Feb 17, 2014: Snow on Tues, T-Storms Possible Fri

Forecast Highlights:

rad22The cold and snowy pattern which has been in place since late January continues into the beginning of this week as another storm affects the region on Tuesday, producing 2-6 inches of snow away from coastal locations mainly during the morning hours. A quick change in conditions will occur by the late week with rain, strong wind gusts and even thunderstorms possible on Friday, followed by a gradual return to colder weather by the end of the month into early March (image credit: PSU e-Wall).

 

 


 

Tonight – Tomorrow: Snow Returns

rad_2.17nam_2.17

Posted above from left to right are the latest surface analysis and radar composite from WPC, and the initialized NAM 500 millibar heights and vorticity from NCEP MAG; both are valid at 18z (1pm). A strong high pressure is currently positioned over the area, leading to fair skies and continued cool temperatures in the upper 20s to low 30s. Since the high pressure was not positioned overhead until this afternoon, with a northwest wind having continued last night before subsiding this morning, conditions were not ideal for strong radiational cooling, and temperatures in NW NJ and interior SE NY only fell into the low 10s. The main highlight over the US is an area of low pressure near Illinois tracking ESE, with widespread heavy precipitation associated with strong upward vertical motion ahead of a strong shortwave trough over Iowa, under regions of positive vorticity advection. Most of this precipitation is falling in the form of snow, with reports of heavy thundersnow in parts of Illinois earlier this afternoon. With a progressive, zonal flow aloft behind this shortwave, indicated by the flat 500mb geopotential height contours outlined in black to the west of this feature, the shortwave will continue to progress eastward while remaining slightly negatively tilted, as indicated by the SSE-NNW orientation of the trough axis. The surface low pressure will track northeast into western NY state early on Tuesday morning before a coastal low pressure develops just south of New York City, continuing NNE up the coast. While the low pressure will be fairly weak and compact, strong lifting ahead of the shortwave will be capable of producing a short period of moderate to heavy precipitation across the region.

 

18z run of the 4k-NAM model at hour 21, at 15z Friday (10am), depicting 700mb geopotential heights, relative humidity (green shaded areas) and vertical velocity, also referred to as omega (brown shaded areas). Negative omega values can be found near NYC and Long Island, indicating strong upward motion supporting the development of moderate to heavy precipitation (image credit: NCEP MAG).

nam-hires_namer_021_700_rh_htThe main uncertainty regarding the impact in the area is the development of the coastal low pressure and the anticipated warm air advection, which will influence the total snowfall amounts and how far inland rain extends from the coast. The high resolution model guidance, such as the NMM/ARW and the earlier runs of the high-resolution 4k-NAM, indicate over 0.50 inch of precipitation over most of the area, which would support widespread moderate to heavy snow totals across the area, while the global models, such as the GFS and ECMWF, are drier but have been trending slightly wetter with their recent runs, with at least 0.25 to 0.35 inch of liquid-equivalent precipitation over NYC. Some of these high-resolution models have a tendency to depict too much precipitation, and another area of uncertainty is regarding the exact positioning of the strongest upward vertical velocity – as shown to the left, the latest 18z run of the 4k-NAM depicts this near and southeast of NYC, which is ultimately where heavy precipitation develops and expands as it tracks northeast along with the surface low pressure. At this time, I am siding closer to this scenario, with the heavier precipitation from about NYC into Long Island and further NNE into Connecticut and most of New England.

The next area of uncertainty is the rate of warming and how it will affect precipitation types; surface temperatures will be in the 20s at the onset of precipitation, supporting accumulating snow, but due to the high pressure departing to the east, an increasing SSE flow will set up in the boundary layer from the surface to about 850mb, leading to warm air advection. Coastal southern locations in Long Island are most likely to quickly change to rain with little accumulations, although the moderate to heavy rate of precipitation may at least initially offset the marginal boundary layer temperatures. As the morning hours progress, warmer temperatures should be able to more easily spread inland, with Long Island and NYC expected to change over to rain, possibly including NE NJ and coastal CT, before precipitation ends in the early afternoon hours. This mixing is expected to limit snow totals in Long Island to mostly under 2-3 inches, with the highest totals in the area likely over CT away from the immediate coast.

