Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 202230 AAA
AFDPSR

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

.UPDATE...UPDATED AVIATION DISCUSSION.

&&

.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...
UPPER LOW CONTINUES TO DRIFT SOUTHEASTWARD DOWN THE SOUTHERN CA
COAST...RESULTING IN A PROMINENT SOUTHWESTERLY FLOW ACROSS THE
REGION. MEANWHILE...CONDITIONS ARE CONSIDERABLY DRIER THAN THEY WERE
24 HOURS AGO. DEWPOINTS ARE AS LOW AS THE MID 40S ACROSS SE CA AND
LATEST GPS-IPW AT TEMPE WAS MEASURED AT 1.3 INCHES. THIS IS YIELDING
VERY LITTLE CAPE ACROSS THE LOWER DESERTS...WHICH IS ALSO EVIDENT ON
SATELLITE WITH A DEARTH OF TOWERING CU. UPPER LEVEL CONVERGENCE
ASSOCIATED WITH THE RIGHT FRONT QUADRANT OF AN APPROACHING JET
STREAK IS ALSO INHIBITING CONVECTION. CONSEQUENTLY...POPS WERE
LOWERED TO AROUND 10 PERCENT FOR TONIGHT.

AFOREMENTIONED 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.

&&

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

THUNDERSTORM ACTIVITY EXPECTED TO REMAIN ISOLATED ACROSS THE REGION
THIS AFTERNOON/EVENING...MAINLY HUGGING THE MOUNTAINS ACROSS EASTERN
AND NORTHERN AZ...BETTER ACTIVITY ACROSS JTNP IN SOUTHEAST CA. NOT
OUT OF THE QUESTION TO SEE A ROGUE STORM MAKE IT INTO THE LOWER AZ
DESERTS THIS EVENING...BUT CONFIDENCE TOO LOW TO INTRODUCE WIND
SHIFTS/OUTFLOW INTO THE TAFS AT THIS TIME. MID-HIGH CLOUDS INCREASE
OVERNIGHT ACROSS THE REGION AS AN UPPER LEVEL DISTURBANCE MOVES
ACROSS SOUTHWEST ARIZONA...WITH ISOLD-WIDELY SCATTERED SHOWERS AND
THUNDERSTORMS BEGINNING TO AFFECT TERMINALS AFTER 13Z THURSDAY.

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...HIRSCH
PREVIOUS DISCUSSION...AJ
AVIATION...MEYERS
FIRE WEATHER...LEINS




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