Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 131750
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
1150 AM MDT MON APR 13 2015

.SHORT TERM...TODAY THROUGH WEDNESDAY NIGHT

TODAY WILL FEATURE A PLEASANT SPRING DAY AS A FLAT TRANSIENT
RIDGE OF HIGH PRESSURE MOVES IN FROM THE WEST ALONG WITH WARMER
TEMPS. THEN ON TUESDAY WINDS WILL PICK UP AS THE PRESSURE GRADIENT
INCREASES OUT AHEAD OF AN APPROACHING TROUGH NOW OFF THE PACIFIC
COAST AT 140W. THIS WEATHER FEATURE WILL BE STRONG ENOUGH WITH A
CORRESPONDING INCREASING PRESSURE GRADIENT THAT WILL BE ENOUGH TO
POTENTIALLY INCREASE THE WINDS TO HIGH WIND CRITERIA ACROSS THE
SOUTHERN THIRD OF THE CWA TUESDAY AS ADVERTISED BY NAM MOS
GUIDANCE AND THE LOCAL WRF. DECIDED TO ISSUE A HIGH WIND WATCH FOR
SWEETWATER AND SOUTHERN LINCOLN COUNTIES AS WELL AS THE GREEN MTNS
AND THE EAST SLOPE OF THE WIND RIVER MTN FROM 15Z TUE THROUGH 00Z
WED IN SPITE OF THE FACT THAT THE SFC PRESSURE GRADIENT IS NOT
IMPRESSIVE AND NEITHER ARE THE NUMBER OF H7 DECIMETER LINES. IN
ADDITION...THESE SOUTHWEST WINDS WILL BE USHERING IN AN RH OF 10
TO 15 PERCENT FOR MUCH OF THE LOWER ELEVATIONS EAST OF THE DIVIDE.
WENT AHEAD AND ISSUED A FIRE WEATHER WATCH FROM 19Z TUE THROUGH
02Z WED FOR THESE AREAS THAT PRESENTLY HAVE CRITICAL FUELS.

MEANWHILE...IN WESTERN WY...RAIN SHOWERS AND HIGH ELEVATIONS SNOW
SHOWERS WILL OCCUR AS THE TROUGH TRACKS EAST ACROSS THE PAC NW AND
GREAT BASIN. CANT RULE OUT SOME ISOLD THUNDER TUE AFT/EVE WITH
NEGATIVE LIFTED INDICES AND SOME WEAK CAPE IN N WY.

ON TUE NIGHT...A LOW WITHIN THE THE TROUGH WILL CUT OFF OVER EASTERN
IDAHO. THIS PARTICULAR CIRCULATION/COLD POOL IS ORIGINATING OFF THE
COAST OF ALASKA WHERE PRESENT H5 TEMPS ARE -40C. SO AS THIS WEATHER
COMPLEX APPROACHES THE AREA...RAIN WILL TURN TO SNOW ACROSS THE
ENTIRE CWA TUE NT AS THE PRECIP SPREADS TO AREAS EAST OF THE DIVIDE
AS THE H5 CLOSED CIRCULATION TRACKS SOUTH SOUTHEAST ALONG THE
IDAHO/WY BORDER ACDG TO THE GFS. THE ECMWF TRACKS IT FARTHER SW
ACROSS THE SALT FLATS. QG FORCING WILL FOLLOW THE TRACK OF THE
LOW. IN ADDITION TO THE SNOW...POST FRONTAL WINDS WILL INCREASE
FROM THE NNW AS 9MB 3 HOURLY SFC PRESSURE RISES TAKE PLACE
RESULTING IN THE POTENTIAL FOR LOW VSBYS...DRIFTING AND BLOWING
SNOW MANY AREAS. WEDNESDAY APPEARS TO BE THE SNOWIEST PERIOD IN
ASCTN WITH THIS WX SYSTEM GIVEN THE ISENTROPIC LIFT WRAPPING
AROUND THE BACK SIDE OF THE LOW AND THE QG FORCING ASSOCIATED WITH
THE LOW ITSELF. IN ADDITION...THE TRACK OF THE LOW FAVORS
SUBSTANTIAL SNOWFALL ON THE EAST SLOPE OF THE WIND RVR MTNS AS
WELL AS THE ADJACENT LANDER FOOTHILLS SO PAINTED MORE TOTAL
SNOWFALL IN THOSE AREAS. MOST AREAS WILL SEE SNOW ALL DAY WED. THE
ABOVE MENTIONED DYNAMICS WILL SEED THE RELATIVELY SHORT LIVED
UPSLOPE FLOW EAST OF THE DIVIDE WED AFT/EVE. THE SNOWFALL WILL END
FROM THE NW LATER WED NIGHT. AS FAR AS SNOW TOTALS GO...WENT
MAINLY WITH 4 TO 6 INCHES ACROSS THE MTN ZONES EXCEPT WENT 6 TO 10
INCHES FOR NOW ON THE EAST SLOPE OF THE WIND RIVER MTNS BECAUSE OF
THE AFOREMENTIONED FAVORED TRACK OF THE H5 LOW. HAVE INDICATED 1
TO 2 INCHES IN THE WESTERN VALLEYS AND CENTRAL AREAS EXCEPT 6 INCHES
IN LANDER. HOWEVER...DECIDED TO GO WITH 6 TO 12 INCHES IN THE BIG
HORN MTNS WHERE THE MID LEVEL CIRCULATION WILL DEVELOP.

