203 FXUS65 KRIW 092124 AFDRIW AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE RIVERTON WY 324 PM MDT WED OCT 9 2013 .SHORT TERM...TONIGHT THROUGH THURSDAY WV IMAGERY SHOWING BAROCLINIC LEAF/SHORTWAVE EXITING THE STATE FOR POINTS NORTH INTO MT...TAKING MORNING UPR LVL LIFT AND MORNING SHOWERS WITH IT. OTHERWISE...JET/SW PLUNGING DOWN THE WRN SIDE OF THE LW AXIS...EFFECTIVELY DIGGING AND INCREASINGLY CLOSING OFF THE MAIN WAVE WELL SOUTH INTO CNTRL CALIFORNIA TODAY. OTHER THAN SOME MODERATE INSTABILITY TODAY OVR THE WRN AND SRN PORTIONS OF THE CWA...THIS WILL BE OUR MAIN FEATURE TO FOLLOW THRU TOMORROW. FIRST OFF...ALTHOUGH THE FIRST PART OF REGIONAL ASCENT ASSOCIATED WITH THE ABOVE MENTIONED BAROCLINIC LEAF IS LEAVING US...MUCH BROADER LARGE-SCALE ASCENT IS STARTING TO TAKE FORM ALOFT AS THE MAIN TROF TO OUR WEST BEGINS TO ENLARGE TODAY...CREATING INCREASINGLY DIFLUENT FLOW ALOFT. ADDITIONALLY...SUFFICIENTLY COLD MID LVL TEMPS AND DIABATIC/DIURNAL SFC HEATING UNDER AT LEAST PARTIALLY CLEARING SKIES W AND S...COMBINED WITH OROGRAPHIC LIFT...FAVORABLE LL FLOW FROM BUILDING SFC LOW TO THE SW...AND MODEST MOISTURE...WILL GIVE US A MODERATELY FAVORABLE ENVIRONMENT WITH UNSTABLE LAPSE RATES FOR CONVECTION BY EARLY THIS AFTN...WITH AT LEAST ISOLATED CHANCES FOR THUNDER. OTHERWISE A FEW HIGH MOUNTAIN SNOW SHOWERS CAN BE EXPECTED OVERNIGHT...MAINLY WEST OF THE DIVIDE AND ABOVE 9000 FEET...WITH LITTLE TO AROUND AN INCH OF ACCUMULATION BY SUNRISE. THURSDAY...THE CLOSED LOW AND JET STREAK/SW WILL ROUND THE BOTTOM OF THE WRN LW TROF AND MAKE A RIGHT TURN...OR LEFT TURN DEPENDING ON YOUR PERSPECTIVE...AND BEGIN HEADING FOR THE CNTRL ROCKIES. MID LVL MOISTURE ADVECTION WILL OCCUR DURING THE DAY...BECOMING SLOWLY SATURATED FROM THE TOP DOWN BY SUNSET. THE GREAT BASIN SFC LOW CURRENTLY DEVELOPING NOW WILL BECOME SOMEWHAT SPLIT AND WEAKEN OVER OUR REGION THURSDAY AS THE THE UPR LVL FLOW PATTERN BECOMES BETTER SUITED TO EVOLVING A VIGOROUS LEE LOW IN E CNTRL CO...LEAVING US WITH SOMEWHAT WEAKER FORCING. HOWEVER...THIS IS NOT TO SAY THAT WE WILL NOT RECEIVE PRECIP...WE WILL...JUST NOT TO THE EXTENT OF THE LAST COUPLE OF WEEKS. ACTUALLY MOST OF THE PRECIP WE GET TOMORROW DURING THE DAY ACROSS THE ENTIRE CWA WILL BE IN THE FORM OF CONVECTIVE SHOWERS...WITH AT LEAST ISOLATED THUNDER POSSIBLE IN BOTH JOHNSON AND NATRONA COUNTIES. STRATIFORM PRECIP...BOTH LIQUID AND FROZEN...WILL NOT ARRIVE UNTIL LATER THURSDAY NIGHT. OTHERWISE...ACCUMULATING