Feb 12, 2014: Heavy Snow, Wind Tomorrow

Forecast Highlights:

snowmapVery cold temperatures were observed again this morning away from urban areas, falling as low as -13 degrees in Sussex, NJ. Temperatures have since rebounded into the 20s, but with cloud cover expected to increase later today into the overnight hours as a developing low pressure currently producing a significant snow and ice storm in the southeast US tracks up the coast, producing widespread heavy snow and wind across the region on Thursday into early Friday. Another minor snow event may be possible on Saturday, but with a gradual warming trend anticipated by next week.

 

 


 

Tonight – Friday Morning: Heavy Snow, Wind, Some Rain/Ice

radsfcus_exp_newAfter this morning’s cold interior temperatures, falling well below zero in NW NJ and interior SE NY, attention turns to the southeast US, where a developing low pressure is producing widespread snow and ice in the same locations which were significantly affected by the late January storm, which can be seen to the left from the latest analysis and radar composite from the Weather Prediction Center. There are currently two shortwave troughs over the south central US, the southern stream near northern Louisiana and the northern stream near Kansas. The latter feature is diving southeast and will phase tonight with the southern stream, leading to a strong mid to upper level trough over the SE US, which in response to amplifying ridging to its east will become negatively tilted and later close off while tracking up the coast, producing widespread heavy snow along the I-95 corridor. The upcoming storm was particularly difficult for some of the model guidance to handle, notably the NAM and GFS which were continuously too weak and suppressed with the low pressure, not coming into a closer agreement with the remainder of the model guidance until today’s runs. While slight uncertainties remain, there is higher confidence of yet another major winter storm affecting the area.

 

12z GFS at hour 6, for 18z today (2pm), depicting the setup for the storm in its development stage; the southern stream shortwave is over Missouri, with the northern stream feature over Kansas diving southeast. The two streams will phase tonight, aiding in the deepening of the surface low pressure as it tracks up the coast (image credit: PSU e-Wall).

f06Model Analysis: As last night’s update noted, the ECMWF, despite earlier appearing to be a western outlier, and with a lower reliability than usual considering its verification this winter, has done rather well with the handling of this storm so far, with a low pressure track close to the coast supporting a changeover to sleet and/or rain at some point for the eastern half of the area, while the NAM and especially the GFS were too far southeast, but have since trended more in line with the remainder of the model guidance with a stronger and more amplified 500mb trough closing off closer to the coast. The main uncertainty in the synoptic scale with the model guidance is regarding the western extent of the front end banding and the northern extent and strength of the back-end comma head snows, which will be discussed in more depth in the next section. At this time, I am siding towards the CMC, UKMET and ECMWF models with some weight given to the 12z GFS and 18z NAM in today’s outlook, with both models having come closer to a consensus. The RGEM has been consistently the northernmost model, and has been discounted as a significant inland and amplified outlier.

 

18z run of the high-resolution 4k NAM at hour 18, valid at 12z Thursday (7am), depicting geopotential heights, relative humidity and omega, a variable describing vertical velocity (lifting) in the 700mb pressure level. The brown-filled shapes indicate strong upward vertical velocity, supporting heavy snow banding affecting the area between 6am-12pm with the heaviest snow rates near NYC/NE NJ and further east/NE (image credit: NCEP MAG).

nam-hires_namer_018_700_rh_htImpact Analysis: The storm will produce widespread significant impacts up the East Coast, ranging from significant snow and ice in the southeast US to over a foot of snow along and west of the I-95 corridor in the northeast US. The cold air mass aloft exited the region last night, and with a more marginal cool air mass left in place, temperatures quickly went up into the 20s during the early afternoon hours. As the precipitation shield spreads in from the south, cloud cover will increase this evening as temperatures slightly fall into the upper 10s-low 20s inland and the mid 20s elsewhere into the early overnight hours. Light snow is expected to develop towards 12-2am from south to north, with temperatures cold enough to support snow across the area into most of New Jersey. As strong warm air advection occurs in the mid levels from the south, with strong lifting and frontogenesis at 850-700mb, a band of heavy snow is expected to track northeast through the Maryland, SE PA and central NJ region in the early morning hours before progressing through the area between at least 6am and 12pm. With strong dynamics, very heavy snow rates of 1-2 inches per hour can be expected, with snow rates perhaps above 2″/hour in parts of NE NJ, NYC, northern Long Island and southern CT. As the warm air advection continues, coastal Long Island will be the first spot to change over to rain with timing likely between 11am-12pm, with the mixing line gradually shifting inland, reaching the rest of Long Island and NYC by at least 1-2pm and NE NJ/southern CT by 2-3pm. By the time the temperature column warms up enough to support sleet or rain outside of coastal Long Island, however, the heaviest banding will have mostly departed the area with only light precipitation rates expected, leading to only light rain/sleet showers as the freezing line likely reaches at least NE NJ, far SE NY and southern CT.

