Special Weather Statement, NWS Riverton, WY

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
327 AM MDT SUN SEP 20 2009

.SHORT TERM...TODAY THROUGH TUESDAY NIGHT

WELL DEFINED TROUGH WILL MOVE SOUTHEASTWARD FROM SOUTHWESTERN CANADA
INTO CENTRAL MT TODAY WITH THE GFS AND NAM IN GOOD AGREEMENT ON THE
PROGRESSION OF THE TROUGH. A LEADING SHORTWAVE AND ASSOCIATED
PACIFIC COLD FRONT WILL MOVE ACROSS WESTERN WY THIS MORNING AND
ACROSS CENTRAL WY THIS AFTERNOON. SCATTERED SHOWERS AND
THUNDERSTORMS WILL MOVE THROUGH WESTERN WY THIS MORNING WITH LOW
LEVEL CONVERGENCE ALONG THE COLD FRONT AND WEAK INSTABILITY.
CONVECTIVE ACTIVITY WILL DIMINISH DURING THE AFTERNOON WEST OF THE
DIVIDE...AS GFS AND NAM SHOW DRIER AND MORE STABLE AIR MOVING INTO
THE REGION FROM THE WEST. SCATTERED SHOWER AND THUNDERSTORM ACTIVITY
WILL MOVE INTO CENTRAL WY THIS AFTERNOON...AS THE COLD FRONT MOVES
EAST OF THE DIVIDE WHERE LIFTED INDICES ARE PROGGED TO BE SLIGHTLY
BELOW 0 C. STRONG H25 JET CORE OF 100+ KT TO THE WEST OF THE TROUGH
AXIS WILL CAUSE THE TROUGH IN CENTRAL MT THIS EVENING TO DIG SHARPLY
SOUTHEASTWARD ACROSS EASTERN WY TONIGHT. INCREASING LOW AND MID
LEVEL MOISTURE ACROSS JOHNSON AND NATRONA COUNTIES COMBINED WITH
NORTHERLY UPSLOPE FLOW AND MODERATE QG FORCING WILL PRODUCE LIGHT TO
MODERATE PRECIPITATION TONIGHT. POWERFUL SECONDARY COLD FRONT WILL
CAUSE H7 TEMPERATURES TO FALL TO AROUND MINUS 6 C OVER JOHNSON
COUNTY BY 03Z AND OVER NATRONA COUNTY BY 09Z...AND SNOW LEVELS WILL
FALL TO AROUND 6500 FEET. A TREND TO SHIFT THE BAND OF HEAVIEST
PRECIPITATION INTO FAR EASTERN WY AND THE WESTERN DAKOTAS WAS NOTED
IN THE 00Z RUNS OF THE GFS AND NAM. LOWERED PRECIPITATION AMOUNTS
SLIGHTLY IN EASTERN PORTIONS OF CENTRAL WY. THE BIGHORN MOUNTAINS
ARE EXPECTED TO RECEIVE BETWEEN 3 AND 6 INCHES OF SNOW THROUGH 15Z
MONDAY...WHILE CASPER MOUNTAIN COULD RECEIVE 2 TO 4 INCHES THROUGH
18Z. THE LOWER ELEVATIONS OF JOHNSON AND NATRONA COUNTIES COULD SEE
SOME MINOR ACCUMULATIONS...BUT AT THIS TIME IT APPEARS MORE LIKELY
THAT THESE AREAS WILL SEE RAIN CHANGING TO A MIXTURE OF RAIN AND
SNOW WITH BOUNDARY LAYER TEMPERATURES STAYING SEVERAL DEGREES
ABOVE FREEZING.

