000 FXUS65 KPSR 270421 AFDPSR AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE PHOENIX AZ 920 PM MST THU NOV 26 2009 .SYNOPSIS... DRY AND WARM WEATHER CONTINUES THROUGH FRIDAY. HOWEVER...A STRONG LOW PRESSURE SYSTEM WILL MOVE THROUGH THE REGION THIS WEEKEND AND INTO MONDAY...BRINGING COOLER TEMPERATURES...LOCALLY BREEZY CONDITIONS...AND A CHANCE OF RAIN AND MOUNTAIN SNOW SHOWERS TO THE HIGHEST PEAKS...WITH LESSER CHANCES OF PRECIPITATION FOR THE DESERTS. SLIGHTLY WARMER TEMPERATURES ARE EXPECTED TUESDAY...BUT AN UNSETTLED WEATHER PATTERN RETURNS BY THE MIDDLE OF THE WEEK. && .DISCUSSION... FEW CLOUDS OVER THE ERN CWA TONIGHT FROM A BROAD CYCLONIC CIRCULATION OVER SONORA. ANTICIPATED STORM SYSTEM FOR THIS WEEKEND IS NOW DIGGING INTO THE PACIFIC NORTHWEST. NO CHANGES TO THE FORECAST FOR TONIGHT. 00Z GFS APPEARS TO HAVE INITIALIZED THE SYSTEM A BIT TOO FAR TO THE EAST AS COMPARED TO THE NAM...WHICH RESULTS IN A SLIGHTLY FASTER AND EASTWARD-DISPLACED LOW THROUGH SUNDAY AT WHICH POINT THE MODELS PHASE TOGETHER. BOTH MODELS ARE BREAKING-OUT LIGHT PRECIP EVEN BY SATURDAY MORNING NOW IN RESPONSE TO THE STRONG JET STREAK /120 KTS/ THAT DEVELOPS ON THE SOUTH SIDE OF THE SYSTEM ALONG WITH OROGRAPHIC LIFTING. AS USUAL...WOULD EXPECT THE BL TO TAKE SOME TIME TO SATURATE BEFORE PRECIPITATION BECOMES MORE EFFICIENT IN REACHING THE GROUND SO SATURDAY MORNING MAY BE A BIT AGGRESSIVE YET. MID SHIFT WILL FULLY ASSESS THE 00Z SUITE OF MODEL DATA AND MAKE ANY NECESSARY ADJUSTMENTS. PREVIOUS DISCUSSION... FRIDAY AND SATURDAY...THE MOST RECENT RUNS OF THE GFS/ECMWF/NAM ARE IN GOOD AGREEMENT...SUGGESTING THAT THE FLOW DEEPENS/AMPLIFIES FRIDAY...PINCHING OFF A CLOSED LOW THAT CENTERS OVER SOUTHERN CA/THE LOWER COLORADO RIVER VALLEY SATURDAY. THE TRAJECTORY OF THIS LOW SKIRTS ALONG THE WEST COAST...BUT IT SHOULD BE ABLE TO TAP SUBTROPICAL MOISTURE AND SPREAD IT ACROSS OUR CWA. THICKNESS VALUES FALL 100-150M DURING THE DAY SATURDAY...AND A MUCH COLDER AIRMASS ENTERS THE REGION...-22 TO -28C 500MB TEMPS BY THE EVENING HOURS. AS SUCH...MAX TEMPS WILL DROP QUITE A BIT ACROSS OUR CWA. EXPECT HIGHS TO DROP BELOW SEASONAL NORMALS AND INTO THE UPPER 60S TO LOW 70S SATURDAY OVER MOST DESERT LOCALES. WITH THE FAVORABLE POSITION OF THE LOW AND INCREASED FORCING AND MOISTURE...EXPANDED SLIGHT CHANCE POPS INTO THE DESERTS SATURDAY AFTERNOON...BETTER CHANCES OVER SOUTHERN GILA COUNTY OVERNIGHT AND CONTINUING INTO SUNDAY. WITH THE COLDER OVERNIGHT LOWS...THERE WILL BE A SLIGHT CHANCE