Flash Flood Warning 1, NWS Great Falls, MT

SOUTHWEST AND NORTH CENTRAL MONTANA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREAT FALLS MT
300 PM MDT MON MAY 28 2001

.SHORT TERM...MODELS ARE IN GENERALLY GOOD AGREEMENT WITH THE LARGE
SCALE FEATURES. THE ETA CONTINUES TO BE THE MOST CONSISTENT WITH ITS
SOLUTION WHILE THE OTHERS HAVE WAFFLED A BIT. BIGGEST DIFFERENCE BETWEEN
THE MODELS IS THE PRESENCE OF A VORTMAX OVER SOUTH CENTRAL MONTANA. THIS
FEATURE IS PICKED UP ON BY THE AVN MODEL BUT NOT BY THE OTHERS. I REALLY
CANT SEEM TO FIND THIS FEATURE ON THE SATELLITE BUT IT STILL CANT BE
IGNORED AS IT COULD BE AN EFFECTIVE TRIGGER FOR CONVECTION. A COMPROMISE
OF THE MODELS SEEMS LIKE THE BEST APPROACH.

PACIFIC LOW NOW SITTING OFF THE BRITISH COLUMBIA COAST WILL ADVANCE
EASTWARD DURING THE PERIOD. A STRONG VORTMAX WILL ROTATE AROUND THE MAIN
LOW TRAVELING THROUGH WASHINGTON STATE AND THEN ACROSS SOUTHWEST
ALBERTA. THE SYSTEM WILL EFFECTIVELY KNOCK DOWN THE RIDGE THAT HAD BEEN
IN PLACE...REPLACING IT WITH A FAST WESTERLY FLOW. LOW WILL TRACK A COLD
FRONT ACROSS THE CWA BUT IT WILL BE A SLOW MOVER AS 500MB WINDS EARLY IN
THE PERIOD ARE UNDER 20 KTS. FRONT AND ASSOCIATED AREA OF SURFACE
CONVERGENCE WILL REACH WESTERN MONTANA LATE THIS AFTERNOON AND ONLY MAKE
IT TO THE VICINITY OF GREAT FALLS BY TUESDAY MORNING AND FINALLY EXITING
THE STATE LATE TUESDAY AFTERNOON. TEMPERATURES CONTINUE TO WARM RAPIDLY
AND AS SUCH THE ATMOSPHERE IS DESTABILIZING QUICKLY. MESO-ETA IS
CONSISTENT WITH KEEPING LIFTED INDICES OF -4 ACROSS THE AREA THIS
EVENING WITH CAPES IN EXCESS OF 1200 J/KG. HOWEVER THE CURRENT LAPS DATA
HAS CAPE VALUES OVER 2000 J/KG POOLING IN THE SOUTHWEST AND NORTHWEST
PARTS OF THE STATE. CONVECTION IS RAPIDLY CRANKING UP.

ANOTHER CONCERN IS DIFLUANCE ALOFT. THIS AREA MOVES FROM THE SOUTHWEST
TO THE NORTH CENTRAL PART OF THE STATE DURING THE EVENING. THE AREA IS
NOT AS IMPRESSIVE AS EARLIER RUNS BUT STILL WORTH NOTING. PRECIPITABLE
WATER OVER THE COUNTY WARNING RANGES FROM BETWEEN 2/3 AND 3/4 OF AN
INCH. BIGGEST CONCERN HERE WILL BE RUN OFF OVER THE BURN AREAS. AN SPS
HAS ALREADY BEEN ISSUED TO ADDRESS THIS CONCERN. THE FRONT AND
CONVERGENCE ZONE WILL BE THE MAIN FOCUS BUT THE PRECIP COULD BE MORE
SIGNIFICANT IF THE AVN IS INDEED CORRECT WITH THE VORTMAX.

ONCE THE TROF EXITS THE REGION THE ATMOSPHERE DRYS OUT AND DESTABILIZES
FAIRLY RAPIDLY BUT A FEW LEFTOVER SHOWERS OVER THE EASTERN ZONES EARLY
TUESDAY CAN NOT BE RULED OUT. HEIGHTS DECREASE SO WILL DROP TEMPS ACROSS
THE BOARD ON TUESDAY AND KEEP IT COOL TUESDAY NIGHT. MODELS ARE ALSO
HINTING AT A WEAK SHORTWAVE FOR TUESDAY NIGHT BUT MOISTURE IS LIMITED
AND WILL KEEP IT DRY. DOWNSLOPE RETURNS ON WEDNESDAY AFTERNOON AS A NEW
RIDGE STARTS TO BUILD OVER THE PACIFIC NORTHWEST. TEMPS WILL REBOUND A
BIT BUT NORTHWEST FLOW ALOFT WILL CONTINUE. MAV TEMPS GENERALLY
ACCEPTED. GOUDSWARD

EXTENDED...LATEST AVN IN LINE WITH MRF RUN FROM LAST NIGHT AS UPPER
RIDGE MOVES OVER AREA ON THU. LIKE YESTERDAY OTHER MODELS ARE SLOWER
THAN THE MRF MOVING THE UPPER RIDGE AXIS OVER THE FORECAST AREA BUT NOW
ARE FASTER THAN THEY WERE YESTERDAY. OTHER MODELS NOW MOVE UPPER RIDGE
AXIS OVER AREA THU NIGHT RATHER THAN ON FRI. WILL GO WITH GENERALLY DRY
WEATHER FOR THU AND FRI. THEREAFTER INTO THE WEEKEND UKMET/ECMWF/MRF
HAVE SW FLOW ALOFT OVER THE AREA AS AN UPPER TROUGH INTENSIFIES OVER THE
W COAST STATES. CMC AND NOGAPS ARE MORE PROGRESSIVE WITH THE UPPER
TROUGH. SINCE MRF/ECMWF/UKMET TEND TO BE MORE ACCURATE THAN THESE LATTER
TWO MODELS WILL GO WITH MRF/ECMWF/UKMET IDEA OF SW FLOW ALOFT. PROBLEM
THEN BECOMES WHEN AND WHERE CONVECTION WILL BE. AVN HAS A SHORTWAVE
SKIRTING FAR NW MT FRI NIGHT WITH DOWNSLOPE DEVELOPING OVER THE NRN
PLAINS. THIS COULD INHIBIT CONVECTION OVER THE N AT LEAST FOR SAT. ON
THE OTHER HAND ECMWF HAS A FAIRLY STRONG SHORTWAVE MOVING THROUGH THE
AREA LATE FRIDAY NIGHT AND SAT. ALTHOUGH MRF IMPLIES A DRY FORECAST FOR
SAT AND SUN IN THE N WILL HEDGE AND MENTION A SLIGHT CHANCE OF SHOWERS.
WILL GO WITH A CHANCE OF SHOWERS OR THUNDERSTORMS IN THE S. BLANK

CCF NUMBERS
GTF BU 046/060 034/072 045 055200
CTB BU 044/056 032/070 040 055200
HLN BU 044/062 034/070 042 055100
BZN BU 042/064 032/071 038 055100
WEY BU 036/058 028/068 033 055100
DLN BU 043/066 034/071 038 054100
HVR BU 050/068 036/073 046 056300
LWT BU 046/060 035/070 042 056300