Special Weather Statement, NWS Missoula, MT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MISSOULA MT
231 PM MDT WED JUN 1 2011

...SIGNIFICANT RAINFALL AND COOLER TEMPERATURES TONIGHT THROUGH
THURSDAY EVENING...

...MUCH WARMER AND SUNNY THIS COMING WEEKEND...

.DISCUSSION...AN UPPER LEVEL LOW CURRENTLY OVER NORTHERN
CALIFORNIA WILL SWING INLAND THIS EVENING AND OVER NORTH CENTRAL
IDAHO AND WESTERN MONTANA LATE TONIGHT THROUGH THURSDAY. THIS
SYSTEM HAS QUITE A BIT OF MOISTURE ASSOCIATED WITH IT AND THE
FORECASTED TRACK IS EXPECTED TO BRING WIDESPREAD MODERATE TO HEAVY
RAIN ACROSS THE REGION. WIDESPREAD ACCUMULATIONS OF .25 TO 1.0
INCHES OF RAIN ARE EXPECTED. HOWEVER...A 700 MB LOW WILL ENHANCE
PRECIPITATION OVER THE CLEARWATER MOUNTAINS THROUGH WEST CENTRAL
MONTANA AND ALONG THE CONTINENTAL DIVIDE FROM DRUMMOND NORTH. THIS
AFOREMENTIONED REGION COULD SEE LIQUID PRECIPITATION AMOUNTS OF
1.5 TO 2.0 INCHES...WITH SOME LOCAL ENHANCEMENT OVER THE HIGHER
TERRAIN. THE COOLER AND RELATIVELY DRY CONDITIONS THE FIRST HALF
OF THIS WEEK HAVE ALLOWED MANY RIVERS AND STREAMS TO RECEDE.
HOWEVER...THIS WEATHER SYSTEM WILL CHANGE THAT WITH THE HIGH
PRECIPITATION AMOUNTS EXPECTED. PERSONS WITH INTERESTS OR LIVING
NEAR RIVERS AND STREAMS WILL WANT TO PAY CLOSE ATTENTION TO LEVELS
AND LATER FORECASTS FOR POTENTIAL HYDROLOGIC HEADLINES.

A MUCH COOLER AIR MASS WILL ACCOMPANY THIS SYSTEM SO SNOW LEVELS
WILL LOWER TO NEAR 5000 FEET BY THURSDAY AFTERNOON. SNOW WILL
OCCUR AT MOUNTAIN PASSES...BUT LITTLE TO NO ACCUMULATIONS ARE
EXPECTED ON ROAD SURFACES SINCE ITS PASSAGE IS DURING THE DAYLIGHT
HOURS. TEMPERATURES THURSDAY WILL RUN 15 TO 20 DEGREES BELOW
AVERAGE MOST LOCATIONS. THE EXCEPTION WILL BE LEMHI COUNTY AND
SOUTHWEST MONTANA WHERE THE LEAST AMOUNT OF PRECIPITATION IS
EXPECTED. ALSO...WITH INCREASED TEMPERATURES MORE INSTABILITY WILL
BE AVAILABLE FOR ISOLATED AFTERNOON AND EARLY EVENING
THUNDERSTORMS OVER THIS SAME REGION.

LATE THURSDAY NIGHT AND FRIDAY THE WEATHER SYSTEM WILL SHIFT
NORTHEAST OF THE CONTINENTAL DIVIDE. RETURN FLOW ON THE BACK SIDE
OF THE SYSTEM WILL KEEP UNSETTLED AND COOL CONDITIONS OVER WESTERN
MONTANA THROUGH FRIDAY. THE BROAD AREA OF PRECIPITATION WILL SHIFT
ACROSS NORTHWEST MONTANA BY FRIDAY MORNING. COLD AIR ALOFT WILL
KEEP A THREAT OF SHOWERS ACROSS THE REST OF WESTERN MONTANA AND
NORTH CENTRAL IDAHO.

