Area Forecast Discussion
Issued by NWS Phoenix, AZ

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000
FXUS65 KPSR 191616
AFDPSR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PHOENIX AZ
915 AM MST SAT APR 19 2014

.SYNOPSIS...
A PACIFIC STORM SYSTEM WILL CONTINUE TO AFFECT THE AREA TODAY WITH
MORE EXTENSIVE CLOUDINESS AND LOCALIZED CHANCES FOR SHOWERS AND
THUNDERSTORMS. THE BEST CHANCES FOR RAINFALL WILL OCCUR ACROSS
CENTRAL AND EASTERN ARIZONA THROUGH THIS AFTERNOON. TEMPERATURES
TODAY WILL END UP NEAR SEASONAL VALUES...HOWEVER QUICKLY WARM BACK
INTO THE 90S EARLY NEXT WEEK AFTER THE STORM SYSTEM PASSES TO THE
NORTHEAST.

&&

.DISCUSSION...
UPPER LEVEL PACIFIC LOW PRESSURE SYSTEM CONTINUED TO TREK STEADILY
EASTWARD ACROSS THE DESERT SOUTHWEST THIS MORNING...WITH THE LOW
CENTER MOVING ACROSS SOUTHWEST/SOUTHCENTRAL AZ AT 5 AM. SYSTEM IS
MODERATELY UNSTABLE...WITH H5 CORE TEMPS NEAR MINUS 18C...AND TUS
SOUNDING AT 12Z INDICATED A LIFTED INDEX NEARLY MINUS 5. TUS RAOB
ALSO SHOWED AN INVERTED VEE CHARACTER TO THE SOUNDING...WITH DRY AIR
REMAINING IN THE LOWER LAYERS...AND AS SUCH MIXED LAYER CAPE IS
RATHER LOW...MOSTLY BELOW 250 J/KG. VAPOR IMAGERY AT 8 AM INDICATED
THE PRESENCE OF A DISTINCT VORT MAX TREKKING ACROSS FAR NRN GULF OF
CA...AND INTO SERN AZ...AND IN THE PVA AREA AHEAD OF THE VORT MAX AN
AREA OF SHOWERS ALONG WITH ISOLD THUNDERSTORMS HAD DEVELOPED. THIS
AREA OF CONVECTION WILL LIFT TO THE NORTHEAST BUT SHOULD MOSTLY
REMAIN TO THE SOUTH AND EAST OF OUR CWA FOR THE REST OF TODAY. SREF
PROBABILITIES OF CAPE...WHETHER IT IS MOST UNSTABLE...SURFACE BASED
OR MIXED LAYER...SUGGEST BEST THREAT FOR SIGNIFICANT CONVECTION WILL
STAY SOUTH AND EAST OF OUR CWA...AND SOUTHEAST OF THE GREATER
PHOENIX AREA...DURING THE DAY TODAY. HOWEVER...THERE IS ALSO THE
THREAT FOR ISOLATED TO SCATTERED INSTABILITY SHOWERS OR EVEN
THUNDERSTORMS ACROSS THE CENTRAL DESERTS TODAY AS THE COLD CORE
ALOFT MOVES OVERHEAD. POP FORECASTS THAT WERE ALREADY IN PLACE
LOOKED GOOD...HOWEVER WE RAISED POPS SLIGHTLY ALONG THE BORDER WITH
TUCSON INTO THE 30-40 PERCENT BALLPARK TO BETTER BLEND WITH THEIR
FORECASTS...AND TO ACCOUNT FOR THE POSSIBILITY OF CONVECTION
ASSOCIATED WITH THE VORT MAX LIFTING NEWD AND INTO THE CENTRAL
DESERTS.

FOR THE REST OF TODAY...CHANCES FOR SHOWERS OR CONVECTION WEST OF
MARICOPA COUNTY LOOK RATHER SLIM...AS DRIER AND MORE SUBSIDENT AIR
WORK IN ON THE BACK SIDE OF THE PACIFIC LOW. CHANCES FOR SHOWERS
ACROSS THE GREATER PHOENIX AREA TODAY WILL STAY IN THE 20-30 PERCENT
BALLPARK...AND WE CANNOT RULE OUT AN ISOLATED THUNDERSTORM AT SOME
POINT. OTHERWISE...EXPECT PARTLY TO MOSTLY CLOUDY SKIES
TODAY...ALONG WITH HIGH TEMPERATURES NEAR SEASONAL NORMALS.