THE TROUGH WILL CONTINUE TO DROP SOUTHWARD INTO WESTERN NEB MONDAY
AFTERNOON WITH BOTH THE GFS AND NAM CLOSING OFF THE CIRCULATION.
LIGHT PRECIPITATION IS EXPECTED TO END ACROSS NATRONA COUNTY AROUND
18Z. COLD CORE OF AIR OVER WESTERN AND CENTRAL WY MONDAY WILL RESULT
IN HIGH TEMPERATURES 8-12 F BELOW NORMAL. AS THE UPPER LOW CONTINUES
TO MOVE SOUTHWARD ACROSS WESTERN NEB AND INTO WESTERN KS MONDAY
NIGHT...STRONG SUBSIDENCE ON THE NORTHWESTERN SIDE OF THE UPPER LOW
WILL PRODUCE CLEARING ACROSS WY. CLEAR SKIES...LIGHT WINDS AND
DEWPOINTS IN THE LOWER TO MID 20S MONDAY NIGHT WILL PRODUCE STRONG
RADIATIONAL COOLING IN THE LOWER ELEVATIONS OF CENTRAL AND WESTERN
WY WITH A GOOD CHANCE OF A HARD FREEZE ACROSS MUCH OF THE REGION.
LARGE UPPER HIGH CENTERED OVER THE PAC NW WILL BUILD OVER WY FROM
THE NORTHWEST TUESDAY. THIS UPPER HIGH WILL BRING CLEAR SKIES AND
DRY WEATHER TO THE REGION TUESDAY WITH HIGH TEMPERATURES RETURNING
TO NEAR NORMAL.

.LONG TERM...WEDNESDAY THROUGH SUNDAY
TEMPERATURES WILL RECOVER TO NEAR NORMAL IN MOST AREAS WEDNESDAY
AS AN UPPER LEVEL LOW PRESSURE SYSTEM IN EASTERN COLORADO IS
SUPPLANTED BY HIGH PRESSURE MOVING INTO THE INTERMOUNTAIN REGION
FROM THE NORTHWEST. COOLER TEMPS FOR THE EASTERN AND SOUTHERN
ZONES AS THE COOLER AIRMASS TO THE SOUTH RETAINS SOME INFLUENCE
THROUGH THURSDAY. GFS KEEPS H7 TEMPS FOR SWEETWATER COUNTY IN THE
0 TO +2 C RANGE. WHILE ECMWF IS A BIT WARMER AT 2-4 C. HIGH
PRESSURE WILL REMAIN IN PLACE THOUGH FRIDAY. GFS CUTS OFF ANOTHER
UPPER LOW LATE FRIDAY AND PUSHES A COLD FRONT THROUGH THE STATE
EARLY SATURDAY. EURO HAS YET TO RESOLVE A SIMILAR FEATURE. IN ANY
CASE...GFS HAS LITTLE MOISTURE TO GO WITH POTENTIAL DYNAMICS SO
HAVE NOT INCLUDED POPS FOR THE SATURDAY/SUNDAY PERIOD.

&&

.AVIATION...

A TROUGH OF LOW PRESSURE AND ASSOCIATED COLD FRONT WILL MOVE ACROSS
THE AREA TODAY.  ISOLATED TO SCATTERED SHOWERS WILL PRECEDE THE COLD
FRONT WEST OF THE DIVIDE THIS MORNING...THE SHOWERS MORE NUMEROUS
ACROSS THE SOUTHWEST ALONG KRKS-KBPI ROUTES WHERE LOCAL MVFR
CONDITIONS MAY BE ENCOUNTERED. RAPIDLY CLEARING SKIES AND
INCREASING NW WIND WILL OCCUR IN THE WEST BEHIND THE FRONT IN THE
AFTERNOON.

EAST OF THE DIVIDE...ISOLATED TO WIDELY SCATTERED SHOWERS AND
THUNDERSTORMS WILL DEVELOP AHEAD OF A COLD FRONT DROPPING SOUTH OUT
OF MONTANA THIS AFTERNOON.  THE SHOWERS AND THUNDERSTORMS WILL BE
MOST NUMEROUS VCNTY AND EAST OF A KCOD-KWRL-KCPR LINE.  LOWERING
CIGS AND MORE WIDESPREAD PRECIPITATION WILL FOLLOW THE FRONT SUNDAY
EVENING WITH SNOW LEVELS LOWERING TO NEAR 6500 FEET BY EARLY MONDAY
MORNING.  EXPECT AREAS OF MVFR/IFR CONDITIONS AND MOUNTAIN
OBSCURATIONS EAST OF THE DIVIDE AFTER 06Z MONDAY.

&&

.FIRE WEATHER...

