Severe Thunderstorm Warning 1, NWS Great Falls, MT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREAT FALLS MT
317 PM MDT TUE MAY 20 2008

.DISCUSSION...

TONIGHT THROUGH THURSDAY...COLD FRONT EXTENDED FROM NW MONTANA
THROUGH SW IDAHO AT 20Z AND WILL CONTINUE MARCHING EAST. IT SHOULD
BE NEAR A HAVRE-LEWISTOWN LINE NEAR MIDNIGHT. STILL EXPECTING
SHOWERS AND THUNDERSTORMS TO ACCOMPANY THE FRONT. ALSO EXPECT A
BAND OF SHOWERS AND POSSIBLY THUNDERSTORMS BEHIND THE FRONT DUE TO
A WEATHER DISTURBANCE ALOFT. ON WEDNESDAY THERE WILL BE MOIST
SOUTHERLY FLOW ALOFT. WEATHER DISTURBANCES ARE EXPECTED TO AFFECT
MAINLY THE EASTERN FORECAST ZONES. INHERITED FORECAST WHICH HAD
LIKELY OR HIGHER POPS IN THE EAST AND CHANCE POPS OVER THE WESTERN
PLAINS AND VALLEYS STILL LOOKS GOOD. THEN AS AN UPPER LOW DEVELOPS
OVER THE GREAT BASIN WEDNESDAY NIGHT AND THURSDAY THE FLOW ALOFT
WILL TURN TO THE SOUTHEAST AND BRING PRECIPITATION TO WESTERN
PORTIONS OF THE FORECAST AREA AS WELL. EXPECT WIDESPREAD RAIN BY
THURSDAY. LIFT WILL BE GENERATED BY UPSLOPE FLOW PARTICULARLY OVER
THE PLAINS AS WELL AS LIFT OVER A COLDER AIRMASS OVER WESTERN
PORTIONS OF THE FORECAST AREA. AS HAS BEEN ADVERTISED EXPECT SNOW
LEVELS TO LOWER TO NEAR 6000 FEET BY LATE TONIGHT IN THE WEST AND
CONTINUE ABOUT THAT LEVEL THROUGH THURSDAY. BLANK

THURSDAY NIGHT THROUGH SUNDAY NIGHT...A VERY WET AND UNSETTLED
WEATHER PATTERN LOOKS TO BE SHAPING UP FOR THE HOLIDAY WEEKEND OVER
MUCH OF NORTH CENTRAL AND SOUTHWEST MONTANA. BOTH THE GFS AND ECMWF
ARE IN GOOD AGREEMENT THROUGH SATURDAY...THEN THEY DIVERGE A BIT
SAT NIGHT THRU SUN...BUT THEY ARE SHOWING SIMILAR RESULTS. THUS
EXPECT WET CONDITIONS FRIDAY AND SATURDAY AS AN AN UPPER LEVEL LOW
OVER SOUTHERN IDAHO CONTINUES TO ROTATE SHOWERS UP INTO CENTRAL
MONTANA. ON SATURDAY NIGHT THE GFS WEAKENS THE STORM SYSTEM A
BIT...WHILE THE ECMWF CONTINUES TO MAINTAIN THE UPPER LOWS STRENGTH
AND ALSO MOVE THE CENTER OF THE LOW TOWARDS CENTRAL MONTANA. GIVEN
THAT CLIMATOLOGY THIS IS THE WETTEST TIME OF THE YEAR...THE ECMWF IS
PREFERRED AND A WET/COOL WEEKEND IS POSSIBLE. RIGHT NOW PRECIP
CHANCES ARE ONLY IN THE SLIGHT CHANCE/CHANCEY CATEGORY FOR SATURDAY
NIGHT THROUGH SUNDAY NIGHT...BUT IF THE ECMWF CONTINUES TO LOOK LIKE
IT WILL BE THE CORRECT MODEL...THEN POPS COULD BE RAISED
SIGNIFICANTLY INTO THE HIGH CHANCE/LIKELY CATEGORY BY TOMORROW FOR
THE REMAINDER OF THE WEEKEND. (BASICALLY THE CURRENT FORECAST IS
GOING FOR SCATTERED SHOWERS THROUGH THE WEEKEND...BUT A WASHOUT IS
POSSIBLE IF THE ECMWF MODEL IS CORRECT.) OTHERWISE...EXPECT
GENERALLY CLOUDY CONDITIONS AND WELL BELOW NORMAL TEMPERATURES
ACROSS THE REGION FROM FRIDAY THROUGH SUNDAY. BRUSDA

