Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 041620
AFDPSR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PHOENIX AZ
920 AM MST FRI SEP 4 2015

.SYNOPSIS...
SUFFICIENT MOISTURE WILL REMAIN OVER MUCH OF ARIZONA TODAY PROVIDING
GOOD SHOWER AND THUNDERSTORM CHANCES...PARTICULARLY ACROSS THE
EASTERN HALF OF THE STATE. SOMEWHAT DRIER AIR WILL MOVE INTO THE
REGION OVER THE WEEKEND...REDUCING RAINFALL CHANCES VERY SLIGHTLY.
WHILE SOME THUNDERSTORM CHANCES WILL PERSIST DURING THE FIRST HALF
OF NEXT WEEK...A SUBSTANTIALLY WARMER AND DRIER AIRMASS WILL SPREAD
INTO THE AREA FOR THE LATTER HALF OF THE WEEK.

&&

.DISCUSSION... AT 9 AM THIS MORNING A DEEP UPPER TROF WAS SITUATED
TO THE NORTHWEST OF ARIZONA...CENTERED ALONG THE PAC NORTHWEST
COAST. THIS TROF WAS GIVING A SOUTHWESTERLY FLOW ALOFT TO SERN CA
AND ARIZONA...AND THIS FLOW WAS TAPPING MAINLY MID AND HIGH LEVEL
MOISTURE FROM TROPICAL STORM KEVIN LOCATED OFF THE SRN TIP OF BAJA
CA. THE STREAM OF MOISTURE BEING SHED FROM KEVIN AND PULLED TO THE
NORTH/NORTHEAST ACROSS EASTERN ARIZONA WAS CLEARLY EVIDENT IN THE
LATEST IR IMAGERY. AS OF 9 AM THE THICKER CLOUD COVER WAS LOCATED
OVER SERN AZ...WITH JUST PARTLY SUNNY SKIES SEEN OVER MUCH OF SOUTH
CENTRAL AND SOUTHWEST ARIZONA. 12Z SOUNDINGS SHOWED AMPLE MOISTURE
IN PLACE OVER EASTERN AZ WITH PWAT VALUES AT BOTH PHX AND TUS NEAR
1.5 INCHES. SURFACE DEWPOINTS THIS MORNING OVER THE CENTRAL DESERTS
WERE MOSTLY IN THE 60S. DESPITE THE AMPLE MOISTURE...INSTABILITY WAS
LACKING AS THE SOUNDINGS SHOWED A VERY MOIST ADIABATIC PROFILE WITH
MODEST MID LEVEL LAPSE RATES AND RATHER SMALL AMOUNTS OF CAPE.

FOR THE REST OF THE MORNING AND INTO THE AFTERNOON...WEAK SHORT WAVE
ENERGY WILL CONTINUE TO FLOW THROUGH THE SOUTHWEST FLOW ALOFT...BUT
NO MAJOR SHORT WAVES ARE EXPECTED TO PASS ACROSS THE AREA. LATEST
HRRR GUIDANCE IS VERY LOW KEY IN REGARDS TO CONVECTIVE DEVELOPMENT
THROUGH LATE AFTERNOON. HOWEVER...A JET STREAK RACING NORTHEASTWARD
ACROSS THE STATE WILL PLACE SOUTH CENTRAL AZ IN THE FAVORED RIGHT
REAR QUADRANT BY 5 PM THIS AFTERNOON...HELPING TO SUPPORT SHOWER OR
THUNDERSTORM DEVELOPMENT. ALL IN ALL WE ARE LOOKING AT 20-30 PERCENT
CHANCES FOR SHOWERS OR STORMS AS THE AFTERNOON PROGRESSES...MAINLY
MARICOPA COUNTY EASTWARD. OF COURSE THE BEST CHANCES ARE OVER THE
HIGHER TERRAIN TO THE EAST OF PHOENIX. MOSTLY SUNNY OR SUNNY SKIES
ARE ON TAP FOR THE FAR SOUTHWEST AZ DESERTS AND ACROSS SOUTHEAST
CALIFORNIA. CURRENT FORECASTS HAVE BEEN TWEAKED FOR SKY AND WEATHER
TRENDS AND LOOK TO BE IN GOOD SHAPE FOR NOW.

