Area Forecast Discussion
Issued by NWS Phoenix, AZ

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000
FXUS65 KPSR 221112
AFDPSR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PHOENIX AZ
410 AM MST MON SEP 22 2014

.SYNOPSIS...
SLIGHT CHANCES FOR SHOWERS AND THUNDERSTORMS WILL BE CONFINED TO THE
HIGHER MOUNTAINS OF SOUTH-CENTRAL ARIZONA THROUGH THURSDAY.
TEMPERATURES WILL TREND UPWARD A FEW DEGREES BY WEDNESDAY. AN
APPROACHING PACIFIC WEATHER SYSTEM LIKELY AFFECTING THE DESERT
SOUTHWEST ON FRIDAY THROUGH SUNDAY WILL LEAD TO A RETURN OF STORM
CHANCES ALONG WITH COOLER TEMPERATURES.

&&

.DISCUSSION...
TODAY THROUGH WEDNESDAY...
THE LOW PRESSURE SYSTEM THAT SPREAD DRIER AIR ACROSS MUCH OF THE
REGION YESTERDAY WAS CENTERED NEAR THE UTAH/IDAHO BORDER EARLY THIS
MORNING. THE EASTWARD ADVANCE OF THE DRIER AIR HAS SLOWED ITS
EASTWARD PROGRESS PER IR IMAGERY SHOWING LINGERING CLOUDINESS OVER
THE EASTERN THIRD OF ARIZONA...WITH EVEN SOME LINGERING CONVECTION
NEAR THE NEW MEXICO BORDER. PRECIPITABLE WATER TREND...PER BLENDED
TPW IMAGERY...SHOWS THIS AS WELL. THE LOW IS UNDERCUTTING A RIDGE
CENTERED OVER THE CANADIAN ROCKIES. MEANWHILE...A COLD FRONT WAS
PUSHING SOUTHWARD ACROSS THE GREAT PLAINS AND BEGINNING TO PUSH UP
AGAINST THE SOUTHERN ROCKIES. OVER THE NORTH PACIFIC THERE IS
SOMETHING OF A REX PATTERN WITH TROUGHING...AND EMBEDDED LOWS...OVER
THE NORTHEAST PACIFIC AND A NARROW HIGH AMPLITUDE RIDGE UPSTREAM OF
IT AND JET ENERGY UNDERCUTTING THE RIDGE.

MODELS SHOW THE UPPER LOW TO OUR NORTH CONTINUING TO SHIFT EASTWARD
WITH A RIDGE AXIS BUILDING OVER THE INTERIOR WESTERN STATES. THEY
ALSO AGREE THAT THERE WILL BE ONLY MODEST ADDITIONAL DRYING OVER THE
FORECAST AREA AS THE BACKDOOR FRONT WILL TEND TO COUNTERACT DRIER
WESTERLY WINDS. THIS LEAVES EAST-CENTRAL AND SOUTHEAST ARIZONA WITH
PRETTY DECENT MOISTURE. THE RESULT FOR OUR FORECAST AREA WILL BE
JUST ENOUGH MOISTURE LEFT OVER OUR HIGHER TERRAIN ZONE TO HANG ON TO
SLIGHT CHANCE POPS THERE THROUGH WEDNESDAY. TEMPS NUDGE UPWARD AT
LEAST A FEW DEGREES ABOVE NORMAL BY WEDNESDAY.

