Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 240602
AFDTFX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREAT FALLS MT
1155 PM MDT THU APR 23 2015

UPDATED AVIATION SECTION

.UPDATE...
UPDATED FORECAST IS OUT. NO MAJOR CHANGES THIS EVENING. I DID ADD
A FEW LIGHT RAIN SHOWERS SOUTH AND EAST OF GREAT FALLS...BUT ANY
PRECIPITATION WILL BE VERY LIGHT TONIGHT. THE REST OF THE FORECAST
LOOKS ON TRACK FOR TONIGHT.

LOOKING AHEAD TO FRIDAY...THE MAIN CONCERN WILL BE
SHOWERS/THUNDERSTORMS DEVELOPING DURING THE AFTERNOON OVER
SOUTHWEST MT. THESE STORMS WILL SPREAD INTO CENTRAL MT MAINLY EAST
OF GREAT FALLS BY EVENING. SOME OF THESE STORMS COULD PRODUCE
BRIEF HEAVY RAIN. TEMPERATURES SHOULD BE A FEW DEGREES COOLER OVER
THE REGION ON FRIDAY BECAUSE OF CLOUD COVER/PRECIP. BRUSDA

&&

.AVIATION...
UPDATED 0555Z.

VFR CONDITIONS ARE GENERALLY EXPECTED ACROSS NORTH CENTRAL AND
SOUTHWEST MONTANA THROUGH AT LEAST FRIDAY EVENING, UNLESS OTHERWISE
NOTED. A FEW LINGERING SHOWERS/THUNDERSTORMS ALONG A LINE FROM KDLN
THROUGH KBZN AND KLWT WILL DIMINISH THROUGH 09Z. A FEW SHOWERS MAY
ALSO MOVE THROUGH THE KHVR AREA THROUGH 09Z AS WELL. OTHERWISE, A
DISTURBANCE EJECTING FROM THE MAIN UPPER TROUGH OVER VANCOUVER
ISLAND WILL BRING INCREASING CLOUDINESS AND A CHANCE FOR
SHOWERS/THUNDERSTORMS TO MOST OF THE AREA, ESPECIALLY AFTER 18Z AS A
WEAK COLD FRONT MOVES ACROSS THE AREA. THE GREATEST THREAT FOR
SHOWERS WILL BE ALONG AND SOUTH OF LINE FROM KHLN TO KGTF TO KHVR.
THE BEST CHANCE OF THUNDERSTORMS WITH GUSTS UP TO 35 KT AND
OCCASIONAL MOUNTAIN TOP OBSCURING MVFR CONDITIONS ALONG AND SOUTH OF
A KHLN TO KLWT LINE. BREEZY WESTERLY WINDS WILL ALSO ACCOMPANY THE
FRONTAL PASSAGE. SHOWERS AND WINDS WILL DECREASE FROM WEST TO EAST
AFTER 00Z AS THE DISTURBANCE BEGINS EXITING THE AREA.
COULSTON

