Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 010525
AFDPSR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PHOENIX AZ
1025 PM MST MON AUG 31 2015

.SYNOPSIS...
SHOWERS AND THUNDERSTORMS WILL TREND DOWN FROM WEST TO EAST THE REST
OF THE NIGHT. LINGERING STORM CHANCES WILL CONTINUE EAST OF THE LOWER
COLORADO RIVER VALLEY TUESDAY THROUGH THURSDAY. MOISTURE IS EXPECTED
TO INCREASE LATE IN THE WEEK WITH WITH STORM CHANCES OVER A BROADER
AREA FRIDAY THROUGH THE WEEKEND. A GRADUAL COOLING TREND IS ALSO
ANTICIPATED THIS WEEK THROUGH THURSDAY.

&&

.DISCUSSION...
ISOLATED WEAK STORMS LATE THIS AFTERNOON NEAR AND SOUTH OF THE
ESTRELLAS QUICKLY BLEW UP INTO MUCH MORE OVER METRO PHOENIX. THE
INITIAL STRONG STORM PRODUCED A DOWNBURST OVER THE ESTRELLAS. THEN AN
EVEN STRONGER SUBSEQUENT STORM FROM THAT OUTFLOW BLEW UP NEAR THE
STATE FAIRGROUNDS. THOSE COMBINED OUTFLOWS IN ADDITION TO OUTFLOWS FROM
PINAL COUNTY...INITIALLY SOUTH OF FLORENCE...AS WELL AS GILA COUNTY
HAVE HELPED TO CREATE WIDESPREAD SHOWERS AND THUNDERSTORMS ACROSS
NORTHEAST MARICOPA COUNTY AND PINAL COUNTY. WITH A WESTERLY COMPONENT
TO THE STEERING FLOW...STORMS ARE GENERALLY MOVING TO THE EAST BUT
BOUNDARY INTERACTIONS ARE AIDING ISOLATED REDEVELOPMENT IN AREAS
THAT ALREADY HAD STORM ACTIVITY. DESPITE AN ENVIRONMENT THAT HAD
MODEST PRECIPITABLE WATER AND SURFACE DEW POINTS...STORMS HAVE BEEN
ABLE TO PRODUCE FLASH FLOODING. AS OF THIS WRITING...THE ACTIVITY HAS
ENDED FOR MUCH OF THE METRO AREA WITH NORTHERN AND EASTERN PERIPHERY
STILL LINGERING BUT MORE ACTION OVER NORTHERN PINAL COUNTY. THIS WILL
LARGELY SETTLE DOWN BY MIDNIGHT. IT REMAINS TO BE SEEN HOW THE MCS
OVER SONORA INFLUENCES THINGS LATE TONIGHT. HRRR HAS BEEN INDICATING
LATE NIGHT DEVELOPMENT ASSOCIATED WITH THAT SYSTEM BUT IT HAS NOT
HANDLED TONIGHT/S ACTIVITY VERY WELL. STEERING FLOW SHOULD KEEP
DEBRIS CLOUDS FROM BEING TOO THICK BUT SURFACE DEW POINTS LOOK LIKE
THEY SHOULD INCREASE TUESDAY...ESPECIALLY OVER SOUTHWEST ARIZONA.
TONIGHT/S ACTIVITY SHOULD LEAVE BEHIND FAIRLY STABLE CONDITIONS FOR
TUESDAY. BUT WITH MOISTURE AVAILABLE AND PERHAPS ACTIVITY OVER
SOUTHWEST ARIZONA...SOUTH-CENTRAL ARIZONA MAY HAVE ANOTHER
OPPORTUNITY FOR STORMS TUESDAY. LOTS MORE FOR THE NIGHT SHIFT TO
EVALUATE.

&&

.PREVIOUS DISCUSSION ISSUED 233 PM...
LATEST STREAMLINE ANALYSIS SHOWS THAT THE RIDGE HAS DRIFTED SLIGHTLY
TO THE EAST...BUT OVERALL VERY LITTLE HAS CHANGED SYNOPTICALLY OVER
THE PAST FEW DAYS. INTERESTING TO NOTE THAT THE NCAR ENSEMBLE PICKED
UP ON THE MORNING ACTIVITY ACROSS GILA COUNTY AND CONSEQUENTLY MORE
WEIGHT WAS ASSIGNED TO THIS GUIDANCE THROUGH THE EVENING.