.PREVIOUS DISCUSSION...
EARLY MORNING WV IMAGERY SHOWS A SEASONABLY DEEP COLD CORE LOW OVER
THE PACIFIC NW...WITH THE LONGITUDINAL TROUGH AXIS EXTENDING ALL THE
WAY SOUTH TO 20N. THIS SCENARIO IS CREATING AN UNUSUALLY STRONG
MERIDIONAL JET THROUGHOUT THE SWRN CONUS WITH EMBEDDED SHORTWAVES
STREAMING NORTH FROM THE OUTER FRINGES OF TROPICAL STORM KEVIN (WEST
OF THE SRN TIP OF THE BAJA PENINSULA). MEANWHILE...IR SATELLITE
LOOPS SHOWED NUMEROUS GRAVITY WAVES PULSING OUTWARDS FROM CONVECTION
EARLIER THURSDAY...WITH RESULTANT TEMPORARY SUBSIDENCE IN THEIR WAKE
OVER MUCH OF ARIZONA. INTERESTINGLY...00Z/03Z REGIONAL SOUNDING DATA
SAMPLED A DISTINCT LAYER OF DRIER AIR (7-8 G/KG) IN THE H8-H7 LEVELS
HOVERING ABOVE MORE IMPRESSIVE SHALLOW SFC MOISTURE.
HOWEVER...OBJECTIVE ANALYSIS AND FORECAST SOUNDINGS INDICATE MIXING
AND ADVECTIVE PROCESSES MODIFYING THIS BOUNDARY LAYER THIS MORNING
CLOSER TO 12 G/KG.

VERY INTERESTING AND TRICKY FORECAST FOR THE NEXT 24-48 HOURS AS
NUMEROUS SUBTLE FEATURES AND PROCESSES WILL MAKE OR BREAK THE
FORECAST. DURING THE MORNING HOURS...AN OVERWHELMING AMOUNT OF HIGH
RESOLUTION MODELS INITIATE ELEVATED CONVECTION THROUGH MUCH OF CNTRL
ARIZONA IN ASSOCIATION WITH A SPEED MAX AND JET DIVERGENCE FIELD.
THERE IS SOME EVIDENCE IN MODEL FORECASTS OF THETA-E ADVECTION AND
WEAK ISENTROPIC ASCENT...ALBEIT FOCUSED MORE NORTH TOWARDS
FLAGSTAFF. WITHIN THE MESOANALYSIS FIELDS...THERE IS HINTS OF A
VORTICITY CENTER AND -10C H5 COLD POCKET PROPAGATING NORTH WITH THE
JET SEGMENT...AND INDEED RECENT SATELLITE IMAGES SHOW A
CORRESPONDING CIRRUS STREAK AND ACCAS FIELD BLOSSOMING THROUGH
SWRN/CNTRL ARIZONA. THUS...FEEL MODEL OUTPUT DOES LEND CREDENCE TO
SHOWERS THIS MORNING LIFTING FROM THE PHOENIX VICINITY INTO NORTHERN
ARIZONA.

AMOUNT AND SCOPE OF AFTERNOON THUNDERSTORM DEVELOPMENT MAY HINGE ON
ANY ATMOSPHERIC CONTAMINATION (OR LACK THEREOF) AND POTENTIAL
LIMITED INSOLATION FROM MORNING SHOWERS AND CLOUD COVER. HOWEVER
GIVEN THE STRONGER THAN NORMAL WINDS ALOFT...ANY DEBRIS CLOUDS AND
SHOWERS WILL LIKELY BE ADVECTED INTO NORTHERN ARIZONA QUICKLY
ALLOWING FOR RAPID AIRMASS RECOVERY AND BOUNDARY LAYER WARMING. WITH
THE BOUNDARY LAYER CONSERVATIVELY HOLDING 11 G/KG MOISTURE
LEVELS...COOLER TEMPERATURES AND STEEP LAPSE RATES ALOFT...AND
DECENT SFC WARMING...MLCAPES AROUND 1000 J/KG WILL BE ATTAINABLE.
DEEP LAYER BULK SHEAR 25-30KT AND ADEQUATE FORWARD STORM MOTION MAY
BE ABLE TO TRANSLATE HIGHER MOMENTUM AIR TO THE SFC IN MORE ROBUST
CONVECTION...HOWEVER DCAPE VALUES BELOW 1000 J/KG MAY PRECLUDE A
MORE SUBSTANTIAL WIDESPREAD SEVERE THREAT.

