Severe Thunderstorm Warning 83, NWS Great Falls, MT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREAT FALLS MT
925 AM MDT TUE JUL 22 2008

.DISCUSSION...

UPDATE FORTHCOMING. HAVE MADE SOME ADJUSTMENTS TO THE POP GRIDS TO
BEETER ALIGN WITH NEIGHBORING OFFICES. ACTIVE WEATHER DAY STILL
ANTICIPATED ACROSS THE AREA. SPC HAS SOUTHWEST MONTANA AND EASTERN
PORTIONS OF THE COUNTY WARNING AREA UNDER A SLIGHT RISK TODAY. RADAR
RETURNS INDICATE LARGE AREA OF SHOWERS AND ISOLATED THUNDERSTORMS
WEST OF THE DIVIDE AND MOVING NORTHEAST ALREADY. REMAINDER OF
FORECAST LOOKS ON TRACK. EMANUEL

&&

.AVIATION...
UPDATED 1200Z.
EXPECT GENERALLY VFR CONDITIONS THROUGH TONIGHT. MOST AREAS WILL
EXPERIENCE SHOWERS/THUNDERSTORMS DURING THE PERIOD FROM 12Z TODAY
TO 12Z WEDNESDAY. IT IS HARD TO SAY EXACTLY HOW THE SHOWERS AND
THUNDERSTORMS WILL PLAY OUT DURING THE PERIOD...BUT HAVE A GENERAL
IDEA THAT SCATTERED SHOWERS AND THUNDERSTORMS WILL MOVE INTO SW
MONTANA THIS MORNING THEN SPREAD SLOWLY NORTHEAST THIS AFTERNOON
AND EVENING AND BECOME MORE NUMEROUS AS THEY DO SO. THERE WILL BE
A RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON AND EVENING...MAINLY
SOUTHEAST OF A LINE FROM HAVRE TO GREAT FALLS TO MISSOULA. SOME OF
THE SHOWERS AND THUNDERSTORMS WILL PRODUCE HEAVY RAIN WITH MVFR
AND IFR CONDITIONS THIS AFTERNOON THROUGH TONIGHT. ALSO AM FAIRLY
CONFIDENT THAT THERE WILL BE AN AREA OF RAIN/SHOWERS/THUNDERSTORMS
AFFECTING KGTF AND KCTB TO THE CONTINENTAL DIVIDE NEAR AND AFTER
MIDNIGHT WITH AGAIN THE POSSIBILITY OF MVFR/IFR CONDITIONS.
CONFIDENCE IN FORECAST WIND DIRECTIONS IS LOW BUT AT LEAST MOST WINDS
UNAFFECTED BY SHOWERS/THUNDERSTORMS WILL BE NEAR OR LESS THAN 10
KNOTS. BLANK

&&

.PREV DISCUSSION...ISSUED 339 AM MDT TUE JUL 22 2008

SEVERE THUNDERSTORMS WILL BE POSSIBLE THIS AFTERNOON AND
EVENING...AS A VERY MOIST AND VIGOROUS UPPER LEVEL TROUGH APPROACHES
THE REGION.

