AREA FORECAST DISCUSSION

 
 FXUS65 KPSR 250332
 AFDPSR
 
 AREA FORECAST DISCUSSION
 NATIONAL WEATHER SERVICE PHOENIX AZ
 832 PM MST TUE NOV 24 2009
 
 .SYNOPSIS...
 HIGH PRESSURE ALOFT WILL KEEP CONDITIONS DRY AND WARM THROUGH FRIDAY.
 A LOW PRESSURE SYSTEM WILL BRING COOLER AND BREEZY CONDITIONS TO THE 
 AREA THIS WEEKEND...WITH BELOW NORMAL TEMPERATURES LIKELY BY SUNDAY.
 
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 .DISCUSSION...
 CURRENT FORECAST LOOKS GOOD. MINOR CHANGES MADE TO THE WIND FORECAST 
 FOR TONIGHT INTO EARLY TOMORROW. RUC/NAM/WRF SUGGESTING MORE NELY 
 WINDS ALONG THE FOOTHILLS N TO E OF PHX AS WELL AS THE SW-FACING 
 SLOPES OF THE CHOCOLATE MTNS IN SE CA. THESE WINDS WILL KEEP 
 TEMPERATURES UP A BIT TONIGHT IN THOSE AREAS...BUT MORE OPEN DESERT 
 AREAS SHOULD SEE LESS WIND AND RADIATE VERY EFFICIENTLY.
 
 PREVIOUS DISCUSSION...
 LATEST WATER VAPOR IMAGERY INDICATES STRONG UPPER LOW OVER THE 
 CENTRAL US...WITH UPPER RIDGE BUILDING OVER THE PACIFIC NORTHWEST. 
 WEAK VORT MAX SPINNING NEAR 28N 135W IS MOVING SLOWLY 
 EASTWARD...WITH ASSOCIATED HIGH CLOUDINESS SPREADING TOWARDS BAJA 
 CA. UPPER RIDGE WILL CONTINUE TO BUILD EASTWARD...LEADING TO A 
 SLIGHT WARMING TREND THROUGH THURSDAY. AFOREMENTIONED VORT MAX WILL 
 EVENTUALLY MOVE INLAND...AND MAY ADVECT SOME MID-LEVEL 
 MOISTURE/INCREASED CLOUDINESS MAINLY ACROSS SOUTHEAST ARIZONA. 
 
 POTENTIALLY BIG CHANGES THEN TAKE SHAPE FRIDAY INTO THE WEEKEND. 
 SHORTWAVE TROUGH OF LOW PRESSURE WILL MOVE INTO THE PAC NW AND 
 DEEPEN AS STRONG UPPER JET MOVES INLAND. EVOLUTION OF THE FEATURE 
 THEN BECOMES MORE UNCERTAIN...AS ECWMF HAS STRONGER UPPER JET WITH 
 FURTHER SUBSEQUENT DEEPENING...BRINGING A DEEP 545 DAM H5 LOW OVER 
 EASTERN AZ. GFS MAINTAINS THE FEATURE AS AN OPEN WAVE...WITH A 
 FASTER EASTWARD PROGRESSION AND STRONGER FORCING REMAINING TO THE 
 NORTH. AT ANY RATE...COOLING TREND LOOKS FAIRLY LIKELY AT THIS 
 POINT...WITH POTENTIALLY BREEZY CONDITIONS AFFECTING MAINLY WESTERN 
 AZ AND SOUTHEAST CA. MORE UNCERTAINTY EXISTS WITH THE DEGREE OF 
 COOLING TAKING PLACE...PARTICULARLY SUNDAY/MONDAY. IF CONFIDENCE 
 INCREASES IN THE OPEN WAVE SOLUTION...THEN IT IS LIKELY THAT MAX 
 TEMPS FOR SUNDAY WILL NEED TO BE ADJUSTED UPWARD SOME. MOISTURE IN 
 ANY CASE LOOKS TO BE LIMITED...WITH THE BEST CHANCES /ALBEIT SLIGHT/ 
 FOR ANY PRECIPITATION CONFINED MAINLY TO THE HIGHER TERRAIN OF 
 EASTERN AZ SATURDAY NIGHT INTO SUNDAY.
 
