Feb 28, 2014: Cold, Snowy Start To March

Forecast Highlights:

f90Record cold temperatures were observed across most of the area this morning, with Central Park, while not breaking any record low, falling to an unseasonably cold 9 degree low. Highs today will only reach the upper 10s-low 20s before rebounding into the low 30s on Saturday and Sunday. Another snowstorm is expected to affect the region on Sunday night into Monday, only the latest addition to what is already one of the snowiest winters on record (image credit: PSU e-Wall).

 

 


 

Today – Saturday: Cold Slightly Moderates; February Recap

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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 12z (7am). Focusing on the eastern US, a strong shortwave trough associated with the strong piece of the polar vortex over southern Canada passed through the region last night, with a period of strong northwesterly winds behind an arctic front having produced scattered snow squalls aiding in strong cold air advection. With a source of frigid temperatures just north of the US/Canada border, unseasonably cold temperatures were observed across the region last night with near record cold temperatures, even considered much colder than average by January standards. Temperatures fell into the single digits to low 10s across most of the area, including a low of 1 in Sussex, NJ and 9 degrees in Central Park, which while above the record low of 5 set in 1934, was well below the average low temperature of 31 degrees. Record lows were broken in four stations: Bridgeport, CT (9, old record 10 from 1950), Islip, NY (10, old record 13 from 1994), JFK (11, old record 15 from 1950), and LaGuardia (10, old record 14 from 1994). This is only the latest addition to what is a notably colder and snowier winter than average, as excluding today, February up to the 27th was 2.9 degrees below average in Central Park. With no additional snow expected this month, the February total snowfall is 29.0″, securing its spot as the 5th snowiest month and 2nd snowiest February on record; both records are held by February 2010 with 36.9″ of snow.

As shown in the 500mb map above, the shortwave trough has lifted out of the region and is currently near Atlantic Canada, supporting an upper level westerly flow aiding in a moderation of temperatures aloft, but with a surface high pressure helping to prevent a significant warming of surface temperatures, with highs today only peaking in the upper 10s to low 20s across the area. This is expected to break record low maximum temperature records for most locations today, and is well below the average high of 44 degrees in Central Park. With the high pressure still over the area tonight with fairly light winds, strong radiational cooling is expected in typical rural locations, including the Long Island Pine Barrens, with temperatures falling into the low-mid 0s in these locations, and the upper 0s-mid 10s elsewhere in the area. The high pressure will shift offshore on Saturday as a weak low pressure races east through southeastern Canada, dragging a cold front through the area with a southwesterly flow in the lower levels aiding in warming surface temperatures into the low-mid 30s, which while much warmer than today’s temperatures is still nearly 10 degrees below average.

 

Sunday – Monday: Storm Expected, Snow Totals Still Uncertain

Attention next turns to the West Coast, where a major extratropical cyclone is affecting California with significant rainfall and strong winds, helping to ease the severe drought conditions in the West which has remained generally dry most of the winter. The strong upper level low will continue to progress ESE towards southern California, but is expected to weaken and open up into a shortwave trough as it enters the southwestern US. Meanwhile, the strong polar vortex will remain stationed near southeastern Canada, keeping a strong upper level WSW flow in place across the eastern half of the US, aiding in the shortwave continuing to track ENE. From there, the question becomes how much interaction this shortwave has with the northern stream feature near the US/Canada border and the extent of suppression from the polar vortex over Canada, which will determine if the area sees a major snowstorm or simply a light-moderate snow event.

 

0z run of the GFS at hour 90, at 18z Monday (1pm), depicting moderate to heavy snow affecting the area. This run was the middle ground of today’s GFS runs, with the 6z run further north with the heavy snow and the 12z GFS placing NYC near the northern end of the heavy snow axis (image credit: PSU e-Wall).

