Area Forecast Discussion
Issued by NWS Phoenix, AZ

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000 FXUS65 KPSR 252107 AFDPSR AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE PHOENIX AZ 205 PM MST SUN JAN 25 2015 .SYNOPSIS... A LOW PRESSURE SYSTEM WILL MOVE INTO THE AREA MONDAY BRINGING AN INCREASE IN MOISTURE AND LIGHT RAIN SHOWERS. THIS DISTURBANCE WILL MOVE NORTH OF THE REGION ON TUESDAY...AND IN ITS WAKE ABOVE NORMAL TEMPERATURES AND HIGHER THAN NORMAL HUMIDITIES WILL PREVAIL. ANOTHER LOW PRESSURE SYSTEM WILL BRING ADDITIONAL MOISTURE AND ANOTHER CHANCE FOR PRECIPITATION LATE IN THE WEEK THROUGH THE WEEKEND. && .DISCUSSION... AN UPPER LOW NEAR 26N 120W HAS NEARLY STALLED AS THE PACIFIC NW BLOCKING RIDGE AXIS SHIFTS FURTHER INLAND. THIS LOW WILL EVENTUALLY BE PICKED UP WITHIN THE SOUTHWEST FLOW ON THE UPSTREAM SIDE OF THE RIDGE...OPENING AND PROPAGATING NORTHWARD THROUGH SRN CALIFORNIA MONDAY. AS THIS TRANSFORMATION MATERIALIZES...DEEP MERIDIONAL FLOW WILL DEVELOP THROUGH MUCH OF THE FORECAST AREA WITH STRONG MOISTURE TRANSPORT...THETA-E ADVECTION...AND ISENTROPIC ASCENT GRADUALLY SPREADING THROUGH THE REGION. ANOMALOUSLY HIGH PWATS BETWEEN 0.75 AND 1.25 INCHES WILL SURGE NORTHWESTWARD (EQUATING TO 2-3 NORMALIZED STANDARD DEVIATIONS ABOVE NORMAL) RESULTING IN FAIRLY WIDESPREAD LIGHT RAIN SHOWERS FOR MOST LOCATIONS MONDAY INTO MONDAY NIGHT. MOST RECENT 12Z OPERATIONAL MODELS AND VARIOUS 09Z/12Z/15Z ENSEMBLE AND HIGH RESOLUTION MODELS DEFINITIVELY SUGGEST MORE BACKING OF WINDS THROUGH THE ATMOSPHERIC COLUMN COINCIDENT WITH THE LOW LEVEL THETA-E PLUME SHIFTING MORE TO THE WEST. AS A RESULT...SATURATED ASCENT STRUCTURE LOOKS MORE ROBUST THROUGH THE WESTERN HALF OF THE CWA...WHILE DRIER DOWNSLOPING BOUNDARY LAYER AIR ATTEMPTS TO INHIBIT A FASTER NORTHEASTWARD SPREAD OF THE RAIN SHIELD INTO CNTRL ARIZONA. HAVE REFINED PRECIPITATION CHANCES MORE DISTINCTLY TO REFLECT THESE PROCESSES...WHILE ALSO CONSIDERING EFFECTS ON FORECAST RAINFALL TOTALS. 09Z/15Z ENSEMBLE MEANS HAVE GENERALLY ACKNOWLEDGED THE CHANGES WITH RESPECT TO AREAS OF BETTER ASCENT...WITH QPF TOTALS ON THE ORDER OF 0.25-0.50 AROUND THE YUMA VICINITY...BUT ONLY 0.10-0.25 THROUGHOUT CNTRL ARIZONA. THE LOW PRESSURE SYSTEM WILL ACCELERATE NORTHWARD TUESDAY WITH DOWNGLIDE SUBSIDENCE...AND HIGH PRESSURE RIDGING DRIFTING NORTHEAST. DESPITE THE SINKING MOTION....MODEL CROSS-SECTIONS INDICATE THAT RESIDUAL MOISTURE JUST ABOVE THE BOUNDARY LAYER...AND LOW CLOUDS WILL BE PREVALENT IN THE MORNING BEFORE DISSIPATING LATER IN THE AFTERNOON. LOCAL HI-RES WRFS ALSO SUGGEST THAT A BRIEF ISOLATED SHOWER COULD BE POSSIBLE IN THE EARLY MORNING...PARTICULARLY NORTH AND EAST OF PHOENIX IN AN UPSLOPE REGIME. ABOVE NORMAL TEMPERATURES AND HIGHER THAN NORMAL HUMIDITIES REPLETE WITH OCCASIONAL THICKER MID/HIGH CLOUDS WILL PREVAIL THROUGH MUCH OF THE WEEK AS A RIDGE BECOMES REESTABLISHED TO SOUTH ACROSS MEXICO. GLOBAL MODELS CONTINUE TO SUGGEST THAT ANOTHER PACIFIC LOW PRESSURE SYSTEM WILL AFFECT THE REGION LATE IN THE WEEK. THIS SYSTEM APPEARS SLOWER THAN ITS PREDECESSOR AND HAS THE POTENTIAL TO PRODUCE PERIODIC PRECIPITATION