Alert archive / Maine / Mesoscale Discussions in 2021

Mesoscale Discussion 1754

September 15, 202112 counties in Maine; 10 counties in New Hampshire; Essex County, Vermont

The Storm Prediction Center issued this mesoscale discussion at Sep 15, 2021, 11:59 AM EDT for 12 counties in Maine; 10 counties in New Hampshire; Essex County, Vermont. It was in force for 2 hr.

The area

The outline as issued.

The facts

Issued: Sep 15, 2021, 11:59 AM EDT (15:59 UTC)
Expired: Sep 15, 2021, 2:00 PM EDT
In force: 2 hr
Issued by: the Storm Prediction Center
Concerning: Severe potential...watch likely
Chance of a watch: 95%

All 23 counties

Androscoggin County, ME, Cumberland County, ME, Franklin County, ME, Kennebec County, ME, Lincoln County, ME, Oxford County, ME, Penobscot County, ME, Piscataquis County, ME, Sagadahoc County, ME, Somerset County, ME, Waldo County, ME, York County, ME, Belknap County, NH, Carroll County, NH, Cheshire County, NH, Coos County, NH, Grafton County, NH, Hillsborough County, NH, Merrimack County, NH, Rockingham County, NH, Strafford County, NH, Sullivan County, NH, Essex County, VT

The radar

The Portland,ME radar (KGYX), 52 km from the middle of the area, began a volume at 16:00 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.

The watches that followed

Watches SPC issued over any of the same counties within four hours of this discussion.

The nearest sounding before it

Gray, ME (KGYX), launched 12:00Z, 4.0 hours before the alert went out and 52 km from the middle of its area. This is the routine balloon launch nearest before it, the air the forecaster had in hand, not a measurement taken for the alert.

Skew-T log-p

Temperature in red, dew point in green, the mixed-layer parcel dashed.

Hodograph

The wind turning with height, colored by layer: ground to 1 km, 1 to 3 km, 3 to 6 km, 6 to 10 km.

Mixed-layer CAPE: 101 J/kg
Mixed-layer CIN: -167 J/kg
Mixed-layer LCL: 468 m above ground
0-6 km bulk shear: 54 kt
0-1 km storm-relative helicity: 299 m²/s²
0-3 km storm-relative helicity: 318 m²/s²
Significant tornado parameter: 0.0
Supercell composite: 0.3

Open this sounding in Storm Lab

The text

As transmitted.

542 
ACUS11 KWNS 151600
SWOMCD
SPC MCD 151559 
MEZ000-NHZ000-MAZ000-VTZ000-151800-

Mesoscale Discussion 1754
NWS Storm Prediction Center Norman OK
1059 AM CDT Wed Sep 15 2021

Areas affected...portions of MA...NH and ME

Concerning...Severe potential...Watch likely 

Valid 151559Z - 151800Z

Probability of Watch Issuance...95 percent

SUMMARY...Thunderstorms will increase in coverage and intensity
through the afternoon. Damaging wind gusts and some hail will be the
main hazards. A severe thunderstorm watch will likely be needed by
18z.

DISCUSSION...Convection is increasing ahead of a cold front near the
Canadian border at midday. Stronger heating across southern New
England to the south/west of a surface warm front has allowed for
more rapid destabilization compared to much of ME, which remains
under heavy stratus. As the upper trough shifts northeast through
the afternoon, and the cold front slowly advances east/southeast,
the warm front will lift northeast across ME. This will allow
surface dewpoints from the mid 60s to near 70 F to overspread much
of New England, and MLCAPE is forecast to increase to around
500-1500 J/kg by mid-afternoon. 

Strong vertical shear is already in place over the region, with
effective shear magnitudes greater than 40 kt evident in regional
VWP data. Deep-layer southwesterly flow parallel to the surface
front will support mainly lines/bowing segments with damaging wind
gusts being the main hazard expected through this evening. However,
southerly near-surface winds will contribute to somewhat enlarged
low-level hodographs. Rich boundary-layer moisture also will support
stronger 0-3 km instability from southern portions of VT/NH into far
southern ME/northern MA. A tornado or two could occur across this
area, especially if any more discrete cellular activity can persist.

..Leitman/Thompson.. 09/15/2021

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

ATTN...WFO...CAR...GYX...BOX...BTV...ALY...

LAT...LON   42537182 42647231 42877251 43307247 43867242 44407217
            44947145 45716981 46046896 46126848 45836803 45546789
            45026803 44696825 43497016 42537182

SPC that day

Everything else SPC issued on the same UTC day.

Issued	Product	Office	Counties	Lasted
Sep 15, 1:15 PM EDT	Severe Thunderstorm Watch	SPC	Androscoggin, Cumberland, Franklin, Kennebec, Knox, Lincoln and 7 more	6 hr
Sep 15, 4:38 PM EDT	Mesoscale Discussion	SPC	Androscoggin, Cumberland, Kennebec, Oxford, Penobscot, Sagadahoc and 2 more	67 min
Sep 15, 10:33 AM EDT	Mesoscale Discussion	SPC	Addison, Bennington, Chittenden, Orange, Rutland, Washington and 2 more	2 hr
Sep 15, 11:55 AM EDT	Severe Thunderstorm Watch	SPC	Addison, Bennington, Caledonia, Chittenden, Essex, Lamoille and 6 more	8 hr

Nearby

Mesoscale Discussions in Maine, 2021 Mesoscale Discussions in New Hampshire, 2021 Mesoscale Discussions in Vermont, 2021 Every mesoscale discussion in 2021

Sources

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.