Severe Thunderstorm Warning 67, NWS Glasgow, MT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GLASGOW MT
935 AM MDT MON AUG 23 2004

.SHORT TERM...FOR NORTHEAST MONTANA...
A COOL AND WET DAY ACROSS OUR NORTHERN ZONES. A DEEP SURFACE
LOW JUST WEST OF MILES CITY THIS MORNING WILL CONTINUE TO TRACK EAST
AND INTO WESTERN SOUTH DAKOTA THIS AFTERNOON. AN UPPER LOW VISIBLE
ON WATER VAPOR PUSHING THROUGH WESTERN MONTANA WILL LIFT NORTHEAST
TODAY...AND SHOULD BE ACROSS NORTHEAST MONTANA BY LATE IN THE DAY.
THE 12Z KGGW SOUNDING INDICATES A DEEP SOUTHERLY FLOW ACROSS EASTERN
MONTANA WITH PRECIPITABLE WATER VALUES IN EXCESS OF AN INCH. THE
KGGW 88-D AND SURFACE OBSERVATIONS INDICATING THAT NUMEROUS SHOWERS
ARE FALLING ACROSS THE CWA...MAINLY ACROSS THE NORTHERN HALF OF THE
CWA. LITTLE IN THE WAY OF LIGHTNING DURING THE LAST HOUR OR
TWO...WITH ONLY A COUPLE OF LIGHTNING STRIKES ACROSS NORTHWEST
PHILLIPS COUNTY. AS THE SURFACE LOW MOVES EAST AND THE MID AND UPPER
LOWS MOVE ACROSS THE AREA EXPECT THE FOCUS TO BE ACROSS OUR NORTHERN
TIER OF ZONES THIS AFTERNOON. A DRY SLOT WILL LIKELY WORK INTO
SOUTHEAST MONTANA...AND EXPECT LITTLE IN THE WAY OF SHOWER ACTIVITY
IN THE SOUTH. IN COORDINATION WITH SURROUNDING OFFICES HAVE MADE
SOME ADJUSTMENTS TO THE POPS TODAY AND HAVE A LARGE GRADIENT IN THE
POPS FROM NORTH TO SOUTH. ALSO PLAN TO LOWER MAXIMA TODAY ACROSS OUR
NORTHERN ZONES WHERE CLOUDS WILL HANG TOUGH AND THE RAIN WILL BE THE
STEADIEST. CB

.PREVIOUS DISCUSSION...FOR NORTHEAST MONTANA TODAY THRU WED...
A WET DAY SHAPING UP...AS THE MODELS ARE ALL CONSIDERABLY WETTER
THAN PREVIOUS RUNS...WITH THE STRONG UPPER LEVEL TROUGH IN IDAHO TO
INTENSIFY AND MOVE NE ACROSS OUR AREA TODAY. MOST MODELS CLOSE THIS
TROUGH OF FOR A TIME AS IT MOVES ACROSS OUR NORTH THIS AFTERNOON.
THIS USUALLY MEANS LIKELY OR GREATER POPS. THEY ALSO CLOSE AT 700 MB
AND LOWER LEVELS AS WELL...BECOMING MORE VERTICAL WITH AN ORGANIZING
SURFACE LOW NOW CENTERED NEAR LEWISTOWN AND NEARLY STATIONARY THIS
MORNING.

CURRENT SATELLITE PICTURES CONFIRMING THESE IDEAS AND LOOKING
IMPRESSIVE WITH MOISTURE AND DEVELOPMENT OF CLOSING UPPER LOW. KGGW
88D VWP PROFILE SHOWING DEEP SOUTHERLY FLOW TO JETSTREAM LEVEL.
MODELS INCREASE MEAN RH TO TO 70-90 PCT ACROSS NORTH HALF OF CWA
THIS AFTERNOON...NOT TOO COMMON IN AUGUST. PRECIP WATER CLIMBS TO 1
TO 1.2 INCHES THIS AREA. MODELS IN REASONABLE AGREEMENT...PREFER THE
GFS WITH OMEGA AND ETA WITH MEAN RH THOUGH. ALL IN ALL NOT YOUR
TYPICAL LATE AUGUST DAY IN NE MT. INSTABILITY QUITE MARGINAL FOR
THUNDERSTORMS WITH MOSTLY CLOUDY AND COOL CONDITIONS...BUT ENOUGH
INSTABILITY AND FORCING WITH UPPER TROUGH TO KEEP ISOLATED
THUNDERSTORM MENTION...WHILE OTHERWISE RAISING POPS A
LOT...ESPECIALLY NORTH. CURRENTLY...THE DRY SLOW IS MOVING ACROSS
THE AREA WHICH HAS PUSHED ISOLATED LINGERING SHOWERS FROM OVERNIGHT
TO OUR NORTHERN BORDER. THIS SHOULD NOT BE A FACTOR BY LATE MORNING
THOUGH.

