Area Forecast Discussion
Issued by NWS Phoenix, AZ

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000
FXUS65 KPSR 180310
AFDPSR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PHOENIX AZ
810 PM MST THU APR 17 2014

.SYNOPSIS...
A PACIFIC STORM WILL MOVE INTO THE AREA STARTING TONIGHT BRINGING
INCREASING MOISTURE AND CLOUDS. BY FRIDAY AFTERNOON THIS SYSTEM WILL
SUPPORT SLIGHT CHANCES FOR SHOWERS FROM THE COLORADO RIVER EAST
ACROSS SOUTH CENTRAL ARIZONA AND INTO THE HIGHER TERRAIN. DAYTIME
TEMPERATURES FOR FRIDAY AND SATURDAY WILL FALL TO MORE SEASONAL
VALUES. AFTER THE STORM AND CLOUDS MOVE TO THE EAST ON
SUNDAY...TEMPERATURES QUICKLY WARM BACK INTO THE 90S FOR EARLY NEXT
WEEK WITH MOSTLY SUNNY SKIES.

&&

.DISCUSSION...
CONSIDERABLE HIGH CLOUDS WERE BEGINNING TO STREAM INTO THE REGION
FROM THE WEST THIS EVENING...ASSOCIATED WITH THE APPROACHING PACIFIC
TROF. MODELS CONTINUE TO MOVE THIS SYSTEM INTO ARIZONA LATE FRIDAY
FRIDAY THROUGH SATURDAY NIGHT AND IS A CONCERN. A SYSTEM AS
DYNAMICAL AS THIS...MOVING OVER A VERY WARM AFTERNOON BOUNDARY LAYER
ALWAYS HAS POTENTIAL FOR GREAT INSTABILITY...INCLUDING THUNDERSTORMS
IF MOISTURE IS AVAILABLE. EVEN ONLY MID LEVEL MOISTURE OVER A VERY
DRY BOUNDARY LAYER COULD GENERATE HIGH BASED SHOWERS/STORMS
RESULTING IN STRONG DOWNBURST WINDS AND AREAS OF BLOWING DUST ON THE
DESERTS.

AFTER REVIEWING THIS EVENINGS COASTAL WEATHER BALLOON SOUNDINGS
THIS SYSTEM WAS STILL TOO FAR WEST TO MEASURE ANY APPRECIABLE
MOISTURE. HOWEVER...UPPER LEVEL STREAMLINES DRAWN FROM 00Z VAPOR
IMAGERY LOOPS SHOW HIGHLY DIFLUENT FLOW ALOFT JUST OFF THE SOUTHERN
CA/NORTHERN BAJA COASTS...INDICATING INCREASING VERTICAL MOTION. AND
UNDER THIS DIFLUENT FIELD...OFFSHORE PRECIP WATER AMOUNTS CONTINUED
TO GROW WITH EVERY PASSING HOUR...I.E. A LARGE FIELD OF 0.75 TO 0.90
INCHES WITH EXPANDING AREAS OF 1.05 INCHES.

SUPPOSING AN INCREASED MOISTURE FIELD WITH A DYNAMICAL SYSTEM...
INCLUDING WARM DESERT TEMPERATURES...AN ARGUMENT CAN BE MADE FOR
EXPANDING A SHWR/TSTM THREAT OVER A LARGE AREA SOUTHWEST AND SOUTH
CENTRAL AZ. UPDATED FORECASTS...INCLUDING A HAZARDOUS WEATHER
OUTLOOK...WERE SENT EARLIER TO REFLECT THIS.

ON SATURDAY THE UPPER LEVEL COLD POOL WITH CONSIDERABLE MID LEVEL
MOISTURE CIRCULATING AROUND THE LOW CENTER SHOULD CONTINUE A THREAT
FOR SHOWERS AND THUNDERSTORMS INTO SATURDAY EVENING.

PREVIOUS DISCUSSION...213 PM MST...
AS THE LOW APPROACHES THE DESERT SOUTHWEST TONIGHT...EXPECT CLOUDS
TO THICKEN FROM THE SOUTHWEST TO THE NORTHEAST LATE TONIGHT INTO
FRIDAY MORNING. SKIES WILL LIKELY BECOME MOSTLY CLOUDY AND POSSIBLY
CLOUDY AT TIMES DURING MUCH OF THE DAYTIME HOURS FRIDAY. MODELS SHOW
WEAK TO MODERATE DIFLUENCE ALOFT AND SUFFICIENT OMEGA IN A NEAR
SATURATED 600-400MB LAYER STARTING FRIDAY MORNING. AM EXPECTING SOME
HIGH BASED...12-15K FEET...SHOWERS TO DEVELOP SOMETIME FRIDAY
MORNING INTO EARLY FRIDAY AFTERNOON...ESPECIALLY ALONG AND JUST
BEHIND THE LEADING EDGE OF THE MID LEVEL MOISTURE PLUME. SINCE THE
SUB-CLOUD LAYER WILL BE DRY...NOT QUITE BONE DRY...NOT EXPECTING TOO
MANY LOCATIONS TO SEE RAIN HITTING THE GROUND...MAINLY HIGHER
TERRAIN LOCALES.

