Area Forecast Discussion
Issued by NWS Phoenix, AZ

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000
FXUS65 KPSR 292117
AFDPSR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PHOENIX AZ
215 PM MST THU JAN 29 2015

.UPDATE...AVIATION DISCUSSION...

&&

...SIGNIFICANT RAIN EVENT POSSIBLE LATER TONIGHT INTO SATURDAY...

.SYNOPSIS...
LOW PRESSURE WILL APPROACH THE AREA TODAY...SETTING THE STAGE FOR A
SIGNIFICANT RAIN EVENT ACROSS THE REGION. RAIN CHANCES INCREASE
DRAMATICALLY REGION WIDE TONIGHT AND CONTINUE INTO SATURDAY...MAINLY
EAST OF THE LOWER COLORADO RIVER VALLEY. SNOW LEVELS WILL REMAIN
HIGH...LOWERING TO NEAR 7000 FEET BY SATURDAY MORNING. ANTICIPATE
COOLER HIGH TEMPERATURES AS WELL. DRY CONDITIONS TAKE OVER SUNDAY
THROUGH WEDNESDAY ALONG WITH A GRADUAL WARMING TREND.

&&

.DISCUSSION...
CLOUD MASS ASSOCIATED WITH A PACIFIC WEATHER SYSTEM CONTINUES TO
STREAM OVER THE SOUTHWEST STATES. CLOUDS WILL THICKEN DURING THE
DAY. LATEST MODELS CONTINUE TO SHOW ISOLATED RAINFALL DEVELOPING
THIS AFTERNOON OVER HIGHER TERRAIN AREAS SUCH AS GILA COUNTY. THEN
IN THE EVENING SCATTERED LIGHT SHOWERS START SPREADING OVER THE
LOWER ELEVATIONS FOLLOWED BY MORE WIDESPREAD DEVELOPMENT LATE
TONIGHT AND FRIDAY MORNING. THE NAM INDICATES A LULL FRIDAY
AFTERNOON BUT OTHERS INDICATE LIGHT SHOWERS CONTINUING THROUGH THE
AFTERNOON. OF NOTE...THERE IS FAIRLY GOOD AGREEMENT ON THE
DYNAMICAL FORCING OCCURRING PRIMARILY TONIGHT AND FRIDAY MORNING AND
TRENDING DOWN FRIDAY AFTERNOON. HOWEVER...THERE ARE CONSIDERABLE
MODEL QPF DIFFERENCES FOR EACH 6 HOUR PERIOD BETWEEN 00Z FRIDAY TO
00Z SUNDAY. THIS IS LARGELY A RESULT OF DIFFERENCES IN THE DETAILS
ON JUST WHEN AND WHERE VORTICITY MAXIMA MOVE THROUGH. THE MAIN
FORECAST TRENDS REMAIN ON TRACK. MAY NEED TO INCREASE POPS OVER FAR
SOUTHWEST ARIZONA AND SOUTHEAST CALIFORNIA BUT THOSE AREAS ARE
EXPECTED TO CONTINUE TO BE ON THE LOWER END OF THE PRECIP RANGE FOR
OUR FORECAST AREA. MORE LATER.

&&

.PREVIOUS DISCUSSION ISSUED 305 AM MST/205 AM PST...
DEEP AND VIGOROUS UPPER LEVEL TROF CONTINUED TO TAKE SHAPE OFF BAJA
CA EARLY THIS MORNING...AND THE SSWLY FLOW AHEAD OF THE TROF COULD
BE SEEN SPREADING CONSIDERABLE MID/HIGH CLOUDS NORTHWARD AND INTO
SERN CA AND ARIZONA. LATEST BLENDED TOTAL PWAT IMAGERY SHOWED VALUES
IN EXCESS OF 1.5 INCHES LOCATED OFF OF THE SRN TIP OF BAJA...AND
MUCH OF THIS MOISTURE WILL BE ADVECTED INTO ARIZONA LATER TONIGHT
AND ALL DAY FRIDAY. FOR THE REST OF TODAY WE ARE LOOKING AT
CONSIDERABLE MID/HIGH CLOUDS...FOR MOSTLY CLOUDY OR CLOUDY
SKIES...WITH SLIGHT CHANCES FOR AFTERNOON SHOWERS DEVELOPING OVER
THE HIGHER TERRAIN OF BOTH SRN GILA COUNTY AND JOSHUA TREE NP.
DESPITE THE CLOUDS...LOW LEVEL SLY FLOW AND WARM AIR ADVECTION WILL
KEEP HIGH TEMPS WELL ABOVE SEASONAL NORMALS. HIGHS IN THE CENTRAL
DESERTS INCLUDING THE GREATER PHOENIX AREA WILL LIKELY HOVER IN THE
MIDDLE 70S.

