Area Forecast Discussion
Issued by NWS Phoenix, AZ

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000 FXUS65 KPSR 030057 AFDPSR AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE PHOENIX AZ 557 PM MST WED SEP 2 2015 .UPDATE...UPDATED SHORT TERM FORECAST AND AVIATION DISCUSSIONS... && .SYNOPSIS... ANOTHER DISTURBANCE IN THE FLOW WILL CONTINUE TO PULL ADDITIONAL MOISTURE NORTHWARD INTO THE REGION...INCREASING THUNDERSTORM CHANCES ACROSS SOUTH CENTRAL ARIZONA TONIGHT AND THURSDAY. AS THE PATTERN BECOMES MODIFIED OVER THE WEEKEND...REDUCED SHOWER AND THUNDERSTORM CHANCES THROUGH SUNDAY BEFORE ANOTHER UPSWING IN MONSOON ACTIVITY RETURNS EARLY NEXT WEEK. && .DISCUSSION... DRY AIR HAS SUCCESSFULLY PUSHED THROUGH MUCH OF SOUTH-CENTRAL ARIZONA THIS AFTERNOON WITH THE 00Z KPSR RAOB ONLY SHOWING 1.10 INCHES OF PRECIPITABLE WATER. MORNING CLOUDINESS LIMITED HEATING TODAY AND WITH SURFACE DEW POINTS DROPPING INTO THE UPPER 40S OVER THE LOWER DESERTS...VERY LITTLE TO NO INSTABILITY TO WORK WITH. HAVE LOWERED POPS FOR THIS EVENING...BUT STILL KEEPING AT LEAST A SLIGHT CHANCE IN AS STORM ACTIVITY OVER NORTHERN MEXICO AND SOUTHEAST ARIZONA MAY MAKE ITS WAY UP HERE BY MIDNIGHT OR SO. THE DEEP MOISTURE SHOULD ADVECT NORTHWARD INTO SOUTH-CENTRAL ARIZONA BY LATE THURSDAY MORNING WITH NO CHANGES NEEDED WITH THE THURSDAY FORECAST. && .PREVIOUS DISCUSSION... EARLY AFTERNOON CONVECTION IS UNDERWAY ACROSS SOUTHEASTERN ARIZONA AND NORTH-CENTRAL ARIZONA...AS WAS FAIRLY WELL ADVERTISED BY THE LOCAL WRF HIGH RESOLUTION MODEL. WHILE CENTRAL ARIZONA HAD BEEN MOSTLY CLOUDED OVER THROUGH THE MORNING HOURS...SOME POCKETS OF CLEARING AND WARMING HAVE ALLOWED TEMPERATURES TO REACH RIGHT AROUND THE CENTURY MARK AND ALSO OF NOTE OUR DEWPOINT TEMPERATURES HAVE DROPPED SUBSTANTIALLY WITH SKY HARBOR MEASURING NEARLY 15 DEGREES DRIER AS OF 2130Z COMPARED WITH YESTERDAY. CURRENT AREA OF CONCENTRATED MOISTURE /PW PLUME/ REMAINS ACROSS THE SOUTHERN THIRD OF THE STATE. AS WAS SEEN EARLIER TODAY IN THE IR SATELLITE IMAGERY...AN MCV/VORT MAX FEATURE WAS SEEN OVER EASTERN ARIZONA AND A BROAD DIFFLUENT PATTERN STILL REMAINS ACROSS THE CENTRAL AND SOUTHERN HALF OF ARIZONA. MOISTURE STREAMING NORTHWARD FROM THE TROPICAL FEATURE WELL SOUTH OF BAJA CONTINUES TO BE ADVECTED INTO SONORA AND SOUTHERN ARIZONA. AT THIS TIME...NO PROGRESS HAS BEEN MADE IN THE STRENGTHENING OF THIS FEATURE. HOWEVER...WITH THE UPPER LEVEL TROUGH PUSHING ONSHORE OF CALIFORNIA AND THE HIGH BEING POSITIVELY TILTED...A FAVORABLE SOUTH- SOUTHWEST FLOW WILL REMAIN IN PLACE THROUGH THE EARLY PART OF NEXT WEEK. SEVERAL SMALL DISTURBANCES IN THE FLOW ALONG WITH UPPER LEVEL DIVERGENT FLOW SHOULD BE THE TRIGGER NEEDED TO INITIATE CONVECTION THE NEXT TWO EVENINGS. TIMING WITH CONVECTION HAS BEEN TRICKY...AS WITH MOST MONSOON FORECASTING...AND EVENING TO LATE NIGHT OR OVERNIGHT STORMS ARE VERY POSSIBLE. LOCATION SEEMS TO FAVOR MARICOPA COUNTY AND PLACES EAST WITH THE SOUTHWEST FLOW...AND A MUCH DRIER AIRMASS TO THE WEST SHOULD