Area Forecast Discussion
Issued by NWS Grand Junction, CO

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000
FXUS65 KGJT 211653
AFDGJT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GRAND JUNCTION CO
1053 AM MDT THU AUG 21 2014

.UPDATE...
ISSUED AT 1049 AM MDT THU AUG 21 2014

MID AND HIGH LEVEL CLOUDS DRAW FROM TROPICAL STORM LOWELL WILL
CONTINUE SPREADING NORTH ACROSS THE AREA TODAY. HAVE UPDATED THE
FORECAST FOR THE INCREASED CLOUD COVER.

&&

.SHORT TERM...(TODAY THROUGH FRIDAY)
ISSUED AT 420 AM MDT THU AUG 21 2014

A WEAK SHORTWAVE CONTINUED TO SLIDE SLOWLY EASTWARD ACROSS NORTHERN
COLORADO EARLY THIS MORNING GENERATING SHOWERS IN THE VICINITY OF
KCAG AND KEEO. EXTRAPOLATED MOVEMENT OF THIS FEATURE CONSISTENT
WITH MODELS WHICH INDICATED THAT THIS ACTIVITY WILL BE EXITING
EAST OF THE DIVIDE AROUND SUNRISE.

MODELS SUGGESTED A BIT OF A DOWN DAY WITH REGARD TO SHOWERS AND
THUNDERSTORMS. THE PRECIPITABLE WATER VALUE IN THE 00/THU KGJT
SOUNDING HAD FALLEN TO 0.71 INCHES WHILE KSLC AND KFGZ WERE BOTH
AROUND 0.1 INCHES LESS. CONSEQUENTLY...APPEARED DRIER FORECAST
ON TRACK AND THEREFORE BACKED POP VALUES DOWN SOMEWHAT. IN THE
ABSENCE OF SIGNIFICANT FORCING...EXPECT MOIST CONVECTION WILL BE
CONFINED TO THE MOUNTAINS...ESPECIALLY OVER CENTRAL COLORADO AND
NORTHEAST UTAH. STORMS WILL BE LESS LIKELY TO PRODUCE HEAVY RAIN
AND THE FLASH FLOOD THREAT MARGINAL AS A RESULT.

TEMPERATURES SHOULD REBOUND WITH INCREASED SUNSHINE AND FEWER
SHOWERS. THEREFORE...BOOSTED HIGHS A FEW DEGREES OVER THE BIAS
CORRECTED VALUES TO BRING MAX TEMPERATURES TO NEAR GUIDANCE
LEVELS.

THE GFS IS COMING MORE IN LINE WITH THE SLOWER NAM/EC SOLUTIONS
CONCERNING MOVEMENT OF THE SOUTHERN CALIFORNIA LOW...SCHEDULED
TO TRACK ACROSS THE AREA ON FRIDAY. THIS LOW GETS `KICKED` BY A
LOW PRESSURE SYSTEM DROPPING INTO THE PACIFIC NORTHWEST...BUT MORE
ABOUT THE NORTHERN SYSTEM IN THE LONG TERM PERIOD. MODELS SHOW
BETTER AGREEMENT WITH THE CENTER OF THE SOUTHERN LOW OVER
SOUTHEAST AZ MIDNIGHT TONIGHT...THEN MOVING NORTHEAST TO THE FOUR
CORNERS FRI AFTERNOON. IN ADDITION TO THIS DISTURBANCE...SATELLITE
IMAGES AND MODELS INDICATE MOISTURE ASSOCIATED WITH TROPICAL STORM
LOWELL HAS BEEN PICKED UP BY THE SOUTHWEST FLOW AHEAD OF THE LOW.
WHILE I ASSUME LOWELL`S TAP IS MAINLY MID TO HIGH LEVEL...ITS NOSE
MADE IT INTO WESTERN ARIZONA LAST EVENING WHERE IT APPEARED TO
HAVE ENHANCED CONVECTION. NOT SURE HOW MUCH OF LOWELL`S MOISTURE
WILL WILL MAKE IT INTO OUR AREA...BUT FEEL THAT THE MODELS MAY BE
UNDERESTIMATING THE AMOUNT OF MOISTURE EXPECTED OVER OUR FORECAST
AREA BY FRIDAY AFTERNOON. MODEL PWATS GENERALLY DO NOT EXCEED ONE
INCH. IN ANY CASE GOOD INSTABILITY WILL ACCOMPANY THE LOW...AND
WITH THE FAVORED AFTERNOON TIMING WILL ENHANCE THE DAYTIME HEATING
FOR A GOOD UPTICK IN SHOWERS AND THUNDERSTORMS. THIS ACTIVITY
SHOULD BEGIN TO DEVELOP OVER THE SOUTH LATE TONIGHT...WITH THE
SOUTHERN HALF FAVORED THROUGH THE MORNING. CONVECTION WILL SPREAD
NORTHWARD...WITH GOOD COVERAGE AREA-WIDE BY THE AFTERNOON. LOCALLY
HEAVY RAIN WILL BE EXPECTED.

