Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 210409
AFDPSR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PHOENIX AZ
909 PM MST WED AUG 20 2014

.SYNOPSIS...
CHANCES OF STORM ACTIVITY INCREASE ON THURSDAY AS AN UPPER LOW
PRESSURE SYSTEM ALONG THE CALIFORNIA COAST MOVES TOWARDS THE DESERT
SOUTHWEST. DRIER AIR SPREADING IN FROM THE WEST WILL REDUCE
THUNDERSTORM ACTIVITY FROM THIS WEEKEND INTO EARLY NEXT WEEK. BELOW
NORMAL TEMPERATURES ARE FORECAST FROM TODAY THROUGH FRIDAY...WARMING
THIS WEEKEND.

&&

.DISCUSSION...
STORM ACTIVITY TODAY HAS BEEN FOCUSED IN A SOMEWHAT FAVORABLE AREA
OF ACCENT TO THE EAST OF A CLOSED LOW CURRENTLY SITUATED NEAR SAN
DIEGO. INITIALLY...STORMS FIRED OVER THE HIGH TERRAIN AREAS OF
SOUTHERN MARICOPA...NORTHERN YUMA...AND LA PAZ COUNTIES WITH THE LA
PAZ STORM ACTIVITY PRODUCING THE MOST ORGANIZED STORMS. EVEN THOUGH
PWATS ARE MUCH LOWER THAN THE PREVIOUS FEW DAYS...SLOW MOVING AND
SOME BACK BUILDING STORMS ALLOWED FOR A FEW AREAS IN LA PAZ COUNTY
TO SEE SOME HEAVY RAINFALL AND MINOR FLASH FLOODING. SHOWERS AND
THUNDERSTORMS CONTINUE TO MOVE NORTHWARD THROUGH CENTRAL LA PAZ AND
NORTHWEST MARICOPA COUNTIES...ON THE NOSE OF STRONG UPPER LEVEL
MOISTURE ADVECTION. MODELS SHOW THIS MOISTURE SURGE QUITE
WELL...MAINLY FOCUSED ABOVE 15K FEET THROUGH 15Z THURSDAY. ONCE THIS
CURRENT ACTIVITY MOVES NORTHWARD OUT OF OUR CWA BY AROUND 06Z...WE
SHOULD HAVE A BREAK IN THE RAIN THROUGH MUCH OF THE OVERNIGHT. MAY
END UP SEEING SOME LIGHT HIGH BASED SHOWERS OR VIRGA DEVELOP
SOMETIME AFTER DAYBREAK THURSDAY ACROSS SOUTHWEST AND SOUTH-CENTRAL
ARIZONA IN RESPONSE TO THE MOISTURE SURGE AND MODERATE ACCENT AHEAD
OF THE UPPER LEVEL LOW. EXTENSIVE MID AND HIGH LEVEL CLOUD BAND WILL
GRADUALLY SHIFT EASTWARD THROUGH ARIZONA DURING THE DAY
THURSDAY...LEAVING QUITE A BIT OF SUNSHINE FOR THE LOWER COLORADO
RIVER VALLEY AND MUCH OF SOUTHWEST ARIZONA BEFORE STORMS REDEVELOP
BY EARLY TO MID AFTERNOON. HAVE MADE SOME MINOR FORECAST ADJUSTMENTS
FOR TONIGHT...LOWERING POPS TO ONLY 10 PERCENT WEST OF PHOENIX.

&&

.PREVIOUS DISCUSSION...
THE UPPER LOW WILL DRIFT TO A FAVORABLE POSITION THURSDAY
MORNING. VORTICITY FORCED ASCENT WILL INCREASE AHEAD OF THE LOW AND
SHOWERS ARE EXPECTED TO DEVELOP ACROSS SE AZ AND SW AZ THURSDAY
MORNING. LAPSE RATES WILL STEEPEN DURING THE AFTERNOON AS THE
MID-LEVEL COLD POOL (500 MB TEMPERATURES AS LOW AS -11C) TRANSLATES
EASTWARD. ADDITIONAL ASCENT ASSOCIATED WITH THE RIGHT REAR QUADRANT
OF AN ANOMALOUSLY STRONG JET STREAK WILL SUPPORT ADDITIONAL
CONVECTION ACROSS CENTRAL AZ...MAINLY DURING THE AFTERNOON AND EARLY
EVENING. SOME STORMS WILL AGAIN HAVE THE POTENTIAL TO PRODUCE HEAVY
RAIN...GUSTY WINDS...SMALL HAIL AND LOCALIZED FLOODING. BULK SHEAR
MAGNITUDES ALSO SUGGEST THE POTENTIAL FOR ORGANIZED STORMS CAPABLE
OF PRODUCING SEVERE STORMS...MAINLY FROM PHOENIX EASTWARD.

FRIDAY...
ON FRIDAY THE LOW TRACKS ACROSS ARIZONA AND WEAKENS IN THE PROCESS
WITH DRIER AIR MOVING IN BEHIND IT. THUS POPS TREND DOWNWARD FRIDAY.
TEMPERATURES WILL BE NEAR OR SOMEWHAT BELOW NORMAL THROUGH FRIDAY.