With the heavy front-end banding expected during the morning hours, widespread heavy snow totals are expected in the area regardless of the inland extent of the changeover during the afternoon hours. The main uncertainty is regarding the intensity of the heavy snow and the exact timing of the changeover, which will determine how much snow falls with the first round. Based on the above scenario, at least 5 to 10 inches of snow are expected with the morning round from NYC and further north/west and southern CT; Long Island will see a shorter duration of snow than locations further inland, but heavier snow rates may slightly compensate for the shorter timing, with 4 to 8 inches of snow likely. The inland extent of mixing is another area of uncertainty, with the warmer models such as the RGEM bringing the snow/sleet line all the way into NW NJ and the mid Hudson Valley, while the ECM and CMC keep locations north and west of NYC mostly, if not entirely with plain snow. Taking typical model biases into consideration, I am siding slightly closer to the colder models but not completely to that extent, with mixing with light sleet/rain extending up to northeast NJ, Rockland/Westchester counties in SE NY, and southern CT. Light precipitation is likely to continue through at least 7-8pm with temperatures in the low 30s across the area.

The second round of the heavy snow is likely in the late evening into the overnight hours as the surface low pressure center redevelops under the closed mid level low east of New Jersey, with strong positive vorticity advection expected ahead of the mid level low, supporting the formation of a comma head near the east central Mid Atlantic and moving through the area between at least 8pm and 3am, bringing another round of heavy precipitation. Initially, this is likely to start as rain near NYC and further east, but with the passage of the mid level low east of the area, winds aloft will turn northerly, leading to a cooling of the temperature column and a gradual changeover back to snow in NYC and Long Island from west to east. This is currently among the lowest confidence aspects of the outlook, with differences in the model guidance ranging from the ECM keeping the heaviest snow north of the area, to the RGEM and 4k NAM depicting the heavy snow over NJ and near/north of NYC. This aspect of the outlook is tough to predict with high precision, although at this time this appears to support heavy snow developing again over parts of the area with the potential for an additional 2-6 inches of snow. At this time, I sided towards a further east comma head development with the heavier snows from northern NJ into NYC, SE NY and Connecticut, with an additional 2-5 inches of snow, although this aspect of the outlook has a lower confidence level and is subject to some changes. Snow is then expected to end by 2-4am from southwest to northeast.

 

snowmapRegional Impacts: Focusing on the larger picture across the region, the front end banding will produce the most significant snow totals across the region on a widespread scale, extending from Virginia into Maine. The model guidance differs with the northern extent, although the latest model runs have trended towards a less significant northward expansion of the heavy snow associated with the morning round. This trend would support the northern end of the initial heavy snow extending up to at least Maryland into eastern PA, the lower Hudson Valley, Massachusetts and coastal Maine. Locations north of the axis above may begin with a period of light snow, but with snow expected to weaken and become more scattered in nature in the evening and early overnight hours with poor snow rates as the strongest dynamics remain to the south and east. The second round is expected to produce another round of moderate to heavy snow, but with the exact location still somewhat uncertain; at this time, I am siding towards the comma head heavy snow setting up from central New Jersey further north/NE into the lower-mid Hudson Valley, western CT/LI, and central New England. Locations further east would see some snow but with lighter accumulations expected. Based on the above scenario, the latest snow map has been posted to the left; the greatest areas of uncertainty include the positioning of the 10″+ inch axis, how far heavy snow extends east into Long Island, and the northern extent of heavy snow. At this time, I extended the northern end of the heavy snow slightly south of the latest model guidance, although the potential is there for much lower than forecast totals with a tighter precipitation gradient from about north central PA into the northern Hudson Valley, northern NY state and NW Vermont, with snow totals in this axis largely depending on the comma head on Thursday night, which I am currently siding south of most of the global model guidance with. Slight revisions may be made to this map tonight and tomorrow morning. Strong winds are expected near coastal regions as well given the tight pressure gradient, with NE winds becoming northerly at 15-30 mph expected mainly along the coast.

