Mesoscale Discussion 710

Issued

May 12, 2017, 1:35 PM MDT (19:35 UTC)

Expired

May 12, 2017, 4:00 PM MDT

In force

2 hr

Issued by

the Storm Prediction Center

Chance of a watch

20%

Counties Beaverhead County, MT, Broadwater County, MT, Cascade County, MT, Chouteau County, MT, Deer Lodge County, MT, Glacier County, MT, Hill County, MT, Jefferson County, MT, Lewis and Clark County, MT, Liberty County, MT, Madison County, MT, Pondera County, MT, Powell County, MT, Silver Bow County, MT, Teton County, MT, Toole County, MT

The Great Falls radar (KTFX), 41 km from the middle of the area, began a volume at 19:35 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.

609 
ACUS11 KWNS 121936
SWOMCD
SPC MCD 121935 
MTZ000-122200-

Mesoscale Discussion 0710
NWS Storm Prediction Center Norman OK
0235 PM CDT Fri May 12 2017

Areas affected...Portions of northern and western Montana

Concerning...Severe potential...Watch unlikely 

Valid 121935Z - 122200Z

Probability of Watch Issuance...20 percent

SUMMARY...Portions of the northern Rockies and adjacent High Plains
are being monitored for an increase in isolated severe thunderstorm
potential this afternoon, continuing into the evening. Watch
issuance is presently unlikely, though convective and environmental
trends will continue to be monitored.

DISCUSSION...Columnar stretching in the lee of the northern Rockies,
attendant to a midlevel south-southwesterly speed maximum grazing
the eastern periphery of a northwest U.S. cyclone, is encouraging
weak surface cyclogenesis between Great Falls and Cut Bank. Weak
return moisture coils from the plains of eastern and northern MT,
through the northern semicircle of the developing cyclone along the
Hi-Line toward the northern Rocky Mountain Front Range west of Cut
Bank. This corridor of moisture -- characterized by dewpoints in the
middle 40s -- is supporting MLCAPE around 250-750 J/kg -- with
buoyancy largely aided by steep midlevel lapse rates (8.0 C/km in
the 700-500-mb layer per the 18Z Great Falls sounding). The
strongest buoyancy will likely remain confined to parts of
north-central and northwest MT -- specifically east of the Front
Range.

DCVA/ascent attendant to the midlevel speed maximum -- coupled with
diurnally enhanced orographic circulations and baroclinic
circulations attendant to differential heating peripheral to a
western MT stratus shield -- will continue to support an uptick in
convective coverage and intensity during the next several hours.
Subsequent activity will spread north-northeastward and
northeastward into the evening hours.

Convection is already producing lightning north of Helena, and
additional convection developing over the Bitterroot and Lewis
Ranges will spread into the plume of greater (albeit modest)
moisture/buoyancy during the 20Z-00Z time frame. With 50-60 kt of
effective shear, and the steep midlevel lapse rates, supercell
structures and small bowing line segments could produce isolated
severe hail/wind, especially from the plains of Teton, Pondera, and
Glacier Counties eastward toward Liberty and western Chouteau
Counties and vicinity. This focused corridor will be the most likely
area for the greatest severe potential later this afternoon into the
evening. 

Elsewhere, deeper boundary-layer circulations and related lesser
low-level moisture/buoyancy will tend to limit the severe risk.
However, high-based storms could develop farther south toward the
southwest MT mountains as stronger ascent eventually emerges over
these locations. Locally severe wind gusts (and perhaps marginally
severe hail) may accompany this activity farther south.

However, overall across the entire area, the magnitude of buoyancy,
and the areal extent of relatively more substantive buoyancy, are
not forecast to be sufficiently large for more than isolated severe
coverage. Regardless, if a more substantial severe risk were to
eventually evolve -- especially over the plains of northern MT --
prospects for Watch issuance could later increase.

..Cohen/Goss.. 05/12/2017

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

ATTN...WFO...TFX...MSO...

LAT...LON   48940999 48301015 46581109 44861188 44751305 45311339
            46001344 47471335 48651324 49011215 49061101 48940999

Mesoscale Discussions in Montana, 2017Every mesoscale discussion in 2017

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.