Area Forecast Discussion
Issued by NWS Great Falls, MT

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000
FXUS65 KTFX 251549
AFDTFX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREAT FALLS MT
945 AM MDT SAT APR 25 2015

.UPDATE... WEATHER SYSTEM THAT BROUGHT RAIN/SNOW YESTERDAY EVENING
THROUGH THIS MORNING IS OVER THE EASTERN THIRD OF MT. ISOLATED
RESIDUAL SHOWERS WILL CONTINUE OVER SOUTHWEST MT AND AREAS ALONG AND
EAST OF WHITE SULPHUR SPRINGS-TO-HAVRE LINE THIS MORNING, WITH ONLY
VERY LIGHT ADDITIONAL PRECIPITATION EXPECTED.  MAIN FORECAST
ADJUSTMENT THIS MORNING WAS TO DECREASE PRECIP CHANCES AND AMOUNTS
THROUGH MIDDAY FOR LOCATIONS EAST AND SOUTH OF GREAT FALLS TO BETTER
MATCH LATEST RADAR AND SATELLITE IMAGERY. REST OF CURRENT FORECAST
IN GOOD SHAPE AT THIS TIME.
WARANAUSKAS

&&

.DISCUSSION...

TODAY THROUGH MONDAY...THE UPPER LEVEL TROUGH OF LOW PRESSURE OVER
THE PACIFIC NORTHWEST WILL CONTINUE THE VERY MOIST SOUTHWESTERLY
FLOW ALOFT OVER NORTH CENTRAL AND SOUTHWEST MONTANA TODAY. THE
SLOW-MOVING DISTURBANCE WITH ITS WIDESPREAD RAIN AND SNOW ALONG
AND EAST OF A DILLON TO HAVRE LINE WILL GRADUALLY LIFT NORTHEAST
OUT OF THE AREA BY THIS AFTERNOON. HOWEVER, ADDITIONAL SNOW
AMOUNTS OF AN INCH OR LESS ARE EXPECTED AT LOWER ELEVATIONS OF
SOUTHWEST MONTANA AND INTO THE LEWISTOWN AREA, AS WELL AS OVER
MOST MOUNTAIN PASSES, WHILE ANOTHER 2 TO 3 INCHES ARE EXPECTED IN
THE MOUNTAINS. AFTER THE DISTURBANCE EXITS THE AREA, BREAKS IN THE
CLOUDS MAY ALLOW JUST ENOUGH HEATING TO CAUSE THE AIRMASS TO
BECOME SLIGHTLY UNSTABLE, WHICH, IN TURN WILL ALLOW SCATTERED
SHOWERS TO REDEVELOP, MAINLY OVER THE MOUNTAINS. A FEW WEAK
THUNDERSTORMS ARE ALSO POSSIBLE IN FAR SOUTHWEST MONTANA.
OVERNIGHT INTO SUNDAY, THE UPPER TROUGH WILL ELONGATE AND BEGIN TO
SPLIT AS IT MOVES EAST INTO MONTANA AND COMES UP AGAINST AN UPPER
LEVEL RIDGE OF HIGH PRESSURE, SENDING THE MAIN ENERGY SOUTH INTO
SOUTHERN IDAHO AND SOUTHWEST MONTANA. THE RESULTING MOIST
SOUTHERLY FLOW ALOFT WILL KEEP THE MAIN THREAT FOR SHOWERS OVER
SOUTHWEST AND CENTRAL MONTANA, SOUTH OF A GREAT FALLS TO
LEWISTOWN LINE. PRECIPITATION WILL DECREASE FROM THE WEST
OVERNIGHT AS THE TROUGH MOVES EAST ACROSS THE STATE. A HIGH
PRESSURE RIDGE WILL THEN SPREAD FROM THE PACIFIC NORTHWEST INTO
MONTANA ON MONDAY. KEEPING THE AREA DRY. DAYTIME TEMPERATURES
WILL REMAIN NEAR NORMAL (HIGHS MOSTLY MID 40S AND 50S) THROUGH
SUNDAY WITH THE PASSAGE OF THE TROUGH, THEN THEY WILL WARM MOSTLY
INTO THE 60S ON MONDAY.  COULSTON

MONDAY NIGHT THROUGH SATURDAY...THE PERIOD STARTS OUT DRY AND
MILD AS A POSITIVE TILT UPPER RIDGE BUILDS OVER THE AREA. BY
TUESDAY NIGHT...THE RIDGE AXIS WILL MOVE TO EASTERN MONTANA AS AN
UPPER TROF APPROACHES NORTHWEST MONTANA. THE ECMWF IS FASTER WITH
THIS FEATURE THAN EITHER THE GFS OR CANADIAN MODELS BUT THE MODELS
AGREE THAT ANY PRECIPITATION WILL BE CONFINED TO THE ROCKY
MOUNTAIN FRONT. WEDNESDAY AND THURSDAY WILL FIND INCREASING
SOUTHWEST FLOW ALOFT AS THE MODELS CONTINUE TO BRING UPPER
DISTURBANCES TO THE WEST COAST. AGAIN, PRECIPITATION BOTH DAYS
WILL REMAIN CONFINED TO THE ROCKY MOUNTAIN FRONT AND THE MOUNTAINS
OF SOUTHWEST MONTANA. THE MODELS DEPICT...WITH SOME VARIATION IN
DETAILS...AN UPPER TROF APPROACHING WESTERN MONTANA SATURDAY. WITH
AVAILABLE MOISTURE AND AMPLE LIFT...SCATTERED SHOWERS WILL RESULT.
TEMPERATURES WILL REMAIN ABOVE SEASONAL AVERAGES THROUGH THE WEEK
WITH HIGHS GENERALLY IN THE UPPER 60S TO LOWER 70S. AFTERNOON
READINGS SATURDAY WILL DROP BACK CLOSER TO SEASONAL AVERAGES WITH
CLOUD COVER AND SCATTERED RAIN EXPECTED.

&&

.AVIATION...
UPDATED 1200Z.
SOUTHWEST FLOW ALOFT WILL CONTINUE TO BRING MOISTURE
AND INSTABILITY TO THE AREA. SCATTERED RAIN AND SNOW SHOWERS THIS
MORNING WILL DECREASE AND BECOME ISOLATED RAIN SHOWERS AFTER 18Z.
SKIES WILL PARTIALLY CLEAR AFTER 00Z BEFORE CONDITIONS BEGIN TO
DETERIORATE AGAIN AFTER 09Z DUE TO LOW CEILINGS AND SNOW SHOWERS.
MVFR CONDITIONS PREVAIL DUE TO LOW CEILINGS AND PRECIPITATION. VFR
CONDITIONS WILL PREVAIL AFTER 17Z WITH INCREASING MVFR CONDITIONS
AGAIN AFTER 09Z. MOUNTAINS OBSCURED AT TIMES.

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  56  34  53  33 /  20  10  40  10
CTB  54  28  53  28 /  20  20  30  10
HLN  58  36  56  32 /  30  40  70  10
BZN  54  34  52  28 /  50  50  60  30
WEY  53  26  52  23 /  40  70  70  10
DLN  53  35  52  30 /  40  70  80  10
HVR  62  32  58  30 /  50  10  10  10
LWT  55  35  52  33 /  50  20  40  20

&&

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

$$

WEATHER.GOV/GREATFALLS



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