Northeast Weather Blog

Last Updated: 7:57 PM GMT on November 16, 2009 — Last Comment: 2:14 AM GMT on November 22, 2009

	Nor'easter to deliver unusually early snowfall

Posted by: sullivanweather, 6:09 AM GMT on October 15, 2009

Previous blog - October 2009, coldest on record? may be reposted after this week's wild weather. After this weekend temperatures across the country may moderate for several days which would put breaking the record likely out of reach. Until then I will keep an updated month-to-date anomaly chart here at the top of the blog.

Fig.1 - Month-to-date temperature anomalies for October 2009. (Credit: NOAA Climate Prediction Center)

Current watches, warnings and advisories.

Fig.2 - Current watches, warning and advisories issued by the National Weather Service. Courtesy of NOAA.

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Forecast Discussion

Synopsis - Issued 10/15/09 @2:10am

A sprawling dome of surface high pressure currently bridges south-central Canada and the northern Great Lakes region with the eastern periphery nosing into the Northeastern United States, responsible for the very chilly air entrenched over the northern third of the US at present. Meanwhile, a series of Pacific disturbances carrying the remnants of once Super Typhoon Melor are riding a fast (120-140kts) zonal jet stream centered near 40°N across the country. As this energy reaches the East Coast a northern stream shortwave will begin to sharpen a trough over the Eastern US causing low pressure to deepen while moving up the coast. This will spread a variety of precipitation types across the Northeastern US, along with wind and coastal flooding. Across the interior elevated terrain, there may be some hefty early-season snowfall accumulations. Then, not to be outdone, the northern stream shortwave which causes the buckling of the jet will form its own coastal low pressure as it crosses the Appalachians. This will continue the unsettled weather along the coast as a second area of low pressure forms and sticks around through the weekend. As this is happening areas to the north along the international border should manage to remain under the influence of the aforementioned high, remaining dry and chilly. The low should finally begin to pull offshore by Monday with moderating temperatures, though readings will continue to run below normal.

Near-term - Issued 10/15/09 @2:10am

Current Northeastern US satellite loop depicts the region in between systems at present, with one strong storm center off the Labrador Coast (Tuesday morning’s trough), one shortwave moving off the southern Mid-Atlantic Coast and another system moving across the Midwest. Despite being triangulated between storms the region still lies under an abundance of clouds which has managed to keep temperatures a little milder than previously anticipated, although they are still quite chilly. There are some clear skies from the Mohawk Valley east to central/southern New England but for the most part, mostly cloudy skies are to be expected this overnight. Aloft, 850mb temps are anomalously cold for Mid-October, with readings ranging from -10°C towards the US-Canadian border, to near 0°C down towards the Mason-Dixon line with surface readings generally in the 20’s north and 30’s south (except 40’s along the southern coastal plain). Temperatures should drop several more degrees before reaching their daily mins as cold air continues to drain south from the dome of high pressure spanning the southern Provinces of Canada.

Short-term - Issued - 10/15/09 @2:10am

As day breaks on Thursday much of the southern half of the region will be socked in with clouds while areas to the north actually begin to clear some as confluent flow over northern New England and southern Canada strengthens. Precipitation should begin to develop around daybreak across southwestern Pennsylvania along and to the south of I-80 as deep-layer ascent increases due to increasing PVA combined with a coupled jet structure and low-level thermal advection. This area of precipitation will rapidly expand eastward and slowly edge north to the New York/Pennsylvania border during the morning and early afternoon hours. Due to the dry airmass over the region, several hours of evaporational cooling will occur during the early morning hours, lowering temperatures as the precipitation starts. This will be crucial for areas where critical thicknesses aloft will be marginal to support snowfall, such as the region from central Pennsylvania, northern New Jersey and southeastern New York along and to the south of the I-80/84 corridors. Current thinking is that evaporational cooing during the first couple hours of the storm will lower temperatures in the boundary layer to support snowfall then as the day progresses and precipitation intensity increases due to increasing lift (omega progged @ -5 to -10 microbars/sec), UVM and dynamic cooling of the column will counterbalance Mid-October insolation and tend to keep precipitation in the form of snow, although this will be highly dependent on precipitation intensity and elevation. Snowfall may even occur in the valleys across this region but accumulations during the daylight hours will be hard to come by and any periods of lower intensity precipitation will tend to change over to rain. Once in the vicinity of the Pennsylvania Turnpike the majority of precipitation will be in the form of rain at elevations below 1,500’, although occasionally a few sleet pellets or wet snowflakes may mix in at times. Thermal profiles aloft are very marginable for snow in this region and boundary layer temperatures may be a couple degrees too warm for all snow during the daylight hours. Once north of I-80 extending to extreme south-central New York State precipitation should primarily be in the form of snow with the exception being the lowest valleys. Temperatures aloft in this region are much cooler but the same dynamics neighbors to the south will see won’t be in place this far north and moisture will be much more limited (precipitable water contents nearly half of those to the south). Hence, precipitation will be lighter and with marginal boundary layer temperatures rain could mix in at times. As far as accumulations go, most areas along and to the north of I-80/84 should get at least a coating on grassy surfaces with upwards of a couple inches as one gains elevation, mainly over 1,500’. Areas to the south of I-80/84 accumulations will be a bit higher as more QPF is expected but one will have to climb higher to see them. Elevations over 2,000’ should get a solid 3-5” of the white stuff while elevated areas between 1,500’ and 2,000’ could walk away with an inch or two. Below 1,500’ there will be little in the way of accumulations as much of the precipitation will be in the form of rain or very wet snow that should melt on contact. For areas along the coastal plain the forecast is a much simpler one, rain and wind.