&&

.PREV DISCUSSION...
/ISSUED 610 PM MDT THU APR 23 2015/

TONIGHT THROUGH SATURDAY...A BROAD UPPER LEVEL TROUGH WILL REMAIN
IN PLACE OVER MUCH OF THE WESTERN U.S. KEEPING CONDITIONS
SOMEWHAT UNSETTLED THROUGH THE UPCOMING WEEKEND. SEVERAL
CIRCULATION CENTERS ARE EMBEDDED WITHIN THE BROAD TROUGH WITH THE
MAIN CENTER LOCATED OFF VANCOUVER ISLAND WHILE THE SOUTHERN
EXTENSION OF THE TROUGH AXIS MOVES EAST ACROSS THE DESERT SW.
MOISTURE STREAMING NE FROM THE SW TROUGH IS JUST CLIPPING FAR SW
MT...WHERE A FEW SHOWERS AND AN ISOLATED THUNDERSTORM ARE POSSIBLE
THROUGH EARLY THIS EVENING...MAINLY SOUTH OF LINE FROM DILLON TO
BOZEMAN. OTHERWISE...DRY CONDITIONS WILL PREVAIL TONIGHT WITH
CLOUD-COVER INCREASING FROM WEST TO EAST OVERNIGHT. THE UPPER
LEVEL LOW OFF THE PACIFIC NW COAST GRADUALLY MOVES INLAND FRIDAY
AND SATURDAY WITH AN EMBEDDED SHORTWAVE ROTATING THROUGH THE
NORTHERN ROCKIES AND MT FRIDAY NIGHT. SW FLOW ALOFT WILL MOISTEN
FRIDAY WITH PRECIPITATION CHANCES INCREASING AND SHOWERS BECOMING
NUMEROUS FRIDAY AFTERNOON AND EVENING ALONG A SWATH FROM SW TO NE
ACROSS THE FORECAST AREA. A FEW ISOLATED THUNDERSTORMS ARE ALSO
POSSIBLE FRIDAY AFTERNOON/EVENING...MAINLY SOUTH OF A LINE FROM
HELENA TO LEWISTOWN WHERE THE AIRMASS WILL BECOME SOMEWHAT
UNSTABLE. SOME DRYING OCCURS BEHIND THE SHORTWAVE ON SATURDAY BUT
ADDITIONAL ENERGY WILL MOVE INTO SW MT BY SATURDAY AFTERNOON
BRINGING AN INCREASE IN SHOWER ACTIVITY AGAIN. HOENISCH

SATURDAY NIGHT THROUGH THURSDAY...THE PERIOD BEGINS WITH AN UPPER
TROF APPROACHING WESTERN MONTANA. A STRONG DIFFLUENT SOUTHWESTERLY
FLOW ALOFT AHEAD OF THE TROF WILL BRING MOISTURE AND INSTABILITY
ACROSS THE AREA. SCATTERED SHOWERS AND ISOLATED THUNDERSTORMS WILL
LIKELY DEVELOP SATURDAY NIGHT. THE TROF BEGINS TO SPLIT AS IT
CROSSES THE CONTINENTAL DIVIDE EARLY SUNDAY BUT ENOUGH ENERGY
REMAINS WITHIN THE NORTHERN BRANCH TO CONTINUE SHOWER ACTIVITY
ACROSS SOUTHWEST AND CENTRAL MONTANA. THE AIRMASS DRIES OUT SUNDAY
NIGHT AND MONDAY AS A POSITIVE TILT UPPER RIDGE BUILDS ACROSS THE
AREA. DRY AND STABLE CONDITIONS CONTINUE TUESDAY THOUGH THE MODELS
BEGIN TO DIVERGE WITH THEIR SOLUTIONS. THE GFS AND CANADIAN MODELS
KEEP THE RIDGE AXIS ALONG THE CONTINENTAL DIVIDE WHILE THE ECMWF
PUSHES THE AXIS FURTHER EAST IN RESPONSE TO AN UPPER TROF MOVING
INTO THE PACIFIC NORTHWEST. WEDNESDAY REMAINS DRY AS THE GFS
MAINTAINS THE RIDGE OVER THE AREA AND THE CANADIAN AND ECMWF...WHILE
BRINGING SHORTWAVE ENERGY ACROSS SOUTHERN CANADA...STILL HAVE HIGH
500 MB HEIGHTS OVER THE AREA. MODELS IN POOR AGREEMENT THURSDAY
BUT WILL LEAN TOWARD THE GFS SOLUTION WHICH DEVELOPS SOUTHWEST
FLOW ALOFT ON THE BACK SIDE OF THE UPPER RIDGE AND DEVELOPS
SHOWERS ACROSS PORTIONS OF SOUTHWEST MONTANA. TEMPERATURES WILL BE
NEAR SEASONAL AVERAGES TO START THE PERIOD BUT WILL CLIMB ABOVE
SEASONAL AVERAGES MONDAY THROUGH THURSDAY.

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  42  57  38  55 /  10  50  60  10
CTB  38  56  34  53 /  10  10  10  10
HLN  39  56  34  57 /  10  60  60  20
BZN  37  60  35  54 /  20  50  70  30
WEY  30  53  29  48 /  20  40  50  40
DLN  38  55  34  52 /  10  50  60  20
HVR  40  63  41  59 /  10  20  50  30
LWT  39  59  37  53 /  20  60  70  20

&&

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

$$

WEATHER.GOV/GREATFALLS


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