Area Forecast Discussion
Issued by NWS Phoenix, AZ

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000
FXUS65 KPSR 050950
AFDPSR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PHOENIX AZ
250 AM MST SAT SEP 5 2015

.SYNOPSIS...
OCCASIONALLY THICK HIGH CLOUDS WILL INFLUENCE AND POSSIBLY LIMIT
THUNDERSTORM CHANCES THIS WEEKEND...HOWEVER SOME SHOWERS WILL STILL
OCCUR. RAINFALL CHANCES WILL LINGER THROUGH THE FIRST PART OF NEXT
WEEK...THOUGH WIDESPREAD ORGANIZED THUNDERSTORM COMPLEXES ARE NOT
LIKELY. TEMPERATURES WILL WARM BACK ABOVE NORMAL BY THE LATTER HALF
OF THE WEEK...THOUGH CANNOT COMPLETELY RULE OUT MORE SCATTERED
SHOWERS THROUGHOUT THE WEEK.

&&

.DISCUSSION...
A NWRN CONUS PRONOUNCED H5 NEGATIVE HEIGHT ANOMALY HAS TRANSLATED
INTO ERN OREGON EARLY THIS MORNING...AND WILL LIFT INTO THE NRN
ROCKIES THIS AFTERNOON PROVIDING A CONTINUATION OF UNUSUALLY STRONG
SWLY JET ENERGY THROUGHOUT THE SOUTHWEST CONUS THROUGH CNTRL
ROCKIES. MEANWHILE...EAST PACIFIC TROPICAL STORM KEVIN WAS RAPIDLY
BECOMING POST-TROPICAL WITH DEEP CONVECTION WELL DISPLACED FROM THE
CENTRAL SFC CIRCULATION UNDER STRONG SHEAR PROFILES. MID/HIGH CLOUDS
FROM KEVIN HAVE BEEN STREAMING ACROSS MUCH OF ARIZONA THE PAST 24
HOURS...AND WILL CONTINUE TO BE A PLAYER IN THE FORECAST TODAY.

SEVERAL CONFLICTING SIGNALS THIS MORNING YIELD A COMPLICATED AND
RATHER LOW CONFIDENCE FORECAST FOR THE NEXT 24-36 HOURS.
FIRST...DEBRIS CLOUDS FROM KEVIN WILL REMAIN RATHER THICK FOR THE
FIRST PART OF THE DAY HINDERING INSOLATION AND MAINTAINING POOR
MIDLEVEL LAPSE RATES (AS SEEN IN REGIONAL 00Z SOUNDING DATA).
HOWEVER...MESOANALYSIS SHOWS A POOL OF MUCH HIGHER PWATS JUST SOUTH
OF THE INTERNATIONAL BORDER THAT COULD CONCEIVABLY BE PULLED NORTH
WITH THE STRONGER DEEP LAYER SWLY FLOW. HAVE SERIOUS RESERVATIONS
CONCERNING GLOBAL MODEL INITIALIZATIONS AT 00Z...AS BOUNDARY LAYER
MOISTURE PROFILES SEEM SOMEWHAT OVERDONE (FORECAST NEAR 14 G/KG
NIGHT BE CLOSER TO 12 G/KG)...AND RELATIVE HUMIDITY FIELDS DO NOT
SEEM TO CAPTURE THE FULL EXTENT OF MID/HIGH LEVEL TROPICAL MOISTURE.

THESE INITIALIZATION QUESTIONS AFFECT FULL UTILIZATION AND
CONFIDENCE IN HIGH RESOLUTION MODELS INCORPORATING THIS DATA...AND
SUBSEQUENT ENSEMBLE APPROACH THROUGHOUT THE DAY. HAVE SOME
PREFERENCE TOWARDS THE RECENT RAP/HRRR OUTPUT GIVEN APPARENT
PRESENCE OF A SHORTWAVE/VORTICITY CENTER CROSSING THE BAJA AND AIMED
TOWARDS SERN ARIZONA...WHILE ALSO COLLOCATED THICKER MID/HIGH LEVEL
MOISTURE. WITH SOMEWHAT LIMITED INSOLATION AND POOR LAPSE
RATES...FEEL LIGHT/MODERATE SHOWERS WITH SOME EMBEDDED THUNDER WOULD
BE THE MOST LIKELY OUTCOME. GIVEN THE SETUP...ANY THUNDERSTORMS
WOULD BE WEAKER IN NATURE (I.E. STRONGER WIND GUSTS NOT
LIKELY)...AND AT THIS TIME SIGNALS FOR HEAVY RAINFALL ARE LIMITED.

