FXUS65 KGJT 242259
AFDGJT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GRAND JUNCTION CO
359 PM MST THU DEC 24 2009

.SHORT TERM...CHRISTMAS EVE THROUGH SATURDAY

THE MIDWEST DEEP UPPER LEVEL LOW KEEPS A HOLD ON THIS REGION THROUGH
THESE PERIODS. A COLD NORTH FLOW WILL PRODUCE SNOW OVER THE NW
COLORADO MTNS AND SOME GUSTY WINDS OVER HIGHER EXPOSED TERRAIN AND
SOUTH-FACING SLOPES. EXPECT VERY COLD MORNINGS WITH VALLEY
INVERSIONS STRENGTHENING.

THE NW COLORADO MTNS LOOK TO BE SETTING UP FOR A CLASSIC
OVERRUNNING EVENT. A STRONG SPOKE OF ENERGY ROTATES AROUND THE
WESTERN EDGE/EASTERN COLORADO OF THE UPPER LOW ON FRIDAY. MID-LEVEL
MOISTURE THAT ERODED TODAY BEGINS TO RETURN TONIGHT. FLOW BACKS
FROM NW TO MORE NNW ON FRIDAY. FORCING IS LIMITED BUT OROGRAPHICS
WILL FAVOR NORTH TO NW-FACING SLOPES. MOST IMPORTANTLY MTN
TEMPERATURES REMAIN IN THE EFFICIENT SNOW PRODUCTION ZONE OF 0F TO
10F. BY FRIDAY AFTERNOON AND EVENING A MOIST WARM ADVECTION AT 650-
550MB OR 11-16KFT MSL BECOMES ISOTHERMAL AT AROUND -18 TO -20C.
THIS FAVORS LARGE DENDRITE GROWTH THAT WILL SHOW LITTLE RIMING OR
ICING ON THEIR WAY TO THE SURFACE. WHERE WINDS ARE RELATIVELY LIGHT
SUCH AS NORTH-FACING SLOPES...SNOW RATIOS COULD REACH 30:1 OR
BETTER. CLASSIC CHAMPAGNE POWDER. FOR THE WINDIER AREAS...SNOW
FLAKES WILL BE FRACTURED AND COMPACTED FOR LOWER SNOW RATIOS. THIS
SCENARIO MAY CONTINUE THROUGH SATURDAY WITH THE 18Z MODEL RUN
FAVORING THAT PERIOD FOR SNOW PRODUCTION.

.LONG TERM...SATURDAY NIGHT THROUGH THURSDAY...
BETWEEN-MODEL CONSISTENCY HAS BEEN VERY GOOD FOR THE LAST COUPLE
RUNS. AT THE BEGINNING OF THE LONG TERM PERIOD...THE NARROW HIGH
PRESSURE RIDGE OVER THE WEST BEGINS TO GET UNDERCUT...AND THE TIGHTLY
WOUND-UP CENTRAL CONUS LOW WEAKENS SLIGHTLY. AS THIS HAPPENS...THE
WHOLE PATTERN SHIFTS EAST. AS A RESULT THE STRONG NORTHERLY FLOW OVER
OUR FORECAST AREA WEAKENS BRINGING TO AN END ANY OROGRAPHIC SHOWERS
ALONG THE NORTHERN DIVIDE.

BY MONDAY MORNING THE FLOW BECOMES SOUTHWEST AS ENERGY ALONG THE
SOUTHERN STREAM STRENGTHENS AND FORMS A CLOSED LOW OFF THE
BAJA/SOUTHERN CA COAST. THIS LOW WILL THEN TRACK ACROSS THE DESERT
SOUTHWEST. THERE ARE SOME DIFFERENCES IN PLACEMENT AND TIMING BETWEEN
THE GFS AND THE ECMWF IN THE HANDLING OF THE LOW AS IT MOVES
EASTWARD. BUT AGREEMENT IS GOOD IN THE MAIN SYNOPTIC PATTERN. THE GFS
IS MORE SOUTHERLY AND FASTER KEEPING THE TRACK OF THE LOW CENTER...
AND BASE OF THE TROUGH...JUST SOUTH OF THE U.S./MEXICAN BORDER
THROUGH MONDAY NIGHT. IT THEN EJECTS THE TROUGH NORTHEASTWARD INTO TX
ON TUE. THE ECMWF ON THE OTHER HAND IS NOT AS FAR SOUTH...KEEPING THE
LOW CENTER NORTH OF THE BORDER AND ACROSS SOUTHERN AZ...WITH BASE OF
THE TROUGH OVER NEW MEXICO ON TUE. THIS SOLUTION SPREADS SOME
MOISTURE/QPF UP INTO THE 4-CORNERS FROM THE SOUTHWEST ON TUE...BUT
OTHERWISE LIMITS ANY QPF TO AREAS ALONG THE U.S./MEXICAN BORDER
BEFORE THAT TIME FRAME. THE GFS PRETTY MUCH KEEPS ANY QPF ASSOCIATED
WITH THIS LOW WELL SOUTH OF OUR FORECAST AREA.

AFTER THIS SYSTEM PASSES TO OUR SOUTH...NORTHWEST FLOW WILL AGAIN SET
UP AS HIGH PRESSURE BUILDS OVER THE WEST COAST. THIS WILL ALLOW THE
POSSIBILITY OF LIGHT OROGRAPHIC SHOWERS OVER THE FAVORED NORTHERN
SLOPES AS EMBEDDED WAVES PASS IN THE FLOW.

TEMPS WILL BE QUITE COLD AT THE BEGINNING OF THE PERIOD...THEN MODIFY
AS THE PATTERN SHIFTS EAST. THE FIRST TO WARM WILL BE THE HIGHER
ELEVATIONS. COLD AIR WILL STAY TRAPPED IN THE WESTERN VALLEYS...
ESPECIALLY CENTRAL AND NORTH...GRADUALLY WARMING TOWARDS THE END OF
THE PERIOD.

&&

.AVIATION...

PATCHY FOG CAN BE EXPECTED TO REFORM AFTER 06Z TONIGHT FOR AREAS
AROUND KVEL...KEGE...AND PERHAPS KRIL AND KASE. CIGS AND
VSBY ARE EXPECTED TO LOWER AGAIN TONIGHT OVER THE CENTRAL AND NRN CO
MTS EFFECTING ILS CONDITIONS FOR KEGE AND KASE ON FRIDAY. SNOW
SHOWERS BECOME NUMEROUS AFTER 12Z ALONG THE NORTHERN COLORADO
MTNS CREATING MVFR TO IFR CIGS AND VSBYS OVER THE CONTINENTAL
DIVIDE.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...

CO...WINTER WEATHER ADVISORY FOR SNOW AND BLOWING SNOW 6AM FRIDAY
     THROUGH 6AM SATURDAY FOR ZONES 4/10/13.
UT...NONE.

$$

SHORT TERM...JOE
LONG TERM....EH
AVIATION.....JOE

$$