Damaging Wind in Montana, May 8, 2001

SOUTHWEST AND NORTH CENTRAL MONTANA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREAT FALLS MT
315 PM MDT TUE MAY 8 2001

SHORT TERM LOOKS FAIRLY CERTAIN...BUT MODEL DISCREPANCY IS CAUSING
CONFIDENCE PAST TONIGHT TO BE SOMEWHAT LOW. WINDS HAVE PICKED UP AS
EXPECTED AHEAD OF UPPER TROUGH EXTENDING FROM WASHINGTON/OREGON WEST TO
JUST OFF THE PACIFIC COAST...WITH A COLD FRONT JUST AHEAD OF THE TROUGH.
ONE SHORTWAVE MOVING NORTHEAST OVER NORTHEASTERN WASHINGTON AND
SOUTHEASTERN BRITISH COLUMBIA IS BRINGING INCREASING CLOUDINESS TO
WESTERN MONTANA...BUT STRONG DOWNSLOPE LIMITING CLOUDINESS EAST OF THE
ROCKIES. REGARDLESS...LOW AND MID LEVELS DRY (AS EVIDENCED BY 12Z
INVERTED-V SOUNDING AT TFX)...SO POPS SHOULD BE LOW WITH ANY CONVECTIVE
SHOWER DEVELOPMENT THIS EVENING. INSTABILITY IS GREATEST OVER SOUTHWEST
AND CENTRAL MONTANA THIS EVENING (-1 TO -3 LIFTED INDICES)...SO WILL GO
WITH MENTION OF ISOLATED THUNDERSTORMS THERE WITH A POTENTIAL FOR GUSTY
WINDS. WILL GENERALLY GO WITH SLIGHT CHANCE/ISOLATED POPS THIS EVENING.
AS THE BROAD TROUGH MOVES OVER THE AREA OVERNIGHT...EXPECT THE COLD
FRONT TO STAY JUST AHEAD OF IT...AS THE COLD FRONT SEEMS TO BE PARALLEL
TO THE WINDS ALOFT AHEAD OF THE TROUGH. THEREFORE...EXPECT FRONTAL
PASSAGE AROUND 00Z AT CUT BANK...WITH THE FRONT POSITIONED AT A HAVRE-
GREAT FALLS-HELENA LINE BY 06Z. THE FRONT SHOULD EXIT THE NORTHERN ZONES
BY 09Z...BUT WILL LINGER OVER THE SOUTHWEST THROUGHOUT THE NIGHT. WEST
WINDS OF 35 TO 40 KT AT 700 MB WILL CAUSE GUSTS OF 40 TO 45 MPH WITH THE
FRONTAL PASSAGE...BUT WINDS WILL DECREASE CONSIDERABLY AFTERWARD. A
SECONDARY COLD FRONT WILL DROP OUT OF CANADA AFTER 06Z...COOLING
TEMPERATURES CONSIDERABLY OVER LAST NIGHT ACROSS THE NORTHERN ZONES. THE
TROUGH AXIS SHOULD BE POSITIONED OVER THE CENTRAL PART OF THE CWA AROUND
06Z...WITH ANOTHER SHORTWAVE AND INCREASING MID LEVEL MOISTURE MOVING
INTO THE AREA IN A NORTHWEST FLOW ALOFT ON THE BACK SIDE OF THE TROUGH.
THIS WILL BRING A BETTER CHANCE OF PRECIPITATION TO THE CWA BEGINNING
AFTER MIDNIGHT. THIS WILL CARRY OVER INTO THE MORNING HOURS...WITH A
COOLER AIRMASS OVER THE AREA AND A LACK OF DYNAMICS LIMITING THE CHANCE
FOR PRECIPITATION IN THE AFTERNOON. THE BEST CHANCE WILL BE IN THE
SOUTHWEST...WHERE THE FRONTAL BOUNDARY WILL STILL BE HANGING AROUND. A
DRIER AIRMASS ASSOCIATED WITH A WEAK UPPER RIDGE WILL MOVE OVER THE AREA
LATER WEDNESDAY THROUGH WEDNESDAY NIGHT...FOR A LESSER CHANCE OF
PRECIPITATION. ANOTHER SHORTWAVE TROUGH MOVES INTO THE AREA FROM THE
PACIFIC LATE WEDNESDAY NIGHT INTO THURSDAY FOR A BETTER CHANCE OF
PRECIPITATION...ESPECIALLY OVER THE SOUTHWEST AND CENTRAL ZONES WHERE AN
INVERTED SURFACE TROUGH DEVELOPS OUT OF WEAK CYCLOGENESIS OVER THE
SOUTHWESTERN CONUS. TEMPERATURES SHOULD STAY FAIRLY CONSISTENT
THROUGHOUT THE PERIOD...AS THE AIRMASS AND 1000-500 MB THICKNESSES
CHANGE LITTLE.  COULSTON

EXTENDED...MODELS IN GOOD AGREEMENT ON GENERAL TREND INTO THE WEEKEND
BUT THEN THERE ARE SOME DIFFERENCES FOR EARLY NEXT WEEK. MODEL CONSENSUS
IS FOR NW FLOW OVER THE AREA ON FRI AHEAD OF AN UPPER RIDGE.  RIDGE AXIS
MOVES OVER THE AREA LATE FRIDAY NIGHT OR EARLY SATURDAY BUT THE ECMWF IS
A LITTLE SLOWER WITH THE PASSAGE OF THE RIDGE AXIS. THEREAFTER MRF
BRINGS A WEAK POSITIVELY TILTED SHORTWAVE THROUGH THE NRN PORTION OF THE
AREA EARLY MONDAY. IT HAS SOME MOISTURE AHEAD OF SHORTWAVE OVER THE SRN
ZONES SUN INTO MON. BY TUE MRF HAS STRONG UPPER RIDGE BACK OVER THE AREA
WITH WARMING TEMPERATURES AND DRY CONDITIONS. LOOKING AT THE OTHER
MODELS THEY TAKE THE SHORTWAVE MORE INTO CANADA AND EXTRAPOLATION OF
THEIR SOLUTIONS SUGGEST FORECAST AREA WILL STAY UNDER SW FLOW ALOFT.
SINCE THEIR SOLUTIONS LEAN TOWARDS KEEPING AT LEAST SW MT UNDER SW FLOW
ALOFT WILL KEEP IN A SLIGHT CHANCE OF SHOWERS OR THUNDERSTORMS FOR TUE
AS WELL FOR THE SRN ZONES. WILL GO WITH A DRY FORECAST FOR THE NRN
ZONES. BLANK

CCF NUMBERS
GTF EE 039/065 036/063 037 10221
CTB BE 034/060 032/058 033 10221
HLN EW 036/065 036/065 038 10322
BZN WW 034/069 034/067 036 10332
WEY WW 026/061 026/058 028 10332
DLN EE 033/069 033/067 035 10322
HVR BB 036/064 034/067 040 10221
LWT EE 037/066 035/066 038 10321