.LONG TERM...(FRIDAY NIGHT THROUGH WEDNESDAY)
ISSUED AT 420 AM MDT THU AUG 21 2014

FRIDAY EVENING WILL REMAIN QUITE CONVECTIVE...WITH ACTIVITY OVER
THE NORTH POSSIBLY PEAKING IN THE MID EVENING. THEN BEYOND
MIDNIGHT SHOWERS AND THUNDERSTORMS WILL DIMINISH. THE NORTHWEST
LOW WILL MOVE INTO THE NORTHERN GREAT BASIN FRIDAY NIGHT...WITH A
SHORTWAVE ROTATING THROUGH THE INTERMOUNTAIN WEST ON SAT. THIS
DISTURBANCE WILL BRUSH OUR FORECAST AREA ADDING ITS SYNOPTIC
INSTABILITY TO THE DIURNAL EFFECTS. IN ADDITION THE UPPER JET WILL
ADD SUPPORT AS IT NOSES INTO THE NORTHWEST CORNER SAT
MORNING...THEN ACROSS THE NORTH THROUGH THE DAY. MODELS SHOW JUST
A TAD LESS MOISTURE ON SAT...BUT ENERGY FROM THE UPPER DISTURBANCE
SHOULD MAKE FOR ANOTHER ACTIVE DAY. EXPECT THE NORTH TO BE
FAVORED.

TIMING OF THE SHORTWAVE MOVING THROUGH THE BASE OF THE BROAD
SHALLOW AMPLITUDE LONGWAVE TROUGH OVER THE WESTERN US VERY
SIMILAR BETWEEN THE EC AND GFS SATURDAY NIGHT. THIS FEATURE SHOULD
KEEP SHOWERS AND THUNDERSTORMS GOING THROUGH MIDNIGHT OVER MUCH OF
WESTERN COLORADO. SUBSIDENCE...DRYING...AND DIURNAL COOLING IN THE
WAKE OF THE SHORTWAVE WILL BRING MOIST CONVECTION TO END SOON
AFTER MIDNIGHT. ENOUGH RESIDUAL MOISTURE TO FUEL AFTERNOON/EVENING
THUNDERSTORMS SUNDAY AFTERNOON AND EVENING THOUGH COVERAGE WILL BE
LIMITED.

SIGNIFICANT DIFFERENCES BEGIN TO APPEAR BETWEEN THE GFS AND EC AT
THE BEGINNING OF THE WEEK. BOTH INDICATED DEEPENING OF THE
LONGWAVE TROUGH WEST OF THE FORECAST AREA HOWEVER...THE ECMWF
CARVES A MUCH DEEPER POSITIVELY TILTED TROUGH OVER THE PACIFIC
NORTHWEST. IN CONTRAST...THE GFS TROUGH WAS SHALLOWER AND LESS
SHARP WITH THE AXIS EXTENDING SOUTH-SOUTHWESTWARD ACROSS THE GREAT
BASIN. THIS DOESN/T APPEAR TO HAVE MUCH IMPACT MONDAY WITH BOTH
MODELS INDICATING DIURNALLY DRIVEN SHOWERS AND THUNDERSTORMS...
MAINLY OVER THE HIGHER TERRAIN.

FORECAST CONFIDENCE FALLS OFF CONSIDERABLY MOVING INTO THE MIDDLE
OF THE WEEK AS THE ECMWF CLOSES OFF A LOW OVER NORTHERN
CALIFORNIA WHILE THE GFS SWEEPS THE TROUGH FROM THE GREAT BASIN TO
THE HIGH PLAINS BY TUESDAY EVENING. THE ECMWF THEN SLOWLY MOVES
THE LOW ACROSS THE NORTHERN INTERMOUNTAIN REGION THROUGH MIDWEEK.
AS A RESULT...THE EC INDICATED A SIGNIFICANT MONSOONAL BURST
TUESDAY AND WEDNESDAY WHILE THE GFS CARRIES A MORE CONTINENTAL
AIRMASS ACROSS THE NORTH MONDAY NIGHT AND TUESDAY WITH DRY
NORTHWEST FLOW SETTING UP ON WEDNESDAY. GIVEN SIGNIFICANT MODEL
DIFFERENCES WAS FORCED TO WALK THE LINE BETWEEN THESE SOLUTIONS
PRODUCED FROM THE BLENDED MODEL OUTPUT. THIS IMPACTS THE
TEMPERATURE FORECAST AS WELL AND WILL LARGELY CONFINE HIGHS AND
LOWS TO WITHIN A FEW DEGRESS OF PERSISTENCE FOR NOW.

&&

.AVIATION...(FOR THE 18Z TAFS THROUGH 18Z FRIDAY AFTERNOON)
ISSUED AT 1049 AM MDT THU AUG 21 2014

DRIER AIR AT LOWER LEVELS WILL CONTINUE TO MOVE INTO THE FORECAST
AREA THE REST OF TODAY AND TONIGHT...LIMITING THUNDERSTORM
DEVELOPMENT TO MAINLY THE CENTRAL COLORADO MOUNTAINS AND EASTERN
UINTA MOUNTAINS. AS A RESULT...EXCEPT FOR GUSTY OUTFLOW WINDS...
ITS UNLIKELY THAT TAF SITES WILL EXPERIENCE THUNDERSTORMS DIRECTLY
OVER THEIR AIRPORT COMPLEXES.

STORMS WILL LARGELY DISSIPATE BY MIDNIGHT. HOWEVER...A SURGE OF
MOISTURE...SOME OF WHICH ORIGINATES FROM TROPICAL STORM LOWELL...
WILL SWEEP INTO THE FOUR CORNERS REGION LATE TONIGHT GENERATING
NOCTURNAL SHOWERS AND THUNDERSTORMS. THERE IS A SLIGHT CHANCE THIS
ACTIVITY COULD IMPACT KTEX OR KDRO BEFORE 12Z/FRI.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...
CO...NONE.
UT...NONE.
&&

$$

UPDATE...CC
SHORT TERM...NL/EH
LONG TERM...EH/NL
AVIATION...CC



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