FORECAST MODELS CONTINUE TO BE VERY CONSISTENT WITH THE OVERALL
WEATHER PATTERN FOR THE WEEKEND...STALLING A LARGE AREA OF LOW
PRESSURE OFF THE CALIFORNIA COAST. THIS WILL PRODUCE A SIGNIFICANT
WARMING TREND FOR THE WEEKEND WITH MANY AREAS EXPECTED TO
EXPERIENCE THEIR WARMEST CONDITIONS OF THE YEAR THUS FAR. BUT
HOWEVER NICE THE WEEKEND CONDITIONS...DRAMATIC WARMING WILL ONLY
FURTHER EXACERBATE OUR FLOODING POTENTIAL WITH ENHANCED SNOW MELT
POSSIBLE. RISES IN THE RIVERS...CREEKS AND STREAMS ARE TO BE
EXPECTED THROUGH THE WEEKEND AND INTO NEXT WEEK AND CITIZENS
ACROSS ALL OF WESTERN MONTANA AND CENTRAL IDAHO SHOULD KEEP
ABREAST OF DEVELOPING HYDROLOGICAL ISSUES.

FORECAST MODELS CONTINUE TO STRUGGLE WITH HOW QUICKLY TO BRING
THE UPPER LOW INLAND AND INTO THE NORTHERN ROCKIES EARLY NEXT
WEEK. THE ECMWF REMAINS THE QUICKER...MORE PROGRESSIVE MODEL WHILE
THE GFS/GFS ENSEMBLE MEAN/CANADIAN AND TO A LESSER EXTENT THE
ECMWF ENSEMBLE MEAN ARE SLOWER. THUS HAVE LEANED A BIT MORE
TOWARDS THE WARMER AND SLOWER SOLUTIONS THINKING THAT THE LARGER
HEMISPHERIC SCALE CONDITIONS (A LARGE BLOCKING RIDGE OF HIGH
PRESSURE UP STREAM OVER THE CENTRAL ATLANTIC OCEAN) WILL FAVOR THE
SLOWER SOLUTIONS. BY MID WEEK ALL LONG RANGE MODELS AGREE THAT A
LARGE TROUGH OF LOW PRESSURE WILL BE SITUATED OVER THE WESTERN
UNITED STATES RESULTING IN BELOW NORMAL TEMPERATURES AND ABOVE
AVERAGE CHANCES FOR PRECIPITATION. THIS LOW COULD PERSIST THROUGH
LATE NEXT WEEK GIVEN THE PERSISTENT UP STREAM BLOCKING OVER THE
CENTRAL ATLANTIC.

&&

.AVIATION...CELINGS ARE GOING TO CONTINUE TO LIFT THIS AFTERNOON
BUT ABUNDANT LOW LEVEL MOISTURE WILL KEEP SOME CLOUDS AROUND TO
OBSCURE TERRAIN. SOME THUNDERSTORMS MAY DEVELOP IN AN UNSTABLE
ATMOSPHERE ACROSS LEMHI COUNTY AND INTO SOUTHWEST MONTANA. KMSN
AND KBTM ARE THE AIRFIELDS EXPECTED TO BE MOST IMPACTED BY
CUMULONIMBUS FROM 01/20Z-02/03Z. AN UPPER LEVEL LOW WILL MOVE OVER
THE REGION TONIGHT CAUSING WIDESPREAD SHOWERS AND LOWERED CEILINGS
IMPACTING NORTH CENTRAL IDAHO AND WEST CENTRAL/SOUTHWEST AIRFIELDS
THROUGH THURSDAY MORNING.

&&

NOTE: FLOOD WATCHES...WARNINGS AND ADVISORIES WILL NOT BE LISTED BELOW.

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...NONE.
ID...NONE.
&&

SHORT TERM...MEAD
LONG TERM....DICKERSON
AVIATION...LOEFFELBEIN

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