Mesoscale Discussion 854

Issued

Jun 5, 2021, 2:33 PM EDT (18:33 UTC)

Expired

Jun 5, 2021, 4:30 PM EDT

In force

117 min

Issued by

the Storm Prediction Center

Concerning

Severe potential...watch possible

Chance of a watch

40%

Counties Androscoggin County, ME, Cumberland County, ME, Kennebec County, ME, Lincoln County, ME, Oxford County, ME, Sagadahoc County, ME, Waldo County, ME, York County, ME, Belknap County, NH, Carroll County, NH, Coos County, NH, Grafton County, NH, Essex County, NY, Addison County, VT, Caledonia County, VT, Chittenden County, VT, Essex County, VT, Lamoille County, VT, Orange County, VT, Washington County, VT

The Portland,ME radar (KGYX), 65 km from the middle of the area, began a volume at 18:35 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.

535 
ACUS11 KWNS 051834
SWOMCD
SPC MCD 051833 
MEZ000-NHZ000-VTZ000-NYZ000-052030-

Mesoscale Discussion 0854
NWS Storm Prediction Center Norman OK
0133 PM CDT Sat Jun 05 2021

Areas affected...Portions of Northern New England

Concerning...Severe potential...Watch possible 

Valid 051833Z - 052030Z

Probability of Watch Issuance...40 percent

SUMMARY...There is uncertainty in convective evolution in northern
New England. A WW is possible this afternoon, but timing is still
uncertain. Observational trends will continue to be monitored.

DISCUSSION...There remains a fair amount of uncertainty as to the
exact evolution of convection this afternoon across northern New
England. 12Z CAM guidance presents a wide array of possible
outcomes. Trends in surface observations and visible satellite
depict at least two plausible scenarios. First, a band of elevated
convection near Burlington, VT, continues to move southeastward
producing little to no lightning. This features, in some guidance,
is forecast to intensify as it encounters greater instability to the
southeast near the southern Maine and New Hampshire. In those
locations, temperatures have already risen to near 90 F with
dewpoints currently holding in the upper 50s to low 60s F. Another
area of potential development is likely in northern New Hampshire
into adjacent portions of Maine along a differential heating
boundary. There, cumulus have continued to deepen of the last hour
and that trend should continue. Given the steep low-level lapse
rates and moderate flow off the surface, strong/damaging wind gusts
are the primary threat with storms this afternoon. Objectively
analyzed 40-50 kts of effective shear will promote organized storms,
some perhaps supercellular. Discrete storms would be capable of
marginally severe hail and perhaps a tornado. A WW is possible this
afternoon, though timing is uncertain. Observational trends would
suggest development in perhaps 2-3 hours. These trends will continue
to be monitored.

For portions of northern Maine, uncertainty is higher given
remaining cloud cover and cooler temperatures. The most probable
scenario in this region is that storms will initiate somewhere in
the St. Lawrence River vicinity and move southeastward into the
region. This activity would more likely occur later this afternoon
given the inhibition that is yet to be overcome.

..Wendt/Guyer.. 06/05/2021

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

ATTN...WFO...CAR...GYX...BTV...

LAT...LON   44147415 44697256 45066974 45146881 44686841 44096931
            43687017 43447051 43367120 44147415

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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.