Dec 31, 2013: Snow Returns Thursday

Forecast revised at 3pm with an updated weekend outlook

Forecast Highlights:

f72Cold temperatures return in time for the new year along with a possibility of scattered snow showers late this afternoon, with highs in the 30s today and tomorrow. A snow event is expected on Thursday into Thursday night with light to moderate accumulations, followed by frigid temperatures by Friday and Saturday with sub-zero lows possible inland. A quick warm up is likely by early next week with another potential system, but with no sustained warm up in sight (Image credit: PSU e-Wall).


 


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Monday, December 30 Observations:

12.30.13Behind Sunday’s nor’easter which produced heavy rain with mild temperatures, a cold air mass entered the region with drier conditions and more sunshine, but surface temperatures failed to cool down as quickly, remaining warmer than average in the upper 30s inland and low-mid 40s for the rest of the area.

The highest temperature was 47 degrees in JFK airport, and the coolest high temperature was 39 degrees in multiple locations.

 

 


 

Today – Wednesday: Cold, Some Flurries

A broad upper level trough remains over the region, with colder than average temperatures expected to continue today with highs in the low to mid 30s. A shortwave trough will move through later this afternoon, spreading scattered snow showers, locally moderate, into the interior Northeast; while most will stay north of the area, some snow is possible over the NYC area, especially north of NYC, late this afternoon. Strong westerly winds at 15-25 mph will develop in the evening and early overnight hours, with mostly cloudy skies, 25 degrees and wind chills in the mid 10s expected in NYC for the beginning of 2014. Partly sunny skies will continue into Wednesday with highs in the upper 20s to low 30s with lighter winds.

 

Thursday – Friday Morning: Some Snow, Wind Expected

The first widespread storm system of 2014 is expected to affect the region on Thursday into early Friday, producing widespread snow across the region. While some uncertainty remains, confidence is increasing on light to moderate snow accumulations, with the most significant snow likely focused towards eastern parts of the area and southern New England.

6z GFS at hour 42, for 0z Thurs (7pm Weds), showing the southern shortwave over Texas, leading northern shortwave over Oklahoma, and the second northern stream energy over North Dakota (Image credit: PSU e-Wall).

f42Storm Synopsis: Three shortwave troughs are involved in this time frame; a southern stream shortwave over Texas, a northern stream shortwave currently entering the NW US, and a second area of shortwave energy over western Canada that later interacts with the NW US shortwave. The latter shortwave will dive southeast, picking up some moisture from the Gulf of Mexico while a broad surface low pressure develops over the southeastern US, with overrunning snow spreading into NY state and south-central New England. By Thursday, the Canadian shortwave digs south into the US, phasing with the leading north stream shortwave over Tennessee by that time, with the surface low pressure deepening as it tracks northeast off the coast, staying southeast of the 40N/70W benchmark.

 

Impact Analysis: Until this point, the main uncertainty has been regarding the handling of the shortwaves involved, as the southern shortwave has continued to trend stronger and somewhat slower while the leading north stream shortwave has trended towards digging further south and is a bit sharper, with more widespread rain over the southeastern US. This has also resulted in a trend towards a somewhat flatter and further south system, with the recent model runs trending drier over the NYC area, especially the ECM which yesterday depicted a major snowstorm but was not very consistent, ultimately showing a less significant event with its 0z run last night. At this point, the leading northern shortwave is currently entering the NW US, and should be sampled into the model guidance for the 12z and 0z runs tonight; while sampling does not always result in major changes, at least some minor changes are possible considering it was previously over the northeast Pacific with a lack of data sampling. Current areas of uncertainty include the sharpness of the leading northern shortwave, the handling of another weak shortwave upstream expected to enter the western US in about 42 hours, and the Canadian energy expected to phase with the leading shortwave.

Precipitation will initially spread into NY State and south-central New England on Thursday morning in the form of overrunning light-moderate snow, with at least 2-4 inches of snow likely by the evening hours. Yesterday there was some uncertainty in the location of the overrunning snow axis, although it currently appears to stay mostly, if not entirely north of the area, resulting in the lowering of precipitation probabilities on Thursday and keeping accumulations over 1 inch mainly north of NYC. At this time I maintained some continuity with light snow on Thursday, although it is possible parts of the area, mainly over New Jersey and NYC, may stay dry through Thursday evening, which would somewhat limit snow accumulation potential. The main event occurs on Thursday night, however, as more widespread precipitation develops across the region as the low pressure deepens off the coast, with a tightening pressure gradient leading to a strong northerly wind funneling down frigid temperatures over Canada into the region, leading to a changeover to snow across the Mid Atlantic region down to the coast.

