Area Forecast Discussion
Issued by NWS Phoenix, AZ

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000
FXUS65 KPSR 210457
AFDPSR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PHOENIX AZ
945 PM MST MON APR 20 2015

.SYNOPSIS... MOSTLY CLEAR SKIES AND ABOVE NORMAL TEMPERATURES ARE
EXPECTED THROUGH TUESDAY. BIG CHANGES ARE POSSIBLE WEDNESDAY INTO
FRIDAY AS AN APPROACHING STORM SYSTEM IS FORECAST TO BRING COOLER
TEMPERATURES AND A CHANCE OF SHOWERS AND THUNDERSTORMS...MAINLY ON
THURSDAY. HIGH PRESSURE AND WARMER TEMPERATURES WILL EVENTUALLY
SETTLE BACK INTO THE REGION FOR THE COMING WEEKEND.

&&

.DISCUSSION...

TONIGHT...WATER VAPOR SATELLITE IMAGERY/REGIONAL UPPER AIR PLOTS
THIS EVENING SHOW A WEAK UPPER LEVEL LOW SPINNING OFF THE WEST
COAST...THE 564DM CENTER AROUND 33N 120W. SOME MEAGER AMOUNTS OF LOW-
MID LEVEL MOISTURE WERE DRAWN NORTHWARD OVER THE BAJA PENINSULA
AHEAD OF THE LOW THIS AFTERNOON...AND WITH THE OROGRAPHIC LIFT...A
FEW ISOLATED THUNDERSTORMS FORMED OVER THE BAJA SPINE SOUTHWEST OF
IMPERIAL AND CALEXICO JUST BEFORE 01Z THIS EVENING. THESE
STORMS...AND ALL OF THE MID AND UPPER LEVEL CLOUDS OVER NORTHERN AND
SOUTHEAST ARIZONA QUICKLY DISSIPATED WITH THE APPROACH OF SUNSET AND
THE LOSS OF SURFACE HEATING. WITH THE CLEAR SKIES...TEMPERATURES
WILL CONTINUE TO COOL OFF INTO THE UPPER 50S TO LOW 60S TUESDAY
MORNING FOR MOST DESERT LOCATIONS.

TUESDAY...THIS TROUGH IN THE SOUTHERN BRANCH OF SPLIT FLOW IS
PROGGED TO SLOWLY MOVE EAST AND FORM A CLOSED LOW OVER SOUTHERN
CALIFORNIA BY TUESDAY AFTERNOON. WITH A DRIER SOUTHWESTERLY FLOW
ALOFT AND PALTRY MIXING RATIOS OVER THE BULK OF OUR FORECAST
AREA...PRECIPITATION IS BASICALLY NIL...JUST SCATTERED MID-HIGH
CLOUDS TUESDAY AFTERNOON. THE BEST CHANCES FOR ANY
SHOWERS/THUNDERSTORMS WILL STILL BE ALONG THE COASTAL MOUNTAINS IN
SOUTHWEST CA...WITH MAYBE SOME ISOLATED DEVELOPMENT OVER THE
MOUNTAINS IN NORTHERN/EASTERN ARIZONA. AS HEIGHTS CONTINUE TO TAPER
OFF...DAYTIME MAX TEMPS MAY FALL A DEGREE OR TWO AS WELL.
STILL...HIGHS WILL BE WELL ABOVE NORMAL WITH A FEW OF THE WARMEST
DESERTS REACHING TO AROUND 90 DEGREES.

WEDNESDAY...PROGS ARE IN GOOD AGREEMENT THAT A DEEPER UPPER TROF
WILL DEVELOP JUST OFF THE SRN CA/NRN BAJA COAST ON WEDNESDAY WITH A
CLOSED UPPER LOW SETTING UP NEAR 120 W 30 N...OFF THE FAR SERN CA
COAST. AGAIN...LITTLE CHANGE IN THE SENSIBLE WEATHER IS EXPECTED
ACROSS THE AREA AS DRY SOUTHWEST FLOW ALOFT PERSISTS. THERE MAY BE
JUST ENOUGH MID LEVEL MOISTURE...INSTABILITY AND LIFT OVER THE
HIGHER TERRAIN OF JOSHUA TREE NP TO SUPPORT A SLIGHT CHANCE FOR
AFTERNOON SHOWERS OR THUNDERSTORMS. WE SHOULD SEE HIGH TEMPS
CONTINUE TO DROP OFF A BIT AND MOST DESERTS WILL TOP OUT IN THE MID
TO UPPER 80S UNDER MOSTLY SUNNY SKIES.

