Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 270534 AAA
AFDPSR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PHOENIX AZ
1035 PM MST THU FEB 26 2015

.UPDATE...UPDATED AVIATION DISCUSSION.

.SYNOPSIS...
DRY WEATHER WITH ABOVE NORMAL TEMPERATURES ARE EXPECTED THROUGH THE
END OF THE WEEK. HOWEVER...LOW PRESSURE WILL APPROACH THE AREA THIS
WEEKEND BRINGING GUSTY WINDS AND WIDESPREAD RAINFALL TO THE AREA
ALONG WITH MUCH COOLER TEMPERATURES. COOL AND UNSETTLED WEATHER IS
LIKELY TO CONTINUE INTO EARLY NEXT WEEK.

&&

.DISCUSSION...
ANOTHER VERY PLEASANT DAY FOR OUTDOOR ACTIVITIES ACROSS OUR CWA
TODAY AS A DRY NORTHWESTERLY FLOW ALOFT KEPT SKIES MAINLY
CLEAR...EXCEPT FOR SOME PATCHY HIGH CLOUDINESS PASSING OVERHEAD FROM
TIME-TO-TIME. PHOENIX SKY HARBOR SAW A HIGH OF 77F TODAY...WHICH IS
4 DEGREES ABOVE THE NORMAL FOR THIS DATE. AFTER ANOTHER NIGHT WITH
LOWS IN THE 40S AND 50S...WE SHOULD SEE ONE MORE DRY DAY WITH HIGHS
IN THE 70S TO NEAR 80 ON FRIDAY BEFORE A MAJOR CHANGE IN THE WEATHER
OCCURS THIS WEEKEND...AS AN UPPER LOW...WHICH IS JUST NOW LOCATED
JUST WEST OF VANCOUVER ISLAND...MOVES SOUTHWARD ALONG THE WEST COAST
ON FRIDAY AND FRIDAY NIGHT. THE LATEST 00Z NAM...AND 00Z GFS ARE NOW
SHOWING LIGHT SHOWERS...AND HIGHER ELEVATION SNOWS DEVELOPING ON
SATURDAY AND SATURDAY NIGHT OVER THE HIGHER TERRAIN OF NORTHERN AND
CENTRAL ARIZONA...WITH A FEW SHOWERS OVER THE LOWER DESERTS...AS AN
INCREASINGLY MOIST WESTERLY FLOW IS OROGRAPHICALLY LIFTED INTO THE
REGION. MORE SUBSTANTIAL RAINS OVER THE LOWER DESERTS...WITH HEAVY
PRECIP POSSIBLE OVER THE HIGHER TERRAIN LOOK TO BE IN THE CARDS FOR
SUNDAY THROUGH MONDAY AS THE MAIN FRONTAL BAND/UPPER JET PROVIDE THE
NECESSARY LIFT OF WHAT WILL BE A VERY MOIST AIRMASS (PWATS AROUND
1.00 INCH)...AS A BAND OF LIGHT-MODERATE RAIN/SNOW (WITH EMBEDDED
HEAVIER SHOWERS) MOVES SLOWLY ACROSS ARIZONA. AS FAR AS THE SHORT
TERM IS CONCERNED...INHERITED FORECASTS FOR TONIGHT AND FRIDAY LOOK
GOOD...AND NO FURTHER UPDATES ARE PLANNED.

&&

.PREVIOUS DISCUSSION...
EARLY AFTERNOON SATELLITE IMAGERY REVEALING MOSTLY CLEAR SKIES FOR
THE SOUTHERN HALF OF ARIZONA AND MOST OF SOUTHEAST CALIFORNIA. THE
UPPER LEVEL RIDGE OF HIGH PRESSURE IS STILL IN PLACE ACROSS THE
EASTERN PACIFIC...DRIVING A NORTHWEST FLOW OF DRY AIR INTO THE
SOUTHWEST STATES. INCREASING MID AND HIGH CLOUDS OVER THE NEXT 24
HOURS WITH THE PASSAGE OF A SHORTWAVE FEATURE AND BROAD AREA OF
HEIGHT FALLS WILL START FILTERING INTO THE SOUTHWEST. TEMPERATURES
TOMORROW WILL BE SIMILAR TO THAT OF TODAY...SEVERAL DEGREES ABOVE
NORMAL.

