Winter Storm in Arizona, January 1, 2015

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PHOENIX AZ
902 PM MST WED DEC 31 2014

...FREEZING TEMPERATURES LIKELY AT MANY LOWER DESERT LOCATIONS
TONIGHT...AND AGAIN ON THURSDAY NIGHT...

.SYNOPSIS...
THE STRONG AND VERY COLD STORM SYSTEM THAT HAS BROUGHT RAIN AND LOW
ELEVATION SNOWFALL TO THE DESERT SOUTHWEST WILL SLOWLY MOVE OFF TO
THE EAST LATE TONIGHT THROUGH THURSDAY. IN ITS WAKE...CLEARING SKIES
AND A VERY COLD AIRMASS WILL ALLOW TEMPERATURES TO FALL TO NEAR OR
BELOW FREEZING AT MANY LOWER DESERT LOCATIONS TONIGHT...AND AGAIN ON
FRIDAY NIGHT. HIGH PRESSURE BUILDING IN FROM THE WEST WILL THEN
BRING WARMER TEMPERATURES...AND DRY CONDITIONS FROM SUNDAY ONWARD
THROUGH AT LEAST THE MIDDLE PART OF NEXT WEEK.

&&

.DISCUSSION...
STRONG MID AND UPPER LEVEL LOW CIRCULATION NOW SKIRTING THE CA-AZ-NV
TRIPLE POINT...HEADING INTO NORTHWESTERN AZ THIS EVENING. 88D AND
TDWR RADAR SCOPES STILL SHOW SCATTERED SHOWER ACTIVITY SPANNING FROM
EAST OF GILA BEND...THROUGH NW PINAL COUNTY AND ACROSS THE
EASTERN/NORTHEASTERN PERIPHERY OF THE PHX METRO. LIGHT RAIN
CONTINUES ON AND OFF AT AREA ASOS/AWOS SITES...WITH MOSTLY TRACE
AMOUNTS AND AN OCCASIONAL HUNDREDTH OR TWO MEASURING OUT.

IR IMAGERY THIS EVENING SHOWS THE VARYING LAYERS OF CLOUDS OVER THE
REGION...WITH BROAD AREAS OF STRATUS AND CIRRIFORM CLOUDS COVERING
MUCH OF SOUTH-CENTRAL AZ. SOME AREAS OF STATUS LINGER ALONG AND WEST
OF THE COLORADO RIVER...BUT COVERAGE NOT NEARLY AS THICK OR
WIDESPREAD AS ACROSS AZ. THE FEW AIRCRAFT SOUNDINGS FROM KPHX WITH
BOTH T AND TD TRACES SHOW A THICK SATURATED LAYER OVER AND EAST OF
PHOENIX BEGINNING AROUND 600MB TO A THOUSAND OR SO FEET OFF THE
SURFACE. SNOW LEVELS HAVE DROPPED TOWARDS THE 2KFT LEVEL AND HAVE
SEEN SOME ISOLATED REPORTS OF MIXED PRECIP OVER SOME OF THE FOOTHILL
AREAS OF THE PHX AREA (NOT INCLUDING NORTH SCOTTSDALE...WHICH
ALREADY SAW SOME FLAKES DURING THE DAYLIGHT HOURS). GIVEN PROXIMITY
OF THE UPPER LOW...COLD CORE AND CONTINUE MID-LEVEL JET SUPPORT
(50KTS ON THE KTWC 00Z BALLOON)...WILL KEEP SLIGHT CHANCE FOR
SHOWERS FROM PHX-GILA BEND EASTWARD THROUGH THE EVENING WITH
CONTINUED SHOWER ACTIVITY PERSISTING WELL TOWARDS SUNRISE AND AFTER
FOR THE MOUNTAINS EAST OF PHOENIX.

ADDED MENTION OF PATCHY FOG OVERNIGHT OVER SOME OF THE MARICOPA AND
NW PINAL COUNTY LOWER ELEVATIONS THROUGH SUNRISE. HAVE RECEIVED A
FEW FOG REPORTS FROM THE EASTERN PERIPHERY OF THE PHOENIX METRO AND
ANTICIPATED DRYING FROM THE WEST IS NOT MOVING AS FAST AS EARLIER
THOUGHT. OVERNIGHT HUMIDITIES WILL CLIMB QUICKLY AS TEMPERATURES
CONTINUE TO FALL AND FOG DEVELOPMENT WILL CONTINUE AND EXPAND IN
COVERAGE. REST OF THE OVERNIGHT FORECAST REMAINS ON TRACK WITH THE
FREEZE WARNING FOR TONIGHT/THURSDAY AM CONTINUING AS WELL AS THE
WINTER STORM WARNING FOR AZZ024/SOUTHERN GILA COUNTY.