.PREVIOUS DISCUSSION...
TWO DISTINCT VORTICITY CENTERS CAN BE SEEN IN REGIONAL RADAR/WV
SATELLITE IMAGERY EARLY THIS MORNING ROTATING ABOUT THE LARGER SCALE
CIRCULATION CENTER/COLD CORE PROGRESSING INTO SWRN ARIZONA. THE
INITIAL VORT MAX HAS LIFTED TO NEAR LAKE HAVASU UNDERGOING
SUBSTANTIAL STRETCHING AND DEFORMATION...WHILE A SECONDARY
INTENSIFYING VORTICITY CENTER/PV ANOMALY CORE WAS LIFTING TO THE
NORTH ACROSS THE BAJA PENINSULA AHEAD OF THE COLD CORE ALOFT AND
COLLOCATED WITHIN THE CORRIDOR OF A STRONGER JET STREAK. RECENT
SATELLITE LOOPS ALSO INDICATE MODEST CLOUD TOP COOLING AHEAD OF THIS
LATTER VORTICITY CENTER WITH OBJECTIVE ANALYSIS DEPICTING INCREASING
DIFLUENCE AND ASCENT BEGINNING TO ENVELOPE THE REGION. IN FACT...A
NICE CLUSTER OF THUNDERSTORMS HAVE FORMED IN NERN SONORA WHERE JET
DYNAMICS HAVE BECOME JUXTAPOSED WITH A SMALL LOW LEVEL MOISTURE
SURGE...STEEPENING LAPSE RATES...AND RESULTANT MUCAPES AOA 500 J/KG.

WHILE PROPAGATION TRAJECTORIES WILL TAKE THIS INITIAL CLUSTER OF
STORMS ONLY THOUGH FAR SERN ARIZONA THIS MORNING...OVERALL MIDLEVEL
LAPSE RATES WILL RAPIDLY STEEPEN (AOA 7.5 C/KM) ACROSS MUCH OF THE
FORECAST AREA AS THE COLD CORE MOVES DIRECTLY OVERHEAD. COMBINED
WITH PERSISTENT UVV...SFC HEATING...AND MOISTURE ADVECTION...THE
ATMOSPHERE ACROSS THE EASTERN PARTS OF THE CWA WILL BECOME
SUPPORTIVE FOR SUSTAINED DEEP CONVECTION THROUGH THE AFTERNOON
HOURS. SREF PROBABILITIES STILL STRONGLY SHOW ARGUMENTS FOR LI/S
NEAR -2C AND SBCAPES BETWEEN 250 AND 500 J/KG BY EARLY AFTERNOON.
HIGH RESOLUTIONS MODELS ALSO FORECAST ISOLD/SCT SHOWERS AND STORMS
TO MATERIALIZE THROUGH CNTRL AND ERN ARIZONA AHEAD OF THE VORTICITY
CENTER AND WITHIN THE CYCLONIC CURVATURE REGION OF THE JET.

WHILE ALL THESE THERMODYNAMIC PARAMETERS APPEAR FAVORABLE FOR MORE
WIDESPREAD CONVECTION...BOUNDARY LAYER MOISTURE WILL STILL BE
PROBLEMATIC (PARTICULARLY AT LOWER ELEVATION LOCATIONS) WITH MIXING
RATIOS ONLY APPROACHING 7 G/KG ADVECTING INTO THE AREA. THUS...WITH
CLOUD BASES ONLY FALLING TO AROUND THE H7 LEVEL...SUBSTANTIAL
SUBCLOUD DRY AIR WILL STILL EXIST AND ONLY THE MORE WIDELY SPACED
HEAVIER SHOWERS MAY LEAD TO ACCUMULATING RAINFALL. WIDESPREAD SVR
WEATHER IS NOT EXPECTED WITH ANY STORMS THAT DO FORM AS BULK 0-6KM
SHEAR WILL BECOME ALMOST NON-EXISTENT WITHIN THE LARGER SCALE
CIRCULATION CENTER AND 0-1KM SHEAR VECTORS WILL BE SIMILARLY
WEAK...HOWEVER MUCH LIKE FRIDAY AFTERNOON EVAPORATIONAL COOLING AND
DESCENT OF RAIN COOLED AIR COULD LEAD TO VERY ISOLATED STRONGER WIND
GUSTS.

THE CONGLOMERATE OF VORTICITY MAXIMA AND OVERALL CIRCULATION CENTER
WILL WEAKEN AND LIFT NORTHEAST SUNDAY AS HEIGHT RISES AND SUBSIDENCE
SLOWLY BUILD INTO THE SWRN CONUS. ENOUGH RESIDUAL MOISTURE AND
COOLER AIR ALOFT MAY BE PRESENT OVER ERN ARIZONA TO SUPPORT SOME
ISOLD AFTERNOON SHOWERS...PRIMARILY OVER THE WHITE MOUNTAINS BUT
POTENTIALLY CLIPPING THE SAN CARLOS AREA. OTHERWISE...H5 HEIGHTS AOA
582DM WILL BECOME MAXIMIZED ON MONDAY AFTERNOON YIELDING HIGHS IN
THE MID/UPPER 90S FOR LOWER ELEVATION LOCATIONS (AND POSSIBLY
TOUCHING 100 IN PREFERRED LOCATIONS OF FAR WRN ARIZONA).

