Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 231016
AFDPSR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PHOENIX AZ
315 AM MST TUE SEP 23 2014

.SYNOPSIS...
THERE WILL BE A SLIGHT CHANCE FOR SHOWERS AND THUNDERSTORMS CONFINED
TO THE HIGHER MOUNTAINS OF SOUTH-CENTRAL ARIZONA TODAY AND
THURSDAY...WITH DRY CONDITIONS EXPECTED WEDNESDAY. TEMPERATURES WILL
BE AT LEAST A FEW DEGREES ABOVE NORMAL DURING THIS TIME. A PACIFIC
WEATHER SYSTEM WILL LEAD TO A RETURN OF STORM CHANCES ACROSS MUCH OF
THE REGION FRIDAY INTO SUNDAY ALONG WITH COOLER TEMPERATURES.

&&

.DISCUSSION...
TODAY THROUGH THURSDAY...
WATER VAPOR IMAGERY SHOWS AN UPPER LOW CONTINUING TO SHIFT EASTWARD
ONTO THE GREAT PLAINS AND RIDGING BECOMING MORE CONSOLIDATED OVER
THE INTERIOR WESTERN STATES. UPSTREAM IS A DEEPENING TROUGH WITHIN
WHICH IS A WELL DEVELOPED LOW CENTERED OFF THE PACIFIC NORTHWEST
COAST. BLENDED TPW IMAGERY SHOWS MOISTURE HANGING ON OVER MAINLY
SOUTHEAST ARIZONA. EARLY MORNING METARS INDICATE A BIT OF A REVERSAL
IN THE DEW POINT TRENDS. METARS ALSO SHOW AND EASTERLY ORIENTED
PRESSURE GRADIENT OVER PORTIONS OF NEW MEXICO AND ARIZONA ASSOCIATED
WITH REMNANTS OF A BACK DOOR FRONT. MODELS CONTINUE TO SHOW A RIDGE
BUILDING OVER OVER THE SOUTHWEST WITH THE MID LEVEL ANTICYCLONE
BECOMING CENTERED NEAR THE AZ/UT BORDER BY WEDNESDAY. WHAT IS LEFT
OF MEANINGFUL MOISTURE OVER OUR FORECAST AREA WILL TREND DOWN MORE
AS NORTHERLY FLOW ALOFT ADVECTS SOME DRIER AIR. HOWEVER...MODELS
INDICATE THAT THERE WILL STILL BE ENOUGH MOISTURE BY MIDDAY/EARLY
AFTERNOON OVER ZONE 24 TO GET ISOLATED STORM ACTIVITY. SO HELD ON TO
SLIGHT CHANCES THERE. WEDNESDAY LOOKS TO BE THE DRIEST DAY WITH NO
STORM CHANCES FORECAST FOR OUR AREA. BY THURSDAY...A PACIFIC TROUGH
DEEPENS ALONG/NEAR THE WEST COAST WHICH BEGINS A NORTHEASTWARD SHIFT
OF THE RIDGE. THIS ALSO ALLOWS A FOR AN INVERTED TROUGH OVER TEXAS
TO SHIFT WESTWARD A LITTLE BIT. WITH MORE EASTERLY FLOW...SOME
MOISTURE EXPANDS WESTWARD FROM SOUTHEAST ARIZONA BUT AT THE SAME
TIME...SOME DRIER AIR FROM THE NORTH TEMPORARILY MIXES IN AS WELL.
THE NET EFFECT IS A RETURN OF SLIGHT CHANCE POPS TO ZONE 24. AS FOR
TEMPERATURES...ANTICIPATE A LITTLE MORE WARMING TODAY AND WEDNESDAY
WITH LITTLE CHANGE THURSDAY.

