Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 230421
AFDPSR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PHOENIX AZ
921 PM 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...
RADAR SCOPES WENT QUIET FAIRLY EARLY THIS EVENING WITH THE ONLY
STORM ACTIVITY IN OUR FORECAST TODAY OVER THE SOUTHERN GILA COUNTY
HIGHER TERRAIN. WESTERLY WINDS THROUGH MUCH OF THE ATMO COLUMN
PUSHED STORMS TO THE EAST AND TOWARDS A MORE FAVORABLE MOISTURE
SURFACE ALONG THE AZ/NM BORDER. AFTERNOON MIXING DROPPED DEWPOINTS
READINGS INTO THE 30S AND 40S ACROSS SOUTHEAST CA AND WESTERN
AZ...WITH STILL RICH UPPER 50 AND 60 DEGREE READINGS SCATTERED
THROUGHOUT POCKETS OF THE PHX METRO AND SOLIDLY POINTS EAST. WITH
CONTINUED DRY AIR INTRUSION FROM THE WEST AS SEEN FROM WATER VAPOR
IMAGERY AND RAPIDLY CLEARING SKIES...ZEROED OUT THE POP FORECAST FOR
THE REST OF THE EVENING. NO OTHER UPDATES NEEDED THIS
EVENING...DISCUSSION BELOW STILL APPLIES FOR MIDWEEK AND BEYOND.

&&

.PREVIOUS DISCUSSION...
THROUGH THURSDAY...WIDELY SCATTERED THUNDERSTORM ACTIVITY HAS POPPED
UP THIS AFTERNOON ACROSS PORTIONS OF EASTERN ARIZONA...SOME ACTIVITY
EXTENDING INTO WESTERN COLORADO AND FORMING ALONG THE TRAILING AXIS
OF THE UPPER LEVEL TROUGH CURRENTLY MOVING EAST ACROSS THE NORTHERN
TIER OF THE CONUS. THUNDERSTORM ACTIVITY WILL REMAIN RELEGATED TO
EASTERN ARIZONA THIS AFTERNOON...WITH ONLY SLIGHT CHANCES OF
PRECIPITATION OVER SOUTHERN GILA COUNTY INTO THE EVENING HOURS. WITH
THE DRIER WESTERLY FLOW IN PLACE OVER SOUTHEAST CA AND MOST OF
ARIZONA...PRECIPITATION CHANCES WILL BE NIL FOR THE LOWER DESERTS
UNTIL THE END OF THE WEEK.

MODELS CONTINUE TO SUGGEST THAT THE UPPER LEVEL RIDGE AXIS WILL
ESTABLISH AND REMAIN OVER THE INTERMOUNTAIN WEST THROUGH AT LEAST
WEDNESDAY...ALONG WITH SOME MODEST DRYING OF THE ATMOSPHERE THAT
WILL TAKE PLACE. THE ANTICYCLONIC FLOW WILL SUPPORT A FEW LOW-MID
LEVEL MOISTURE SEEPS FROM TIME TO TIME...RECYCLING MOISTURE UNDER
THE HIGH...JUST ENOUGH MOISTURE TO KEEP A DAILY ROUND OF ISOLATED TO
WIDELY SCATTERED THUNDERSTORMS OVER THE HIGHER TERRAIN OF SOUTH
CENTRAL ARIZONA IN THE FORECAST FOR THE NEXT FEW DAYS. DAYTIME
TEMPERATURES WILL ALSO TREND UPWARD TO AROUND 5-7 DEGREES ABOVE
NORMAL THROUGH MID-WEEK...TOPPING OUT IN THE 104 TO 108 DEGREE RANGE
FOR THE LOWER DESERTS...AS THE RIDGE AMPLIFIES AHEAD OF A LARGE
EASTERN PACIFIC TROUGH DROPPING SOUTH OUT OF THE GULF OF ALASKA.
WEDNESDAY AND THURSDAY WILL LIKELY BE OUR WARMEST DAYS THIS WEEK...
AROUND 105 FOR PHOENIX...AND PROBABLY THE LAST OF THE 100 DEGREE
HEAT AND THE HOT TEMPERATURES UNTIL NEXT YEAR. SAY GOODBYE TO
MONSOON 2014.

FRIDAY THROUGH SUNDAY...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
TROPICAL MOISTURE BEING PULLED NORTHWARD...ALTHOUGH THE ECMWF IS
MORE GENEROUS. THUS FRIDAY LOOKS TO BE THE BEGINNING OF A WINDOW OF
OPPORTUNITY FOR A MAJORITY OF OUR FORECAST AREA TO HAVE AT LEAST
SOME CHANCE OF SHOWERS/THUNDERSTORMS INTO THE WEEKEND...WITH THE
BEST TIMING FOR PEAK PRECIPITATION SLATED FOR SATURDAY.

BOTH MODELS CONTINUE TO 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...

CLEAR SKIES AND DRY SFC CONDITIONS EXPECTED THROUGH THE TAF PERIOD
WITH DIURNALLY DRIVEN WINDS REMAINING AOB 10KTS. NO WEATHER IMPACTS
TO AVIATION OPERATIONS EXPECTED.

AVIATION DISCUSSION NOT UPDATED FOR AMENDED TAFS.

&&

.FIRE WEATHER...
THURSDAY THROUGH MONDAY...
WESTERLY FLOW ALOFT TO KEEP MOISTURE LEVELS LIMITED ON
THURSDAY...WITH ONLY A SLIGHT CHANCE FOR STORMS OVER THE HIGHER
TERRAIN NORTH AND EAST OF PHOENIX. MOISTURE LEVELS...AND CHANCES FOR
SHOWERS/THUNDERSTORMS TO INCREASE SOMEWHAT ON FRIDAY...THEN
DRAMATICALLY SO ON SATURDAY AND EARLY SUNDAY AS WINDS ALOFT BECOME
MORE SOUTHERLY AHEAD OF A PACIFIC WEATHER SYSTEM THAT WILL BE
APPROACHING THE AREA FROM THE WEST...PULLING TROPICAL MOISTURE BACK
NORTHWARD INTO THE REGION. WIDESPREAD WETTING RAINS...COOLER
TEMPERATURES...AND BREEZY TO WINDY CONDITIONS ARE POSSIBLE FROM
SATURDAY INTO EARLY SUNDAY AS THE 1ST SFC COLD FRONT OF THE EARLY
FALL SEASON MOVES ACROSS THE REGION. MUCH DRIER AIR TO OVERSPREAD
THE REGION FROM WEST TO EAST FROM LATE SUNDAY INTO MONDAY AS HIGH
PRESSURE BUILD INLAND BEHIND THE FRONT. MINIMUM HUMIDITIES ACROSS
THE LOWER DESERTS...WHICH COULD FALL AS LOW AS 15 PERCENT ON
FRIDAY...ARE EXPECTED TO REMAIN MAINLY AOA 25 PERCENT ON SUNDAY AND
MONDAY...BEFORE FALLING BACK DOWN INTO THE 15-25 PERCENT RANGE ON
MONDAY.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...

AZ...NONE.

CA...NONE.

&&

$$

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

PREVIOUS DISCUSSION...NOLTE
DISCUSSION...MEYERS/AJ
AVIATION....NOLTE
FIRE WEATHER...PERCHA





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