FXUS65 KPSR 050428
AFDPSR
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PHOENIX AZ
930 PM MST FRI DEC 4 2009
.SYNOPSIS...
COLD TEMPERATURES CAN BE EXPECTED OVERNIGHT...WITH CONSIDERABLE HIGH
CLOUDS SPREADING INTO THE REGION THIS WEEKEND. UNSETTLED
WEATHER...AND A CHANCE OF SHOWERS CAN BE EXPECTED MOST OF NEXT
WEEK...AS A SERIES OF LOW PRESSURE SYSTEMS MOVE INTO THE WESTERN
STATES.
&&
.DISCUSSION...
FORECAST MIN TEMP FOR SKY HARBOR IS 36 DEGREES. LAST TIME IT GOT
THAT COLD WAS DEC 28 LAST YEAR...WHEN THE TEMPERATURE REACHED 34
DEGREES.
CURRENT FORECAST TEMPS ARE IN VERY GOOD SHAPE. DEWPOINTS WERE A
LITTLE ON THE LOW SIDE AND MINOR ADJUSTMENTS WERE MADE TO THE
OVERNIGHT PERIOD. FOCUS REMAINS ON THE RAINFALL FOR NEXT WEEK.
PREVIOUS DISCUSSION...
COOL AND DRY CONDITIONS WILL PERSIST OVERNIGHT INTO TOMORROW AS THE
WESTERN UNITED STATES REMAINS CAUGHT IN BETWEEN STORM SYSTEMS.
EXPECTING OVERNIGHT MINIMUMS WILL BE NEAR FREEZING TO BELOW FREEZING
IN MANY LOWER DESERT LOCATIONS...BUT FREEZE WARNING THRESHOLDS FOR
TEMPORAL/AREAL COVERAGE ARE NOT EXPECTED TO BE MET. PREVIOUS
THINKING WAS THAT HIGH CLOUDS MAY KEEP MINIMUM TEMPERATURES A COUPLE
DEGREES WARMER THIS EVENING...BUT IT LOOKS LIKE THAT COOLING SHOULD
GO UNABATED FOR MOST OF THE NIGHT IN THE EASTERN CWA /SOUTH-CENTRAL
AZ/. OUT WEST...KEPT MINIMUMS A BIT HIGHER AS CLOUDS WILL LIKELY
ARRIVE EARLIER.
SIGNIFICANT CHANGE IN THE WEATHER PATTERN THAT HAS BEEN WELL
ADVERTISED IS STILL ON TRACK. THE FIRST UPPER LOW CAN BE SEEN
CLEARLY ON TODAY/S IR IMAGERY...DIVING SOUTHWARD ACROSS BRITISH
COLUMBIA...AND WILL CONTINUE UNTIL IT REACHES ITS MOST SOUTHWARD
EXTENT NEAR NORTHERN UTAH. A SECOND SYSTEM WILL DEEPEN THE
LARGE-SCALE TROUGH AS IT TAKES A TRACK DOWN THE PACIFIC COAST. THIS
WILL BE A VERY STRONG WINTER-TYPE SYSTEM...WITH 500 HPA HEIGHTS
BOTTOMING AROUND 520 DM...AND AN ASSOCIATED 150+ KT JET...DEPICTING
A VERY TIGHT BAROCLINIC GRADIENT. AS IS NOT ALWAYS THE CASE FOR THIS
PART OF THE COUNTRY...PLENTY OF AVAILABLE MOISTURE WILL GET DRAWN
INTO THIS SYSTEM FROM THE ERN PACIFIC. GOES TOTAL PRECIP WATER
IMAGERY INDICATES ABUNDANT MOISTURE POOL NEAR 130W / 25N.
