Dust Devil in Arizona, May 13, 2011

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PHOENIX AZ
956 AM MST FRI MAY 13 2011

.SYNOPSIS...
ADDITIONAL WARMING CAN BE EXPECTED TODAY WITH HIGHS ON THE LOW
DESERTS IN THE UPPER 90S TO NEAR 100. A COOLING TREND...WITH BREEZY
TO WINDY CONDITIONS AT TIMES...WILL BEGIN SATURDAY AND CONTINUE INTO
EARLY NEXT WEEK AS LOW PRESSURE SYSTEMS FROM THE PACIFIC MOVE
THROUGH THE WESTERN STATES. OTHERWISE...EXPECT A CONTINUATION OF DRY
WEATHER WITH MOSTLY CLEAR NIGHTS AND SUNNY DAYS.

&&

.DISCUSSION...
CLEAR/SUNNY SKIES OVER THE ENTIRE FORECAST AREA THIS MORNING.
YUMA/TUCSON/FLAGSTAFF SOUNDINGS INDICATED BOUNDARY LAYER WARMED BY
4-5C DURING THE PAST 24 HRS. SLIGHT BOUNDARY LAYER DRYING OCCURRED
ACROSS CENTRAL AZ...WITH PRECIPITABLE WATER LESS THAN ONE QUARTER
INCH...WHILE VERY SLIGHT MOISTENING OCCURRED OVER FAR SOUTHEAST
CALIFORNIA...WHERE PRECIPITABLE WATER WAS NEAR TWO THIRDS INCH. AS
OF 9 AM MST/PDT...24 HR TEMP CHANGE AVERAGED ABOUT +5F OVER MOST OF
THE CWA...WHILE SFC DEW POINT TRENDS VARIED (RANGE FROM -4F TO
+6F...WITH LITTLE CHANGE OVER SOUTH-CENTRAL AZ SITES). CURRENT
FORECAST HIGHS LOOK TO BE ON TARGET...WITH MAX TEMPS OVER THE LOWER
DESERT RANGING FROM THE UPPER 90S TO JUST ABOVE 100.

WEAK RIDGING PERSISTED ALOFT OVERNIGHT...WITH 500 MB HEIGHTS NEAR
5800 GPM THIS MORNING. HOWEVER...BUT A WELL-DEFINED UPPER LOW...BEST
SEEN IN WV IMAGERY...WAS APPROACHING THE SOUTHERN CALIFORNIA COAST.
THIS LOW WILL CONTINUE TO MOVE TOWARD THE NORTHEAST TODAY...THEN
WEAKEN AS IT MOVES EAST ACROSS ARIZONA OVERNIGHT. LIMITED LOW-MID
LEVEL MOISTURE EXISTS WITH THIS SYSTEM...SO ONLY EXPECT SCTD MAINLY
MID-HIGH CLOUDS ACROSS FAR SOUTHEAST CA THIS AFTERNOON...AND OVER
PORTIONS OF WEST-CENTRAL AZ OVERNIGHT. MODEST COLD ADVECTION OCCURS
AS THIS SYSTEM MOVES THROUGH...WITH 12Z NAM/GFS FORECAST SOUNDINGS
SHOWING BOUNDARY LAYER COOLING BY 4C OVER SOUTH-CENTRAL AZ BETWEEN
THIS EVENING AND SATURDAY EVENING...AND 2C OVER FAR SE CALIFORNIA/
FAR SW AZ. CURRENT FORECAST HANDLES THESE TEMP TRENDS REASONABLY
WELL...ONLY VERY MODEST CHANGES TO GRIDDED DATABASE THIS
MORNING.