MODEL GUIDANCE REMAINS IN EXCELLENT AGREEMENT THAT WE WILL BE
LOOKING AT A VERY WET AND UNSETTLED PERIOD...ESPECIALLY FOR SOUTH
CENTRAL ARIZONA...FOR AT LEAST 2 DAYS BEGINNING LATER TONIGHT AND
CONTINUING THROUGH THE DAY ON SATURDAY. THE BEST WINDOW FOR HEAVY
RAIN OVER EASTERN ARIZONA WILL BE LATE TONIGHT THROUGH FRIDAY
EVENING. GFS/NAM/ECMWF HAVE SHOWED EXCELLENT RUN TO RUN...AND
INTER-MODEL CONSISTENCY FOR THE PAST COUPLE OF DAYS...AND A BLEND OF
THE MODELS CONTINUES TO BE USED FOR THE FORECAST. THE FIRST IN A
SERIES OF SHORT WAVES MOVES OUT OF THE DEEPER TROF AND INTO ARIZONA
LATER TONIGHT...THEN LIFTS OUT TO THE NORTH/NORTHEAST DURING THE DAY
FRIDAY. BY 12Z FRIDAY POPS WILL BECOME LIKELY TO CATEGORICAL ACROSS
SOUTH CENTRAL AZ...WITH CHANCE POPS EXTENDING WEST INTO SERN CA.
VERY HIGH POPS PERSIST ALL DAY FRIDAY EAST OF THE CO RIVER VALLEY...
WITH GOOD CHANCES FOR SHOWERS INTO SERN CA. BY FRIDAY AFTERNOON...
PROGS INDICATE THAT A CLOSED UPPER LOW WILL FORM OVER FAR SERN CA
NEAR THE NRN BAJA/SRN CA BOUNDARY...AND THIS LOW WILL TRANSLATE VERY
SLOWLY EAST/SOUTHEAST OVER THE NEXT 24-36 HOURS. A SERIES OF VORT
SPOKES WILL ROTATE AROUND THE UPPER LOW...KEEPING PERIODS OF
ENHANCED LIFT GOING ACROSS THE AREA...AND WITH MOISTURE REMAINING
VERY HIGH...CHANCES FOR SHOWERS WILL STAY ELEVATED FRIDAY NIGHT
THROUGH THE DAY ON SATURDAY.

PWAT VALUES ARE FORECAST TO REACH BETWEEN 1 AND 1.25 INCHES OVER THE
LOWER DESERTS FRIDAY...AND WILL STAY CLOSE TO 1 INCH THRU THE DAY
SATURDAY. PWAT VALUES IN THE TUS AREA FRIDAY ARE FORECAST TO CLIMB
TO NEARLY 1 INCH...AND THAT WOULD BE CLOSE TO RECORD LEVELS FOR THIS
TIME OF YEAR. PWAT VALUES OVER THE SOUTH CENTRAL AZ FRIDAY ARE
FORECAST TO BE 4-5 STANDARD DEVIATIONS ABOVE NORMAL. GIVEN THE VERY
HIGH MOISTURE VALUES...STRONG UPPER FORCING...AND MODEST LEVELS OF
AFTERNOON INSTABILITY BOTH FRIDAY AND SATURDAY...WE CAN EXPECT
SIGNIFICANT STORM TOTAL RAINFALL ACCUMULATIONS. RAINFALL STORM
TOTALS OVER THE AZ DESERTS WILL RANGE FROM ONE QUARTER INCH IN YUMA
COUNTY TO AROUND 1 INCH IN THE GREATER PHOENIX AREA...WITH THE
HEAVIEST AMOUNTS LIKELY TO OCCUR OVER THE HIGHER TERRAIN AREAS OF
FAR NORTHERN AND EASTERN AZ. RAIN TOTALS OVER SOUTHERN GILA COUNTY
WILL BE EVEN HIGHER AND COULD TOTAL BETWEEN 1.5 AND 2 INCHES BEFORE
THE EVENT ENDS SATURDAY NIGHT.