 

Forecast for NYC Area: Increasing clouds are expected tonight as temperatures fall into the upper 10s-low 20s inland and the mid 20s elsewhere, with light snow developing between 12-2am from south to north. Snow rates will gradually intensify, with heavy snow expected between 6am and 12pm. During this time period, snow rates of 1-2 inches per hour are expected, perhaps over 2 inches per hour in parts of NE NJ, NYC, southern CT and northern Long Island. A gradual changeover to rain is expected in Long Island from 11am near the coast to 1pm elsewhere, and a changeover to light rain/sleet in NYC, NE NJ, SE NY and southern CT by 1-3pm. A period of dry slotting is then expected through the evening hours with light rain/sleet showers for most location and light snow in interior locations, with moderate to heavy precipitation developing again after 8-10pm, especially west of central Long Island/CT; this may initially start with rain near NYC and further east before gradually changing to snow as temperatures slightly cool down. Snow is then expected to end between 2am and 4am from southwest to northeast.

This storm is expected to produce the most significant single-storm snow totals of the winter across the area, with widespread snow totals over 10-12 inches expected. At this time, the snow outlook is for 5-9 inches in Long Island, 7-12 inches in NYC and SE CT, and 10-16 inches in northern NJ, SE NY and the rest of southern CT. Depending on the development of the second round, totals locally over 14-16 inches cannot be ruled out along the heavy snow axis labeled in the snow map above, including parts of northern NJ, SE NY and western CT. Windy conditions are expected as well, up to 15-30 mph from the NE turning north especially near coastal locations. This snow will not be of the dry, fluffy composition as with the last few storms but will be heavy and wet, leading to the possibility of scattered downed trees and power outages. It is important to note that despite the anticipated changeover to sleet and rain in the late afternoon hours, the strong heavy snow banding in the morning and late evening hours is still expected to lead to significant snow totals, with only minimal melting likely from rain as the changeover occurs during the timing of a dry slot moving through the area. There remain some uncertainties in the outlook; the biggest uncertainties at this time include the potential for higher than forecast snow totals over Long Island, and the positioning of the banding from the evening round over the area which will determine snow totals from NYC and further north/west.

This is the final storm forecast discussion. Frequent storm updates will be posted throughout the day on Thursday into Thursday night.

 

Friday – Beyond: More Cold/Some Snow, Then Gradually Warming

After a prolonged period of frequent snowstorms and cold temperatures, it appears tomorrow’s storm is the last of the major snowstorms for a while. Continuing the theme of the progressive pattern in the upper levels, another strong shortwave trough will reach the region on Friday night, digging through the southeast US before becoming negatively tilted offshore, aiding in the development and rapid deepening of a low pressure off the coast. This trough is expected to take too long to become negatively tilted to lead to another significant storm in the region, with the most significant impact expected in Nova Scotia while scattered precipitation affects the area on Friday night. As the rising heights aloft ahead of this shortwave will lead to a southwesterly flow, warmer temperatures are expected on Friday, peaking in the 35-40 degree range for most of the area. Regardless of daytime temperatures, overnight lows are expected to cool down enough overnight to support light snow showers, with at least a dusting of snow possible with any accumulations remaining below 1/2 inch to possibly 1 inch at most, although the latter scenario does not have model support at this time.

The last cold air mass for a while will return for Sunday and Monday, leading to another short lasting round of well below average temperatures. Highs on Saturday are expected to peak in the upper 20s to low 30s before crashing into the single digits inland and 10s near NYC and further south/east overnight, with highs on Sunday peaking only in the low-mid 20s inland and the mid-upper 20s elsewhere with a breezy west wind at 10-15 mph. A strong high pressure will then expand into the area on Sunday night, leading to another setup of strong radiational cooling with lows likely in the low 10s in NYC and the single digits elsewhere, falling below zero in interior NW NJ and SE NY.

More significant changes in the global pattern are expected into next week, most notably including a strong upper level low near the Gulf of Alaska which has been absent this winter, leading to a more zonal flow over North America supporting a gradual warming trend into next week. Some scattered rain/snow showers are possible on Monday night as a warm front passes through, with highs gradually warming up into the 30s by the middle of next week and possibly the 50s by the end of next week into next weekend.

 

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