.PREVIOUS DISCUSSION...
STRONG HIGH PRESSURE RIDGE ALOFT IS IN PLACE OVER THE DESERT
SOUTHWEST EARLY THIS MORNING...AND THIS SYSTEM WILL BRING ANOTHER
DAY OF SHARPLY WARMER TEMPERATURES UNDER SUNNY SKIES. 2 AM IR
IMAGERY SHOWED CLEAR SKIES ACROSS THE CWA. LATEST PLOT DATA SHOWED
60M 500MB HEIGHT RISES...AND THE LATEST SOUNDINGS SHOWED MORE WARM
AIR PRESENT IN THE MID LEVELS. GOOD MIXING TODAY WILL BRING THAT AIR
TO THE SURFACE...LOOK FOR HIGHS TO CLIMB INTO THE UPPER 90S TO
AROUND 100 OVER THE WARMEST LOWER DESERTS. ALSO THE VERY DRY NATURE
OF THE ATMOSPHERE...WITH PWAT VALUES BELOW ONE QUARTER INCH...WILL
HELP THE HIGH TEMPS CLIMB WELL ABOVE SEASONAL NORMALS TODAY.

LATEST VAPOR LOOP SHOWED AN AREA OF LOW PRESSURE OFF THE NRN
BAJA/FAR SRN CA COAST MOVING TOWARDS THE NORTHEAST...BEING STEERED
BY A DEEP UPPER TROF NOW DEVELOPING OFF THE PAC NW COAST. THIS
SYSTEM HAS MODERATELY STRONG DYNAMICS/Q CONVERGENCE/PVA...AND WILL
TRACK QUICKLY ACROSS THE CWA LATE TODAY AND TONIGHT...BEFORE
WEAKENING AND MOVING OFF TO OUR EAST ON SATURDAY. THIS WILL BE A DRY
SYSTEM...IR IMAGERY SHOWS LITTLE CLOUDINESS ASSOCIATED WITH THE
FEATURE...SO DESPITE THE DYNAMICS NO PRECIP IS EXPECTED. MAY SEE
SOME MID/HI CLOUD TONIGHT INTO SATURDAY BUT NOTHING EXTENSIVE. MAIN
EFFECT OF THE LOW WILL BE TO LOWER TEMPS A HANDFUL OF DEGREES
SATURDAY. PROGS/MOS AGREE HIGHS WILL FALL BACK INTO THE MID 90S OVER
MUCH OF THE LOWER DESERT SATURDAY UNDER MOSTLY SUNNY SKIES.

THE AFOREMENTIONED DEEP TROF DEVELOPING OFF THE PAC NW COAST WILL BE
THE MAJOR PLAYER IN OUR WEATHER LATER THIS WEEKEND INTO THE MIDDLE
PORTION OF NEXT WEEK...AS IT GIVES US A SHARP COOLING TREND WITH
TIME ALONG WITH INCREASED WIND...MAINLY DURING THE AFTERNOON AND
EVENING HOURS. LATEST GFS AND ECMWF ARE IN PRETTY GOOD AGREEMENT
WITH THEIR HANDLING OF THE TROF NEXT SEVERAL DAYS AND A BLEND OF THE
TWO PROGS WILL BE USED AS THE BASIS FOR THE FORECAST PACKAGE. SUNDAY
INTO MONDAY THE TROF WILL GRADUALLY STRENGTHEN AND SHIFT SLOWLY
EAST...AS THIS OCCURS THE RIDGE OVERHEAD WILL FLATTEN AND SHIFT EAST
ALLOWING FOR COOLER TEMPERATURES TO OVERSPREAD THE AREA. FIRST
SHORTWAVE MOVING AROUND THE DEEP LOW WILL PASS THRU THE WESTERN CWA
SUNDAY...THE ASSOCIATED COLD FRONT WILL LOWER HIGH TEMPS 8-10
DEGREES OVER THE WESTERN ZONES. HIGHS WILL FALL INTO THE LOW TO MID
80S FROM THE FAR SWRN AZ DESERTS WESTWARD. LESSER COOLING IS
EXPECTED ACROSS SOUTH CENTRAL AZ AND HIGHS IN THE PHOENIX AREA
SHOULD STAY IN THE LOWER 90S. ADDITIONAL COOLING WILL OCCUR MONDAY
AS HEIGHTS/THICKNESSES CONTINUE TO FALL.

