Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 210936
AFDPSR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PHOENIX AZ
236 AM MST TUE APR 21 2015

.SYNOPSIS...
THE REGION WILL GET ONE MORE DAY OF ABOVE NORMAL TEMPERATURES BEFORE
A BROAD AREA OF LOW PRESSURE BRINGS CHANGES STARTING ON
WEDNESDAY...LASTING THROUGH THE END OF THE WORK WEEK. WE CAN EXPECT
CONSIDERABLY COOLER TEMPERATURES FOR THURSDAY AND FRIDAY ALONG WITH
CHANCES FOR SHOWERS AND POSSIBLY A THUNDERSTORM BOTH DAYS. HIGH
PRESSURE AND WARMER TEMPERATURES WILL EVENTUALLY SETTLE BACK INTO
THE REGION FOR THE COMING WEEKEND.

&&

.DISCUSSION...
SOUTHERLY FLOW ALOFT HAS SETTLED OVER THE REGION AHEAD OF AN
APPROACHING UPPER LEVEL LOW THAT IS CURRENTLY CENTERED JUST OFF THE
SOUTHERN CALIFORNIA COAST. CURRENT SATELLITE IMAGERY REVEALS LITTLE
IN THE WAY OF ANY MID AND UPPER LEVEL MOISTURE NEAR AND AROUND THE
LOW CIRCULATION AS ONLY OCEAN STRATUS IS PRESENT. WATER VAPOR
IMAGERY INDICATES VERY DRY AIR AROUND THE SOUTHERN PERIPHERY OF THE
BROAD TROUGH...EXTENDING INTO SOUTHERN ARIZONA. THERE WAS ENOUGH
BOUNDARY LAYER MOISTURE TO SPARK OFF A FEW SHOWERS AND THUNDERSTORMS
IN A FAVORED VERTICAL ACCENT AREA JUST EAST OF THE LOW CIRCULATION
IN AN AREA OVER THE NORTHERN BAJA MOUNTAINS LATE YESTERDAY
AFTERNOON. AS THIS FIRST LOW CIRCULATION DRIFTS EASTWARD TODAY
MOVING OVER SOUTHERN CALIFORNIA BY THIS AFTERNOON...MODEL GUIDANCE
SHOWS THE POSSIBILITY OF MORE SHOWERS AND THUNDERSTORMS...MAINLY
FOCUSED OVER THE INLAND MOUNTAINS OF SOUTHERN CALIFORNIA. SLIGHT
CHANCE POPS FOR OUR JTNP AREA ARE IN THE FORECAST FOR THIS
AFTERNOON. ELSEWHERE...DRY CONDITIONS WILL REMAIN WITH SOME SLIGHT
COOLING AS THE LOW ENTERS THE REGION. HIGHS THIS AFTERNOON SHOULD
PEAK IN THE MIDDLE TO UPPER 80S ACROSS OUR WESTERN DESERTS AND UPPER
80S IN SOUTH-CENTRAL ARIZONA.

THE EVOLUTION OF THE BROAD LOW PRESSURE TROUGH REMAINS SOMEWHAT
UNCERTAIN AS MODEL DIFFERENCES REMAIN WITH SUBTLE CHANGES FROM RUN
TO RUN. COMPARED TO THE PREVIOUS COUPLE DAYS MODEL RUNS...THE LATEST
12Z AND 00Z MODEL RUNS OF THE GFS AND EUROPEAN BOTH SCALE BACK THE
STRENGTH OF THE SECOND SHORTWAVE TROUGH FORECAST TO MOVE THROUGH THE
REGION ON THURSDAY. THIS SLIGHTLY WEAKER WAVE SOLUTION LIKELY WON/T
AFFECT HOW MUCH LOW LEVEL MOISTURE MAKES ITS WAY INTO ARIZONA AS
THIS HAS NOT CHANGED...BUT WEAKER BROAD SCALE UPPER LEVEL FORCING
AND SLIGHTLY WEAKER UPSLOPE LOW LEVEL SOUTHERLY FLOW KEEPS US FROM
RAISING POPS ANY FURTHER. THURSDAY AFTERNOON INTO THURSDAY NIGHT
STILL LOOKS TO BE THE BEST TIME FRAME FOR ANY SHOWERS AND MAYBE A FEW
THUNDERSTORMS WITH THE HIGHEST POPS OVER THE HIGHER TERRAIN NORTH
AND NORTHEAST OF PHOENIX. THERE IS STILL QUITE A BIT OF DOUBT
WHETHER THE LOWER DESERTS WILL SEE MUCH IN THE WAY OF MEASURABLE
RAINFALL.

TEMPERATURES WILL TAKE A BIG HIT FOR THURSDAY WITH HIGHS MOST LIKELY
IN THE UPPER 70S TO AROUND 80 ACROSS THE DESERTS...BUT WITH THE
POSSIBILITY OF SHOWERS DURING THE DAY SOME LOCATIONS MAY STRUGGLE TO
REACH ABOVE THE LOW TO MID 70S. THE MAIN SHORTWAVE TROUGH SHOULD
QUICKLY EXIT THE AREA LATE THURSDAY NIGHT...BUT GUIDANCE CONTINUES
TO SHOW AT LEAST A SLIGHT CHANCE OF SOME LINGERING SHOWERS OVER
SOUTH-CENTRAL ARIZONA INTO FRIDAY MORNING AND POSSIBLY INTO THE
AFTERNOON.

THE OVERALL SYNOPTIC PATTERN REMAINS PROGRESSIVE GOING INTO THE
WEEKEND WITH ANOTHER WEAK SHORTWAVE TROUGH MOVING THROUGH SOMETIME
FRIDAY NIGHT OR SATURDAY. FOR NOW...THIS FEATURE SEEMS TO BE TOO
WEAK TO ADD IN ANY POPS FOR SATURDAY...BUT WE WILL HAVE TO KEEP ANY
EYE ON HOW THE MODELS RESOLVE THIS FEATURE IN COMING MODEL RUNS.
TEMPERATURES WILL GRADUALLY RECOVER THIS WEEKEND WITH HIGHS BACK TO
AROUND NORMAL BY SUNDAY. EVEN WARMER AIR IS SHOWN BY BOTH THE GFS
AND EUROPEAN FOR EARLY NEXT WEEK AS A RATHER STRONG PACIFIC RIDGE
SLIDES OVER THE WESTERN UNITED STATES. THIS MAY RESULT IN HIGH
TEMPERATURES WELL INTO THE 90S BY THE EARLY TO MIDDLE OF NEXT WEEK.

&&

.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...KUHLMAN
AVIATION...MEYERS
FIRE WEATHER...PERCHA



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