CONVECTIVE POTENTIAL FOR THE FIRST SEVERAL DAYS OF NEXT WEEK STILL
APPEARS A BIT MEAGER AS DEEP LAYER WINDS VEER SOMEWHAT MORE WESTERLY
IN RESPONSE TO NRN STREAM TROUGHING PROPAGATING INTO THE NRN
ROCKIES. THERE IS GOOD MODEL AGREEMENT THAT SUBTROPICAL RIDGING WILL
RETROGRADE WESTWARD INTO THE SWRN CONUS ALLOWING WARMING ALOFT
CONCURRENT WITH BOUNDARY LAYER MOISTURE FALLING TO SEMI-MARGINAL
LEVELS (9-10 G/KG). WHILE SUFFICIENT TO SUPPORT DEEP CONVECTION OVER
HIGHER TERRAIN...SHOULD THESE FORECASTS BE ACCURATE...MEASURABLE
RAINFALL AT LOWER DESERT ELEVATIONS MAY BE DIFFICULT TO COME BY.

UNCERTAINTY FOR THE LATTER HALF OF NEXT WEEK HAS INCREASED
SUBSTANTIALLY AS SPREAD AMONG OPERATIONAL AND ENSEMBLE MEMBERS GROWS
NOTICEABLY. THE MAIN CULPRIT IS A TROPICAL DISTURBANCE CURRENTLY NEAR
15N 105W WHICH NATIONAL HURRICANE CENTER PREDICTS HAS A 70% CHANCE
OF GROWING INTO A NAMED CYCLONE (NAME WOULD BE LINDA). A SUBSET OF
GFS ENSEMBLE MEMBERS ALONG WITH THE 00Z ECMWF INTENSIFY THIS SYSTEM
AND TRACK THE CENTER ALONG THE BAJA PENINSULA WHILE ALSO PULLING
COPIOUS MOISTURE NORTH UP THE GULF OF CALIFORNIA WITH FAVORABLE LOW
LEVEL WINDS. HOWEVER...SEVERAL OTHER ENSEMBLE MEMBERS...THE 00Z
OPERATIONAL GFS...AND THE CONSENSUS FORECAST TRACKS THE SYSTEM WEST
INTO THE OPEN PACIFIC WHILE ALLOWING THE SUBTROPICAL RIDGE TO SETTLE
WEST OF THE CALIFORNIA COAST (PLACING ARIZONA IN STRONGER DRY NWLY
FLOW). HAVE STUCK WITH THE DRIER CONSENSUS/MEAN FORECAST FOR THE
TIME BEING UNTIL BETTER TROPICAL ORGANIZATION AND TRACKING CAN BE
ATTAINED WITH THIS POTENTIAL TROPICAL STORM.

&&

.AVIATION...
SOUTH-CENTRAL ARIZONA INCLUDING KPHX...KIWA...AND KSDL...
SCT-BKN MID CLOUDS AOA 10KFT CAN BE EXPECTED THROUGH THE REST OF
TONIGHT WITH A FEW LINGERING SHOWERS POSSIBLE ACROSS THE NORTH AND
WEST PART OF PHOENIX INTO LATE EVENING. WINDS WILL GRADUALLY SHIFT TO
AN EASTERLY DIRECTION AFTER MIDNIGHT AND REMAIN EITHER EASTERLY OR
SOUTHEASTERLY THROUGH SATURDAY AFTERNOON. ANOTHER CHANCE OF SCATTERED
SHOWERS AND THUNDERSTORMS WILL BE PRESENT LATE SATURDAY AFTERNOON AND
EVENING.

SOUTHEAST CALIFORNIA/SOUTHWEST ARIZONA INCLUDING KIPL AND KBLH...
SKIES GENERALLY CLEAR WITH A FEW MID-HIGH CLOUDS DURING PERIOD.
WINDS WILL REMAIN LESS THAN 12 KTS AT KIPL AND SOMEWHAT VARIABLE
WHILE A PERSISTENT SOUTHERLY WIND AT KBLH WILL BE GUSTY DURING THE
DAYTIME HOURS.

AVIATION DISCUSSION NOT UPDATED FOR AMENDED TAFS.

&&

.FIRE WEATHER...
MONDAY THROUGH FRIDAY...
STORM CHANCES WILL CONTINUE OVER SOUTH-CENTRAL ARIZONA BEFORE
TRENDING DOWN SIGNIFICANTLY WEDNESDAY. HUMIDITIES WILL ALSO BEGIN TO
TREND DOWN FROM WEST TO EAST ACROSS THE ENTIRE FORECAST AREA NEXT
WEEK...BEGINNING AS EARLY AS MONDAY. A GRADUAL WARMING TREND WILL
ENSUE DURING THE PERIOD CLIMBING TO SEVERAL DEGREES ABOVE NORMAL BY
NEXT THURSDAY.

&&

.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...MO
AVIATION...KUHLMAN
FIRE WEATHER...MEYERS


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