Flash Flood in Washington, August 4, 2010

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SPOKANE WA
220 AM PDT WED AUG 4 2010

.SYNOPSIS...
WARM TEMPERATURES...SHOWERS AND THUNDERSTORMS CAN BE EXPECTED ACROSS
THE FORECAST AREA THROUGH FRIDAY. SATURDAY WILL SEE A BREAK IN
THE WEATHER BEFORE A PACIFIC FRONT BRINGS COOLER TEMPERATURES AND
A THREAT OF SHOWERS FOR SUNDAY INTO NEXT WEEK.

&&

.DISCUSSION...
TODAY THROUGH THURSDAY...A MOIST AND UNSTABLE ENVIRONMENT WILL
REMAIN OVER THE FORECAST AREA THROUGH THIS PERIOD AS THE UPPER
LEVEL TROUGH LINGERS OVER THE REGION. AMPLE MOISTURE OVER THE
REGION WILL BRING ANOTHER ROUND OF CONVECTION TODAY AND THURSDAY
ACROSS A WIDE EXPANSE OF THE FORECAST AREA.

FOR TODAY...THE COLD CORE OF THE UPPER LEVEL LOW WILL SHIFT OVER
CENTRAL WASHINGTON BY THIS AFTERNOON. THIS KEEPS THE ATMOSPHERE
OVER THE INLAND NORTHWEST VERY UNSTABLE. PW"S WILL AGAIN PUSH AND
INCH...SO THERE IS PLENTY OF MOISTURE TO WORK WITH. IN ADDITION A
STRONG SHORT WAVE WILL MOVE AROUND THE BODY OF THE LOW AND
INCREASE THE FORCING ACROSS THE EASTERN ZONES BY LATE THIS
AFTERNOON. ROTATION AROUND THE LOW WILL KEEP EASTERLY FLOW INTO
THE CASCADES AGAIN TODAY AND WITH LITTLE SHEAR...WE ARE EXPECTING
MORE SLOW MOVING THUNDERSTORMS WITH LOCALIZED BUT VERY HEAVY RAIN
AGAIN THIS AFTERNOON. AS SUCH A FLASH FLOOD WATCH WAS PUT UP FOR
THIS AFTERNOON AND EVENING FOR OUR WESTERN ZONES. MEANWHILE
SOUTHWESTERLY FLOW ACROSS THE EASTERN ZONES WILL INCREASE LIFT.
MUCAPES ARE ON THE ORDER OF 800-100J/KG AND THE 0-6KM SHEAR
INCREASES TO 20-30KTS. THIS SHOULD SHOULD PROVIDE BETTER
ORGANIZATION OF CONVECTION ACROSS THE AREA AND POPS AND COVERAGE
WERE INCREASED ACCORDINGLY. THERE WILL BE THE POSSIBILITY OF
SEVERE THUNDERSTORMS IN THESE DUE TO WINDS AND LARGE HAIL.
CONVECTION WILL LINGER WELL INTO THE EVENING HOURS. TEMPERATURES A
LITTLE WARMER TODAY ACROSS THE AREA.

FOR THURSDAY THE CLOSED LOW OPENS INTO A WAVE AS A SHORTWAVE
DISTURBANCE DROPS DOWN THE BACK SIDE OF THE TROUGH. THIS WILL
ALLOW HEIGHTS TO INCREASE AND RESULT IN TEMPERATURES WARMING A FEW
MORE DEGREES. THE ATMOSPHERE WILL REMAIN UNSTABLE...ESPECIALLY
ACROSS THE HIGHER TERRAIN OF THE CASCADES...NORTHEAST AND
PANHANDLE MOUNTAINS. THE STEERING FLOW WILL BE OUT OF THE SOUTHWEST
20-30KT AND SLOW MOVING THUNDERSTORMS AND FLASH FLOODING SHOULD
NOT BE A THREAT. HOWEVER SHEAR DOES INCREASE ACROSS THE AREA AND
THERE WILL BE THE POTENTIAL FOR ORGANIZED STRONG THUNDERSTORMS
CONTAINING HAIL. /TOBIN

THURSDAY NIGHT THROUGH THE WEEKEND...A RELATIVELY TROUGHY PATTERN
LOOKS TO BE IN STORE ACROSS THE INLAND NORTHWEST. THE CONTINUAL
LACK OF A FOUR CORNERS HIGH THIS SUMMER IS THE LIKELY CULPRIT IN A
VERY OFF KILTER LARGE SCALE PATTERN THAT SEEMS TO BE PERSISTING IN
PERPETUITY ACROSS THE ENTIRE WESTERN UNITED STATES. THE
RAMIFICATIONS OF THIS FOR OUR AREA OVER THE END OF THE WORK WEEK
AND WEEKEND PERIOD SEEM TO BE RATHER CONTINUAL INFLUXES OF PACIFIC
ENERGY AND MOISTURE AT REGULAR INTERVALS. THIS LEADS TO
UNSEASONABLY HIGH POPS...PARTICULARLY OVER THE MOUNTAINS...AND
TEMPERATURES THAT SEEM RELATIVELY STUCK IN THE 80S.

