FXUS65 KGJT 081503 AAA AFDGJT AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE GRAND JUNCTION CO 802 AM MST THU JAN 8 2004 ...UPDATED FOR DENSE FOG THIS MORNING... .DISCUSSION... SHORT TERM UPDATE...DENSE FOG HAS OVERSPREAD THE VALLEYS OF EAST- CENTRAL UTAH AND WEST-CENTRAL COLORADO EARLY THIS MORNING. COMBINATION OF FRESH SNOWPACK...SHALLOW MOIST LAYER AND SUBSIDENCE ABOVE THIS LAYER HAS LED TO THE DENSE FOG. WILL EXPAND DENSE FOG ADVISORY TO INCLUDE THE GREEN RIVER TO MOAB AREAS (ALONG HIGHWAY 191)...AND THE LOWER UNCOMPAHGRE RIVER VALLEY FROM DELTA TO MONTROSE (INCLUDES CEDAREDGE). THE ADVISORY WILL RUN THROUGH 1000 AM AT THIS TIME. DJN SHORT TERM (REST OF TODAY-FRIDAY NIGHT)...SNOW SHOWERS CURRENTLY HANGING ON ACROSS NRN MTNS PER CDOT ROAD REPORTS IN W-NW MOIST OROGRAPHIC FLOW WITH MARGINAL CONVECTIVE INSTABILITY. SHORTWAVE MOVING ACROSS SW WY WILL ALSO BRUSH NRN ZONES EARLY THIS MORNING BUT SHOULD PASS BY AROUND 12Z. NOSE OF UPPER JET THEN SWEEPS ACROSS THE NORTH DURING THE DAY FOR SOME HIGH CLOUDS AND BIT OF UPPER DIVERGENCE AS RIDGE BEGINS TO AMPLIFY OVER THE GREAT BASIN...MAINTAINING A CHANCE OF SNOW SHOWERS NRN MTNS THIS AFTN. PCPN SHOULD END BY TONIGHT AS JET MOVES EAST AND ATMS CONTINUES TO STABILIZE...WITH UPPER RIDGE BECOMING THE DOMINANT FEATURE FOR ERN UT/WRN CO TONIGHT THROUGH FRI NIGHT. MAIN FORECAST CHALLENGE THE NEXT FEW DAYS WILL BE MAX TEMPS FOR THE VALLEYS. THIS MOST RECENT STORM TOO WEAK TO FLUSH OUT MANY OF THE VALLEYS...ESPECIALLY CNTL VALLEYS...OF REMNANT COLD AIR AND MOISTURE. STABLE CONDITIONS TO PREVAIL WITH BUILDING RIDGE...SO WON'T SEE ANY MIXING THROUGH AT LEAST FRI NIGHT. THUS...WHILE HIGHER ELEVATIONS BENEFIT FROM THE WARMING ALOFT AT 700 MB...SNOW COVER AND STAGNANT AIR WILL RETARD WARMUP IN MANY VALLEYS. HAVE LOWERED MAX TEMPS FOR MUCH OF THE I-70/US-50 CORRIDORS TODAY AND FRI. AREAS OF FOG DEVELOPING ALONG I-70 IN ERN UT AND WRN CO AS WELL OTHER AREAS. ISSUED A DENSE FOG ADVISORY FOR THE GRAND VALLEY (CO ZONE 6) UNTIL MID-MORNING AS VISIBILITY HAS DROPPED TO 1/4 MI OR LESS AT THE AIRPORT. WILL HANDLE OTHER AREAS WITH NOWCASTS UNTIL MORE SPOTTER REPORTS AVAILABLE. ANY EVAPORATION/SUBLIMATION OF SNOW COVER WILL ADD TO TRAPPED MOISTURE...AND WILL KEEP SOME MENTION OF FOG FOR NRN/CNTL VALLEYS TONIGHT. STABILIZING CONDITIONS AND DECREASING WINDS ALOFT WILL CREATE GENERALLY POOR SMOKE DISPERSAL THROUGH FRIDAY NIGHT FOR FIRE WEATHER CONCERNS. LONG TERM (SATURDAY THROUGH THURSDAY)... SATURDAY AND SUNDAY...GFS AND ECMWF HAVE BACKED OFF CONSIDERABLY REGARDING THIS WEEKENDS SHORT WAVE. IF CORRECT...THERE WON/T BE ANY SNOW ANYWHERE IN OUR FORECAST AREA. WITHOUT THIS SHORT WAVE... CANNOT EXPECT THE MIXING OF TRAPPED COLD AIR IN THE SNOW COVERED LOWER VALLEYS...LIKE THE GRAND AND GUNNISON VALLEYS. IN THE POSSIBILITY THAT SHORT WAVE AMPLIFIES AND DIGS INTO COLORADO... REMOVED MENTION OF ANY SNOW FOR SATURDAY...BUT HAVE CONTINUED "SLIGHT CHANCE" WORDING FOR MOUNTAIN SNOW FOR SUNDAY AFTERNOON. EXPECT WARMING TREND FOR MOST AREAS...WITH THE NOTABLE EXCEPTION OF VERNAL...GUNNISON...AND CITIES ALONG THE COLORADO RIVER BELOW EAGLE. HAVE UNDERCUT MEX MOS GUIDANCE OF 5 TO 10 AT THOSE LOCATIONS. MONDAY THROUGH THURSDAY...FORECASTS BECOME MORE PROBLEMATIC FOR DAYS 5 THROUGH 8. GFS HAS PERSISTENT NORTHWEST FLOW WITH EMBEDDED SHORT WAVES. THE ECMWF ISN/T HAS AMPLIFIED WITH THE H5 PATTERN ACROSS THE U.S. DECIDED TO LEAN TOWARD THE MORE AMPLIFIED GFS SINCE BLOCKING RIDGE OVER THE YUKON AND STRONG POLAR VORTEX CENTERED OVER THE HUDSON BAY WILL BE SIGNIFICANT PLAYERS WITH THE LARGE SCALE PATTERN. THEREFORE...WILL MENTION "SLIGHT CHANCE OF SNOW" EACH DAY FOR THE NORTHERN MOUNTAINS. BUT IT WILL BE POSSIBLE THAT NONE OF THE SHORT WAVES WILL BE STRONG ENOUGH TO SCOUR OUT THE COLD DENSE AIR ALONG THE COLORADO AND GUNNISON RIVERS. && .GJT WATCHES/WARNINGS/ADVISORIES... .CO...DENSE FOG ADVISORY THROUGH 1000 AM MST FOR ZONES 6/11. .UT...DENSE FOG ADVISORY THROUGH 1000 AM MST FOR ZONE 27. && JAD/PF/DJN