000
FXUS65 KPSR 280427
AFDPSR
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PHOENIX AZ
925 PM MST MON DEC 27 2010
.SYNOPSIS...
PARTLY CLOUDY SKIES WILL GRADUALLY GIVE WAY TO RAIN AND SNOW LATER
THIS WEEK. RAIN IS POSSIBLE ACROSS THE REGION ON WEDNESDAY WITH A
CHANGEOVER TO SNOW ACROSS THE HIGHER TERRAIN NORTHEAST OF PHOENIX ON
THURSDAY. MUCH COLDER TEMPERATURES WILL SETTLE INTO THE REGION ON
FRIDAY AND WILL LINGER THROUGH EARLY NEXT WEEK.
&&
.PREVIOUS DISCUSSION...
CLOUDS HAVE DISSIPATED RAPIDLY IN THE SUBSIDENCE BEHIND THE TROUGH
AXIS. HOWEVER...THE CLEARING TREND WILL BE SHORT-LIVED AS HIGH
CLOUDS IN THE EXIT REGION OF AN APPROACHING JET STREAK ENCROACH ON
THE AREA. TEMPERATURES THIS EVENING ARE GENERALLY CLOSE TO THOSE 24
HOURS AGO...BUT WITH DRIER AIR ACROSS SE CA...COLDER CONDITIONS ARE
EXPECTED THERE OVERNIGHT. MINOR ADJUSTMENTS WERE MADE TO THE
SHORT-TERM TEMPERATURES/DEWPOINTS THIS EVENING. OTHERWISE...FCST
REMAINS ON TRACK.
&&
.DISCUSSION...
RAPID CLEARING WAS NOTED PER VISIBLE SATELLITE IMAGERY AS OF 2
PM...WITH MOSTLY CLEAR SKIES FROM PHOENIX WESTWARD. SOME LINGERING
CLOUD COVER REMAINED ACROSS SOUTHERN GILA COUNTY...BUT THIS WAS
ERODING QUICKLY AS A WEAK TROUGH CONTINUED TO MOVE THROUGH THE AREA.
THE CLEARING SKIES ALLOWED TEMPERATURES TO REACH THE UPPER 60S
ACROSS SOUTHEAST CALIFORNIA AND THE MID 60S AROUND THE PHOENIX AREA.
CONDITIONS WILL REMAIN DRY FOR TOMORROW AS WEAK RIDGING SETS UP
OVERHEAD. ANOTHER MOSTLY SUNNY/PARTLY CLOUDY DAY IN STORE WITH NEAR
NORMAL TEMPS IN THE MID/UPPER 60S. MAIN FORECAST CONCERNS ARE
THREEFOLD TODAY...PRECIP CHANCES AND TIME OF ONSET ON
WEDNESDAY...RAIN/SNOW TRANSITION TIME ON THURSDAY...AND VERY COLD
TEMPERATURES FRIDAY MORNING.
MODELS CONTINUE TO SHARE A COMMON SOLUTION SHOWING DEEP LOW PRESSURE
MOVING THROUGH CENTRAL CALIFORNIA ON WEDNESDAY AND DRAGGING A COLD
FRONT THROUGH THE REGION. THERE SHOULD BE A DECENT AMOUNT OF
MOISTURE WITH THIS SYSTEM AS IT MOVES ONSHORE BUT A FAIR AMOUNT
SEEMS TO DRY UP AS IT CROSSES THE MOUNTAINS OF CA/NV AND ARRIVES IN
AZ. THAT BEING SAID...THERE SHOULD BE MODERATELY MOIST UPSLOPE FLOW
OUT OF THE SOUTHWEST AHEAD OF TIME AS A SHORTWAVE ROUNDS THE BASE OF
THE UPPER TROUGH. PRECIPITABLE WATER CLIMBS TO 0.80 INCHES WEDNESDAY
AFTERNOON AS A RESULT...APPROXIMATELY 1-2 STDEV ABOVE NORMAL. I AM
EXPECTING RAIN TO BEGIN WEDNESDAY MORNING ACROSS SOUTHEAST
CALIFORNIA AND WESTERN ARIZONA WITH INCREASING CHANCES THROUGH THE
AFTERNOON AND EVENING HOURS. ATMOSPHERE SHOULD BE SUFFICIENTLY WARM
WITH SNOW LEVELS SUFFICIENTLY HIGH THAT WEDNESDAY\S PRECIP WILL FALL
IN THE FORM OF RAIN. AMOUNTS SHOULD RANGE FROM A THIRD TO HALF AN
INCH ACROSS THE LOWER DESERTS OF THE PHOENIX METRO AREA...WITH AN
INCH POSSIBLE NORTHEAST OF PHOENIX. AMOUNTS WILL BE SUBSTANTIALLY
LESS ACROSS SOUTHEAST CALIFORNIA AS RAIN SHADOWING...ALONG WITH THE
TRAJECTORY OF THE UPPER LOW...WILL SEVERELY LIMIT THE AMOUNT OF RAIN
NEAR IMPERIAL/YUMA. IN SHORT...FAIRLY CONFIDENT IN A WIDESPREAD LOW
TO MODERATE-QPF EVENT. POPS WERE RAISED A BIT...GENERALLY 40-50
PERCENT IN THE WEST AND 80-90 PERCENT IN THE EAST.
