Area Forecast Discussion
Issued by NWS Phoenix, AZ

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000
FXUS65 KPSR 180941
AFDPSR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PHOENIX AZ
240 AM MST FRI APR 18 2014

.SYNOPSIS...
A PACIFIC STORM SYSTEM WILL MOVE INTO THE AREA STARTING TODAY
BRINGING EXTENSIVE CLOUDS...INCREASING MOISTURE AND LOCALIZED
CHANCES FOR SHOWERS THROUGH SATURDAY AFTERNOON. THROUGH
TONIGHT...ANY RAINFALL WILL BE FOCUSED MAINLY OVER HIGHER TERRAIN
LOCATIONS...WITH SPRINKLES MORE LIKELY ACROSS THE DESERTS. BETTER
CHANCES FOR SHOWERS AND THUNDERSTORMS WILL OCCUR ON SATURDAY ACROSS
CENTRAL AND EASTERN ARIZONA. DAYTIME TEMPERATURES TODAY AND SATURDAY
WILL FALL TO MORE SEASONAL VALUES...HOWEVER QUICKLY WARM BACK WELL
INTO THE 90S EARLY NEXT WEEK AFTER THE STORM SYSTEM HAS PASSED.

&&

.DISCUSSION...
A SEASONABLY VIGOROUS SHORTWAVE TROUGH HAS PROGRESSED TO NEAR THE
121W LATITUDE EARLY THIS MORNING...WITH THE SMALL PV ANOMALY CORE
TUCKED IN THE BASAL PORTION OF THE WAVE AROUND 25N. OBJECTIVE
ANALYSIS SUGGESTS A -16C TO -18C H5 COLD CORE REMAINS SITUATED IN
THE UPSTREAM NLY FLOW QUADRANT...WITH THE EASTERN SECTION OF THE
WAVE STRUCTURE CHARACTERIZED BY DEEP MERIDIONAL WARM MOIST
ADVECTION. THIS CONFIGURATION MAY MEAN FURTHER DEEPENING AND FUTURE
NEGATIVE TILT EJECTION OF THE WAVE INTO THE FORECAST AREA. JET
DIFLUENCE AHEAD OF THE WAVE LOOKS IMPRESSIVE ON WV IMAGERY PARTIALLY
TAPPING A NARROW PLUME OF SUBTROPICAL MOISTURE AND BECOMING
MANIFESTED IN A BAROCLINIC LEAF TYPE STRUCTURE (FURTHER EVIDENCE OF
THE POTENCY OF THIS SYSTEM).

WITH MOIST MERIDIONAL FLOW AND UVV IN THE 600-400MB LAYER ALREADY
ESTABLISHED AHEAD OF THE WAVE...THICKER CLOUD COVER WILL HOLD OVER
THE REGION TODAY. BOTH GOES AND AMSU TOTAL PWAT ANALYSIS SHOW A
CORRIDOR OF 1.0 INCH AMOUNTS ADVECTING TOWARDS THE FORECAST AREA
ORIGINATING WEST OF THE BAJA CORRIDOR. FORECAST SOUNDINGS SUGGEST
MUCH OF THIS MOISTURE IS EITHER ALOFT ABOVE THE H7 LAYER...OR VERY
SHALLOW BELOW THE H9 LAYER WITH NOTABLE DRY AIR TRAPPED BETWEEN. IN
ADDITION...WITH THE INTRODUCTION OF THIS MOISTURE...MIDLEVEL LAPSE
RATES WILL SUFFER LIMITING ANY INSTABILITY BASED AID TOWARDS DEEPER
VERTICAL MOTIONS. THUS..DESPITE THE ROBUST SYNOPTIC SCALE ASCENT
THROUGHOUT THE DAY IN ASSOCIATION WITH VORTICITY ADVECTION...MUCH OF
THE HYDROMETEORS MAY BE LOST AS VIRGA. EXCEPTIONS TO THIS ANALYSIS
MAY BE ACROSS SERN CALIFORNIA WHERE THE COLD CORE ALOFT WILL BEGIN
TO INTERACT WITH THE HEATED SFC ALLOWING BETTER INSTABILITY LATE IN
THE AFTERNOON...AND OVER HIGH TERRAIN NORTHEAST OF PHOENIX WHERE THE
SUBCLOUD DRY LAYER WILL BE NATURALLY REDUCED BY ELEVATION.
REGARDLESS...WITH THE PRESENCE OF EXTENSIVE VIRGA AND VERTICAL
MOTIONS THROUGHOUT THE COLUMN...GUSTY OUTFLOW WINDS REMAIN A CONCERN
THROUGHOUT THE AREA DURING THE DAYLIGHT HOURS.