SCATTERED SHOWERS CONTINUE TO DEVELOP WEST OF THE CONTINENTAL DIVIDE
THIS MORNING IN CONJUNCTION WITH A LEAD SHORTWAVE TROUGH...AS CLOUD
COVERAGE INCREASES ACROSS THE ENTIRE AREA. AS THE DAY
PROGRESSES...LIFT ASSOCIATED WITH THE PRIMARY TROUGH WILL SPREAD
ACROSS THE REGION FROM SOUTHWEST TO NORTHEAST. SHOWERS AND
THUNDERSTORMS ARE EXPECTED TO FORM BY LATE MORNING OVER SOUTHWEST
MONTANA...AND MOVE NORTHEASTWARD ACROSS THE REMAINDER OF THE REGION
THIS AFTERNOON AND EVENING. WITH COPIOUS AMOUNTS OF PACIFIC AND
MONSOONAL MOISTURE POOLING INTO THE REGION...IT WON/T TAKE MUCH OF A
KICKER TO GET STORMS GOING EARLY IN THE DAY...WHICH HAS THE
POTENTIAL TO LIMIT OVERALL SOLAR HEATING AND INSTABILITY. WIND
DIRECTIONS WILL BE FAIRLY UNIFORM WITH HEIGHT OVER SOUTHWEST
MONTANA...WHERE THE STRONGEST FLOW ALOFT AND THREAT FOR ORGANIZED
STRONG/SEVERE STORMS IS EXPECTED TO BE. THE MAIN THREAT INITIALLY
WILL BE LARGE HAIL AS COOLER AIR ALOFT WITH THE TROUGH APPROACHES
THE REGION...THOUGH THE DAMAGING WIND THREAT WILL INCREASE WITH TIME
AS CONVECTION BECOMES MORE ORGANIZED. THE SEVERE THREAT WILL EXTEND
NORTHEASTWARD TOWARD GREAT FALLS AND LEWISTOWN BY LATE AFTERNOON.
ALSO OF CONCERN IS THE HEAVY RAIN POTENTIAL...AS THE ATMOSPHERE WILL
BE VERY MOIST. WHILE A FLASH FLOOD WATCH WAS CONSIDERED...ESPECIALLY
FOR BURN AREAS...A MAJOR MITIGATING FACTOR FOR THE FLASH FLOOD
POTENTIAL WILL BE STRONG WINDS ALOFT AND FAST STORM MOTION...WITH
STORMS FORECAST TO MOVE TO THE NORTHEAST AT 15 TO 25 KNOTS.
HOWEVER...ANY CASES OF STORMS TRAINING OVER THE SAME LOCATIONS WILL
NEED TO BE CAREFULLY MONITORED FOR A FLASH FLOOD THREAT. AS
INSTABILITY DIMINISHES THIS EVENING...THE MAIN THREAT WILL
TRANSITION TO A HEAVY RAIN POTENTIAL...AS SHOWERS AND THUNDERSTORMS
DIMINISH FROM SOUTHWEST TO NORTHEAST...WITH ONLY ISOLATED ACTIVITY
EXPECTED BY DAWN ON WEDNESDAY. COOLER TEMPERATURES ARE EXPECTED
TODAY WITH INCREASED CLOUDINESS...AND TONIGHT AS WELL ONCE THE COLD
FRONT WILL HAVE CROSSED THE REGION. ANOTHER...ALBEIT
WEAKER...SHORTWAVE TROUGH WILL MOVE ACROSS THE FORECAST AREA
WEDNESDAY AFTERNOON/EVENING...RESULTING IN ISOLATED SHOWERS AND
THUNDERSTORMS...MAINLY OVER THE NORTHWEST HALF OF THE FORECAST AREA.
SOME OF THE STORMS COULD BE STRONG...GIVEN THE INCREASED
INSTABILITY...ESPECIALLY WITH LINGERING LOW LEVEL MOISTURE FROM THE
PRECEDING PRECIPITATION EVENT AND STRONG FLOW ALOFT MOVING THROUGH
THE REGION ONCE AGAIN. OTHERWISE...TEMPERATURES WILL BE NEAR
SEASONAL AVERAGE WITH PARTLY CLOUDY SKIES ON WEDNESDAY. FOR
THURSDAY...SINKING AIR DOWNSTREAM OF AN UPPER LEVEL RIDGE BUILDING
OVER THE WESTERN CONUS WILL LIMIT THE CHANCE FOR THUNDERSTORMS TO
FAR EASTERN PORTIONS OF THE FORECAST AREA...WITH TEMPERATURES NEAR
SEASONAL AVERAGES UNDER PARTLY CLOUDY SKIES. COHEN

THURSDAY NIGHT THROUGH SUNDAY NIGHT...MODELS FORECAST AN UPPER RIDGE
TO DEVELOP OVER THE AREA THURSDAY NIGHT AND CONTINUE OVER THE AREA
INTO FRIDAY. WITH THE UPPER RIDGE APPROACHING THE FORECAST AREA
THURSDAY NIGHT MODELS DO NOT FORECAST PRECIPITATION AND FEELING
CONFIDENT ENOUGH TO REMOVE ITS MENTION FROM THE FORECAST. LATEST
MODEL RUNS ARE ALSO DRY ON FRIDAY BUT PREVIOUS RUNS OF SOME OF THE
MODELS HAD PRECIPITATION SO WILL LEAVE THE SLIGHT CHANCE OF
PRECIPITATION FORECAST FOR SW MT AND THE NW MOUNTAINS. THEREAFTER
MODELS FORECAST SW FLOW ALOFT BUT DIFFER ON DETAILS OF WEAK EMBEDDED
SHORTWAVES. ALTHOUGH LATEST MODEL RUNS ARE MOSTLY DRY AM RELUCTANT
TO DROP THE MENTION OF PRECIPITATION AS IT DOES NOT TAKE MUCH
MOISTURE/LIFT THIS TIME OF YEAR TO GENERATE A FEW THUNDERSTORMS.
ENDED UP MAKING ONLY SOME MINOR CHANGES TO INHERITED CHANCE OF
PRECIPITATION FORECASTS. BLANK

MONDAY AND TUESDAY...LATEST MODELS CONTINUE TO DEPICT A WEST TO
SOUTHWEST FLOW OVER WEST AND CENTRAL MONTANA DURING THIS PERIOD.
MONSOONAL MOISTURE IS EXPECTED TO MOVE IN FROM THE SOUTH AND PROVIDE
A CHANCE FOR SHOWERS AND THUNDERSTORMS FOR THE SOUTHWEST MOUNTAINS
AND POSSIBLY THE NORTH CENTRAL PLAINS. USED CLIMO POPS THROUGH THE
PERIOD AND KEPT TEMPERATURES NEAR SEASONAL. SAUCIER