NEXT TUESDAY INTO WEDNESDAY...THE DEEP LOW ALONG THE PAC NW COAST
OPENS UP AND SHIFTS INLAND OVER THE WESTERN CONUS...AND THICKNESS
VALUES ARE FORECAST TO FALL NEARLY 90M FROM MONDAY AFTERNOON THRU
WED AFTERNOON ACROSS THE CENTRAL AZ DESERTS. AS SUCH...HIGHS WILL
TAKE ANOTHER SHARP DROP AND FALL INTO THE MID 80S IN MANY AREAS
INCLUDING THE GREATER PHOENIX AREA. LATEST MEX MOS AS WELL AS BIAS
CORRECTED GUIDANCE FROM THE GFS AND ECMWF SUPPORT THESE TEMPERATURE
DROPS. HAVE LOWERED OUR HIGHS TEMPS A HANDFUL OF DEGREES FOR TUE AND
WED TO REFLECT THIS CHANGE. SEEING AS HOW JUST A FEW DAYS AGO HIGHS
FELL INTO THE MID 70S IN THE PHOENIX AREA...HIGHS IN THE MID 80S IS
NOT UNREASONABLE AT ALL. THIS SHOULD STILL BE A DRY WEATHER PATTERN
BUT WE HAVE SOME CLIMO TYPE SINGLE DIGIT POPS GOING OVER MOST OF THE
CWA BOTH DAYS JUST IN CASE.

AS THE TROUGH CONTINUES TO TRACK TO THE EAST THURSDAY...AND HIGHER
PRESSURE ALOFT BUILDS IN FROM THE WEST...TEMPS WILL START TO WARM UP
AND MUCH OF THE LOWER DESERT SHOULD SEE HIGHS CLIMB INTO THE UPPER
80S TO NEAR 90 UNDER MOSTLY SUNNY SKIES.

&&

.AVIATION...
SOUTH-CENTRAL ARIZONA AND SOUTHEAST CALIFORNIA...INCLUDING KPHX KIWA
KIPL KBLH AND SURROUNDING AIRFIELDS...

CLEAR WITH A FEW CLOUDS ABOVE 20 THSD FT SPREADING INTO AZ THIS
EVENING. SFC WINDS LIKELY TO VEER TO SOUTHWEST AT ALL TAF SITES AND
BECOMING GUSTY AT TIMES TO 15-20 KTS AFT 20Z.

AVIATION DISCUSSION NOT UPDATED FOR AMENDED TAFS.

&&

.FIRE WEATHER...
SUNDAY THROUGH THURSDAY...
AN INCREASE IN AFTERNOON WIND FROM THE SOUTHWEST IS EXPECTED
SUNDAY...AS THE LOW PRESSURE SYSTEM MOVES THROUGH NORTHERN
CALIFORNIA. TEMPERATURES WILL LIKELY DROP EVEN MORE...AND RH
VALUES ARE EXPECTED TO CLIMB A BIT EACH DAY THROUGH WEDNESDAY
ALTHOUGH NO RAIN IS EXPECTED.

BREEZY TO WINDY WEATHER IS STILL EXPECTED THROUGH THE MIDDLE OF THE
WEEK. THE STRONGEST WINDS WILL BE IN SOUTHEAST CALIFORNIA AND IN THE
HIGHER TERRAIN. CRITICAL FIRE WEATHER THRESHOLDS ARE NOT EXPECTED
AT THIS TIME...BUT WIND GUSTS OF 30 MPH ARE POSSIBLE.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...

AZ...NONE.

CA...NONE.

&&

NATIONAL WEATHER SERVICE PHOENIX IS ON THE INTERNET AT

WEATHER.GOV/PHOENIX

DISCUSSION...DG/CB
AVIATION...SIPPLE
FIRE WEATHER...SIPPLE