Jan 30, 2014: Wintry Mix Expected Next Weds

Slightly revised 12am

Forecast Highlights:

gfs_namer_138_850_temp_mslp_precipThe cold air mass that has been present over the region over the last few days is finally beginning to moderate, with the warmest temperatures since mid January on the way for the region as a low pressure tracks north of the area, leading to highs reaching the 40s to low 50s on Sunday. A cool down will then occur for next week with highs back into the 30s as a storm affects the region on Wednesday with a wintry mix, while monitoring the potential for a “surprise” snowstorm on Monday (Image credit: NCEP MAG).



Wednesday, January 29 Observations:

1.29.14A coastal low that produced a significant snow/ice storm in the southeastern US brushed parts of the area early on Wednesday morning, producing light snow for most locations with up to 1/2″ to 1″ in NYC and 1-4″ in Long Island. The low moved out by the morning hours, with clearing skies for the rest of the day as temperatures remained colder than average, peaking in the low to mid 20s across the area.

The highest temperature was 26 degrees in New Haven, CT, and the coolest high temperature was 21 degrees in Andover, NJ and Morristown, NJ.



Tonight – Monday: Warmer Weekend, “Surprise” Snow Possible on Monday

usfntsfcwbgThe latest surface analysis from the Weather Prediction Center (WPC) is posted to the left, valid as of 21z Thursday (4pm). Some notable features to make note of include the area of high pressure positioned east of the area, which was positioned over the region as of last night into the morning, providing the region with clear skies and allowing for strong radiational cooling to occur; temperatures this morning fell down to -6 degrees in Westhampton Beach, NY in Long Island and -4 degrees in Sussex, NJ. A cold front can be seen over the Midwest, which will significantly weaken before reaching the region as favorable dynamics lift well to the north into Canada. As this occurs, the area of low pressure over the Rockies associated with precipitation will continue to track to the east, as the associated shortwave trough aloft continues to carve out a trough over the western US while aiding in the establishment of a broad southwesterly flow over the eastern half of the US. The low pressure is expected to emerge near Missouri and rapidly track to the northeast, which will keep precipitation north of the NYC area but will lead to increased warm air advection from the southwest, bringing the warmest temperatures since mid January into the region.

Mostly cloudy skies are expected on Saturday with high temperatures rising into the upper 30s to low 40s with a SSE wind transitioning to south and later overnight to the SW. With a continued SW flow overnight, temperatures will struggle to fall below the mid to upper 30s for most locations, with the cold front expected to move through on Sunday afternoon with the possibility of isolated showers in the morning. The raw model guidance indicates temperatures peaking in the mid 40s, but often underestimates temperatures during dry frontal passages with a westerly wind direction; along with the model guidance trending slower with the frontal passage, the temperature outlook for Sunday has been upped, and temperatures are generally expected to peak in the 46-53 degree range on Sunday before falling into the upper 30s for Sunday evening for the Super Bowl in East Rutherford, NJ.

18z GFS at hour 90, for 12z Monday (7am), depicting the mid level shortwave moving through with rain over North Carolina. This has the potential to trend northwest with moderate snow possible up to New Jersey and NYC (image credit: PSU e-Wall).

f90Monday’s outlook is a rather complicated one, with some aspects that need to be closely monitored which may lead to precipitation affecting the area. With the front pushing southeast of the area on Sunday night, the GFS progresses the front well offshore with only isolated precipitation over Virginia on Monday, while the ECM depicts a weak wave of low pressure with light snow mostly south of the area but brushing NYC. This event is associated with a shortwave trough currently off the NW US coast, which enters the southwest US on Saturday and with the southwesterly flow aloft races towards the NYC area on Monday. The model guidance has been slowly trending slower with the cold front on Sunday, thus the increase in the forecast temperatures, and placing the frontal boundary closer to the area on Monday morning. If typical model trends from the recent pattern continue to play out with the shortwave ending up stronger and/or slightly sharper than modeled, the potential exists again for a trend towards a low pressure closer to the coast, with a period of moderate snow possible on Monday morning into the early afternoon with light to moderate snow accumulations; the highest probability of this occurring is south of the area, but could extend into the area as well in the scenario where the northwest trend is more aggressive than currently forecast. At this time, a 50% chance of light snow has been introduced into the 8-day outlook, and this will continue to be closely monitored over the next 1-2 days.


