Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 311542
AFDTFX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREAT FALLS MT
942 AM MDT SUN MAY 31 2015

.DISCUSSION...
STILL EXPECT SCATTERED SHOWERS AND THUNDERSTORMS TO DEVELOP OVER
SOUTHWEST AND CENTRAL MONTANA THIS AFTERNOON IN LINE WITH THE
CURRENT FORECAST. ALSO IN LINE WITH THE CURRENT FORECAST THERE
SHOULD BE ENOUGH INSTABILITY FOR SOME OF THE STORMS TO HAVE SMALL
HAIL AND GUSTY WINDS. AS FOR CHANGES TO THE FORECAST FOR THIS
MORNING LOWERED THE CHANCES OF SHOWERS SLIGHTLY OVER SOUTHWEST
MONTANA. IN LINE WITH SATELLITE IMAGERY LOWERED CLOUD COVER OVER
MUCH OF THE PLAINS. FOR THIS AFTERNOON RAISED HIGH TEMPERATURES
SLIGHTLY FROM GALLATIN COUNTY TO JUDITH BASIN AND FERGUS COUNTIES.
BLANK

&&

.AVIATION...

UPDATED 1140Z.

THE AIRMASS WILL BECOME UNSTABLE THIS AFTERNOON AND CAUSE THE CHANCE
FOR SHOWERS AND THUNDERSTORMS TO ALSO INCREASE. LOCATIONS SOUTH OF A
KHLN-KGTF-KLWT LINE WILL HAVE MORE MOISTURE AVAILABLE FOR
PRECIPITATION AND WILL GENERALLY HAVE SCATTERED TO NUMEROUS SHOWERS
AND THUNDERSTORMS THROUGH THE AFTERNOON AND EARLY EVENING. LOCATIONS
TO THE NORTH WILL BE DRIER AND SCATTERED TO ISOLATED THUNDERSTORMS
ARE EXPECTED. LOCAL IFR CONDITIONS AT KCTB WILL IMPROVE TO VFR BY
MID-MORNING. LOCAL MVFR CONDITIONS ARE POSSIBLE NEAR SHOWERS AND
THUNDERSTORMS THIS AFTERNOON AND EVENING. ELSEWHERE VFR CONDITIONS
WILL PREVAIL.

&&

.PREV DISCUSSION...
/ISSUED 542 AM MDT SUN MAY 31 2015/

TODAY THROUGH TUESDAY...UNSETTLED WEATHER WILL CONTINUE ACROSS
NORTH CENTRAL, CENTRAL, AND SOUTHWEST MONTANA THROUGH THIS
PERIOD. AN UPPER LEVEL RIDGE OF HIGH PRESSURE WILL BUILD INTO THE
AREA TODAY INTO TONIGHT, WARMING TEMPERATURES UP TO 10 DEGREES
ABOVE NORMAL. A WEAK DISTURBANCE WILL COMBINE WITH MODERATE
INSTABILITY DURING THE AFTERNOON AND EARLY EVENING HOURS TO CAUSE
ISOLATED TO SCATTERED SHOWER AND THUNDERSTORM DEVELOPMENT OVER THE
MOUNTAINS, WHICH WILL GRADUALLY MOVE OUT ONTO THE PLAINS. WIND
SPEED SHEAR WILL BE MARGINALLY STRONG, SO CANNOT RULE OUT THE
POSSIBILITY OF GUSTY WINDS AND SMALL HAIL, MAINLY OVER SOUTHWEST
AND CENTRAL MONTANA. STORMS WILL THEN DIMINISH LATE THIS EVENING
AND OVERNIGHT WITH THE LOSS OF DAYTIME HEATING. AN UPPER LEVEL
TROUGH OF LOW PRESSURE WILL THEN MOVE INTO THE PACIFIC NORTHWEST
FOR MONDAY THROUGH TUESDAY, PUTTING THE AREA UNDER A MUCH MORE
MOIST AND UNSTABLE SOUTHWESTERLY FLOW ALOFT. A DISTURBANCE WILL
EJECT FROM THE TROUGH AND MOVE OVER THE CONTINUED WARM AND
UNSTABLE AIRMASS MONDAY INTO MONDAY NIGHT, BRINGING WIDESPREAD
SHOWERS AND THUNDERSTORMS. AN INVERTED SURFACE TROUGH WILL ALSO
MOVE FROM THE ROCKIES EAST INTO WYOMING AND SOUTHEAST MONTANA,
WHICH WILL SHIFT WINDS MORE EASTERLY AND DRAW MOISTURE FROM THE
GULF OF MEXICO INTO THE STATE. THE RESULTING MODERATE TO STRONG
WIND SHEAR (BOTH SPEED AND DIRECTIONAL) WILL COMBINE WITH THE DEEP
MOISTURE TO BRING AT LEAST A SLIGHT CHANCE OF SEVERE THUNDERSTORMS
TO PORTIONS OF CENTRAL MONTANA (EAST OF A BOZEMAN TO GREAT FALLS
TO HAVRE LINE). HEAVY RAIN, DAMAGING WINDS, AND LARGE HAIL WILL
ALL BE POSSIBLE IN THIS AREA, WITH A LESSER CHANCE OF THESE WEST
OF THE AREA. IN THE WAKE OF THE SURFACE TROUGH, COOLER
TEMPERATURES WILL MOVE INTO THE AREA FOR TUESDAY, BRINGING THEM
CLOSER TO NORMAL, ALONG WITH A LESSER CHANCE FOR STRONG STORMS.
HOWEVER, DEEP PACIFIC MOISTURE PAIRED WITH ANOTHER DISTURBANCE IN
THE SOUTHWEST FLOW WILL CONTINUE WIDESPREAD SHOWERS WITH A CHANCE
OF THUNDERSTORMS INTO TUESDAY. THIS EXTENDED PERIOD OF POTENTIALLY
HEAVY SHOWERS AT TIMES MAY CAUSE RIVERS, CREEKS, AND STREAMS TO
RISE, BUT SIGNIFICANT FLOODING IS NOT EXPECTED AT THIS TIME.
COULSTON

