Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 221126
AFDTFX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREAT FALLS MT
520 AM MDT FRI MAY 22 2015

UPDATED AVIATION DISCUSSION

.DISCUSSION...

TODAY THROUGH SUNDAY...A BROAD CYCLONIC CIRCULATION PRESENTLY MOVING
ASHORE OVER SOUTHERN CALIFORNIA IS FORECAST TO DRIFT SLOWLY
NORTHEAST ACROSS UTAH ON SATURDAY...AND EASTERN WYOMING ON SUNDAY.
THE SYSTEM IS NEARLY EQUIVALENT BAROTROPIC...PROVIDING WEAK SHEAR
BUT JUST ENOUGH INSTABILITY TO SUPPORT WIDESPREAD GENERAL
THUNDERSTORMS. SPECIFIC TO OUR CWA...AFTERNOON SHOWERS AND
THUNDERSTORMS WILL CONTINUE TO REMAIN ACTIVE OVER THE SOUTHWEST
GIVEN DEEPER MOISTURE AND ELEVATED HEATING SOURCES OVER HIGHER
TERRAIN. SMALL HAIL AND GUSTY WINDS WILL AGAIN BE POSSIBLE IN A
FEW OF THESE SLOW MOVING THUNDERSTORMS...ESPECIALLY SATURDAY
AFTERNOON AS MID- LEVEL LAPSE RATES EXCEED 7 DEG/KM. DEEP-LAYER
EASTERLY FLOW CONTINUES OVER NORTH CENTRAL MONTANA. THIS IS
ASSOCIATED WITH DRIER AIR AND A MID-LEVEL INVERSION THAT WILL
SUPPRESS DEVELOPMENT OF SHOWERS IN THIS AREA. HOWEVER...ON
SATURDAY AFTERNOON A SUBTLE INVERTED SHORTWAVE TROUGH ROTATES
AROUND THE PARENT LOW TO HELP WEAKEN THE INVERSION AND ALLOW
SHOWERS AND THUNDERSTORMS TO SPREAD INTO SOUTHERN PORTIONS OF
NORTH CENTRAL MONTANA INCLUDING THE GREAT FALLS AREA. SHOWERS WILL
BE LESS WIDESPREAD ON SUNDAY AS THE PRIMARY TROUGH MOVES FURTHER
WAY INTO THE NORTHERN PLAINS. NUTTER

SUNDAY NIGHT THROUGH FRIDAY...MODELS CONTINUE TO BE IN AGREEMENT IN
A GENERAL SENSE WITH PRECIPITATION TRENDS AT LEAST INTO MIDWEEK.
THAT IS SHOWER ACTIVITY SHOULD BE MAINLY ISOLATED SUNDAY NIGHT AND
MEMORIAL DAY THEN PICK UP SOME INTO MIDWEEK. WITHIN A PARTICULAR
MODEL THERE IS GENERAL CONSISTENCY FROM RUN-TO-RUN BUT THE GFS AND
ECMWF CONTINUE TO EXHIBIT DIFFERENCES BETWEEN EACH OTHER WITH THEIR
FORECASTS ALOFT. LIKE LAST NIGHT THE ECMWF TAKES AN UPPER LOW NEAR
VANCOUVER ISLAND SUNDAY NIGHT SOUTHEAST AND WEAKENS IT AS IT MOVES
INTO CENTRAL MONTANA BY MID-WEEK. THE GFS TAKES THE UPPER LOW
SOUTHEAST EVER SO SLOWLY AND THEN FOR A PERIOD HAS IT QUASI-
STATIONARY BEFORE WEAKENING IT AND MOVING IT INTO MONTANA ON FRIDAY.
DUE TO MODEL DIFFERENCES WITH DETAILS OF THEIR PRECIPITATION
FORECASTS WILL GO WITH THE BROADBRUSH IDEA FROM TUESDAY ON OF
SCATTERED SHOWERS EXCEPT ISOLATED SHOWERS TOWARDS THE CANADIAN
BORDER. OVER THE FORECAST AREA THERE WILL BE ISOLATED THUNDERSTORMS
DURING THE LATE AFTERNOON AND EARLY EVENING BUT DO NOT ANTICIPATE
ANY SEVERE STORMS AT THIS TIME. BLANK

&&

.AVIATION... UPDATED 1125Z.

EXPECT VFR CONDITIONS. THE EXCEPTION TO THIS IS THAT THERE COULD BE
PATCHY FOG IN THE VALLEYS OF SOUTHWEST MONTANA EARLY THIS MORNING.
ALTHOUGH MVFR/IFR VISIBILITIES IN FOG CAN NOT BE TOTALLY RULED OUT
AT KBZN AND KHLN..THINK THE THREAT OF FOG AT THOSE TWO SITES IS LOW
ENOUGH THAT IT WAS NOT INCLUDED IN THE RESPECTIVE TAF FORECASTS.
ISOLATED SHOWERS WILL DEVELOP OVER THE MOUNTAINS OF SOUTHWEST
MONTANA THIS MORNING AND POSSIBLY AS FAR NORTH AS THE LITTLE BELT
MOUNTAINS. SHOWERS WILL INCREASE IN COVERAGE THIS AFTERNOON AND
SCATTERED THUNDERSTORMS WILL DEVELOP. SHOWERS/THUNDERSTORMS WILL
MOVE SLOWLY AND MAINLY WESTWARD. THEY WILL ALSO BE MAINLY NEAR THE
MOUNTAINS. SHOWER AND THUNDERSTORMS ARE UNLIKELY NORTHWEST OF A LINE
FROM KGTF TO KHVR. SHOWERS AND THUNDERSTORMS WILL DIMINISH THIS
EVENING BUT COULD LINGER OVER CENTRAL MONTANA AFTER MIDNIGHT. BLANK

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  69  46  66  43 /  10  20  40  20
CTB  69  41  68  38 /   0  10  20  10
HLN  71  47  67  46 /  20  20  70  40
BZN  68  42  63  41 /  30  20  60  40
WEY  60  35  58  35 /  50  50  60  30
DLN  66  44  64  43 /  30  30  40  30
HVR  73  42  73  41 /  10  20  20  10
LWT  67  45  63  42 /  30  40  50  20

&&

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

$$

WEATHER.GOV/GREATFALLS


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