The second shortwave disturbance will reach the coast during the afternoon hours, providing a shot in the arm for the developing coastal low pressure east of Delmarva with the central pressure of this low dropping from ~1006mb Thursday afternoon to ~1000mb by the wee hours of Friday morning. Nothing too impressive but this will tighten the pressure gradient between the low offshore and the high over southern Canada resulting in a stronger northeasterly fetch. This will also help to transport Atlantic moisture back across the Northeast in the cold conveyor flow to the north of the developing cyclone. Additionally, ageostrophic flow should promote cold air drainage into the storm during the evening hours, helping to push the rain/snow line south and to lower elevations, possibly 500’ by midnight. QPF during the overnight is a solid two to four tenths of an inch across the southern half of the Northeast, mainly south of the NY/PA border region. With the lack of insolation snow will begin to accumulate in areas that failed to see accumulations during the daylight hours while the elevated terrain will continue to add to their totals. Accumulations should range from 1-4” depending on elevation during the overnight. This additional snowfall combined with snowfall from earlier on Thursday would result in 4-7” at elevations over 2,000’ by daybreak on Friday with 2-5’ of snowfall for elevations from 1,000-2,000’. There should even be a coating to an inch down in the valleys, possibly more if mesoscale features line up. Meanwhile, strong high pressure to the north will keep precipitation at bay across the northern half of the region, although there will be an increase in high cloudiness. Temperatures will be extraordinarily chilly across the region with highs on Thursdays likely remaining in the 30’s across the interior with only 40’s down along the coastal plain. Southern New Jersey may break into the low 50’s towards late in the afternoon as marine air comes ashore but even these readings are well below normal. In some cases areas of central Pennsylvania will see highs more than 20 degrees below the average for Mid-October. Temperatures during the overnight period will drop only a couple degrees across the southern half of the region where clouds and precipitation should limit the diurnal range. Areas to the north where high pressure will be in control should see a more marked fall in temperatures. Especially if the high cloud deck thins at all. Here readings should fall into the low to mid 20’s with teens across the higher elevations.

By Friday morning the situation begins to get more complicated as the first storm heads out to sea as a second area of low pressure begins to take shape over the central Appalachians. This second area of low pressure will begin to tap into the cold conveyor of moisture in the long east-northeasterly fetch coming off the North Atlantic directly into the Northeastern US. As dynamics increase once again within the vicinity of this secondary low precipitation will once again increase in intensity across central Pennsylvania to the northwest of the 700mb low track in the area of strongest mid-level frontogenesis. As was the situation on Thursday, the transition zone will lie in pretty much the same region and precipitation type will once again be determined by elevation and precipitation intensity. This low will reach the coast Friday night and begin to turn up the coast on Saturday with precipitation remaining confined to within 100-150 miles of the low track. Several more inches of heavy wet snowfall may coat the elevated terrain of central Pennsylvania through southeastern New York. With most of the trees still carrying much of their foliage the possibility exists for major tree damage along with the potential for power outages. Along the coastal plain precipitation should remain in the form of rain throughout much of the time during the second system, although some wet snow may mix in at times late Friday night in the tier of counties away from the coast. All the while northern New York and New England continue to be shielded from the adverse weather by the strong surface high. Once again, temperatures Friday through Saturday will run 10-25 degrees below normal for highs and 5-10 degrees below normal for lows.

Fig.3 - Total snowfall through Saturday AM.
More snow is expected after this period but will not be included in this map.

Mid/long-term - Coming soon

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Radar: Northeast Region Loop

NE radar

Fig.4 - Radar loop of the Northeast region. Courtesy of Weather Underground.

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Local SST's

Fig.5 - Sea-surface temperatures off the Northeast Coast. Courtesy of NOAA.

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