Dec 4, 2013: Wintry Mix Expected Sunday Night

Forecast Highlights:

SundayPrecipWarmer than average temperatures continued across the area today, reaching the upper 40s to low 50s for highs again. Unlike the last few nights, however, temperatures will struggle to fall below the 40s tonight and gradually rise as a warm front moves through, with temperatures surging into the upper 50s-low 60s on Thursday. A cold front will move through with two waves of precipitation, the second possibly producing snow in interior areas on Friday night, followed by a colder weekend and a wintry mix likely on Sunday night.

<< December 2 | December 3 | December 4 >>

Tuesday, December 3 Observations:

12.3.13Widespread cloud cover persisted across the region with some clearing in the cloud cover at times as a coastal low developed well to the east of the region, with a continuation in the warmer than average temperature pattern that started with the beginning of the month as temperatures slightly warmed up compared with Monday, reaching the upper 40s to low 50s across the area. The highest temperature was 55 degrees in Newark, NJ, and the coolest high temperature was 44 degrees in Montgomery, NY.




Tonight – Friday: Warm Thursday, Then Cooling Down With Rain, Interior Snow

namfntsfcwbgSetup Overview: The latest surface pressure and front analysis from the Weather Prediction Center (WPC) posted to the left shows a narrow area of high pressure over New England, with the coastal low pressure that developed east of the area yesterday currently located over Newfoundland after having produced widespread heavy rain/snow in Atlantic Canada. A strengthening low pressure is located over Wisconsin with a minimum pressure near 995mb; this low will continue to deepen as it tracks northeast into Canada, bringing a warm front through the region with much warmer temperatures for Thursday, rising into the upper 50s to low 60s across the area. The most extreme weather of the week focuses over the Midwest, however, where a frigid air mass displaced over southern Canada with the aid of anomalously strong -EPO blocking over the northeast Pacific will spill southward behind the departing Wisconsin low pressure and its cold front, with widespread sub-zero temperatures expected in that region. With ridging persisting aloft over the eastern US, however, the cold front will slowly track east towards the region, producing two rounds of rain; the first will accompany the frontal passage on Friday morning, and the second will be behind the front on Friday night with a wave of low pressure developing along the front. The second wave is expected to produce widespread moderate snow over the Ohio Valley into Pennsylvania and southern NY state; most of the NYC area is expected to see plain rain, but with interior NW NJ and SE NY possibly changing over to sleet and snow towards the end of the storm as colder air aloft enters the area.

Forecast for NYC Area: Mostly cloudy skies will continue tonight with isolated showers and temperatures initially struggling to fall below the mid 40s before rising later overnight. Mostly cloudy skies are expected on Thursday with highs in the upper 50s to low 60s across the area. Some showers are expected overnight, especially north of NYC, as the cold front slowly approaches the area with temperatures remaining steady in the mid 50s before falling by the morning, with highs on Friday in the low to mid 50s across the area. In between the two waves of rain, dry conditions are expected for most of Friday but with developing rain towards the evening, mainly after 5-7 PM, with moderate rain expected to continue through the late overnight hours, drying out by Saturday morning. At least 1/2 to 1 inch of rain is expected, locally over 1 inch. With colder air slowly entering the area aloft and at the surface, a changeover to sleet and snow is possible in interior NW NJ and SE NY after 1-2 am before precipitation gradually ends, although accumulations are generally likely to remain under an inch.


Saturday – Monday: Colder Weekend; Wintry Mix Sunday Night

Behind the wave of low pressure on Friday night, drier conditions and partly sunny skies will return for Saturday with partly sunny skies and highs in the upper 30s inland and low 40s elsewhere. Widespread lows in the 20s are expected overnight, falling into the low 20s inland, mid-upper 20s for the north/west suburbs, southern CT and Long Island, and the upper 20s near NYC and the coast. By Sunday, however, cloud cover will increase as the next storm system approaches the area. A shortwave trough currently near western Canada will dive southward towards the SW US by Sunday, which will then interact with a strong upper level low tracking southwards through Canada, dragging another frigid air mass with it into the northern US. As this occurs, a dual-centered low pressure will lift out of the southern US and track northeast towards the region, with one center tracking through Kentucky and Ohio while the second tracks along the coast.

