Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 230602
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
1202 AM MDT SAT APR 23 2016

.UPDATE...ADDED 06Z AVIATION DISCUSSION.

&&

.AVIATION.../06Z ISSUANCE/

WEST OF THE CONTINENTAL DIVIDE...KJAC/KRKS/KPNA/KBPI AIRPORTS

AN UPPER LEVEL LOW PRESSURE SYSTEM AND ASSOCIATED COLD FRONT WILL
SPREAD NMRS LOW VFR/MVFR SHRA AND WDLY SCT TSRA TO VCNTY OF THE
CONTINENTAL DIVIDE BY 18Z.  SNOW LEVELS WILL BE INITIALLY NEAR 9KFT
WITH MTN OBSCURATIONS BECOMING WIDESPREAD IN THE AFTERNOON.
WIDESPREAD IFR RASN WITH SNOW LEVELS LOWERING TO NEAR 7KFT ARE
EXPECTED VCNTY AND NORTH OF A KJAC-KDUB LINE BEHIND THE COLD FRONT
AFTER 00Z SUNDAY WITH SCT-NMRS SHRASN SOUTH OF THIS LINE. SNOW
LEVELS WILL LOWER TO MOST VALLEY FLOORS AFTER 06Z SUNDAY WITH SNOW
SHOWERS REMAINING MORE NMRS ACROSS THE NORTHWEST.

EAST OF THE CONTINENTAL DIVIDE...KCPR/KCOD/KRIW/KWRL/KLND AIRPORTS

SOUTH TO SOUTHWEST SFC WINDS WILL INCREASE 16Z-20Z SATURDAY AHEAD OF
AN APPROACHING UPPER LEVEL LOW PRESSURE SYSTEM AND ASSOCIATED COLD
FRONT.  WDLY SCT TSRA/SHRA IS EXPECTED TO SPREAD EAST OF THE DIVIDE
INTO CENTRAL WYOMING 20-22Z SATURDAY WITH MORE ORGANIZED STRONGER
STORMS EXPECTED ALONG AND NORTH OF A FRONTAL BOUNDARY VCNTY WY/MT
LINE. LOWERING CIGS AND INCREASING SHRA ARE EXPECTED THROUGH
SATURDAY NIGHT AS THE UPPER LEVEL LOW PRESSURE SYSTEM MOVES OVER THE
AREA.  SNOW LEVELS WILL LOWER TO BTWN 7-8KFT ACROSS THE 6Z-12Z
SUNDAY WITH MTNS BECOMING MOSTLY OBSCURED...ESPECIALLY THE ABSAROKA
AND BIGHORN RANGES.

&&

.PREV DISCUSSION... /ISSUED 221 PM MDT FRI APR 22 2016/

SHORT TERM...SUNDAY NIGHT THROUGH SATURDAY

BEGINNING OF FCST...NEGATIVELY TILTED TROF ROTATING THRU THE NRN
ROCKIES INTO THE HIGH/NRN PLAINS. BOTH CYCLO AND FRONTO-GENESIS WILL
BE ONGOING ACROSS THE REGION...WITH THE LEE LOW DEVELOPING OVER NERN
WY AND A INCREASINGLY BETTER DEFINED FRONT FOLLOWING IT FROM WEST TO
EAST ACROSS THE FA. GUSTY WEST TO SOUTHWEST WIND...WITH SOME GUSTS
45 TO TO 50 KTS...WILL BE FLOWING ACROSS MUCH OF CNTRL AND SRN WY AT
THIS TIME. PRECIP TOO WILL BE ONGOING...WITH STRATIFORM AND TERRAIN
FORCED EMBEDDED CONVECTIVE PRECIP WEST OF THE DIVIDE...AND
RELATIVELY DRY CONVECTIVE WITH LITTLE PRECIP...HIGH BASED
CLOUDS/THUNDER...AND VERY GUSTY WINDS EAST OF THE DIVIDE. THESE
SHOWERS AND THUNDERSTORMS EAST OF THE DIVIDE WILL BE OF THE LOW
CAPE HIGH UNIDIRECTIONAL SHEAR TYPE...MOVING ALONG AT A QUICK PACE
FROM THE SOUTH SOUTHWEST. AS THE AMBIENT WINDS ACROSS MUCH OF THIS
AREA WILL ALREADY BE GUSTING 3O+KTS AT TIMES...THERE COULD BE AN
ISOLATED SVR GUST FROM TIME TO TIME DURING THE EARLY SATURDAY
EVENING PERIOD. HAIL SHOULD BE VERY SMALL GRAUPEL TYPE...IF AT
ALL. BEST CHC FOR SIGNIFICANT PRECIP WILL OCCUR ACROSS NW WY WITH
SNOW LEVELS STARTING OUT ABOVE 9.5K FT AND FALLING TO THE VALLEY
FLOORS BY/AFTER MIDNIGHT. AN INCH TO 4 PLUS INCHES CAN BE EXPECTED
ACROSS THE YELLOWSTONE AREA AND PORTIONS OF THE NORTHERN ABSAROKAS
BY THRU SUNDAY MORNING.