MUCH COLDER AIR WILL MOVE INTO THE REGION EARLY THURSDAY MORNING
BEHIND THE COLD FRONT. THIS WILL ALLOW SNOW LEVELS TO DROP TO AROUND
3000 FEET ACROSS THE AREA THURSDAY MORNING AND TO AROUND 2000 FEET
LATER IN THE DAY. LINGERING RAIN SHOWERS SHOULD MIX AND CHANGE TO
SNOW ACROSS SOUTHERN GILA COUNTY AS WELL AS THE NORTHERN AND EASTERN
EXTENT OF THE PHOENIX METRO /SCOTTSDALE...CAVE CREEK...EAST
MESA/...ALTHOUGH SOME QUESTIONS REMAIN REGARDING HOW MUCH LINGERING
PRECIPITATION THERE WILL BE ONCE THE COLD AIR ARRIVES. GFS SUGGESTS
A BRIEF CHANGEOVER FOLLOWED BY DRY CONDITIONS...EUROPEAN MODEL
SUGGESTS A MORE PROLONGED SNOW EVENT ESPECIALLY AROUND GLOBE. GIVEN
LAST WEEK\S STORM...IT SEEMS THAT THE MODELS HAVE BEEN BRINGING IN
PRECIP TOO FAST AND ENDING IT TOO FAST. THEREFORE I AM INCLINED TO
HOLD ONTO PRECIP CHANCES A BIT LONGER THAN THE GFS AND CONTINUE
SNOWFALL ACROSS SOUTHERN GILA COUNTY THURSDAY NIGHT. I DO NOT HAVE
ACCUMULATING SNOWFALL IN THE FORECAST FOR THE DESERT FLOORS BUT SOME
RIDGE TOPS COULD SEE LIGHT ACCUMS ON THURSDAY. THIS MAY NEED TO BE
RE-ADDRESSED IF IT BECOMES EVIDENT THAT PRECIP WILL BE LINGERING
WELL INTO THURSDAY NIGHT.
AS OF NOW IT APPEARS SKIES WILL CLEAR THURSDAY EVENING BEHIND THE
LOW WITH VERY COLD AIR EXPECTED TO SETTLE OVER THE ENTIRE REGION
EARLY FRIDAY MORNING. LOCAL RESEARCH SUGGESTS THAT 500MB TEMPS OF
-24C...700MB TEMPS BELOW -10C...ALONG WITH 850MB TEMPS OF -3C WILL
RESULT IN SUB FREEZING TEMPS AT SKY HARBOR...ALL OF THESE CRITERIA
ARE MET FROM ALL AVAILABLE MODEL GUIDANCE. I WILL CONTINUE WITH
SUB-FREEZING TEMPS IN THE FORECAST FOR EARLY FRIDAY MORNING AND IT
APPEARS THAT THE ENTIRE AREA MAY REACH FREEZE WARNING CRITERIA. THE
FLY IN THE OINTMENT WOULD BE LINGERING RAIN/SNOW SHOWERS AND CLOUD
COVER WHICH WOULD KEEP TEMPS JUST ABOVE THE FREEZING MARK.
I WILL NOT BE ISSUING ANY WINTER STORM/FREEZE WATCHES TODAY...BUT WE
WILL ISSUE A SPECIAL WEATHER STATEMENT HIGHLIGHTING THE UPCOMING
CHANGE IN THE WEATHER. IF THINGS REMAIN ON TRACK...WINTER
STORM/FREEZE WATCHES WILL BECOME NECESSARY BUT FOR THE TIME BEING
THE SPS SEEMS JUSTIFIED.
THE WEATHER WILL REMAIN ON THE CHILLY SIDE THROUGH THE WEEKEND AND
INTO EARLY NEXT WEEK. SOME SUGGESTION...ESPECIALLY FROM THE EUROPEAN
MODEL...THAT TEMPS EARLY NEW YEARS DAY WILL ALSO DROP BELOW THE
FREEZING MARK ALTHOUGH THIS MODEL IS THE OUTLIER ON SATURDAY AND I
KEPT LOWS ABOVE FREEZING. BELOW NORMAL TEMPERATURES...ALONG WITH DRY
CONDITIONS WILL PREVAIL THROUGH EARLY NEXT WEEK.
&&
.AVIATION...
SOUTH-CENTRAL ARIZONA INCLUDING KPHX AND KIWA AIRFIELDS...
NO AVIATION WX CONCERNS WITH LIGHT WINDS AND INCREASING HIGH CLOUDS
THROUGH TUE EVENING.
SOUTHEAST CALIFORNIA AND SOUTHWEST ARIZONA...KIPL AND KBLH...
NO AVIATION WX CONCERNS WITH LIGHT WINDS AND INCREASING HIGH CLOUDS
THROUGH TUE EVENING.
AVIATION DISCUSSION NOT UPDATED FOR AMENDED TAFS.
&&
.FIRE WEATHER...
WEDNESDAY THROUGH SUNDAY...
A MAJOR CHANGE IN THE PATTERN WILL TAKE PLACE THIS WEEK...AS LOW
PRESSURE DEVELOPS TO THE NORTH AND BRINGS MORE CLOUDS...SHOWERS AND
COLDER WEATHER TO THE AREA. BREEZY CONDITIONS ARE EXPECTED...WITH
WIND SPEEDS UP TO 20 MPH...EXCEPT GUSTS TO 35 MPH ARE POSSIBLE IN
IMPERIAL COUNTY LATE WEDNESDAY. COLD TEMPERATURES AND VERY DRY AIR
WILL OVERSPREAD THE AREA BY NEW YEARS EVE WITH DAYTIME HIGHS IN
PHOENIX ONLY IN THE UPPER 40S. DRY AND WARMER WEATHER IS LIKELY BY
SUNDAY.
&&
.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...NONE.
CA...NONE.
&&
$$
NATIONAL WEATHER SERVICE PHOENIX IS ON THE INTERNET AT
WEATHER.GOV/PHOENIX
DISCUSSION...HIRSCH/LEINS
AVIATION...HIRSCH
FIRE WEATHER...SIPPLE
