Feb 24, 2016: Severe Weather in February

Forecast Highlights:

2.25_0230The winter of extremes continues to live up to its name, with a rare February severe weather outbreak currently ongoing across the East Coast. With a warm front having moved through earlier this evening, temperatures are surging into the upper 50s and low 60s, with sufficient lifting and elevated instability to maintain a narrow but intense squall line over the region. The squall line will move through the area later tonight, capable of producing heavy rainfall and damaging wind gusts over 50 to 60 mph.

 

 


 

Latest 2015-16 Winter Extreme: Severe Weather

The winter of 2015-16 hasn’t seen many significant winter-time systems affect the region, unlike the last two winters, but nonetheless has its share of extremes, ranging from the warmest December on record by a large margin, to Central Park’s 2nd biggest snowstorm on record, to Central Park’s first sub-0 degree reading since 1994 and its coldest February temperature since 1969, only to be followed 48 hours later by 50s and thunderstorms. Severe weather is the latest addition to this list, as an intense squall line with a history of producing widespread wind damage over the Mid Atlantic continues to approach the area.

Meteorological Analysis of Severe Weather Outbreak

A typical February often brings more in the way of cold and snow than warmth and thunderstorms. Occasionally, low pressures do tracks inland of the tri-state area, which places the East Coast within the warm sector of the cyclone, and in some of these cases conditions are favorable enough to support a severe weather outbreak. Such winter-time severe weather events are largely facilitated by synoptic-scale forcing, given the strong mid-upper level dynamics aiding in the development and deepening of the surface cyclone, and are often characterized by high vertical wind shear and low instability, given the strong low-level jets typically found east of the low pressure and the low instability given widespread cloud cover, relatively low wintertime sun angle and moist vertical profile.

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12z NAM initialization, valid at 1200 UTC (7am EST) this morning, depicting 500 hPa heights and absolute vorticity.

Yesterday’s Analysis: As the 500 hPa analysis above indicates, this system was associated with a deep upper-level low, which originated as a shear vorticity maximum digging over the western United States early this week before transitioning into curvature vorticity over Texas yesterday. Strong cyclonic vorticity advection downstream of this upper-level low, as well as upper-tropospheric divergence and ascent near the left-exit quadrant of the jet stream, facilitated surface cyclogenesis downstream of this trough, with the low pressure quickly deepening over the southern United States.

A strong southerly flow in the warm sector of this system advected a warm and moist air mass into the Gulf of Mexico coast ahead of the developing cold front, with the moistening and warming of the thermal profile resulting in increased instability with surface-based convective available potential energy (CAPE) having exceeded 1500 J/kg in some areas. In addition, significant 0-6km directional and speed shear values were observed, with the combination of the two having resulted in an environment favorable for severe thunderstorm cells that ultimately produced numerous tornadoes over Louisiana and Mississippi. Directional shear refers to the change in wind direction with height from the surface to 6 kilometers above the ground, with a wind profile turning from southeasterly near the surface to southwesterly aloft indicative of warm air advection and a curved hodograph, often an indication of a tornado potential. Speed shear refers to the change in wind speed from the surface to 6km above ground, with a rapid increase in wind speed just above ground level indicative of the presence of a low-level jet, which can be indicative of a strong wind gust potential as heavier thunderstorms are capable of mixing down some of the strong winds aloft to the surface.

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0000 UTC 25 February (7pm EST tonight) sounding from Upton, NY. Refer to the Storm Prediction Center for more information on reading Skew-Ts.

Today’s Analysis: As the low pressure continued to track northeast through Ohio, its associated warm front progressed northeast and moved through the area late this afternoon, accompanied by a rapid increase in temperatures from the 40s into the 50s near NYC, Long Island and CT and the low 60s over New Jersey with a stronger southerly wind. The air mass within the warm sector of the cyclone is also anomalously moist, with dew points having climbed into the low 60s west of NYC. The latest sounding from Upton, NY in Long Island, posted above, depicts a shallow surface inversion, characterized by temperatures and wind increasing with height, and a conditionally unstable layer above the inversion with lapse rates near the moist-adiabatic lapse rate. Due to the inversion in place, there is no surface-based instability, as an air parcel lifted from the surface will remain colder than the environmental temperature. An air parcel lifted from the 925 hPa level, which is just above the inversion, however, would remain slightly warmer than the environmental temperature throughout much of the troposphere, indicative of slightly positive buoyancy and the presence of elevated instability. This is further supported by the most unstable CAPE (MUCAPE) calculation, or the maximum CAPE that can be yielded from an air parcel lifted from any level of the troposphere, which in the sounding above is 652 J/kg.

The sounding above indicates very high surface-1km vertical wind shear of 57 knots. Unlike yesterday’s severe weather event in the Gulf of Mexico coast, however, the vertical wind shear is primarily dominated by the speed shear component, indicating that the primary risk with the squall line over the region is damaging wind gusts. The low-level jet in this case is maximized just above the inversion, with winds increasing to 61 knots (70 mph) at 969 hPa, or just 285 meters above ground level. The low-level inversion will prevent that wind from fully mixing down to the surface, although heavier showers accompanying the approaching squall line will be capable of mixing down some of the low-level jet winds to the surface, resulting in the potential for damaging wind gusts over 50-60 mph. This has been reflected by upstream observations, as the latest SPC storm reports page depicts a wide swath of damaging wind observations from North Carolina into Pennsylvania. Nonetheless, slight low-level directional shear, especially near the coast south of NYC, does indicate that an isolated tornado potential still exists.

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Latest regional radar mosaic, from the Pennsylvania State University e-Wall.

Current Analysis & Forecast: The latest regional radar, posted above, depicts a well-defined squall line ahead of the cold front, indicative of the strong synoptic-scale forcing primarily responsible for fueling this severe weather event. The SPC Storm Reports page depicts widespread reports of wind damage from North Carolina into Pennsylvania, which is not a common occurrence for any time of the year, especially during February. Numerous tornadoes were also reported over North Carolina and southeast Virginia, where the low-level directional wind shear component was greater.

The squall line will continue to move northeast through the tri-state area tonight, maintained by strong synoptic-scale forcing associated with the cold front and the aforementioned elevated instability, with timing around 10-11 PM west of NYC and 11 PM to 1 AM east of NYC. The squall line is expected to be accompanied by up to 1/2 inch of rain in a short period of time, along with damaging wind gusts in excess of 50-60 mph.

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