Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 011145
AFDTFX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREAT FALLS MT
544 AM MDT MON JUN 1 2015

...STRONG TO SEVERE THUNDERSTORMS POSSIBLE THIS AFTERNOON AND
EVENING...

.DISCUSSION...

TODAY THROUGH WEDNESDAY...THE UPPER LEVEL TROUGH OF LOW PRESSURE
OFF THE PACIFIC NORTHWEST COAST WILL CONTINUE TO SLOWLY MOVE INTO
THE PACIFIC NORTHWEST THROUGH THE PERIOD. THE RESULTING MOIST AND
UNSTABLE SOUTHWESTERLY FLOW ALOFT WILL MOVE A SUCCESSION OF
DISTURBANCES THROUGH THE AREA. TODAY, MANY FACTORS WILL COME
TOGETHER TO BRING A CHANCE FOR STRONG TO SEVERE THUNDERSTORMS.
DEEP PACIFIC MOISTURE WILL COMBINE WITH WARMER THAN NORMAL
TEMPERATURES TO PROVIDE STRONG INSTABILITY FOR THUNDERSTORM
POTENTIAL. A JET MAX ASSOCIATED WITH A DISTURBANCE WILL PROVIDE
LIFT AND WIND SPEED SHEAR, WHICH ENHANCES THE CHANCE FOR STRONG
WIND GUSTS. A PASSING SURFACE TROUGH WILL PROVIDE A FOCUSING
MECHANISM FOR STORM DEVELOPMENT AND DIRECTIONAL WIND SHEAR, WHICH
WOULD HELP CAUSE STORM ROTATION. ALSO, LOW LEVEL MOISTURE WILL BE
DRAWN INTO THE AREA FROM THE GULF OF MEXICO, WHICH WOULD COMBINE
WITH COOLER AIR ALOFT TO INCREASE THE CHANCE FOR HAIL AND HEAVY
RAIN. AS A RESULT OF ALL THIS, THE STORM PREDICTION CENTER HAS PUT
ALMOST ALL OF THE FORECAST AREA UNDER A THREAT FOR SEVERE STORMS,
AND HAVE ADDRESSED THE THREATS FROM THESE STORMS IN THE FORECAST.
THE AREA SOUTH OF A CUT BANK TO HAVRE LINE HAS A CHANCE FOR SEVERE
STORMS, BUT THE GREATEST CHANCE WILL BE EAST OF A DILLON TO GREAT
FALLS TO HAVRE LINE, INCLUDING THE LEWISTOWN AREA. IN ADDITION,
HEAVY RAINS FROM THESE STORMS MAY CAUSE LOCALIZED FLOODING OF
URBAN AREAS AND SMALL CREEKS AND STREAMS. WILL MONITOR THIS
SITUATION FOR POTENTIAL FLOODING STATEMENTS. AS THE DISTURBANCE
AND SURFACE TROUGH MOVE EAST OF THE AREA OVERNIGHT INTO TUESDAY,
THE THREAT FOR SEVERE STORMS WILL DECREASE, BUT THE CONTINUED
MOIST AND DIFFLUENT SOUTHWESTERLY FLOW ALOFT WILL KEEP A GOOD
CHANCE OF SHOWERS OVER THE AREA, ESPECIALLY ALONG THE HI-LINE,
WHERE THE TRAILING EDGE OF THE SURFACE TROUGH WILL LINGER. THE
AIRMASS WILL DESTABILIZE AGAIN, KEEPING A CHANCE FOR THUNDERSTORMS
AS WELL. AFTER A BRIEF RESPITE FROM STORMS TUESDAY NIGHT, ANOTHER
DISTURBANCE WILL BRING ANOTHER CHANCE FOR SHOWERS AND
THUNDERSTORMS ON WEDNESDAY. HIGH TEMPERATURES WILL WARM INTO THE
70S AGAIN TODAY, BUT THEY WILL COOL MOSTLY INTO THE 60S FOR
TUESDAY AND WEDNESDAY.  COULSTON

