Damaging Winds in Montana, August 12, 2009

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GLASGOW MT
304 PM MDT WED AUG 12 2009

.SHORT TERM...FOR NORTHEAST MONTANA...TONIGHT THROUGH FRIDAY...
MAIN SURFACE FRONT HAS PUSHED INTO SOUTHERN MONTANA TODAY WITH
COOLER AIRMASS LINGERING JUST TO THE NORTH OF THE STATE. AIRMASS
OVER EASTERN MONTANA REMAINS HOT AND DRY SO WEAK ELEVATED
CONVECTIVE CELLS MOVING THROUGH THE SOUTHWEST FLOW ALOFT HAVE
PRODUCED LITTLE MORE THAN ERRATIC GUSTY WINDS. MID TO LOW LEVEL
MOISTURE WILL BE INCREASING AS THE UPPER TROF OFF THE BC COAST
SWINGS TOWARD THE PACIFIC NORTHWEST. THIS WILL GRADUALLY INCREASE
THE CHANCE OF SHOWERS OR THUNDERSTORMS ALONG THE MOSTLY STATIONARY
SURFACE FRONT FROM LATE THURSDAY INTO FRIDAY.

COLDER AIR WILL ALSO BE SLOWLY FILTERING SOUTHWARD OVER THE NEXT
FEW DAYS AS TROF DIGS OVER THE PACIFIC NORTHWEST. INCREASED CLOUD
COVER AND MOISTURE WILL ALSO RESULT IN COOLER TEMPERATURES BY
FRIDAY. EBERT

.LONG TERM...FRIDAY NIGHT THROUGH WEDNESDAY...
AN UPPER LEVEL TROUGH WILL DIVE SOUTHEASTWARD OUT OF THE GULF OF
ALASKA LATER THIS WEEK AND REACH THE NORTHERN ROCKIES AND NORTHERN
HIGH PLAINS BY THE UPCOMING WEEKEND. THIS WILL SEND A COLD FRONT
THROUGH NORTHEAST MONTANA BRINING MUCH COOLER TEMPERATURES FOR
SATURDAY AND SUNDAY. THUNDERSTORMS ARE POSSIBLE WITH THE FRONTAL
PASSAGE FRIDAY NIGHT...ESPECIALLY ACROSS THE NORTH UNDER THE
STRONGEST DYNAMICAL FORCING ASSOCIATED WITH A SHORTWAVE
TROUGH. ANOTHER SHORTWAVE TROUGH FOLLOWS QUICKLY BEHIND THE FIRST
WAVE TRACKING FROM WYOMING INTO THE DAKOTAS. THIS HAS THE POTENTIAL
TO BRING ANOTHER PERIOD OF SHOWERS TO THE LOCAL AREA. THE THERMAL
PROFILE WILL BE STABLE BEHIND THE COLD FRONT...SO THUNDER IS NOT
ANTICIPATED. THE UPPER LOW SLOWLY LIFTS NORTHEAST SUNDAY. GFS AND
GFS ENSEMBLES ARE MORE PROGRESSIVE WITH THE DEPARTURE OF THE
TROUGH THAN THE ECMWF AND GEM...WHICH HOLD ONTO MORE RAINFALL ON
SUNDAY. THE GFS/GFS ENSEMBLE SOLUTION IS MORE REASONABLE FOR THIS
TIME OF YEAR...AND IS FAVORED FOR SUNDAY. A HIGHLY AMPLIFIED UPPER
RIDGE BEGINS TO BUILD OVER THE WEST COAST NEXT WEEK INDUCING
NORTHWEST FLOW OVER THE HIGH PLAINS. THIS WILL KEEP TEMPERATURES
BELOW NORMAL ALONG WITH MAINLY DRY CONDITIONS. AJZ

PREVIOUS LONG TERM DISCUSSION...
A COOL AND UNSETTLED WEEKEND COMING UP. ALL MODELS SHOW A LARGE
AND STRONG UPPER TROUGH FROM THE WEST TO MOVE ACROSS THE AREA.
SOME MODELS HAVE BEEN INDICATING THE TROUGH MAY CLOSE OFF AS IT
MOVES ACROSS MT. PAST 2 RUNS OF ECMWF LEADING THE WAY WITH THIS
IDEA...BUT 00Z RUN MORE PROGRESSIVE THAN PREVIOUS RUN AND IS NOT
QUITE SO WET. 00Z CMC LOOKING WET WITH A SIMILAR SOLUTION. GFS AND
MOST OF ITS ENSEMBLES ARE MORE OPEN ON THE TROUGH. WOULD LIKE TO
SEE MORE MODELS HAVING A CLOSED SYSTEM BEFORE GOING LIKELY POPS
FOR ANY PERIOD...ESPECIALLY DUE TO THE SIZE OF TROUGH AND ENERGY
POTENTIALLY BEING QUITE SPREAD OUT...AS WELL AS AUGUST NOT A VERY
FAVORABLE TIME OF YEAR FOR CLOSED SYSTEMS. WILL STICK WITH GENERAL
TRENDS ON POPS OF PREVIOUS LONG TERM FORECAST. HOWEVER...ALL
MODELS DO LOOK MOIST WITH SIGNIFICANT QPF FOR FRI NIGHT/SAT IN
PARTICULAR...SO HAVE RAISED POPS BOTH PERIODS...THE SUM OF WHICH
DOES ADD UP TO A LIKELY OCCURRENCE IN THE FRI NIGHT/SAT 24 HOUR
PERIOD.

A STRONG COLD FRONT WILL GENERATE THUNDERSTORMS FRI NIGHT...THEN
MUCH COOLER AIRMASS ARRIVES FOR LESS INSTABILITY SAT. THICKNESSES
DROP TO THE UPPER 550S DM. WHILE THE MAIN PART OF THE TROUGH MOVES
OUT OF THE AREA SAT NIGHT...A SLIGHTLY CYCLONIC FLOW WILL LINGER
THRU MON...SO PROBABLY A FEW SHOWERS AS INDICATED BY MODELS...AND
WILL HAVE POPS SUN THRU TUE IN 10-15 PCT RANGE. THICKNESSES STAY
IN THE 550S TO LOWER 560S DM...SO HIGHS REMAIN IN THE 70S SUNDAY
AND MON.

MOST MODELS SHOW UPPER RIDGING TUE AND WED...FOR DRY AND WARMER
WEATHER. 00Z GFS HAD A TROUGH ARRIVING FROM THE NW WHICH IS
DIFFERENT FROM OTHER MODELS OR PREVIOUS RUNS AND SO IS REJECTED.
SIMONSEN

&&

.AVIATION...
VFR CONDITIONS WILL PREVAIL THROUGH THURSDAY WITH SCATTERED CLOUDS
AT OR ABOVE 10K FT. ISOLATED THUNDERSTORMS ARE POSSIBLE
OVERNIGHT...AND THURSDAY AFTERNOON AND EVENING. ANY THUNDERSTORMS
TONIGHT WILL HAVE VERY LITTLE IMPACT ON CEILING AND
VISIBILITY...BUT COULD PRODUCE AN ISOLATED STRONG WIND GUST.
MOISTURE WILL INCREASE THURSDAY AFTERNOON...SO THUNDERSTORMS HAVE
A POTENTIAL TO PRODUCE RAIN AND COULD HAVE A MINOR IMPACT ON
CEILING AND VISIBILITY. AJZ

&&

.GLASGOW WATCHES/WARNINGS/ADVISORIES...NONE.

&&

WEATHER.GOV/GLASGOW