Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 061745
AFDTFX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREAT FALLS MT
1145 AM MDT MON JUL 6 2015

AVIATION SECTION UPDATED

.UPDATE... LATEST SATELLITE IMAGERY SHOWS THE FORECAST AREA DIVIDED
ALMOST IN HALF WITH MOSTLY CLEAR SKIES FROM GREAT FALLS NORTH TO THE
US/CAN BORDER AND MOSTLY OVERCAST SKIES ALONG/SOUTH OF LINE FROM
HELENA TO LEWISTOWN.  SOME OF THE OVERCAST IS STARTING TO SLOWLY
DISSIPATE AND EXPECT THAT TREND TO CONTINUE THROUGH EARLY AFTN.
RESIDUAL MOISTURE FROM THE OVERCAST AND SOME SPOTTY MOISTURE
DRIFTING NORTH OUT OF IDAHO WILL GENERATE SOME ISOLATED SHOWERS AND
THUNDERSTORMS OVER THE MTNS IF FAR SOUTHWEST MT THIS AFTN/EVE, BUT
REMAINDER OF OUR REGION WILL BE RAIN-FREE TODAY.  CURRENT FORECAST
IN GOOD SHAPE...JUST A FEW CHANGES TO INCREASE CLOUD COVERAGE IN THE
OVERCAST TO MATCH SATELLITE IMAGERY.
WARANAUSKAS

&&

.DISCUSSION...

QUICK UPDATE TO THIS MORNING`S FORECAST...FOG HAS MOVED FARTHER
SOUTH THAN ORIGINALLY EXPECTED (INTO WEST YELLOWSTONE), SO HAVE PUT
MENTION OF AREAS OF FOG INTO THE FORECAST FOR ALL VALLEYS OF NORTH
CENTRAL AND SOUTHWEST MONTANA. STILL EXPECTING FOG TO DISSIPATE LATE
THIS MORNING.  COULSTON

TODAY THROUGH WEDNESDAY...THE SHORTWAVE TROUGH IN THE NORTHWEST
FLOW ALOFT THAT BROUGHT LOW CLOUDS AND LIGHT RAIN/SHOWERS TO THE
FORECAST AREA YESTERDAY AND OVERNIGHT IS EXITING THE AREA. THIS IS
ALLOWING SKIES TO AT LEAST PARTIALLY CLEAR FROM THE NORTHWEST. THE
AFOREMENTIONED RAIN IS LEAVING A VERY MOIST AIRMASS AT THE
SURFACE, SO AM KEEPING MENTION OF AREAS OF FOG IN THE FORECAST
THROUGH MID MORNING WHERE SKIES SHOULD CLEAR OUT. THIS AFTERNOON,
SKIES SHOULD BE MOSTLY CLEAR, BUT A WEAK DISTURBANCE WILL MOVE
OVER SOUTHWEST MONTANA, ALLOWING A FEW WEAK SHOWERS/THUNDERSTORMS
TO DEVELOP THERE THROUGH THE EARLY EVENING HOURS. NORTHERLY WINDS
AT THE SURFACE AND ALOFT MAY BRING SOME SMOKE TO THE EASTERN
PORTION OF THE FORECAST AREA, BUT AM NOT EXPECTING IT TO REDUCE
VISIBILITY SIGNIFICANTLY, SO HAVE LEFT IT OUT OF THE FORECAST FOR
NOW. ANOTHER SHORTWAVE TROUGH WILL MOVE INTO MONTANA LATE TONIGHT
INTO TUESDAY. THE RESULTING CYCLONIC NORTHWESTERLY FLOW ALOFT
WILL BRING A GOOD CHANCE OF SHOWERS AND THUNDERSTORMS TO THE AREA
BY TUESDAY. ANOTHER SURGE OF CANADIAN HIGH PRESSURE AT THE SURFACE
WILL BRING NORTHEASTERLY SURFACE WINDS, SO THE BEST CHANCE WILL BE
ALONG THE NORTH AND EAST FACING SLOPES. THE SHORTWAVE WILL BE SLOW
TO EXIT THE AREA TUESDAY NIGHT INTO WEDNESDAY, LEAVING A CHANCE
OF SHOWERS AND THUNDERSTORMS MAINLY OVER THE AREA SOUTH OF A GREAT
FALLS TO LEWISTOWN LINE. WITH THE PASSAGE OF THIS SHORTWAVE,
INSTABILITY AND WIND SHEAR ARE FAIRLY WEAK, SO AM NOT EXPECTING
ANY OF THE THUNDERSTORMS DURING THE PERIOD TO BE SEVERE. THE COOL
CANADIAN HIGH PRESSURE THAT WILL LINGER OVER THE AREA WILL KEEP
TEMPERATURES A FEW DEGREES BELOW NORMAL.  COULSTON

