821
FXUS65 KGGW 042055
AFDGGW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GLASGOW MT
255 PM MDT SAT JUL 4 2009
SHORT TERM...FOR NORTHEAST MONTANA...TONIGHT THROUGH MONDAY
CURRENT GOES WATER VAPOR IMAGERY REVEALS WESTERLY FLOW ALOFT OVER
NORTHEAST MONTANA WITH A FEW IMPULSES EMBEDDED WITHIN THE LONGWAVE
FLOW...ONE OF WHICH IS TRIGGERING A FEW THUNDERSTORMS OVER CENTRAL
MONTANA THIS AFTERNOON. AT THE SURFACE...AN AREA OF HIGH PRESSURE
IS CENTERED OVER SOUTH CENTRAL SASKATCHEWAN WITH THE SURFACE RIDGE
BISECTING THE LOCAL FORECAST AREA. THIS HIGH PRESSURE SYSTEM HAS
BEEN STRONGER THAN EXPECTED...WITH AFTERNOON TEMPERATURES RUNNING
BELOW FORECAST VALUES AT THIS POINT. THE STRENGTH OF THE SURFACE
HIGH IS SUGGESTIVE OF MODEST SUBSIDENCE ALOFT...WHICH AT THIS
POINT HAS PREVENTED THE WAVE OVER CENTRAL MONTANA FROM ADVANCING
EASTWARD. ANOTHER SHORTWAVE IS TRIGGERING CONVECTION OVER
SOUTHWEST MONTANA WHERE THE ATMOSPHERE IS UNSTABLE. THIS AREA OF
INSTABILITY DOES REACH INTO THE FAR SOUTHWEST ZONES WHERE A FEW
THUNDERSTORMS COULD DRIFT IN FROM THE WEST LATE THIS AFTERNOON
INTO THIS EVENING. A FEW STORMS COULD CONTAIN MODEST WIND GUSTS
AND SMALL HAIL. THUNDERSTORM POTENTIAL APPEARS LIMITED ACROSS THE
REMAINDER OF THE AREA UNTIL THE HIGH PRESSURE RIDGE SLIDES EAST.
THE AREA OF PVA OVER SOUTHWEST MONTANA WILL MOVE ACROSS NORTHEAST
MONTANA OVERNIGHT. MEANWHILE...AN UPPER RIDGE WILL STRENGTHEN OVER
THE CONTINENTAL DIVIDE AS A VIGOROUS UPPER TROUGH DRIVES TOWARD
THE PACIFIC NORTHWEST. THIS WILL INDUCE A BAROCLINIC ZONE OVER THE
NORTHEAST CORNER OF THE CWA. THIS WILL BECOME THE FOCAL POINT FOR
A FEW SHOWERS OR THUNDERSTORMS IN THE NORTHEAST ZONES LATE TONIGHT
AND EARLY SUNDAY. TEMPERATURES WILL WARM WELL INTO THE 80S SUNDAY
FOR MOST OF THE AREA AS A THERMAL RIDGE BUILDS NORTH INTO CENTRAL
MONTANA. THE EXCEPTION WILL BE THE FAR NORTHEAST NEAR THE FRONTAL
ZONE WHERE TEMPERATURES WILL BE A FEW DEGREES COOLER.
THE UPPER RIDGE AXIS PASSES OVERHEAD SUNDAY NIGHT WITH SOUTHWEST
FLOW ALOFT DEVELOPING AHEAD OF THE APPROACHING PACIFIC TROUGH
MONDAY. THERE IS STILL SOME UNCERTAINTY IN THE POSITIONING OF THE
STRONGEST DYNAMICS. THE ECMWF...GEM...AND UKMET BRING THE
STRONGEST DYNAMICS INTO CENTRAL MONTANA MONDAY AFTERNOON...WHILE
THE GFS AND NAM SEND THE VIGOROUS SHORTWAVE INTO THE CANADIAN
PRAIRIES. NONETHELESS THE GFS AND NAM DO INDICATE MID-LEVEL
CONVERGENCE...WHICH PHASES WITH A SURFACE LOW OVER CENTRAL MONTANA
AND SOUTHEASTERLY LOW-LEVEL FLOW TO THE EAST DRAWING MOISTURE INTO
THE AREA. SO THE PATTERN DOES LOOK FAVORABLE FOR STRONG TO SEVERE
STORMS. MONDAY WILL BE ANOTHER WARM DAY AS THE THERMAL RIDGE
REMAINS OVER THE AREA. AJZ
LONG TERM...MONDAY NIGHT THROUGH SATURDAY
GFS MOVING TOWARD THE SLIGHTLY MORE AMPLIFIED ECMWF SOLUTION FOR
THE EXTENDED PERIOD. FORECAST WAS ALREADY TRENDED TOWARD THE ECMWF
SO FEW CHANGES MADE. TROF MOVES THROUGH THE REGION MID TO LATE
WEEK WITH CHANCES OF THUNDERSTORMS...THE RIDGE REBOUNDS INTO THE
WEEKEND FOR A RETURN TO DRIER CONDITIONS. EBERT
PREVIOUS LONG TERM DISCUSSION...UPPER RIDGE MOVES TO THE EAST
MONDAY NIGHT AND TUESDAY. A SURFACE TROUGH OVER THE AREA COMBINED
WITH SHORTWAVES ON SOUTHWEST FLOW ALOFT WILL BRING A CHANCE OF
SHOWERS AND THUNDERSTORMS.
MODELS DIFFER ON HOW TO BRING UPPER TROUGH THAT IS FORECASTED TO
BE OFF THE WASHINGTON COAST ON TUESDAY INLAND. THE ECMWF BRINGS THE
TROUGH EAST INTACT PASSING THROUGH EASTERN MONTANA WEDNESDAY
NIGHT. THE GFS SIGNIFICANTLY WEAKENS THE TROUGH AND WAITS UNTIL
FRIDAY TO GET IT TO EASTERN MONTANA. THE ECMWF IS SHOWING BETTER
RUN TO RUN CONSISTENCY WITH THIS SYSTEM. IT LOOKS LIKE THERE COULD
BE A GOOD CHANCE OF SHOWERS AND THUNDERSTORMS WHEN THE TROUGH
COMBINED WITH 100 KNOT JET MOVES THROUGH WEDNESDAY NIGHT. WEAK
SHORTWAVES BEHIND THE TROUGH COULD BRING ANOTHER CHANCE OF SHOWERS
AND THUNDERSTORMS THURSDAY.
THE MODELS DIFFER BETWEEN GFS HAVING ZONAL FLOW WITH WAVES AND
ECMWF HAVING UPPER RIDGE GOING INTO NEXT WEEKEND. WILL MENTION A
SLIGHT CHANCE OF THUNDERSTORMS ON SATURDAY FOR NOW.
AVIATION
VFR CONDITIONS EXPECTED TO DOMINATE AT THE TAF SITES OVER THE NEXT
24 HOURS. HOWEVER THERE IS A CHANCE OF A SHOWER AND/OR
THUNDERSTORM OVERNIGHT THAT COULD BRING BRIEF PERIODS OF MVFR
CONDITIONS. RAE
GLASGOW WATCHES/WARNINGS/ADVISORIES
NONE.
WEATHER.GOV/GLASGOW
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