TUESDAY NIGHT THROUGH SATURDAY...A BROAD AREA OF WEAK LOW
PRESSURE WILL BE ACROSS THE WESTERN US TUESDAY NIGHT. THE AIRMASS
WILL BECOME SLIGHTLY UNSTABLE WEDNESDAY AFTERNOON ALTHOUGH MOST
OF THE AVAILABLE MOISTURE WILL BE NORTH OF THE CANADIAN BORDER. A
DEEPER LOW WILL MOVE OVER THE PACIFIC NORTHWEST THURSDAY. THIS
WILL DEEPEN MAINLY OVER CALIFORNIA BUT MOIST SOUTHWEST FLOW AHEAD
OF THIS SYSTEM WILL BRING MOISTURE INTO SOUTHWEST MONTANA. THE
LOW PRESSURE CENTER OVER CALIFORNIA WILL BECOME CUT-OFF AND
MOISTURE ASSOCIATED WITH THIS SYSTEM WILL GENERALLY REMAIN SOUTH
OF THE ZONES. HOWEVER, A LOW PRESSURE SHORTWAVE WILL BE CROSSING
THE PACIFIC NORTHWEST FRIDAY AND APPROACH THE ROCKIES. THIS WILL
LOWER HEIGHTS ALOFT AND BRING WIDESPREAD INSTABILITY TO THE
REGION. THE AIRMASS WILL DRY AND STABILIZE SATURDAY AS THE
SHORTWAVE MOVES EAST OF THE ZONES ALTHOUGH THE SOUTHERN PORTIONS
OF SOUTHWEST MONTANA WILL BECOME UNSTABLE AGAIN. HOWEVER, WITH
HIGH PRESSURE CONTINUING TO BUILD ALOFT, THE AIRMASS WILL ALSO BE
DRYING IN THIS REGION AND THUNDERSTORM ACTIVITY SHOULD NOT BE
WIDESPREAD. THE AIRMASS WILL BE TRENDING WARMER THROUGH THE
PERIOD. TEMPERATURES WILL REMAIN NEAR OR SLIGHTLY BELOW NORMALS
THROUGH THURSDAY THEN BEGIN WARMING ABOVE NORMALS. TEMPERATURES
SATURDAY WILL BE 5 TO 10 DEGREES ABOVE SEASONAL NORMALS. ZELZER

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  79  55  75  53 /  30  20  80  70
CTB  76  48  74  48 /  20  10  70  80
HLN  78  56  76  52 /  40  20  80  60
BZN  78  52  77  49 /  40  40  60  60
WEY  71  44  71  41 /  40  40  40  50
DLN  76  53  73  47 /  40  20  70  50
HVR  80  52  82  53 /  20  30  40  80
LWT  78  55  76  51 /  40  40  80  70

&&

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

$$

WEATHER.GOV/GREATFALLS


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