IN THE FRIDAY-SUNDAY TIME FRAME. OFFICIAL FORECAST IS WEIGHTED TOWARDS THE LATEST ECMWF...WHICH IS EXHIBITING RELATIVELY LOW ENSEMBLE SPREAD. HOWEVER...PRECISE TIMING REMAINS UNCERTAIN AND CHANCE POPS HAVE BEEN UTILIZED FRIDAY THROUGH SATURDAY NIGHT. THEREAFTER...MODEL CONSENSUS SUGGESTS LOWER PROBABILITIES FOR SUNDAY. && .AVIATION... SOUTH-CENTRAL ARIZONA INCLUDING KPHX...KIWA AND KSDL... LITTLE TO NO AVIATION IMPACTS THROUGH THIS EVENING...WITH JUST SOME HIGH CLOUDINESS SPREADING OVER THE PHX AREA. CIGS ARE THEN EXPECTED TO LOWER TO AROUND 10K FEET LATE TONIGHT...WITH THESE CIGS HOLDING NEAR THESE LEVELS THROUGH MONDAY AFTERNOON AS LOW PRESSURE BEGINS TO APPROACH FROM THE SOUTH. LIGHT RAIN...WITH CIGS AOB 6K FT TO FINALLY MOVE INTO THE REGION BY EARLY EVENING ON MONDAY. WINDS TO REMAIN MAINLY EASTERLY THROUGH THE THIS PERIOD WITH SPEEDS MAINLY AOB 6 KNOTS...PERHAPS EVEN BECOMING RATHER LIGHT/VARIABLE AT TIMES. FOR PLANNING PURPOSES...LIGHT RAIN THAT IS EXPECTED TO MOVE INTO THE PHX AREA EARLY MONDAY EVENING TO CONTINUE INTO EARLY TUES MORNING. CIGS DURING THIS PERIOD ARE EXPECTED TO REMAIN MAINLY IN THE 4-6K FT RANGE...WITH FEW-SCT SCUD LAYERS DROPPING AS LOW AS 2-3K FT AT TIMES. ALTHOUGH THE RAIN IS EXPECTED TO END TUES MORNING...LIGHT WINDS AND A VERY MOIST AIRMASS IS EXPECTED TO ALLOW CIGS AOB 6K FT TO LINGER THROUGH AT LEAST MIDDAY TUES...WITH ONLY SLOW CLEARING EXPECTED ON TUES AFTERNOON. WINDS TO REMAIN ON THE LIGHT SIDE...MAINLY AOB 6 KTS THROUGH TUESDAY MIDDAY...MAINLY BLOWING FROM AN EASTERLY DIRECTION. SOUTHEAST CALIFORNIA AND SOUTHWEST ARIZONA INCLUDING KIPL AND KBLH... NO AVIATION IMPACTS THROUGH MONDAY MORNING WITH MOSTLY CLEAR SKIES TODAY AND INCREASING CLOUDS TONIGHT. WINDS WILL REMAIN LIGHT AND GENERALLY FAVOR A WESTERLY OR NORTHERLY DIRECTION. RAIN AND LOWER CIGS WILL THEN MOVE INTO THE KIPL AREA BY MIDDAY MONDAY...AND INTO THE KBLH AREA BY MON AFTERNOON AS LOW PRESSURE APPROACHES FROM THE SOUTH...WITH CIGS AS LOW AS 3-5K FT AND VIS AS LOW AS 3-5 MILES AT TIMES DURING THIS RAIN EVENT...WITH WINDS FAVORING AT NORTHERLY DIRECTION AT KBLH...AND WESTERLY AT KIPL. AVIATION DISCUSSION NOT UPDATED FOR AMENDED TAFS. && .FIRE WEATHER... TUESDAY THROUGH SATURDAY... NEAR TO SLIGHTLY ABOVE NORMAL TEMPERATURES EXPECTED THROUGH THE PERIOD WITH WEDNESDAY AND THURSDAY BEING THE WARMEST DAYS. FAIRLY MOIST CONDITIONS WILL EXIST AS SOUTHERLY FLOW PERSISTS ACROSS THE DESERT SOUTHWEST. AN EXITING LOW PRESSURE SYSTEM EARLY TUESDAY WILL BRING LINGERING RAIN CHANCES THROUGH TUESDAY AFTERNOON...WHILE ANOTHER LOW PRESSURE SYSTEM IS FORECAST TO BRING MORE RAIN CHANCES FOR THURSDAY NIGHT THROUGH POSSIBLY SATURDAY. THE MOIST CONDITIONS WILL ONLY LEAD TO MINIMUM HUMIDITIES EACH AFTERNOON IN THE 25 TO 45 PERCENT RANGE. WITH THE EXCEPTION OF SOME OCCASIONAL EARLY MORNING OR AFTERNOON BREEZINESS...WINDS WILL REMAIN LIGHT THROUGH THE ENTIRE PERIOD. && .PSR WATCHES/WARNINGS/ADVISORIES... AZ...NONE. CA...NONE. && $$ VISIT US ON FACEBOOK...TWITTER...AND AT WEATHER.GOV/PHOENIX DISCUSSION...MO/HIRSCH AVIATION...PERCHA FIRE WEATHER...KUHLMAN

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