Flash Flood in Montana, May 27, 2001

SOUTHWEST AND NORTH CENTRAL MONTANA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREAT FALLS MT
300 PM MDT SUN MAY 27 2001

.SHORT TERM...ETA/NGM CONTINUE THEIR TREND OF COMING MORE IN LINE WITH
PREVIOUS RUNS OF THE AVN WHICH IS DROPPING THE PACIFIC UPPER LOW FURTHER
SOUTH AND SHARPENING UP THE TROF. AVN HAS BEEN CONSISTENT WITH THIS
SCENARIO FOR THE PAST FEW DAYS. AVN IS STILL THE MOST AMPLIFIED BUT
SINCE THE OTHER MODELS ARE COMING MORE IN LINE WITH IT...IT MAY HAVE
BEEN THE RIGHT SOLUTION ALL ALONG. SATELLITE IMAGERY SHOWS THE PACIFIC
LOW NEAR 145W/55N AND A FULL LATITUDE TROF EXTENDING SOUTHWARD TO
ANOTHER MUCH WEAKER LOW CENTER NEAR 125W/30N. THESE FEATURES LINE UP
WELL WITH THE AVN PLACEMENT OF THEM SO SEE NO REASON NOT TO GO WITH THE
AVN SOLUTION.

MODELS CONTINUE OVERALL TREND OF MOVING THE UPPER LEVEL RIDGE AXIS...
CURRENTLY OVER THE CENTRAL PART OF THE STATE...EASTWARD IN RESPONSE TO
THE AFOREMENTIONED STRONG PACIFIC LOW APPROACHING THE CANADIAN COAST.
THE RESULT IS A MOIST UNSTABLE SOUTHWEST FLOW WITH SEVERAL SHORTWAVES
EMBEDDED IN THE FLOW. ONE OF THESE WAVES IS CURRENTLY IN BEAVERHEAD
COUNTY...FIRING UP CONVECTION OVER THE SOUTHWEST PART OF THE STATE.
AIRMASS CONTINUES TO DESTABILIZE WITH LIFTED INDICES OF -4 AND CAPES IN
EXCESS OF 1200 J/KG FORECASTED FOR A GOOD PORTION OF THE SOUTHWEST AS
THE SHORTWAVE MOVES EAST/NORTHEAST INTO AN AREA OF UPPER LEVEL
DIVERGENCE. MODELS INDICATE LI'S OF -1 AS FAR NORTHEAST AS GREAT FALLS.
THE BEST DYNAMICS STAY OUT OF THE NORTHERN ZONES BUT SCATTERED SHOWERS
STILL CAN NOT BE RULED OUT. BIGGEST THREAT FROM THESE STORMS APPEARS TO
BE GUSTY WINDS. CONVECTION WILL CONTINUE THROUGH THE EVENING AS THE
SHORTWAVE WEAKENS AND DAYTIME INSOLATION WANES.

IT LOOKS LIKE A REPEAT PERFORMANCE FOR MONDAY AFTERNOON AS ANOTHER
SHORTWAVE EJECTS OUT OF THE TROF. THIS WAVE PRETTY MUCH FOLLOWS THE SAME
PATH AS THE ONE TODAY BUT A LITTLE MORE TO THE SOUTHEAST. THE MOST
UNSTABLE AIR IS FURTHER TO THE EAST BUT ANOTHER ROUND OF AFTERNOON
CONVECTION SEEMS LIKELY. THE STRONG TROF SWINGS THROUGH THE AREA MONDAY
NIGHT AND TUESDAY WITH THE BEST DYNAMICS REMAINING NORTH OF THE BORDER.
THICKNESSES CRASH BEHIND THE TROF WITH COLD AIR ADVECTION...VALUES AS
LOW AS 540 DCM ARE SEEN MONDAY MORNING. BEST CHANCES OF WET WEATHER
TUESDAY WILL BE ACROSS THE HI LINE CLOSER TO THE LOW CENTER. SURFACE HI
PRESSURE BUILDS ACROSS THE REGION MONDAY AFTERNOON WITH GOOD DOWNSLOPE
GRADS DEVELOPING. STRONG GUSTY WINDS SEEM LIKELY ON TUESDAY REGION WIDE
BUT WILL STILL CARRY A MENTION OF SHOWERS IN THE UNSTABLE AIRMASS IN
SPITE OF THE DOWNSLOPE FLOW. MAV NUMBERS LOOK GOOD ON MONDAY BUT WILL
UNDERCUT BY A FEW DEGREES ON TUESDAY. GOUDSWARD

EXTENDED...MODELS IN UNUSUALLY GOOD AGREEMENT WITH GENERAL TRENDS.
MODELS START OFF AREA WITH NW FLOW ALOFT ON WED THEN TRANSITION TO SW
FLOW ALOFT BY THE WEEKEND. DIFFERENCES AMONGST THE MODELS CENTER MAINLY
ON WHEN THE UPPER RIDGE AXIS WILL MOVE THROUGH THE AREA. MRF APPEARS TO
BE THE FASTEST OF THE MODELS WITH THE RIDGE AXIS AS IT MOVES IT OVER THE
AREA ON THU WHEREAS ECMWF AND UKMET DELAY RIDGE AXIS UNTIL FRI. ALTHOUGH
LATEST AVN IS JUST AS FAST AS LAST NIGHTS MRF WITH THE UPPER RIDGE AXIS
SINCE MOST OTHER MODELS ARE SLOWER WITH IT WILL GO WITH THEIR SLOWER
SOLUTIONS. LATEST AVN HAS A SOMEWHAT MOIST AIRMASS FOR WED. ALTHOUGH
THERE WILL BE LOW LEVEL DOWNSLOPE...WITH NW FLOW ALOFT WILL KEEP IN A
SLIGHT CHANCE OF SHOWERS FOR THE PLAINS WITH A BETTER CHANCE FOR THE
ADJOINING MOUNTAINS. AS RIDGE APPROACHES AND MOVES OVER AREA BY FRI WILL
GO WITH DRY CONDITIONS FOR THU AND FRI WITH WARMING TEMPERATURES. WITH
SW FLOW EXPECTED FOR NEXT WEEKEND WILL GO WITH A CHANCE OF SHOWERS OR
THUNDERSTORMS. BLANK

CCF NUMBERS
GTF EW 048/076 042/058 035 054433
CTB EW 044/072 040/056 033 053344
HLN EW 050/075 042/060 037 055433
BZN EW 042/073 038/058 032 056433
WEY EW 03M8/062 036/056 030 056433
DLN EW 044/074 040/060 036 054433
HVR EW 052/078 042/058 037 053344
LWT EW 048/074 040/056 034 055433