Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 031752 RRA
AFDPSR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PHOENIX AZ
1052 AM MST THU SEP 3 2015

.UPDATE...TO AVIATION DISCUSSION...

&&

.SYNOPSIS...
INCREASING SOUTHWESTERLY FLOW ALOFT IS EXPECTED TO PULL TROPICAL
MOISTURE FROM THE EASTERN PACIFIC NORTHWARD INTO THE
REGION...INCREASING THUNDERSTORM CHANCES ACROSS SOUTH CENTRAL
ARIZONA TODAY AND FRIDAY. AS THE WINDS ALOFT BECOME MORE WESTERLY
OVER THE WEEKEND...SOMEWHAT DRIER AIR IS EXPECTED TO MOVE INTO THE
REGION AND WILL REDUCE SHOWER AND THUNDERSTORM CHANCES THROUGH
SUNDAY. A TEMPORARY RETURN TO MORE SOUTHERLY FLOW WILL THEN BRING AN
UPSWING IN MONSOON ACTIVITY EARLY NEXT WEEK BEFORE A MID-WEEK PERIOD
OF QUIET THUNDERSTORM ACTIVITY.

&&

.DISCUSSION...
PER IR SATELLITE IMAGERY FROM LAST EVENING THROUGH THIS EARLY
MORNING...IT APPEARS A NEAR PERSISTENT STREAM OF CLOUDS AND MOISTURE
WAS PRESENT FROM THE BAJA PENINSULA STRETCHING NORTHEAST THROUGH SE
ARIZONA AND INTO THE WESTERN HALF OF NEW MEXICO. THIS BODES WELL AS
THE UPPER LEVEL TROUGH CONTINUES TO PUSH ONSHORE TO
CALIFORNIA...PINCHING THE SOUTHWEST FLOW AND INCREASING UPPER LEVEL
SOUTHWEST WINDS THROUGH ARIZONA TO 50-60 KTS AT 300MB. THE PLUME OF
HIGH MOISTURE REMAINS OVER SONORA THIS MORNING...SLOWLY INCHING
NORTHWARD AS TUCSON SAW A SUBTLE INCREASE IN THE 12Z RAOB THIS
MORNING TO JUST OVER 1.5 INCHES WHILE PHOENIX REMAINED BELOW 1.4
INCHES...AN INCREASE FROM THE 00Z SOUNDING BUT A SHARP DROP FROM THE
PREVIOUS DAY`S 12Z LAUNCH. TEMPERATURES AGAIN ARE RUNNING RIGHT
ABOUT THE SAME AS THE PREVIOUS DAY WITH DEWPOINTS ON THE
RISE...COMPARED WITH YESTERDAY AS WE SAW A GRADUAL DECLINE IN
MOISTURE THROUGH THE DAY. STARTING THE DAY WITH A SIGNIFICANT CLEAR
SLOT IN THE WAY OF CLOUDS OVER SOUTHWEST AND SOUTH-CENTRAL ARIZONA I
BELIEVE WILL HELP AIDE IN CONVECTIVE INITIATION LATER THIS
AFTERNOON...WHAT WE LACKED YESTERDAY. MINOR CHANGES TO THE FORECAST
PACKAGE FOR TIMING OF THUNDERSTORM DEVELOPMENT BUT THE TREND LOOKS
ON TRACK.

