Mar 3, 2013: Nor’easter Expected, Impact Uncertain

Forecast Highlights:

After yesterday’s unexpected light snow event at night, less cloud cover was observed today with highs peaking in the upper 30s to low 40s across the area. Slightly warmer temperatures are expected for Monday and Tuesday ahead of the next storm, expected to affect the region from Wednesday through Thursday night and possibly Friday. Significant impacts are likely from this storm in the Mid Atlantic region, although there is uncertainty with the exact storm impact in the NYC area.



[notice]A poll has been opened in the right side of the blog about the mid week storm. The poll will remain open until late Tuesday night, and will be closed before the storm begins.[/notice]

Tonight – Tuesday: Slightly Warmer

The deep trough over the region this weekend continues to flatten out as the cold air mass in place moderates, with temperatures slightly warming up on Monday and Tuesday. A breezy NW wind will continue through Monday night, with partly cloudy skies expected for Monday with highs in the upper 30s to low 40s across the area. As the next storm approaches the region, winds will shift to the NE overnight into Tuesday, with partly sunny skies and highs in the low to mid 40s inland and mid to upper 40s for the rest of the area.

Wednesday – Friday: Nor’easter Expected

The next storm to affect the region has its origins in the northern US with a low pressure currently located near Montana, which will track southeast over the next two days producing a widespread winter storm in the north central US. The low pressure will then track through the Ohio Valley on Tuesday, with a secondary low developing on Tuesday night into Wednesday, becoming a potentially significant coastal low especially for the Mid Atlantic, but with impacts in the area still uncertain.

Today’s model trends have been more variable compared to yesterday. Even though yesterday’s models were in agreement with the storm staying almost entirely south of the area, some of today’s runs, most notably the 18z GFS, trended further north with more precipitation in the area. The storm is approximately 3 days, or 66-72 hours, away from reaching the region; typically, this far away from a storm, additional changes still take place with the model guidance, but typically decrease in magnitude as the storm continues to approach. The GFS, CMC and SREF ensemble means remain north of the operational model runs with more significant impacts across the region, although it is also important to consider that ensemble means can be skewed by a few significant outlier members, such as those showing the heaviest precipitation with a major snowstorm over and north of NYC which is highly unlikely to be the case.

Another area of uncertainty is with the precipitation types; a stale cool air mass is expected during this time frame with a marginal boundary layer even with a north/NE wind, as temperatures during this time frame in the Northeast and Atlantic Canada peak in the upper 30s and 40s for highs and fail to drop much below freezing at night. As a result, heavy snow accumulations outside of the higher elevations especially in the Mid Atlantic into the area would depend on strong dynamic cooling with heavy precipitation rates, with a heavy wet snowstorm likely for places that do end up within the major snowstorm zone.

There are several main players in this setup; a strong shortwave currently over the northwestern US expected to dive SE into the Mid Atlantic, an area of vorticity over the southwestern US, a weakening upper level low near New England as a weakening block to its north shifts to the south, and another area of vorticity over southern Canada. The strong shortwave over the NW US will continue to track SE towards the Mid Atlantic region as the vort over the southwestern US merges with it. This part of the forecast is generally nailed down, although uncertainty continues regarding the interaction between the shortwave, the weakening ULL in New England, and the vort over south central Canada. The 18z GFS handled this in a way that resulted in a different outcome than the earlier runs; instead of the storm continuing east and offshore, it takes a track more towards the northeast, up the coast. The ECMWF remains on the southern end of today’s guidance, with hardly any precipitation north of Maryland, although it also shows the vort over southern Canada diving through the Northeast US and towards the storm, which currently does not have much support from the other models.

Preliminary Forecast: There is still some uncertainty with the forecast, and at this time it is too early to go with full confidence with the forecast. However, the overall idea at this time is for the most significant impacts from this storm staying south of the area towards the Mid Atlantic, consisting of a heavy wet snowstorm inland and heavy rain, strong winds and coastal flooding for the coast, including locations that were significantly affected by Sandy from the NJ coast and further south which need to pay close attention to this storm. At this time, my preliminary forecast for the NYC area is for occasional periods of light-moderate rain/snow, especially east and south of NYC, with more rain expected for coastal areas and wet snow inland. The forecast is still subject to some revisions, however, and it is possible that the storm ends up further north, in which case a more significant storm would affect the area with rain for the coast and a wet snowstorm inland. Regardless of how far north or south the storm is, however, windy conditions are still likely across the area, especially from NYC and further east with gusts up to or above 40 mph likely. Stay tuned for more information on this storm.

Longer Range Update:

Once tracking offshore, the coastal low pressure will significantly slow down and drift east, slowly away from the region, with mostly cloudy skies and breezy conditions lasting through at Thursday night or Friday. Temperatures will gradually warm up by Friday and the weekend with more sunshine likely, rising well into the 40s and possibly the 50s by the weekend, although this warm up is not likely to be long lasting with the potential for another storm towards early-mid next week. Stay tuned for more information on the longer range.

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