SATURDAY THROUGH TUESDAY...
MODELS SHOW AN INCREASE IN MOISTURE BEHIND THE DEPARTING LOW WITH
POSSIBLY SOME TANGENTIAL INFLUENCE FROM WHAT IS NOW T.S. LOWELL. GFS
SHOWS MORE BROAD TROUGHING OVER THE WESTERN STATES THAN DOES THE
ECWMF WHICH HAS THE RIDGE AXIS FURTHER A BIT FURTHER NORTH THROUGH
MONDAY. AS A RESULT...ECWMF HAS MORE MOISTURE AROUND. ON TUESDAY AND
WEDNESDAY...BOTH MODELS BRING A NORTHEAST PACIFIC RIDGE
INLAND...ECMWF IS MORE AMPLIFIED. MEANWHILE...BOTH MODELS CONTINUE
TO DEPICT YET ANOTHER TROPICAL STORM OFF SOUTHERN BAJA AND DEEPER
MOISTURE OVER OUR FORECAST AREA. IT REMAINS TO BE SEEN JUST HOW THE
MOISTURE PLAYS OUT. HELD ON TO LOW GRADE MONSOON SCENARIO...SLIGHT
CHANCE POPS LIMITED TO SOUTH-CENTRAL ARIZONA...WITH SUBTLE WESTWARD
EXPANSION OF POPS FOR WEDNESDAY.

&&

.AVIATION...
SOUTH-CENTRAL ARIZONA INCLUDING KPHX...KIWA...AND KSDL...

LINGERING SHOWER AND THUNDERSTORM ACTIVITY TO THE WEST OF PHOENIX
WILL GRADUALLY PUSH NORTHWARD BY LATE EVENING...BUT SOME OUTFLOWS
WILL KEEP WESTERLY WINDS ACROSS THE PHOENIX AREA FOR A FEW MORE
HOURS BEFORE DOWNSLOPE EASTERLIES TAKE OVER. CIG LEVELS WILL
GRADUALLY LOWER THROUGH NOON THURSDAY TO AROUND 15K FEET. THERE IS
LOW TO MEDIUM CONFIDENCE OF SOME LIGHT SHOWERS OR VIRGA
AFFECTING AREA TERMINALS SOMETIME BETWEEN 12Z-18Z. AFTERNOON AND
EVENING STORM CHANCES ARE EXPECTED TO MAINLY BE FOCUSED ACROSS
SOUTHWEST ARIZONA...BUT CAN/T RULE OUT A FEW STORMS AFFECTING AREA
TERMINALS LATE THURSDAY AFTERNOON AND EARLY EVENING. EVEN THOUGH
CONFIDENCE IN THESE STORMS IS FAIRLY LOW AT THIS POINT...GUSTY
OUTFLOW WINDS AFFECTING TERMINALS WILL BE THE MORE LIKELY THREAT.

SOUTHWEST ARIZONA AND SOUTHEAST CALIFORNIA INCLUDING KIPL AND KBLH...

UPPER LEVEL LOW WILL PUSH THROUGH THE REGION TONIGHT INTO THURSDAY
WITH A PERIOD OF MORNING SHOWERS AND ISOLATED THUNDERSTORMS POSSIBLE
AROUND 12Z. ADDITIONAL STORM DEVELOPMENT IS EXPECTED DURING THE
AFTERNOON ON THURSDAY...MAINLY ACROSS THE LOWER COLORADO RIVER
VALLEY AND SOUTHWEST ARIZONA. FAIRLY CONFIDENT GUSTY OUTFLOW WINDS
WILL AFFECT KBLH SOMETIME MID TO LATE AFTERNOON THURSDAY...WITH A
LOWER POTENTIAL FOR THUNDERSTORM RAINS AFFECTING THE TERMINAL.

AVIATION DISCUSSION NOT UPDATED FOR AMENDED TAFS.

&&

.FIRE WEATHER...
FRIDAY THROUGH TUESDAY...
STORM CHANCES BEGIN TRENDING DOWNWARD FRIDAY THROUGH EARLY NEXT WEEK
AS WESTERLY FLOW ALOFT TAKES HOLD ACROSS THE AREA. SLIGHTLY DRIER
AIR WILL MOVE INTO THE REGION FROM THE WEST WHICH SHOULD RESULT IN
STORMS IN THE MORE CLIMATOLOGICALLY FAVORED AREAS EAST OF PHOENIX.
LOOKS TO BE A LOW-GRADE MONSOON THROUGH EARLY NEXT WEEK. HUMIDITIES
WILL REMAIN SOMEWHAT ELEVATED WITH FAIR TO GOOD OVERNIGHT RECOVERIES
EXPECTED. WINDS WILL FAVOR TYPICAL DIURNAL DIRECTIONS.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...

AZ...NONE.

CA...NONE.

&&

$$

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

DISCUSSION...KUHLMAN
PREVIOUS DISCUSSION...HIRSCH/AJ
AVIATION...KUHLMAN
FIRE WEATHER...LEINS







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