Feb 4, 2014: More Snow, Ice Tomorrow

Forecast Highlights:

cld14Following yesterday’s major snowstorm, only a short lasting break is anticipated from the snow before another significant system enters the area tonight into Wednesday, producing another round of moderate to heavy snowfall accompanied by ice and rain, but with less snow for most compared with Monday. A slightly longer duration of dry conditions is expected for the late week before the next potential storm affects the region on Sunday night into Monday (Image credit: PSU e-Wall).




Tonight – Wednesday: Heavy Snow, Significant Ice Expected

radsfcus_exp_newOne of the biggest snowstorms of the winter affected the area yesterday, with totals as high as 8-10 inches in localized areas in northern NJ, NYC and Long Island. Dry conditions briefly returned today with partial sunshine as high temperatures climbed into the low to mid 30s. To the left is the latest surface pressure analysis and national radar mosaic from the Weather Prediction Center (WPC), updated as of 21z (4pm). A wide variety of impacts can be seen in the map to the left, ranging from heavy snow over the Plains into the western Ohio Valley to heavy rain and thunderstorms to the south, as moisture from the Gulf of Mexico is being pulled into the system which will help to produce heavy precipitation as the system continues to track ENE. A notable aspect to make note of, however, is that there is no organized surface low pressure associated with this system at this time, but with a strong mid level neutrally-tilted shortwave trough at 500mb and a closed mid level low at 850mb near northeast Oklahoma, tracking ENE. Later in the overnight hours, a surface low pressure will develop near Louisiana, tracking northeast towards western Pennsylvania before transferring to a secondary low pressure near the coast of New Jersey, producing widespread heavy precipitation over the region late tonight into tomorrow with impacts ranging from heavy snow, moderate to heavy rain, and significant icing.


High-resolution 22z HRRR model at hour 14, for 12z Monday (7am), depicting heavy snow in northern parts of the area, and sleet/freezing rain further south. The primary low is over eastern Ohio, and the secondary low is developing near southern NJ (Image credit: PSU e-Wall; the HRRR is a new feature on the e-Wall site).

cld14Model Analysis: The model guidance has been much more consistent with this storm than the most recent major snowstorms, which were not even depicted to produce much, if any snow in the area just 2-3 days in advance. With the area overall close to the borderline between mostly non-frozen and frozen precipitation, however, slight shifts in the forecast led to slightly increased uncertainty regarding exact snow and ice accumulations. Over the last 2-3 days, the model guidance shifted south of where it depicted the storm in the medium range, with widespread 6-10 inches across most of the area, including Long Island, changing over to sleet and freezing rain later on Wednesday morning before ending. Today’s model guidance, however, followed the trend observed with practically every storm over the last month and trended stronger and further north with the system. While the shortwave trough was only slightly more amplified than originally modeled, the model guidance was a little too suppressed with the polar vortex and thus confluence over southern Canada, while underestimating the intensity of the ridging off the coast of the southeast US, which allowed for slightly larger height rises ahead of the shortwave and allowing for a slightly stronger and further north system than modeled over the last 1-2 days especially by the GFS, and at an earlier point by the ECM/CMC as well. The model guidance is now generally in agreement with the overall scenario for tomorrow, but with uncertainties remaining in the smaller details relating to the amount of snow and ice that accumulates.


Impact Analysis: The storm will produce another round of significant wintry precipitation across the region, but unlike Monday, freezing rain and ice accumulations will be a significant concern as well. With cold temperatures in the low to mid levels, precipitation type is expected to fall as snow across the area with the onset of precipitation; with precipitation over Ohio and the squall line over Mississippi already slightly faster than modeled, light snow is expected to develop between 12-2am from west to east. With the aid of favorable dynamics, including strong mid level lifting and frontogenesis moving through the region, a band of heavy precipitation is expected to develop between at least 3am through 10am, focusing between south central PA-central NJ northward into just south of Albany, NY and further east along southern Massachusetts. With strong lifting expected, this precipitation is expected to fall as heavy snow over the area, with snow rates likely up to 1-2 inches per hour north of I-80 in NJ into SE NY and south-central CT, approaching 2 inches per hour in these northern areas. This banding will be monitored for the potential for slightly higher snow totals than forecast.

As the mid level low reaches western Pennsylvania and southern NY state, the NYC area will be to the east of the mid level low; due to anticyclonic rotation around low pressures, the area will be under southerly winds in the 850mb-700mb layer, or roughly 1.5-3 kilometers above ground level, leading to mid level warm air advection as warmer air from the south is transported into the area, with temperatures in this layer warming above freezing. The surface low pressure, however, will remain south of the mid level low, reaching western Pennsylvania before transferring to the coast near New Jersey; this will keep the area north of the low pressure, with winds near the surface from the northeast, keeping cold temperatures trapped near the surface and taking longer to erode than the mid levels. In this situation, where surface temperatures rise at a much slower pace than the mid level temperatures, the temperature column is supportive of sleet and freezing rain, where snow aloft melts as it passes through the warm layer, and depending on the depth of the warm layer will either partially melt, before refreezing into sleet as it reaches the surface, or entirely melt before landing on the surface as a supercooled rain droplet, freezing on contact with most surfaces, otherwise referred to as freezing rain. In this case, it is important to note that the peak of the warm layer is expected above the 850mb layer, so while temperatures at 850mb may be below freezing, precipitation type may still fall as sleet.

