Area Forecast Discussion
Issued by NWS Phoenix, AZ

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000
FXUS65 KPSR 281207
AFDPSR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PHOENIX AZ
507 AM MST SAT FEB 28 2015

.UPDATE...UPDATED AVIATION AND FIRE WEATHER DISCUSSIONS...

&&

.SYNOPSIS...
LOW PRESSURE WILL SLOWLY APPROACH THE AREA THIS WEEKEND...BRINGING
MOISTURE INTO THE REGION ALONG WITH INCREASING CHANCES OF SHOWERS
SUNDAY AND MONDAY. PERIODS OF HEAVY RAIN ARE POSSIBLE AT TIMES
ESPECIALLY SUNDAY NIGHT THROUGH MONDAY AFTERNOON. COOLER
TEMPERATURES WILL ALSO ACCOMPANY THIS STORM WITH BELOW NORMAL
READINGS EXPECTED MONDAY THROUGH LATE WEEK.

&&

.DISCUSSION...

...A PROLONGED WET WINTER WEATHER EVENT IS IN STORE FOR THE DESERT
SOUTHWEST SUNDAY THROUGH LATE MONDAY...

EARLY MORNING WATER VAPOR IMAGERY REVEALS DEEPENING LOW PRESSURE
OVER OREGON WITH AN INITIAL SHORTWAVE TROUGH DEVELOPING OVER
PORTIONS OF NEVADA AND CENTRAL CALIFORNIA. TO THE SOUTH ACROSS
ARIZONA AND SOUTHERN CALIFORNIA...THE LOW HAS STARTED ADVECTING
SOMEWHAT HIGHER PWATS AND SURFACE DEWPOINTS INTO THE AREA...EVEN
STARTING TO SEE SOME SHOWERS ON RADAR ACROSS CENTRAL ARIZONA NEAR
THE MID LEVEL MOISTURE GRADIENT. HOWEVER ACROSS THE LOWER DESERTS
THIS MORNING...THERE WAS ONLY A SUBTLE INCREASE IN CLOUD COVER. 09Z
TEMPERATURES WERE QUITE A BIT WARMER THAN THIS TIME FRIDAY WITH MANY
LOCATIONS STILL IN THE LOWER 60S.

A COUPLE OF THINGS TO ADDRESS THIS MORNING WITH RESPECT TO THE
POTENTIAL FOR RAINFALL ACROSS THE AREA THIS WEEKEND INTO EARLY NEXT
WEEK. AS HAS BEEN THE CASE FOR THE LAST SEVERAL DAYS...ALL
OPERATIONAL MODELS ARE IN AGREEMENT THAT THE UPSTREAM LOW WILL SAG
SOUTHWARD...DEEPEN...AND RETROGRADE WEST OF SAN DIEGO THIS WEEKEND.
THIS STILL LOOKS ON TARGET HOWEVER THERE ARE ONCE AGAIN SIGNALS IN
THIS MORNING`S 00Z GUIDANCE TO SUGGEST PRECIP WILL BE SLOW TO ARRIVE
ACROSS THE AREA...ESPECIALLY IN THE PHOENIX METRO.

CONCEPTUALLY...MUCH OF TODAY WILL BE SPENT MOISTENING THE BOUNDARY
LAYER AS THE LOW IS STILL SEVERAL HUNDRED MILES AWAY AND HAS YET TO
TAP INTO THE DEEPER/MORE SUBSTANTIAL SUBTROPICAL MOISTURE SOURCE
SOUTH OF BAJA CALIFORNIA. IN ALL LIKELIHOOD THERE WILL BE SCATTERED
SHOWERS TO THE NORTH AND EAST OF PHOENIX WITH A FEW ISOLATED SHOWERS
ACROSS THE METRO...BUT THAT`S ABOUT IT. THIS IDEA IS SUPPORTED BY
ALL OPERATIONAL 18Z AND 00Z DETERMINISTIC MODELS AND IS ALSO WITHIN
THE WINDOW OF THE LATEST /07Z AND 08Z/ HRRR RUNS. MADE SOME DOWNWARD
ADJUSTMENTS TO THE POPS FOR TODAY ESPECIALLY ACROSS THE LOWER
DESERTS ALONG WITH SOME UPWARD ADJUSTMENTS TO TEMPERATURES. NOT
GOING TO REMOVE THE MENTION OF PRECIP ALTOGETHER FOR THE PHOENIX
AREA AS THERE COULD EASILY BE 20-30 PERCENT COVERAGE ACROSS THE
METRO. BUT FOR THE MOST PART TODAY LOOKS TO BE MORE DRY THAN WET.