A PACIFIC COLD FRONT WILL MOVE ACROSS WESTERN WYOMING THIS MORNING
AND CONTINUE ACROSS CENTRAL WYOMING THIS AFTERNOON. DAYTIME
TEMPERATURES WILL BE 10-14 F COOLER THAN TEMPERATURES ON
SATURDAY...AND AFTERNOON RELATIVE HUMIDITIES WILL BE IN THE UPPER
TEENS AND 20S IN THE LOWER ELEVATIONS. SCATTERED SHOWERS AND
THUNDERSTORMS ARE EXPECTED ALONG THE COLD FRONT IN  THE MORNING WEST
OF THE DIVIDE AND IN THE AFTERNOON EAST OF THE DIVIDE. A STRONGER
COLD FRONT WILL MOVE ACROSS THE REGION TONIGHT. IT WILL BRING MUCH
COOLER AIR TO THE REGION LATE TONIGHT AND MONDAY. HIGH TEMPERATURES
MONDAY WILL BE 8-12 F BELOW NORMAL. UPSLOPE FLOW AND INCREASING
MOISTURE EAST OF THE DIVIDE LATE TONIGHT AND MONDAY MORNING IS
EXPECTED TO BRING WETTING RAIN/SNOW TO JOHNSON AND NATRONA COUNTIES.
SNOW LEVELS WILL BE AROUND 6500 FEET LATE TONIGHT AND MONDAY MORNING
WITH 3-6 INCHES POSSIBLE IN THE BIGHORN MOUNTAINS AND 2-4 INCHES ON
CASPER MOUNTAIN.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
NONE.

&&

SHORT TERM...SMITH
LONG TERM...BERC
AVIATION...AEM
FIRE WEATHER...SMITH

HAZARDOUS WEATHER OUTLOOK
NATIONAL WEATHER SERVICE RIVERTON WY
501 AM MDT SUN SEP 20 2009

WYZ001>020-022>030-211101-
ABSAROKA MOUNTAINS-BIGHORN MOUNTAINS SOUTHEAST-
BIGHORN MOUNTAINS WEST-CASPER MOUNTAIN-CODY FOOTHILLS-
EAST SWEETWATER COUNTY-FLAMING GORGE-
GREEN MOUNTAINS AND RATTLESNAKE RANGE-JACKSON HOLE-LANDER FOOTHILLS-
NATRONA COUNTY LOWER ELEVATIONS-NORTH BIG HORN BASIN-
NORTHEAST JOHNSON COUNTY-OWL CREEK AND BRIDGER MOUNTAINS-
ROCK SPRINGS AND GREEN RIVER-SALT RIVER AND WYOMING RANGES-
SOUTH LINCOLN COUNTY-SOUTHEAST BIG HORN BASIN-
SOUTHEAST JOHNSON COUNTY-SOUTHWEST BIG HORN BASIN-STAR VALLEY-
TETON AND GROS VENTRE MOUNTAINS-UPPER GREEN RIVER BASIN-
UPPER GREEN RIVER BASIN FOOTHILLS-UPPER WIND RIVER BASIN-
WIND RIVER BASIN-WIND RIVER MOUNTAINS EAST-WIND RIVER MOUNTAINS WEST-
YELLOWSTONE NATIONAL PARK-
501 AM MDT SUN SEP 20 2009

THIS HAZARDOUS WEATHER OUTLOOK IS FOR WESTERN AND CENTRAL WYOMING.

.DAY ONE...TODAY AND TONIGHT

SCATTERED SHOWERS AND THUNDERSTORMS ARE EXPECTED WEST
OF THE DIVIDE THIS MORNING AND EAST OF THE DIVIDE THIS AFTERNOON.
THUNDERSTORMS WILL BE CAPABLE OF PRODUCING GUSTY WIND...SMALL HAIL
AND LIGHTNING.

A STRONG COLD FRONT WILL MOVE SOUTHWARD ACROSS CENTRAL WYOMING
TONIGHT. MOISTURE ASSOCIATED WITH THE FRONT AND UPSLOPE
FLOW BEHIND THE FRONT WILL BRING MEASURABLE RAIN AND SNOW TO AREAS
EAST OF THE DIVIDE TONIGHT. SNOW LEVELS WILL FALL TO AROUND 6500
FEET LATE TONIGHT. 2 TO 4 INCHES OF SNOW ARE EXPECTED IN THE BIGHORN
MOUNTAINS AND 1 TO 2 INCHES ON CASPER MOUNTAIN.

.DAYS TWO THROUGH SEVEN...MONDAY THROUGH SATURDAY

UPSLOPE FLOW WILL CONTINUE THROUGH MONDAY MORNING IN EASTERN
PORTIONS OF CENTRAL WYOMING WITH 3 TO 6 INCHES OF TOTAL SNOWFALL
POSSIBLE IN THE BIGHORNS AND 2 TO 4 INCHES POSSIBLE ON CASPER
MOUNTAIN.