CHANCES INCREASE FRIDAY AFTERNOON AND EARLY FRIDAY EVENING...BUT
OVERALL COVERAGE WILL BE LIMITED AND POPS STILL ONLY IN THE SLIGHT
CHANCE CATEGORY AND MAINLY COVERING THE HIGHER TERRAIN. THE WARM AND
DRY SUB-CLOUD LAYERS MAY BRING THE POTENTIAL FOR GUSTY WINDS AND
OUTFLOWS IF A STORM GETS ORGANIZED ENOUGH. THE MAIN UPPER LOW
CIRCULATION WILL HANG BACK ACROSS SOUTHEAST CALIFORNIA FRIDAY
AFTERNOON...THEN SLOWLY PROGRESSING INTO CENTRAL ARIZONA DURING THE
DAY SATURDAY. SLIGHT CHANCE POPS WILL REMAIN THROUGH FRIDAY NIGHT
INTO SATURDAY EVENING ACROSS SOUTH-CENTRAL ARIZONA WITH CHANCE POPS
ACROSS THE HIGHER TERRAIN IN GILA COUNTY. QPF AMOUNTS FOR THE ENTIRE
EVENT WILL BE NEAR ZERO ACROSS THE LOWER DESERTS TO AROUND A TENTH
OF AN INCH ACROSS GILA COUNTY.

DRY WESTERLY FLOW TRANSITIONS INTO THE AREA SUNDAY...QUICKLY WARMING
TEMPERATURES BACK INTO THE 90S FOR EARLY NEXT WEEK. BROAD AREA OF
PACIFIC TROUGHING STILL PROGGED TO SWEEP INTO THE WEST LATE MONDAY
INTO TUESDAY...ALTHOUGH LATEST MODEL RUNS VARY IN THEIR DEPTH AND
STRENGTH AS IT MOVES IN FROM THE NORTHEAST PACIFIC. THE SET-UP STILL
LOOKS PROMISING FOR A RETURN TO COOLER (AND MORE SEASONAL)
TEMPERATURES BY MIDDLE PART OF NEXT WEEK...BUT ANY CHANCES FOR RAIN
LOOK PRETTY REMOTE. MAINTAINED SOME INFLUENCE OF CLIMATOLOGY IN THE
POP GRIDS GIVEN THE PRESENCE OF TROUGHING...BUT UNLESS THE MODELS
EXPAND THE SOUTHERN EXTENT OF THE SYSTEM...EVEN THOSE SINGLE DIGIT
VALUES MAY BE OVERDONE.

&&

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

THROUGH 17Z FRI...BKN CLDS AOA 25 THSD MSL. LIGHT WIND UNDER 9
KNOTS. FROM 17Z FRI TO 23Z FRI...THICKENING AND LOWERING CLOUDS
BECOMING BKN 10 THSD AGL BY 19Z. AN INCREASING CHANCE OF SHOWERS AND
ISOLATED THUNDERSTORMS. INCREASING SOUTHWEST WIND 5 TO 15 KNOTS...
BUT GUSTY AND ERRATIC NEAR THUNDERSTORM DOWNBURSTS.

SOUTHEAST CALIFORNIA AND SOUTHWEST ARIZONA INCLUDING KIPL AND KBLH...
THROUGH 16Z FRI...BKN CLDS AOA 25 THSD MSL. LIGHT SOUTH WIND 5 TO 10
KNOTS. FROM 16Z FRI TO 23Z FRI...THICKENING AND LOWERING CLDS
BECOMING BKN 12 THSD AGL BY 18Z. A SLIGHT CHANCE OF SHOWERS AND
ISOLATED THUNDERSTORMS. INCREASING SOUTH TO SOUTHWEST WIND 5 TO 15
KNOTS...HOWEVER BECOMING ERRATIC AND GUSTY NEAR TSTM DOWNBURSTS.

AVIATION DISCUSSION NOT UPDATED FOR AMENDED TAFS.

&&

.FIRE WEATHER...
SUNDAY THROUGH THURSDAY...
WESTERLY FLOW WILL BRING A WARMING AND DRYING TREND TO THE REGION
THROUGH TUESDAY. THEREAFTER...A COOLING TREND IS EXPECTED AS AN AREA
OF LOW PRESSURE MOVES THROUGH THE WESTERN STATES. THE BIGGEST IMPACT
WITH THIS SYSTEM WILL BE AN INCREASE IN WIND...PARTICULARLY ON
WEDNESDAY. LOW RHS DURING THE MIDWEEK PERIOD WILL ALSO PRESENT AN
INCREASED FIRE DANGER THREAT.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...

AZ...NONE.

CA...NONE.

&&

$$

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

DISCUSSION...VASQUEZ/KUHLMAN/NOLTE
AVIATION...VASQUEZ
FIRE WEATHER...HIRSCH











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