SUNDAY...NOW FULLY MATURE CYCLONE WILL BE LOCATED ACROSS WRN SD IN
THE MORNING...TILTED TO THE WEST WITH THE UPR PORTION OVR ERN WY.
WHILE GULF MOISTURE WILL BE OF LITTLE CONSEQUENCE IN THIS
CASE...THERE WILL BE SOME ENHANCEMENT IN THE RETURN MOISTURE OUT OF
THE NRN PLAINS/UPR MIDWEST THAT WAS REINFORCED SLIGHTLY...EARLIER ON
SATURDAY...ADDING TO EPAC MOISTURE ALREADY IN PLACE ACROSS THE
NRN/NW STATES AND SRN CA PROVINCES. COLD FRONT SHOULD BE LOCATED
FROM THE SFC LOW OVR SD...SW INTO ERN WY...THEN SW INTO NRN CO.
PRECIP AXIS WILL SHIFT TO THIS AREA E OF THE DIVIDE...AT THE FRONT
AS WELL AS POST FRONTAL. GOOD UPSLOPE FLOW WILL MOSTLY BE LIMITED TO
JOHNSON COUNTY AND POINTS NORTH...WITH VEERING PROFILES TO THE SOUTH
AND WEST OF THIS AREA THRU MUCH OF THE MORNING...WITH BACKING
PROFILES AND CAA IN THE AFTERNOON AND EVENING AS ANOTHER FRONT MOVES
THRU CNTRL WY FROM THE NORTH BEFORE STALLING OUT FOR A WHILE. PRECIP
WILL BE MORE SPREAD OUT SUNDAY AS A WHOLE...BUT THE MOST SIGNIFICANT
AMOUNTS WILL BE FOUND ACROSS THE ERN FA...JOHNSON AND NATRONA
COUNTIES SUNDAY MORNING AND AFTERNOON. ATTM...SNOWFALL LOOKS TO BE
LIMITED TO THE SRN AND CNTRL FA...INCLUDING MAINLY SWEETWATER...SRN
FREMONT...AND NATRONA COUNTIES.  IT SHOULD BE PRIMARILY TOO WARM
ACROSS JOHNSON COUNTY FOR MUCH SNOW. SNOW ACCUM SUNDAY WILL
GENERALLY RANGE FROM A TRACE TO PERHAPS AN INCH AND A HALF ACROSS
THE LOWER ELEVATIONS OF CNTRL AND SRN WY...WITH FROM A HALF INCH TO
PERHAPS 4 INCHES OR SO ACROSS THE MOUNTAINS EAST OF THE DIVIDE.
SYSTEM EXITS TO THE E INTO THE PLAINS SUNDAY NIGHT THRU MONDAY
MORNING AHEAD OF THE NEXT APPROACHING SPRING SYSTEM FROM THE WEST.

MONDAY AFTERNOON THRU FRIDAY...ANOTHER NEGATIVELY TILTED TROF WITH
DEVELOPING CLOSED TO CUT-OFF LOW MOVES OVR THE GREAT BASIN...WITH
INCREASING DIFLUENT FLOW SPREADING OVR WY INCREASE LIFT. A
DEVELOPING SFC LOW ACROSS NRN CO/SWRN WY REGION WILL ALSO AID IN
ALLOWING SHOWERS TO RAPIDLY DEVELOP OVR MUCH OF THE FA...WITH BEST/
HEAVIEST PRECIP POTENTIAL LIKELY IN THE WARM SECTOR BETWEEN
DEVELOPING COLD FRONT OUT WEST AND NEAR A (PSEUDO) WARM FRONT
LIFTING OVR CNTRL/ERN WY MONDAY EVEN IN/NIGHT. THEN...THRU AT LEAST
WED MORNING...THIS SYSTEM...WITH AN ASSOCIATED STRONG JET...WILL
CONTINUE ACROSS THE CNTRL/SRN ROCKIES...INDUCING A LEE SIDE TROF IN
WY WHILE INITIATING STRONG LEE SIDE CYCLOGENESIS TUESDAY OVR SERN
CO/NERN NM. THIS WILL INITIATE A SVR WX OUTBREAK ACROSS THE
SRN/CNTRL PLAINS REGION...WHILE PULLING ANOTHER COLD FRONT THRU WY
WITH INCREASING PVA OVERHEAD MOVING THRU WRN/NRN WY. THIS WILL KEEP
PRECIP CHCS GOOD BOTH AHEAD OF AND POST FRONTAL THRU AT LEAST WED
MORNING. IT MAY BE COLD ENOUGH TO SNOW ACROSS THE NRN FA WED
MORNING. THEN A BRIEF PERIOD OF SHORTWAVE RIDGING
OCCURS...STABILIZING THE AIRMASS AND DIVERTING MOISTURE TO THE N OF
WY UNTIL FRIDAY MORNING...PLUS OR MINUS IN THE TIMING. THE LAST DAY
OF THE FORECAST PERIOD IS ALSO WHEN THE MDLS BEGIN TO REALLY DIVERGE
IN SOLUTIONS BOTH TEMPORALLY AND...SPATIALLY...AND TO A LESSER
EXTENT DYNAMICALLY. HOWEVER...THE CURRENT TREND REMAINS SIMILAR
BETWEEN MDLS IN THAT MOISTURE AND INCREASING INSTABILITY WILL RETURN
TO THE FA...PROVIDING ANOTHER POSSIBLE GOOD ROUND OF PRECIP
FRIDAY/FRIDAY NIGHT.