WEDNESDAY NIGHT THROUGH SUNDAY...A BROAD UPPER LEVEL TROF OF LOW
PRESSURE WILL BE OVER THE WESTERN US WEDNESDAY NIGHT. THE
SOUTHERN PORTION OF THIS TROF WILL DEEPEN OVER CALIFORNIA
THURSDAY. THE AIRMASS WILL REMAIN RELATIVELY MOIST ALOFT AND EAST
TO SOUTHEAST LOW LEVEL FLOW WILL HELP KEEP THE LOWER LEVELS MOIST
AS WELL THROUGH THE NIGHT. A SHORTWAVE IN THE SOUTHWEST FLOW
ALOFT WILL APPROACH THE ROCKIES BY EARLY THURSDAY MORNING AND
SHOULD PROLONG EVENING SHOWERS INTO THE EARLY MORNING HOURS. THE
AIRMASS WILL DRY A BIT THURSDAY NIGHT AND FRIDAY BUT MOISTURE
AHEAD OF THE LOW PRESSURE OVER CALIFORNIA WILL BE AFFECTING
SOUTHWEST MONTANA. THE AIRMASS WILL BECOME UNSTABLE ESPECIALLY
ACROSS CENTRAL AND SOUTHERN PORTIONS OF THE CWA. THE LOW PRESSURE
CENTER OVER CALIFORNIA WILL GRADUALLY MOVE NORTHEAST AND APPROACH
SOUTHERN IDAHO BY SUNDAY NIGHT. MOISTURE WILL CONTINUE TO MOVE
INTO THE FORECAST AREA AS THIS SYSTEM APPROACHES AND AFTERNOON
SHOWERS AND THUNDERSTORMS WILL CONTINUE THROUGH SUNDAY.
TEMPERATURES WILL BE NEAR NORMALS AS THE PERIOD BEGINS WEDNESDAY
NIGHT AND THURSDAY. THE AIRMASS TRENDS WARMER THROUGH THE PERIOD
AND TEMPERATURES WILL 10 DEGREES ABOVE NORMALS BY SUNDAY. ZELZER

&&

.AVIATION...
UPDATED 1141Z.

AN UPPER LEVEL LOW PRESSURE SHORTWAVE WILL MOVE THROUGH CENTRAL AND
SOUTHWEST MONTANA TODAY. THIS WILL BRING WIDESPREAD SHOWERS AND
THUNDERSTORMS TO THE AREA. THESE THUNDERSTORMS MAY PRODUCE HEAVY
RAIN AND PEA-SIZED HAIL. LOCAL MVFR CONDITIONS ARE POSSIBLE NEAR
THESE SHOWERS AND THUNDERSTORMS. ALSO, LARGER HAIL SOUTH OF A KCTB-
KHVR LINE IS ALSO POSSIBLE THROUGH THE AFTERNOON AND EARLY EVENING.
LOCAL IFR/MVFR CONDITIONS ARE POSSIBLE AFTER MIDNIGHT DUE TO FOG AND
LOW CIGS. CONDITIONS WILL IMPROVE SLOWLY TUESDAY MORNING.

&&

.HYDROLOGY...
UPDATED 01/0940Z.

A FEW RIVERS ACROSS NORTH CENTRAL AND SOUTHWEST MONTANA ARE
CURRENTLY RUNNING WITHIN A FOOT OF ACTION STAGE, INCLUDING THE BIG
HOLE RIVER NEAR MELROSE, THE GALLATIN RIVER NEAR LOGAN AND THE
GALLATIN GATEWAY, TENMILE CREEK NEAR HELENA, THE DEARBORN RIVER
NEAR CRAIG, THE SUN RIVER NEAR SIMMS AND VAUGHN (LIKELY DUE MORE
TO INCREASED RELEASES FROM GIBSON DAM), BADGER CREEK NEAR
BROWNING, AND THE ST. MARY RIVER NEAR THE CANADIAN BORDER NEAR
BABB. AM MILDLY CONCERNED THAT THE EXPECTED PRECIPITATION OVER THE
NEXT FEW DAYS COULD CAUSE THESE LOCATIONS, AND OTHERS, TO APPROACH
FLOOD STAGE. CURRENT FORECASTS FROM THE MISSOURI BASIN RIVER
FORECAST CENTER DO NOT INDICATE THIS, BUT WILL CONTINUE TO
MONITOR THE FORECASTS FOR POTENTIAL UPDATES TO THIS SITUATION.

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  74  52  63  44 /  90  70  70  40
CTB  73  48  57  40 /  70  90  90  40
HLN  76  52  68  48 /  90  70  60  40
BZN  76  48  67  44 /  80  70  60  50
WEY  71  41  61  37 /  50  50  50  40
DLN  72  47  66  44 /  90  50  50  40
HVR  81  52  64  44 /  30  90  80  40
LWT  77  51  65  44 /  80  90  80  40

&&

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

$$

WEATHER.GOV/GREATFALLS


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