.LONG TERM...THURSDAY THROUGH MONDAY

LARGE UPPER LOW WILL BE DIGGING SOUTH OF THE 4-CORNERS REGION AT THE
START OF THIS PERIOD WITH MOST OF THE PCPN LONG GONE. ONE EXCEPTION
MAY BE IN THE FAR S OR SERN ZONES SO KEEPING SOME POPS IN THESE
AREAS THURSDAY MORNING YET. OTHERWISE...A CHILLY START TO THE DAY
AND A COOL AFTERNOON FOR MOST AREAS. NICE COUPLE DAYS THEN FRIDAY
AND SATURDAY WITH BUILDING HEIGHTS AND GENERALLY LIGHT FLOW ALOFT
WITH LIMITED SFC PRESSURE GRADIENT. WHAT HAPPENS LATE IN THE WEEKEND
INTO MONDAY IS THE BIGGER CHALLENGE. THE LARGE UPPER LOW WILL LIKELY
STAY FAR ENOUGH SOUTH TO HAVE LIMITED IF ANY INFLUENCE AFTER
THURSDAY EVEN THOUGH THE ECMWF WANTS TO LIFT IT NE AHEAD OF IT`S
SUNDAY SYSTEM. THAT SUNDAY SYSTEM IS THE BIG QUESTION MARK. THE GFS
REMAINS WITH A GLANCING BLOW SHORTWAVE WELL OFF TO THE NE WITH SOME
WEAK RIPPLES IN DEVELOPING NW FLOW FOR POSSIBLE LATE DAY CONVECTION
SUNDAY AND MONDAY. WEAK SURFACE FRONT MAY HELP FOCUS SOME
SHOWERS/ISOLD TSTMS IN THIS SCENARIO. THE 00Z ECMWF IS A WHOLE OTHER
STORY. THE NEW RUN HAS A STRONG SWD MOVING TROUGH THAT TURNS INTO A
NEARLY CLOSED MID LEVEL LOW OVER WYO COUNTRY DURING THE DAY SUNDAY
WITH POTENTIAL STORM STORM ALONG AND EAST OF THE DIVIDE. MONDAY
WOULD BE COLD AND UNSTABLE WITH SNOW SHOWERS IN ROUGHLY THE SAME
AREA. LOOKING AT THE H5 HEMISPHERIC CHARTS SHOWS A SIMILAR PATTERN
OUT THROUGH ABOUT DAY 4 BEFORE DIVERGENCE. THEY BOTH HAVE A STRONG
TROUGH PUMPING UP A RIDGE ALONG THE COAST BUT FOR SOME REASON THE
GFS IS SLOW TO ALLOW THIS DISTURBANCE MOVING THROUGH THE FLOW TO DIG
SWD. THE ECMWF SCENARIO OR AT LEAST A STRONGER S TO SEWD MOVING
TROUGH SEEMS MORE PLAUSIBLE BASED ON THE LONGWAVE PATTERN. HARD TO
BITE WHOLEHEARTEDLY INTO THIS SCENARIO AS THIS WOULD REQUIRE
LOWERING HIGHS INTO THE 30S AT BEST SUNDAY AND GETTING EVERYONE ELSE
ON BOARD. NUDGING FORECAST IN THIS DIRECTION TONIGHT AND WILL NEED
TO WATCH THIS EVOLUTION VERY CLOSELY AND SEE HOW IT LOOKS AGAIN
TONIGHT. THE GEM IS STRONGER AND FURTHER WEST ABOUT HALF WAY IN THE
MIDDLE BETWEEN THE ECMWF AND GFS. THE ECMWF ENSEMBLES SUPPORT A
STRONGER TROUGH THAN THE OP GFS ALSO. SOME GFS ENSEMBLE MEMBERS ALSO
SUPPORT MORE TROUGHING FURTHER WEST. WE WILL DEFINITELY NEED TO
WATCH THE LATTER PART OF THIS FORECAST FOR THE POTENTIAL FOR ANOTHER
MODERATE TO STRONG SEWD DIGGING TROUGH IN THE SUN/MON TIMEFRAME.