OTHER CONVECTIVE ALLOWING GUIDANCE CONTINUES TO SUGGEST THAT
ISOLATED/SCATTERED THUNDERSTORMS WILL AGAIN DEVELOP ACROSS THE
PHOENIX AREA AND POINTS EASTWARD THIS EVENING...AIDED BY SUFFICIENT
LOW-LEVEL MOISTURE...BOUNDARY PROPAGATION FROM THE SOUTH AND
RELATIVELY COOL TEMPERATURES ALOFT ASSOCIATED WITH A TROUGH TO THE
WEST. LATEST ACARS SOUNDINGS SHOW THAT STEERING FLOW HAS VEERED
SLIGHTLY TO THE SOUTHWEST...WHICH AT LEAST CONCEPTUALLY SHOULD FOCUS
ACTIVITY SOMEWHAT FURTHER EAST THAN IN PREVIOUS NIGHTS. A WARM LAYER
NEAR 700 MB IS ALSO EVIDENT...WHICH IS LIKELY RESULTING IN
CONSIDERABLE CONVECTIVE INHIBITION AND MOSTLY CLEAR SKIES ACROSS THE
PHOENIX AREA.

U OF A WRFS HAVE PERFORMED WELL THE PAST FEW EVENINGS AND AGAIN
INDICATE THAT SCATTERED ACTIVITY IS POSSIBLE ACROSS THE PHOENIX AREA.
FORECAST POPS WERE INCREASED TO 20-30 PERCENT ACROSS PORTIONS OF
SOUTH-CENTRAL AZ FOR TONIGHT. MOST AREAS WILL NOT SEE MEASURABLE
RAINFALL...THOUGH LIGHTNING...GUSTY WINDS AND AREAS OF BLOWING DUST
ARE MORE PROBABLE.

FORECAST CONFIDENCE TUESDAY THROUGH THURSDAY IS LOW. DETERMINISTIC
GUIDANCE TAKEN AT FACE VALUE WOULD SUGGEST A DOWNWARD TREND IN STORMS
THROUGH MID WEEK...HOWEVER PROBABILISTIC TOOLS SUGGEST THE BL WILL
REMAIN MOIST WITH MODEST INSTABILITY ACROSS THE REGION. OF COURSE THE
MISSING INGREDIENT IS A TRIGGER TO KICK OFF STORMS IN THE FIRST PLACE
AND NONE OF THE DETERMINISTIC GUIDANCE INDICATE MUCH IN THE WAY OF
INVERTED TROUGHS OR LARGER SCALE FEATURES. WILL HANG ONTO SOMEWHAT
ELEVATED POPS /HIGHER THAN CLIMO/ THROUGH MID WEEK ON THE PREMISE
THAT STORMS WILL BE TRIGGERED BY COLLIDING OUTFLOWS.

