March 27, 2016: Cold Start to April Expected

Forecast Highlights:

e_tmpprs_f192Despite a slight cool down in the last few days, this month remains on track to end up among the top 5 warmest Marches on record in NYC, especially following several days last week with highs in the low 70s. Temperatures are expected to rebound into the upper 50s and 60s again by the midweek period, accompanied by rain and wind, but with remarkable model consistency for a potentially significant early April cold surge, with well below normal temperatures possible around the April 3-5 time frame.




This Week’s Outlook: Fluctuating Temperatures, Some Rain Expected


12z NMM hour 24, valid at 1200 UTC Monday, 28 March, depicting simulated reflectivity and precipitation types.

Monday: A fluctuating pattern is expected this week, alternating between sunshine and cool temperatures and brief surges of warmth accompanied by clouds and rain. The forecast period will begin with the latter, as a trough currently near Illinois will become negatively tilted as it approaches the region. Two low pressures are forecast to deepen, the first over Michigan and southern Quebec in association with strong cyclonic vorticity advection downstream of the trough, and the second offshore along the coastal baroclinic zone and further enhanced by weak jet streak coupling. The tri-state area will remain in between these low pressures with generally weak forcing for ascent, with scattered showers expected to develop tonight into Monday morning with rain totals generally below 1/4 inch.

With the coastal low inducing a northeasterly flow over the area, the warm front associated with the northern low is expected to remain to the south, with temperatures hovering in the upper 40s to low 50s until the occluded front progresses through the area towards the mid-late afternoon hours. The frontal passage will be accompanied by a shift to a strong westerly wind due to a strong pressure gradient west of the low pressure, gusting up to 40 mph, with temperatures briefly rising into the mid to upper 50s and possibly the low 60s in northeast NJ as the low-level inversion is mixed out, before rapidly cooling into the upper 30s to low 40s overnight.




0z NAM hour 42, valid at 1800 UTC Tuesday, depicting temperatures in the upper 40s. The NAM, as with most models, is often biased too cold with a downsloping northwesterly flow.

Tuesday: A brief shift into a transient cool spell is expected to occur as the low pressures phase and rapidly deepen offshore, with a strong pressure gradient persisting over the region maintaining a northwesterly wind up to 20-30 mph with gusts up to 40 mph at times. Considering that there’s higher terrain upwind over northwestern New Jersey, the associated downsloping and adiabatic warming will slightly offset the strong cold air advection, with high temperatures expected to reach the low to mid 50s.

Colder temperatures are expected overnight as a surface high pressure moves overhead, with clearing skies and decreasing winds allowing for a radiational inversion to develop north and west of NYC where temperatures may decrease into the upper 20s locally. With the high pressure centered west of NYC, a lingering light northwesterly wind will likely be sufficient to maintain a low-level mixed layer over NYC, Long Island and Connecticut, generally in the upper 30s in NYC, mid 30s in Long Island, and low to mid 30s in southern Connecticut.



Wednesday-Friday: A deep upper-level low is expected to emerge out of the West Coast, ultimately accelerating to the northeast in response to a digging trough upstream over the upper Midwest region. The strongest forcing for ascent via cyclonic vorticity advection and upper-level jet streak dynamics will be concentrated over Michigan into central Quebec, where the low pressure is expected to track, with a strong southwesterly flow in the warm sector resulting in warm and moist air advection over the region. Temperatures are expected to rise into the mid to upper 50s on Wednesday with mostly sunny skies, with highs in the low to mid 60s possible on Thursday depending on the extent of cloud cover especially early in the day. Scattered showers are expected on Thursday evening and night ahead of and along the cold front; thunderstorms are possible given weak elevated instability, but with the forecast environment unfavorable for severe thunderstorms. The timing of the front remains somewhat uncertain, which will influence Friday’s high temperatures; a faster front would allow for more clearing but more cold air advection, while a slower front would be accompanied by more cloud cover, rain and cooler temperatures.





Early April: Significant Cold Surge Potential


12z GFS hour 84, valid at 0000 UTC 31 March, depicting 500 hPa geopotential heights and standardized anomalies.

Over the last few days, rapid cyclogenesis occurred over the north-central Pacific Ocean in association with a powerful upper-tropospheric low and poleward exit jet streak region, with the upper level low expected to slowly drift east over the next few days. Continued warm and moist air advection associated with the southerly flow downstream of the trough, in addition to mid-upper tropospheric latent heating, will contribute to significant ridge amplification over the northeast Pacific Ocean and western Canada over the next few days. The GFS 500 hPa geopotential height forecast above depicts a 576 dam ridge building into British Columbia, with standardized anomalies upwards of 3 sigma implying that the forecast geopotential heights are above the 99.7th percentile.




Relative measure of predictability for the GEFS mean hour 168, valid at 0000 UTC 3 April. The darkest orange shade indicates over a 71% probability of the event occurring. Image from NCEP.

As the ridge builds into western Canada, an upper-level low associated with a frigid air mass will be progressing east over northern Canada, with the result an enhanced geopotential height gradient and a strong northwesterly jet, acting to significantly amplify the downstream trough over the eastern half of Canada. This aspect of the forecast has been remarkably consistent among both the deterministic and ensemble models over the last several days, as the image above from the GEFS relative measure of predictability shows. Consistency, both between runs and model guidance, is not necessarily an indicator of accuracy, as exemplified by last week’s nor’easter when the ECMWF ensembles were underdispersive with their depiction on a major snowstorm, although this case is likely associated with a higher degree of predictability, especially considering that a widespread cold air outbreak occurs at a larger scale than a snowstorm, and in this case is influenced by the upstream ridge over the northeast Pacific Ocean, itself an anomalous feature that has a high likelihood of verification.




12z GFS hour 192, valid at 1200 UTC Monday, 4 April, depicting 850 hPa temperatures between -15C and -20C, which is unseasonably cold for early April.

The main uncertainty at this time is the timing and magnitude of the cold air in the region. Given the aforementioned frigid air mass in northern Canada, 850 hPa temperatures of -20C to -30C will be advected southeastward towards the US-Canada border. Such values that are more typical of a mid-winter cold surge; based on the latest GFS forecast, such values would be over 3 standard deviations below normal. The raw GFS 2-meter temperature outlook depicts highs in the low 30s and overnight lows in the 10s on April 3-4; for reference, the average for Central Park is a high of 57 and low of 41. However, medium-range model guidance is often biased too cold, with the GFS often too fast with the onset of cold. Taking these biases into consideration would support the potential for highs in the upper 30s to low 40s, with lows in the 20s inland of NYC, which is at least 10-15 degrees below average.

Given the progressive nature of the upper-level flow, the cold surge is expected to remain short lasting. As the western coast ridge deamplifies and the trough lifts out of the region, temperatures will quickly rebound into the 50s and possibly 60s by the April 5-7 time frame, further aided by ridging downstream of a developing low pressure in the Rockies, but with a lack of support for any prolonged warm or cold period at this time.

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