&&

.PREVIOUS DISCUSSION.../225 AM MST 3 SEPTEMBER/...
TODAY THROUGH FRIDAY...
ALTHOUGH YESTERDAY TURNED OUT TO BE MUCH LESS ACTIVE WEATHERWISE
THEN WHAT WAS EXPECTED...DUE TO DRIER AIR BEING ADVECTED INTO SOUTH-
CENTRAL AZ BY MORE WESTERLY MID-LEVEL WINDS...IT STILL APPEARS THAT
TODAY...AND TO A LESSER EXTENT ON FRIDAY WILL SEE MORE CONVECTIVE
ACTIVITY...AS A RETURN TO MORE SOUTHERLY LOW-MID LEVEL FLOW IS
EXPECTED TO PULL AT LEAST SOME MOISTURE BACK NORTHWARD INTO SOUTH-
CENTRAL ARIZONA. ALTHOUGH THE LATEST GFS MODEL RUN NOW ONLY RAISES
PWAT VALUES TO AROUND 1.50 ACROSS THE PHX AREA...A MORE FAVORABLE
VERTICAL SHEAR ENVIRONMENT AND OUTFLOWS FROM STORMS TO THE SOUTH IS
EXPECTED TO ALLOW AT LEAST SOME THUNDERSTORM ACTIVITY TO DEVELOP
FROM CENTRAL MARICOPA COUNTY EASTWARD. THIS FORECAST IS ALSO BEING
SUPPORTED BY THE LATEST (00Z) WRF4KM/NMM6KM HIGH-RES MODEL
RUNS...WITH BOTH OF THEM SHOWING A VERY SHARP WESTERN EDGE TO THE
CONVECTIVE ACTIVITY OVER CENTRAL MARICOPA COUNTY...WITH MUCH DRIER
AIR TO THE WEST GREATLY LIMITING THUNDERSTORM DEVELOPMENT. ALTHOUGH
THE STRONG WINDS ALOFT (50KT AT 300MB) WILL LIMIT RAINFALL AMOUNTS
DUE TO THE RAPID MOVEMENT OF INDIVIDUAL CELLS...THESE STRONG WINDS
WILL LIKELY ENHANCE OUTFLOW WINDS FROM THESE STORMS...WITH SOME OF
THEM APPROACHING...OR EVEN REACHING SEVERE LIMITS.

ALTHOUGH AVAILABLE MOISTURE WILL BE SOMEWHAT LESS ON FRIDAY...EVEN
STRONGER WINDS ALOFT WILL HELP TO DESTABILIZE THE ATMOSPHERE
FURTHER...WHICH SHOULD ALLOW AT LEAST SOME STORMS TO DEVELOP FROM
PHOENIX EASTWARD. THE COMBINATION OF COOLER AIR ALOFT AND INCREASED
CLOUDINESS/CONVECTIVE ACTIVITY WILL HELP TO HOLD HIGH TEMPS DOWN
ACROSS THE REGION BOTH DAYS...WITH MANY LOWER DESERT LOCATIONS
ACROSS SOUTH-CENTRAL AZ SEEING HIGHS ONLY IN THE MID-UPPER 90S.

SATURDAY AND SUNDAY...
SOMEWHAT LESS CONVECTIVE ACTIVITY IS EXPECTED DURING THIS TIME
PERIOD AS DRIER AIR WORKS ITS WAY EASTWARD INTO SOUTH-CENTRAL AZ.
STILL...AT LEAST SLIGHT CHANCE POPS ARE WARRANTED EACH DAY ACROSS
THE LOWER DESERTS OF SOUTH- CENTRAL AZ AS PWAT LEVELS REMAIN IN THE
1.0-1.3 INCH RANGE AND THE VERTICAL SHEAR PROFILE REMAINS FAVORABLE
THROUGH THE COLUMN.

MONDAY THROUGH TUESDAY...
ALTHOUGH FORECAST CONFIDENCE LEVELS ARE STILL RATHER LOW THIS FAR
OUT...A INCREASE IN AVAILABLE MOISTURE AS THE FLOW BELOW 500MB
BECOMES A BIT MORE SOUTHERLY IS EXPECTED TO ALLOW PRECIP CHANCES TO
RISE A BIT DURING THIS TIME PERIOD. SOMEWHAT WARMER AIR ALOFT WILL
ALSO ALLOW TEMPS TO WARM TO ABOVE NORMAL VALUES.

WEDNESDAY...
THE GFS AND THE EURO ARE NOW BOTH SHOWING INCREASING WESTERLY FLOW
ACROSS THE REGION AS THE MAIN UPPER-LEVEL HIGH CENTER SHIFTS TO OUR
SOUTH DURING THIS TIME PERIOD...WITH NOTICEABLY DRIER AIR MOVING
INTO THE REGION...WHICH SHOULD GREATLY REDUCE THUNDERSTORM ACTIVITY
ACROSS SOUTH-CENTRAL AZ.