SNOW SHOWERS WILL START OUT WELL ABOVE 9000 FEET IN THE MORNING BEFORE DECREASING TO AROUND 8000 FEET WEST OF THE DIVIDE BY LATE AFTERNOON. EAST OF THE DIVIDE...THE SNOW LVL WILL GENERALLY ONLY DECREASE TO AROUND 8500 FEET AT THE MOST OVER THE SAME PERIOD OF TIME. GENERALLY...SNOW ACCUMULATIONS DURING THE DAY...AND ABOVE THESE ELEVATIONS...WILL ONLY AMOUNT TO AROUND AN INCH OR TWO. .LONG TERM...THURSDAY NIGHT THROUGH WEDNESDAY THE FIRST PART OF THE PACIFIC TROUGH WILL BE LIFTING NORTHEAST INTO WESTERN NEBRASKA WITH COLD NORTHWEST FLOW DEVELOPING OVER WESTERN AND CENTRAL WYOMING BEHIND THE TROUGH. A NORTHERN PORTION OF THE MAIN TROUGH WILL BE MOVING SOUTHEAST FROM WESTERN MONTANA IN THE NORTHWEST FLOW DEEPENING THE CYCLONIC CURVATURE OVER THE AREA. SOME SNOW SHOWERS...WITH A RAIN SNOW MIX IN THE LOWER ELEVATIONS WILL RESULT. THE SHOWERS WILL BE MOST NUMEROUS IN THE MOUNTAINS... ESPECIALLY IN THE NORTH. COME CSI BANDS OF SHOWERS MAY ENHANCE PRECIPITATION AMOUNTS IN THE LOWER ELEVATIONS IN THE LEE OF THE MOUNTAINS AS LOW LEVEL CONVERGENCE ZONES DEVELOP. THE NORTHWEST FLOW AND SHOWERS WILL REMAIN OVER THE AREA FRIDAY MORNING WITH DECREASING SHOWERS FROM WEST TO EAST AS A SHORT WAVE HIGH PRESSURE RIDGE OVER SOUTHERN IDAHO DRIFTS EAST OVER THE AREA. TEMPERATURES WILL BE COOL. SHOWERS WILL BECOME ISOLATED WITH PARTIAL CLEARING THROUGH FRIDAY NIGHT INTO SATURDAY MORNING. SATURDAY WILL SEE CONTINUED CLEARING IN THE MORNING WITH WARMING BY AFTERNOON. A LINE OF SHOWERS FROM NORTHERN UTAH...EASTERN IDAHO AND WESTERN MONTANA WILL MOVE INTO THE WEST IN THE AFTERNOON AS SOUTHWEST FLOW DEVELOPS AHEAD OF THE NEXT PACIFIC TROUGH. SNOW SHOWERS WILL INCREASE ACROSS THE WEST SATURDAY NIGHT AND SPREAD EAST ACROSS THE NORTH. SNOW SHOWERS...WITH SOME RAIN SNOW MIX IN THE LOWER ELEVATIONS WILL INCREASE AND SPREAD ACROSS THE AREA SUNDAY AFTERNOON AND EVENING. PRECIPITATION SHOULD CHANGE TO ALL SNOW BY LATE EVENING AND CONTINUE THROUGH THE NIGHT. MONDAY WILL BE COLDER WITH AREAS OF SNOW MOST NUMEROUS IN THE NORTH AND EAST AS NORTH NORTHEAST FLOW SPREADS OVER THE AREA AS THE UPPER TROUGH PASSES TO THE EAST. SNOW LIGHT SNOW WILL LINGER THROUGH THE NIGHT IN THE NORTH AND EAST WHILE DECREASING THROUGH MONDAY NIGHT. ISOLATED SNOW SHOWERS WILL LINGER IN THE EAST TUESDAY MORNING THEN CLEARING WILL OCCUR FROM WEST TO EAST. TUESDAY NIGHT WILL BE COLD WITH AT LEAST PARTIAL CLEARING. SOME FOG MAY DEVELOP IN THE BASINS AND VALLEYS DEPENDING ON THE SNOWFALL