FXUS65 KGJT 112243
AFDGJT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GRAND JUNCTION CO
342 PM MST FRI DEC 11 2009
...SNOW TO BEGIN TONIGHT OVER THE SOUTH...WITH THE HEAVIEST SNOW
WAITING UNTIL LATE SATURDAY THROUGH SUNDAY NIGHT...
.SHORT TERM...TONIGHT THROUGH SUNDAY...
MID TO HIGH LEVEL MOISTURE HAS BEEN MOVING OVER THE FORECAST AREA
FROM THE SOUTHWEST TODAY AS THE FIRST IN A SERIES OF WEATHER
DISTURBANCES APPROACH. MOISTURE WILL CONTINUE TO DEEPEN THIS EVENING
WITH SNOW DEVELOPING OVER THE SOUTHERN MOUNTAINS BY MID EVENING. BY
LATE TONIGHT SNOW WILL SPREAD NORTHWARD...WITH A CHANCE OF SNOW
DEVELOPING IN THE VALLEY AREAS AS ISENTROPIC UPGLIDE BRINGS THIS
MOIST AIR MASS OVER THE COLD AIR TRAPPED NEAR THE SURFACE. WHILE THE
COLD AIR WILL LARGELY STAY IN PLACE...IT WILL BECOME MODIFIED DUE TO
THE ADDITIONAL MOISTURE AND CLOUDS.
THEN ON SATURDAY THE MOIST SOUTHWEST FLOW WILL CARRY ANOTHER WEAK
WAVE OVER THE AREA. SNOW WILL GENERALLY BE LIGHT THROUGH THE EARLY
PART OF THE DAY WITH BREAKS IN THE PRECIPITATION EXPECTED. THE
SOUTHERN MOUNTAINS WILL CONTINUE TO SEE THE BEST SNOWFALL.
THE MOISTURE WILL GRADUALLY DEEPEN BY LATE AFTERNOON WITH SNOW
BECOMING MORE WIDESPREAD. THE UPPER JET SHIFTS OVER THE FOUR-CORNERS
FROM THE SOUTHWEST BY THE EVENING. AS A RESULT UPPER LEVEL
DIVERGENCE...AND BETTER OROGRAPHICS WILL ENHANCE SNOWFALL AMOUNTS. IN
ADDITION ANOTHER WAVE WILL PASS EARLY SUNDAY. WHILE SOME EROSION OF
THE COLD POOL AT THE SURFACE IS EXPECTED...MODELS SHOW NO STRONG
SURFACE FEATURE TO SCOUR OUT THE TRAPPED AIR. THEREFORE THE MOISTURE
RIDING OVER THE COLD AIR AT THE SURFACE WILL BE LIFTED. ALSO
TEMPERATURES WILL STAY MORE FAVORABLE FOR OPTIMAL DENDRITIC GROWTH.
THESE CONDITIONS CONTINUE THROUGHOUT SUNDAY AS THE THE UPPER JET AND
THE ENERGETIC FLOW REMAIN OVER THE REGION...AND THE MAIN TROUGH
APPROACHES FROM THE WEST.
HAVE ISSUED A WINTER STORM WATCH FOR MOST OF THE MOUNTAINS...AND FOR
THE FOOTHILLS AND VALLEYS SOUTHWEST OF THE SAN JUAN MOUNTAINS ALONG
THE NM BORDER. WHILE THERE MAY BE SOME FUTURE ADJUSTMENTS TO THE
START AND END TIMES OF THE STORM...I HAVE NOT SEGMENTED THE WATCH
IN ORDER TO KEEP THE MESSAGE SIMPLE. SINCE ONLY LIGHT SNOW IS
EXPECTED THROUGH EARLY SATURDAY HAVE WAITED UNTIL THE LATE AFTERNOON
TO START THE WATCH.
.LONG TERM...SUNDAY NIGHT THROUGH FRIDAY...
STORM TRANSITIONS TO COLD ADVECTION SUN NIGHT AS THE MAIN UPPER WAVE
AXIS MOVES TO THE EAST AND OROGRAPHIC FORCING BECOMES DOMINANT. WINDS
ALOFT SHIFT FROM THE SW TO WESTERLY AND MOISTURE CONTENT /SPECIFIC
HUMIDITY/ BEGINS TO DECREASE...BUT BUFR DATA AND TIME-HEIGHT SECTIONS
SHOW THE AIRMASS REMAINING NEARLY SATURATED THROUGH SUN NIGHT...SO
EXPECT SNOW TO REMAIN WIDESPREAD ACROSS THE MTNS BUT SHIFTING MORE
INTO THE NRN MTNS BY MON MORNING. HOW LONG SNOW PERSISTS IN THE
FOOTHILLS FROM CORTEZ TO PAGOSA DEPENDENT TO A LARGE EXTENT ON HOW
MUCH OF WEST OR WNW DIRECTION THE FLOW TAKES ON.
CHANCE OF SNOW WILL CONTINUE TO DECREASE ON MONDAY AS HIGH PRESSURE
BUILDS IN FOR MUCH OF NEXT WEEK...WITH INVERSIONS RETURNING TO LIMIT
VALLEY MIXING. FLAT RIDGE TAKES OVER TUE-WED AS EMBEDDED DISTURBANCES
PERIODICALLY BRUSH THE N. WITH CONTINUED EXTENSIVE SNOW COVER PRESENT
IN THE VALLEYS...WILL UNDERCUT MOS TEMP GUIDANCE STILL. RIDGE
AMPLIFIES OVER THE FAR WEST THU-FRI RESULTING IN MORE NWLY FLOW AND
MAY LET ANY EMBEDDED DISTURBANCES DROP OVER THE AREA.
&&
.AVIATION...
CLOUDS WILL CONTINUE TO LOWER THROUGH THIS EVENING WITH MVFR CIGS
DEVELOPING OVER THEM MOUNTAINS AFTER 06Z. AREAS OF IFR CIGS/VSBY IN
-SN OVER THE MOUNTAINS AFTER 06Z.
&&
.GJT WATCHES/WARNINGS/ADVISORIES...
CO...WINTER STORM WATCH FROM SATURDAY AFTERNOON THROUGH MONDAY
MORNING FOR COZ003-004-009-012-013-017>019-021>023.
UT...WINTER STORM WATCH FROM SATURDAY AFTERNOON THROUGH MONDAY
MORNING FOR UTZ025-028.
&&
$$
SHORT TERM...EH
LONG TERM....JAD
AVIATION.....JAD
