Area Forecast Discussion
Issued by NWS Phoenix, AZ

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000
FXUS65 KPSR 200320 AAA
AFDPSR

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE PHOENIX AZ
820 PM MST TUE AUG 19 2014

.UPDATED...AVIATION DISCUSSION.

&&

.SYNOPSIS...
SCATTERED SHOWERS AND THUNDERSTORMS WILL REMAIN A THREAT THROUGH
THURSDAY AS AN AREA OF LOW PRESSURE 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...
ISOLATED SHOWERS AND THUNDERSTORMS CONTINUED TO LINGER THROUGH PARTS
OF THE PHOENIX AREA AT 730 PM MST IN THE WAKE OF A HEAVY RAIN EVENT
TUESDAY MORNING AND AFTERNOON. THE STRONGEST ACTIVITY AT 730 PM WAS
IN THE PHOENIX WEST VALLEY WHERE A CONVECTIVE WIND GUST TO 53 MPH
WAS MEASURED AT LUKE AFB. ISOLATED CONVECTION SHOULD CONTINUE TO
DEVELOP THE REMAINDER OF THE EVENING...ALONG THE OUTFLOWS WHICH ARE
MOVING EAST INTO A VERY HIGH THETA-E FIELD LAYED DOWN BY MORNING
RAINS. SHOWERS SHOULD END BY MIDNIGHT...BUT A SLIGHT CHANCE WILL
CONTINUE OVERNIGHT.

THE EARLY SEASON COOL PACIFIC LOW PRESSURE SYSTEM DEVELOPING OVER
THE CENTRAL AND SOUTHERN CA COAST...AND RESPONSIBLE FOR A VERY
DYNAMICAL PATTERN AND WIDESPREAD RAIN IN SOUTH CENTRAL AZ TUESDAY
MORNING...WILL BE MOVING INTO AZ BY THURSDAY AFTERNOON...ACCORDING
TO THE GFS MODEL. WE THINK THE GFS SOLUTION IS MORE BELIEVABLE THAN
THE SLOWER MOVING TROF IN THE EUROPEAN MODEL SOLUTION.

THE GFS MODEL CONTINUES TO KEEP AZ UNDER A STRONGER THAN NORMAL
SOUTHWESTERLY FLOW ALOFT AS THE SOUTHERN CA TROF APPROACHES.
SOUTHWESTERLY FLOW ALOFT IS AWAYS WORRISOME...IT CAN EASILY PRODUCE
BOUTS OF UPPER LEVEL SUPPORT/DYNAMICS ANYTIME. SINCE THERE WILL BE
INCREASED SUNSHINE WEDNESDAY...AND HUMIDITY LEVELS WILL CONTINUE IN
THE MONSOON RANGE...THE SLIGHT CHANCE OF SHWR/TSTM FORECAST ACROSS
OUR ENTIRE FORECAST AREA THROUGH WEDNESDAY NIGHT LOOKS OK.

HOWEVER...IF THE GFS MODEL IS CORRECT...THE SOUTHERN CA TROF MOVING
INTO AZ THURSDAY COULD PRODUCE ANOTHER BOUT OF WIDESPREAD SHOWERS
AND THUNDERSTORMS WITH HEAVY RAIN. THE COLD CORE CENTER WITH THE
TROF DROPS 500 MB TEMPERATURES INTO THE MINUS 10 TO 12 RANGE...
UNUSUAL ON THE DESERTS FOR MID AUGUST...ALONG WITH INCREASED UPPER
LEVEL DYNAMICS AND WEAK VERTICAL WIND SHEAR FOR A FEW ORGANIZED
THUNDERSTORMS. AGAIN...THIS IS AN ATYPICAL WEATHER PATTERN FOR
SOUTHWEST AND SOUTH CENTRAL AZ AND SHOULDNT BE DISCOUNTED.

WE WILL PASS THIS ANALYSIS TO THE MID SHIFT FOR FURTHER REVIEW SINCE
A NEW FORECAST PACKAGE WILL BE ISSUED. A NEW EUROPEAN MODEL FORECAST
INTO OUR COMPUTERS SHORTLY AFTER MIDNIGHT MAY CONFIRM THE ABOVE
THINKING.

