Special Weather Statement, NWS Phoenix, AZ

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PHOENIX AZ
315 PM MST FRI AUG 7 2015

.UPDATE...TO AVIATION DISCUSSION...

&&

.SYNOPSIS...
SHOWERS AND THUNDERSTORMS WILL BE POSSIBLE EAST OF PHOENIX
TODAY...BUT STORM CHANCES WILL WANE ACROSS THE ENTIRE AREA THIS
WEEKEND AS DRIER AIR MOVES INTO THE AREA. A RETURN TO A MORE TYPICAL
MONSOON FLOW IS EXPECTED BY THE MIDDLE OF NEXT WEEK...ALTHOUGH STORM
CHANCES WILL REMAIN SLIGHTLY BELOW NORMAL FOR MID AUGUST.

&&

.DISCUSSION...
SKIES HAVE REMAINED MOSTLY CLEAR ACROSS SOUTHERN CALIFORNIA SINCE
BEFORE SUNRISE...AND SOME CLOUDS HAVE FILLED IN ACROSS LA PAZ AND
EASTERN YUMA COUNTY. AN EVIDENT DRY AIR MASS RESIDES ON THE EASTERN
SIDE OF THE SAN DIEGO MOUNTAINS...DEW POINT READINGS IN IMPERIAL
COUNTY ARE IN THE 20S AND 30S WHILE IN YUMA AZ THE DEWPOINT HAS
FALLEN FROM 72F EARLY THIS MORNING TO 62F EARLY THIS AFTERNOON.
LOOKING FARTHER EAST...THUNDERSTORM ACTIVITY ACROSS SOUTHERN GILA
COUNTY STARTED AS EARLY AS 11AM AND CONTINUES TO PERSIST THIS
AFTERNOON...AS EXPECTED ACROSS HIGHER TERRAIN.

A NOD TO THE HIGH RESOLUTION VERSION OF THE NAM-WRF MODEL WHICH HAD A
GOOD HANDLE ON INITIALIZATION THIS MORNING...SO FAR THE FORECAST HAS
PLAYED OUT NICELY. UNIDIRECTIONAL STEERING FLOW OUT OF THE SOUTHWEST
AND DECENT SHEAR /15-20 MPH/ PAIRED UP WITH 1.6 - 1.7 INCH OF
PRECIPITABLE WATER ARE INGREDIENTS FOR STORMS TO THE EAST THIS
AFTERNOON AND EVENING. THANKS IN PART TO THE OFF SHORE UPPER LEVEL
LOW OFF OF CALIFORNIA...DIFFLUENCE ACROSS CENTRAL AND EASTERN ARIZONA
AMONG THE UPPER LEVELS OF THE ATMOSPHERE AND DEEP MIXING WILL CREATE
STRONG DOWNDRAFT SIGNATURES AS STORMS COLLAPSE. STRONG WINDS AND
HEAVY RAIN ARE GOING TO BE THE MAJOR HAZARDS FOR THIS AFTERNOON AND
EVENINGS STORMS. SOME BLOWING DUST WILL ALSO POSE A THREAT AS STORMS
COLLAPSE OVER OPEN DESERT. ONLY MINOR EDITS TO THE FORECAST GRIDS
INCLUDING ADDING DUST THIS AFTERNOON AND SLIGHTLY INCREASING POPS.

&&

.PREVIOUS DISCUSSION.../ISSUED AT 247 AM MST 7 AUG/....

NOT SURPRISINGLY...PERSISTENT SOUTHWESTERLY FLOW ACROSS THE AREA
THROUGH THE WEEKEND INTO EARLY NEXT WEEK WILL LEAD TO WELL BELOW
NORMAL PRECIP CHANCES ACROSS THE AREA. HARD TO JUSTIFY POPS ANYWHERE
NEAR CLIMO THIS WEEKEND. A FEW ISOLATED SHOWERS ARE POSSIBLE EAST OF
GLOBE...BUT THAT'D BE ABOUT IT.