STARTING OUT ON THURSDAY NIGHT...A WEAKLY POSITIVELY TILTED TROUGH
WILL BE SET TO MOVE ON SHORE. THE UPPER JET STREAM ASSOCIATED WITH
THIS SYSTEM TOPS OUT AT A MEAGER 50 KTS OVER THE NORTHERN
MOUNTAINS OF WASHINGTON BY FRIDAY MORNING. THE RIGHT ENTRANCE
REGION FROM THIS WEAK JET STREAK COUPLED WITH EVEN WEAKER
DIFFERENTIAL PVA DRIVE UPWARD MOTION FIELD IS GENERALLY RELEGATED
TO THE EASTERN THIRD OF THE AREA FROM LATE THURSDAY NIGHT THROUGH
FRIDAY. WHILE THE TROUGH IS VERY SLOW TO MOVE THROUGH THE AREA
BECAUSE OF ITS EXCEEDING WEAK POSITIVE TILT...INSTABILITY POOLING
AHEAD OF THE WAVE ON FRIDAY LOOKS TO YIELD AROUND 500 J/KG OF
SURFACE CAPE ACROSS THE IDAHO PANHANDLE. TAKING THE CAPE ALONG
WITH 30 KTS OF 0-6 KM SHEAR DOES SEEM TO LEND TO SOME THUNDER
DEVELOPMENT...HOWEVER A VERY WEAK FOCUSING MECHANISM SEEMS TO
MEAN THAT ANYTHING BEYOND SCATTERED COVERAGE WILL BE TOUGH TO COME
BY.

A VERY BRIEF PERIOD OF SHORT WAVE RIDGING WILL LIKELY DRY THINGS
OUT BY FRIDAY NIGHT...HOWEVER YET ANOTHER MORE WELL DEFINED SHORT
WAVE IS ON ITS HEELS BY SATURDAY. THE 00Z GFS HAS SLOWED FROM ITS
PREVIOUS RUNS WITH THIS SYSTEM SIGNIFICANTLY. IT IS ALSO
SIGNIFICANTLY DEEPER THAN ALL OF ITS 00Z COUNTERPARTS. THE 00Z
ECMWF TRACKS A MUCH WEAKER/FASTER WAVE THROUGH THE AREA LARGELY
DRY ON SATURDAY...WHILE THE 00Z CANADIAN GLOBAL HAS SLOWED EVEN
MORE THAN THE GFS BUT CARRIES A WEAKER SYSTEM. DUE TO THE FACT
THAT NO MODEL MANAGED TO GENERATE ANYTHING OF NOTE OVER THE AREA
ON SATURDAY...THE FORECAST WAS LARGELY DRIED OUT...HOWEVER
SIGNIFICANT FORECAST CHANGES SEEM LIKELY BEYOND SATURDAY AS THE
MODELS SORT OUT HOW THEY DEAL WITH THIS SYSTEM.

BEYOND THE WEEKEND...LARGE SCALE TROUGHING OVER THE PACIFIC
NORTHWEST LOOKS TO EVOLVE INTO A SPRAWLING CLOSED UPPER LOW OFF
THE CALIFORNIA COAST BY THE MIDDLE OF NEXT WEEK. THE 00Z CANADIAN
AND ECMWF BOTH TRENDED IN SIMILAR DIRECTIONS IN DEPICTING THIS
SOLUTION...WITH EVEN THE 00Z GEFS MEAN SQUARELY IN LINE WITH ITS
TWO INTERNATIONAL COUNTERPARTS. THE 00Z GFS RUN WAS A WILD OUTLIER
IN BRINGING TROUGHING ON SHORE...AND IS NOT FAVORED AT THIS TIME.
AS A RESULT...TEMPERATURES ARE LIKELY TO WARM AGAIN THROUGHOUT THE
WEEK NEXT WEEK WITH 850 MB TEMPERATURES AGAIN HEADED TO 25C AND
HIGHER. /FRIES

&&

.AVIATION...
06Z TAFS... JUST A FEW THUNDERSTORMS LEFT AT 06Z OVER THE WATERVILLE
PLATEAU. EXPECT THESE TO SLOWLY END BTWN 06Z AND 10Z LEAVING ONLY
RAIN AND SHOWERS ALONG THE EAST SLOPES. NO THUNDER OR RAIN EXPECTED
AT THE TAFS SITES THRU 18Z. STARTING AT 18Z CONVECTIVE ACTIVITY
EXPECTED TO DEVELOP ALONG EXISTING INSTABILITY AXIS FROM THE METHOW
VALLEY TO NEAR ST MARIES ID. THUNDER... HEAVY RAIN AND POSSIBLY
SMALL HAIL EXPECTED WITH THESE STORMS BTWN 21Z AND 03Z. JL

&&

.PRELIMINARY POINT TEMPS/POPS...
SPOKANE        88  61  91  63  88  60 /  50  50  20  20  20  20
COEUR D'ALENE  86  57  90  58  87  55 /  50  50  20  30  30  20
PULLMAN        87  53  92  56  87  51 /  30  20  20  40  40  30
LEWISTON       95  65  99  66  95  61 /  10  10  20  30  30  20
COLVILLE       90  55  93  56  91  53 /  60  60  30  30  20  20
SANDPOINT      83  53  88  54  85  52 /  40  40  30  30  20  20
KELLOGG        84  56  87  58  84  55 /  60  60  30  40  40  30
MOSES LAKE     94  61  96  61  93  58 /  40  20  10  10  10  10
WENATCHEE      93  66  95  65  91  62 /  50  40  20  10  10   0
OMAK           92  61  94  59  93  57 /  60  60  50  50  10  10

&&

.OTX WATCHES/WARNINGS/ADVISORIES...
ID...NONE.
WA...FLASH FLOOD WATCH FROM 11 AM PDT THIS MORNING THROUGH THIS
     EVENING EAST SLOPES NORTHERN CASCADES...OKANOGAN VALLEY...
     WATERVILLE PLATEAU...WENATCHEE AREA.

&&