 BEYOND THE WEEKEND...UPPER RIDGING IS EXPECTED TO BUILD AGAIN OVER 
 THE WESTERN CONUS...WITH MOST OF THE SOUTHWEST UNITED STATES UNDER 
 DRY NORTHERLY FLOW ALOFT. 
 
 &&
 
 .AVIATION...
 
 THROUGH 00Z THURSDAY...HIGH PRESSURE ALOFT WILL CONTINUE TO BRING 
 CLEAR SKIES AND LIGHT DIURNAL WINDS TO KPHX...KIWA...AND KIPL. WINDS 
 WILL REMAIN DOWNSLOPE FROM THIS EVENING THROUGH WEDNESDAY AFTERNOON.
 
 AVIATION DISCUSSION NOT UPDATED FOR AMENDED TAFS.
 
 &&
 
 .FIRE WEATHER...
 FRIDAY THROUGH TUESDAY...
 UNCERTAINTY EXISTS REGARDING EVOLUTION...STRENGTH...AND TRACK OF  
 LOW PRESSURE SYSTEM THIS WEEKEND...WITH DIVERGENT SOLUTIONS OFFERED 
 BY ECMWF VERSUS GFS AND CANADIAN. ECMWF CALLS FOR A MORE AMPLIFIED 
 SOLUTION...WITH A STRONG UPPER LOW EXPECTED TO DEVELOP OVER THE 
 SOUTHWEST UNITED STATES...WHILE THE GFS AND CANADIAN FORECAST A 
 MODERATELY STRONG BUT PROGRESSIVE SHORT WAVE TROUGH TO PASS 
 SOUTHEAST THROUGH ARIZONA. CURRENT FORECAST LEANS HEAVILY ON THE 
 00Z/12Z ECMWF GUIDANCE. FROM PREVIOUS DISCUSSION...A NORTH PACIFIC 
 LOW PRESSURE SYSTEM WILL BRING SIGNIFICANT COOLING BY THIS WEEKEND 
 WITH INCREASING WIND EACH DAY FRIDAY THROUGH SUNDAY. BY 
 SUNDAY...STRONG GUSTY NORTH WINDS ARE LIKELY ACROSS SOUTHEAST 
 CALIFORNIA AND THE WESTERN DESERTS...WITH PEAK GUSTS OVER 30KT 
 EXPECTED DOWN THE LOWER COLORADO RIVER VALLEY. HIGH TEMPERATURES MAY 
 FALL AROUND 15 DEGREES BY SUNDAY COMPARED WITH THE PEAK HIGH 
 TEMPERATURES THANKSGIVING DAY. HIGHS SUNDAY ARE EXPECTED TO BE IN 
 THE LOW TO MID 60S IN MOST LOWER ELEVATION DESERT LOCATIONS. THIS 
 WILL BE A RATHER DRY SYSTEM FOR OUR AREA...BUT RELATIVE HUMIDITY 
 WILL BE HIGHER MOSTLY BECAUSE OF THE COOLING. LESS WIND IS EXPECTED 
 ON MONDAY...ALTHOUGH BREEZY NORTH WINDS WILL CONTINUE OVER 
 SOUTHEASTERN CALIFORNIA AND SOUTHWEST ARIZONA. TEMPERATURES WILL 
 WARM SLIGHTLY BUT REMAIN BELOW SEASONAL NORMALS...AND VERY DRY AIR 
 WILL PERSIST ACROSS THE REGION.
 
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 .PSR WATCHES/WARNINGS/ADVISORIES...
 
 AZ...NONE.
 
 CA...NONE.
 
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 NATIONAL WEATHER SERVICE PHOENIX IS ON THE INTERNET AT
 
 WEATHER.GOV/PHOENIX
 
 DISCUSSION...ROGERS
 AVIATION...DG
 FIRE WEATHER...INIGUEZ/DG/CB/ELLIS/ROGERS
 
 
 

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