I would love to be able to say I know exactly what is going to transpire over the next five days and I am extremely confident in the forecast I am going to issue. The problem is that I can't say that and likely won't be able to say that until Thursday night. Over the past 48 hours, I've been following the track of a strong Sub Tropical disturbance moving into southern California and a strong Polar disturbance and eventual Polar Vortex diving into Ontario. The combination of these two features have the potential to produce a significant snowfall or a complete miss depending on how each feature interacts with each other. Naturally, the model guidance the past few days have produced a variety of solutions from a significant heavy snowfall for all of the northern Mid Atlantic to a complete miss with no snowfall on yesterday's 12Z guidance. The latest 00Z guidance has trended a bit further north, and the Philadelphia office of the National Weather Service noted the same error that I noticed this morning:
"
THE GFS INITIALIZATION OF THE CLOSED LOW THAT WAS OVER THE OHIO
VALLEY LAST NIGHT AND OVER MAINE AT 00Z WAS 30 METERS HIGHER THAN ITS 24 HOUR FORECAST. THE SIGNIFICANCE OF THIS IS THAT THE MODEL BIAS OF HANGING ON TO LOW HEIGHTS OR POLAR VORTICES TOO LONG IN THAT PART OF NOAM IS STILL OCCURRING."
The water vapor satellite image this morning now has the Sub Tropical disturbance and Polar disturbance beginning to enter the United States. Note the strength of the Sub Tropical disturbance which is driving moisture northward over the Southwestern United States and appears to be very well developed. Also note that the Polar jet stream has established a well developed trough stretching from the Rockies towards the Mid Atlantic. The observations above are important because of the potential implications on how model guidance overall is handling the development of the Polar Vortex and therefore the idea of suppressing the storm
over the southern Mid Atlantic.
The most damning argument against the suppression idea of at least the mid level moisture is trends of the indices and the forecast of those indices over the next five days. When I look for a pattern that features suppression, I usually look for a rapidly falling North Atlantic Oscillation, sometimes coupled with a
falling Arctic Oscillation. What this basically illustrates is whether a strong upper low will be present either over Ontario, Quebec, or the Canadian Maritime.
However, instead I see an argument to allow the Sub Tropical disturbance to "turn the corner" and have a more substantial impact on the northern Mid Atlantic. The NAO is
currently slightly negative to almost neutral, with a forecast for a slight dip in the index in the next 5 days, but nothing to argue for a strong suppression. Meanwhile, the AO is in a period of rapidly rising, likely meeting an apex of the positive trend in the next 2 days followed by another decline. The point however is the AO will actually end up slightly more positive the next few days than current observations. Again, this is an argument against suppression. I also took a look at the PNA observations and ensemble guidance. A weak positive PNA environment is in place with more neutral conditions expected the next 5 days. The overall average of guidance does produce a ridge axis over Montana, which does argue for a stronger Sub Tropical disturbance than currently forecasted as the magnitude of the trough should be sharper, this correction was starting to unfold in the 00Z guidance.
However, there are other considerations to discuss with the forecast, namely the impact of dry air. After seeing other storms from the past fall through this current winter being impacted by the influence of dry air, you can bet I am taking that into consideration. The air mass in place by Friday morning is a true Arctic air mass with very low dew points, like below the mid 10's for many locations, to be in place. The impact of this dry air will be an agent to "suppress" low level moisture from reaching many locations over the northern interior in my opinion even if the storm does end up further north. So while plenty of mid level moisture may be able to push north on Saturday morning into the Hudson Valley, the result may be a long period of frustrating virga while many snow lovers look at a radar that says it's snowing when it is not.
At this time, I am not confident enough to issue even preliminary snowfall accumulations. However, I am issuing a second threat map to cover the thoughts above. Given the data above, I think there is potential for this storm to end up further north with a significant amount of moisture impacting at least the Philadelphia metropolitan area and possibly the New York City metropolitan area on Friday night through Sunday morning. At this time, I think the air mass will be too dry for locations like northeastern Pennsylvania, extreme northern New Jersey, the Hudson Valley, and interior Connecticut to get an accumulating snowfall. The strength of the Arctic air mass is not going to be under forecasted by this forecaster this time and I think the magnitude of dry air in place will limit the push of snowfall to the north. However, on the other side of the coin, those in southern New Jersey, especially southeast of Philadelphia better not let their guard down. A slight shift of this storm to the north would lead to a moderate to heavy snowfall with temperatures in the 20's at the surface and around -8°C at 850 MB, leading to a high liquid to snow ratio.
The point here is that the best course of action is to take a wait and see approach on just how strong the Polar Vortex and Sub Tropical disturbances really are. If the actual observations and trends continue, I think that much of the Philadelphia metropolitan area along with central and southern New Jersey still stand at having an accumulating snowfall Friday night through Sunday. However, locations north and west of Philadelphia and New York City are likely not to be impacted by this storm given the upper level pattern and the strength of the dry air I'm expecting with the Arctic air mass.
After this storm exits, high pressure will take hold Sunday afternoon through Tuesday with scattered clouds, dry conditions and temperatures running near to slightly below normal.
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