THAT OCCURS WITH THIS SYSTEM. THE MODELS SHOW ENOUGH DISAGREEMENT WITH THE STRENGTH AND TRACK OF THIS SYSTEM TO INCREASE THE UNCERTAINTY IN THE FORECAST. WEDNESDAY WILL BE MOSTLY CLEAR AND COOL WITH COLD HIGH PRESSURE SETTLING IN OVER THE AREA. && .AVIATION.../00Z ISSUANCE/ WEST OF THE CONTINENTAL DIVIDE...VFR CONDITIONS WILL PREVAIL THROUGH 10Z THURSDAY BEFORE CONDITIONS DETERIORATE TO MVFR OR BELOW. ISOLATED SHOWERS WITH HIGH ELEVATION SNOW SHOWERS...GENERALLY ABOVE 9000 FEET...WILL OCCUR THIS EVENING IN AND NEAR TERMINALS ADJACENT TO MOUNTAINS. ISOLATED THUNDER WILL WILL ALSO BE A SLIGHT POSSIBILITY TILL 02Z. OTHERWISE...LOWERED CLOUD BASES IN ADDITION TO AN INCREASE IN SHOWER ACTIVITY WILL OCCUR BY 12Z THURSDAY AT MANY TERMINALS...BRINGING SOME TERMINALS DOWN TO MVFR. SNOW SHOWERS WILL ALSO BE POSSIBLE IN THE MOUNTAIN AREAS...LOWERING FROM 9000 FEET TO 8000 FEET MSL THROUGH THE DAY. EAST OF THE CONTINENTAL DIVIDE...VFR CONDITIONS WILL MOSTLY PREVAIL THROUGH 12Z THURSDAY...ALTHOUGH THERE ARE CURRENTLY INDICATIONS THAT KCPR MAY DROP TO 5 - 6SM BR BEFORE 10Z. OTHERWISE...LOWERED CLOUD BASES IN ADDITION TO AN INCREASE IN SHOWER ACTIVITY WILL OCCUR BY 14Z AT MANY TERMINALS...BRINGING SOME TERMINALS DOWN TO MVFR FOR SHORT PERIODS. SNOW SHOWERS WILL ALSO BE POSSIBLE IN THE MOUNTAIN AREAS...LOWERING FROM 9200 FEET TO 8500 FEET MSL THROUGH THE DAY. && .FIRE WEATHER... REST OF TODAY...NO REAL FIRE WEATHER CONCERNS THROUGH SATURDAY...AS MORE FALL/WINTER WEATHER RETURNS TODAY. HOWEVER...AN ISOLATED THUNDERSTORM OR TWO WILL BE POSSIBLE OVER OUR WESTERN AND SOUTHERN DISPATCH ZONES THROUGH SUNDOWN. TEMPERATURES ARE DOWN 10 TO 15 DEGREES OVER THOSE ON TUESDAY EAST OF THE DIVIDE...AND DOWN 5 TO 10 DEGREES WEST OF THE DIVIDE. MINIMUM RH VALUES ARE UP FOR ALL DISPATCH AREAS TODAY. PRECIPITATION CHANCES WILL ALSO INCREASE OVERNIGHT TONIGHT...LASTING THROUGH MUCH OF FRIDAY. MOUNTAINS SNOWS CAN ALSO BE EXPECTED...WITH SNOW LEVELS DROPPING FROM AROUND 9000 FEET TONIGHT...TO AROUND 7000 TO 7500 FEET ON FRIDAY. SNOW TOTALS BY THE END OF FRIDAY WILL RANGE FROM 4 TO 10 INCHES IN THE WIND RIVER MOUNTAINS...2 TO 6 INCHES IN THE ABSAROKAS...AND FROM 4 TO 12 INCHES IN THE BIG HORN MOUNTAINS. OTHERWISE AND ELSEWHERE...EXPECT A TENTH TO TWO TENTHS OF AN INCH OF LIQUID PRECIPITATION THROUGH THE SAME PERIOD AT LOWER ELEVATIONS. && .RIW WATCHES/WARNINGS/ADVISORIES... NONE. && $$ SHORT TERM...BRAUN LONG TERM...C.BAKER AVIATION...BRAUN FIRE WEATHER...BRAUN