FXUS65 KTWC 051029
AFDTWC

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TUCSON AZ
330 AM MST SAT DEC 5 2009

.SYNOPSIS...MUCH COOLER AND DRIER AIR HAS SETTLED INTO THE AREA. 
THIS HAS RESULTED IN VERY COLD TEMPERATURES THIS MORNING ACROSS 
SOUTHEAST ARIZONA. A STRONG WINTER STORM SYSTEM PUSHING SOUTH OUT OF 
CANADA WILL MOVE SOUTH ALONG THE WEST COAST THIS WEEKEND...THEN 
TRACK THROUGH THE GREAT BASIN MONDAY. THIS SYSTEM WILL RESULT IN 
WIDESPREAD VALLEY RAIN AND SIGNIFICANT MOUNTAIN SNOW ACROSS ALL OF 
SOUTHEAST ARIZONA MONDAY NIGHT AND TUESDAY.

&&

.DISCUSSION...THE SHORT TERM REMAINS RATHER QUIET AS A COLD AND DRY 
AIRMASS RESIDES ACROSS SOUTHEAST ARIZONA. TEMPERATURES CONTINUE TO 
FALL EARLY THIS MORNING TOWARDS THE FREEZING MARK HERE IN THE TUCSON 
METRO AREA...WITH WELL BELOW FREEZING READINGS OBSERVED ALREADY IN 
THE VALLEYS OF SANTA CRUZ AND COCHISE COUNTIES. THERE IS A FREEZE 
WARNING CURRENTLY IN EFFECT FOR SOUTHEAST PINAL AND EASTERN PIMA 
COUNTIES UNTIL 9 AM MST. ALTHOUGH HIGH CLOUDS CONTINUE TO INVADE THE 
STATE FROM THE WEST...THE LOW DEWPOINTS HAVE ALLOWED FOR SIGNIFICANT 
COOLING TO OCCUR...WITH VALUES LIKELY DIPPING BELOW FREEZING AROUND 
DAYBREAK. 

OTHERWISE...A QUIET WEEKEND IS ON TAP WITH WEATHER VERY TYPICAL OF 
EARLY DECEMBER WITH HIGHS IN THE LOWER TO MID 60S FOR THE LOWER 
ELEVATIONS. HOWEVER...THIS TRANQUIL PATTERN WILL DRASTICALLY CHANGE 
EARLY NEXT WEEK...AS A MUCH MORE ACTIVE PATTERN DEVELOPS ACROSS THE 
WEST COAST.
  
