FXUS65 KRIW 250944
AFDRIW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
244 AM MST FRI DEC 25 2009
.SHORT TERM...TODAY THROUGH SUNDAY NIGHT
WATER VAPOR SATELLITE IMAGERY AT 08Z SHOWS A DEEP UPPER LOW ACROSS
THE SOUTH CENTRAL PLAINS WITH A CENTER NEAR SW MO AND ANOTHER DEEP
CLOSED LOW OVER WESTERN SD/WESTERN NEB. THE SOUTHERNMOST UPPER LOW
HAS ENTRAINED CONSIDERABLE GULF OF MEX MOISTURE INTO ITS CIRCULATION
AND TRANSPORTED THIS MOISTURE AS FAR NORTH AS THE DAKOTAS AND
SOUTHERN MANITOBA. THE NORTHERN UPPER LOW HAS TRANSPORTED A PORTION
OF THIS MOISTURE INTO WESTERN ND...WHERE LIGHT SNOW HAS BEEN FALLING
FOR 4-8 HOURS SINCE YESTERDAY EVENING. AS THE TWO UPPER LOWS MERGE
TODAY OVER CENTRAL NEB AND MOVES SLOWLY EASTWARD OVER MO...MOISTURE
WILL BE WRAPPED AROUND THE CIRCULATION ACROSS EASTERN WY AND EASTERN
CENTRAL WY. GFS AND NAM INDICATE THE ONSET OF ISENTROPIC UPGLIDE
OVER JOHNSON AND NATRONA COUNTIES AT AROUND 12Z...HOWEVER SNOWFALL
WILL BE VERY LIGHT THROUGH 18Z WITH SPECIFIC HUMIDITY ONLY AROUND
0.5 G/KG. ISENTROPIC LIFT INCREASES ALONG WITH MOISTURE ADVECTION
ACROSS NATRONA AND JOHNSON COUNTIES AFTER 18Z AND CONTINUES THROUGH
06Z SUNDAY. BEST PERIOD OF SNOWFALL ACROSS NATRONA AND JOHNSON
COUNTIES APPEARS TO BE MOST PROBABLE FROM 00Z SATURDAY THROUGH 18Z
SATURDAY. HAVE DECIDED TO ISSUE WINTER WEATHER ADVISORIES FOR
NATRONA AND JOHNSON COUNTIES FROM 18Z TODAY THROUGH 00Z SUNDAY.
SHORT RANGE MODEL GUIDANCE HAS TRENDED TOWARD A SOLUTION BRINGING
MOISTURE FARTHER WEST OVER THE PAST 24 HOURS...AND ISENTROPIC LIFT
ACROSS THE SOUTHERN BIG HORN BASIN...FREMONT COUNTY...THE GREEN
MOUNTAINS AND THE RATTLESNAKE RANGE TONIGHT THROUGH 18Z SATURDAY ARE
EXPECTED TO PRODUCE LIGHT TO MODERATE SNOWFALL ACROSS THESE AREAS
WITH THE BEST CHANCE FOR ADVISORY AMOUNTS IN THE LANDER FOOTHILLS.
THE SLOW MOVEMENT OF THE CONSOLIDATED UPPER LOW IN THE CENTRAL AND
UPPER MIDWEST TODAY THROUGH SATURDAY COMBINED WITH A NORTHERLY TO
NORTHEASTERLY FLOW ALOFT WILL KEEP A COLD AIR MASS IN PLACE ACROSS
WY. HIGHS TODAY AND SATURDAY WILL BE 12 TO 18 F BELOW NORMAL. AS THE
UPPER LOW PROGRESSES SLOWLY EASTWARD SUNDAY...H7 TEMPERATURES WILL
MODIFY. WARMING ALOFT SUNDAY WILL RESULT IN MORE NORMAL TEMPERATURES
IN THE HIGHER ELEVATIONS SUNDAY...BUT INVERSIONS IN THE BASINS EAST
OF THE DIVIDE WILL KEEP DAYTIME TEMPERATURES WELL BELOW NORMAL.
