Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 301145
AFDTFX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREAT FALLS MT
545 AM MDT SAT MAY 30 2015

.DISCUSSION...

TODAY THROUGH MONDAY...MODELS CONTINUE TO INDICATE THAT THIS WILL
BE AN UNSETTLED PERIOD WITH AT LEAST A CHANCE OF SHOWERS AND
THUNDERSTORMS, MAINLY DURING THE AFTERNOON HOURS. A FEW SHOWERS
ARE LINGERING THIS MORNING OVER THE HAVRE AREA AS A WEAK
DISTURBANCE EXITS THE AREA. FOR TODAY, SCATTERED TO NUMEROUS
SHOWERS AND THUNDERSTORMS WILL DEVELOP BY THIS AFTERNOON AND
CONTINUE INTO THIS EVENING. A WEAK DISTURBANCE WILL MOVE THROUGH
THE UPPER LEVEL RIDGE OF HIGH PRESSURE OVER THE AREA AND COMBINE
WITH DEEP PACIFIC MOISTURE AND INCREASING INSTABILITY. WIND SPEED
SHEAR WILL BE MARGINAL FOR SEVERE STORMS, BUT A SURFACE BOUNDARY
WILL MOVE NORTHEAST THROUGH SOUTHWEST MONTANA INTO CENTRAL MONTANA
THIS AFTERNOON, CREATING INCREASING DIRECTIONAL SHEAR. THE
STRONGEST STORMS WILL BE FOCUSED ALONG THE LEADING EDGE OF THIS
BOUNDARY (INCLUDING MUCH OF NORTH CENTRAL/CENTRAL MONTANA), SO
HAVE KEPT MENTION OF GUSTY WINDS AND SMALL HAIL WITH STORMS. THE
STORMS WILL WEAKEN AND DECREASE OVERNIGHT AS THE DISTURBANCE EXITS
THE AREA. THE UPPER RIDGE WILL REBUILD INTO THE AREA ON SUNDAY AND
KEEP THE AIRMASS A BIT DRIER AND MORE STABLE. HOWEVER, THE
RESULTING WARMING TEMPERATURES WILL CAUSE A FEW WEAK STORMS TO
DEVELOP DURING THE AFTERNOON AND EVENING HOURS. MARGINAL WIND
SHEAR WILL CONTINUE INTO THE DAY, BUT MOISTURE WILL BE LACKING, SO
ANY STRONGER STORMS THAT DEVELOP WILL MAINLY HAVE A GUSTY WIND
THREAT. THE RIDGE WILL THEN BEGIN TO SHIFT TO THE EAST ON MONDAY,
AS A DEEP UPPER LEVEL LOW PRESSURE TROUGH MOVES INTO THE PACIFIC
NORTHWEST. THE RESULTING SOUTHWEST FLOW ALOFT WILL TAP INTO DEEP
PACIFIC MOISTURE AND MOVE ANOTHER DISTURBANCE OVER THE AREA,
LIKELY BRINGING SHOWERS AND THUNDERSTORMS TO THE AREA ONCE AGAIN.
CONTINUED WARM TEMPERATURES WILL HELP DESTABILIZE THE AIRMASS AND
A SURFACE TROUGH MOVING THROUGH THE AREA WILL ENHANCE WIND SHEAR.
WILL NEED TO MONITOR THIS SITUATION FOR POTENTIALLY STRONG
THUNDERSTORMS. TEMPERATURES TODAY WILL BE SLIGHTLY WARMER THAN
YESTERDAY, BUT THEY WILL WARM TO BETWEEN 5 AND 10 DEGREES ABOVE
NORMAL FOR SUNDAY AND MONDAY WITH MILD OVERNIGHT LOWS.  COULSTON