THE BIG WEATHER CHANGE WILL START SATURDAY AS THE UPPER LEVEL LOW
DIGS DOWN THE WEST COAST AND THE JET STREAM NOSES INTO SOUTHEAST
CALIFORNIA AND SOUTHWEST ARIZONA. A MODEST AMOUNT OF MOISTURE
INCREASE OVER THE AREA STARTING SATURDAY WILL HELP INITIATE A
PROLONGED PERIOD OF UNSETTLED WEATHER SATURDAY THROUGH MONDAY.
PRECIPITABLE WATER VALUES CLIMB TO OVER HALF AN INCH BY SATURDAY
MORNING...AND CONTINUE TO RISE TO OVER AN INCH ACROSS SOUTHCENTRAL
ARIZONA BY MONDAY. A 115 KT JET NOSES INTO SOUTHWEST ARIZONA WHICH
CREATES A TIGHT TEMPERATURE GRADIENT...500 MB TEMPS SPANNING -15C TO
-26C...AND WILL INCREASE THE WIND SPEEDS MAINLY ACROSS SOUTHEAST
CALIFORNIA FOR SATURDAY. WINDS GUSTING SATURDAY BETWEEN 30 TO 35 KTS
INTERMITTENTLY AS THE FRONT PUSHES THROUGH WILL APPROACH WIND
ADVISORY CRITERIA BUT AT THIS TIME THINK WE`LL HOLD OFF ON ANY
HEADLINES. FROM LATE SATURDAY THROUGH MOST OF MONDAY...THE COLD
FRONT APPEARS TO STALL AND HAVE A VERY SLOW PROGRESSION AS THE LOW
BEGINS TO STRETCH OUT. BY LATE MONDAY...THE TROUGH WILL BEGIN TO
FLATTEN OUT AND LIFT EASTWARD...WITH ANOTHER WAVE OF ENERGY ROTATING
AROUND THE TROUGH TUESDAY.

AS WITH ANY MAJOR PRECIPITATION EVENT...THE TIMING AND EXACT AMOUNTS
ARE OF HIGH QUESTION. OPERATIONAL MODEL GUIDANCE SUGGESTS PRECIP
WILL BEGIN AFTER 12Z SATURDAY ACROSS NORTHERN ARIZONA AND SOUTHERN
CALIFORNIA AND WILL FILL IN ACROSS THE LOWER DESERTS LATE SATURDAY
INTO SUNDAY...AND AGAIN LINGER THROUGH LATER IN THE DAY MONDAY. AS
MODELS CONTINUE TO MAKE ADJUSTMENTS...12Z SUNDAY THROUGH 12Z MONDAY
SEEMS TO HAVE A DECENT HEAVY RAIN TIME PERIOD WITH THE 00Z MONDAY
THROUGH 12Z MONDAY POTENTIALLY HOLDING THE HIGHER RAIN AMOUNTS. POP
GRIDS WERE ADJUSTED ACCORDINGLY...AS WELL AS SOME MINOR CHANGES TO
THE QPF GRIDS. THIS STORM IS PANNING OUT FOR A DECENT CHANCE OF 1 TO
1.25 INCHES AROUND THE PHOENIX AREA AND VALUES UPWARDS OF 1.5 TO 2
INCHES ALONG THE HIGHER TERRAIN NORTH AND EAST OF PHOENIX...OVER A
36 TO 48 HOUR TIME PERIOD.

MONDAY WILL BE A TRANSITION DAY AS THE COLD CORE OF THE SYSTEM MAKES
SOME PROGRESS EASTWARD. 500 MB TEMPS DIP INTO -23C FIRST OVER
PORTIONS OF JOSHUA TREE NATIONAL PARK AND INTO CENTRAL ARIZONA BY
00Z TUESDAY. THE TIMING OF PRECIPITATION RIGHT NOW LOOKS TO BE
SLIGHTLY OFF WHEN THE COLD CORE PUNCHES THROUGH SOUTHEAST
CALIFORNIA...SO THE RAIN-SNOW MIX SHOULD NOT ACCUMULATE IN JOSHUA
TREE. HOWEVER...WHEN THE COLD CORE MOVES INTO CENTRAL ARIZONA LATE
MONDAY INTO TUESDAY A CONVECTIVE PRECIP THREAT WILL REMAIN INTO THE
HIGHER TERRAIN AREAS NORTH AND EAST OF PHOENIX. RAIN TO SNOW
ACCUMULATION BETWEEN 00Z TUESDAY AND 00Z WEDNESDAY WILL LIKELY BE
RESTRICTED TO ELEVATIONS OF SOUTHERN GILA COUNTRY ABOVE 5500 FT. ALL
THIS SAID...AT THIS TIME...NOT TOO CONCERNED WITH MAJOR FLOODING OR
ANY HEADLINE PRODUCTS AT THIS TIME.


