000
FXUS65 KTWC 270426
AFDTWC

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TUCSON AZ
925 PM MST THU NOV 26 2009

.SYNOPSIS...HIGH PRESSURE ALOFT WILL PROVIDE WARM AND DRY CONDITIONS 
AGAIN FRIDAY. A VIGOROUS PACIFIC LOW PRESSURE SYSTEM WILL THEN MOVE 
INTO THE AREA THIS WEEKEND INTO MONDAY. THIS SYSTEM WILL BRING A 
CHANCE OF RAIN AND MOUNTAIN SNOW AS WELL AS MUCH COOLER TEMPERATURES 
SUNDAY AND MONDAY. DRY CONDITIONS TO PREVAIL TUESDAY AND WEDNESDAY 
THOUGH DAYTIME TEMPERATURES REMAINING BELOW NORMAL.

&&

.DISCUSSION...SATELLITE IMAGERY THIS EVENING SHOWED A CLEAR TO 
PARTLY CLOUDY SKY ACROSS SOUTHEAST ARIZONA. WEAK UPPER LOW TO OUR 
SOUTH WILL CONTINUE TO SPREAD MID-HIGH LEVEL CLOUDS NORTH...MAINLY 
EAST OF TUCSON. HAVE ADJUSTED CLOUD COVER IN GRID/TEXT BASED 
FORECASTS FOR TONIGHT PERIOD TO REFLECT THIS THINKING. ONE MORE WARM 
NOVEMBER DAY BEFORE A VIGOROUS LOW PRESSURE SYSTEM IMPACTS THE AREA 
OVER THE WEEKEND. CHECK PREVIOUS DISCUSSION BELOW FOR DETAILS.

&&

.AVIATION...VARIABLE CLOUDINESS...MAINLY EAST OF TUCSON...AT OR 
ABOVE 20K FT AGL INTO FRIDAY MORNING. SURFACE WIND KTUS VICINITY 
THRU 01Z ELY/SELY 10-20 KTS AND GUSTY...AT KOLS/KDUG AIRFIELDS 
GENERALLY ELY WIND 5-15 KTS. SURFACE WIND INTO FRIDAY MORNING 
ELY/SELY 5-15 KTS. AVIATION DISCUSSION NOT UPDATED FOR TAF 
AMENDMENTS.

&&

.FIRE WEATHER...REMAINING DRY FRIDAY WITH MINIMUM HUMIDITIES MOSTLY 
BELOW 15 PERCENT AND LESS WIND VERSUS THANKSGIVING DAY. A VIGOROUS 
PACIFIC LOW PRESSURE SYSTEM WILL MOVE ACROSS SOUTHERN CALIFORNIA 
SATURDAY AND TOWARD NORTHERN SONORA MEXICO BY EARLY MONDAY. THIS 
SYSTEM WILL BRING A CHANCE OF RAIN AND MOUNTAIN SNOW TO THE AREA... 
ESPECIALLY SATURDAY AFTERNOON INTO MONDAY. BREEZY TO WINDY 
CONDITIONS ARE ALSO FORECAST SATURDAY. HOWEVER...CRITICAL FIRE 
WEATHER THRESHOLDS ARE NOT EXPECTED TO BE MET SINCE RELATIVE 
HUMIDITIES ARE FORECAST ABOVE 15 PERCENT. COOLER SATURDAY WITH MUCH 
COOLER TEMPERATURES TO OCCUR SUNDAY AND MONDAY. DRY CONDITIONS TO 
PREVAIL TUESDAY THROUGH THURSDAY.

&&

.PREVIOUS DISCUSSION...FOCUS OF ATTENTION THEN SHIFTS TOWARD 
VIGOROUS LOW PRESSURE SYSTEM TO BE NEAR THE NRN CALIFORNIA COAST FRI 
AFTERNOON...THEN PROGGED TO MOVE SEWD TOWARD THE SWRN AZ-NRN BAJA 
CALIFORNIA REGION BY EARLY SUN. THERE WERE DIFFERENCES BETWEEN THE 
26/12Z GFS/ECMWF SOLUTIONS REGARDING THE EVOLUTION/PROGRESSION OF 
THIS SYSTEM. TAKEN AT FACE VALUE THE GFS WOULD ARGUE FOR THE BEST 
CHANCE OF PRECIP TO OCCUR SAT NIGHT...THEN MID-LEVEL DRYING OR A 
SEMBLANCE OF A DRY-SLOT ON SUN. UPPER LOW THEN WOULD STALL OVER NRN 
SONORA MON FOR ANOTHER ROUND OF PRECIP. MEANWHILE...THE ECMWF WAS 
MORE-PROGRESSIVE WITH THE SEWD TRAJECTORY ACROSS SRN CALIFORNIA AND 
SRN AZ SAT-SUN...THEN UPPER LOW TO STALL OVER NRN SONORA MON BEFORE 
EJECTING EWD MON NIGHT AND TUE. AT ANY RATE...

HAVE OPTED FOR A CHANCE OF SHOWERS ACROSS MUCH OF THE AREA SAT NIGHT 
AND SUN MORNING. SNOW LEVELS BY SUN MORNING EXPECTED TO LOWER TO 
AROUND 6000-6500 FEET. FORECAST SNOW ACCUMULATIONS BY SUN MORNING ON 
MT LEMMON AROUND 2 INCHES...3 INCHES FOR MT GRAHAM...AND A LITTLE 
OVER 4 INCHES AT HANNAGAN MEADOW...WHICH WOULD BE LOW-END ADVISORY 
CRITERIA ABOVE 7000 FEET FOR THE WHITE MOUNTAINS. FOR NOW WILL 
REFLECT SLIGHTLY LOWER SNOW ACCUMS SUN...SUN NIGHT...AND MON THOUGH 
CHANCE-CATEGORY POPS FROM TUCSON EWD WITH LESSER CHANCES WEST. BOTH 
GFS/ECMWF INDICATE THIS SYSTEM TO EJECT EWD MON NIGHT THEN WLY/NWLY 
FLOW ALOFT TO OCCUR TUE-THUR. FOR NOW WILL REJECT THE GFS SCENARIO 
OF UPPER LOW TO MOVE ACROSS NRN AZ NEXT WED NIGHT-THUR.

ABOVE NORMAL DAYTIME TEMPS AGAIN FRI AHEAD OF THE APPROACHING STORM 
SYSTEM. COOLER SAT THEN QUITE COOL SUNDAY-TUESDAY WITH DAYTIME HIGH 
TEMPS AVERAGING ABOUT 10 DEGS OR SO BELOW NORMAL. WARMER WED WITH 
HIGH TEMPS TRENDING CLOSE TO NORMAL NEXT THUR.

&&

.TWC WATCHES/WARNINGS/ADVISORIES...NONE.

$$

WEATHER.GOV/TUCSON