A STRONG TROUGH OF LOW PRESSURE OVER MT AND ID THIS MORNING WILL BE 
REINFORCED BY ANOTHER SHORTWAVE MAKING ITS WAY SOUTH THROUGH 
WESTERN CANADA INTO THE PACIFIC NORTHWEST THIS AFTERNOON. THESE TWO 
SYSTEMS WILL BE RESPONSIBLE FOR CARVING OUT A MEAN TROUGH ACROSS THE 
WEST COAST THIS WEEKEND. IT IS THIS SECOND SYSTEM THAT TRACKS SOUTH 
ALONG THE COASTLINE INTO NORTHERN CALIFORNIA BY MONDAY MORNING. THIS 
TROUGH WILL THEN TRACK EAST THROUGH THE GREAT BASIN MONDAY NIGHT AND 
TUESDAY WITH THE TAIL END OF THIS SYSTEM AFFECTING OUR NECK OF THE 
WOODS. THE SOUTHERN PERIPHERY OF THE MEAN TROUGH WILL ABSORB A LOW 
THAT IS CAUGHT UNDER THE MEAN EASTERN PACIFIC RIDGE NUDGING ITS WAY 
NORTH INTO THE GULF OF ALASKA. THE MOISTURE ASSOCIATED WITH THIS 
PACIFIC SYSTEM WILL WORK IT WAY EAST INTO THE STRONG DYNAMICS OF THE 
TROUGH RESULTING IN WIDESPREAD VALLEY RAIN AND SIGNIFICANT MOUNTAIN 
SNOWFALL TO SOUTHEAST ARIZONA MONDAY NIGHT INTO TUESDAY. EXPECTING 
TOTAL STORM SNOWFALL AMOUNTS BETWEEN 6 AND 12 INCHES BETWEEN 6000 
FEET AND 7000 FEET...WITH 12 TO 18 INCHES POSSIBLE ABOVE 7000 
FEET.STRONG WINDS WITH THIS SYSTEM WILL RESULT IN SOUTHWEST TO WEST 
WINDS OF 20 TO 30 MPH WITH GUSTS TO 40 MPH IN THE HIGHER ELEVATIONS 
WITH THE PASSAGE OF THIS SYSTEM.THESE WINDS WILL RESULT IN AREAS OF 
BLOWING SNOW IN THE MOUNTAINS...REDUCING VISIBILITIES BELOW A  
QUARTER MILE AT TIMES. CONFIDENCE IS HIGH THAT THIS SYSTEM WILL 
SIGNIFICANTLY IMPACT SOUTHEAST ARIZONA. NOT ONLY IS THE ORIENTATION 
OF THE DEVELOPING SYSTEM HARD TO SHAKE LOOSE...BUT BOTH THE GFS AND 
ECMWF MODELS HAVE SHOWN NICE RUN TO RUN CONSISTENCY. THEREFORE...I 
HAVE ISSUED A WINTER STORM WATCH FOR ELEVATIONS ABOVE 6000 FEET FOR 
MOST LOCATIONS ACROSS SOUTHEAST ARIZONA.

ZONAL FLOW IN THE WAKE OF THIS SYSTEM WILL CONTINUE TO BRING A 
SERIES OF DISTURBANCES INTO THE DESERT SOUTHWEST THROUGH THE WORK 
WEEK. PLENTY OF FETCH TO GRAB MOISTURE FROM IN THE PACIFIC WILL 
RESULT IN ANOTHER CHANCE OF HEAVY RAINFALL AND MOUNTAIN SNOWFALL 
THURSDAY AND FRIDAY. UNFORTUNATELY THE NATURE OF ZONAL FLOW WILL 
MAKE IT PROBLEMATIC TO HIT THE EXACT TIMING OF THIS SECOND SYSTEM AS 
ANY DISTURBANCE WILL QUICKLY TRACK EAST THROUGH THE AREA.  

&&

.AVIATION...HIGH THIN CLOUDS ACROSS ARIZONA TODAY AS THE LARGE SCALE 
PATTERN BEGINS TO CHANGE. WINDS GENERALLY LESS THAN 10 KTS AT THE 
SURFACE TODAY WITH NORTHWEST TO NORTH WINDS OVER 15 KTS ABOVE 10K 
FEET. AVIATION DISCUSSION NOT UPDATED FOR TAF AMENDMENTS.

&&

.FIRE WEATHER...THE DRY AIR ACROSS SOUTHEAST ARIZONA THIS WEEKEND WILL BE 
CHANGING TO A MUCH WETTER PATTERN THE FIRST OF NEXT WEEK. THE LARGE 
SCALE PATTERN IS SHIFTING TO MORE ZONE FLOW BY WEDNESDAY. THIS 
CHANGE WILL ALLOW MIDLATITUDE SYSTEMS TO AFFECT SOUTHERN ARIZONA 
WITH AN INCREASE IN WIND AND PRECIPITATION MONDAY NIGHT AND TUESDAY. 
UNTIL THIS CHANGE FULLY TAKES PLACE...DRY CONDITIONS WITH A DIURNAL 
WIND PATTERN WILL PERSIST.

&&

.TWC WATCHES/WARNINGS/ADVISORIES...

FREEZE WARNING UNTIL 9 AM MST THIS MORNING FOR AZZ029-033.

WINTER STORM WATCH MONDAY NIGHT AND TUESDAY ABOVE 6000 FEET FOR 
AZZ019-030-033>035.

$$

WEATHER.GOV/TUCSON