FORECAST CONFIDENCE ISN`T MUCH HIGHER LATER IN THE FORECAST PERIOD.
IT WOULD APPEAR THAT MOISTURE WILL BE DRAWN INTO THE REGION FROM THE
TROPICS LATE IN THE WEEK /FRIDAY-SATURDAY/ ALTHOUGH EXACTLY WHERE
THIS AXIS OF DEEPEST MOISTURE RESIDES IS STILL UP FOR DEBATE.
00Z DETERMINISTIC GFS AND ECMWF SUGGEST ANOMALOUSLY HIGH PWATS AND
MIXING RATIOS WILL OVERSPREAD MUCH OF THE DESERT SOUTHWEST WHEREAS
GEFS AND NAEFS KEEP THIS MOISTURE SOUTH OF THE INTERNATIONAL BORDER
AND SHIFT IT TOWARD THE AZ/NM BORDER. GIVEN THE MAGNITUDE OF THE
700-300MB FLOW DURING THIS TIME PERIOD /AROUND 30-40KTS/...I`M
SKEPTICAL THE MOISTURE WILL REMAIN CONFINED TO THE BORDER AND POINTS
SOUTH. DEWPOINTS HAVE BEEN BUMPED UP LATE IN THE WEEK AND POPS HAVE
ALSO BEEN ELEVATED A BIT OVER INHERITED VALUES /MORE IN LINE WITH
CLIMO/. COULD END UP SEEING ELEVATED RAIN CHANCES LATE IN THE WEEK
INTO THE WEEKEND...BUT COULD JUST AS EASILY SEE INCREASE CLOUD COVER
AND DRY CONDITIONS DEPENDING ON WHERE THE MOISTURE AXIS POSITIONS
ITSELF. THUS CONFIDENCE IN PRECIP CHANCES IS LOW. FAIRLY HIGH
CONFIDENCE IN LOWER TEMPERATURES LATE IN THE WEEK WITH SOME OF THE
RAW UN-CORRECTED GUIDANCE POINTING TO HIGHS IN THE UPPER 90S. LOWERED
TEMPERATURES A FEW DEGREES FROM FRIDAY ONWARD ACCORDINGLY.

&&

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

MAIN BAND OF STORMS AS OF 05Z IS SHIFTING EASTWARD OUT OF THE METRO
PHOENIX AREA BUT ISOLATED REDEVELOPMENT CONTINUES. ANTICIPATE MOST OF
THE ACTIVITY OVER THE LOWER DESERTS WILL DISSIPATE BY 07Z-08Z.
EAST AND SOUTHEAST SURFACE WINDS SHOULD PREDOMINATE REST OF NIGHT AND
WELL INTO THE DAY TUESDAY.

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

ANTICIPATE CLEAR SKIES EXCEPT FOR SOME VERY MINOR CIRRUS. TYPICAL
WARM SEASON WIND PATTERNS WILL PREVAIL...THOUGH THE FAVORED DAYTIME
SOUTH AND SOUTHEAST WINDS MAY BE A BIT STRONGER THAN USUAL TUESDAY.
ALSO ANTICIPATE SOME ISOLATED STORMS OVER SOUTHWEST ARIZONA TUESDAY
AFTERNOON.

AVIATION DISCUSSION NOT UPDATED FOR AMENDED TAFS.

&&

.FIRE WEATHER...
WEDNESDAY THROUGH SUNDAY...
A LOW PRESSURE SYSTEM ACROSS THE PACIFIC NORTHWEST WILL DISPLACE THE
MONSOON HIGH SLIGHTLY TO THE SOUTH AND EAST THROUGH THE WEEK. THE
TREND WILL BE TOWARDS COOLER CONDITIONS ACROSS THE DESERT SOUTHWEST
WITH HIGH TEMPERATURES NEAR OR SLIGHTLY BELOW SEASONAL NORMALS EACH
DAY. HOWEVER SUFFICIENT MOISTURE WILL LINGER ACROSS EASTERN ARIZONA
TO KEEP AT LEAST A SLIGHT CHANCE FOR THUNDERSTORMS FROM PHOENIX EAST
WEDNESDAY INTO THURSDAY. MINIMUM RH VALUES WILL FALL BELOW 15
PERCENT WEDNESDAY AND THURSDAY ACROSS SOUTHEAST CALIFORNIA WITH
VALUES ABOVE 15 PERCENT EXPECTED EAST OF THE LOWER COLORADO RIVER.
FOR FRIDAY INTO THE WEEKEND...LATEST GUIDANCE MORE INSISTENT UPON A
RETURN TO DEEPER MOISTURE...HIGHER HUMIDITY AND BETTER RAIN
CHANCES...PARTICULARLY ACROSS SOUTH CENTRAL ARIZONA.

&&

.SPOTTER INFORMATION STATEMENT...
SPOTTERS ARE ENCOURAGED TO REPORT CONDITIONS ACCORDING TO STANDARD
OPERATING PROCEDURES.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...NONE.
CA...NONE.
&&

$$

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

DISCUSSION...AJ
PREVIOUS DISCUSSION...HIRSCH/LEINS
AVIATION...AJ
FIRE WEATHER...CB/HIRSCH



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