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AREA FORECAST DISCUSSION


 
 FXUS65 KPSR 271155
 AFDPSR
 
 AREA FORECAST DISCUSSION
 NATIONAL WEATHER SERVICE PHOENIX AZ
 455 AM MST FRI FEB 27 2015
 
 .UPDATE...UPDATED AVIATION AND FIRE WEATHER DISCUSSIONS...
 
 &&
 
 .SYNOPSIS...
 DRY WEATHER WITH ABOVE NORMAL TEMPERATURES ARE EXPECTED THROUGH THE 
 END OF THE WEEK. HOWEVER...LOW PRESSURE WILL APPROACH THE AREA THIS 
 WEEKEND BRINGING GUSTY WINDS AND WIDESPREAD RAINFALL TO THE AREA 
 ALONG WITH MUCH COOLER TEMPERATURES THROUGH EARLY NEXT WEEK. COOL 
 AND UNSETTLED WEATHER IS LIKELY TO CONTINUE INTO THE MIDDLE OF THE 
 WEEK.
 
 &&
 
 .DISCUSSION...
 DRY NORTHWESTERLY FLOW PERSISTS ACROSS THE REGION THIS MORNING WITH 
 ONLY A FEW HIGH CLOUDS MOVING THROUGH ARIZONA AS OF 0945Z. THESE 
 CLOUDS HAVE HAD A NEGLIGIBLE IMPACT ON TEMPERATURES WITH MOST DESERT 
 LOCALES IN THE MID 40S TO LOWER 50S. SHOULD BE ABLE TO SEE SEVERAL 
 MORE DEGREES OF COOLING BEFORE DAYBREAK WITH LIGHT WINDS ACROSS THE 
 AREA. IN THE SHORT TERM THROUGH THIS AFTERNOON...THE FORECAST 
 REMAINS LARGELY UNCHANGED FOR TODAY...MOSTLY SUNNY WITH TEMPS IN THE 
 MID/UPPER 70S. 
 
 FORECAST CONCERNS ARE CENTERED ON A POTENT WINTER STORM THAT IS 
 EXPECTED TO MAKE ITS WAY THROUGH THE PACIFIC NORTHWEST AND OFF THE 
 SOUTHERN CALIFORNIA COAST THIS WEEKEND. THE SYSTEM IS LOCATED WEST 
 OF WASHINGTON STATE PER WATER VAPOR IMAGERY...IT WILL GRADUALLY MOVE 
 SOUTHWARD TODAY AND WILL EVENTUALLY CARVE OUT A RATHER LARGE UPPER 
 LEVEL CIRCULATION BY SATURDAY EVENING. ONCE THE LOW DEEPENS IT WILL 
 ADVECT A SUBSTANTIAL AMOUNT OF MOISTURE INTO THE AREA. WITH PWATS 
 EXPECTED TO RISE FROM ROUGHLY 0.3 INCHES TODAY TO 0.8-0.9 INCHES AND 
 INTEGRATED WATER VAPOR TRANSPORT VALUES APPROACHING THE 99TH 
 PERCENTILE AT TIMES THIS WEEKEND...CONFIDENCE CONTINUES TO INCREASE 
 THAT THIS WILL BE A PROLONGED WET WINTER WEATHER EVENT ACROSS THE 
 DESERT SOUTHWEST.
 
