Heavy Snow in Utah, December 29 to 30, 2001

WESTERN COLORADO & EASTERN UTAH AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GRAND JUNCTION CO
249 PM MST SAT DEC 29 2001

LIGHT SNOW SHOWERS CONTINUE TO FALL THIS AFTERNOON OVER MOST OF OUR
SOUTHERN ZONES AS DEEP PACIFIC MOISTURE OVERRUNS COLDER SFC.
AMOUNTS HAVE THUS FAR BEEN ON THE LIGHT SIDE...GENERALLY LESS THAN 2
INCHES. SATELLITE...UPSTREAM RADARS AND SFC OBSERVATIONS SHOW MORE
MOISTURE WORKING INTO THE SRN TWO-THIRDS OF THE FORECAST AREA
TONIGHT. USING DISTANCE TOOLS IN AWIPS...TIMING BACK EDGE OF SYSTEM
TO MOVE THROUGH BY MIDDAY SUNDAY...ALTHOUGH BULK OF ACTIVITY
EXPECTED TO OCCUR THROUGH THE OVERNIGHT HOURS TONIGHT.

SEVERAL FACTORS TO THINK ABOUT CONCERNING TOTAL SNOWFALL AMOUNTS
INCLUDING LACK OF STRONG DYNAMICS...LITTLE JET SUPPORT AND WEAK
OROGRAPHICS.  HOWEVER...DID SOME RUNS OF THE OROGRAPHIC SNOW MODEL
WITH WEAK FLOW BUT DEEP MOISTURE...AND HAVE COME UP WITH LOW END SNOW
ADVISORY CRITERIA FOR MOST MOUNTAIN LOCATIONS SOUTH OF INTERSTATE 70.
WITH ISENTROPIC UPGLIDE IN A VERY MOIST ATMOSPHERE COMBINED WITH
PROLONGED NATURE OF EVENT AND OPTIMUM H7 TEMPS AROUND -6 C...BELIEVE
WE HAVE ENOUGH TO GENERATE ADVISORY SNOWFALL FOR MOST MTNS S OF THE
INTERSTATE...SO WL GO IN THIS DIRECTION. WL ALSO INCLUDE PORTIONS OF
THE 4-CORNERS...MAINLY IN THE VCNTY OF COLORADO ZONE 21 DUE TO
FAVORABLE...ALBEIT WEAK...FLOW FOR THIS AREA. REMAINDER OF THE
FORECAST AREA WILL SEE SOME SHOWER ACTIVITY THROUGH SUNDAY...ALTHOUGH
AMOUNTS ARE EXPECTED TO BE ON THE LIGHT SIDE.

ACTIVITY WL WIND DOWN SUNDAY AFTERNOON AS DEEPER MOISTURE ERODES. NW
FLOW THEN RESUMES SUNDAY NIGHT AND MONDAY WITH A CLIPPER-TYPE SYSTEM
DROPPING INTO THE PLAINS FROM SRN CANADA.  MODELS HAVE BEEN FAIRLY
CONSISTENT WITH DRIVING THIS WAVE SOUTH ALONG THE FRONT RANGE
MONDAY...WITH SOME COLD AIR POSSIBLY SPILLING WEST OF THE DIVIDE.  A
SECOND SYSTEM RIDES THROUGH THE RIDGE TO OUR WEST AT THE SAME TIME...
BUT WEAKENS CONSIDERABLE AS IT APPROACHES.  WL NEED TO CONTINUE A
MORE BROAD-BRUSH APPROACH FOR NOW...TO ACCOUNT FOR MORE UNSETTLED
CONDITIONS AS WE MOVE INTO THE NEW YEAR. JDC

EXTENDED (WED-SAT)...MODELS IN GENERAL AGREEMENT ON REBUILDING
WESTERN RIDGE WED AND THU IN THE WAKE OF TUE/S WAVE. WITH RIDGE AXIS
TO THE WEST...WILL BE IN NORTHERLY BUT DRIER FLOW. SFC HIGH OVER
ERN UT/WRN CO WED AND THU POINTS TOWARD MORE STABLE AND STAGNANT AIR
IN VALLEYS. WED MORNING LOOKS QUITE COLD...THEN GRADUAL MODERATION.
MORE PAC ENERGY TO IMPACT RIDGE LATER IN THE PERIOD. MRF BRINGS WAVE
INTO PAC NW WHILE DROPPING A LOW SE ACROSS CA THU IN SOMEWHAT SPLITTY
FLOW. CA LOW FARTHER WEST THAN IN PREV MRF RUN. UK LEANING TOWARD MRF
PATTERN...WHILE 28/12Z EC RUN BRINGS WAVE INTO DESERT SW ON FRI. WILL
LEAN TOWARD MRF AT THIS TIME...BUT IF SOUTHWARD PLUNGE OF COLDER AIR
AFTER WED IS LESS...THEN SOUTHERN STREAM MAY REMAIN MORE INTACT AND
CARRY WAVE INLAND. MRF PUSHES NORTHERN WAVE THROUGH LATER FRI AND
EARLY SAT...WITH MAIN EFFECT LIKELY ACROSS CENTRAL AND NORTH.
HOWEVER...WILL LEAVE IN THE MENTION OF SNOW ACROSS ENTIRE AREA FOR
FRI AND SAT DUE TO PATTERN UNCERTAINTY. JAD

WESTERN COLORADO AND EASTERN UTAH EMERGENCY MANAGERS SHOULD MAINTAIN
A HEIGHTENED LEVEL OF WEATHER AWARENESS THROUGH EARLY SUNDAY
AFTERNOON DUE TO PERSISTENT LIGHT SNOWFALL...ESPECIALLY OVER THE
MOUNTAINS SOUTH OF INTERSTATE 70.

.GJT...SNOW ADVISORY UNTIL NOON SUNDAY FOR COLORADO ZONES 009-
       012-017>021 AND UTAH ZONE 028.