FRIDAY NIGHT THOUGH SATURDAY AFTERNOON...THE COLD CORE ALOFT PULLS
TO THE EAST OF THE SHORTWAVE AND BECOMES BETTER ALIGNED WITH THE PV
ANOMALY AS THE BULK OF THE ENTIRE WAVE LIFTS AND EJECTS OVER
ARIZONA. DURING THIS TRANSITION...RENEWED VORTICITY ADVECTION AND
ASCENT WILL ROTATE THROUGH THE TROUGH WHILE LAPSE RATES STEEPEN IN
THE PRESENCE OF RESIDUAL MOISTURE. A FEW HIGH RESOLUTION MODELS AND
SREF PROBABILITIES SUGGEST THE FIRST WAVE OF UVV/SHOWERS ROTATING
NORTH INTO CNTRL ARIZONA AS EARLY AS SUNRISE SATURDAY. SFC HEATING
UNDER THE COLD CORE DURING THE DAY MAY VERY WELL LEAD TO SUBSTANTIAL
INSTABILITY (SBCAPE AOA 250 J/KG AND LI/S AOB -2)...ESPECIALLY OVER
HIGHER TERRAIN. CONCEPTUALLY AND EVIDENTIALLY THROUGH MODEL
OUTPUT...POPS WERE INCREASED SIGNIFICANTLY FOR POINTS EAST OF
PHOENIX SATURDAY AFTERNOON.

THE AFOREMENTIONED WAVE WILL RAPIDLY WEAKEN AND LIFT INTO THE CNTRL
ROCKIES/PLAINS ON SUNDAY...ALLOWING RIDGING TO QUICKLY REBUILD OVER
THE FORECAST AREA. H5 HEIGHTS AOA 582DM WILL BECOME MAXIMIZED ON
MONDAY AFTERNOON YIELDING HIGHS IN THE MID/UPPER 90S FOR LOWER
ELEVATION LOCATIONS. THE NEXT PROGRESSIVE PACIFIC WAVE WILL MOVE
ONSHORE TUESDAY AFTERNOON...THOUGH MODEL OUTPUT AND TRENDS KEEP THE
VAST MAJORITY OF JET ENERGY AND MOISTURE FROM CNTRL CALIFORNIA
NORTHWARD. IF ANYTHING...THE FORWARD SPEED AND ARRIVAL INTO THE WRN
CONUS MAY BE A BIT SLOWER...AND HAVE DELAYED THE LOCAL ONSET OF
FROPA AND COOLING TO BEYOND TUESDAY. THE GREATEST IMPACT OF THIS
SYSTEM FOR ARIZONA MAY BE GUSTY WINDS ALONG AND IMMEDIATELY BEHIND
THE FRONTAL BOUNDARY AS THE PRESSURE GRADIENT TIGHTENS TUESDAY
AFTERNOON AND EVENING. SOUTHEAST CALIFORNIA AND THE LOWER COLORADO
RIVER VALLEY WOULD LIKELY EXPERIENCE THE MOST PROLONGED AND FREQUENT
HIGHER GUSTS.

WHILE HEIGHTS ALOFT WILL BE TEMPERED BACK INTO A 570-576DM RANGE FOR
THE REMAINDER OF THE WEEK UNDER QUASI-ZONAL FLOW...FEEL THE NET
EFFECT ON TEMPERATURES WILL ONLY BE LIMITED TO KEEPING DAILY
READINGS NEAR CLIMATOLOGY. ENSEMBLE SPREAD IS RELATIVELY LOW IN THE
DAY 5-8 RANGE...SO FORECAST CONFIDENCE IS SOMEWHAT HIGHER THAN
AVERAGE. AT THIS TIME...PRECIPITATION CHANCES APPEAR ALMOST
NON-EXISTENT FOR PRETTY MUCH THE ENTIRETY OF NEXT WEEK.

&&

.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...MO
AVIATION...VASQUEZ
FIRE WEATHER...HIRSCH





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