FXUS65 KRIW 052323
AFDRIW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
423 PM MST SAT DEC 5 2009
SHORT TERM...TONIGHT THROUGH TUESDAY
INTERESTING FORECAST THROUGH THE SHORT TERM AS ONE-TWO PUNCH
AFFECTS THE AREA. CURRENT UPPER LOW IS SPINNING OVER FAR SRN ID
WITH SOME REDEVELOPMENT INTO FAR NRN UT. NEW BAROCLINIC LEAF IS
FORMING OVER SWRN WY WITH THE ERN FRINGES AFFECTING THE LANDER
FOOTHILLS AND SOUTH TO THE NRN RED DESERT. FIRST BAROCLINIC LEAF
FORMED RAPIDLY THIS MORNING AND GAVE THE CENTRAL ZONES A QUICK
SHOT OF SHOW WITH UP TO 2 INCHES REPORTED IN LANDER AND ONE HALF
TO ONE INCH IN THE WIND RIVER BASIN AND OVER TO CASPER. BPI
AIRPORT IS CURRENTLY REPORTING HEAVY SNOW UNDER THE NEW BAROCLINIC
LEAF. MUCH OF THE SOUTH AND CENTRAL PORTIONS ARE IN A FAVORABLE
LOCATION FOR SNOW THROUGH SUNDAY DUE TO A COMBINATION OF QG
FORCING... FRONTOGENESIS AND DEVELOPING ISENTROPIC LIFT. DETAILS
ARE STILL A TRICKY AS TO WHERE SOME OF THESE HEAVIER BANDS SET UP
BUT STEADIER SNOW WILL DEVELOP OR CONTINUE ACROSS ALL OF THE
ADVISORY AREA THROUGH SUNDAY. THE FORCING GRADUALLY INCREASES
OVERNIGHT AND PEAKS SUNDAY MORNING BEFORE SOME WEAKENING OCCURS
LATER SUNDAY. UPSLOPE WILL ALSO INCREASE DRAMATICALLY THROUGH THE
EVENING AND BECOME TWICE (10MB TO 20MB) AS STRONG AS IT IS
CURRENTLY. STRONG NE WINDS WILL BECOME A CONCERN FROM PORTIONS OF
THE WIND RIVER BASIN BUT ESPECIALLY OVER SOUTH PASS...GREEN MTNS
AND I80 CORRIDOR TONIGHT AND SUNDAY. THESE AREAS WILL HAVE TO BE
WATCHED FOR WHITEOUT CONDITIONS IF ENOUGH SNOW IS FALLING. AIRMASS
IS TOO COLD FOR FAVORABLE DENDRITIC SNOW GROWTH BUT WE DO SEE SOME
SLIGHTLY WARMER AIR TRYING TO SNEAK UP FROM THE SE LATER TONIGHT
AND SUNDAY IN MODERATELY STRONG ISENTROPIC UPGLIDE. EVEN THIS
MORNING...WE HAD ABOUT AN HOUR OF GOOD SIZE FLAKES AND CLUSTERS.
EVENING CREW WILL NEED TO WATCH THE BAROCLINIC LEAF IN THE SWRN
AND DEPENDING ON ITS EVOLUTION...THE ADVISORIES MAY NEED TO BE
EXTENDED TO INCLUDE THE UPPER GREEN AND SALT...WYOMING RANGES.
NEXT SYSTEM DROPS QUICKLY INTO THE BACK SIDE OF THIS TROUGH AND
SETS UP THE SRN AND ERN ZONES FOR ANOTHER POTENTIALLY SIGNIFICANT
STORM SYSTEM MONDAY INTO AT LEAST TUESDAY MORNING. SIMILAR FORCING
IS PRESENT AND POSSIBLY EVEN THE RIGHT ENTRANCE REGION OF A JET
STREAK FOR THE SECOND HALF OF THIS EVENT. THE MOST FAVORABLE AREA
SEEMS TO BE THE RKS TO CPR CORRIDOR BUT LANDER AND POSSIBLY
RIVERTON WILL ALSO HAVE TO BE WATCHED. WILL LIKELY NEED AT LEAST
ANOTHER ADVISORY AND POSSIBLY MORE DEPENDING ON HOW THINGS COME
TOGETHER. VERY COLD AIRMASS WILL BECOME ENTRENCHED OVER THE AREA
WITH H7 TEMPS DROPPING TO BETWEEN -20C AND -26C ACROSS THE NRN TWO
THIRDS THROUGH SUNDAY BEFORE SOME MODERATION EDGES IN FROM THE
SOUTH BUT IT STILL REMAINS -10C TO -20C ACROSS THE STATE MONDAY
AND COLDER AGAIN TUESDAY. HIGHS WILL STRUGGLE INTO THE SINGLE
DIGITS IN MOST AREAS SUNDAY WITH LITTLE MODERATION MONDAY EXCEPT
IN THE SOUTHWEST.
LONG TERM...TUESDAY NIGHT THROUGH SATURDAY
THE MAIN PESKY TROUGH WILL FINALLY PUSH EAST INTO THE PLAINS DURING
THE DAY ON TUESDAY. SOME SNOW WILL LIKELY BE ONGOING TUESDAY
MORNING OVER THE SOUTH AND EASTERN SECTIONS OF THE FORECAST AREA.
