Feb 15, 2012: Watching This Upcoming Weekend

Forecast Highlights:


- Light rain returns again tomorrow
- Warmer again, near 50 degrees Friday/Saturday
- Weekend uncertainty - big storm potential exists


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Cloudy skies were observed across most of the area yesterday and today as a weak storm moved through the region, which produced more widespread rain showers than originally expected, covering the majority of the area. Temperatures were in the mid to upper 40s across the area today, still slightly warmer than average. Temperatures will generally stay in the mid 40s to lower 50s through Saturday, although uncertainty significantly increases by the weekend, when a storm potential exists for parts of the region, possibly including the area.


Thursday - Saturday Outlook: Mild, Rainy Tomorrow


Another weak but slightly more organized low pressure will move into the region tomorrow, bringing widespread rain with temperatures peaking in the lower to mid 40s across the area. Isolated flurries may be possible inland by the end of the storm, but no snow accumulation is expected. Friday and Saturday will both bring mostly to partly sunny skies with high temperatures reaching the mid to upper 40s inland and the upper 40s to lower 50s across the rest of the area.


Sunday Uncertainty: Big Storm Or Nothing?


It has been apparent that a storm would be likely to affect parts of the East Coast this weekend since the last update two days ago. Since then, however, uncertainty has significantly increased, with some models showing a big rainstorm for the area and heavy snow in the interior Northeast, and others keeping the storm even south of North Carolina. While there is too much uncertainty to make any high confidence forecast, there are some hints on the model guidance regarding the potential scenarios for this storm.


Set-Up For Storm: Up until this point, most of the storm potentials for the area were either expected to produce rain or stay offshore, with the storms containing the snow potentials likely to be weak. This time, however, there are several key parts to the set up that make it more supportive of an East Coast storm than the previous potentials. The most significant difference is a low pressure near Newfoundland with weak ridging near the Davis Strait. This low pressure position, known as a 50/50 low for its position at 50N, 50W, is often found during winter storms in the Mid Atlantic and Northeast. In addition, the southern shortwave in the southern US is much stronger and more moist than most of the others so far this winter, meaning that should phasing take place, the potential would be there for a big storm to develop containing heavy precipitation.

The main issue with this set up is the lack of a strong, established western ridge to result in more amplification, and the generally progressive flow. In addition, storms this winter have had trouble phasing at the right location for the I-95 corridor to see a big snowstorm, and as will be discussed below in more details, the GFS shows a nearly ideal scenario for parts of the area to see a snowstorm. The main question right now is regarding the location and timing of the northern and southern streams, and where phasing takes place, if any and if so then how much. Should less phasing take place, the progressive flow would prevent the storm from moving up the coast, with the result a wet yet mild storm moving ENE in the southern, possibly central Mid Atlantic producing rain and some snow but no big snowstorm. If phasing does take place, however, then a stronger storm would be able to move up the coast and produce heavy snow and rain for the region.


Model Overview: The model guidance over the last two days has been variable with the storm, especially with today's models. The southernmost models today were the ECMWF and the 12z CMC. The ECMWF, which 2 nights ago showed a big snowstorm for the Mid Atlantic and a moderate storm for parts of the area, has stayed far to the south of the area with the storm today, with precipitation barely making it north of North Carolina. Last night's 0z CMC run showed snow making it up to southern NJ, although the 12z run significantly shifted south, keeping the storm entirely in the Southeast. The northern models are currently the GFS and the UKMET; the UKMET shifted north with its 12z run to show a scenario close to a big snowstorm for parts of the area, with the GFS the most extreme with such a solution. Although the 6z GFS run was further south, the 0z and 12z runs had a significant snowstorm with over 10 inches north and west of NYC, while the 18z run was wetter and warmer, with heavy rain in the immediate NYC area and heavy snow in the interior Northeast. The NAM is slowly trending north, but for now remains in between the northern and the southern solutions.


To the left, I posted an image from the 12z GFS run from the NCEP model page (direct link to 12z GFS). This image shows a heavy wet snow further north/west of NYC, with heavy rain in NYC and further south/east. This is the result of a phasing solution where the storm phases at the ideal location for the area to see snow; with the uncertainty regarding the set up, the phase has to be nearly ideal for the area to see snow, as any slight shifts would either turn this into a big rainstorm or keep the storm to the south of the area. In contrast to this run, the 12z CMC keeps precipitation entirely to the south of the North Carolina/Virginia border. Timing differences are also clear with the models, as at the same time that the 12z GFS has the storm emerging off the Mid Atlantic coast, the CMC only has the storm near New Orleans. The latest 0z GFS run may serve as proof that the 12z and 18z GFS runs were outliers, as the run still phases the storm but has it further southeast.


Latest Thoughts On Storm: As shown with the models and the storm set up, there is too much uncertainty right now to make any high confidence forecast regarding this storm. However, while the GFS is likely too extreme with this storm, at least a slight north trend is possible with the southern models. For now, I am going with a slightly further north scenario than the southern models, with at least a 30-40 percent chance of light rain or snow for Sunday. The bigger risk with this set up is for the storm to miss the area to the south, instead producing rain and some snow in the southern and central Mid Atlantic, but the less likely but still possible scenario is where more phasing takes place, with a big storm producing either rain/snow or heavy snow for the area. The latter solution does not have as much support and for now the first solution of a more suppressed low is favored, but will be watched in case it becomes more likely. Stay tuned for more information on the weekend outlook with tomorrow's update.