Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 230940
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
240 AM MST FRI JAN 23 2015

.SHORT TERM...TODAY THROUGH SUNDAY NIGHT

THE OVERALL PATTERN FOR THE SHORT TERM PERIOD KEEPS A DIRTY RIDGE TO
OUR WEST WITH WYOMING IN THE PATH OF SEVERAL WEAK WAVES RIDING THE
PERIPHERY.

THE FIRST SHORT WAVE IS ALREADY MOVING INTO FORECAST AREA THIS
MORNING. TYPICAL ISENTROPIC SIGNATURES ARE NOT TOO STRONG WITH THIS
FIRST WAVE...SPECIFIC HUMIDITY IS HIGH BUT THE UPGLIDE PATTERN IS
NOT PARTICULARLY STRONG AS THE FLOW IS MORE PARALLEL TO THE
ISENTROPES THIS MORNING. THERE IS HOWEVER SOME QG FORCING AT THE
FRONT OF THE WAVE THAT COULD STIR SOMETHING UP. LOOKING UPSTREAM
THERE IS NOT A LOT SHOWING UP ON THE MONTANA OR IDAHO RADARS. WENT
AHEAD AND INCREASED POPS A BIT ACROSS CENTRAL WYOMING AS THERE COULD
BE SOMETHING THAT MAKES IT TO THE GROUND...WITH SLIGHT CHANCE POPS
IN THE MOUNTAINS WITH SOME HIGHER POPS IN JOHNSON COUNTY WHERE THERE
ACTUALLY IS A WEAK ISENTROPIC UPGLIDE SIGNATURE THIS MORNING. THIS
IS PROBABLY PRETTY GENEROUS GIVEN HOW LITTLE THIS WAVE HAS GOING FOR
IT. OVERALL THIS DISTURBANCE SHOULD RESULT IN LITTLE MORE THAN SOME
OVERCAST SKIES FOR MOST. THERE WILL BE SOME CLEARING BEHIND THE WAVE
WITH ANOTHER MOSTLY CLEAR AND MOSTLY NIGHT EXPECTED TONIGHT...ALONG
WITH WAA AND POTENTIALLY MORE BASIN FOG FOR SATURDAY MORNING. KEPT
THE FOG OUT OF THE GRIDS FOR TONIGHT AS THE SURFACE GRADIENT MAY BE
ENOUGH TO KEEP THINGS FROM GETTING TOO STAGNANT BUT WOULD NOT BE
SURPRISED TO SEE SOME RIVER VALLEYS FOG IN BRIEFLY.

THE NEXT WAVE IS ON THE DOORSTEP FOR SATURDAY WITH FLOW SHIFTING
NORTHWEST. THERE WILL NOT BE MUCH OF A COLD POOL...NO QG FORCING TO
SPEAK OF WITH THIS ONE EITHER. HOWEVER THERE IS A MUCH BETTER
ISENTROPIC UPGLIDE SIGNATURE WITH THIS ONE IN ADDITION TO A LITTLE
HELP FROM THE LEFT EXIT REGION OF THE JET. THERE COULD BE A FEW
PERIODS OF RAIN AND SNOW BUT THIS IS NOT EXPECTED TO BE A HIGH
IMPACT SYSTEM IN MOST AREAS. THERE IS SOME CONCERN OF A BRIEF WIND
EVENT AROUND BUFFALO AND PINEY CREEK AS FLOW ALIGNS TO THAT SWEET
SPOT FOR THIS AREA...A FEW WIND GUSTS COULD EXCEED 55MPH ON SATURDAY
AFTERNOON...700MB FLOW IS DEFINITELY STRONGER TO THE NORTH AND
EAST BUT WITH THE JET MOVING OVERHEAD THIS SITUATION WILL BE WATCHED
TO SEE IF THAT STRONGER FLOW TRIES TO MIGRATE WEST OR IF THE WRONG
SIDE OF THE JET MOVES INTO POSITION AND BEGINS TO ENACT MORE
DOWNWARD FORCING. OTHERWISE...LEE SIDE TROUGHING HOLDS AT THE
SURFACE WITH DECENT MIXING OF MILD MID LEVEL TEMPERATURES EXPECTED
DURING THE DAY ON SATURDAY. MANY LOCATIONS EAST OF THE DIVIDE WILL
BE ABOVE FREEZING DURING THE DAY ON SATURDAY WITH MANY AREAS TOPPING
OUT INTO THE MID TO UPPER 40S.

