Flash Flood in Utah, September 19, 2004

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GRAND JUNCTION CO
214 PM MDT SUN SEP 19 2004

.SHORT TERM (TONIGHT THROUGH MONDAY NIGHT)...STRONG PACIFIC TROUGH
WITH UNSEASONABLY COLD AIR CONTINUES TO BARREL DOWN ACROSS THE
PACIFIC NORTHWEST AND GREAT BASIN. ELKO NV (ELEV 5067 FT) REPORTED
-SN AT 16Z TODAY. TROPICAL MOIST SURGE ACROSS THE AREA TODAY
PRODUCED IMPRESSIVE RAINFALL AMOUNTS THROUGH 19Z...0.72 REPORTED AT
KGJT...AND SEVERAL SITES IN THE SW SAN JUAN MTS HAVE >2 INCHES OF
RAIN IN THE LAST 24 HOURS. EXPECTING WIDESPREAD HEAVY RAIN TO
DIMINISH THIS EVENING SO WILL LET FLASH FLOOD WATCH EXPIRE AT
600 PM MDT TONIGHT. SOUTHEAST UTAH WILL BE REMOVED FROM THE WATCH AT
THE 330 PM ISSUANCE.

LATEST WV SATL IMAGERY SHOWING WELL-DEFINED BACK EDGE OF DRIER MID
LEVEL AIR INTRUDING EASTERN UTAH. EXPECT DRY SLOT TO WORK ACROSS
WESTERN COLORADO BY SUNSET THIS EVENING. PRECIP WILL END FROM WEST TO
EAST AS DYNAMICS WEAKEN AND MOISTURE DECREASES. OVERNIGHT...LACK OF
STRONG MID LEVEL FORCING AND DEEP MOISTURE WILL MINIMIZE PRECIP
CHANCES HOWEVER...100+KT UPR JET OVERTOP THE AREA COUPLED WITH WEAK
EJECTING SHORTWAVES TRAVERSING THE AREA...ISOLATED TO SCATTERED
SHOWERS LIKELY TO PERSIST. SOUTHWEST H7 FLOW 20-35KTS WILL ADD
OROGRAPHIC LIFT IN THE MOUNTAINS.

12Z/ETA SHOWS COLD FRONT EDGING INTO EXTREME NE UTAH AND NW COLORADO
LATE TONIGHT. THIS COULD BE MORE OF A PRE-FRONTAL TROUGH BUT THE
700MB 0C ISOTHERM GETS VERY CLOSE TO THE EASTERN UINTA MTNS. SNOW
LEVELS WILL LOWER TO AROUND 8500 FT...BUT ONLY EXPECTING SCATTERED
PRECIP...SO ANY SNOW ACCUMULATION WILL REMAIN LESS THAN 2 INCHES.

MONDAY AND MONDAY NIGHT...TROUGH WILL SLOWLY MIGRATE SOUTHEAST
MONDAY BUT REMAIN WELL WEST OF THE FORECAST AREA. FORECAST AREA
WILL REMAIN IN MODERATE SOUTHWEST FLOW BUT WE HAVE NOW LOST THE GOOD
MOISTURE TAP...SO EXPECT CONVECTION TO BE SCATTERED AT BEST. ETA
HINTS AT A SURFACE BOUNDARY/COLD FRONT BEING DRAPED FROM COLO/WYOMING
BORDER DOWN ACROSS CENTRAL UTAH. GOOD UPR LVL JET DYNAMICS IN PLACE
WITH MODERATE 0-6KM SHEAR WILL SUPPORT SCT STRONG THUNDERSTORMS...
ESPECIALLY ALONG AND AHEAD OF THIS LOW LVL BOUNDARY. COOLER AIR WILL
REMAIN OVER EXTREME NE UTAH/NW COLO WHERE A MORE STRATIFORM AIR MASS
WILL EXIST. BY MONDAY NIGHT...H5 LOW SHOWS SIGNS OF CLOSING OFF OVER
WESTERN UTAH. COLD FRONTAL BOUNDARY WILL EXTEND FROM THE TAVAPUTS/
BOOKCLIFFS IN EAST-CENTRAL UTAH TO STEAMBOAT SPRINGS. SOUTH OF THIS
BOUNDARY...EJECTING MID LVL WAVES AND UPR JET SUPPORT WITHIN MOIST/
UNSTABLE AIR MASS WILL CONTINUE TO PRODUCE SCATTERED CONVECTION.
NORTH OF THE FRONT...A COOLER/MORE STRATIFIED AIR MASS WILL EXIST
WITH OVERUNNING MOISTURE CREATING AREAS OF RAIN/SNOW. SNOW LEVELS
WILL LOWER TO AROUND 8K FT...WITH SOME ACCUMULATIONS POSSIBLE. NOT
LOOKING AT HEAVY PRECIP EVENT WITH TROUGH STILL OVER THE GREAT
BASIN...SO WILL NOT GO WITH ANY WINTER HIGHLIGHTS.