FOR MOUNTAIN SNOW SHOWERS OVER THE HIGHEST PEAKS OF JTNP IN SE CA...AND HILLTOP IN ZONE 24/SRN GILA COUNTY. SUNDAY AND MONDAY...THE LOW DROPS SOUTHEAST AND INTO NORTHWESTERN MEXICO. THE BEST MOISTURE AND DYNAMICS STILL EXISTS OVER SOUTH-CENTRAL AZ...SO KEEPING A CHANCE FOR SHOWERS OVER THE HIGHER TERRAIN EAST OF PHOENIX. DAYTIME TEMPS CONTINUE TO COOL A FEW DEGREES SUNDAY... ALTHOUGH STILL TOPPING OUT 6-9 DEGREES BELOW SEASONAL NORMALS. ALSO...AS THE LOW TRACKS FURTHER SOUTH AND WEST...GRADIENTS WILL NOT BE AS TIGHT ACROSS THE WESTERN CWA SO LESS WIND IS FORECAST AND IT APPEARS AT THIS TIME THAT WINDS WILL NOT MEET ADVISORY LEVELS. QUITE A BIT OF WRAP AROUND MOISTURE IS ADVERTISED TO MOVE INTO OUR SOUTH-CENTRAL ZONES SUNDAY NIGHT INTO MONDAY MORNING. DESPITE UVVS DECREASING...ELECTED TO KEEP SLIGHT CHANCE TO CHANCE POPS OVER THE HIGHER TERRAIN OF OUR EASTERN ZONES...MAINLY FROM PHOENIX EASTWARD. TUESDAY AND WEDNESDAY...THE UPPER LOW WILL PUSH OFF TO THE EAST... LEAVING A BROAD BAGGY TROF TO LINGER OVER THE DESERT SOUTHWEST. MODELS ARE NOT IN GOOD AGREEMENT AT THIS TIME...THE GFS SUGGESTS THAT ANOTHER LOW UNDERCUTS THE BUILDING RIDGE ALONG THE WEST COAST TUESDAY...WHILE THE ECMWF HINTS AT YET ANOTHER SYSTEM THAT SLIDES THROUGH THE PAC NW INTO THE GREAT BASIN BY WEDNESDAY. EITHER SCENARIO WILL OFFER A BRIEF PERIOD OF WARMER TEMPERATURES AND A RETURN TO AN UNSETTLED WEATHER PATTERN BY MID-WEEK. && .AVIATION... PHX AREA... EASTERLY WINDS TO CONTINUE ONCE AGAIN THROUGH THE NIGHT AND INTO FRIDAY MORNING. EXPECTING THE EASTERLY WINDS TO DECREASE DURING THE EARLY PART OF FRIDAY AFTERNOON AND POSSIBLY SHOW SOME SIGNS OF TRYING TO SWITCH TO THE SOUTHWEST. SE CA/SW AZ... WEST TO NORTH WINDS THIS EVENING WILL WEAKEN DURING THE OVERNIGHT HOURS...BECOMING LIGHT AND VARIABLE FOR MUCH OF FRIDAY. AVIATION DISCUSSION NOT UPDATED FOR AMENDED TAFS. && .FIRE WEATHER... SATURDAY THROUGH WEDNESDAY... A LOW PRESSURE SYSTEM WILL MOVE THROUGH THE AREA LATE THIS WEEKEND ...WITH COOL AND BREEZY CONDITIONS SUNDAY THROUGH MONDAY. THERE IS A THREAT FOR MOUNTAIN SHOWERS AND VALLEY RAIN ACROSS THE ENTIRE FORECAST AREA...WITH MUCH DRIER CONDITIONS EXPECTED BY MID WEEK. BREEZY NORTHERLY WINDS ARE POSSIBLE ON THE BACKSIDE OF THE TROUGH...PARTICULARLY OVER THE WESTERN CWA. && .PSR WATCHES/WARNINGS/ADVISORIES... AZ...NONE. CA...NONE. && $$ NATIONAL WEATHER SERVICE PHOENIX IS ON THE INTERNET AT WEATHER.GOV/PHOENIX DISCUSSION...INIGUEZ/MEYERS/CB AVIATION...INIGUEZ FIRE WEATHER...WANEK