FXUS65 KPSR 242321
AFDPSR

AREA FORECAST DISCUSSION 
NATIONAL WEATHER SERVICE PHOENIX AZ
420 PM MST THU DEC 24 2009

.SYNOPSIS...
NORTHWEST FLOW ALOFT WILL BRING A CONTINUATION OF MOSTLY CLEAR SKIES 
AND BELOW NORMAL TEMPERATURES THROUGH SATURDAY. BELOW-FREEZING 
OVERNIGHT LOWS ARE EXPECTED AT COLDER SOUTH-CENTRAL ARIZONA DESERT 
SITES BOTH CHRISTMAS MORNING AND SATURDAY MORNING. WEST TO SOUTHWEST 
FLOW ALOFT WILL BRING AN INCREASE IN HIGH CLOUDS AND SLIGHTLY MILDER 
DAYTIME HIGHS ON SUNDAY. EXPECT CLOUDS TO THICKEN AND INCREASE 
SUNDAY NIGHT INTO MONDAY...WITH CONSIDERABLE CLOUDINESS AND A SLIGHT 
CHANCE FOR SHOWERS MONDAY INTO TUESDAY...AS A PACIFIC LOW PRESSURE 
SYSTEM TRACKS EAST MAINLY ACROSS NORTHWEST MEXICO.  

&&

.DISCUSSION...
IN THE SHORT TERM...TONIGHT THROUGH SATURDAY...EXPECT A CONTINUATION 
OF DRY CONDITIONS WITH BELOW NORMAL TEMPERATURES...AND LITTLE DAY TO 
DAY CHANGE IN MIN/MAX TEMPERATURES...AS A POWERFUL STORM SYSTEM 
REMAINS OVER THE CENTRAL UNITED STATES. NUMERICAL MODELS IN GOOD 
AGREEMENT THAT OUR REGION WILL STAY IN COOL NORTH-NORTHWEST FLOW 
ASSOCIATED WITH THE PRINCIPAL POLAR JET STREAM ON THE BACKSIDE OF 
THAT STORM. DURING THE DAY ON SUNDAY...SPLIT FLOW WILL REDEVELOP 
OVER THE EPAC...WITH THE SOUTHERN BRANCH OF THE JET TAKING AIM ON 
SOUTHERN CALIFORNIA AND NORTHWEST MEXICO.  

IN EXTENDED...NUMERICAL GUIDANCE IN GENERAL AGREEMENT THAT BEST 
UPPER LEVEL FORCING ASSOCIATED WITH NEXT STORM IN THE SOUTHERN 
BRANCH OF THE JET WILL TRACK FROM OFF THE SOUTHERN CALIFORNIA COAST 
MONDAY MORNING...TO NEAR THE CENTRAL BAJA CALIFORNIA COAST MONDAY 
EVENING...AND EAST ACROSS THE SOUTHWEST UNITED STATES AND NORTHWEST 
MEXICO TUESDAY. IN ADVANCE OF THE UPPER TROUGH OF LOW PRESSURE... 
SOUTHERLY LOW-MID LEVEL FLOW DEVELOPS OVER ARIZONA...WHICH SUPPORTS 
ENHANCED MOIST ADVECTION. HOWEVER...THE ASSOCIATED UPPER JET WILL 
TRACK WELL TO OUR SOUTH...WHICH LIMITS DEGREE OF Q-G FORCING/ASCENT 
OVER OUR REGION. FOR NOW...HAVE GONE WITH SLIGHT CHANCE POP AT MOST 
LOCALES MONDAY-TUESDAY...WITH BEST CHANCE FOR MEASURABLE RAIN OVER 
THE WESTERN CWA MONDAY...AND EASTERN CWA (INCLUDING THE GREATER 
PHOENIX AREA) MONDAY NIGHT INTO TUESDAY MORNING. BASED ON OUTPUT 
FROM 12Z ECMWF/18Z GFS...MIDSHIFT MAY WANT TO CONSIDER INCREASING 
POP OVER MUCH OF SOUTH CENTRAL AZ...ESPECIALLY THE HIGHER 
TERRAIN...MONDAY NIGHT/EARLY TUESDAY.  

&&

.AVIATION...
SOUTH-CENTRAL AZ INCLUDING KPHX/KIWA...
MOSTLY CLEAR SKIES AND LIGHT BREEZES THROUGH 18Z CHRISTMAS DAY. 

SOUTHEAST CALIFORNIA AND SOUTHWEST ARIZONA INCLUDING 
KIPL/KBLH...CLEAR BLO 25 THSD FT TODAY. WINDS WILL DIMINISH SLOWLY 
AFT 25/02Z...WITH LIGHT NORTHERLY FLOW SUPPORTED BY LOW-LEVEL 
PRESSURE GRADIENT AND TOPOGRAPHY...AND NOT BE QUITE AS STRONG ON 
CHRISTMAS DAY.

AVIATION DISCUSSION NOT UPDATED FOR AMENDED TAFS. 

&&

.FIRE WEATHER...
SATURDAY THROUGH WEDNESDAY...
DRY AND COOL WEATHER WILL PREVAIL THROUGH THE WEEKEND. THE RH
WILL INCREASE EARLY NEXT WEEK AHEAD OF A PACIFIC LOW PRESSURE
SYSTEM. EXPECT CONSIDERABLE CLOUDINESS AND A SLIGHT CHANCE OF 
SHOWERS MONDAY THROUGH TUESDAY.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...

AZ...NONE.

CA...NONE.

&&

$$

NATIONAL WEATHER SERVICE PHOENIX IS ON THE INTERNET AT

WEATHER.GOV/PHOENIX

DISCUSSION...DG/ELLIS
AVIATION...DG/SIPPLE
FIRE WEATHER...SIPPLE