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  88  57  81  53 /  60  60  20  20
CTB  83  53  76  48 /  40  60  20  20
HLN  85  60  86  57 /  60  50  20  20
BZN  85  55  87  52 /  70  60  20  20
WEY  70  46  75  45 /  50  40  20  20
DLN  77  51  81  50 /  70  30  20  20
HVR  90  60  79  53 /  30  50  20  20
LWT  87  57  79  52 /  50  50  20  20

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

SHORT TERM...EMANUEL/COHEN
LONG TERM...BLANK/SAUCIER
AVIATION...BLANK

WEATHER.GOV/GREATFALLS

HAZARDOUS WEATHER OUTLOOK...UPDATED
NATIONAL WEATHER SERVICE GREAT FALLS MT
249 PM MDT TUE JUL 22 2008

MTZ008-015-055-231230-
BEAVERHEAD-MADISON-GALLATIN-
249 PM MDT TUE JUL 22 2008

THIS HAZARDOUS WEATHER OUTLOOK IS FOR PORTIONS OF SOUTHWEST
MONTANA.

.DAY ONE...THIS AFTERNOON AND TONIGHT

A SEVERE THUNDERSTORM WATCH IS IN EFFECT FOR
BEAVERHEAD...MADISON...AND GALLATIN COUNTIES UNTIL 9 PM MDT.

SHOWERS AND THUNDERSTORMS WILL CONTINUE ACROSS SOUTHWEST MONTANA
IN ADVANCE OF A VERY MOIST PACIFIC STORM SYSTEM. SOME OF THE
THUNDERSTORMS MAY BECOME SEVERE...WHICH MEANS THEY MAY PRODUCE
HAIL GREATER THAN THE SIZE OF DIMES AND DAMAGING WINDS GREATER
THAN 55 MPH. ALL THUNDERSTORMS WILL BE CAPABLE OF PRODUCING HEAVY
RAINFALL. SHOWERS AND THUNDERSTORMS ARE EXPECTED TO CONTINUE INTO
THE EVENING...BEFORE DIMINISHING AFTER MIDNIGHT.

.DAYS TWO THROUGH SEVEN...WEDNESDAY THROUGH MONDAY

ISOLATED SHOWERS AND THUNDERSTORMS WILL BE POSSIBLE WEDNESDAY
AFTERNOON AND EVENING. SOME OF THESE STORMS WILL HAVE THE
POTENTIAL TO PRODUCE WIND GUSTS TO 50 MPH AND HAIL TO THE SIZE OF
DIMES.

.SPOTTER INFORMATION STATEMENT...

WEATHER SPOTTERS ARE ENCOURAGED TO REPORT SIGNIFICANT WEATHER
CONDITIONS ACCORDING TO STANDARD OPERATING PROCEDURES.

MTZ009>014-044>054-231230-
NORTHERN ROCKY MOUNTAIN FRONT-EASTERN GLACIER-HILL-CASCADE-
CHOUTEAU-CENTRAL AND SOUTHERN LEWIS AND CLARK-TOOLE-LIBERTY-
EASTERN PONDERA-BLAINE-SOUTHERN ROCKY MOUNTAIN FRONT-
EASTERN TETON-JUDITH BASIN-FERGUS-JEFFERSON-BROADWATER-MEAGHER-
249 PM MDT TUE JUL 22 2008

THIS HAZARDOUS WEATHER OUTLOOK IS FOR PORTIONS OF CENTRAL
MONTANA...NORTH CENTRAL MONTANA AND SOUTHWEST MONTANA.

.DAY ONE...THIS AFTERNOON AND TONIGHT

SHOWERS AND THUNDERSTORMS ARE EXPECTED TO DEVELOP THIS AFTERNOON
IN ADVANCE OF A VERY MOIST PACIFIC STORM SYSTEM. SOME OF THE
THUNDERSTORMS WILL BE CAPABLE OF PRODUCING LARGE HAIL AND DAMAGING
WINDS...ESPECIALLY ALONG AND SOUTH OF A LINE FROM HELENA TO GREAT
FALLS TO LEWISTOWN. ALL THUNDERSTORMS WILL BE CAPABLE OF PRODUCING
HEAVY RAINFALL. SHOWERS AND THUNDERSTORMS ARE EXPECTED TO CONTINUE
INTO THE EVENING...BEFORE DIMINISHING AFTER MIDNIGHT.

.DAYS TWO THROUGH SEVEN...WEDNESDAY THROUGH MONDAY

ISOLATED SHOWERS AND THUNDERSTORMS WILL BE POSSIBLE WEDNESDAY
AFTERNOON AND EVENING. SOME OF THESE STORMS WILL HAVE THE
POTENTIAL TO PRODUCE WIND GUSTS TO 50 MPH AND HAIL TO THE SIZE OF
DIMES.

.SPOTTER INFORMATION STATEMENT...

WEATHER SPOTTERS ARE ENCOURAGED TO REPORT SIGNIFICANT WEATHER
CONDITIONS ACCORDING TO STANDARD OPERATING PROCEDURES.