THINGS BEGIN TO CLEAR OUT ON SUNDAY AS THE RIDGE AXIS TO OUR WEST
BEGINS TO MIGRATE EASTWARD. IT WILL BRING IN SOME VERY MILD AIR FOR
THIS TIME OF YEAR...AROUND +4C AT H7 INTO THE WEST BY SUNDAY
EVENING...WHICH IS IN THE 99TH PERCENTILE FOR THIS TIME OF
YEAR...ABOUT 10 DEGREES ABOVE NORMAL. A DECENT GRADIENT WILL REMAIN
THROUGH THE DAY AT THE SURFACE AS THE HIGH TRIES TO MOVE EAST...THIS
WILL KEEP THINGS MIXED IN THE MORE WIND PRONE AREAS AND WILL LIKELY
LEAD TO A RATHER PLEASANT AND MILD DAY WITH SOME OF THE BREEZIER
SPOTS SOARING INTO THE LOW 50S. HOWEVER AS HIGH PRESSURE TAKES OVER
OVERNIGHT...THE WINDS WILL CALM A BIT AND THAT COMBINED WITH THE WAA
WILL LIKELY LEAD TO SOME OVERNIGHT VALLEY AND BASIN FOG LATE SUNDAY
NIGHT.

.LONG TERM...MONDAY THROUGH FRIDAY...

SYNOPSIS...UNSEASONABLE MILD AND DRY CONDITIONS UNDER A RIDGE OF
HIGH PRESSURE WILL PREVAIL OVER MOST OF THE AREA MONDAY AND
TUESDAY...THOUGH STEEP INVERSIONS WILL KEEP COLD AIR TRAPPED IN
WESTERN VALLEYS.  A PACIFIC STORM SYSTEM WILL LIFT ACROSS THE AREA
TUESDAY NIGHT AND WEDNESDAY WITH CHANCES OF RAIN AND SNOW
SHOWERS...MAINLY ACROSS THE WEST AND SOUTH.  AN UNSETTLED AND COOLER
NORTHWEST FLOW WILL FOLLOW THIS SYSTEM ON THURSDAY WITH SOME
LINGERING ISOLATED TO SCATTERED SNOW SHOWERS.

DISCUSSION...NEGATIVELY TILTED RIDGE AXIS IS FORECAST TO EXTEND FROM
THE SOUTHERN PLAINS NW INTO THE INTERMOUNTAIN WEST ON MONDAY WITH
5-6C H7 THERMAL RIDGE ACROSS WYOMING AND LIGHT WIND.  THE MAIN
FORECAST CHALLENGE WITH THIS VERTICAL PROFILE WILL BE THE STEEPNESS
OF SHALLOW INVERSIONS OVER THE CENTRAL BASINS AND FAR WEST VALLEYS.
FULL MIXING OF THIS COLUMN WOULD PUT SURFACE TEMPERATURES WELL INTO
THE 60S AT 5KFT WITH NORTHEAST JOHNSON COUNTY AND THE CODY FOOTHILLS
SET AT ONLY A FEW DEGREES BELOW FULL MIXING.  SNOW COVER WILL BE THE
BIGGEST FACTOR ON HOW WARM LOWER CENTRAL BASINS GET IN THIS PATTERN
WITH BETTER MIXING OVER THE WEEKEND LIKELY TO FURTHER ELIMINATE MUCH
OF THE OLD SNOW.  SETTING HIGHS IN THESE AREAS AROUND 20 DEGREES
BELOW FULL MIXING POTENTIAL (MOSTLY 40S FOR HIGHS) SEEMS
REASONABLE.  THE WESTERN VALLEYS WILL SEE THE STEEPEST INVERSIONS
AND THE BEST CHANCES FOR FOG FORMATION MONDAY AND TUESDAY.

BAJA UPPER LOW WILL GET KICKED INTO BACKSIDE OF RIDGE MONDAY AND
TUESDAY...LIFTING ACROSS THE GREAT BASIN BEFORE TURNING EAST ACROSS
THE CENTRAL ROCKIES TUESDAY NIGHT/WEDNESDAY.  GFS CONTINUES TO BE ON
THE FAST AND STRONGER END OF GUIDANCE WITH SYSTEM IMPACTING WYOMING
TUESDAY NIGHT AND WEDNESDAY MORNING.  ECMWF KEEPS THE MAIN TROUGH
BACK OVER THE GREAT BASIN THROUGH 12Z WEDNESDAY...SHEARING IT OUT
ACROSS THE AREA DURING THE DAY.  SLOWER SOLUTION ALMOST ALWAYS WINS
OUT IN A SW LOW KICKER/KICKEE SCENARIO.  THIS SYSTEM...GIVEN ITS
ORIGINS...WILL BE RELATIVELY WARM (SNOW LEVELS AROUND 6KFT) AND
PRECIPITATION AMOUNTS SHOULD BE LIGHT.

