Mesoscale Discussion 1074

Issued

Jun 5, 2012, 1:56 PM MDT (19:56 UTC)

Expired

Jun 5, 2012, 4:00 PM MDT

In force

2 hr

Issued by

the Storm Prediction Center

Chance of a watch

95%

Counties Blaine County, MT, Cascade County, MT, Chouteau County, MT, Fergus County, MT, Glacier County, MT, Hill County, MT, Judith Basin County, MT, Lewis and Clark County, MT, Liberty County, MT, Petroleum County, MT, Phillips County, MT, Pondera County, MT, Teton County, MT, Toole County, MT

The Great Falls radar (KTFX), 49 km from the middle of the area, began a volume at 19:58 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.

• Tornado Watch 360Jun 5, 2:35 PM MDT

796 
ACUS11 KWNS 051956
SWOMCD
SPC MCD 051956 
MTZ000-052200-

MESOSCALE DISCUSSION 1074
NWS STORM PREDICTION CENTER NORMAN OK
0256 PM CDT TUE JUN 05 2012

AREAS AFFECTED...PARTS OF CNTRL/N-CNTRL MONTANA

CONCERNING...SEVERE POTENTIAL...TORNADO WATCH LIKELY 

VALID 051956Z - 052200Z

PROBABILITY OF WATCH ISSUANCE...95 PERCENT

SUMMARY...A TORNADO WATCH WILL BE ISSUED BY 2100Z ACROSS PARTS OF
CNTRL AND N-CNTRL MONTANA...AS THE THREAT FOR TORNADIC SUPERCELLS
INCREASES THIS AFTERNOON AND CONTINUES INTO THE EVENING.

DISCUSSION...A 1000-MB SUB-SYNOPTIC SCALE LOW IS BECOMING
INCREASINGLY BETTER-DEFINED PER SFC OBSERVATIONS ACROSS N-CNTRL
MT...WITH A CLOSED CIRCULATION CENTERED OVER CHOUTEAU COUNTY.
CYCLOGENESIS IS BEING SUPPORTED IN THIS AREA BY INCREASING
LARGE-SCALE ASCENT -- HEIGHT FALLS APPROACHING 60 GEOPOTENTIAL
METERS PER 12 HOURS -- IN ADVANCE OF A POTENT/NEGATIVELY TILTED
SHORTWAVE TROUGH MOVING ACROSS THE NRN/CNTRL GREAT BASIN. LOW-LEVEL
MOISTURE TRAJECTORIES EMANATING FROM THE LOWER PLAINS OF MT/DAKOTAS
ARE MAINTAINING A ZONE OF DEWPOINTS IN THE MIDDLE/UPPER 50S TO THE
NORTH/NORTHEAST OF A FRONTOGENETIC BOUNDARY/WARM FRONT ARCING
EWD/SEWD FROM THE LOW INTO SERN MT. THIS ZONE OF ABNORMALLY RICH
LOW-LEVEL MOISTURE FOR SUCH HIGH ELEVATIONS IS CIRCULATING AROUND
THE WRN AND SRN SEMICIRCLE OF THE LOW JUST EAST OF THE NRN ROCKY
MOUNTAIN FRONT AND INTO AREAS JUST NORTH OF THE LITTLE AND BIG BELT
MOUNTAINS NEAR GREAT FALLS...WITH ONLY MODESTLY LOWER DEWPOINTS
FARTHER SOUTH INTO THE HELENA VALLEY.

AREAS OF STRONG INSOLATION/DIABATIC HEATING EAST OF THE CONTINENTAL
DIVIDE AMIDST THE RICH LOW-LEVEL MOISTURE WILL ALLOW MLCAPE VALUES
TO CLIMB TO 1000-1750 J/KG. MEANWHILE...THE SHALLOW/STABLE NATURE OF
THE RELATIVELY COOL AIR MASS WEST OF THE CONTINENTAL DIVIDE --
DELINEATED BY THE OVERCAST LOW-CLOUD DECK -- IS PRESENTLY BEING
BLOCKED BY THE NRN ROCKIES AND IS NOT EXPECT TO EMERGE ONTO THE
PLAINS UNTIL DEEPER COLD AIR ASSOCIATED WITH THE TROUGH MAKES MORE
SUBSTANTIAL EWD PROGRESS TONIGHT. FURTHERMORE...THE BARRIER-JET-LIKE
NATURE OF THE SFC FLOW ON THE WRN/SWRN FLANK OF THE LOW WILL LIKELY
MITIGATE DOWNSLOPE EFFECTS OVER AREAS JUST EAST OF THE ROCKY
MOUNTAIN FRONT. THESE FACTORS WILL ALLOW MODERATE INSTABILITY TO BE
MAINTAINED AND JUXTAPOSED WITH FAVORABLE VERTICAL WIND PROFILES FOR
SUSTAINED SUPERCELLS THIS AFTERNOON INTO THIS EVENING...WITH NEARLY
9-C/KM 700-500 MB LAPSE RATES EVIDENT IN THE 18Z GLASGOW AND 19Z
GREAT FALLS SPECIAL RAOBS AND 50-65 KT OF EFFECTIVE BULK SHEAR.
THESE SPECIAL RAOBS ALREADY INDICATE APPRECIABLE LOW-LEVEL SHEAR.

STORMS WILL CONTINUE TO DEVELOP OVER THE HIGHER TERRAIN OF
SWRN/CNTRL MONTANA DURING THE NEXT COUPLE OF HOURS BEFORE ADVANCING
NWD/NEWD INTO N-CNTRL MT THROUGH THE EVENING. DAMAGING WIND GUSTS
AND SEVERE HAIL...SOME POTENTIALLY SIGNIFICANT...WILL BE LIKELY WITH
THE SUPERCELLS. STRONG VERTICAL DIRECTIONAL/SPEED SHEAR WILL EXIST
OWING TO AN INCREASING SELY/ELY CURRENT WITHIN THE 850-700-MB LAYER
FEATURING SPEEDS EXCEEDING 40 KT BY 00Z ATOP SFC WINDS RELATIVELY
BACKED COMPARED TO THOSE ALOFT. THIS WOULD SUPPORT EFFECTIVE
STORM-RELATIVE HELICITY VALUES IN EXCESS OF 400 M2/S2. THE MOST
SUBSTANTIAL TORNADO THREAT WILL EXIST WITHIN AN AREA BOUNDED BY
GREAT FALLS...CUT BANK...AND HAVRE INVOF MAXIMIZED SFC VORTICITY
NEAR THE LOW. STORMS COULD EVENTUALLY EVOLVE INTO BOWING LINE
SEGMENTS WITH LINE ECHO WAVE PATTERN STRUCTURES THROUGH THE EVENING.
THE STRONGEST STORMS WILL LIKELY HAVE CROSSED NORTH OF THE
INTERNATIONAL BORDER INTO ALBERTA AND SASKATCHEWAN BY 05Z...THOUGH A
FEW STRONG/ELEVATED STORMS MAY PERSIST IN THE WAKE OF THE INITIAL
ACTIVITY THROUGH THE NIGHT.

..COHEN/MEAD.. 06/05/2012

...PLEASE SEE WWW.SPC.NOAA.GOV FOR GRAPHIC PRODUCT...

ATTN...WFO...GGW...TFX...

LAT...LON   49000863 48590779 47390786 46930813 46790957 46851118
            46581200 46821238 47051269 47681284 47831270 48261285
            48451322 48851325 48991236 49000863

Mesoscale Discussions in Montana, 2012Every mesoscale discussion in 2012

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.