.AVIATION.../18Z ISSUANCE/

VFR CONDITIONS...UNLIMITED CIGS...WILL PREVAIL THROUGH 18Z TUESDAY.
S-SW SFC WINDS AROUND 10KTS WILL PREVAIL THIS AFTERNOON WITH
SOUTH SOUTHWEST SFC WINDS INCREASING TO 20 TO 30 LATE TUESDAY
MORNING OVER AND NEAR THE EAST SLOPES OF THE MOUNTAINS AND THROUGH THE
WIND CORRIDOR FROM ROCK SPRINGS TO CASPER.

.FIRE WEATHER...

..FIRE WEATHER WATCH FOR PORTIONS OF NORTHERN AND CENTRAL WYOMING
EAST OF THE DIVIDE FOR TUESDAY AFTERNOON AND EARLY EVENING...

TODAY WILL BE DRY AND WARMER WITH LIGHT WIND SPEEDS. TONIGHT WILL
BE DRY WITH LIGHT WIND. TUESDAY WILL BE WARMER WITH WINDY
CONDITIONS DEVELOPING IN THE AFTERNOON WITH LOW RELATIVE HUMIDITY
ACROSS THE SOUTH AND AREAS EAST OF THE CONTINENTAL DIVIDE. THIS
WILL INCREASE THE FIRE DANGER IN PARTS OF THE REGION EAST OF THE
CONTINENTAL DIVIDE. THE FAR WEST WILL SEE SOME RAIN AND HIGH
MOUNTAIN SNOW SHOWERS TUESDAY AFTERNOON. THE PRECIPITATION WILL
SPREAD TO AREAS EAST OF THE DIVIDE TUESDAY NIGHT AND TURN TO
MOSTLY ALL SNOW. MOUNTAIN SNOW AND LOW ELEVATION RAIN AND SNOW
WILL CONTINUE WEDNESDAY INTO WEDNESDAY EVENING. EXPECT SIGNIFICANT
SNOW ACCUMULATION IN MANY AREAS ALONG WITH SIGNIFICANT MOISTURE IN
ALMOST ALL AREAS.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
HIGH WIND WATCH FROM TUESDAY MORNING THROUGH TUESDAY AFTERNOON
WYZ015-019-027>030.

FIRE WEATHER WATCH FROM TUESDAY AFTERNOON THROUGH TUESDAY
EVENING WYZ275-276-280>283.

&&

$$

SHORT TERM...LIPSON
LONG TERM...SKRBAC
AVIATION...C.BAKER
FIRE WEATHER...LIPSON






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