DETERMINISTIC AND PROBABILISTIC GUIDANCE SUGGESTS A RETURN TO A MORE
TYPICAL/FAVORABLE MONSOON FLOW /EAST AND SOUTHEAST WINDS IN THE
700-300MB LAYER/ FROM TUESDAY ONWARD. THAT BEING SAID...MOISTURE
APPEARS TO BE LACKING WITH PWATS ONLY RISING TO NORMAL VALUES
/AROUND 1.25 INCHES/. WITHOUT ANY OBVIOUS TRIGGERS IN THE FLOW AND
BL MOISTURE WITHIN A FEW PERCENT OF NORMAL...IT DOESN'T APPEAR
ANYTHING MUCH HIGHER THAN CLIMO POPS ARE WARRANTED AT THIS POINT.
INHERITED FORECAST ALREADY HAD THIS IDEA AND WAS ONLY ADJUSTED
SLIGHTLY.

&&

.AVIATION...
SOUTH-CENTRAL ARIZONA INCLUDING KPHX...KIWA...AND KSDL...
BROAD TS COVERAGE OVER THE MTNS ACROSS EASTERN AZ ARE SENDING STORM
OUTFLOWS AND BUILDING SCATTERED TS ACTIVITY INTO THE LOWER DESERT
ELEVATIONS. SOME ISOLATED TO SCATTERED TS COVERAGE WILL BUBBLE SOUTH
OF PHOENIX AND LIKELY PULSE UP AS STORM OUTFLOWS RUN UNDERNEATH
THEM. PRIMARY THREAT FOR THE REMAINDER OF THE EVENING WILL BE
ERRATIC WINDS AND SUDDEN SHIFTS BECAUSE OF THE STORM COVERAGE AND
THEIR ASSOCIATED WINDS. ONCE STORMS DO DEVELOP THEY ARE MOVING NE-
WARD SO ANYTHING ADDITIONAL STORMS THAT POP OVER SW MARICOPA COULD
BE DRAWN INTO THE PHX AREA THROUGH THE EVENING. SAVE FOR OUTFLOW
DRIVEN WINDS...WINDS OFF THE SFC TO MAINTAIN SOME S-SW HEADING BUT
MAY NOT BE REALIZED AT THE SFC. SCT-BKN MID LEVEL CB AND CIGS TO
LINGER INTO THE EVENING HOURS...WITH SKIES GRADUALLY CLEARING FROM
THE W-SW LATER INTO THE EVENING.

SOUTHEAST CALIFORNIA/SOUTHWEST ARIZONA INCLUDING KIPL AND KBLH...
DRY WESTERLY SFC FLOW WILL PERSIST ELEVATED WEST BREEZES AND SCOUR
OUT THE ELEVATED DEWPOINTS...SHUTTING OFF ANY PTNL FOR STORMS FOR
THE WESTERN TERMINALS TODAY AND INTO THE WEEKEND. SOME HIGH LEVEL
PASSING CU WILL GNLY BE THE EXTENT OF SKY COVERAGE...OTHERWISE
CONDITIONS WILL BE SKC.

AVIATION DISCUSSION NOT UPDATED FOR AMENDED TAFS.

&&

.FIRE WEATHER...
MONDAY THROUGH FRIDAY...
INCREASING MOISTURE INTO EARLY NEXT WEEK WILL PUSH CHANCES OF
SHOWERS AND THUNDERSTORMS INTO THE SOUTHERN AND WESTERN ARIZONA
DESERTS. MINIMUM RELATIVE HUMIDITIES OF 10 TO 20 PERCENT RANGE WILL
BE COMMON ACROSS THE DESERTS THIS WEEKEND WITH SOME IMPROVEMENT
GOING INTO EARLY NEXT WEEK. DRIEST LOCATION WILL BE SOUTHEAST
CALIFORNIA THIS WEEKEND WITH MINIMUM RH VALUES INTO THE SINGLE
DIGITS. HIGH TEMPERATURES WILL BE AROUND OR SLIGHTLY ABOVE NORMAL
THROUGH THE PERIOD. WINDS WILL BE LIGHT THROUGH MUCH OF THE PERIOD
FOLLOWING TYPICAL DIURNAL DIRECTIONS EXCEPT FOR AFTERNOON UPSLOPE
BREEZES GUSTING AT TIMES UP TO 20 MPH.

&&

.SPOTTER INFORMATION STATEMENT...
SPOTTERS ARE ENCOURAGED TO REPORT CONDITIONS ACCORDING TO STANDARD
OPERATING PROCEDURES.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...NONE.
CA...NONE.
&&

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

DISCUSSION...DEWEY/LEINS
AVIATION...NOLTE
FIRE WEATHER...KUHLMAN/CB

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