Severe Thunderstorm Warning 52, NWS Grand Junction, CO

WESTERN COLORADO & EASTERN UTAH AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GRAND JUNCTION CO
300 AM MDT MON OCT 23 2000

CLOSED LOW CONTINUES TO SPIN OVER NORTHWESTERN MEXICO WITH DEEP
SOUTHERLY MOIST FLOW ALONG THE BORDER OF ARIZONA AND NEW MEXICO.
EVENING MODEL SUITE OFFERING DIFFERENT SOLUTIONS.. SO WILL USE A
COMBINATION OF THE ETA AND AVN MODELS.  ETA SLOWER WITH EASTWARD
PROGRESSION OF CLOSED LOW.  BUT IT ALSO EJECTS A WAVE THAT REACHES
THE NORTHERN PLAINS BY TUESDAY AFTERNOON.. AND DOING SO THE ETA
SOLUTION WEAKENS UPPER CORE A LITTLE FASTER ONCE IT REACHES THE
FORECAST AREA.  NO OTHER MODEL SHOWED THIS EJECTING WAVE AND WILL
DISREGARD.  THE AVN MODEL HAS BEEN THE FASTEST MODEL OF TRYING TO
ENTRAIN CLOSED LOW INTO THE WESTERLIES.  DPROG/DT CLEARLY SHOWS THE
AVN WAS TOO FAST WITH THIS IDEA IN LAST EVENINGS 00Z MODEL RUN.. AND
BELIEVE IT CONTINUES TO BE TOO FAST.  NEXT EASTERN PACIFIC SHORT WAVE
TOO FAR UPSTREAM TO HAVE ANY IMMEDIATE IMPACT ON CLOSED LOW.  PREFER
THE SLOWER ETA BUT LIKE THE SLIGHTLY STRONGER CLOSED LOW OF THE AVN.
THINK THAT THE CLOSED LOW WILL BE SOMEWHERE OVER EASTERN UTAH BY
TUESDAY AFTERNOON.

GIVEN THE FOLLOWING CONDITIONS ABOVE.. DEEP MOIST LAYER REACHES THE
SOUTHERN ZONES TODAY AIDED BY JET STREAK THAT HAS ROUNDED THE BASE
OF THE TROUGH.  SOUTHWEST COLORADO WILL BE IN A REGION OF UPPER
LEVEL DIVERGENCE AND LEFT FRONT QUAD OF JET WITH DEEP MOIST LAYER
IN PLACE.. THEREFORE HIGHEST PRECIPITATION AMOUNTS WILL OCCUR IN THE
SAN JUAN FOOTHILLS AND FOUR CORNERS AREA.  FREEZING LEVEL STAYS ABOVE
10K FEET TODAY.. BUT DIABATIC PROCESSES IN AREAS OF HEAVIER
PRECIPITATION WILL LOWER SNOW LEVELS BETWEEN 9000 TO 9500 FEET.  SNOW
ADVISORY FOR THE SAN JUANS AND PORTIONS OF THE ELK MOUNTAINS (ZONE
12) LOOKS GOOD.  FORECASTING SNOW AMOUNTS WILL BE A CHALLENGE DUE TO
HIGH WATER CONTENT AND VARIABLE SNOW LEVEL.. BUT CURRENT AMOUNTS IN
LAST FORECAST PACKAGE ARE REASONABLE.  SNOW LEVELS LOWER TONIGHT TO
AROUND 8500 FEET AS COOLER AIR FILTERS INTO THE FOUR CORNERS REGION..
BUT JET SUPPORT WILL BE WEAKENING AND THEREFORE AREAL COVERAGE OF
RAIN OR SNOW WILL BE DECREASING.  WILL NOT EXTEND THE SNOW ADVISORY
AND LET DAY SHIFT REEVALUATE.

ON TUESDAY.. UPPER COLD CORE OUGHT TO BE IN THE VICINITY OF EASTERN
UTAH FOR CONTINUATION OF SHOWERS. BUT WITH LOWER PRECIPITABLE WATER
AND LIGHTER WIND ALOFT.. PRECIPITATION AMOUNTS WILL BE LOW.

FRISBIE

WESTERN COLORADO AND EASTERN UTAH EMERGENCY MANAGERS OVER SOUTHWEST
COLORADO SHOULD MAINTAIN A HIGH LEVEL OF WEATHER AWARENESS TODAY
DUE TO SNOW OVER THE HIGHER MOUNTAINS ABOVE 9000 FEET.

.GJT...SNOW ADVISORY ABOVE 9500 FEET CO ZN 12/18/19 TODAY.