&&

.PREVIOUS DISCUSSION /ISSUED AT 230 PM MST/130 PM PST/...
THE STORM THAT HAS SO FAR BROUGHT A VERY COLD AIRMASS...ALONG WITH
SHOWERS AND EVEN SOME SNOW OVER THE LOWER DESERTS OF SW AND
SOUTH-CENTRAL ARIZONA...AND THE LOWER COLORADO RIVER
VALLEY...CONTINUES TO AFFECT THE REGION THIS AFTERNOON. THE MAIN
MOISTURE PLUME...THAT IS MARKING THE LEADING EDGE OF THE COLD AIR
POOL IS NOW MOVING ACROSS SOUTH-CENTRAL AZ. BEHIND THIS
PLUME...DRIER AIR WORKING ITS WAY FROM THE SW IS CLEARING SKIES
ACROSS SW AZ AS THE PARENT UPPER LOW CONTINUES TO MOVE ACROSS SE
CA. TEMPERATURES ACROSS THE LOWER DESERTS ARE HOLDING IN THE 40S AT
THIS HOUR AT MOST LOWER DESERT LOCATIONS ACROSS SOUTH-CENTRAL AND SW
AZ...WITH A FEW LOCATIONS EVEN SEEING TEMPS IN THE UPPER 30S...AND
LOWER 50S BEING OBSERVED ACROSS SE CA. THE LATEST SHORT-RANGE HI-RES
HRRR MODEL OUTPUT IS INDICATING THAT THE MAIN PRECIP BAND WILL
CONTINUE TO PROGRESS SLOWLY EASTWARD THROUGH THE REST OF THIS
AFTERNOON AND INTO TONIGHT...WITH MOST PRECIP BEING CONFINED TO
SOUTHERN GILA COUNTY BY THIS EVENING...WITH SIGNIFICANT SNOWFALL
LIKELY TO CONTINUE OVER THE HIGHER TERRAIN THROUGH THE REMAINDER OF
THE EVENING HOURS BEFORE TAPERING OFF LATER TONIGHT AND EARLY
THURSDAY. ALTHOUGH THE BULK OF THE PRECIP HAS ENDED ACROSS THE LOWER
DESERTS...SOME REDEVELOPMENT OF LIGHT SPRINKLES...OR FLURRIES IS
POSSIBLE EARLY THIS EVENING ACROSS SOUTH-CENTRAL ARIZONA AS THE MAIN
UPPER LOW PASSES OVER THE REGION. SNOW LEVELS WILL LIKELY CONTINUE
TO REMAIN QUITE LOW...AROUND 2000FT...AS 850MB TEMPS FALL INTO THE
-4 TO -6C RANGE THIS EVENING.

A WIDESPREAD FREEZE EVENT NOW LOOKS ALMOST CERTAIN FOR THE NEXT FEW
NIGHTS ACROSS THE LOWER DESERTS AS THE COLD AIR MASS SETTLES INTO
THE REGION BEHIND THE UPPER LOW CENTER. MANY RURAL DESERT LOCATIONS
WILL LIKELY SEE TEMPS FALL BELOW FREEZING TONIGHT AS SKIES CLEAR
LATER THIS EVENING AND 500MB TEMPS FALL INTO THE -25 TO -29C
RANGE...WITH THE ONLY THING KEEPING THE PHX URBAN CORE FROM SEEING
FREEZING TEMPERATURES AS WELL WILL BE SOME LINGERING
CLOUDINESS/MOISTURE OVER THE REGION. DRIER AIR...THAT HAS ALREADY
MOVED INTO THAT REGION...IS EXPECTED TO ALLOW MUCH OF IMPERIAL
COUNTY TO SEE A FREEZE AS WELL...WITH SEVERAL HOURS OF TEMPS AOB
FREEZING EXPECTED. THURSDAY NIGHT WILL LIKELY BE EVEN COLDER ACROSS
SOUTH-CENTRAL AZ AS LIGHT WINDS...CLEAR SKIES...AND A DRIER AIRMASS
WILL RESULT IN NEAR-IDEAL RADIATIONAL COOLING CONDITIONS...IN SPITE
OF SOME SLIGHT WARMING ALOFT. ALTHOUGH FURTHER WARMING ALOFT IS
EXPECTED ON FRIDAY NIGHT...AT LEAST SOME OF THE COLDER DESERT
LOCATIONS WILL LIKELY STILL SEE A THIRD NIGHT OF FREEZING TEMPS.