.LONG TERM (TUESDAY THROUGH SUNDAY)...
AN UPPER TROUGH WILL STILL BE OVER THE FORECAST AREA TUESDAY. JET
DYNAMICS PROGGED TO BE LINGERING OVER WESTERN COLORADO AND THE MID
LEVEL COLD POOL WILL KEEP FAIRLY STEEP LAPSE RATES IN PLACE FOR A
CONTINUED CHANCE OF PRECIPITATION AND HIGHER ELEVATION SNOW. COULD BE
LOOKING AT SNOW LEVELS NEAR 7K FEET ACROSS THE NORTH AND CLOSER TO 9-10K
FEET ACROSS THE SOUTH WITH LIGHT ACCUMULATIONS. COLD FRONT MAY BE HARD
TO IDENTIFY AT THIS POINT...BUT IT SHOULD BE THROUGH THE FORECAST AREA
BY LATE TUESDAY.

SOME DIVERGENCE IN MODEL SOLUTIONS ON THE EASTWARD PROGRESSION OF
THE TROUGH TUESDAY NIGHT AND WEDNESDAY. WILL LEAN TOWARD A MORE
PROGRESSIVE SOLUTION WHICH WOULD TAKE THE TROUGH AXIS EAST OF THE
FORECAST AREA BY WEDNESDAY EVENING WITH NORTHWEST FLOW ADVECTING
SOME DRIER AIR INTO THE AREA. WILL TAPER OFF POPS FROM WEST TO EAST
TO ACCOUNT FOR THIS DRYING AND MORE STABLE CONDITIONS. COLD
TEMPERATURES TUESDAY NIGHT WITH LOWS IN THE 30S TO AROUND 40 FOR THE
LOWER VALLEYS. ENOUGH LOW LEVEL MOISTURE AND CLOUDS SHOULD BE AROUND
TO PREVENT A WIDESPREAD FREEZE FOR THE LOWER VALLEYS. DAYTIME HIGHS
TUESDAY AND WEDNESDAY WILL BE SEVERAL DEGREES BELOW NORMAL.

A WARMING TREND WITH DRY CONDITIONS IS EXPECTED THURSDAY THROUGH
SATURDAY AS A RIDGE BUILDS OVER THE AREA. WILL LEVEL OFF TEMPERATURES
SUNDAY DUE TO A WEAK TROUGH PROGGED TO MOVE ACROSS THE GREAT BASIN
AND LOWER HEIGHTS ACROSS THE AREA. TAL

&&

.GJT WATCHES/WARNINGS/ADVISORIES...
.CO...FLASH FLOOD WATCH THROUGH 600 PM MDT TONIGHT FOR SOUTHWEST
      COLORADO...INCLUDING ZONES 19...21...22 AND 23.
.UT...NONE.

