Hail in Montana, July 20, 2001

SOUTHWEST AND NORTH CENTRAL MONTANA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREAT FALLS MT
300 PM MDT FRI JUL 20 2001

SEEMINGLY PERMANENT PATTERN OF EMBEDDED SHORTWAVES IN A SW FLOW
ALOFT TO CONTINUE THROUGH SAT NIGHT WITH AIRMASS CONTINUING
UNSTABLE. MODELS CONTINUE TO BE DIFFERENT FROM ONE ANOTHER ON
SHORTWAVE DETAILS AND ALSO DIFFER FROM RUN TO RUN ON THEIR DETAILS.
SO ALTHOUGH AIRMASS WILL CONTINUE TO BE UNSTABLE AND SOMEWHAT MOIST
WILL KEEP POPS IN THE SCATTERED CATEGORY THROUGH THE 1ST THREE
FORECAST PERIODS. AS FOR SEVERE WEATHER...FOR TONIGHT...AT 18Z
SATELLITE IMAGERY INDICATED A VORTICITY CENTER OVER NE OREGON WITH A
SHORTWAVE EXTENDING SW THROUGH SE OREGON. ETA/MESOETA ARE BETTER THAN
THE NGM AND AVN IN THE HANDLING OF THIS FEATURE. THERE IS A JET
STREAK ASSOCIATED WITH THE FEATURE AND SOME THUNDERSTORMS ARE
BREAKING OUT IN IN THE RIGHT REAR QUADRANT OF THE JET STREAK. EXPECT
THUNDERSTORMS TO CONTINUE TO MOVE NE TOWARDS CENTRAL MT AND THEY MAY
INTENSIFY AS THEY MOVE INTO THE MORE UNSTABLE AIRMASS THERE. CAPES
SHOULD BE AROUND 1000/1500 JOULES/KG. PROBLEM IS ETA/MESOETA HAVE
ONLY A VERY WEAK SHORTWAVE ASSOCIATED WITH THESE THUNDERSTORMS.
MESOETA DOES HAVE A STRONGER SHORTWAVE MOVING NE THROUGH THE AREA
LATER THIS EVENING AND AFTER MIDNIGHT BUT THE PROBLEM IS THAT
SHORTWAVE WILL BE MOVING THROUGH AFTER INSOLATION WILL HAVE GONE. SO
UPSHOT IS NOT SURE HOW MUCH SEVERE WEATHER THERE WILL BE TONIGHT. SPC
HAS CENTRAL ZONES UNDER A RISK OF SEVERE WEATHER AGAIN SAT AFTERNOON
AND NIGHT. AGAIN THIS WILL REMAIN A POSSIBILITY AS AIRMASS WILL BE
QUITE UNSTABLE. TO AID IN THUNDERSTORM DEVELOPMENT MODELS INDICATE
SOME POSSIBLE INTERACTION BETWEEN LEFT FRONT AND RIGHT REAR QUADRANTS
OF TWO SEPARATE JET STREAKS. LOOKING AHEAD TO SUN PATTERN CHANGES A
LITTLE AS FLOW ALOFT BECOMES MORE ZONAL. ETA HINTS AT WEAK RIDGING
ALOFT WHILE AVN HAS CYCLONIC WSW FLOW ALOFT. MODELS HINT AT SOME
POSSIBLE SHORTWAVE ACTIVITY ACROSS THE N WHILE CHECK OF FORECAST
SOUNDINGS INDICATES AIRMASS SHOULD BE WEAKLY UNSTABLE OVER ENTIRE
AREA. WILL GO WITH 20 PCT POPS BUT GO 30 PCT AT HVR. THE REASON FOR
GOING A LITTLE HIGHER THERE IS THEY APPEAR TO BE IN A LITTLE BETTER
POSITION GO GET LIFT FROM SHORTWAVE DYNAMICS.

.EXTENDED...
UPPER LEVEL TROF IS FORECASTED TO MOVE OVER NORTH CENTRAL MONTANA BY
MONDAY AND DISSIPATE...AS UPPER LEVEL FLOW BECOMES MORE ZONAL. THIS
WILL RESULT IN MOSTLY DRY CONDITIONS TUESDAY AND WEDNESDAY WITH
NEAR NORMAL TEMPERATURES. BY THURSDAY A NEW UPPER LEVEL TROF AND
CANADIAN COLD FRONT IS EXPECTED TO DIP DOWN INTO FAR NORTH CENTRAL
MONTANA. THIS WILL RESULT IN ANOTHER ROUND OF SHOWERS AND
THUNDERSTORMS ACROSS THE REGION...WITH SLIGHTLY BELOW NORMAL
TEMPERATURES OVER THE NORTH AND NEAR NORMAL TEMPERATURES IN THE
SOUTH. BRUSDA

CCF NUMBERS
GTF EB 055/080 053/078 048 04444
CTB BB 050/077 050/075 046 04323
HLN EB 053/082 053/080 048 04444
BZN BB 051/083 048/080 045 04444
WEY BB 046/078 043/075 040 04333
DLN BB 052/079 049/077 047 04433
HVR EE 060/088 060/085 052 04544
LWT BB 055/082 053/077 047 04545=