QG FORCING AND FRONTOGENESIS REMAINS STRONG OVER THE AREA
TUESDAY...ESPECIALLY IN THE MORNING. IN WAKE OF THIS
SYSTEM...MAINLY DRY CONDITIONS ARE EXPECTED. HOWEVER...A WEAK
DISTURBANCE RIGHT BEHIND THE TROUGH COULD GIVE THE NORTHWEST SOME
SNOW SHOWERS. TUESDAY NIGHT...COULD BE THE COLDEST NIGHT OF THIS
EARLY WINTER SEASON...AS SURFACE HIGH PRESSURE BUILDS INTO THE
AREA...WHICH SHOULD RESULT IN RELATIVELY LIGHT WINDS. A COUPLE
LIMITING FACTORS ARE THAT MODELS ARE A BIT SLOWER IN CLEARING THAN
LAST NIGHT MODEL RUNS...AND 700MB FLOW WILL BE STRENGTHENING DURING
THE NIGHT. MEDIUM RANGE MODELS FROM WEDNESDAY NIGHT THROUGH FRIDAY
ARE SHOWING NORTHWEST FLOW OVER THE NORTHERN ROCKIES...WITH A MOIST
WESTERLY FLOW DOMINATING THE CENTRAL AND SOUTHERN ROCKIES. THE MAIN
CHALLENGE IS WHAT FLOW WILL HAVE MORE INFLUENCE ON THE FORECAST
AREA. PREFER MORE OF A NORTHWEST FLOW AND A DRIER FORECAST THAN
WHAT THE ECMWF IS CURRENTLY INDICATING. HAVING SAID THAT...HAVE
KEPT CHANCE POPS UNCHANGED WEDNESDAY NIGHT THROUGH FRIDAY OUT
WEST...AS DISTURBANCES IN THE NORTHWEST COULD STILL ALLOW FOR SOME
SNOWFALL...AND SEE WHAT LATER MODELS RUNS SHOW.
TEMPERATURES WILL REMAIN WELL BELOW AVERAGE TUESDAY AND
WEDNESDAY...WITH A TEMPERATURES MODERATING THURSDAY AND FRIDAY...AS
700 MB TEMPS REBOUND TO -5 TO -12 DEG CELSIUS. HOWEVER...INVERSION
TRAPPED...SNOW COVERED BASINS WILL STRUGGLE TO WARM UP. AREAS THAT
CAN MIX OUT...SUCH AS FAVORED WINDY AREAS SHOULD SEE NEAR SEASONAL
TEMPERATURES BY FRIDAY.
AVIATION...
AN AREA OF LOW PRESSURE WILL CONTINUE TO IMPACT MUCH OF THE STATE
TODAY...BRINGING WIDESPREAD SNOWFALL. CONSEQUENTLY...WIDESPREAD MVFR
TO IFR CONDITIONS ARE ANTICIPATED FOR MANY AIRPORTS. SOME SNOWFALL
WILL BE HEAVY THROUGH THE FORECAST PERIOD...EXPECT SOME LIFR
CONDITIONS UNDER THE AREAS OF HEAVIER SNOWFALL. THE WESTERN
MOUNTAINS AND VALLEYS...ESPECIALLY KJAC...WILL SEE A BREAK IN
SNOWFALL AFTER 09Z SUNDAY WITH VFR TO MVFR CONDITIONS. SNOWFALL WILL
REDEVELOP AFTER 18Z SUNDAY. SUNDAY AFTERNOON WILL ALSO SEE THE
DEVELOPMENT OF GUSTY WIND OVER THE WIND CORRIDOR BETWEEN KRKS AND
KCPR. INCREASED WIND OF 20G30KT WILL DEVELOP OVER KRKS AFTER 12Z
SUNDAY...WITH 15G25KT WIND EXPECTED OVER KCPR. THIS INCREASED WIND
WILL CAUSE BLOWING AND DRIFTING SNOW THROUGH THE AFTERNOON.
FIRE WEATHER...
BITTERN COLD STORM SYSTEM WILL BLANKET THE ENTIRE AREA WITH FRIGID
TEMPERATURES THE NEXT FEW DAYS WITH MANY AREAS RECEIVING A COATING
OF SNOW. BRISK TO STRONG NORTHEAST WINDS WILL PREVAIL ACROSS THE
SOUTH AND SOME CENTRAL ZONES THROUGH SUNDAY WITH CONSIDERABLE
BLOWING AND DRIFTING SNOW. WIND CHILLS WILL REACH 20 TO 30 DEGREES
BELOW ZERO. ANOTHER STORM SYSTEM WILL BRING ANOTHER ROUND OF SNOW
MONDAY INTO TUESDAY WITH CONTINUED VERY COLD CONDITIONS AND BRISK
WINDS.
&&
.RIW WATCHES/WARNINGS/ADVISORIES...
WINTER WEATHER ADVISORY UNTIL 5 PM MST SUNDAY FOR WYZ014-015-
017>020-022-027>030.
&&
$$
SHORT TERM...SKRBAC
LONG TERM...WM
AVIATION...BRANHAM
FIRE WEATHER...SKRBAC