 TRENDS IN THE PROBABILISTIC AND DETERMINISTIC GUIDANCE OVER THE PAST 
 FEW DAYS HAVE SUGGESTED THE SYSTEM WILL SLOW DOWN A BIT AS IT 
 APPROACHES THE AREA. THEREFORE...CONFIDENCE IN EXACTLY WHEN THE 
 HIGHEST POPS SHOULD BE DRAWN IN FOR THIS WEEKEND/EARLY NEXT WEEK IS 
 LOW. CONCEPTUALLY...MUCH OF SATURDAY SHOULD BE SPENT MOISTENING THE 
 ATMOSPHERE WITH ONLY A FEW SCATTERED SHOWERS POSSIBLE MAINLY 
 NORTH/EAST OF PHOENIX IN THE HIGHER TERRAIN LOCATIONS. THIS IDEA IS 
 SUPPORTED BY THIS MORNING`S 00Z DETERMINISTIC MODELS AS A NUMBER OF 
 BUFR SOUNDINGS ACROSS THE FORECAST AREA. ALSO WORTH NOTING...WINDS 
 WILL INCREASE QUITE A BIT ON SATURDAY /ESPECIALLY ACROSS SOUTHEAST 
 CALIFORNIA/ AND WITH MANY DESERT LOCATIONS GOING ON A MONTH SINCE 
 OUR LAST MEASURABLE RAIN EVENT...COULD BE SOME PATCHY BLOWING DUST 
 IN THE NORMALLY SUSCEPTIBLE AREAS NEAR OPEN FIELDS. MAY ALSO NEED TO 
 ADDRESS THE NECESSITY OF A WIND ADVISORY FOR PORTIONS OF SOUTHEAST 
 CALIFORNIA AS WELL. 
 
 BL MOISTURE WILL CONTINUE TO DEEPEN SATURDAY NIGHT AND THE 00Z 
 NAEFS/GEFS INDICATE MOST OF SUNDAY WILL BE A WASH. 
 HOWEVER...QUASI-STATIONARY UPPER LOWS OFF THE CALIFORNIA COAST CAN 
 BE PARTICULARLY TRICKY IN TERMS OF TIMING PRECIP MOVING INTO THE 
 AREA AND MY CONFIDENCE IN EXACTLY HOW SUNDAY WILL PLAY OUT IS 
 LOW...ESPECIALLY ACROSS THE PHOENIX AREA SEVERAL HUNDRED MILES AWAY 
 FROM THE CIRCULATION CENTER. AS THE ENSEMBLES CONTINUE TO INDICATE 
 70-90 POPS ACROSS THE DESERTS...I WILL HANG ONTO SIMILAR VALUES. 
 
 00Z NAM/GFS/ECMWF ALL INDICATE THE MID LEVEL CIRCULATION WILL MOVE 
 THROUGH THE AREA MONDAY MORNING WITH A FRONTAL BAND OF SHOWERS 
 AFFECTING THE LOWER DESERTS AROUND 12Z MONDAY. THIS IS A BIT SLOWER 
 THAN THE MODELS WERE SHOWING YESTERDAY AND I INCREASED POPS QUITE A 
 BIT FOR MONDAY TO ACCOUNT FOR THIS SLOWER SOLUTION. THE BEST Q 
 CONVERGENCE AND ISENTROPIC ASCENT IS INDICATED DURING THIS PERIOD 
 AND THE ENSEMBLES ALSO INDICATE ELEVATED POPS DURING THIS TIMEFRAME. 
 ALSO DREW IN THE HIGHEST QPF DURING THE DAY MONDAY WITH PRECIP 
 RAPIDLY SHIFTING EAST OF PHOENIX BY MONDAY EVENING. MEANWHILE...SNOW 
 LEVELS WILL FALL IMMEDIATELY BEHIND THE FRONTAL BAND AND THERE 
 SHOULD BE A RAIN/SNOW MIX EAST OF PHOENIX LATE MONDAY NIGHT. WITH 
 SNOW LEVELS EXPECTED TO FALL TO AROUND 6000FT MONDAY NIGHT...ONLY 
 THE HIGHEST PEAKS WOULD SEE ACCUMULATING SNOWFALL. 
 
 STILL SOME UNCERTAINTY WITH RESPECT TO A POST-FRONTAL SHORTWAVE 
 TROUGH AND ITS IMPACTS ON TUESDAY. THIS ONE WILL BE MUCH LESS 
 ORGANIZED THAN THE PREVIOUS UPPER LOW BUT IF ENOUGH MOISTURE LINGERS 
 ACROSS THE AREA...THERE COULD CERTAINLY BE ADDITIONAL SHOWERS MAINLY 
 NORTH/EAST OF PHOENIX TUESDAY/TUESDAY NIGHT. WITH SNOW LEVELS AROUND 
 5500FT BY THIS TIME...COULD BE LIGHT ACCUMS AS WELL ALTHOUGH 
 CONFIDENCE IS WAY TOO LOW TO PIN DOWN EXACT SNOWFALL AMOUNTS AT THIS 
 TIME. MADE SOME SLIGHT UPWARD ADJUSTMENTS TO POPS FROM THE COLORADO 
 RIVER EASTWARD. PRECIP CHANCES SHOULD BEGIN TO WANE QUICKLY ON 
 WEDNESDAY AS DRY NORTHWESTERLY FLOW TAKES HOLD ACROSS THE WESTERN 
 CONUS.
 
