Mar 10, 2014: Rain Weds, Then Cold & Windy

Forecast Highlights:

f60After a brief cool down yesterday and scattered snow showers this morning, another brief warm up is expected with highs in the mid-upper 40s today and the mid-upper 50s on Tuesday. A significant storm system will affect the region with rain and wind in the NYC area and snow well to the north, near northern New England, but with a flash freeze expected overnight as temperatures crash into the 20s by Thursday, along with strong winds gusting to near or over 40 mph. This cold will be brief as well, with a warm up in time for next weekend.




Today – Tuesday: Mild Temperatures Return


Posted above from left to right are the latest available surface analysis and radar composite from WPC, and the initialized GFS 500 millibar heights and vorticity from NCEP MAG; both are valid at 12z (7am). The scattered snow showers that affected the area this morning can be seen above, which have since shifted offshore. A relatively zonal flow can be seen in the 500mb map over the northern US; at the 300 millibar pressure layer, not shown above, a relatively strong jet streak is positioned over the US/Canada border near Minnesota and North Dakota, supporting a continued strong upper level westerly flow. The shortwave trough near southern Manitoba, Canada will rapidly track ESE into the northern Northeast, exiting the region tomorrow with rising heights aloft as a weak frontal boundary stalls near the area. With the westerly flow advecting a warmer air mass into the region, a brief but significant warm up in temperatures is expected on Tuesday, with partly sunny skies and highs peaking in the mid to upper 50s.


Wednesday: Rapidly Deepening Low Pressure Passes Nearby; Rain, Wind Expected

A significant low pressure system will affect the region on Wednesday, with the most significant impacts expected across the region since the February 13-14 storm. There are three key features that lead to the development of the system; the upper level low in northern Mexico, which will open up into a shortwave trough as it track ENE through the southeastern US and is not expected to fully phase with the system, and more importantly a robust shortwave trough entering the NW US tracking ESE, and the northern stream from Canada which will dig south towards the US. Both are expected to meet up near the Ohio Valley, where phasing is expected as the trough becomes neutrally and later negatively tilted, aiding in strong positive vorticity advection with significant height falls ahead of the trough. Along with strong cold air advection associated with yet another cold air mass from Canada, as well as the northeast US region placed under the left exit and right entrance quadrants of two jet streaks, favoring upper level divergence which further aids in the deepening of surface pressures, a rapidly deepening surface low pressure is expected to track through the region, with minimum pressure as low as nearly 980mb over southern New England.

12z GFS at hour 60, on 0z Thursday (8pm Weds), depicting the low pressure over eastern PA with light rain over the area and heavy snow well to the north. The possibility exists for an even further north outcome than the one depicted on the GFS (image credit: PSU e-Wall).

f60Model Analysis: Yesterday’s update noted the main bust risk was for a further north and amplified system; today’s model runs have continued this trend, with some runs such as the latest 18z NAM going as far as the low pressure tracking over Massachusetts with the heavy snow axis limited to far northern New England. While on the warm end of the model guidance, this is not an unreasonable scenario, although it is unlikely at this time. As noted yesterday, the model guidance continues to correct towards a stronger and more amplified northern stream with an earlier and more aggressive phase aiding in this trend, and are still continuing to slightly adjust towards a stronger northern stream. Greatest odds are currently placed towards the model guidance remaining the same or slightly edging north; the possibility of a south shift cannot be ruled out but there is little support for enough of a shift to re-introduce snow across the area except for scattered snow showers at the back end. The main uncertainty at this time is if the NAM had gone too far or if this north trend will continue; this can sometimes be difficult to assess, as with the March 2-3 storm when the south trend continued right until the onset of precipitation, despite appearing to be too far south initially. Taking the above trends into consideration, along with yesterday’s outlook having sided with a further north and warmer scenario than the model guidance depicted, the latest 8-day forecast remains similar to that of yesterday, with the main changes being adjusting towards slightly warmer temperatures and including a south wind across the entire area, which is likely to enter the warm sector at some point.


Forecast for NYC Area: Despite the storm failing to produce much snow across a large part of the region, it will nonetheless be a significant storm system with a relatively low minimum pressure compared with typical storm tracks over the northeast US, likely near 980 millibars. The low pressure will rapidly deepen as it tracks ENE through Pennsylvania, southern NY state and CT, with a swath of precipitation developing north of the area during the day while occasional periods of light rain are expected in the NYC area mainly in the mid-late afternoon hours. The warm front will slowly shift north, likely reaching NYC and coastal Long Island by the mid-late afternoon before briefly passing north into the lower Hudson Valley and southern CT by the evening hours. In the warm sector, a strong south-SSW wind is expected with gusts up to or over 40 mph likely, primarily near NYC and coastal Long Island; temperatures are likely to surge into the 60s up to central NJ, but at this time temperatures in the area are expected to peak in the low-mid 50s in NYC and northern NJ, approaching 60 degrees southwest of NYC, and in the mid 40s to low 50s in SE NY and southern CT. With strong warm air advection associated with the strong southerly low-mid level flow east of the low pressure, the rain/snow mix line is expected to reach its northernmost extent by the evening, likely extending into Albany or the Adirondacks in northern NY state into coastal-central Maine.

The most significant impacts from the storm in the area will be overnight, however, as the low pressure tracks north of the area. As yet another frigid air mass surges southward through Canada into the region with strong cold air advection, a very tight pressure gradient is expected to the west of the low pressure, with a very sharp temperature gradient ranging from 40s and 50s to its east and 10s and 20s just to its west. A brief period of moderate to locally heavy rain is possible with the frontal passage, with temperatures around 12-2am rapidly falling, perhaps supporting a brief changeover to non-accumulating snow showers with the back end and why a slight chance of snow has been maintained overnight. Temperatures may fall by 20 or more degrees in only 2 hours, leading to a flash freeze as any leftover rain quickly freezes into black ice. With the strong pressure gradient, a strong northwesterly wind up to 20-35 mph is expected, with gusts up to 40-50mph likely. Overnight lows are expected to reach the low-mid 20s near NYC, Long Island and CT, and the 10s for most of northern NJ and SE NY.


Thursday – Beyond: Cold/Windy Thursday, Then Warm Weekend

Significant temperature swings will continue through the remainder of the forecast period, as an anomalously cold air mass will be overhead on Thursday with mostly cloudy skies, NW winds at 20-30 mph gusting up to or over 40 mph, and high temperatures only peaking in the 25-31 degree range, which is nearly 20 degrees below average. Along with the strong winds, wind chill values are expected to remain in the single digits and low-mid 10s for most of the day. Meanwhile, the upper level trough will rapidly depart with significant height rises aloft ahead of the next approaching shortwave in the Midwest, with a surface low pressure near southern Canada aiding in a low level southwesterly flow, bringing about a quick warm up. Highs on Friday are expected to reach the low-mid 40s, with Saturday’s highs peaking in the low-mid 50s.

Yet another cool down is expected early-mid next week as the polar vortex likely approaches southeastern Canada again, with highs likely back into the 30s. The model guidance depicts this time period as possibly producing widespread precipitation in the region, but with the possible approach of the polar vortex hinting at possible suppression. Uncertainty remains high in this time period, and the positioning of the polar vortex is subject to change as well.

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