THE CURRENT THINKING IS THAT DUE TO ONLY MODEST INSTABILITY...AND A
CORRESPONDINGLY REDUCED THREAT FOR THUNDERSTORMS...RAINFALL TOTALS
WILL ACCRUE MORE GRADUALLY OVER TIME...LIMITING ANY THREAT FOR FLASH
FLOODING. WE DO NOT INTEND TO ISSUE FLASH FLOOD WATCHES AT THIS
TIME. EXCESSIVE RAINFALL WILL MOST LIKELY LEAD TO SOME URBAN
FLOODING ISSUES...AND CAUSE AREA WASHES TO FLOW WITH TIME.

OF COURSE...HIGH TEMPS FRIDAY AND SATURDAY WILL FALL SUBSTANTIALLY
DUE TO CLOUDS...SHOWERS AND THICKNESS COOLING...AND WE WILL SEE
HIGHS OVER THE CENTRAL DESERTS FALL INTO THE UPPER 50S TO LOW 60S
ACROSS SOUTH CENTRAL ARIZONA. HIGHS WILL BE SLIGHTLY WARMER OUT
WEST...MORE IN THE MID 60S BALLPARK. A FEW DEGREES OF WARMING IS
EXPECTED SATURDAY BUT READINGS WILL STAY BELOW SEASONAL NORMALS.

SUNDAY THROUGH WEDNESDAY...
SUNDAY LOOKS TO BE A DRY. PATCHY FOG IS A POSSIBILITY BUT THERE MAY
BE ENOUGH LINGERING CLOUDS TO HINDER THE RADIATIONAL COOLING
NECESSARY.  THE RIDGE SLOWLY BUILDS INTO THE WEST MONDAY THROUGH
WEDNESDAY FOR A GRADUAL WARMING TREND...AND WE CAN EXPECT DESERT
HIGHS TO REACH BACK INTO THE LOW TO MID 70S BY NEXT WEDNESDAY.

&&

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

A PACIFIC LOW PRESSURE SYSTEM WILL MOVE INTO THE REGION FROM THE
SOUTHWEST...IMPORTING A SIGNIFICANT AMOUNT OF SUBTROPICAL MOISTURE
AND RAIN INTO THE GREATER PHOENIX AREA TONIGHT AND FRIDAY. POSSIBLE
THUNDERSTORMS FRIDAY AFTERNOON BETWEEN 18Z FRI AND 23Z FRI. CIGS
LOWERING WITH RAIN THROUGH THE PERIOD BECOMING 8 THSD OVC BY 00Z
FRI...6 THSD OVC BY 06Z FRI...AND WIDE-SPREAD 2-3 THSD BY 12Z FRI.


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

A PACIFIC LOW PRESSURE SYSTEM WILL MOVE INTO THE REGION FROM THE
SOUTHWEST...IMPORTING A SIGNIFICANT AMOUNT OF SUBTROPICAL MOISTURE
AND RAIN MAINLY INTO AZ TONIGHT AND FRIDAY...BUT SPILLING INTO
SOUTHEAST CA AND THE COLORADO RIVER VALLEY. CIGS LOWERING WITH
SHOWERS THROUGH THE PERIOD BECOMING 8 THSD OVC BY 00Z FRI...6 THSD
OVC BY 06Z FRI...AND BKN 3 THSD OVC BY 12Z FRI.

AVIATION DISCUSSION NOT UPDATED FOR AMENDED TAFS.

&&

.FIRE WEATHER...
SATURDAY THROUGH WEDNESDAY...
RAIN SHOWERS WILL LINGER OVER PORTIONS OF THE REGION THROUGH
SATURDAY. GREATEST LIKELIHOOD FOR SHOWERS WILL BE OVER SOUTH-CENTRAL
ARIZONA...PARTICULARLY OVER THE HIGHER TERRAIN NORTH AND EAST OF THE
PHOENIX METROPOLITAN AREA. THERE IS A SLIGHT CHANCE OF
THUNDERSTORMS...MAINLY SATURDAY AFTERNOON. NORTHWEST WINDS WILL ALSO
PREVAIL OVER THE REGION ON SATURDAY...KEEPING TEMPERATURES SEVERAL
DEGREES COOLER THAN BELOW NORMAL. A WARMING AND DRYING TREND IS THEN
EXPECTED SUNDAY THROUGH WEDNESDAY...AS A RIDGE BUILDS BACK IN ACROSS
THE EASTERN PACIFIC.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...

AZ...NONE.

CA...NONE.

&&

$$

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

DISCUSSION...AJ
PREVIOUS DISCUSSION...CB
AVIATION...VASQUEZ
FIRE WEATHER...MCLANE









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