STARTING FRIDAY...MODELS REMAIN IN GOOD AGREEMENT...SHIFTING THE
UPPER LOW EASTWARD OUT OF OUR REGION AND ALLOWING A GRADUAL INCREASE
IN UPPER HEIGHTS. THE HIGH AMPLITUDE RIDGE THAT HAS BEEN OVER THE
EASTERN PACIFIC THE PAST SEVERAL DAYS WILL BEGIN TO TILT EASTWARD
INTO THE DESERT SOUTHWEST BY EARLY NEXT WEEK. THERE IS SOME MODEL
DISAGREEMENT AT THIS POINT...BUT OVERALL LONG RANGE MODELS SUGGEST
INCREASED HEIGHTS WHICH POINTS TO A SUBSTANTIAL WARM-UP. HIGHS
SHOULD BOUNCE BACK TO NEAR NORMAL BY NEXT MONDAY AND POSSIBLY REACH
INTO THE LOWER 70S ACROSS THE LOWER DESERTS BY THE MIDDLE OF NEXT
WEEK. DRY CONDITIONS WILL BE THE RULE AFTER THURSDAY WITH NO
SIGNIFICANT LOW PRESSURE SYSTEMS THREATENING OUR REGION THROUGH THE
MIDDLE OF NEXT WEEK.

&&

.AVIATION...
SOUTH-CENTRAL ARIZONA INCLUDING KPHX...KIWA AND KSDL...

BROAD AREA OF LOWER LEVEL STRATUS COVERS MUCH OF THE LOWER CENTRAL
AZ DESERTS...WITH VARYING LAYERS AND BREAKS IN THE CLOUD DECK. WILL
LOOK FOR PERIODS OF MVFR CIGS AT PHX AREA TERMINALS THROUGH THE
EVENING...BUT CONDITIONS WILL NOT REMAIN AS SOCKED IN AS THEY WERE
DURING THE DAY. CIGS AROUND THE 5500-6000FT LAYER MAY HOLD THROUGH
SUNRISE AND TERRAIN OBSCURATIONS WILL ALSO PERSIST. WX RADAR SCOPES
STILL SHOW SCATTERED LIGHT SHOWERS OVER EASTERN PORTIONS OF THE PHX
METRO THAT MAY PASS OVER THE TERMINALS...BRIEFLY REDUCING CIGS BUT
NOT AMOUNTING TO MUCH BY WAY OF WETTING TARMAC/APRON SFCS. WESTERLY
WINDS 10KTS OR LESS WILL HOLD INTO THE OVERNIGHT HOURS...BEFORE
ATTEMPTING TO SWITCH TO EASTERLY NEAR OR AFTER 01/12Z...OTHERWISE
BECOME VARIABLE WITH SPEEDS 6 KTS OR LESS.

GIVEN THE COOLING AIRMASS AND ELEVATED DEWPOINTS...EXPECT ATLEAST
VCFG NEAR KIWA AND KSDL BY SUNRISE...WITH LESS CONFIDENCE OF ANY
SUBSTANTIAL FOG DEVELOPING NEAR KPHX.

SOUTHEAST CALIFORNIA AND SOUTHWEST ARIZONA INCLUDING KIPL AND KBLH...

PATCHES OF LOW TO MID LEVEL CIGS CONTINUE TO CIRCULATE AROUND THE
UPPER LOW THAT HAS NOW PUSHED INTO NW AZ. CIGS GNLY WILL HOLD ABOVE
8KFT FOR THE REMAINDER OF THE EVENING AS SOME DRIER AIR CONTINUES TO
WORK INTO THE REGION. WINDS TO HOLD NORTHERLY HEADINGS WHILE
SUBSIDING IN THEIR ELEVATED SPEEDS...OCCASIONALLY BECOMING NEAR CALM
AS SKIES CLEAR BY SUNRISE.

AVIATION DISCUSSION NOT UPDATED FOR AMENDED TAFS.

&&

.FIRE WEATHER...
FRIDAY THROUGH TUESDAY...
MOSTLY CLEAR SKIES AND MUCH BELOW NORMAL TEMPERATURES ARE EXPECTED
FRIDAY THROUGH SUNDAY...WARMING SOMEWHAT MONDAY AND TUESDAY. MINIMUM
RELATIVE HUMIDITIES FRIDAY WILL RANGE FROM 20 PERCENT IN SOUTHEAST
CALIFORNIA TO 50 PERCENT IN SOUTHERN GILA COUNTY...DECREASING MONDAY
AND TUESDAY BY 15 TO 20 PERCENT RANGE AREA-WIDE. LIGHT WIND.

AVIATION DISCUSSION NOT UPDATED FOR AMENDED TAFS.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...

AZ...FREEZE WARNING FROM 11 PM MST/10 PM PST THIS EVENING TO 9 AM
     MST/8 AM PST THURSDAY AZZ020>022-026>028...CAZ033.

     FREEZE WARNING FROM 11 PM THURSDAY TO 9 AM MST FRIDAY
     AZZ020>023-026>028.

     WINTER STORM WARNING UNTIL NOON MST THURSDAY ABOVE 3500 FEET
     FOR AZZ024.

&&

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DISCUSSION...NOLTE
PREVIOUS DISCUSSION...PERCHA/KUHLMAN
AVIATION....NOLTE
FIRE WEATHER...VASQUEZ