A WELL DEFINED STOUT PACIFIC WAVE WILL CRASH ASHORE TUESDAY ACROSS
THE PACIFIC NORTHWEST...EVENTUALLY PROGRESSING INTO THE CNTRL/NRN
ROCKIES. MODEL OUTPUT REMAINS VERY CONSISTENT IN KEEPING THE TROUGH
BASE LIMITED TO FAR NORTHERN ARIZONA...WITH THE ENTIRE FORECAST AREA
STUCK IN SUBSIDENT ANTI-CYCLONIC FLOW. BY FAR...THE GREATEST IMPACT
WILL BE STRONG GUSTY WINDS ALONG AND IMMEDIATELY BEHIND THE LOW
LEVEL FRONTAL BOUNDARY AS THE PRESSURE GRADIENT TIGHTENS
SUBSTANTIALLY TUESDAY AFTERNOON AND EVENING. SOUTHEAST CALIFORNIA
AND THE LOWER COLORADO RIVER VALLEY WOULD LIKELY EXPERIENCE THE MOST
PROLONGED AND FREQUENT HIGHER WIND GUSTS.

OPERATIONAL AND ENSEMBLE MODEL SPREAD IS SOMEWHAT HIGHER THAN SEEN
RECENTLY FOR THE END OF NEXT WEEK...HOWEVER STILL REMAINS RELATIVELY
SMALL IN A SEASONAL SENSE. FLAT RIDGING AND/OR QUASI-ZONAL FLOW
SHOULD PREDOMINATE THE GENERAL PATTERN FOR THE MAJORITY OF TIME WITH
H5 HEIGHTS HOVERING NOT FAR FROM 576DM. THIS MODEL AVERAGE SOLUTION
FOR HEIGHTS ALOFT IS SLIGHTLY HIGHER THAN PREVIOUS MODEL
ITERATIONS...AND CONSEQUENTLY HAVE EDGED FORECAST HIGHS SLIGHTLY
WARMER TO JUST ABOVE CLIMATOLOGICAL LEVELS. THERE IS STILL NO
EVIDENCE SUPPORTING ANY PRECIPITATION MUCH OF NEXT WEEK...WITH ONLY
VERY LIMITED OPPORTUNITY POSSIBLY ARRIVING NEXT WEEKEND.

&&

.AVIATION...
SOUTH-CENTRAL ARIZONA INCLUDING KPHX...KIWA...AND KSDL...
A LOW PRESSURE SYSTEM WILL TRACK EAST ACROSS CENTRAL AZ THIS PERIOD.
EXPECT LIGHT WINDS AND BROKEN CLOUD DECKS GENERALLY AOA 10KFT WITH
ISOLATED SHOWERS/THUNDERSTORMS THIS MORNING NEAR KPHX-KIWA-KSDL...
ACTIVITY PERSISTING INTO THE AFTERNOON HOURS. STRONG GUSTY ERRATIC
WINDS POSSIBLE NEAR VIRGA SHAFTS AND STORMS. SKIES IMPROVE AND CLEAR
OUT LATER THIS EVENING AFTER 03Z AS LOW MOVES EAST.

SOUTHEAST CALIFORNIA/SOUTHWEST ARIZONA INCLUDING KIPL AND KBLH...
LIGHT WINDS AND SCATTERED TO BROKEN CLOUD DECKS AOA 12KFT EXPECTED
THROUGH 18Z. SKIES CLEAR OUT BY EARLY AFTERNOON FOR MOSTLY CLEAR
SKIES THE REMAINDER OF THE TAF PERIOD INTO SUNDAY MORNING.

AVIATION DISCUSSION NOT UPDATED FOR AMENDED TAFS.

&&

.FIRE WEATHER...
MONDAY THROUGH FRIDAY...
WESTERLY FLOW ALOFT WILL BRING A WARMING AND DRYING TREND TO THE
REGION THROUGH TUESDAY AS A RIDGE OF HIGH PRESSURE ESTABLISHES OVER
THE INTERMOUNTAIN WEST. WELL ABOVE SEASONAL NORMAL HIGH TEMPERATURES
ARE EXPECTED THROUGH TUESDAY. A COOLING TREND ENSUES WEDNESDAY AND
THURSDAY AS A LOW PRESSURE SYSTEM MOVES INTO THE GREAT BASIN AND THE
PACIFIC NORTHWEST. THE BIGGEST IMPACT WITH THIS SYSTEM WILL BE AN
INCREASE IN WIND AREA-WIDE TUESDAY INTO WEDNESDAY...WITH GUSTS UP TO
35 MPH POSSIBLE...AND EVEN STRONGER GUSTS EXPECTED ACROSS SOUTHEAST
CALIFORNIA AND PORTIONS OF THE LOWER COLORADO RIVER VALLEY. MINIMUM
HUMIDITY LEVELS ALSO BOTTOM OUT IN THE SINGLE DIGITS DURING THIS
TIME FRAME OF STRONG WINDS...PRESENTING AN INCREASED FIRE DANGER
THREAT AND THE POSSIBLE ISSUANCE OF RED FLAG WARNINGS IN SOME AREAS.
LIGHT WINDS...WITH DRY AND WARMER CONDITIONS ARE ON TAP THURSDAY AND
FRIDAY AS HIGH PRESSURE ALOFT BUILDS BACK INTO THE AREA.


&&

.PSR WATCHES/WARNINGS/ADVISORIES...

AZ...NONE.

CA...NONE.

&&

$$

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

DISCUSSION...CB
PREVIOUS DISCUSSION...MO
AVIATION...MEYERS
FIRE WEATHER...MEYERS/CB






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