Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 160903
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
303 AM MDT THU APR 16 2015

.SHORT TERM...THURSDAY THROUGH SATURDAY NIGHT

ALL CONCERNS IN THE SHORT RANGE FOCUS AROUND WHAT WILL HAPPEN WITH
OUR UPPER LEVEL DEVIL...AKA THE UPPER LEVEL LOW THAT BROUGHT THE
HEAVY SNOW TO PORTIONS OF THE AREA. AT THIS TIME...SATELLITE IMAGERY
SHOWS IT DROPPING SOUTHWARD THROUGH UTAH. A SPOKE OF ENERGY IS
BRINGING SOME MAINLY LIGHT SNOW TO PORTIONS OF THE AREA.
HOWEVER...THERE IS SOME DRIER AIR TRYING TO PUNCH INTO EASTERN
PORTIONS OF THE AREA. WITH THAT...WE WILL CANCEL THE WARNING FOR
NATRONA COUNTY AT ISSUANCE. IN THE OTHER AREAS WITH HIGHLIGHTS...WE
WILL LET THEM RIDE UNTIL EXPIRATION MORE THAN LIKELY. MOST OF
TODAY CONDITIONS SHOULD SOMEWHAT IMPROVE AS THE LOW CONTINUES TO
DRIFT SOUTHWARD AND DRIER AIR TRIES TO PUNCH IN FROM THE NORTH.
MOST SHOWERS SHOULD RETREAT TO AROUND THE WIND RIVER RANGE TO
CASPER MOUNTAIN THIS AFTERNOON. SOME SUNSHINE COULD RETURN TO THE
NORTH WITH WARMER TEMPERATURES. HOWEVER...THE REST OF THE AREA
WILL SEE CLOUDS AND A FEW SHOWERS LINGER. ROAD PROBLEMS SHOULD BE
MINIMAL THOUGH...AS THE STRONG APRIL SUN SHOULD MELT ANY SNOW ON
CONTACT. THE NEXT QUESTION IS FOR TONIGHT AS ANOTHER SURGE OF
MOISTURE TRIES TO MOVE NORTHWARD. THE QUESTION IS HOW FAR NORTH
WILL IT GO. AND...LIKE YESTERDAY THERE ARE STILL SOME DIFFERENCES.
THE BEST GUESS AT THIS POINT IS MOST STAYS SOUTH OF A RIVERTON TO
CASPER LINE. CONTINUITY HAD THIS REPRESENTED FAIRLY WELL SO WE
MADE FEW CHANGES. AS FOR HIGHLIGHTS...WITH THE CURRENT ONES STILL
GOING WE WILL LET THE DAY SHIFT MAKE THAT DECISION.
MEANWHILE...DRY WEATHER WILL BE FOUND ACROSS THE NORTHWESTERN HALF
OF THE COUNTY WARNING AREA.

ANOTHER PROBLEM IS FRIDAY AS THE LOW TURNS A BIT TO THE EAST AND
THEN NORTHEAST. AT THE SAME TIME...A BIT OF JET ENERGY WILL ROUND
THE BASE OF IT AND HEAD TOWARD THE SOUTHERN AND EASTERN PORTIONS
OF THE COUNTY WARNING AREA. BOTH THE NAM AND GFS TRY AGAIN TO
BRING A SURGE OF MOISTURE NORTHWARD...THOUGH AGAIN...THE EXACT
NORTHERN EXTENT OF IT IS RATHER UNCERTAIN AT THIS TIME. ONE BIG
DIFFERENCE WITH THIS SURGE IS THAT THERE WILL BE SOME WARMER AIR
WITH THIS SURGE WITH 700 MILLIBAR TEMPERATURES RISING TO AROUND
MINUS 1 TO MINUS 2 CELSIUS. THIS WOULD RAISE SNOW LEVELS TO
BETWEEN 7500 AND 8000 FEET SO MUCH OF THE PRECIPITATION ON THIS
DAY WOULD FALL AS RAIN EXCEPT IN THE MOUNTAINS.

THERE IS MORE UNCERTAINTY FOR SATURDAY AS WELL. A SYSTEM MOVING IN
FROM THE NORTHWEST LOOKS TO FINALLY BE ABLE TO KICK THE UPPER LEVEL
LOW AWAY TO THE NORTH AND EAST. HOWEVER...THE NAM AND GFS SHOW A
BAND OF PRECIPITATION ACROSS THE AREA. AND...OF COURSE...THERE ARE
DIFFERENT LOCATIONS OF WHERE IT WILL SET UP. AT THIS POINT WE MADE
FEW CHANGES. THE SYSTEM MOVING IN FROM THE WEST LOOKS TO BRING SOME
MORE PRECIPITATION TO THE WEST AS WELL BUT AT THIS POINT AMOUNTS ARE
UNCERTAIN.