OTHERWISE...FORECASTS LOOK OK. NO UPDATES PLANNED. PREVIOUS
DISCUSSION BELOW STILL APPLIES.



.PREVIOUS DISCUSSION...330 PM MST...
HISTORIC RAIN AND FLOODING OCCURRED ACROSS PORTIONS OF THE PHOENIX
AREA TODAY. THE BIGGEST IMPACT WAS FELT ACROSS NORTHERN MARICOPA
COUNTY WHERE WIDESPREAD RAIN TOTALS OF 2 TO 5 INCHES WERE MEASURED.
A SECOND LINE OF STORMS DEVELOPED EARLY THIS AFTERNOON WHICH
PRODUCED AN ADDITIONAL HALF INCH TO ONE INCH OF RAIN ACROSS CENTRAL
AND NORTH PHOENIX. ELEVATED STREAMFLOWS WERE ALSO OBSERVED ACROSS
THE AGUA FRIA RIVER...SKUNK CREEK AND CAVE CREEK. THE AGUA FRIA
BRIEFLY CRESTED INTO THE MAJOR FLOOD CATEGORY BUT CONTINUES TO
RECEDE. MEANWHILE...CAVE CREEK REACHED A STAGE OF 12.75
FT...ECLIPSING THE PREVIOUS RECORD OF 12.1 FT FROM THE RAIN EVENT
IN JANUARY 2010.

LATEST WV IMAGERY SHOWS DRIER AIR WORKING ITS WAY INTO SOUTHWESTERN
AZ ASSOCIATED WITH THE CYCLONIC FLOW AROUND THE CLOSED LOW ALONG THE
CENTRAL CA COAST. DEWPOINTS CONTINUE TO DROP MAINLY WEST OF MARICOPA
COUNTY...THOUGH EVEN THE GPS-IPW AT TEMPE HAS DROPPED TO 1.6 INCHES.
NEVERTHELESS....SHOWERS HAVE REDEVELOPED ACROSS THE FAVORED HIGHER
TERRAIN AREAS OF EASTERN LA PAZ AND NORTHERN MARICOPA COUNTY. LATEST
MESOANALYSIS SHOWS WIDESPREAD CAPES ABOVE 500 J/KG WITH MINIMAL
INHIBITION STRETCHING FROM EASTERN RIVERSIDE COUNTY TO EASTERN AZ.
LATEST GLOBAL AND CONVECTIVE ALLOWING GUIDANCE SUGGESTS THE MOST
FAVORED AREA FOR ADDITIONAL STORMS WILL BE ACROSS NORTHWESTERN
MARICOPA COUNTY WHERE THE EXISTING INSTABILITY IS JUXTAPOSED WITH
STRONGER THAN NORMAL DEEP LAYER SHEAR. SHOWERS AND THUNDERSTORMS
WILL DISSIPATE THIS EVENING AS CONDITIONS STABILIZE RAPIDLY.

&&

.PREVIOUS DISCUSSION...
WEDNESDAY AND WEDNESDAY NIGHT...
A SIGNIFICANT REDUCTION IN SHOWER/THUNDERSTORM ACTIVITY IS EXPECTED
ON WEDNESDAY AND WEDNESDAY NIGHT AS PWATS FALL INTO THE 1.25-150
INCH RANGE AS SOUTHWESTERLY FLOW ALOFT STRENGTHENS OVER THE REGION
AHEAD OF THE MAIN UPPER LOW CENTER. STILL...ISOLATED TO SCATTERED
STORMS CAN BE EXPECTED...WITH THE BEST CHANCES FOR STORMS BEING OVER
THE HIGHER TERRAIN NORTH AND EAST OF PHOENIX AS STRONGER WINDS ALOFT
INCREASES BULK SHEAR VALUES OVER THE REGION.