MONDAY THROUGH TUESDAY...MODEL DIFFERENCES CONTINUE TO DETRACT FROM
FORECAST CONFIDENCE DURING THIS PERIOD. THE GFS AND ECMWF DISAGREE
IN POSITIONS AND STRENGTH OF SYSTEMS AFFECTING THE REGION DURING
THIS PERIOD...HOWEVER BOTH SOLUTIONS WOULD INDICATE A COOL UNSETTLED
PERIOD. MOSTLY CLOUDY SKIES ARE EXPECTED WITH ISOLATED TO SCATTERED
SHOWERS. TEMPERATURES SHOULD BE A LITTLE BELOW SEASONAL AVERAGES.  EK

&&

.AVIATION...
AN UPPER LEVEL TROUGH AND SURFACE COLD FRONT WILL APPROACH THE
REGION FROM THE WEST AND CROSS THE REGION THIS EVENING. EXPECT
INCREASING CLOUDS AS THE AIRMASS BECOMES UNSTABLE THIS AFTERNOON.
SHOWERS AND THUNDERSTORMS PRECEDING THE COLD FRONT WILL REACH
WESTERN TERMINALS BY 00Z AND EASTERN TERMINALS BY 04Z WITH THE FRONT
CLEARING EASTERN AREAS AROUND 06Z TONIGHT. SHOWERS AND THUNDERSTORMS
WILL BE NUMEROUS ALONG THE COLD FRONT WITH GUSTY SURFACE WINDS AND
SMALL HAIL POSSIBLE. WINDS WILL BE FROM THE SOUTH TO SOUTHEAST
THROUGH THIS AFTERNOON AHEAD OF THE FRONT...SHIFTING TO THE WEST
BEHIND THE FRONT TONIGHT. SHOWERS WILL CONTINUE TONIGHT BEHIND THE
COLD FRONT WITH WIDESPREAD MVFR AND LOCAL IFR CONDITIONS IN SHOWERS
OVER MUCH OF CENTRAL AND SW MT. HOENISCH

&&

.HYDROLOGY...
FOR WEDNESDAY INTO THE WEEKEND RAINFALL TOTALS OF 1 TO 2 INCHES
ARE A GOOD BET OVER MUCH OF THE FORECAST AREA. TOTALS OF 2 TO 3
INCHES ARE POSSIBLE WHILE LOCALLY HIGHER RAINFALL TOTAL ARE NOT
OUT OF THE QUESTION. WITH STREAMS AND SOME RIVERS RUNNING HIGH DUE
TO RECENT SNOWMELT AND NOW AN EXPECTTATION OF SIGNIFICANT
RAINFALL..WILL ISSUE A FLOOD WATCH FROM WEDNESDAY NIGHT THROUGH
SUNDAY FOR THE ENTIRE FORECAST AREA WITH THE EXCEPTION OF NORTH
CENTRAL MONTANA. BLANK

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  45  52  40  48 /  80  50  60 100
CTB  43  53  39  45 /  70  30  80 100
HLN  49  52  45  51 /  80  50  40  80
BZN  44  49  40  50 /  80  80  40  80
WEY  38  42  33  45 /  80  60  60  70
DLN  42  48  36  48 /  70  70  50  70
HVR  49  56  44  53 /  80  70  70 100
LWT  44  50  39  46 /  60  80  80 100

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
FLOOD WATCH FROM WEDNESDAY EVENING THROUGH SUNDAY AFTERNOON FOR
MTZ008>010-012>015-044>046-048>055.