 

Forecast for NYC Area: Light snow is expected to develop tonight between at least 3-5am, remaining fairly light and scattered until heavier snow bands develop around 7-9am; these are primarily expected to be focused over NE NJ, NYC, Long Island and Connecticut, while light to moderate snow continues further inland. A quick changeover to rain is expected near southern and eastern Long Island, transitioning to rain elsewhere in Long Island and NYC by at least 10-11am as surface temperatures warm into the low to mid 30s. Locations further inland are expected to remain with plain snow, with a risk of some light rain at the end near coastal CT and northeast NJ, with precipitation ending by 12-1pm for most locations.

Snow totals of at least 1 to 4 inches are expected across Long Island, ranging from near to below 1 inch near coastal southern and eastern locations and 4 inches near the north and west locations; 2 to 4 inches in NYC; 2 to 5 inches over SE NY, northeast NJ and southern CT, perhaps below 2 inches in western NJ, and around 4-6 inches in interior Connecticut. Snow totals are subject to slight changes, with the biggest areas of uncertainty regarding Long Island, with the possibility for more snow than forecast if warm air advection is not as significant as currently anticipated, and snow totals from NE NJ into NYC and Connecticut which depend on the western extent of the heavy precipitation bands. Occasional storm updates will be posted on Tuesday morning.

 

Rest of Week: Warmer, Thunderstorms/Wind Possible Friday

A temporary break from the cold and snow is expected for the rest of the week into the weekend as the retreating strong cold air, along with a storm track shifted north of the region, supports a warming trend with highs returning into the 40s for Wednesday and Thursday; a few isolated showers are possible on Wednesday with more widespread sunshine likely on Thursday. In a setup not unheard of in winter months but especially interesting given the lack of interior storm tracks and a persistent cold and stormy pattern, a rapidly deepening low pressure will track through the Great Lakes into Canada on Thursday, resulting in significant northward advection of warm, moist air into the central US, leading to a likely development of severe thunderstorms. The cold front will then progress east into the region, resulting in a brief spike in temperatures and the potential for thunderstorms and strong wind gusts on Friday.

As the cold front approaches on Thursday night, a strengthening low level southerly wind will lead to temperatures holding steady in the low-mid 40s overnight before likely spiking into the 50s later into Friday morning and the early afternoon. While the strongest forcing will shift north into Canada, which will result in a lower risk of severe weather over the region, some marginal instability is likely to be present ahead of the cold front, which along with a strengthening low level jet is likely to lead to a line of moderate to possibly heavy rain moving through, which may be accompanied by thunderstorms along with the risk for strong wind gusts over 40 mph. The GFS is currently the most progressive with the cold front, bringing it through by the late morning to early afternoon which would cap high temperatures in the upper 40s-low 50s with a lower risk of thunderstorms; the GFS often has a progressive bias in similar setups, and I am currently siding with the slower ECM and CMC models with the timing of the front towards the mid-late afternoon hours, which supports high temperatures reaching at least the low to mid 50s, along with the risk of thunderstorms with the frontal passage; more instability along with warmer temperatures in the 60s are expected further south towards at least Washington DC and into Virginia, where a slightly higher risk of severe thunderstorms exists.

Yesterday’s discussion analyzed the outlook beyond the late week in more details; despite the anticipated warm up for this week, there are strong indications that winter is not over yet, with a potentially significant surge of cold air possible towards the very end of the month into early March, which may be potentially accompanied by additional snow, but likely not to the magnitude of some of the recent snowstorms.

5 thoughts on “Feb 17, 2014: Snow on Tues, T-Storms Possible Fri

  1. Anonymous Reply

    With the information you have, what is the time frame of the line of heavy rain and thunder?

    • NYC Area Weather Post authorReply

      The model guidance continues to differ with the timing, with the GFS still progressive with rain in the late morning to early afternoon, while the CMC and ECM are slower with the line moving through in the evening. At this time I am leaning closer to the latter with the line of rain and possible thunderstorms moving through during the mid-late afternoon hours.

  2. rinne211 Reply

    Does anyone know the exact time it started snowing this day , Feb. 17th 2014? I need to know the exact time please. Trying to prove something wrong. Thanks

    • NYC Area Weather Post authorReply

      The NYC tri-state area was dry on the 17th; precipitation generally fell early on the 18th. Is there any specific location you are looking for? In Central Park, snow began to fall at exactly 4:59 AM based on the hourly METAR report.

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