Feb 19, 2014: Warmup Begins With Rain/Ice Today

Forecast Highlights:

gfs_42The most sustained warm-up and first widespread rain event in over a month will begin starting today, as a weak low pressure tracks through the area with occasional showers, starting out as light freezing rain/drizzle inland of NYC. Following the passage of this system, a temporary warm up will occur through the remainder of the week, peaking on Friday with highs in the 50s and rain/thunder possible. This will not mark the end of winter, however, as strong indications signal yet another cold outbreak and more snow in the longer range.




Today’s Outlook: Light Rain, Morning Freezing Rain


Posted above from left to right are the latest surface analysis and radar composite from WPC, and the 3-hour forecast of the GFS 500 millibar heights and vorticity from NCEP MAG; both are valid at 9z (4am). Yesterday’s storm system, associated with a compact low pressure but strong mid level shortwave aiding in heavy snow over eastern New England over 8-12 inches, has moved out into Atlantic Canada, while the focus shifts to the next shortwave trough over eastern Illinois, with a broad area of low pressure extending southeastward into Ohio and Kentucky. As of this morning, this area of precipitation has entered central PA into West Virginia, and continues to progress northeastward. While temperatures aloft at the 850 millibar pressure layer are above freezing, surface temperatures remain below freezing across parts of the region, leading to the development of freezing rain and drizzle. The highest risk for up to a tenth of an inch of ice is over central-NE PA into northwest NJ and interior SE NY, where temperatures are still in the 10s.

Wind direction today will remain out of the south/SSE, which will help to gradually warm temperatures and result in a changeover to rain after at least 10am-12pm, while the rest of the area outside of NW NJ/SE NY generally observes light rain, perhaps starting out with a brief period of light freezing rain as temperatures reach and climb above freezing. High temperatures are expected to climb into the mid-upper 30s inland and the upper 30s-low 40s for the rest of the area, with showers mostly ending by the mid-late afternoon hours and drying out by the evening as a breezy westerly wind briefly develops up to 10-15 mph.


Thursday – Friday: Brief Warmth Spike, Rain/Possible Thunder

6z GFS hour 42, at 0z Friday (7pm Thurs), depicting the rapidly deepening low pressure over Wisconsin with blizzard conditions to its west and a strong cold front to its south. This front will reach the area on Friday, resulting in a brief spike of warmth and a line of rain/possible thunder (image credit: PSU e-Wall).

gfs_42The main highlight over the eastern half of the US on Thursday and Friday will be a significant low pressure system, associated with the shortwave trough currently over the NW US, which organizes over Kanasas tonight before tracking northeast and rapidly deepening to reach a minimum pressure near 976mb near Wisconsin tomorrow night, leading to heavy snow and blizzard conditions in the Midwest. Additionally, the system will pull in moist, warm air from the southern US, leading to severe thunderstorms developing near the Plains and Ohio Valley tomorrow evening along the cold front, which then progresses east into the region on Friday.

The warm front will slowly approach during Thursday afternoon and evening, with high temperatures in the low to mid 40s along with isolated rain showers. Warm air advection will continue overnight with a strengthening S-SW flow aloft, aiding in the development of more widespread rain northeast of the area, but with cold surface temperatures leading to freezing rain in the high elevations of New England, northeast of the NYC area. Most of the rain will remain to the north and east, with occasional light rain showers expected overnight as temperatures slightly fall into the mid 30s inland and the upper 30s-low 40s elsewhere, holding steady as the warm front slowly approaches NYC during the overnight hours.

During Friday, weakening forcing is expected as the low pressure becomes vertically stacked over south central Canada and the vorticity maximum lifts to the north, although a strong mid level temperature gradient will persist just behind the front, and along with a strengthening low level jet and some instability ahead of the front will result in a narrow line of moderate to locally heavy rain moving through on Friday, perhaps accompanied by thunder and strong wind gusts. The exact impacts and high temperatures depend on the timing of the front; the GFS and NAM are currently the most progressive with less rain and lower temperatures, while the remainder of the models are slower with the timing; given typical model biases I am siding closer to the slower timing of the front at this time, which would support the line moving through around 12pm-2pm; this would allow temperatures in the late morning to briefly spike into the low-mid 50s near the immediate NYC area and further east, while interior NW NJ/SE NY likely struggle to warm up beyond the mid 40s. The greatest risk of strong thunderstorms exists south of the area, towards southern NJ and further southwest where more instability is expected with highs in the 60s, although the possibility of thunder and strong wind gusts over 40 mph exists for the area as well. Drying conditions are expected by the evening hours with clearing skies overnight and temperatures falling into the upper 20s-low 30s for lows overnight.


Weekend – Next Week: Winter Is Still Not Over

6z run of the GFS ensemble mean for next week, depicting anomalous ridging over northwest Canada and Alaska as a strong piece of the polar vortex dives southward into the Hudson Bay, sending frigid temperatures closer to the US (image credit: PSU e-Wall).

gefsWhile the mid to late week warmup matches well with the time of the year as the end of February approaches, this does not mark the end of winter. As noted in earlier updates, with persistence of this year’s patterns along with the aid of tropical forcing as the MJO propogates further east towards phases often more conductive for colder temperatures in the East, the strong upper level low which has recently settled near the Gulf of Alaska, breaking this year’s trend of strong ridging in this region, will only remain transient and is expected to move out over the next 1-2 days as highly amplified ridging develops over the northeast Pacific and western US region. This will support a re-establishment of a cross polar flow over Canada which has been present for the majority of January and early February, indicating that the region will generally be affected by Canadian cold air masses with a northwesterly flow over the north central US rather than the relatively zonal upper level flow seen more recently. As this zonal flow has mostly removed the strong cold air supply from Canada, the developing amplified ridging into Alaska and northerly flow over western Canada will lead to a frigid air mass surging southward into Canada from the arctic regions as a strong piece of the polar vortex likely extends to the north of the US, which by the weekend will place an anomalously cold air mass, especially for this time of the year, sitting not far to the north of the US/Canada border.

The key question remains how far south will the cold extend, as at this time a similar cold outbreak to that of January when the core of the cold air mass over North America almost entirely spilled into the US appears unlikely. The cold air mass will begin to slowly filter into the region in the weekend with high temperatures slowly falling, reaching the upper 30s-low 40s by Sunday. The latest model guidance indicates a shortwave trough will move through around Monday the 24th; most of the model guidance keeps this time frame dry besides for isolated rain/snow showers, although given trends this winter for model guidance underestimating shortwave troughs, Monday will be monitored for the possibility of more precipitation than currently forecast. A slightly higher potential for a storm system affecting the region exists around the 26-27th time frame as the strong upper level low in Canada approaches the US; while model guidance is far from a consensus at this time, this time period will be monitored as well for the potential for a storm system to affect at least parts of the region. The extent of the cold temperatures that follow this potential system into the end of February and the first few days of March has yet to be determined, which will affect the outcome of whether the region sees moderately below average temperatures or a stronger surge of anomalous cold for this time of the year, although regardless of the exact outcome, colder than average temperatures are expected to continue through at least the first week of March.

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