THURSDAY THROUGH FRIDAY...MODEL GUIDANCE IS STILL HAVING PROBLEMS
WITH TIMING AND MOISTURE AVAILABILITY FOR THE SYSTEM EXPECTED TO
ARRIVE IN ARIZONA DURING THE LAST PORTION OF WORK WEEK...SOMETIME
THURSDAY INTO FRIDAY. RUN TO RUN AND DAY TO DAY THE PRIME TIME FOR
PRECIP SHIFTS...LATER... EARLIER AND BACK AGAIN. LATEST GFS IS NOW
ABOUT 12 HOURS FASTER THAN THE ECMWF AND IS ALSO WETTER. GFS
SUGGESTS PRIME TIME FOR PRECIP IS THURSDAY INTO THURSDAY
EVENING...WITH THE BEST THREAT OVER SERN AZ. ECMWF IS QUITE DRY
THURSDAY INTO THURSDAY EVENING AND SUGGESTS AFTER MIDNIGHT THURSDAY
INTO FRIDAY MORNING. WITH A CLOSED LOW SOLUTION TEND TO FAVOR THE
SLOWER EMCWF BUT DUE TO INCONSISTENCY AND LACK OF CONFIDENCE...OPTED
TO MAKE NO MAJOR CHANGES AND MAINLY BLENDED OUR FORECAST WITH
SURROUNDING OFFICES. WILL KEEP A CHANCE OF SHOWERS GOING ACROSS
SOUTH CENTRAL AZ THURSDAY INTO THURSDAY NIGHT... BECOMING A SLIGHT
CHANCE FRIDAY. SLIGHT CHANCES FOR AFTERNOON THUNDERSTORMS SEEM A
GOOD BET AS GUIDANCE CALLS FOR MODEST CAPE AND LIFTED INDICES DOWN
TO MINUS 2 OR SO. CONFIDENCE IN THE TEMPERATURE FORECAST FOR
THURSDAY AND EVEN FRIDAY IS STILL ON THE LOW SIDE AS A LOT DEPENDS
ON THE CLOUD COVER AND POTENTIAL FOR RAIN. EITHER WAY...THURSDAY
LOOKS LIKE IT WILL BE THE COOLEST DAY WITH HIGHS 5-10 DEGREES BELOW
NORMAL WITH SOME RECOVERY IN TEMPERATURES ON FRIDAY AS THE TROUGH
EXITS TO EAST AND SOME WEAK UPPER LEVEL RIDGING TAKES OVER. THE
COMING WEEKEND SHOULD SEE A RETURN TO MORE TYPICAL LATE APRIL
WEATHER WITH DRY CONDITIONS AND TEMPERATURES BACK NEAR 90 DEGREES
OVER THE LOWER DESERTS.

&&

.AVIATION... SOUTH-CENTRAL ARIZONA INCLUDING KPHX...KIWA...AND
KSDL...SOUTHEAST CALIFORNIA AND SOUTHWEST ARIZONA INCLUDING KIPL AND
KBLH...

MOSTLY CLEAR SKIES AND LIGHT WINDS EXPECTED AT ALL TAF SITES THROUGH
AT LEAST 17Z TUESDAY. SCATTERED MID-UPPER LEVEL CLOUD DECKS AOA
12KFT FORM DURING THE AFTERNOON...AND WINDS BECOME LOCALLY GUSTY UP
TO 20KT OR SO.

AVIATION DISCUSSION NOT UPDATED FOR AMENDED TAFS.

&&

.FIRE WEATHER...
THURSDAY THROUGH MONDAY...
A RATHER STRONG (FOR APRIL) PACIFIC STORM SYSTEM IS STILL EXPECTED
TO BRING MUCH COOLER TEMPERATURES AND SCATTERED SHOWERS...AND AN
ISOLATED THUNDERSTORM TO THE REGION ON THURSDAY AND THURSDAY
NIGHT...WITH MANY LOWER DESERT LOCATIONS SEEING HIGHS RISING ONLY IN
THE 70S ON THURSDAY AFTERNOON. SHOWERS TO LINGER INTO FRIDAY ACROSS
SOUTH-CENTRAL ARIZONA WITH CLEARING SKIES AND SOMEWHAT WARMER
TEMPERATURES ACROSS SOUTHWEST ARIZONA AND SOUTHEAST CA...AS THE
STORM SYSTEM MOVES OFF TO THE EAST. DRY CONDITIONS TO RETURN FROM
SATURDAY INTO NEXT MONDAY...WITH A SLOW WARMING TREND...AS HIGH
PRESSURE GRADUALLY REBUILDS BACK INLAND OVER THE REGION. MINIMUM
HUMIDITIES IN THE 15 TO 25 PERCENT RANGE ON THURSDAY TO SLOWLY DROP
BACK DOWN INTO THE 8 TO 18 PERCENT RANGE BY MONDAY...WITH FAIR TO
GOOD OVERNIGHT RECOVERIES.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...

AZ...NONE.
CA...NONE.

&&

$$

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

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


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