SundayPrecipStorm Analysis: Despite the widespread ridging aloft over the region with above normal 500 millibar heights, however, widespread cold air will be in place initially both aloft and near the surface as some of the late week frigid air mass enters the northeast US behind Friday’s cold front, but in a much more moderated form. With the aforementioned ridging aloft, and the mid level trough axis centered to the west of the region, a southwesterly flow will persist around the 850mb layer, leading to mid level warm air advection (WAA) gradually removing the cold air mass. At the onset of precipitation on Sunday morning, however, a strong surface high pressure will also be positioned over New England, which will prevent surface temperatures from warming up as quickly. With enough cold air initially present, precipitation is expected to develop as snow, starting from Washington DC/Baltimore on Sunday morning and extending into the NYC area and New England by the late afternoon and evening.

With light E/ESE winds bringing warmer temperatures towards the coast, a quick changeover to rain is likely for NYC and coastal locations. Further inland, however, precipitation types become more complicated, and depend on how fast the cold air erodes at the surface and how fast the mid level warming will occur. At this time, the forecast is for light to moderate snow for a several hour period during Sunday evening-early night in northern NJ, SE NY and interior southern CT, which changes over to sleet and then rain likely towards midnight, with moderate rain continuing through Monday morning. NW NJ and SE NY, however, are expected to hold onto low level cold for a longer duration with cold air damming expected, with snow likely at first before changing to sleet and then freezing rain overnight; some of the models depict a full changeover to rain on Monday morning, although with past storms with a similar setup, precipitation could remain frozen towards far NW NJ and parts of Orange county through the end of the steady precipitation on Monday morning. Precipitation types are still subject to some changes, however, as details become clearer over the next few days.

The next uncertainty is regarding precipitation amounts, which will also determine how much snow accumulates prior to the changeover to rain. Earlier model runs today showed more digging of the western trough, with a more amplified and stronger surface low pressure developing in the Ohio Valley with more widespread heavy precipitation; the latest runs, however, have increasingly trended less amplified and thus drier, depicting an area of maximum snowfall south of the area associated with the coastal low pressure, which then weakens and leaves the area with light precipitation and minimal snow accumulations before a quick changeover to ice/rain, while precipitation redevelops to the north of the area with more widespread snow. The precipitation aspect is still uncertain and is subject to change; at this time, I am siding towards the somewhat wetter solutions and included a potential for light to moderate accumulations over interior northern NJ and SE NY, although should the aforementioned scenario verify, minor snow accumulations generally less than an inch would be expected for most prior to the changeover. Another uncertain aspect of the forecast is a potential wave of low pressure developing along the front on Tuesday which the latest GFS runs have depicted, showing additional light snow; at this time, this potential is still uncertain and has not been included in the 8-day forecast, but will continue to be monitored. More information will be posted on this with Thursday’s forecast update.


Preliminary Forecast for NYC Area: At this time, the forecast is for light snow to develop late on Sunday afternoon, starting out as rain in Long Island and rain/snow in NYC, with highs in the mid 30s for most of the area. A quick changeover to rain is likely in NYC and the coast, with north/west suburbs and interior southern CT continuing to see snow/sleet for another few hours before changing over to rain as well, with the possibility for minor accumulations; steady rain will then continue overnight into Monday morning. Further inland towards northwest NJ and SE NY, a longer period of snow is expected, transitioning overnight into sleet and then freezing rain, with a gradual changeover to rain towards the morning but with the possibility that some locations remain frozen for the majority of the event. Precipitation is expected to become lighter after Monday morning, with occasional showers possible during the day before ending towards the evening.

At this time, preliminary thinking is for light snow accumulations in northern NJ, SE NY and interior southern CT, with the possibility of moderate accumulations towards NW NJ and SE NY with several inches of snow possible. Additionally, freezing rain is a risk towards interior areas late on Sunday night into early Monday morning. Accumulations are still uncertain at this point, and depend on how much precipitation spreads into the area before temperatures aloft and at the surface warm up above freezing, which is subject to some changes over the next few days.


Tuesday – Beyond: Colder Ending To Week

The previously mentioned frigid air mass entering the upper Midwest early in the week will surge southeast behind the storm, approaching the region, but will fail to settle in as a ridging signal persist over the southeastern US with a more zonal flow upstream in the western US and a lack of blocking or significant height rises downstream to lead to more significant amplification of the trough; as a result, the arctic air mass with 850mb temperatures near -20C to -30C will remain north of the region, but with the southern end of the cold air mass sliding through the region in the mid-late week, leading to cold temperatures returning for a few days. How cold temperatures end up is still somewhat uncertain and depends on how far south the cold air manages to extend, although at this time high temperatures are likely to return into the 30s, with widespread lows in the 10s or 20s likely.

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