The tough aspect of the forecast is to determine the exact pace of the warming of temperatures aloft and at the surface, which will determine how much snow and ice accumulates. Models often tend to underestimate mid level warming, and some of the latest model runs are particularly quick to warm the mid levels above freezing, with the high-resolution NAM and operational NAM only depicting a very short period of snow before changing to sleet and ice, while the remaining model guidance is somewhat colder, with snow persisting through at least 6-8am before changing to sleet in far NW NJ/SE NY and southern CT, freezing rain elsewhere in northern NJ, coastal CT and northern Long Island, and rain near the coast of NYC and Long Island. The short range high-resolution RAP/HRRR models, however, are much slower with warming of the mid levels, with snow persisting through at least 8-10am with more significant snow accumulations. At this time, I am siding towards a faster warming of the mid levels than the high-resolution models, closer to the ECM, although the potential for slightly more snow than forecast will continue to be monitored. The next question is the northern extent of the changeover to rain during the day; the NAM and RGEM are faster in the changeover, while the short range high-resolution models are slower, keeping locations at least along and north of about I-80 with freezing rain through the end of the event. In cases similar to this, some of the models, especially the GFS, tend to underestimate low level cold, and at this time, I am siding towards a longer duration of freezing rain.


Forecast for NYC Area: Light snow is expected to develop tonight between 12-2am; with temperatures cold enough, the entire area is expected to start out as snow, with periods of heavy snow expected after 3am. Towards 6-7am, coastal Long Island and coastal NYC are expected to change over to a mix of sleet/rain and quickly change to rain afterwards, while the rest of Long Island and northern NJ change over to sleet, with freezing rain then expected after 8-9am. In southeast NY, primarily over Rockland/Orange/Westchester counties, a changeover to sleet is expected after 7-8am, with Rockland and Westchester counties likely to change to freezing rain after 9-10am, while southern CT changes to sleet and freezing rain after about 8-9am as well. This wintry mix is expected to continue through at least 12-2pm before precipitation rates decrease and NYC, Long Island and eastern NE NJ likely change over to light rain for a short period of time. Drier conditions are then expected later in the afternoon and evening with highs in the low-mid 30s across the area.

Based on the latest indications, the snow outlook at this time is 1-4 inches in Long Island, 2-5 inches in NYC, 4-7 inches in most of northern NJ, 5-8 inches in southern CT and NW NJ, and 6-10 inches in SE NY north of Westchester county. Ice accumulations are slightly more uncertain, but with at least 0.10 to 0.25 inch of ice expected in north central and NE NJ into Rockland/Westchester counties, northern Long Island and coastal CT. The highest probability of near/over 0.25 inch of rain is likely near western into north central NJ. As previously noted, slight uncertainties remain with the exact snow and ice totals, which are subject to slight changes. The main uncertainty potentials to be analyzed in depth tomorrow morning include more ice totals than expected over central-NE NJ into northern Long Island, and more snow than expected north of I-80 in New Jersey if mid level temperatures are slightly cooler than forecast. With the anticipated additional snow and freezing rain, the potential is there for scattered power outages. Storm updates will be posted starting in the early morning hours on Wednesday.


Wednesday Night – Beyond: Monitoring Weekend Potential

Dry conditions will return for a slightly longer period of time in the late week as a broad high pressure settles over the region, providing the area with partly sunny skies through Thursday and Friday with highs in the mid 20s-low 30s and lows in the 10s for most, possibly below 10 in inland locations. Towards Saturday, a wave of low pressure is modeled to remain well to the south of the area; the mid level shortwave trough is relatively weak, but given trends for more robust and amplified shortwaves than initially modeled, the potential exists for a further north and slightly stronger low pressure on Saturday. At this time, another major snowstorm similar to that of Monday appears unlikely, although light snow may be possible in the event it does spread north into the area.

The main uncertainty in the longer range outlook is towards Sunday night into Monday; earlier model runs today depicted a major snowstorm affecting the region, although the afternoon set of model runs significantly backed away from this outcome with a much weaker, drier and suppressed system; the origins of this potential system is way north, with the main shortwave in question currently located in northwestern Canada. The model guidance has slightly changed the handling of a strong upper level low in Alaska and an anomalously strong block in the north central Pacific Ocean, which has partially resulted in a weaker storm in this time frame; as this is still in the longer range, however, additional changes will occur with the main shortwave in question, especially as it gets sampled a few days from now as it enters the NW US, when the model guidance should have a better idea. At this time, continuity is kept from yesterday’s outlook with a 60-70% chance of snow, which is subject to change with future updates.

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