BY THIS EVENING THE LOW WILL BE LOCATED WEST OF LOS ANGELES BUT
STILL TOO FAR NORTH TO TAP INTO THE AFOREMENTIONED SUBTROPICAL
MOISTURE PLUME NEAR THE BAJA SPUR. THE MODELS ARE NOW SUGGESTING THE
BEST CHANCE OF PRECIP ACROSS SOUTHEAST CALIFORNIA AND FAR SW ARIZONA
WON`T OCCUR UNTIL DURING THE DAY ON SUNDAY...ABOUT 6-12 HRS SLOWER
THAN PREVIOUSLY INDICATED. FOLLOWING THAT TREND...PRECIP ISN`T
EXPECTED TO MOVE INTO THE PHOENIX AREA UNTIL MUCH LATER AS WELL
/SUNDAY AFTERNOON OR EVENING/. THUS I MADE SOME ADJUSTMENTS TO THE
POPS/QPF AND FLOOD WATCH TIMING FOR THIS EVENT TO BETTER MATCH THE
00Z DETERMINISTIC AND PROBABILISTIC GUIDANCE. I`M NOT REMOVING POPS
ALTOGETHER FOR THE PHOENIX AREA SUNDAY BUT DRASTICALLY TRIMMING
VALUES AND HIGHLIGHTING SUNDAY EVENING THROUGH MID-DAY MONDAY AS THE
PRIME-TIME FOR SHOWERS ACROSS THE AREA. POPS DURING THIS SUNDAY
EVENING-MONDAY TIME PERIOD STILL LOOK GOOD IN THE 80-100 PERCENT
RANGE AS THIS IS WHEN THE UPPER LOW FINALLY GETS A KICK FROM ANOTHER
UPSTREAM TROUGH. ALSO NOTED A SOMEWHAT CONCERNING REDUCTION IN
ENSEMBLE QPF THIS MORNING WITH THE HIGHEST MEMBERS INDICATING AROUND
1 INCH OF PRECIP IN THE PHOENIX AREA...WITH A MEAN MORE IN THE
0.5-0.75 INCH REALM. PERHAPS THIS IS A RESULT OF THE MORE
PROGRESSIVE NATURE OF THE TROUGH VS THE SOMEWHAT NEGATIVELY TILTED
DEPICTION THE MODELS WERE INDICATING THE PAST FEW DAYS.
REGARDLESS...THERE WILL ALMOST CERTAINLY BE LOCALLY HIGHER VALUES
ESPECIALLY IN THE OROGRAPHICALLY FAVORED AREAS NORTH/EAST OF PHOENIX
WITHIN THE INHERITED FLOOD WATCH BUT I WILL MAKE SOME REDUCTIONS TO
THE TOTAL QPF FORECAST ESPECIALLY ACROSS THE SOUTHERN HALF OF THE
FORECAST AREA. WILL ADJUST THE FLOOD WATCH TO RUN FROM 5 PM SUNDAY
TO 5 PM MONDAY.