Tuesday – Wednesday: Wintry Mix, Rain Return

The most significant storm system since January 21 is expected to affect the region on Tuesday night into Wednesday. A more robust shortwave trough will enter the southwestern US on Monday, and with rising heights aloft ahead of the shortwave and behind the departing Monday system over the region, is expected to gain a neutral tilt over Texas before slightly opening up as it reaches the Great Lakes region with the continued progressive flow in the upper levels, while the surface low transfers to a low pressure near the coast. With the system tapping into moisture from the Gulf of Mexico with precipitable water values over 1.5 inch, significant precipitation amounts are expected from this system from the Ohio Valley region into the Northeast. The main uncertainty at this time remains the exact impact in the area, specifically regarding the precipitation types.

18z GFS at hour 120, for 18z Tuesday (1pm), depicting the developing low pressure over the southern US, including the southern shortwave over Oklahoma. This run ends up taking the low north of the area with a little snow/ice to rain (image credit: PSU e-Wall).

f120Model Analysis: The main uncertainty at this time remains with the handling of the shortwave over the southern US, the northern stream shortwave entering the north central US, and the confluence over southeast Canada. A stronger/sharper shortwave with less confluence ahead of it will tend to end up more amplified, leading to a storm track further northwest with little frozen precipitation and widespread rain over the area; meanwhile, a weaker/flatter and slightly faster shortwave with more confluence to the north and less interaction with the northern stream would lead to a more suppressed storm track and/or an earlier and further south transfer to a low pressure near the coast, increasing the amount of snow/ice over the area while likely leading to a slightly weaker low pressure.

Since yesterday’s update, the model guidance has generally trended towards more amplification of the shortwave with a stronger, further west and longer lasting primary low pressure, with the transfer to a coastal low pressure delayed until the low is near or north of NYC. With the mid level low remaining well to the west of the area along with a lack of low level cold compared with recent storms, such a setup would be nearly certain to lead to a changeover to rain in the area following some initial snow for most and freezing rain for interior locations. Typical model biases this winter, including underestimating the strength/amplitude of incoming shortwaves would argue for a scenario similar to the current model guidance to verify, although uncertainty continues with the exact handling of the key players for the system which have yet to be fully sampled into the model guidance.


Preliminary Forecast: At this time, based on the aforementioned factors, the current outlook is for the primary low pressure to track into the eastern Ohio Valley, transferring to a secondary low pressure near NYC which continues to track northeast along the coast. This would support initial front end periods of snow on Tuesday evening and into the the early overnight hours, especially inland of NYC, with the mid level low further west supporting warm air advection in the mid levels, leading to a changeover to rain for most of the area by Wednesday morning and icing for interior parts of the area where low level cold air will take longer to erode. Steady precipitation is likely to end on Wednesday afternoon or evening, with drier conditions overnight.

As the storm is still 5-6 days away, confidence remains highest regarding precipitation falling in the area, but with more uncertainty in the specific details; the above forecast should be considered preliminary and is subject to some changes over the next few days. At this time, it is too early to go into specifics regarding snow and ice accumulations, with the first call for snow/ice totals to be posted on Friday or Saturday. This update sides with the slightly further north scenarios; the potential exists for a slightly colder and more suppressed storm which would lead to some additional snow and especially an increased freezing rain potential for interior areas. More information will be posted on this storm over the next few days as details become clearer.


Late Next Week – Next Week: Cooling Down, Another Storm Potential

The active pattern is expected to continue with a slight cool down likely behind Wednesday’s storm for the late week, with more sunshine for Thursday and Friday as a high pressure builds into the region with high temperatures likely in the upper 20s-mid 30s range. Attention then turns back to the southwestern US where the next shortwave trough will enter, and with the continued southwesterly flow aloft with ridging over the southeast US, is modeled to reach the area by next weekend with precipitation potentially affecting the region again. As this is over a week away, specific details cannot be determined with high confidence at this time, regardless of individual model outputs, although the potential for additional rain or snow exists in this time frame and will continue to be monitored.

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