.LONG TERM...MONDAY THROUGH FRIDAY
MEDIUM RANGE PERIOD BEGINS WITH THE WEAKENING CLOSED HIGH TO THE
NORTH AND A TROUGH SPLITTING WITH MOST OF THE ENERGY DIGGING INTO
THE DESERT SW. MODELS BEGIN TO DIVERGE SIGNIFICANTLY AGAIN TONIGHT
FROM WEDNESDAY ONWARDS. THE ECMWF BUILDS THE RIDGE UP TO THE WEST
AGAIN AND BRINGS ANOTHER ARCTIC COLD FRONT SOUTH ACROSS THE AREA
WEDNESDAY NIGHT. THE GFS DEVELOPS A STRONG AND INCREASINGLY MOST
NW FLOW. THE MONTH OF DECEMBER HAS FAVORED THE ARCTIC SOLUTIONS
BUT CONFIDENCE IS STILL LOW WITH THE RAPID CHANGES IN EACH MODEL
RUN. HAVE KEPT IDEA OF MOISTURE IMPACTING THE WEST IN THIS PATTERN
WEDNESDAY THROUGH FRIDAY WITH A NUDGE TOWARDS EITHER THE ECMWF
ARCTIC OUTBREAK EAST OF THE DIVIDE OR EVEN A STRONG ENOUGH SHORTWAVE
IN THE NW FLOW TO BRING A CHANCE OF SNOW EAST OF THE DIVIDE. JUST
SLIGHT CHANCES FOR NOW EAST OF THE DIVIDE UNTIL MODELS PROVIDE A
LITTLE MORE CONSISTENCY. TEMPERATURES WILL REMAIN NEAR OR BELOW
NORMAL WITH TRAPPED BASIN AND VALLEY INVERSIONS FOR AT LEAST THE
FIRST COUPLE DAYS OF THE MEDIUM RANGE PERIOD.
&&
.AVIATION...
WEST OF THE CONTINENTAL DIVIDE VFR CONDITIONS WILL PREVAIL TODAY
WITH ONLY A FEW SNOW SHOWERS OVER THE MOUNTAINS.
EAST OF THE CONTINENTAL DIVIDE AREAS OF LIGHT SNOW WILL OCCUR TODAY
AND TONIGHT. VFR TO MVFR CONDITIONS WILL OCCUR THROUGH 21Z. AFTER
21Z MVFR TO IFR CONDITIONS WILL PREVAIL ACROSS MUCH OF THE REGION.
KCPR AIRPORT WILL SEE MVFR TO IFR CONDITIONS FOR MOST OF THE DAY
INTO THE EVENING. IFR CONDITIONS WILL DEVELOP AROUND KLND AND KRIW
AIRPORTS BY 00Z SAT AND CONTINUE AFTER 00Z SAT.
&&
.FIRE WEATHER...
CIRCULATION AROUND DEEP LOW PRESSURE IN THE CENTRAL PLAINS TODAY
WILL BRING GUSTY NORTH WINDS TO NORTHEASTERN WYOMING WITH NORTH WIND
IN JOHNSON COUNTY GUSTING TO NEAR 35 MPH. NORTH TO NORTHWEST WIND IN
NATRONA COUNTY WILL GUSTS TO AROUND 25 MPH TODAY. WEST WIND WILL
GUSTS TO AROUND 30 MPH ACROSS SWEETWATER COUNTY AND EASTERN FREMONT
COUNTY. ELSEWHERE ACROSS CENTRAL AND WESTERN WYOMING TODAY WINDS
WILL GENERALLY BE LESS THAN 15 MPH. SNOW WILL BEGIN THIS MORNING
ACROSS JOHNSON AND NATRONA COUNTIES AND BECOME MODERATE AT TIMES
THIS AFTERNOON THROUGH SATURDAY AFTERNOON. INVERSIONS IN THE LOWER
ELEVATIONS WEST OF THE DIVIDE TODAY AND SATURDAY WILL BE FAIRLY
STRONG...WHILE INVERSIONS ARE NOT EXPECTED EAST OF THE DIVIDE.
&&
.RIW WATCHES/WARNINGS/ADVISORIES...
WINTER WEATHER ADVISORY FROM 11 AM THIS MORNING TO 5 PM MST
SATURDAY FOR WYZ010-011-020-022.
&&
$$
SHORT TERM...SMITH
LONG TERM...SKRBAC
AVIATION...AR
FIRE WEATHER...SMITH