MONDAY NIGHT THROUGH FRIDAY...A SHALLOW LOW PRESSURE TROF WILL
MOVE OVER THE PACIFIC NORTHWEST AND MOVE TOWARDS THE ROCKIES
EARLY TUESDAY. THE ASSOCIATED COLD FRONT WILL CROSS THE AREA
TUESDAY AND COOL MOST TEMPS ACROSS THE CWA. THE WESTERN US WILL
REMAIN UNDERNEATH A BROAD AREA OF CYCLONIC FLOW, A VERY SHALLOW
TROF, AND THE AIRMASS WILL REMAIN COOL AND SOMEWHAT MOIST.
ANOTHER LOW PRESSURE SYSTEM WILL MOVE OVER THE WEST COAST AND
INTO THE CYCLONIC FLOW ALOFT THURSDAY. THIS TROF WILL DEEPEN OVER
THE WESTERN US AND INFLUENCE THE WEATHER OVER CENTRAL MONTANA
THROUGH FRIDAY NIGHT. THE ECMWF BEGINS TO DIFFER A BIT FROM THE
GFS ON THURSDAY BY DEEPENING ITS TROF OVER THE PACIFIC NORTHWEST
AND CALIFORNIA. BY FRIDAY, THE ECMWF PLACES A CLOSED LOW OVER
CALIFORNIA. OVERALL, THE TROF IS DEEPER THAN THE GFS BUT HEIGHTS
OVER THE CWA ARE NOT THAT MUCH DIFFERENT BETWEEN THE TWO MODELS.
BY EARLY SATURDAY, THOUGH, THE GFS IS ALREADY BEGINNING TO CREATE
A RIDGE OVER THE ZONES WHILE THE ECMWF CONTINUES THE TROF. THE
GFS SOLUTIONS WILL BRING A COLD FRONT ACROSS CENTRAL MONTANA
FRIDAY, AND WITH QUITE UNSTABLE CONDITIONS, STRONG THUNDERSTORMS
WOULD BE EXPECTED. THE ECMWF, HOWEVER, WOULD KEEP CWA UNDER A
MORE STABLE AIRMASS. TIMING IS A PROBLEM WITH THESE DIFFERING
SOLUTIONS AND WILL TREND TOWARDS A BLEND...WILL MENTION
THUNDERSTORMS ON FRIDAY BUT WITH THE TIMING ISSUES, WILL WAIT
BEFORE STRESSING ANY STRONG DEVELOPMENT. TEMPERATURES WILL REMAIN
NEAR OR JUST BELOW NORMALS THROUGH THURSDAY. THE GFS SOLUTION
WOULD BRING COOLER TEMPS ON FRIDAY BUT WITH THE DIFFERENCES
BETWEEN THE GFS AND ECMWF, WILL HOLD BACK ON ANY STRONG COOLING.
ZELZER

&&

.AVIATION...

UPDATED 1143Z.

A DISTURBANCE MOVING THROUGH MOIST WEST FLOW ALOFT WILL BRING A
PERIOD OF SHOWERS AND THUNDERSTORMS TO CENTRAL AND SOUTHWEST
MONTANA. THE STRONGEST PERIOD OF SHOWERS AND THUNDERSTORMS WILL BE
FROM MID-DAY THROUGH EARLY EVENING. SOME SHOWERS AND THUNDERSTORMS
ARE EXPECTED TO HAVE HEAVY RAIN AS WELL AS SMALL HAIL. LOCAL MVFR
CONDITIONS ARE POSSIBLE NEAR SHOWERS AND THUNDERSTORMS. LOCAL MVFR
CONDITIONS ARE ALSO POSSIBLE TOWARDS SUNRISE SUNDAY MORNING AT KHVR
AND KLWT DUE TO LOW CLOUDS AND FOG.

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  71  50  78  52 /  60  30  30  10
CTB  66  45  76  47 /  70  30  10  10
HLN  75  53  79  56 /  50  40  40  20
BZN  72  51  76  52 /  60  40  40  20
WEY  65  41  71  44 /  40  40  40  30
DLN  71  51  77  53 /  60  40  30  20
HVR  73  47  81  51 /  50  60  30  20
LWT  70  48  76  53 /  60  60  40  30

&&

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

$$

WEATHER.GOV/GREATFALLS



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