Mesoscale Discussion 1319

Issued

Jul 20, 2021, 1:03 PM EDT (17:03 UTC)

Expired

Jul 20, 2021, 3:30 PM EDT

In force

2 hr

Issued by

the Storm Prediction Center

Concerning

Severe potential...watch possible

Chance of a watch

60%

Counties Clinton County, NY, Franklin County, NY, Jefferson County, NY, Lewis County, NY, Monroe County, NY, Orleans County, NY, Oswego County, NY, St. Lawrence County, NY, Grand Isle County, VT

The Ft Drum radar (KTYX), 120 km from the middle of the area, began a volume at 17:05 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.

• Severe Thunderstorm Watch 384Jul 20, 3:05 PM EDT

831 
ACUS11 KWNS 201704
SWOMCD
SPC MCD 201703 
VTZ000-NYZ000-201930-

Mesoscale Discussion 1319
NWS Storm Prediction Center Norman OK
1203 PM CDT Tue Jul 20 2021

Areas affected...Parts of Upstate New York and adjacent portions of
Vermont

Concerning...Severe potential...Watch possible 

Valid 201703Z - 201930Z

Probability of Watch Issuance...60 percent

SUMMARY...An organizing cluster of thunderstorms over parts of
southern Ontario likely will begin impacting the upper St. Lawrence
Valley and Lake Ontario vicinity by 3-5 PM EDT, accompanied by
increasing potential for damaging wind gusts.

DISCUSSION...Associated with larger-scale mid/upper troughing
digging in the St. Lawrence Valley/lower Great Lakes region, forcing
for ascent with a lead speed maximum (30-50 kts in the 500-300 mb
layer) appears to be providing support for an ongoing, well
organized cluster of thunderstorms across southeastern Ontario. 
Perhaps aided by an associated strengthening surface cold pool, this
activity has accelerated some (up to 40 kt) over the past couple of
hours, and may reach the Ottawa vicinity by around 19Z.

Along trailing outflow into the vicinity of its intersection with a
southward advancing cold front, additional thunderstorm development
and intensification is now also well underway.  Supported by
moderate southeasterly low-level inflow of moist air characterized
by CAPE of 1000-2000 J/kg, this seems likely to continue with the
evolution of another upscale growing and organizing cluster/segment
possible.  This may begin impacting the Ontario shores of Lake
Ontario into the Watertown vicinity as early as 20-21Z, perhaps a
bit earlier and more substantively than suggested by the latest
Rapid Refresh.

Although low-level wind fields are generally weak, the effective
downward mixing of higher momentum air aloft, associated with the
well developed/maturing organized convective system and associated
surface cold pools, probably will be accompanied by increasing
potential for strong gusts at least approaching severe limits.

..Kerr/Hart.. 07/20/2021

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

ATTN...WFO...BTV...ALY...BGM...BUF...

LAT...LON   44618000 44557856 44917736 45307677 45407578 44937319
            44477329 44107422 43797486 43407580 43167760 43377902
            44238068 44618000

Mesoscale Discussions in New York, 2021Mesoscale Discussions in Vermont, 2021Every mesoscale discussion in 2021

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.