WILL EXTEND HIGHER POPS INTO THE EVENING HOURS IN OUR NORTH UNTIL
THE MOISTURE WITH UPPER LOW MOVES FAR ENOUGH AWAY LATE TONIGHT FOR
LOW POPS. MODELS INDICATE 850 MB WINDS OF 30-40 KT OVERNIGHT...BUT
DONT SHOW THAT REACHING TO SURFACE ON MOS. LAPSE RATES LOOK QUITE
CLOSE THOUGH OF BEING ABLE TO MIX THIS WIND DOWN...BUT SURFACE
GRADIENTS NOT GREAT AND COOL ADVECTION MODERATE. BEST CHANCE OF THIS
WOULD BE IN NORTH BEFORE CLEARING ARRIVES. THIS TYPE PATTERN
SOMETIMES MISSED AND UNDERFORECAST...BUT FEEL BEST FOR NOW TO GO
ONLY A LITTLE HIGHER THAN MOS AND LET DAYSHIFT LOOK MORE CLOSELY.

STILL SOMEWHAT COOL AND UNSETTLED PATTERN FOR TUESDAY AND WEDNESDAY.
APPEARS THE NEXT SHORTWAVE WILL ARRIVE WEDNESDAY WITH UPPER FLOW
TURNING A LITTLE MORE SW AGAIN...SO WILL RAISE POPS THEN...LOWER
FOR TUESDAY THOUGH. SIMONSEN

.LONG TERM...WEDNESDAY NIGHT THRU SATURDAY...
GOOD MODEL AGREEMENT WITH OVERALL PATTERN...AND GOING FORECAST IN
GOOD SHAPE. CHIEF DIFFERENCE AMONG MODELS IS TIMING...WITH FORECAST
REMAINING BASED MAINLY ON GFS WHICH SHOWED VERY GOOD RUN-TO-RUN
CONTINUITY. CHIEF FEATURE IN FORECAST PERIOD IS WELL DEFINED TROUGH
OVER PACIFIC NORTHWEST AT BEGINNING OF PERIOD THAT IS EXPECTED TO
GRADUALLY TRANSLATE ACROSS REGION THROUGH FRIDAY...JUST AS GRADUALLY
ENDING SHOWERS/THUNDER FROM WEST TO EAST BY FRIDAY NIGHT. FORECAST
CHALLENGE REMAINS POTENTIAL FOR DIFFERENT TIMING OF EVENT. MAIN
TROUGH AXIS SHIFTS INTO WESTERN GREAT LAKES BY SUNDAY WITH MODELS
SHOWING ANOTHER...ALBEIT MORE SHALLOW...TROUGH SWINGING THROUGH
REGION BY SUNDAY. HERE...HAVE DEVIATED FROM GFS TOWARD ECMWF WHICH
KEEPS WAVE FLAT AND FURTHER NORTH IN SOUTHERN CANADA...BASED LARGELY
ON CLIMATOLOGY AND PERCEIVED WET BIAS IN GFS. AFTER PRECIPITATION
ENDS ON FRIDAY...FORECAST DRY BUT SKIES PARTLY CLOUDY DUE TO
PROXIMITY OF RELATIVELY STRONG JET AXIS IN FLOW PATTERN THAT VARIES
SOMEWHAT BUT TRENDS CLOSE TO ZONAL. MAY HAVE MINOR IMPULSES IN THIS
FLOW OVER WEEKEND THAT WILL BE CAPABLE OF GENERATING SHOWERS...BUT
TOO WEAK TO CONSIDER FOR NOW. FORECAST TEMPERATURES WERE CHANGED
LITTLE FROM INHERITED GRIDS DUE TO EXPECTED CLOUDS AND PRECIPITATION
THROUGH FRIDAY...AND QUASI-ZONAL FLOW OVER WEEKEND. DNS

&&

.KGGW WATCHES/WARNINGS/ADVISORIES...NONE.

&&