WEDNESDAY NIGHT THROUGH MONDAY...MODELS ARE IN GOOD AGREEMENT ON
GENERAL TRENDS. EXPECT SCATTERED SHOWERS AND THUNDERSTORMS OVER
SOUTHWEST MONTANA WEDNESDAY EVENING IN PART DUE TO TERRAIN EFFECTS
AND AN UPPER TROUGH. MOISTURE AHEAD OF AN UPPER TROUGH OVER
CALIFORNIA WILL WORK ITS WAY INTO SOUTHWEST MONTANA THURSDAY FOR
MORE SCATTERED SHOWERS AND THUNDERSTORMS. LATER THURSDAY NIGHT THE
LATEST GFS MODEL RUN BRINGS SOME PRECIPITATION INTO CENTRAL
MONTANA BUT THE ECMWF MODEL RUN HAS MORE OF AN UPPER RIDGE OVER
SOUTHWEST MONTANA THAN THE GFS MODEL RUN AND ENDS UP BEING DRY.
FOR FRIDAY BOTH MODEL RUNS BRING A WEATHER DISTURBANCE ALOFT INTO
SOUTHWEST MONTANA DURING THE EVENING AND TO CENTRAL MONTANA LATE
THAT NIGHT. THE LATEST GFS MODEL RUN IS STRONGER WITH THE
DISTURBANCE AND HENCE WETTER THAN THE CORRESPONDING ECMWF RUN.
HAVE STUCK PRETTY CLOSE TO THE IDEA OF SCATTERED SHOWERS AND
THUNDERSTORMS OVER SOUTHWEST MONTANA WITH MORE ISOLATED ACTIVITY
OVER THE PLAINS. SHOULD THE ECMWF COME AROUND TO THE WETTER GFS
THE CHANCES OF PRECIPITATION FOR FRIDAY AND FRIDAY NIGHT WILL NEED
TO BE RAISED. ON SATURDAY THE DISTURBANCE MOVES INTO EASTERN
MONTANA BUT LINGERING INSTABILITY LEADS TO ISOLATED TO SCATTERED
SHOWERS AND THUNDERSTORMS. BOTH MODEL RUNS ARE ON THE SAME PAGE IN
HAVING AN UPPER RIDGE OVER MONTANA SUNDAY AND MONDAY WITH
GENERALLY LESS SHOWER AND THUNDERSTORM ACTIVITY. HIGH TEMPERATURES
WILL BE ABOVE NORMAL BUT DO NOT EXPECT ANY RECORD-BREAKING HEAT
SUCH AS OCCURRED LAST WEEK. BLANK

&&

.AVIATION...
UPDATED 1745Z. WIDESPREAD BKN-OVC CLOUD DECKS BETWEEN
2000-4000 FT STILL COVERING MOST OF THE AREA ALONG AND SOUTH OF A
KHLN-TO-KLWT LINE TO INCLUDE KBZN.  HOWEVER SATELLITE IMAGERY
SHOWING THE DECKS BEGINNING TO BREAK UP DUE TO AFTN HEATING. EXPECT
VFR CONDITIONS WITH A BRIEF PERIOD OF MVFR CEILINGS POSSIBLE VCNTY
KLWT THROUGH 20Z. DECREASING CLOUD COVER IS FORECAST ACROSS THE
REGION AFTER SUNSET.  SURFACE WINDS GENERALLY AT OR BELOW 15 KTS
TODAY.
WARANAUSKAS

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  77  51  79  54 /   0   0  20  50
CTB  75  49  74  48 /   0  20  50  30
HLN  82  57  83  58 /  10   0  20  30
BZN  79  50  80  50 /  10  10  30  20
WEY  73  42  72  41 /  20  20  30  30
DLN  79  52  80  51 /  20  10  20  20
HVR  76  49  81  50 /   0   0  20  30
LWT  73  50  78  53 /  10   0  20  30

&&

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

$$

WEATHER.GOV/GREATFALLS



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