THURSDAY THROUGH SUNDAY...
ON THURSDAY...AMPLIFICATION OF THE FLOW PATTERN LEADS TO A LARGE
PACIFIC TROUGH BEGINNING TO SHIFT SOUTHEASTWARD AND THE RIDGE OVER
US SHIFTING NORTHEASTWARD. IN THE PROCESS...MOISTURE OVER SOUTHEAST
ARIZONA SHIFTS WESTWARD AND SOME DRIER AIR FROM THE NORTH MIXES IN.
STILL ENOUGH MOISTURE ON BALANCE TO MAINTAIN THE SLIGHT CHANCES OVER
ZONE 24 BUT NOT ENOUGH YET TO INTRODUCE POPS TO THE LOWER DESERTS.
BY FRIDAY...GFS AND ECMWF SHOW MORE DISTINCT DIFFERENCES WITH THE
SLOWLY ADVANCING TROUGH. THE GFS IS FASTER WITH THE INLAND MOVEMENT
WHILE THE ECMWF IS DEEPER AND SLOWER. THIS HAS BEEN THE CASE FOR
MULTIPLE RUNS. OF NOTE HOWEVER...THEY BOTH DEPICT DEEPER MOISTURE
BEING PULLED NORTHWARD THOUGH THE ECMWF IS MORE GENEROUS...AS IS
OFTEN THE CASE. THUS FRIDAY LOOKS TO BE THE BEGINNING OF A WINDOW OF
OPPORTUNITY FOR A MAJORITY OF OUR FORECAST AREA TO HAVE AT LEAST
SOME SORT OF CHANCE FOR SHOWERS/THUNDERSTORMS. BECAUSE THE GFS IS
FASTER AND NOT AS DEEP WITH THE LOW/TROUGH...IT DEPICTS A SMALLER
WINDOW OF OPPORTUNITY...ENDING SUNDAY MORNING OVER OUR EASTERNMOST
AREAS...AND KEEPS QPF PRETTY MUCH EAST OF THE LOWER COLORADO RIVER
VALLEY. THE ECMWF DEPICTS QPF INTO SOUTHEAST CALIFORNIA AND ENDS IT
SUNDAY NIGHT. BOTH TRACK THE BRUNT OF THE SYSTEM TO OUR NORTH
KEEPING THE JET AXIS JUST TO OUR NORTH. ACCORDINGLY...WE ONLY GET
BRUSHED WITH QG FORCING BUT MAY HAVE SOME UPPER LEVEL DIFFLUENCE TO
WORK WITH IN ADDITION TO CAPE. STRONGER FLOW ALOFT WILL BRING SOME
VERTICAL WIND SHEAR INTO PLAY AS WELL. BOTH THE GFS AND ECMWF HAVE
SUPPORT FROM THEIR RESPECTIVE ENSEMBLES IN TERMS OF HOW THEY HANDLE
THE TROUGH. THE 00Z GFS APPEARS TO HAVE TRENDED SLIGHTLY SLOWER FROM
THE 12Z RUN. STILL A LOT OF UNCERTAINTY WITH HOW THE TROUGH WILL
PLAY OUT AND WHAT THE MOISTURE AND CAPE WILL BE LIKE.
HOWEVER...ENOUGH TO GO ON IN ORDER TO MAKE UPWARD ADJUSTMENTS TO THE
POPS. TEMPS WILL TREND DOWN AS WELL.


&&

.AVIATION...
SOUTH-CENTRAL ARIZONA INCLUDING KPHX...KIWA...AND KSDL...AND
SOUTHWEST ARIZONA AND SOUTHEAST CALIFORNIA INCLUDING KIPL AND KBLH...

DRY CONDITIONS AND CLEAR TO MOSTLY CLEAR SKIES WILL CONTINUE THROUGH
MONDAY NIGHT. EXPECTING LIGHT WINDS AND FAIRLY TYPICAL DIURNAL WIND
SHIFTS TODAY WITH THE EXCEPTION OF LINGERING WESTERLY WINDS ACROSS
ALL BUT EASTERN PORTIONS OF THE PHOENIX AREA THROUGH AROUND DAYBREAK
THIS MORNING.

AVIATION DISCUSSION NOT UPDATED FOR AMENDED TAFS.

&&

.FIRE WEATHER...
WEDNESDAY THROUGH SUNDAY...
MOISTURE RETREAT DUE TO WESTERLY FLOW WILL KEEP STORM CHANCES
MINIMAL THROUGH THURSDAY AND LIMITED TO THE HIGHER MOUNTAINS OF
SOUTH-CENTRAL ARIZONA. MEANWHILE...TEMPERATURES NUDGE UPWARD A FEW
DEGREES ABOVE NORMAL. HUMIDITIES ON THE LOWER DESERTS DROP BELOW 15
PERCENT THOUGH MAX HUMIDITIES REMAIN FAIR. NO STRONG WINDS ARE
EXPECTED DURING THIS TIME FRAME. ON FRIDAY...A PACIFIC WEATHER
SYSTEM WILL PULL MOISTURE NORTHWARD OVER THE REGION FOR A RETURN OF
STORM CHANCES INTO NEXT WEEKEND...MAINLY EAST OF THE LOWER COLORADO
RIVER VALLEY. IN ADDITION...COOLER TEMPERATURES...HIGHER
HUMIDITIES...AND BREEZY TO LOCALLY WINDY CONDITIONS CAN BE EXPECTED
AS WELL.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...

AZ...NONE.

CA...NONE.

&&

$$

VISIT US ON FACEBOOK...TWITTER...AND AT WEATHER.GOV/PHOENIX

DISCUSSION...AJ
AVIATION...VASQUEZ
FIRE WEATHER...MEYERS/AJ






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