THURSDAY AND FRIDAY...
THE LIKELIHOOD OF A SECOND HEAVY RAIN EVENT ON THURSDAY/FRIDAY NOW
LOOKS TO BE SIGNIFICANTLY LESS...SINCE LATEST MODELS RUNS ARE NOW
SHOWING MUCH LESS TROPICAL MOISTURE...FROM TROPICAL STORM
LOWELL...BEING PULLED NORTHEASTWARD INTO THE REGION. THUS...HAVE
REDUCED POPS SOMEWHAT ON THU/FRI...BUT STILL KEEPING THINGS
BROADBRUSHED A BIT...SINCE THE EURO CONTINUES TO BE SLOWER WITH THE
MOVEMENT OF THE UPPER LOW CENTER...TAKING IT ACROSS AZ ON
FRIDAY...WHILE THE FASTER GFS CONTINUES TO MOVE IT THROUGH ON
THURSDAY.

SATURDAY THROUGH TUESDAY...
THE COMBINATION OF A DRIER WESTERLY FLOW ALONG WITH RIDGING ALOFT
WILL LIKELY KEEP CHANCES FOR THUNDERSTORM ACTIVITY AT CLIMO...OR
BELOW LEVELS DURING THIS PERIOD. HOWEVER...IT STILL APPEARS THAT
THERE WILL BE ENOUGH RESIDUAL MOISTURE AROUND TO KEEP US FROM
ELIMINATING CHANCES FOR POPS ENTIRELY AT THIS POINT...WITH
TEMPERATURES RETURNING TO NEAR...OR SLIGHTLY ABOVE NORMAL VALUES.

&&

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

ISOLATED SHOWERS AND THUNDERSTORMS WILL CONTINUE ACROSS THE PHOENIX
VALLEY THROUGH 06Z. SCT CLOUD DECKS BTWN 6 AND 10 THSD FT AGL...
WITH BKN CLOUDS ABOVE 14 THSD THROUGH 10Z. WEST WINDS WILL DIMINISH
FROM 15 KNOTS AT 04Z WED TO LGT AND VRBL BY 07Z WED. FROM 07Z WED TO
18Z MON...LGT VRBL WNDS. SCT CLDS AOA 10 THSD AGL.



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

THROUGH 18Z WED...MOSTLY CLEAR SKIES. LIGHT WEST TO SOUTHWEST WIND
UNDER 9 KNOTS.

AVIATION DISCUSSION NOT UPDATED FOR AMENDED TAFS.

&&

.FIRE WEATHER...
THURSDAY THROUGH MONDAY...
A SLIGHT CHANCE OF AFTERNOON AND EVENING THUNDERSTORMS WILL CONTINUE
MAINLY EAST OF THE LOWER COLORADO RIVER VALLEY ON THURSDAY WITH A
CHANCE ACROSS THE HIGHER TERRAIN EAST OF PHOENIX. LITTLE CHANGE IS
EXPECTED FRIDAY. HIGH TEMPERATURES BOTH DAYS WILL STAY BELOW
SEASONAL NORMALS...WITH COOLER DESERTS FALLING INTO THE 90S.
HUMIDITY LEVELS WILL STAY QUITE ELEVATED WITH MOST OF THE DESERTS
RECORDING MINIMUM RH VALUES BETWEEN 20 AND 30 PERCENT. A STEADY
SOUTHWEST FLOW ALOFT WILL LEAD TO GRADUAL DRYING AND WARMING
CONDITIONS OVER THE WEEKEND AND INTO EARLY NEXT WEEK. BY SUNDAY
THERE WILL BE JUST A SLIGHT CHANCE OF AFTERNOON AND EVENING
THUNDERSTORMS ACROSS THE HIGHER TERRAIN EAST OF PHOENIX WITH PARTLY
TO MOSTLY SUNNY SKIES EXPECTED ELSEWHERE. DESPITE THE DRYING
TREND...MINIMUM RH VALUES WILL STAY ON THE ELEVATED SIDE AND SHOULD
MOSTLY RANGE FROM 15 TO 25 PERCENT. NO STRONG WINDS ARE EXPECTED
THROUGH THE PERIOD...MAINLY NORMAL DIURNAL WINDS FAVORING THE SOUTH
TO SOUTHWEST LESS THAN 15 MPH.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...

AZ...NONE.

CA...NONE.

&&

$$

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

DISCUSSION...VASQUEZ/PERCHA
AVIATION....VASQUEZ
FIRE WEATHER...CB









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