UPPER RIDGE IS EXPECTED TO REBUILD ALONG THE WEST COAST BEHIND THIS
SYSTEM THURSDAY AND FRIDAY PUTTING WYOMING IN NW FLOW.  SOME WEAK
RIPPLES IN THIS FLOW WILL KEEP A FEW SNOW SHOWERS IN THE MOUNTAINS
AND NORTH.  BOTH THE GFS AND ECMWF SHOW A STRONGER CLIPPER PUSHING
ACROSS NE WYOMING JUST OUTSIDE THIS PERIOD ON FRIDAY NIGHT.


&&

.AVIATION.../12Z ISSUANCE/

A QUICK MOVING SYSTEM WILL SLIDE SOUTHEAST ACROSS THE COWBOY STATE
TODAY WITH CONSIDERABLE CLOUDINESS AND ISOLATED TO LOCALLY SCATTERED
SNOW SHOWERS. MOST OF THIS ACTIVITY WILL OCCUR IN THE HIGHER
TERRAIN...BUT ENOUGH ISOLATED ACTIVITY IS EXPECTED TO PUSH INTO THE
BASIN/VALLEYS TO WARRANT VCSH AT ALL TERMINALS. CEILINGS IN THE
LOWER ELEVATIONS ARE EXPECTED TO RANGE BETWEEN 050-100 KFT AGL FOR
MUCH OF THE DAY...WITH BRIEF PERIODS OF MVFR/IFR CONDITIONS POSSIBLE
IN SNOW SHOWERS. THE EXCEPTION TO THIS RULE COULD BE THE JAC
TERMINAL THIS MORNING...AND HAVE DECIDED TO TEMPO MVFR CONDITIONS.
MOUNTAINS WILL BE OBSCURED AT TIMES. IMPROVING CONDITIONS ARE
EXPECTED FROM NORTH TO SOUTH BETWEEN 20Z AND 02Z...WITH SKIES
QUICKLY BECOMING MAINLY SKC IN MOST AREAS. NOT OUT OF THE
QUESTION...THAT THE FAR WESTERN VALLEYS COULD HANG ONTO A STRATOCU
DECK BETWEEN 025-050 KFT AGL. WHERE SNOWFALL DOES OCCUR...THERE WILL
BE AN INCREASED THREAT OF SOME FOG DEVELOPING. HOWEVER A LIMITING
FACTOR WILL BE INCREASING MID/HIGH LEVEL CLOUDINESS AFTER 06Z AS
ANOTHER FAST MOVING SYSTEM APPROACHES THE REGION. THE BASINS AND
VALLEYS WILL SEE FAIRLY LIGHT WINDS THROUGH THE PERIOD WHILE THE LEE
SIDE OF MOUNTAINS...FOOTHILLS AND WIND CORRIDOR FROM KRKS TO KCPR
WILL BE BREEZY TO WINDY AT TIMES.

&&

.FIRE WEATHER...

HIGH PRESSURE LOOMS TO THE WEST WHILE A FEW WEAK SYSTEMS RIPPLE
AROUND THE HIGH TODAY AND SATURDAY. THEY WILL HAVE MINOR IMPACT ON
THE AREA WITH SOME INCREASING CLOUDS AND SOME BREEZES IN MOST AREAS.
THE MOUNTAINS MAY SEE SOME SNOW WITH A VERY SMALL CHANCE OF A RAIN
OR SNOW SHOWER IN THE LOW ELEVATIONS OF NORTHERN CODY DISPATCH AND
CASPER DISPATCH. TEMPERATURES WILL BE MILD THROUGH THE PERIOD WITH A
WARMING TREND EXPECTED TO LAST INTO NEXT WEEK. WE STILL CAN NOT GET
MIXING HEIGHTS UP TOO MUCH...ESPECIALLY IN LESS WIND PRONE AREAS.
HOWEVER THERE WILL BE SOME HIGH END POOR TO FAIR SMOKE DISPERSAL
POSSIBLE WITH EACH WAVE...EVEN IN THE LESS WINDY AREAS.

&&

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

$$

SHORT TERM...ALLEN
LONG TERM...AEM
AVIATION...WM
FIRE WEATHER...ALLEN






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