AVIATION.../00Z ISSUANCE/

VFR CONDITIONS WILL PREVAIL AT THE AIRPORTS THROUGH THE PERIOD.
SOUTH TO SOUTHWEST SFC WINDS WILL INCREASE TO 15-20KTS ACROSS
MUCH OF THE AREA THIS AFTERNOON. ISOLD TO WDLY SCT SHRA/TSRA ARE
EXPECTED 00Z 06Z IN THE VCNTY OF THE DIVIDE...MAINLY ALONG A
VCNTY KJAC-KCOD ROUTE AND ALSO ISOLD TSRA/SHRA MAY DEVELOP OFF
THE UINTA RANGE AND MOVE NORTH BTWN KEVW-KRKS. MOST OF THIS
ACTIVITY WILL DISSIPATE BY 03Z. AN INCREASING SOUTHERLY FLOW
AHEAD OF AN APPROACHING COLD FRONT WILL BRING AREAS OF LLWS VCNTY
AND WEST OF THE DIVIDE 08Z- 12Z SATURDAY. CLOUDS AND SCATTERED
RAIN AND SNOW SHOWERS WILL INCREASE ACROSS THE WEST.  18Z - 00Z
TO THUNDERSTORMS WILL INCREASE IN THE WEST AND SPREAD EAST.
WESTERN MOUNTAINS WILL BECOME MOSTLY OBSCURED WITH FOG...RAIN AND
SNOW.

FIRE WEATHER...

AS A LARGE LOW PRESSURE TROUGH MOVES INLAND ALONG THE WEST
COAST...INCREASING SOUTHWEST FLOW WILL BRING PACIFIC MOISTURE
AND INCREASING INSTABILITY INTO WESTERN WYOMING.  SHOWERS ARE
EXPECTED EARLY THIS EVENING THEN TAPER OFF BEFORE A STRONGER SURGE
OF MOISTURE MOVES IN EARLY IN THE MORNING. THE WEST WILL THEN SEE
WIDESPREAD SHOWERS BY MID DAY WITH EMBEDDED THUNDERSTORMS IN THE
EARLY AFTERNOON. THIS MOISTURE AND INSTABILITY WILL SPREAD EAST
THROUGH THE AFTERNOON...FIGHTING DOWNSLOPE WINDS TO HOLD TOGETHER
IN THE BIGHORN AND WIND RIVER BASINS. WINDY CONDITIONS WILL BE
PREVALENT...ADDING TO THE GUST POTENTIAL FROM THUNDERSTORMS.
TEMPERATURES WILL FALL FROM WEST TO EAST AS CLOUDS AND
PRECIPITATION SPREAD EAST. THE EAST WILL BE DRY AND WARM INITIALLY
IN JOHNSON AND NATRONA COUNTY WITH THE WARMEST HIGHS EXPECTED.
BEFORE THE MOISTURE MOVES IN ...MIXING HEIGHTS AND WIND WILL
COMBINE FOR VERY GOOD TO EXCELLENT SMOKE DISPERSAL. AFTER THE
PRECIPITATION MOVES IN THE SMOKE DISPERSAL WILL DROP TO FAIR.

&&

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

$$

SHORT TERM...C.BAKER
LONG TERM...BRAUN
AVIATION...AEM
FIRE WEATHER...C.BAKER



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