AS THE SYSTEM APPROACHES THE REGION...LOW/MID LEVEL MOISTURE WILL
BE IMPORTED AS EARLY AS SUNDAY...WITH 850 MB TDS INCREASING TO NEAR
4 DEG C BY MONDAY AFTERNOON. SIGNIFICANT UPSLOPE COMPONENT TO THE
WIND AHEAD OF A DEEPENING SFC/850 MB LOW WILL CONTRIBUTE TO
OROGRAPHIC FORCING AS EARLY AS MONDAY AFTERNOON. IN FACT...850 MB
WINDS MAY INCREASE TO APPROXIMATELY 50 KTS OUT OF THE SW...WHICH
WILL FAVOR PRECIPITATION IN MOUNTAINOUS AREAS TO THE NORTH AND EAST
OF THE PHOENIX AREA. FRONTAL PASSAGE IS EXPECTED TO OCCUR LATE
MONDAY EVENING INTO EARLY TUESDAY MORNING...COINCIDENT WITH AN
UPPER JET STREAK THAT WILL MOVE ACROSS CENTRAL/SOUTHERN CA.
DYNAMICAL FORCING WILL BE IMPRESSIVE WITH THIS SYSTEM...SO
CONFIDENCE IS HIGH THAT A SIGNIFICANT PRECIPITATION EVENT WILL OCCUR
IN CENTRAL AND NORTHERN AZ...PRIMARILY IN THE HIGHER TERRAIN AREAS
OF OUR CWA. ALREADY INCREASED POPS TO LIKELY/CATEGORICAL FOR MOST OF
SOUTH-CENTRAL AZ FOR MONDAY NIGHT/TUESDAY MORNING. THE COLD-CORE OF
THE UPPER LOW WILL REMAIN TO THE NORTH...WITH A TIGHT TEMPERATURE
GRADIENT ALOFT...MEANING THE LOWEST SNOW LEVELS WILL REMAIN OVER
NORTHERN AZ. EXPECTING SNOW LEVELS TO DROP DOWN TO BETWEEN 5000-6000
FT OVER SOUTHERN GILA COUNTY...WHICH SHOULD ONLY AFFECT THE HIGHER
MOUNTAIN PEAKS.
AS THE FRONT PASSES THROUGH THE REGION...TEMPORARY DRYING WILL OCCUR
WEDNESDAY IN ITS WAKE. HOWEVER...MODELS ARE SHOWING MORE CONSISTENCY
IN RECENT RUNS THAT YET ANOTHER SYSTEM WILL MOVE INLAND...AND IT
SHOULD HAVE AMPLE MOISTURE AVAILABLE. THIS SYSTEM DOESN/T LOOK TO BE
AS COLD AS THE FIRST...DUE TO ITS ORIGINS AT LOWER LATITUDES.
NONETHELESS...THE PATTERN WILL REMAIN ACTIVE...SO HAVE OPTED TO
INCREASE POPS A BIT IN THE EXTENDED /DAYS 6 AND 7/.
&&
AVIATION...
NO AVIATION CONCERNS THROUGH AT LEAST 00Z SUN. LIGHT WINDS WITH
INCREASING MID AND HIGH LEVEL CLOUDS EXPECTED...AHEAD OF THE WELL
ADVERTISED APPROACHING LOW PRESSURE SYSTEM LATE IN THE WEEKEND/EARLY
NEXT WEEK.
AVIATION DISCUSSION NOT UPDATED FOR AMENDED TAFS.
&&
.FIRE WEATHER...
SUNDAY THROUGH THURSDAY...
A SERIES OF LOW PRESSURE SYSTEMS WILL MOVE ACROSS THE REGION...
BRINGING INCREASED MOISTURE THAT WILL LEAD TO CHANCES OF DESERT RAIN
AND MOUNTAIN SNOW. BREEZY CONDITIONS...WITH GUSTS UP TO 30 MPH...ARE
EXPECTED AREA-WIDE LATE MONDAY INTO TUESDAY...AND LIGHTER WINDS THE
REST OF THE PERIOD. ANTICIPATE COOLER TEMPERATURES AND HIGHER
HUMIDITY VALUES TO ACCOMPANY THE STRONGER WINDS...AND NOT APPROACH
CRITICAL FIRE WEATHER THRESHOLDS.
&&
.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...NONE.
CA...NONE.
&&
$$
NATIONAL WEATHER SERVICE PHOENIX IS ON THE INTERNET AT
WEATHER.GOV/PHOENIX
DISCUSSION...HIRSCH/ROGERS
AVIATION...WANEK
FIRE WEATHER...MEYERS