.LONG TERM...SUNDAY THROUGH THURSDAY NIGHT

THE PATTERN IN THE LONG TERM REMAINS SOMEWHAT CONVOLUTED AS AN
UPSTREAM RIDGE CONTINUES TO BUILD OVER THE NORTHEAST AND EASTERN
CANADA SUNDAY INTO TUESDAY. AS IT DOES SO...AN UPPER LEVEL LOW
MOVING SOUTHWARD FROM THE YUKON TERRITORY...WILL MERGE WITH THE
SYSTEM THAT IS CURRENTLY GENERATING THE RECENT PRECIP OVER THE
CENTRAL ROCKIES. THIS WILL CAUSE THE LOW TO DEEPEN AS IT MAKES ITS
WAY OVER THE UPPER MIDWEST. THIS...IN TURN...WILL GENERATE A
NORTHWEST FLOW PATTERN OVER THE NORTHERN ROCKIES. A COLD FRONT
ASSOCIATED WITH THE CANADIAN SYSTEM WILL MOVE THROUGH THE AREA ON
SUNDAY...WHICH WILL KEEP A SLIGHT CHANCE FOR RAIN AND SNOW SHOWERS
ACROSS MUCH OF THE FORECAST AREA. THIS ACTIVITY WILL BECOME MORE
ISOLATED BY MONDAY AND TUESDAY.

THE LAST PORTIONS OF THE FORECAST PERIOD REMAIN UNCERTAIN...AS A NEW
PACIFIC STORM SYSTEM BEGINS TO MAKES ITS WAY ONSHORE OVER NORTHERN
CALIFORNIA. MODELS ARE TRENDING FOR THIS SYSTEM TO CUT OFF AS IT
DOES SO. AS A RESULT...WEAK SHORTWAVES ROTATING AROUND THIS SYSTEM
COULD MAKE THEIR WAY OVER THE FORECAST AREA FROM THE SOUTHWEST. THE
PROBLEM IS THAT THE COWBOY STATE WILL BE SQUEEZED BETWEEN THIS NEW
LOW AND THE AFOREMENTIONED NORTHWEST FLOW AND WHICH ONE WILL HAVE
THE STRONGER INFLUENCE. AT THIS TIME...LEFT A SLIGHT CHANCE FOR
RAIN/SNOW SHOWERS WEDNESDAY NIGHT...CHANGING TO RAIN DURING THE DAY
THURSDAY.

&&

.AVIATION.../12Z ISSUANCE/

EAST OF THE DIVIDE...KCOD/KCPR/KLND/KRIW/KWRL ROUTES

WIDESPREAD MVFR AND AREAS OF IFR CONDITIONS IN -SN WILL CONTINUE TO
PREVAIL VICINITY AND SOUTH OF A KCPR-KRIW LINE AND ALONG THE EAST
SLOPES OF THE DIVIDE...VICINITY KCOD-KLND...UNTIL AROUND 12Z. A DRIER
AIRMASS WILL ROTATE NW ACROSS THE AREA BETWEEN 12Z AND 16Z RESULTING
IN CEILINGS GRADUALLY LIFTING AND MOST -SN ENDING. ANOTHER BATCH OF
RAIN AND SNOW SHOWERS MAY ROTATE WESTWARD ACROSS SOUTHERN WYOMING LATE
THURSDAY AFTERNOON AND THURSDAY EVENING...WITH MOST OF THIS
ACTIVITY EXPECTED ALONG AND SOUTH OF A KCPR-KLND LINE THROUGH 12Z
FRIDAY.

WEST OF THE DIVIDE...KBPI/KJAC/KPNA/KRKS ROUTES

VFR CONDITIONS WILL PREVAIL VICINITY AND NORTH OF KJAC WHERE A N-NE
DOWNSLOPE WIND WILL PERSIST THROUGH THE PERIOD.  OCCASIONAL MFVR/IFR
CONDITIONS IN -SN WILL OCCUR ACROSS THE SW QUARTER OF WY...VICINITY AND
SOUTH OF KBPI...THROUGH THE PERIOD.  THE SNOW MAY BECOME MIXED WITH
RAIN THURSDAY AFTERNOON AND EVENING.  GUSTY NE SURFACE WINDS AT 25-35KTS
WILL CONTINUE ACROSS THE INTERSTATE 80 CORRIDOR...VICINITY KRKS.
&&

.FIRE WEATHER...

UNSETTLED WEATHER WILL CONTINUE MAINLY SOUTH OF A RIVERTON TO NORTH
OF CASPER LINE AS AN UPPER LEVEL LOW KEEPS CLOUDS AS WELL AS SOME
RAIN AND SNOW SHOWERS AROUND TODAY AND EVEN INTO FRIDAY AS WELL.
THIS MEANS A MAINLY CLOUDY SKY CONTINUED COOL TEMPERATURES. FURTHER
NORTH AND WEST...EXPECT MORE SUNSHINE AND WARMER TEMPERATURES ALONG
WITH LOWER HUMIDITY. A GUSTY BREEZE MAY BLOW AT TIMES TODAY...BUT
RELATIVE HUMIDITY WILL REMAIN WELL ABOVE CRITICAL LEVELS TODAY.
&&

.RIW WATCHES/WARNINGS/ADVISORIES...
WINTER WEATHER ADVISORY UNTIL 6 AM MDT EARLY THIS MORNING
WYZ008-009-028-030.

WINTER STORM WARNING UNTIL 6 AM MDT EARLY THIS MORNING
WYZ014-015-018-019.

&&

$$

SHORT TERM...HATTINGS
LONG TERM...LAVOIE
AVIATION...AEM
FIRE WEATHER...HATTINGS









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