000 
FXUS65 KPSR 301008
AFDPSR
AZZ020>028-CAZ030>033-301745-

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PHOENIX AZ
245 AM MST TUE DEC 30 2003

SOUTHWEST AND SOUTH-CENTRAL ARIZONA...AND SOUTHEAST CALIFORNIA 
FORECAST DISCUSSION

.SYNOPSIS...
CONSIDERABLE CLOUDINESS WILL CONTINUE TO SPREAD OVER THE REGION 
TODAY AND TONIGHT...AS A LARGE PACIFIC STORM DEVELOPS OFF THE 
SOUTHERN CALIFORNIA COAST. THIS WEATHER SYSTEM WILL APPROACH 
SOUTHERN ARIZONA ON WEDNESDAY...INCREASING THE CHANCE FOR SHOWERS
OVER MAINLY SOUTH CENTRAL ARIZONA LATE WEDNESDAY AFTERNOON AND 
NIGHT. A DRIER AIRMASS WILL MOVE ACROSS SOUTHEAST CALIFORNIA AND 
SOUTH CENTRAL ARIZONA THURSDAY...WITH ANOTHER CHANCE FOR SHOWERS 
FRIDAY NIGHT OR SATURDAY.

&&

.DISCUSSION...
A CONSIDERABLE AMOUNT OF MID AND HIGH LEVEL CLOUDS CONTINUED TO 
STREAM INTO ARIZONA FROM THE SOUTHWEST AT 09Z. MOISTURE WAS BEING  
THROWN INTO THE STATE FROM A DEVELOPING LOW PRESSURE SYSTEM OFF THE 
SOUTHERN CA COAST...ALONG WITH INCREASING JET STREAM WINDS OVER AZ.  

IN THE SHORT TERM (24 HOURS) MODELS CONTINUE TO MOVE THIS OFFSHORE 
LOW PRESSURE SYSTEM (CENTERED ABOUT 430 MILES WEST OF SAN DIEGO AT 
09Z) SOUTH SOUTHEASTWARD TO A POSITION 300 MILES WEST OF CENTRAL 
BAJA BY LATE TUESDAY EVENING. WHILE THIS SYSTEM MOVES SOUTH...MODELS 
FORECAST BETWEEN 120-140 METER 500 MB HEIGHT FALLS EVERY 6 HOUR 
PERIOD THROUGH TUESDAY EVENING. THIS SUGGESTS QUITE A BIT OF 
CONVECTION...WHICH SHOULD INJECT AND THROW MORE MOISTURE INTO THE 
ATMOSPHERE...MORE THAN CURRENT MODELS SUGGEST. MUCH OF THIS DEEP 
MOISTURE WILL LIKELY BE CONTAINED WITHIN THE BROAD 700/500 MB 
CIRCULATION CENTER...AS IT MOVES INTO SOUTH CENTRAL AND SOUTHEAST AZ 
LATE WED AND WED NIGHT (NEW YEARS EVE). WE CURRENTLY HAVE A SLIGHT 
CHANCE FOR SHOWERS THEN...HOWEVER PROBABILITIES MAY BE A LITTLE 
HIGHER THAN THIS. WE THINK SOME MEASURABLE PRECIPITATION WILL FALL. 
THEREFORE WE WILL UPGRADE THE PROBABILITY OF PRECIP IN SOUTH CENTRAL 
AZ WED EVE TO A CHANCE.

IN THE INTERIM HOWEVER... A SWATH OF DEEP MID LEVEL CLOUDINESS WILL 
BRUSH SOUTHEAST CA AND FAR SOUTHWEST AZ (COLORADO RIVER ZONES) FOR A 
SLIGHT CHANCE FOR SPRINKLES OR LIGHT SHOWERS TODAY. 

OTHERWISE...THE LOW PRESSURE SYSTEM COMING UP FROM THE BAJA AREA 
MOVES QUICKLY OUT OF THE STATE THURSDAY...HOWEVER A VERY MOIST 
WESTERLY FLOW IS FORECAST TO SPREAD INTO THE AREA FRIDAY...WITH THE 
POSSIBILITY OF RAIN.  DUE TO THE DIFFERENCES IN MEDIUM RANGE MODEL 
GUIDANCE ON FRIDAY (UKMET...ECMWF...AND GFS)...WE WILL OPT TO LEAVE 
THIS PERIOD ALONE FOR NOW. 
 
&&

.PSR...WATCHES/WARNINGS/ADVISORIES...
AZ...NONE. 
CA...NONE.

&&

$$

02