usually is) and I also think it gets rid of the cold air too quickly (also another bias of the model). The GFS to me has been the most consistent all along for this storm and in addition, the SREF ensemble mean leans toward the GFS as well.

Bottom line: Snow will develop around or a bit after midnight on Tuesday night and continue until about mid afternoon on Wednesday. We will definitely start off as all snow and probably pick up a quick 2-4" inches before any change over occurs. I DO think KUNV will go to sleet for a time and cut down on accumulations. I hope I'm wrong but it's hard for me to buy into all snow with the track of the 850 mb low. Around daybreak I think KUNV changes to a mix of snow and sleet and we pick up about 1-3" of that mess. And as the system moves out maybe and inch or two of snow. So, in the end, my call is for 4-7" of snow/sleet accumulation in State College. Best guess amount is 5".

Have a good night!

PS For those in the NYC area. I think NW of I-287 will see about 3-6". Far NW NJ and the Poconos will see 4-8" (local 10" in the Poconos). And NYC itself will see about 2-4" (mostly at the beginning of the storm).