&&

DJN/TAL

HAZARDOUS WEATHER OUTLOOK...UPDATED
NATIONAL WEATHER SERVICE GRAND JUNCTION CO
802 AM MDT SUN SEP 19 2004

UPDATED TO INCLUDE SEVERE THUNDERSTORM THREAT

COZ001>014-017-018-020-UTZ023>025-027-201200-
ARCHES/GRAND FLAT-CENTRAL COLORADO RIVER BASIN-
CENTRAL GUNNISON AND UNCOMPAHGRE RIVER BASIN-
CENTRAL YAMPA RIVER BASIN-DEBEQUE TO SILT CORRIDOR-
EASTERN UINTA BASIN-EASTERN UINTA MOUNTAINS-
ELKHEAD AND PARK MOUNTAINS-FLAT TOPS-
GORE AND ELK MOUNTAINS/CENTRAL MOUNTAINS-GRAND AND BATTLEMENT MESAS-
GRAND VALLEY-LOWER YAMPA RIVER BASIN-NORTHWEST SAN JUAN MOUNTAINS-
PARADOX VALLEY/LOWER DOLORES RIVER-ROAN AND TAVAPUTS PLATEAUS-
TAVAPUTS PLATEAU-UNCOMPAHGRE PLATEAU AND DALLAS DIVIDE-
UPPER GUNNISON RIVER VALLEY-UPPER YAMPA RIVER BASIN-
WEST ELK AND SAWATCH MOUNTAINS-
802 AM MDT SUN SEP 19 2004

THIS HAZARDOUS WEATHER OUTLOOK IS FOR EAST-CENTRAL AND NORTHEAST
UTAH...AND WEST-CENTRAL AND NORTHWEST COLORADO.

.DAY ONE...TODAY AND TONIGHT...

WIDESPREAD SHOWERS AND THUNDERSTORMS WILL OCCUR TODAY. ISOLATED
SEVERE THUNDERSTORMS ARE POSSIBLE. THE MAIN THREAT FROM THE STORMS
WILL BE WIND GUSTS UP TO 60 MPH...AND HAIL UP TO 3/4 INCH IN DIAMETER.
LOCALLY HEAVY RAIN IS POSSIBLE WITH SOME STORMS. SHOWERS AND
THUNDERSTORMS ARE EXPECTED TO DECREASE THIS EVENING.

.DAYS TWO THROUGH SEVEN...MONDAY THROUGH SATURDAY...

THE CHANCE OF THUNDERSTORMS WILL CONTINUE ON MONDAY.  WITH COLDER
AIR MOVING INTO NORTHEAST UTAH AND NORTHWEST COLORADO ON MONDAY...A
CHANCE OF MOUNTAIN SNOW WILL BE POSSIBLE BY MONDAY NIGHT AND
CONTINUING THROUGH WEDNESDAY.  THE FIRST SIGNIFICANT SNOWFALL OF THE
SEASON MAY OCCUR MONDAY NIGHT AND TUESDAY.

.SPOTTER INFORMATION STATEMENT...

SPOTTER ACTIVATION MAY BE REQUIRED TODAY.

COZ019-021>023-UTZ022-028-029-201200-
ANIMAS RIVER BASIN-CANYONLANDS/NATURAL BRIDGES-
FOUR CORNERS/UPPER DOLORES RIVER-LA SAL AND ABAJO MOUNTAINS-
SAN JUAN RIVER BASIN-SOUTHEAST UTAH-SOUTHWEST SAN JUAN MOUNTAINS-
802 AM MDT SUN SEP 19 2004

THIS HAZARDOUS WEATHER OUTLOOK IS FOR SOUTHEAST UTAH AND
SOUTHWEST COLORADO.

...FLASH FLOOD WATCH THROUGH 600 PM TODAY...

.DAY ONE...TODAY AND TONIGHT...

WIDESPREAD SHOWERS AND THUNDERSTORMS WILL OCCUR TODAY. HEAVY RAIN IS
POSSIBLE ALONG WITH ISOLATED SEVERE THUNDERSTORMS. THE POTENTIAL
EXISTS FOR FLASH FLOODING. WIND GUSTS UP TO 65 MPH AND HAIL UP TO 3/4
INCH IN DIAMETER IS POSSIBLE WITH SOME STORMS.

.DAYS TWO THROUGH SEVEN...MONDAY THROUGH SATURDAY...

THE CHANCE OF THUNDERSTORMS WILL CONTINUE ON MONDAY.  WITH COLDER
AIR SPREADING ACROSS EASTERN UTAH AND WESTERN COLORADO BY MONDAY
NIGHT...A CHANCE OF MOUNTAIN SNOW EXISTS.  THE SNOW LEVEL WILL FALL
BELOW MANY MOUNTAIN PASSES MONDAY NIGHT AND TUESDAY NIGHT.  SOME
ACCUMULATIONS MAY BE POSSIBLE MONDAY NIGHT THROUGH WEDNESDAY...
MAINLY ABOVE 10000 FEET.

.SPOTTER INFORMATION STATEMENT...

SPOTTER ACTIVATION MAY BE REQUIRED TODAY.

TAL