RAINFALL WILL QUICKLY MOVE THROUGH THE AREA MONDAY BUT ONE MORE
UPSTREAM TROUGH WILL DIVE SOUTHWARD TUESDAY AFTERNOON. PRECIP
CHANCES WITH THIS TROUGH WILL BE HIGHEST NORTH/EAST OF PHOENIX BUT
STILL THINK THERE COULD BE SOME SCATTERED SHOWERS ACROSS THE
DESERTS. WILL LEAVE THE FORECAST FOR TUESDAY LARGELY UNCHANGED WITH
20-30 POPS AROUND PHOENIX AND 40-50 POPS IN THE HIGHER TERRAIN.
AFTER THAT...DRY NORTHWESTERLY FLOW WILL PERSIST THROUGH MID
WEEK...FOLLOWED BY LATE WEEK RIDGING/WARMING. TEMPERATURES WILL
REACH THEIR LOWEST VALUES ON MONDAY AND WILL SLOWLY REBOUND BACK
INTO THE UPPER 70S/LOWER 80S BY LATE NEXT WEEK.

&&

.AVIATION...
SOUTH-CENTRAL ARIZONA INCLUDING KPHX...KIWA AND KSDL...

AN APPROACHING WEATHER SYSTEM WILL BRING BREEZY SOUTHWESTERLY WINDS
DURING THE DAYTIME HOURS TODAY WITH GUSTS APPROACHING 25 KNOTS THIS
AFTERNOON. INCREASING LOW LEVEL MOISTURE WILL CAUSE PERIODIC LOW VFR
CIGS THROUGH THIS EVENING AND THEN MORE PERSISTENT BROKEN CIGS AT
5-6K FEET LATE TONIGHT. THERE IS A CHANCE OF FEW-SCT SHOWERS MAINLY
ACROSS THE NORTHERN FRINGES OF THE PHOENIX AREA THROUGHOUT THE
PERIOD WITH INCREASING CHANCES ON SUNDAY. CONFIDENCE IS TOO LOW TO
INCLUDE IN THE TAFS AT THIS TIME...EXCEPT FOR VCSH AT KSDL.

SOUTHEAST CALIFORNIA AND SOUTHWEST ARIZONA INCLUDING KIPL AND KBLH...

LOW PRESSURE APPROACHING FROM THE NORTHWEST WILL KEEP GUSTY
SOUTHWESTERLY WINDS GOING THROUGH EARLY THIS EVENING WITH SOME GUSTS
APPROACHING 35 KNOTS THIS AFTERNOON. THERE IS AN OUTSIDE CHANCE OF
BLOWING DUST POSSIBLE FOR A TIME THIS AFTERNOON AT BOTH TAF SITES.
INCREASING MOISTURE LEVELS THROUGH THE COLUMN WILL ALLOW LOW VFR
CIGS TO DEVELOP THIS EVENING AND PERSIST THROUGH THE OVERNIGHT
HOURS.

AVIATION DISCUSSION NOT UPDATED FOR AMENDED TAFS.

&&

.FIRE WEATHER...
MONDAY THROUGH FRIDAY...
COOL AND WET CONDITIONS WILL BE PRESENT ON MONDAY AND LIKELY INTO
TUESDAY WITH THE BEST CHANCES FOR WIDESPREAD RAINS ON MONDAY. THERE
WILL BE A CHANCE OF A FEW THUNDERSTORMS ON MONDAY. HUMIDITIES WILL
BE WELL ABOVE TYPICAL VALUES THROUGH TUESDAY BEFORE MINIMUM VALUES
FALL BACK INTO THE TEENS STARTING THURSDAY AFTERNOON. PERIODS OF
BREEZY SOUTHWESTERLY WINDS CAN BE EXPECTED ON MONDAY AND AGAIN ON
WEDNESDAY. HIGH TEMPERATURES ACROSS THE DESERTS WILL ONLY REACH INTO
THE 60S THROUGH WEDNESDAY. HIGH PRESSURE BRINGING WARMER AND DRIER
WEATHER SHOULD MOVE IN STARTING LATER WEDNESDAY AND SHOULD PERSIST
THROUGH THE END OF THE WEEK.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...

AZ...FLOOD WATCH FROM SUNDAY AFTERNOON THROUGH MONDAY AFTERNOON
     AZZ021>024.

CA...NONE.

&&

$$

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

DISCUSSION...LEINS
AVIATION...KUHLMAN
FIRE WEATHER...KUHLMAN










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