 AS FOR TEMPERATURES...BELOW NORMAL TEMPERATURES ARE EXPECTED FROM 
 SATURDAY THROUGH MUCH OF THE UPCOMING WEEK. SUNDAY AND MONDAY WILL 
 BE PARTICULARLY COOL WITH HIGHS BARELY GETTING OUT OF THE LOWER 60S 
 IN MOST LOCATIONS. SOME MODERATION WILL TAKE PLACE BY MID WEEK AS 
 DRIER AIR MOVES IN FROM THE NORTHWEST. 
 
 &&
 
 .AVIATION... 
 SOUTH-CENTRAL ARIZONA INCLUDING KPHX...KIWA AND KSDL...
 SOUTHEAST CALIFORNIA AND SOUTHWEST ARIZONA INCLUDING KIPL AND KBLH...
 
 A LOW PRESSURE SYSTEM APPROACHING FROM THE NORTHWEST WILL BRING 
 INCREASING HIGH CLOUDS TODAY AND INTO TONIGHT. LOW LEVEL MOISTURE 
 WILL START INCREASING LATE TONIGHT WHICH SHOULD ALLOW FOR SOME LOWER 
 FEW TO SCT CLOUDS AT AROUND 6K FEET JUST BEFORE SUNRISE SATURDAY. 
 SOUTHWESTERLY WINDS WILL BECOME GUSTY AT TIMES THIS AFTERNOON AND 
 ESPECIALLY ACROSS SOUTHEAST CALIFORNIA. WINDS AT KIPL AND KBLH WILL 
 CONTINUE TO INCREASE THROUGH THE OVERNIGHT HOURS WITH SOME GUSTS UP 
 TO AROUND 25 KTS POSSIBLE.
 
 AVIATION DISCUSSION NOT UPDATED FOR AMENDED TAFS.
 
 &&
 
 .FIRE WEATHER...
 SUNDAY THROUGH THURSDAY...
 COOL AND WET CONDITIONS WILL BE PRESENT THROUGH THE FIRST PART OF 
 THE PERIOD WITH THE BEST CHANCES FOR WIDESPREAD RAINS ON SUNDAY AND 
 MONDAY. THERE WILL BE A CHANCE OF A FEW THUNDERSTORMS ON MONDAY AND 
 POTENTIALLY TUESDAY. HUMIDITIES WILL BE WELL ABOVE TYPICAL VALUES 
 THROUGH TUESDAY BEFORE MINIMUM VALUES FALL BACK TO AROUND 20 PERCENT 
 ON THURSDAY AFTERNOON. PERIODS OF BREEZY SOUTHWESTERLY WINDS CAN BE 
 EXPECTED ON MONDAY AND TUESDAY. HIGH TEMPERATURES ACROSS THE DESERTS 
 WILL ONLY REACH INTO THE 60S SUNDAY THROUGH WEDNESDAY. HIGH PRESSURE 
 AND DRIER WEATHER SHOULD MOVE IN STARTING LATER WEDNESDAY AND SHOULD 
 PERSIST THROUGH THE END OF THE WEEK.
 
 &&
 
 .PSR WATCHES/WARNINGS/ADVISORIES...
 
 AZ...NONE.
 
 CA...NONE.
 
 &&
 
 $$
 
 VISIT US ON FACEBOOK...TWITTER...AND AT WEATHER.GOV/PHOENIX
 
 DISCUSSION...LEINS
 AVIATION...KUHLMAN
 FIRE WEATHER...KUHLMAN
 
 
 
 
 
 

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Phoenix, AZ 85072

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