Revised at 3pm

The evolution of this part of the system depends on the previously noted shortwave troughs, which will determine how much snow falls in the area and how far west the heavier precipitation extends. Current model projections would suggest at least a moderate 3-6 inch event across most of the area, with the axis of heaviest snow over southern New England with up to 8-12 inches possible. Today’s 12z model guidance continued the trend towards a flatter shortwave with a weaker and further east coastal low developing, and while the probability of a trend returning to a major storm depicted on yesterday’s 12z ECM appears low, the potential still remains for somewhat of a trend towards a closer system to the coast. As such, I kept snow totals in line with previous updates, expecting a light to moderate snow event with the possibility of significant (6-8+ inch) accumulations in eastern parts of the area and into NY state and southern New England, with light to moderate snow on Thursday night into early Friday morning providing the majority of the snow accumulations from this system in the area. The possibility of a more significant system cannot be ruled out, however, until all the fine details in the setup have been narrowed down.

 

Forecast for NYC Area: Snow is expected to develop on Thursday morning over SE NY and southern CT, continuing though the evening with light to moderate rates. Over northern NJ, NYC and Long Island, occasional snow showers are likely, but with little, if any accumulations forecast. High temperatures are expected to peak in the mid to upper 20s for most locations. By the overnight hours, steadier snow is expected to develop, with the heavier rates further east towards Long Island and Connecticut, with temperatures crashing into the 10s as a strong northerly wind develops with wind chills falling to near zero. Sustained winds up to 20-30 mph on Tuesday night will lead to low visibility and areas of blowing snow. Snow is expected to end early on Friday morning.

At this time, the outlook is for at least 3 to 6 inches of snow across most of the area, with 4 to 8 inches towards eastern Long Island and SE CT. The snow totals are preliminary, however, and subject to some changes. At this time, I am siding on the higher end of the model guidance, and it is possible totals may be slightly lowered, with the possibility of lower totals than currently forecast over northern NJ. As previously noted, however, the possibility of higher snow totals than currently forecast cannot be ruled out  until all the fine details have been narrowed down, and will continue to be monitored. More information will be posted over the next few days.

 

Friday – Beyond: Frigid Weekend, More Cold Next Week

Regardless of the outcome on Thursday night, frigid temperatures will enter the region for Friday and Saturday. With the polar vortex displaced over southeast Canada, frigid surface temperatures will remain centered just north of the US/Canada border, with the strong northerly winds behind Thursday night’s system allowing some of the cold to drain into the region for Friday and Saturday. With strong cold air advection on Friday, the high temperatures are likely to peak at 12am in the upper 10s-mid 20s across the area, and during the daytime hours are only expected to reach the low-mid 10s inland and the mid-upper 10s for the rest of the area, which along with windy conditions will result in wind chill values near or slightly below zero. With the high pressure building into the region bringing lighter winds overnight, temperatures are expected to fall below zero over interior NW NJ/Orange county, with lows in the single digits elsewhere except for NYC and the immediate coast, forecast to fall to near or slightly below 10 degrees. Wind chill values below zero are expected in the evening, and may approach -10 degrees for interior areas.

The cold air will exit faster than it entered, with a southwest flow by Saturday as ridging builds over the region ahead of another approaching system. High temperatures on Saturday are expected to reach the low to mid 20s across the area, struggling to fall overnight with increased cloud cover before warming up into the mid 30s by Sunday. Another low pressure is expected to track northwest of the area by Monday, with warmer temperatures likely returning into the 40s along with rain.

The main uncertainty in the longer range outlook is towards the second half of next week, when cold temperatures will make a return into the US but with uncertainty regarding the exact setup; some of the model runs, such as yesterday’s ECM, depicted the polar vortex plunging southward into the US with an extreme cold outbreak, while some of the more recent runs depict the vortex as remaining over Canada, but with below to well below average temperatures entering the region again but not as cold as those of Friday-Saturday. At this time, I am siding with the latter scenario, with highs returning into the 20s and lows in the 0s inland and 10s elsewhere, although there remain some uncertainties with next week’s outlook which are subject to change, especially with the handling of the polar vortex and the early week storm.

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