FXUS65 KFGZ 051631
AFDFGZ
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE FLAGSTAFF AZ
930 AM MST SAT DEC 5 2009
.SYNOPSIS...EXPECT INCREASING CLOUDS...COLD TEMPERATURES...AND WINDY
CONDITIONS TODAY AND SUNDAY AHEAD OF A MAJOR WINTER STORM SYSTEM
DEVELOPING NORTH OF ARIZONA. THIS STORM WILL AFFECT NORTHERN AND
CENTRAL ARIZONA BEGINNING LATE SUNDAY NIGHT...THROUGH TUESDAY
MORNING. SNOW LEVELS WILL RANGE FROM 5000 TO 6000 FEET WITH HEAVY
SNOW ACCUMULATIONS ABOVE 6000 FEET...ESPECIALLY ALONG THE MOGOLLON
RIM. AT LOWER ELEVATIONS SOUTH OF THE RIM...EXPECT PERIODS OF
MODERATE TO HEAVY RAINFALL.
&&
.DISCUSSION...A QUICK LOOK AT THE 12Z GFS AND NAM MODELS INDICATES
LITTLE CHANGE FROM PREVIOUS RUNS AND OUR CURRENT FORECASTS REGARDING
THE MAJOR WINTER STORM FOR MONDAY INTO EARLY TUESDAY. THE LATEST
RUNS ARE JUST A FEW HOURS SLOWER ON THE ARRIVAL OF THE MAIN COLD
FRONT MONDAY NIGHT. THIS HAS FEW IMPLICATIONS FOR AREAS ABOVE 6000
FEET SINCE THE ENTIRE PRECIPITATION EVENT SHOULD BE IN THE FORM OF
SNOW. HOWEVER...BETWEEN 5000 AND 6000 FEET...SOME OF THE
PRECIPITATION MAY FALL AS RAIN MONDAY EVENING BEFORE THE FRONT
ARRIVES. WE WILL EXAMINE THE LATEST DATA AND MAKE ANY APPROPRIATE
CHANGES WITH THE AFTERNOON PACKAGE. FOR TODAY THROUGH
SUNDAY...BREEZY AND COLD CONDITIONS WILL CONTINUE WITH A SLIGHT
CHANCE OF LIGHT SNOW SHOWERS LATE TONIGHT INTO SUNDAY.
.PREV DISCUSSION...(400 AM MST FRI DEC 4)...THE PRIMARY SHORT WAVE
WITH THE STORM SYSTEM WILL CONTINUE TO DIG SOUTH ACROSS THE GREAT
BASIN SUNDAY NIGHT AND MONDAY AND WILL MERGE WITH A SERIES OF
SOUTHERN STREAM SHORT WAVES AND ASSOCIATED PACIFIC MOISTURE BEFORE
CROSSING OUR AREA DURING THE DAY MONDAY THROUGH EARLY TUESDAY. THIS
WILL SET THE STAGE FOR A SIGNIFICANT SNOW AND RAIN EVENT MONDAY
THROUGH EARLY TUESDAY WITH SNOW LEVELS AROUND 5000 FEET. THE
FOLLOWING SUMMARIZES CONCERNS WITH THIS SYSTEM:
-SNOW LEVELS AROUND 5000 FEET WITH HEAVIEST ACCUMULATIONS ABOVE 6000 FT
-GIVEN THE STRONG DYNAMICS COINCIDENT WITH MODERATE FRONTOGENESIS...AND
CONSIDERING UPSLOPE NATURE OF THIS EVENT...WE ARE EXPECTING 1-2 INCHES
OF RAINFALL FOR LOCATIONS BELOW THE SNOW LEVEL...UP TO 6 INCHES OF
SNOW ACCUMULATION IN THE PRECIP TYPE TRANSITION ELEVATIONS BETWEEN
5000-6000FT AND AT LEAST 12-18 INCHES OF SNOW WITH ISOLATED HIGHER
AMOUNTS ABOVE 6000 FT THROUGH TUESDAY MORNING.
-SFC WINDS WILL GRADUALLY INCREASE FROM NOW THROUGH THE EVENT. GUIDANCE
SHOWS 700MB WINDS PEAKING AROUND 60 KNOTS BY LATE MONDAY. CONSIDERABLE
BLOWING AND DRIFTING OF SNOW CAN BE EXPECTED OVER THE HIGHER TERRAIN WITH
NEAR BLIZZARD CONDITIONS DURING TIMES OF HEAVY SNOWFALL LATE MONDAY.
DOWNSLOPE WINDS OVER THE LCR WILL WARRANT AT LEAST A WIND ADVISORY AS WE
GET CLOSER TO THE EVENT.
-AFOREMENTIONED DOWNSLOPE FLOW WILL LIMIT SNOW AND RAINFALL AMOUNTS ACROSS
THE LCR...THUS LOWER POPS FOR THAT AREA.
-STRONG SUBSIDENCE IN THE WAKE OF THE TROUGH AXIS WILL QUICKLY SHUT OFF
HEAVY PRECIP FROM WEST TO EAST BETWEEN 08Z-12Z TUESDAY WITH LOW CLOUDS
AND FLURRIES POSSIBLE THE REMAINDER OF THE DAY.
-GIVEN HIGH CONFIDENCE WITH THIS EVENT...WILL ISSUE A WINTER STORM
WATCH WITH THIS PACKAGE FOR ELEVATIONS ABOVE 5000 FEET.
A BREAK IN THE WEATHER IS EXPECTED MID TUESDAY THROUGH EARLY
WEDNESDAY...THEN THE NEXT SHORT WAVE IN THE SOUTHERN STREAM WILL
AFFECT THE SOUTHWEST THURSDAY AND FRIDAY...BRINGING THE RISK FOR
ADDITIONAL RAIN AND SNOW.
&&
.AVIATION...FOR THE 18Z PACKAGE...THROUGH 06Z SUN...VFR CONDITIONS
EXPECTED WITH BKN-OVC CIGS WITH BASES AOA 15 KFT MSL. SOUTHWEST
WINDS WILL INCREASE TO 15 TO 25 KTS WITH GUSTS TO 30 KTS THIS
AFTERNOON. AFT 06Z SUN...HIGH CLOUDS MOVE OFF...REPLACED BY LOW AND
MID LEVEL CLOUDS...LCL MVFR CIGS AND A SLIGHT CHANCE OF -SHRA/-SHSN
ALONG AND NORTH OF THE MOGOLLON RIM. AVIATION DISCUSSION NOT
UPDATED FOR AMENDMENTS.
&&
.FGZ WATCHES/WARNINGS/ADVISORIES...WINTER STORM WATCH FROM MONDAY
MORNING THROUGH TUESDAY MORNING FOR ELEVATIONS ABOVE 5000 FEET.
&&
$$
PUBLIC..................JJ / NP
AVIATION/FIRE WEATHER...41
VISIT WEATHER.GOV/FLAGSTAFF FOR NORTHERN ARIZONA WEATHER INFORMATION.