f90Model Analysis: As of last night’s runs, most of the model guidance depicted a relatively stronger and slightly more amplified southern shortwave trough entering the US, which along with a slightly further north positioning of the polar vortex over southeast Canada aided in a wave of low pressure tracking ENE along a frontal boundary south of the area. Along with a surge of moisture into the region, this supported a swath of heavy snowfall from the Ohio Valley into Pennsylvania, New Jersey and southern New England. Liquid-equivalent precipitation totals were generally near 1 inch on the 6z GFS and 0z ECMWF runs, which along with higher than the standard 10:1 snow to liquid ratios supported at least a foot of snow, perhaps more for some locations. The latest runs of the GFS and ECMWF, however, initialized with the California cyclone slightly further west than modeled earlier, which ends up flattening out more than earlier runs as it enters the western US. The polar vortex also initialized slightly stronger and slightly further south than earlier runs, which also helps to push the frontal boundary further south and result in a somewhat drier and further south axis of precipitation; the 12z GFS and ECMWF generally depicted 0.70″ to 0.75″ of liquid-equivalent precipitation near NYC, mostly from a prolonged 18-hour period of steady moderate snow rates, but with a sharp cutoff in precipitation totals not far to the north of NYC. The other model guidance has been more variable, with the CMC supporting more of a southwesterly orientation to the upper level flow, resulting in the precipitation axis gradually shifting eastward with lower snow totals across the region compared with the GFS/ECM; the NAM’s handling of this system has been inconsistent and is being considered an outlier at this time.

This is a rather delicate setup with regards to the position, strength and timing of the key features involved, and slight run to run discontinuities on the model guidance are leading to somewhat different outcomes near the surface. With the model guidance struggling to narrow down on these small features with more consistent handling, additional relatively minor shifts are likely in the model guidance through tomorrow, with a gradual narrowing down of the range likely especially after the upper level low enters the southwestern US towards a region of better sampling of the upper levels. It is important to keep in mind that the model guidance will always shift around with the fine details of the setup until the short range, and one set of model runs alone should not significantly change the perception of what the storm might do. In this case given the presence of the strong polar vortex suppressing heights over the northeastern US, the main bust risk which would lead to a different outcome than currently forecast is the potential for a weaker and more strung-out system affecting the region, which would produce a weaker moderate snowstorm across the region with totals generally remaining below 6-8 inches. This is currently considered the lower-end scenario. With this greater than usual uncertainty, it is premature at this time to consider any snow outlook a near-guarantee or certainty, whether it’s on the low or high end of the spectrum.

Taking the above into consideration, along with recent model trends/verification and the overall recent pattern, I am siding towards a moderate to significant snowstorm affecting the area at this time from Sunday evening to Monday evening; preliminary thoughts on snow accumulations are included in the next section. Precipitation type is generally expected to be snow with the potential for initial mixing with light rain/sleet on Sunday afternoon and evening before colder air filters into the region from the north. I am placing a high emphasis on that this is a relatively low confidence outlook for a 3-day forecast, and additional changes are possible over the next 1-2 days as the model guidance gains a better handle on the features involved in this setup.

 

Preliminary Forecast for NYC Area: Light precipitation is expected to develop on Sunday afternoon-evening, with high temperatures mostly in the low-mid 30s. Precipitation type may initially begin with light rain/sleet, especially near the immediate NYC area and further east/south, before changing over to snow overnight as temperatures gradually fall into the 20s. Steady moderate snow is expected late overnight through most of Monday, ending by the late afternoon/early evening hours with unseasonably cold high temperatures, generally in the low-mid 20s.

At this time, preliminary thinking on snow accumulations is 4 to 9 inches in southeast NY and southern CT, and 6 to 13 inches for the rest of the area, with snow totals gradually increasing from south to north. At this time, I am placing a relatively wide range for snow accumulations with the higher than usual uncertainty, which will be narrowed down with any future updates, and at this moment I am leaning towards at least slightly higher totals from the lowest end of this range. As previously mentioned, this is a difficult forecast to make, with slight changes leading to changes in the snow outlook for the area and the region. Due to this uncertainty, I sided slightly lower than the earlier data this morning suggested, which would support at least 6-10 inches in northern areas and 8-15 inches elsewhere. A bust risk to watch for, however, would be a scenario with a weaker, more strung-out and slightly drier system with lower snow accumulations for the area than currently forecast. In the event that this bust risk decreases in likelihood, snow totals may be increased with the next forecast update. Another update may be posted later tonight following the remainder of this evening’s model runs; otherwise, the next forecast discussion will be posted on Saturday morning.

 

Rest of Next Week: Cold temperatures are expected to continue into Tuesday and Wednesday in the wake of the early week storm system, with temperatures well below average once again. High temperatures are generally expected to peak in the mid-upper 20s, with overnight lows in the mid-upper single digits towards interior NW NJ/SE NY and the 10s elsewhere. The last time high temperatures remained below 30 degrees in March was in 2009, and before that in 2007. A gradual warm up is expected later next week as a potential low pressure affects the region towards Friday, with less cold air aloft compared with the early week time frame bringing up the potential for mixed precipitation should this storm develop, but with high uncertainty by this time frame considering it is nearly a week away.

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