&&

.AVIATION...
SOUTH-CENTRAL ARIZONA INCLUDING KPHX...KIWA...AND KSDL...
CHALLENGING MONSOON FORECAST TODAY FOR THE PHX AREA TERMINALS WITH A
STILL PLENTIFUL MOISTURE SFC AND FAVORABLE UPPER JET STREAM DYNAMICS
IN PLACE TO SUPPORT STORMS. ONLY TRICK WILL BE TO GET STORMS STARTED
AND ACCESS THE AVAILABLE INSTABILITY OVER THE REGION. STRONG UPPER
LET JET NOSING INTO THE AREA FROM THE SOUTHWEST...WITH OVER 40 TO 50
KNOTS OF WIND ALOFT...WILL HELP POP SOME ACTIVITY EARLY IN THE DAY
WARRANTING VCSH COVERAGE POST MIDDAY...AND ON-GOING AT THE TIME OF
THE WRITING FOR KIWA. WINDS SHOULD FAVOR SE-SSE HEADINGS INTO THE
MIDDAY PERIOD...BUT MAY ALSO REMAIN LIGHT ENOUGH TO RESULT IN
PERIODS OF VRB. MORE ERRATIC WINDS ARE POSSIBLE IN RESPONSE TO ANY
PASSING SHOWERS. MID LEVEL CLOUD COVERAGE TO REMAIN SCT-BKN GNLY
AROUND 10-12KFT.

WILL ALSO MAINTAIN THE TREND FROM THE OVERNIGHT SHIFT OF KEEPING THE
THE IDEA OF VCTS/VCSH AT ALL SITES FROM LATE AFTERNOON ONWARD. MOST
NUMERICAL GUIDANCE BRINGS IN STORMS AND OUTFLOWS AROUND THE 04/00Z.
TIMING THE DEVELOPMENT AND TRACK OF THE OUTFLOW IS CONTINGENT ON
WHERE THE LARGEST COVERAGE OF STORMS DEVELOP...WHICH ATTM LOOK TO BE
BETWEEN OVER PINAL AND NORTHERN PIMA COUNTIES. COULD SEE SCATTERED
SHOWERS AND CIGS AROUND 10KFT LINGER INTO THE OVERNIGHT HOURS.

SOUTHEAST CALIFORNIA/SOUTHWEST ARIZONA INCLUDING KIPL AND KBLH...
ASIDE FROM HIGH CLOUDS...EXPECT MINIMAL WEATHER-RELATED AVIATION
IMPACTS THROUGH FRIDAY MORNING. DIURNAL WINDS WITH OCCASIONAL LATE-
AFTERNOON GUSTINESS ANTICIPATED.

AVIATION DISCUSSION NOT UPDATED FOR AMENDED TAFS.

&&

.FIRE WEATHER...
SATURDAY THROUGH WEDNESDAY...
ELEVATED MOISTURE WILL REMAIN IN PLACE ACROSS ARIZONA THIS WEEKEND
THROUGH THE MIDDLE OF NEXT WEEK...WHEREAS DRIER THAN NORMAL AIR WILL
RESIDE WEST OF THE COLORADO RIVER. HUMIDITIES WILL FALL INTO THE 20-30
PERCENT RANGE EACH AFTERNOON ACROSS ARIZONA AND INTO THE 10-20
PERCENT RANGE TO THE WEST. OVERNIGHT RECOVERIES AT ALL SITES WILL BE
GOOD/EXCELLENT. WINDS WILL FOLLOW TYPICAL UPSLOPE/DOWNVALLEY PATTERNS
OUTSIDE OF THUNDERSTORM INDUCED OUTFLOWS.

&&

.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
PREVIOUS DISCUSSION...PERCHA
AVIATION...NOLTE/LEINS
FIRE WEATHER...LEINS


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