SKIES WILL CLEAR ACROSS CENTRAL AND WESTERN WYOMING MONDAY NIGHT.
WITH CLEAR SKIES...LIGHT WINDS AND A COLD AIRMASS IN PLACE MONDAY
NIGHT...A HARD FREEZE IS LIKELY ACROSS MANY OF THE LOWER ELEVATIONS
OF CENTRAL AND WESTERN WYOMING.

.SPOTTER INFORMATION STATEMENT...

SPOTTERS ARE ENCOURAGED TO REPORT HAIL AND WIND GUSTS IN EXCESS OF 40
MPH TODAY. THANK YOU.

TEAM RIVERTON

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SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE RIVERTON WY
106 PM MDT SUN SEP 20 2009

WYZ012-013-202100-
JACKSON HOLE-TETON AND GROS VENTRE MOUNTAINS-
106 PM MDT SUN SEP 20 2009

AN WEST-TO-EAST ORIENTED BAND OF SHOWERS AND EMBEDDED THUNDERSTORMS
EXISTS FROM NEAR DRIGGS TO JUST NORTH OF MOOSE TO TOGWOTEE PASS.
THIS BAND WILL GRADUALLY DRIFT SOUTH THROUGH 3 PM...AFFECTING TETON
VILLAGE...MOOSE...KELLY...AND THE NORTHERN COMMUNITIES OF JACKSON.
EXPECT BRIEF LIGHT TO MODERATE RAIN...A FEW LIGHTNING STRIKES AND
WIND GUSTS TO 30 MPH.  THERE IS ANOTHER BAND ALONG THE SOUTHERN
YELLOWSTONE PARK BOUNDARY DRIFTING SOUTH AS WELL.

TWB

SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE RIVERTON WY
132 PM MDT SUN SEP 20 2009

WYZ016-202100-
UPPER WIND RIVER BASIN-
132 PM MDT SUN SEP 20 2009

AN WEST-TO-EAST ORIENTED BAND OF SHOWERS AND EMBEDDED THUNDERSTORMS
EXISTS FROM TOGWOTEE PASS TO NORTH OF DUBOIS. THIS BAND WILL
GRADUALLY DRIFT SOUTH THROUGH 3 PM...AFFECTING DUNOIR...UNION PASS
AND DUBOIS.  EXPECT BRIEF LIGHT TO MODERATE RAIN...A FEW LIGHTNING
STRIKES AND WIND GUSTS TO 40 MPH.

TWB

SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE RIVERTON WY
151 PM MDT SUN SEP 20 2009

WYZ015-019-026-030-202030-
EAST SWEETWATER COUNTY-GREEN MOUNTAINS AND RATTLESNAKE RANGE-UPPER
GREEN RIVER BASIN-WIND RIVER MOUNTAINS EAST-
151 PM MDT SUN SEP 20 2009

...SIGNIFICANT WEATHER ADVISORY FOR FREMONT AND SWEETWATER COUNTIES
UNTIL 230 PM MDT...

AT 149 PM MDT...NATIONAL WEATHER SERVICE DOPPLER RADAR INDICATED A
STRONG THUNDERSTORM NEAR OREGON BUTTES...OR 33 MILES EAST OF
FARSON...MOVING EAST AT 20 MPH.

PENNY SIZE HAIL...AND WINDS AROUND 40 MPH ARE POSSIBLE WITH THIS
STORM.

RESIDENTS IN NORTHERN SWEETWATER AND SOUTHERN FREMONT COUNTIES...
INCLUDING THE LOCATION OF OREGON BUTTES...SHOULD MONITOR THE WEATHER
SITUATION CLOSELY. LISTEN FOR POSSIBLE WARNINGS AND BE PREPARED TO
TAKE APPROPRIATE ACTION SHOULD SEVERE WEATHER THREATEN.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

MOVE INDOORS IMMEDIATELY AS THIS STORM APPROACHES YOUR LOCATION. THIS
IS A POTENTIALLY DANGEROUS STORM. IF YOU ARE IN ITS PATH...YOU SHOULD
BE PREPARED FOR GUSTY WINDS...HAIL...AND DEADLY CLOUD TO GROUND
LIGHTNING. IF A STURDY STRUCTURE IS NOT AVAILABLE...SEEK SHELTER IN A
VEHICLE WITH THE WINDOWS UP.

