FXUS65 KPSR 250450
AFDPSR
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PHOENIX AZ
945 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 LOW DESERT
LOCATIONS...ESPECIALLY SOUTH-CENTRAL ARIZONA...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...
24 HOUR TEMP TRENDS THIS EVENING HAVE BEEN A MIXED BAG WITH SOME
LOCATIONS COOLER AND SOME WARMER. MADE SOME MINOR ADJUSTMENTS TO THE
MIN TEMP GRID FOR TONIGHT TO REFLECT TRENDS...SUCH AS ZONE 24 TO
REFLECT THE LOCAL BREEZINESS DUE TO THE GRADIENT ASSOCIATED WITH THE
SURFACE HIGH CENTERED TO THE NORTH. TEMPS WILL BE QUITE CHILLY
TONIGHT WITH MANY LOWER ELEVATION LOCATIONS IN SOUTH-CENTRAL ARIZONA
FLIRTING WITH THE FREEZING MARK BUT DURATIONS AND AREAL COVERAGES
ARE NOT EXPECTED TO MEET FREEZE WARNING CRITERIA. INSTEAD AN SPS WAS
ISSUED THIS AFTERNOON TO HIGHLIGHT THE COLD TEMPERATURES. WILL
REISSUE TO REFLECT SUBTLE CHANGES.
LOOKING AT THE BIGGER PICTURE FLOW PATTERN...A HIGH AMPLITUDE RIDGE
PERSISTS OVER THE WEST COAST THE NEXT COUPLE DAYS WITH A HUGE LOW
PRESSURE SYSTEM AFFECTING MOST OF THE REST OF THE CONUS. OUR TEMPS
WILL STAY COOL. WITH TIME THE RIDGE AXIS SHIFTS EASTWARD AND
UNDERCUTTING FLOW STRENGTHENS WITH A SHORT WAVE PASSING THROUGH
SATURDAY NIGHT FOR SOME THICK MAINLY HIGH CLOUDS AND MORE MILD MIN
TEMPS.
PREVIOUS 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...
LIGHT WINDS AND MOSTLY CLEAR SKIES WITH THIN CIRRUS WILL BE THE RULE
THROUGH FRIDAY EVENING. SOME MORE NOTICEABLE NORTHERLY WINDS WILL
REDEVELOP ALONG THE LOWER COLORADO RIVER VALLEY AGAIN FRIDAY
AFTERNOON WITH SPEEDS OF 10-15 KTS AND OCCASIONAL HIGHER GUSTS.
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...AJ/DG/ELLIS
AVIATION...AJ
FIRE WEATHER...SIPPLE
