Alert archive / Montana / Mesoscale Discussions in 2012

Mesoscale Discussion 1623

July 27, 201211 counties in Montana; 5 counties in South Dakota; 6 counties in Wyoming

The Storm Prediction Center issued this mesoscale discussion at Jul 27, 2012, 2:27 PM MDT for 11 counties in Montana; 5 counties in South Dakota; 6 counties in Wyoming. It was in force for 3 hr.

The area

The outline as issued.

The facts

Issued: Jul 27, 2012, 2:27 PM MDT (20:27 UTC)
Expired: Jul 27, 2012, 5:00 PM MDT
In force: 3 hr
Issued by: the Storm Prediction Center
Chance of a watch: 40%

All 22 counties

Big Horn County, MT, Carbon County, MT, Carter County, MT, Golden Valley County, MT, Musselshell County, MT, Powder River County, MT, Stillwater County, MT, Sweet Grass County, MT, Treasure County, MT, Wheatland County, MT, Yellowstone County, MT, Butte County, SD, Custer County, SD, Fall River County, SD, Lawrence County, SD, Pennington County, SD, Big Horn County, WY, Campbell County, WY, Crook County, WY, Johnson County, WY, Sheridan County, WY, Weston County, WY

The radar

The Billings radar (KBLX), 137 km from the middle of the area, began a volume at 20:31 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.

Severe Thunderstorm Watch 533Jul 27, 4:25 PM MDT

The nearest sounding before it

Riverton, WY (KRIW), launched 12:00Z, 8.5 hours before the alert went out and 231 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: 0 J/kg
Mixed-layer CIN: No buoyant parcel
Mixed-layer LCL: 3638 m above ground
0-6 km bulk shear: 34 kt
0-1 km storm-relative helicity: 119 m²/s²
0-3 km storm-relative helicity: 143 m²/s²
Significant tornado parameter: 0.0
Supercell composite: 0.0

Open this sounding in Storm Lab

The text

As transmitted, in the capitals the NWS used until 2016.

279 
ACUS11 KWNS 272028
SWOMCD
SPC MCD 272027
SDZ000-MTZ000-WYZ000-272300-

MESOSCALE DISCUSSION 1623
NWS STORM PREDICTION CENTER NORMAN OK
0327 PM CDT FRI JUL 27 2012

AREAS AFFECTED...PORTIONS NRN WY...CENTRAL/SRN MT...WRN SD.

CONCERNING...SEVERE POTENTIAL...WATCH POSSIBLE

VALID 272027Z - 272300Z

PROBABILITY OF WATCH ISSUANCE...40 PERCENT

SUMMARY...SVR THREAT SHOULD INCREASE THROUGH MID-LATE AFTERNOON OVER
CORRIDOR FROM S-CENTRAL MT TO NERN WY AND WRN SD.  MAIN CONCERN IS
DAMAGING GUSTS.  SOME PORTION OF THIS AREA MAY REQUIRE WW.

DISCUSSION...MOISTURE CHANNEL IMAGERY INDICATES MID-UPPER LEVEL
SHORTWAVE TROUGH MOVING NEWD ACROSS PORTIONS SERN ID...SWRN MT AND
WRN WY...THROUGH BROADER-SCALE RIDGE POSITION.  THIS FEATURE SHOULD
PIVOT MORE EWD-ENEWD AND SPREAD ITS PLUME OF LARGE-SCALE ASCENT E OF
MOUNTAINS THROUGH REMAINDER AFTERNOON/EVENING.  IN COMBINATION WITH
ONGOING DIABATIC SFC HEATING...EXPECT STEEP LOW-MIDDLE LEVEL LAPSE
RATES TO RESULT.  JUXTAPOSED WITH MRGL MOISTURE -- I.E. SFC DEW
POINTS GENERALLY TO GENERATE MLCAPE 500-1000 J/KG OVER MUCH OF THIS
AREA...LOCALLY APCHG 1500 J/KG.  SIMILAR VALUES OF DCAPE ALSO ARE
EXPECTED...BASED ON MODIFIED RAOBS AND FCST SOUNDINGS...AND PRESENCE
OF WELL-MIXED SUBCLOUD LAYER SUPPORTING MAINTENANCE OF STG-SVR
DOWNDRAFTS TO GROUND LEVEL.  FOCI APPEAR RATHER NEBULOUS AWAY FROM
AREAS OF HIGHER TERRAIN...ALTHOUGH SFC STREAMLINE ANALYSIS SHOWS
BROAD AREA OF GEN CONFLUENCE FROM NERN WY/WRN SD NWWD ACROSS
E-CENTRAL MT.  LOW-LEVEL SHEAR SHOULD REMAIN MAXIMIZED IN CORRIDOR
OF RELATIVELY BACKED FLOW N OF BLACK HILLS...ENHANCING
STORM-RELATIVE INFLOW AND LOW-LEVEL HODOGRAPHS OVER NERN WY/WRN SD
REGION.

..EDWARDS/CORFIDI.. 07/27/2012

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

ATTN...WFO...UNR...CYS...BYZ...RIW...TFX...

LAT...LON   44861039 47011041 45230271 43030280 44861039

SPC that day

Everything else SPC issued on the same UTC day.

Issued	Product	Office	Counties	Lasted
Jul 27, 4:37 PM MDT	Mesoscale Discussion	SPC	Broadwater, Deer Lodge, Granite, Jefferson, Lewis and Clark, Powell and 1 more	53 min
Jul 27, 4:24 PM MDT	Severe Thunderstorm Watch	SPC	Bennett, Butte, Custer, Fall River, Haakon, Jackson and 4 more	4 hr

Nearby

Mesoscale Discussions in Montana, 2012 Mesoscale Discussions in Wyoming, 2012 Mesoscale Discussions in South Dakota, 2012 Every mesoscale discussion in 2012

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.