Feb 25, 2014: Light Snow Tomorrow, Cold Continues

Forecast Highlights:

rad31A colder than average pattern has been established over the region again, with high temperatures in the 20s and low-mid 30s expected to continue into next week. The first upcoming snow event is tomorrow morning with light snow showers, followed by a brief surge of colder temperatures for Thursday night into Friday and isolated snow showers on Saturday. A more widespread storm potential is likely next Monday, however, with additional snow and rain expected across the region (Image credit: PSU e-Wall).




Today – Wednesday: Light Snow Expected


Posted above from left to right are the latest surface analysis and radar composite from WPC, and the initialized NAM 500 millibar heights and vorticity from NCEP MAG; both are valid at 12z (7am). The shortwave trough which yesterday was positioned near North Dakota has progressed to the ESE and is currently near Ohio, with enhanced upward vertical motion ahead of this shortwave supporting areas of light to moderate snow in southern Pennsylvania into Maryland and northern Virginia. This shortwave will continue eastward today, with most of the snow remaining south of the area but with isolated snow showers possible at times early this afternoon mainly south/east of NYC, with otherwise mostly cloudy skies and highs in the upper 20s inland and low 30s elsewhere.

With a strong upper level northwesterly flow aided by the strong block-like feature over Alaska enforcing a northerly wind over western Canada dragging down a strong piece of the polar vortex and a frigid air mass along with it, an active train of shortwaves will continue to approach the region; despite no major snow event this week, multiple minor snow events are expected starting with today. The next event will affect the region on Wednesday morning, as a shortwave currently over the northwest US continues ESE, reaching Ohio by tomorrow morning. Over the last few days, the biggest uncertainty was regarding the exact setup of this shortwave, including its strength and amplitude, which would determine the intensity and track of the low pressure developing offshore and snow totals over the region. A few days ago, the ECMWF and its ensemble mean supported a more significant 3-6 inch snow event, while the remainder of the model guidance was more suppressed with less than 1-2 inches. My forecast from the 22nd noted this, and sided slightly less amplified than the ECMWF but still predicted at least up to 2-4 inches of snow. In this case, despite recurring trends this winter for model guidance to underestimate the amplitude of shortwaves entering the US, the model guidance ended up trending towards a weaker and less amplified shortwave, with a very weak low pressure developing offshore and weak lifting supporting a quick 6-hour period of light snow on Wednesday morning accumulating up to an inch and ending by 12pm for most locations. This case also shows the difficulty in relying solely on the ECM and its ensemble mean, despite their typical higher reliability when in agreement, as both depicted a solution much more amplified than the latest expectation. Even though the ECMWF has been more reliable throughout most of this month, especially with the three major storms in the first half of February, its verification with storm systems has not been strong in most of January and parts of December.


Thursday – Saturday: Cooling Down, Near-Zero Wind Chills Likely

Following Wednesday’s quick light snow event, weak cold air advection overnight will allow for overnight lows to fall into the single digits in interior NW NJ/SE NY and the 10s elsewhere, generally in the upper 10s near NYC. Slight warming is possible by the morning hours as a stronger low pressure approaches northern NY state associated with the southern end of the strong polar vortex. With a weakening northerly flow over western Canada and a lack of blocking near northern Canada/Greenland, along with the polar vortex becoming elongated over southern Canada, preventing further southward displacement and keeping the frigid temperatures trapped north of the US/Canada border, a brief period of strong cold air advection behind the low pressure will drag some of the cold into the region on Thursday night into Friday, supporting a brief period of much colder temperatures along with windy conditions, with a westerly wind at 15-25 mph expected on Thursday, gusting up to 40 mph. Winds will gradually subside overnight as lows fall to near zero in interior NW NJ/SE NY, low-mid 10s in NYC and the coast, and the upper 0s to low 10s elsewhere, with wind chill values in the single digits to near zero likely for most locations and likely falling below zero inland.

A surface high pressure will move through on Friday, keeping high temperatures well below average in the mid 20s for most of the area; this is nearly 20 degrees below the average high temperature for late February. As the polar vortex begins to retreat northward and a weak shortwave trough races east through the region, mostly cloudy skies are expected for Saturday with warmer temperatures, likely peaking in the upper 20s to low 30s, along with isolated snow showers mainly near and north of NYC.


Sunday – Next Week: More Widespread Rain/Snow Expected

The main highlight of the forecast is towards next Monday, as a widespread storm system likely affects the eastern half of the US. Some changes will begin to evolve in the pattern as the southern stream becomes active over California, bringing widespread rain to locations which have been under a drought for the majority of the winter, leading to a trough setting up over the western US, which along with the retreating polar vortex will allow for weak ridging to develop over the southeast US and support a southwesterly flow across much of the central-eastern US. The first such shortwave will enter California on Wednesday, racing east and producing the light snow event on Saturday in the northeast US. A stronger shortwave trough will land onshore around Friday night, and will continue to progress ENE through the central and northeast US while possibly interacting with the northern stream. This type of setup often supports a high probability of precipitation in the northeast US in the longer range, and along with strong support from the model guidance and ensemble members, a 90% chance of precipitation has been added into the 8-day forecast for Monday.

As this is still a week away, there remain many uncertainties with the more specific details, including the exact track, timing and precipitation totals. Given the presence of ridging in the southeast US along with an antecedent cold air mass, this system is expected to produce snow across parts of the region, currently likely to include the area as well, but with the potential for rain to reach the area as well in the more amplified scenario, which is a risk under this type of setup if the system is too amplified. The exact timing is in question as well, with the ECMWF depicting a fairly quick event ending by Monday while the GFS supports a two-wave event stretching into Tuesday, while the model guidance also backed away from the earlier depiction of a high pressure building in on Sunday and now support a frontal boundary stalling nearby keeping snow in the forecast during Sunday into Sunday night. As the key players for this system are still in the Pacific Ocean and northern Canada, regions of poor upper level data sampling, additional changes are expected over the next few days, and a preliminary timing of Sunday night through Monday evening is currently forecast but is subject to change. More information will be posted on this system over the next few days.

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