FRIDAY THROUGH MONDAY...
ON FRIDAY...GFS AND ECMWF SHOW THE PACIFIC TROUGH DEEPENING AND
MOVING A LITTLE FURTHER INLAND WITH THE AXIS ALONG THE PACIFIC
NORTHWEST/NORTHERN CALIFORNIA COAST AND EXTENDING SOUTHWARD. IN THE
PROCESS...FLOW BECOMES MORE SOUTHERLY AND DEEPER MOISTURE BEGINS TO
SPREAD NORTHWARD OVER OUR FORECAST AREA. THE ECMWF IS STILL A BIT
DEEPER AND SLOWER THAN THE GFS BY FRIDAY. THE GFS HAS TRENDED TOWARD
ECMWF IN TERMS OF DEEPER MOISTURE...PER 1000-700 MB MEAN MIXING
RATIOS...BY SPREADING IT INTO CALIFORNIA. CONVERSELY GFS ALSO SHOWS
A DRY POCKET OVER SOUTHEAST ARIZONA FRIDAY. OVER THE WEEKEND...THE
TIMING OF THE EASTWARD MOVEMENT OF THE TROUGH GETS CLOSER BETWEEN
THE GFS AND ECMWF AS THE SYSTEM SLOWS DOWN AND BECOMES A CLOSED LOW
CENTERED OVER THE GREAT BASIN. THEY BOTH CONTINUE TO SHOW THE BRUNT
OF THE SYSTEM AND THE JET AXIS PASSING NORTH OF OUR FORECAST
AREA...THOUGH THE GFS IS A BIT FURTHER SOUTH THAN IN SOME PREVIOUS
RUNS. QG FORCING PER DIV Q FIELD BEGINS IN OUR WESTERN AREAS FRIDAY
NIGHT AND TRACKS EASTWARD DURING THE DAY SATURDAY. HOWEVER...THE
SYSTEM BEGINS TRACKING NORTHEASTWARD DURING THE DAY SATURDAY FOR
MORE OF A BRUSH-BY OF THE DIV Q FOR OUR EASTERN AREAS. ALSO ON
SATURDAY...A WEAKENING COOL FRONT MOVES ACROSS THE FORECAST AREA.
ANOTHER POTENTIAL SOURCE OF DYNAMICAL LIFT WILL BE UPPER LEVEL
DIVERGENCE WHICH LOOKS TO AFFECT MAINLY EASTERN PORTIONS OF OUR
FORECAST AREA...BETTER TO OUR NORTH. THETA-E ADVECTION BRINGS BRINGS
NOTABLE CAPE INTO PLAY. DEEP LAYER SHEAR LOOKS LIKE IT WILL BECOME
STRONG ENOUGH TO GET ORGANIZED STORMS.

BEST WINDOW OF OPPORTUNITY OF OPPORTUNITY OF SHOWERS AND
THUNDERSTORMS FOR OUR FORECAST AREA LOOKS TO BE LATE FRIDAY THROUGH
SUNDAY MORNING AS THINGS TRANSITION FROM WEST TO EAST. THERE ARE OF
COURSE CAVEATS AS IT IS FAR FROM CERTAIN JUST HOW THE SYSTEM WILL
PLAY OUT IN TERMS OF TIMING...TRACK...AND INTENSITY. THIS IN TURN
AFFECTS THE MOISTURE ADVECTION...CAPE...DYNAMICAL FORCING...AND
TEMPERATURE TRENDS. QUALITATIVELY...IT LOOKS LIKE A TRANSITION EVENT
WITH STORM CHANCES AND COOLER TEMPERATURES IN STORE.


&&

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

CLEAR SKIES AND DRY SFC CONDITIONS EXPECTED THROUGH THE TAF PERIOD
WITH DIURNALLY DRIVEN WINDS REMAINING AOB 10KTS. NO WEATHER IMPACTS
TO AVIATION OPERATIONS EXPECTED.

AVIATION DISCUSSION NOT UPDATED FOR AMENDED TAFS.

&&

.FIRE WEATHER...
THURSDAY THROUGH MONDAY...
WESTERLY FLOW ALOFT TO KEEP MOISTURE LEVELS LIMITED ON
THURSDAY...WITH ONLY A SLIGHT CHANCE FOR STORMS OVER THE HIGHER
TERRAIN NORTH AND EAST OF PHOENIX. MOISTURE LEVELS...AND CHANCES FOR
SHOWERS/THUNDERSTORMS TO INCREASE SOMEWHAT ON FRIDAY...THEN
DRAMATICALLY SO ON SATURDAY AND EARLY SUNDAY AS WINDS ALOFT BECOME
MORE SOUTHERLY AHEAD OF A PACIFIC WEATHER SYSTEM THAT WILL BE
APPROACHING THE AREA FROM THE WEST...PULLING TROPICAL MOISTURE BACK
NORTHWARD INTO THE REGION. WIDESPREAD WETTING RAINS...COOLER
TEMPERATURES...AND BREEZY TO WINDY CONDITIONS ARE POSSIBLE FROM
SATURDAY INTO EARLY SUNDAY AS THE 1ST SFC COLD FRONT OF THE EARLY
FALL SEASON MOVES ACROSS THE REGION. MUCH DRIER AIR TO OVERSPREAD
THE REGION FROM WEST TO EAST FROM LATE SUNDAY INTO MONDAY AS HIGH
PRESSURE BUILD INLAND BEHIND THE FRONT. MINIMUM HUMIDITIES ACROSS
THE LOWER DESERTS...WHICH COULD FALL AS LOW AS 15 PERCENT ON
FRIDAY...ARE EXPECTED TO REMAIN MAINLY AOA 25 PERCENT ON SUNDAY AND
MONDAY...BEFORE FALLING BACK DOWN INTO THE 15-25 PERCENT RANGE ON
MONDAY.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...

AZ...NONE.

CA...NONE.

&&

$$

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

PREVIOUS DISCUSSION...AJ
AVIATION....NOLTE
FIRE WEATHER...PERCHA








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