CONVECTIVE POTENTIAL FOR THE WEEKEND AND EARLY NEXT WEEK MAY BE
SOMEWHAT MORE MUTED THAN RECENT DAYS AS DEEP LAYER WINDS VEER
SOMEWHAT MORE WESTERLY AS TROUGHING TRANSLATES INTO THE GREAT BASIN
AND NRN ROCKIES. STILL...BOUNDARY LAYER MOISTURE WILL BE DIFFICULT
TO COMPLETELY SCOUR OUT AND SOME MODEST INFUSION OF MIDLEVEL
MOISTURE PEELING OFF THE REMNANTS OF KEVIN MAY BE POSSIBLE.
HOWEVER...THERE REMAINS SOME DISCONNECT WITH THE MORE QUALITY
TROPICAL MOISTURE TO THE SOUTH...AND THE BEST WV TRANSPORT MAY BE
DEFLECTED AROUND THE FORECAST AREA. IN ADDITION...WITH MID LATITUDE
TROUGHING LIFTING TO THE NORTH AND HEIGHTS ALOFT RISING AHEAD OF THE
REMNANT TROPICAL SYSTEM...MODELS DEPICT SUBSTANTIAL MIDLEVEL WARMING
AND INCREASED CONVECTIVE INHIBITION. KEPT SOME MODEST NEAR
CLIMATOLOGY POPS THROUGH TUESDAY...HOWEVER MEASURABLE RAINFALL AT
LOWER DESERT ELEVATIONS MAY BE DIFFICULT TO COME BY.

SOME MINOR UNCERTAINTY STILL EXISTS FOR THE LATTER HALF OF NEXT WEEK
AS SUBTLE DIFFERENCES IN THE SUBTROPICAL RIDGE POSITION WILL DICTATE
IF AND HOW QUICKLY MOISTURE IS COMPLETELY SCOURED SOUTH OF THE
FORECAST AREA. MESOSCALE PROCESSES AND OUTFLOW FROM SONORAN MCS/S
MAY ALLOW FOR GULF SURGES AND BOUNDARY LAYER MOISTURE
ADVECTION...BUT DEFINITIVELY FORECASTING AND TIMING SUCH EVENTS
WOULD PROVE FRUITLESS AT THIS TIME. REGARDLESS...H5 HEIGHT RISES AOA
590DM WILL RESULT IN TEMPERATURES PEAKING BACK ABOVE NORMAL UNDER
GRADUALLY MORE COMPRESSIONAL SUBSIDENT FLOW.

&&

.AVIATION...
SOUTH-CENTRAL ARIZONA INCLUDING KPHX...KIWA...AND KSDL...
SCT-BKN MID CLOUDS...AOA FL100...CAN BE EXPECTED THIS MORNING WITH
POSSIBLY AN EMBEDDED LIGHT SHOWER. THE SOUTHERN END OF AN UPPER
TROUGH WILL HELP REDEVELOP STORM ACTIVITY OVER ARIZONA AGAIN THIS
AFTERNOON. CONFIDENCE TOO LOW AT THIS POINT TO INSERT TSRA IN
PREVAILING WEATHER OR IN TEMPO GROUPS IN THE TAFS.
OTHERWISE...SURFACE WINDS WILL FAVOR SOUTHEAST AND SOUTH FOR MUCH OF
THE DAY.

SOUTHEAST CALIFORNIA/SOUTHWEST ARIZONA INCLUDING KIPL AND KBLH...
ANTICIPATE FAMILIAR WARM SEASON SURFACE WIND PATTERNS...FAVORING
SOUTHERLY DIRECTIONS. ANY STORM ACTIVITY WILL BE ISOLATED AND
CONFINED TO AREAS EAST OF THE LOWER COLORADO RIVER VALLEY. OTHERWISE
ANTICIPATE ONLY SOME SCATTERED MID TO HIGH LEVEL CLOUDS.

AVIATION DISCUSSION NOT UPDATED FOR AMENDED TAFS.

&&

.FIRE WEATHER...
SUNDAY THROUGH THURSDAY...
STORM CHANCES WILL CONTINUE OVER SOUTH-CENTRAL ARIZONA THROUGH
TUESDAY BEFORE TRENDING DOWN SIGNIFICANTLY BEGINNING WEDNESDAY.
HUMIDITIES WILL BEGIN TRENDING DOWN FROM WEST TO EAST NEXT
WEEK...BEGINNING AS EARLY AS MONDAY. A GRADUAL WARMING TREND WILL
CONTINUE NEXT WEEK 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...CB
PREVIOUS DISCUSSION...MO
AVIATION...AJ
FIRE WEATHER...AJ


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