HINDER ANY ACTIVITY. HOLDING ON TO SLIGHTLY HIGHER THAN CLIMO POPS THROUGH THE NEXT FEW EVENINGS SEEMS REASONABLE. AS THE UPPER TROUGH MOVES FARTHER EAST TOMORROW AND THE FLOW BECOMES SLIGHTLY MORE CONFLUENT...WIND SHEAR REMAINS FAIR WITH 40-50 KTS ABOVE 300MB..AND SLIGHTLY LESS PRECIPITABLE WATER. STORM CHANCES STILL LOOK FAVORABLE TOMORROW AND ESPECIALLY SO IF TODAY DOES NOT PAN OUT TO BE AS ACTIVE. FRIDAY THROUGH SUNDAY... SOMEWHAT LESS CONVECTIVE ACTIVITY IS EXPECTED DURING THIS TIME PERIOD AS SOMEWHAT DRIER AIR WORKS ITS WAY EASTWARD INTO SOUTH- CENTRAL AZ. STILL...AT LEAST SLIGHT CHANCE POPS ARE WARRANTED EACH DAY ACROSS THE LOWER DESERTS OF SOUTH- CENTRAL AZ AS PWAT LEVELS REMAIN IN THE 1.0-1.5 INCH RANGE AND THE VERTICAL SHEAR PROFILE REMAINS FAVORABLE THROUGH THE COLUMN. MONDAY THROUGH TUESDAY... ALTHOUGH FORECAST CONFIDENCE LEVELS ARE RATHER LOW THIS FAR OUT...A INCREASE IN AVAILABLE MOISTURE AS THE FLOW BELOW 500MB BECOMES A BIT MORE SOUTHERLY IS EXPECTED TO ALLOW PRECIP CHANCES TO RISE A BIT DURING THIS TIME PERIOD. SOMEWHAT WARMER AIR ALOFT AS RIDGING BUILDS BACK NORTHWARD INTO THE REGION WILL ALSO ALLOW TEMPS TO WARM TO NEAR...OR SLIGHTLY ABOVE NORMAL VALUES. && .AVIATION... SOUTH-CENTRAL ARIZONA INCLUDING KPHX...KIWA...AND KSDL... DRIER AIR HAS WORKED INTO THE AREA FOR THE TIME BEING WITH ALL STORM ACTIVITY EXPECTED TO REMAIN TO THE SOUTH AND EAST OF THE TERMINALS THROUGH THE EVENING HOURS. THERE IS STILL A SLIGHT POSSIBILITY OF AN ORGANIZED STORM OUTFLOW COULD TRANSITION INTO PHOENIX LATE THIS EVENING...BUT CONFIDENCE IS LOW IN THIS OCCURRING. INCREASING MOISTURE AND DYNAMICS TONIGHT COULD LEAD TO SOME POP-UP SHOWER OR TS ACTIVITY AFTER MIDNIGHT. RATHER VRB SFC WIND PATTERN CONTINUES OVER THE AREA...WITH ERRATIC SHIFTS FROM E TO LIGHT WEST TO VRB AT KPHX. EXPECT THIS TO CONTINUE THIS EVENING BEFORE TYPICAL EASTERLY DRAINAGE WINDS DEVELOP CLOSER TOWARDS THURSDAY EARLY AM. BETTER CHANCES FOR SHOWERS AND THUNDERSTORMS WILL OCCUR FROM LATE THURSDAY AFTERNOON AND THURSDAY EVENING WITH STRONG GUSTY WINDS AND BRIEF HEAVY RAINFALL A POSSIBILITY. SOUTHEAST CALIFORNIA/SOUTHWEST ARIZONA INCLUDING KIPL AND KBLH... HIGH CLOUDS WILL CONTINUE FOR THE WESTERN TERMINALS WITH WESTERLY WINDS DEVELOPING FOR KIPL IN THE EVENING AND SOUTHERLY HEADINGS MAINTAINED FOR KBLH. ANY WEATHER IS EXPECTED TO STAY WELL EAST OF THE TERMINALS ACROSS AZ THIS EVENING. AVIATION DISCUSSION NOT UPDATED FOR AMENDED TAFS. && .FIRE WEATHER... FRIDAY THROUGH TUESDAY... MOISTURE WILL INCREASE ACROSS THE AREA BY THE MIDDLE OF THE WEEK RESULTING IN ELEVATED STORM CHANCES FROM PHOENIX EASTWARD ON THURSDAY. MOISTURE VALUES WILL WANE A BIT INTO THE WEEKEND ALTHOUGH DAILY STORM CHANCES WILL REMAIN PRESENT THROUGH EARLY NEXT WEEK. MINIMUM RH VALUES WILL FALL BELOW 15 PERCENT 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...KUHLMAN PREVIOUS DISCUSSION...DEWEY/PERCHA AVIATION...NOLTE/KUHLMAN FIRE WEATHER...PERCHA/LEINS

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