&&

REPORT HAZARDOUS WEATHER TO THE NATIONAL WEATHER SERVICE OFFICE IN
RIVERTON AT 1-800-211-1448.

LAT...LON 4220 10816 4195 10829 4219 10899 4235 10891
TIME...MOT...LOC 1950Z 287DEG 19KT 4224 10881

T. BAKER

SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE RIVERTON WY
210 PM MDT SUN SEP 20 2009

WYZ028>030-202100-
EAST SWEETWATER COUNTY-FLAMING GORGE-ROCK SPRINGS AND GREEN RIVER-
210 PM MDT SUN SEP 20 2009

...SIGNIFICANT WEATHER ADVISORY FOR SWEETWATER COUNTY UNTIL 300 PM
MDT...

AT 209 PM MDT...NATIONAL WEATHER SERVICE DOPPLER RADAR INDICATED A
STRONG THUNDERSTORM 9 MILES NORTH OF STATE HWY 414 MILE MARKER 140...
OR 11 MILES NORTHWEST OF MANILA...MOVING EAST AT 30 MPH.

PEA SIZE HAIL...AND WINDS AROUND 40 MPH ARE POSSIBLE WITH THIS STORM.

RESIDENTS IN SOUTHWESTERN SWEETWATER COUNTY...INCLUDING THE LOCATIONS
OF STATE HWY 530 MILE MARKER 30 TO 40...SQUAW HOLLOW BOAT RAMP ON
FLAMING GORGE...U.S. HWY 191 MILE MARKER 550 TO 540...SHOULD MONITOR
THE WEATHER SITUATION CLOSELY.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

IF ON OR NEAR FLAMING GORGE...GET OUT OF THE WATER AND MOVE INDOORS
OR INSIDE A VEHICLE. REMEMBER...LIGHTNING CAN STRIKE OUT TO 15 MILES
FROM THE PARENT THUNDERSTORM. IF YOU CAN HEAR THUNDER...YOU ARE
CLOSE ENOUGH TO BE STRUCK BY LIGHTNING. MOVE TO SAFE SHELTER NOW.
DONT BE CAUGHT ON THE WATER IN A THUNDERSTORM

&&

REPORT HAZARDOUS WEATHER TO THE NATIONAL WEATHER SERVICE OFFICE IN
RIVERTON AT 1-800-211-1448.

LAT...LON 4107 10994 4124 10989 4116 10904 4099 10908
      4099 10956
TIME...MOT...LOC 2010Z 281DEG 25KT 4114 10979

T. BAKER

SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE RIVERTON WY
221 PM MDT SUN SEP 20 2009

WYZ005-007-017-202100-
OWL CREEK AND BRIDGER MOUNTAINS-SOUTHWEST BIG HORN BASIN-WIND RIVER
BASIN-
221 PM MDT SUN SEP 20 2009

...SIGNIFICANT WEATHER ADVISORY FOR FREMONT AND HOT SPRINGS COUNTIES
UNTIL 300 PM MDT...

AT 219 PM MDT...NATIONAL WEATHER SERVICE DOPPLER RADAR INDICATED
STRONG THUNDERSTORMS ALONG A LINE EXTENDING FROM 15 MILES WEST OF
U.S. HWY 20/789 MILE MARKER 120 TO STATE HWY 134 MILE MARKER 10...OR
ALONG A LINE EXTENDING FROM 17 MILES SOUTHWEST OF THERMOPOLIS TO 15
MILES NORTH OF RIVERTON...MOVING EAST AT 25 MPH.

PEA SIZE HAIL...AND WINDS AROUND 40 MPH ARE POSSIBLE WITH THESE
STORMS.

RESIDENTS IN CENTRAL FREMONT AND SOUTHERN HOT SPRINGS
COUNTIES...INCLUDING BOYSEN RESERVOIR...SHOSHONI AND MISSOURI
VALLEY...SHOULD MONITOR THE WEATHER SITUATION CLOSELY. LISTEN FOR
POSSIBLE WARNINGS AND BE PREPARED TO TAKE APPROPRIATE ACTION SHOULD
SEVERE WEATHER THREATEN.

REPORT HAZARDOUS WEATHER TO THE NATIONAL WEATHER SERVICE OFFICE IN
RIVERTON AT 1-800-211-1448.

LAT...LON 4357 10791 4315 10801 4315 10865 4357 10855
TIME...MOT...LOC 2020Z 270DEG 20KT 4348 10844 4324 10850

T. BAKER