Snow for the Mid-Atlantic?

Here we go again, a tough forecast lies ahead for midweek as a potent upper-level low currently moving into Texas helps develop a low pressure system that may produce snow for parts of the Mid-Atlantic. I don’t see this being a big deal for the I-95 corridor yet, including much of NJ, but the details are still coming together on this.

The last three runs of the GFS have been aggravating to say the least as this model has been out-of-touch with other global models. The European and Ukmet models have had a storm track that at least threatens snow for the last couple of days, but the GFS keeps changing the surface low track, which affects the cold air arriving in time to produce snow or nothing at all.

06z GFS sea-level pressure and 6-hourly precipitation accumulation valid at 1 pm EST on 12/8/11.

The 06z GFS (last night) showed a flat wave with precipitation clipping the DC metro up through southern NJ Thursday. The cold air is nearby, but precipitation amounts are nothing to write home about. This run is also way too slow compared to other models.

12z GFS sea-level pressure and 6-hourly precipitation amounts valid at 4 am EST on 12/8/11.

The 12z GFS nudged the storm north with higher precipitation amounts from DC up through Ocean County, NJ early Thursday morning. The first blue line is the 540 thickness which we use to describe the possible location of the rain/snow line. At this time stamp, the 540 thickness is approaching the DC-Baltimore area, but it’s still relatively warm in most of NJ and Philadelphia. Notice that the timing has changed as the storm is now about 6 hours faster in this run compared to the 06z run.

18z GFS sea-level pressure and 6-hourly precipitation amounts valid at 4 am EST on 12/8/11.

The 18z GFS continues to nudge the storm northward and shows a much heavier axis of precipitation with DC-Baltimore-S. NJ on the northern edge of this axis with the 540 thickness nearly overhead. This could spell snow, especially at the end of this storm. The timing seems more in check with other models, especially the European, so the event looks to be late Wednesday into Thursday morning.

18z GFS snowfall accumulation map for the event.
18z NAM snowfall accumulation for the event.

The above two maps are the 18z GFS (top) and 18z NAM (bottom) snowfall accumulation maps. The GFS is the more robust solution for the I-95 corridor with up to 4 inches of snow, while the NAM is farther north with the axis and thus, less snow in the same area. For comparison, the European (not allowed to show) has less accumulation, but does have the same axis as the GFS. . .food for thought.

Hydrometeorological Prediction Center (HPC) snowfall probability map for Day 3 (Thursday) highlighting the 4 in axis. Blue outline is 10%.

So, what does the Hydrometeorological Prediction Center (HPC) have out? (I promise this isn’t a plug just because I work with them.) The most recent forecast (above) shows the main axis of snow from the Blue Ridge up through the Shenandoah and interior mountains of the Mid-Atlantic/Northeast. For now, that’s a good start, but these trends will be interesting to watch heading into tomorrow.

So what is my forecast? Rain, to start. I expect rain to pick up in intensity late Wednesday afternoon and may start mixing with or changing to snow from DC-Baltimore up into Central NJ by early Thursday morning. For now, I don’t see anything significant for these areas as the ground is warm (thanks to a mild Fall) and the significant cold air lags behind. This could all change, especially if the storm is stronger than the models depict. I do think there will be a swath of heavier snow north and west of the 95 corridor, so be aware. The models will continue to shift things around, so this will really come down to nowcasting the event.

Regardless, stay safe and thanks for reading!

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