Mesoscale Discussion 21

Issued

Jan 16, 2021, 4:33 AM EST (09:33 UTC)

Expired

Jan 16, 2021, 8:30 AM EST

In force

4 hr

Issued by

the Storm Prediction Center

Concerning

Heavy snow

Counties Coos County, NH, Grafton County, NH, Sullivan County, NH, Essex County, NY, Hamilton County, NY, Saratoga County, NY, Warren County, NY, Washington County, NY, Addison County, VT, Bennington County, VT, Caledonia County, VT, Chittenden County, VT, Orange County, VT, Rutland County, VT, Washington County, VT, Windham County, VT, Windsor County, VT

The Burlington radar (KCXX), 85 km from the middle of the area, began a volume at 09:34 UTC, the scan nearest the moment this went out.

Open the radar archive at this moment

The archived warnings, watches and discussions in force at that minute are drawn over the scan, this one among them.

941 
ACUS11 KWNS 160933
SWOMCD
SPC MCD 160933 
NHZ000-VTZ000-MAZ000-NYZ000-161330-

Mesoscale Discussion 0021
NWS Storm Prediction Center Norman OK
0333 AM CST Sat Jan 16 2021

Areas affected...Parts of northern New York state...Vermont...New
Hampshire and adjacent western Massachusetts

Concerning...Heavy snow 

Valid 160933Z - 161330Z

SUMMARY...Periods of heavy snow, at rates of 1 to 2+ inches per
hour, are expected to develop through 7-10 AM EST, particularly
across the Adirondacks, Champlain Valley, and Green and White
Mountains.

DISCUSSION...A developing surface cyclone off the northern Mid
Atlantic coast is forecast to continue deepening while migrating
inland across Long Island into southern New England through 14-15Z. 
As it does, models indicate a coincident intensification of
southeasterly flow within the warm conveyor (50-70 kt in the 850-70
mb layer) preceding it, accompanied by strengthening warm advection,
associated large-scale ascent and increasing precipitation rates. 
Given the generally sub-freezing thermodynamic profiles across the
region, this is expected to initiate as snow, before precipitation
ends and/or near-surface warming allows for a changeover to rain
east/southeast of the Berkshires and Green/White Mountains by mid to
late morning.

Aided by an influx of precipitable water on the order of .5-.75
inches, periods of heavy snow are expected to develop, particularly
where strengthening orographic forcing contributes to lift across
the higher terrain, including the Adirondacks.  Forecast soundings
suggest that increasingly divergent upper flow will contribute to
periods of intensifying lift within the favorably cold layers
conducive to large dendritic ice crystal growth.  Coupled with weak
destabilization aloft supportive of occasional convective bursts,
heavy snow rates in excess of 1 inch per hour, and at times in
excess of 2 inches per hour, are possible.

..Kerr.. 01/16/2021

...Please see www.spc.noaa.gov for graphic product...

ATTN...WFO...GYX...BTV...ALY...

LAT...LON   42657336 43237456 44057488 44757402 44497252 44517194
            44757122 44187128 43367206 42857260 42657336

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The product and its follow-ups from the Iowa Environmental Mesonet's archive of National Weather Service and Storm Prediction Center text; the sounding from the Mesonet's radiosonde archive; radar from the NEXRAD Level II archive.