SMALL STREAM FLOOD ADVISORY THROUGH THIS AFTERNOON FOR MTZ009.

SMALL STREAM FLOOD ADVISORY THROUGH WEDNESDAY AFTERNOON FOR MTZ008.

&&

SHORT TERM...BLANK
LONG TERM...HOENISCH
AVIATION...HOENISCH

WEATHER.GOV/GREATFALLS

HAZARDOUS WEATHER OUTLOOK...UPDATED
NATIONAL WEATHER SERVICE GREAT FALLS MT
503 PM MDT TUE MAY 20 2008

MTZ008>015-044>055-211200-
BEAVERHEAD-NORTHERN ROCKY MOUNTAIN FRONT-EASTERN GLACIER-HILL-
CASCADE-CHOUTEAU-CENTRAL AND SOUTHERN LEWIS AND CLARK-MADISON-
TOOLE-LIBERTY-EASTERN PONDERA-BLAINE-
SOUTHERN ROCKY MOUNTAIN FRONT-EASTERN TETON-JUDITH BASIN-FERGUS-
JEFFERSON-BROADWATER-MEAGHER-GALLATIN-
503 PM MDT TUE MAY 20 2008

THIS HAZARDOUS WEATHER OUTLOOK IS FOR PORTIONS OF CENTRAL
MONTANA...NORTH CENTRAL MONTANA AND SOUTHWEST MONTANA.

.DAY ONE...TONIGHT

A SMALL STREAM FLOOD ADVISORY IS IN EFFECT FOR NORTHWESTERN
BEAVERHEAD COUNTY THROUGH NOON MDT WEDNESDAY DUE TO RUNOFF FROM
MELTING SNOW. RIVERS...STREAMS...AND CREEKS ALONG THE ROCKY
MOUNTAIN FRONT SOUTH INTO SOUTHWEST MONTANA MAY ALSO RUN NEAR TO
SLIGHTLY ABOVE FLOOD STAGE THROUGH TONIGHT DUE TO SNOW
MELT...WHICH MAY CAUSE VERY LOCALIZED LOW LAND FLOODING.

OTHERWISE...A COLD FRONT WILL MOVE ACROSS SOUTHWEST AND NORTH
CENTRAL MONTANA THIS EVENING. THUNDERSTORMS ARE EXPECTED TO
DEVELOP OVER THE CONTINENTAL DIVIDE AND SOUTHWEST MONTANA AND
MOVE NORTHEAST ACROSS THE AREA TONIGHT. SOME THUNDERSTORMS WILL
BE STRONG WITH GUSTY WINDS...AND POSSIBLY SMALL HAIL. AN ISOLATED
SEVERE THUNDERSTORM WITH DAMAGING WINDS WILL ALSO BE POSSIBLE.

.DAYS TWO THROUGH SEVEN...WEDNESDAY THROUGH MONDAY

PERIODS OF RAIN ARE EXPECTED THROUGH THE WEEKEND AND LOCAL FLOODING
IS POSSIBLE. THEREFORE...A FLOOD WATCH IS IN EFFECT FROM WEDNESDAY
EVENING THROUGH SUNDAY AFTERNOON FOR MOST OF SOUTHWEST AND NORTH
CENTRAL MONTANA...EXCEPT FOR HILL AND BLAINE COUNTIES. FLOOD PRONE
AREAS WILL CONTINUE TO BE MONITORED THROUGH THIS PERIOD.

.SPOTTER INFORMATION STATEMENT...

WEATHER SPOTTERS ARE ENCOURAGED TO REPORT SIGNIFICANT WEATHER
CONDITIONS ACCORDING TO STANDARD OPERATING PROCEDURES.