CONDITIONS REMAIN UNSETTLED INTO TUESDAY AS WELL AS A SECONDARY
TROUGH DIVES THROUGH THE GREAT BASIN INTO ARIZONA. THE IMPACTS OF
THIS TROUGH ARE MUCH LESS CLEAR AS THERE IS A TREMENDOUS AMOUNT OF
SPREAD IN THE ENSEMBLES WITH RESPECT TO THE MAGNITUDE OF THIS TROUGH
AND ITS EXPECTED TRAJECTORY THROUGH THE GREAT BASIN. GFS-BASED
GUIDANCE INDICATES A MORE SOUTHWARD TRAJECTORY ALONG WITH SCATTERED
SHOWERS ACROSS THE AREA WHEREAS THE ECMWF AND ITS MEMBERS KEEP ALL
PRECIP WELL NORTH OF THE FORECAST AREA. GOING TO MAINTAIN POPS FOR
SOME PORTIONS OF THE AREA ON TUESDAY PER THE GFS. WITH ANOMALOUSLY
COLD TEMPERATURES ALOFT IT SHOULD BE EASY FOR SCATTERED SHOWERS TO
DEVELOP WITH A MINIMAL AMOUNT OF FORCING BUT THE FINER DETAILS WILL
NEED TO BE SORTED OUT DOWN THE ROAD. FINALLY BY MID WEEK ANY/ALL
FORCING ASSOCIATED WITH THIS UPPER LOW SHIFTS TO THE EAST AND DRY
NORTHWESTERLY FLOW TAKES HOLD.

FOR AS WARM AS FEBRUARY HAS BEEN...TEMPERATURES THIS WEEKEND INTO
THE FIRST FEW DAYS OF MARCH WILL MORE THAN MAKE UP FOR IT.
CLOUDS/SHOWERS ON SUNDAY AND MONDAY COULD CONCEIVABLY KEEP HIGH
TEMPS IN THE UPPER 50S ACROSS THE DESERTS ALTHOUGH SOME MODERATION
IS EXPECTED BY THE MIDDLE OF THE WEEK. POST-FRONTAL DRY ADVECTION
TUESDAY/WEDNESDAY WILL RESULT IN SOME CHILLY MORNINGS BY MID WEEK
WITH LOWS IN THE MID 30S TO LOWER 40S ACROSS THE DESERTS. DEPENDING
ON HOW QUICKLY SKIES CLEAR AND WINDS DIE DOWN...MAY NEED TO ADDRESS
THE POTENTIAL FOR FREEZE HEADLINES IN SOME PORTIONS OF THE FORECAST
AREA /ESPECIALLY IF THE COOLER AND DRIER ECMWF VERIFIES/.

&&

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

DRY NORTHWESTERLY FLOW ALOFT WILL KEEP DRY CONDITIONS IN PLACE AT
ALL THE TERMINALS THROUGH THE TAF PERIOD. A WEATHER DISTURBANCE
APPROACHING FROM THE NORTHWEST WILL BEGIN TO INCREASE THE HIGH CLOUD
COVERAGE OVER THE REGION ON THURSDAY AFTERNOON/EVENING...AND BEGIN
TO INCREASE THE SOUTHWESTERLY WINDS AT KIPL AND SOUTHERLY WINDS AT
KBLH BY THURSDAY EVENING. NO SIGNIFICANT AVIATION IMPACTS THROUGH
THE TAF PERIOD.

AVIATION DISCUSSION NOT UPDATED FOR AMENDED TAFS.

&&

.FIRE WEATHER...
SATURDAY THROUGH WEDNESDAY...
CONDITIONS WILL TURN MUCH COOLER...WETTER AND UNSETTLED STARTING
SATURDAY AS A STRONG LOW PRESSURE SYSTEM NEARS THE REGION. BREEZY TO
WINDY WESTERLY WINDS CAN BE EXPECTED ACROSS SOUTHEAST CALIFORNIA AND
SOUTHWEST ARIZONA FOR SATURDAY AFTERNOON. RAIN WILL BECOME LIKELY
SATURDAY NIGHT ACROSS SOUTH-CENTRAL ARIZONA AND PERSIST THROUGH
SUNDAY. HIGH TEMPERATURES ACROSS THE DESERTS WILL FALL INTO THE
UPPER 50S TO LOW 60S STARTING SUNDAY...LASTING INTO THE MIDDLE OF
THE WEEK. HUMIDITY LEVELS WILL BE ELEVATED THROUGHOUT THE ENTIRE
PERIOD. A SECOND LOW PRESSURE SYSTEM MAY BRING CONTINUED
PRECIPITATION CHANCES FOR TUESDAY AND WEDNESDAY.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...

AZ...NONE.

CA...NONE.

&&

$$

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

DISCUSSION...PERCHA/DEWEY/LEINS
AVIATION...PERCHA
FIRE WEATHER...KUHLMAN





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