Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 240942
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
342 AM MDT THU APR 24 2014

.SHORT TERM...TODAY THROUGH SATURDAY NIGHT

AN UPPER LOW CONTINUES TO MOVE ACROSS SOUTHERN CANADA...THE SOUTHERN
REACHES OF AN ATTENDANT SHORT WAVE WILL GRAZE THE NORTHERN ZONES
EARLY...BUT THE RADAR LOOKS PRETTY QUIET SO FAR...IT IS DOUBTFUL
THAT THIS WAVE WILL HAVE MUCH IMPACT. FLAT RIDGING BUILDS IN QUICKLY
BEHIND THIS WAVE THIS MORNING WITH WARM AND DRY CONDITIONS EXPECTED
TO HOLD FOR MOST OF THE FORECAST AREA...WITH SOME MOISTURE ALREADY
ARRIVING IN THE FAR WEST AHEAD OF THE NEXT PACIFIC SYSTEM WITH SOME
WAA SHOWERS POSSIBLE ACROSS THE HIGHER ELEVATIONS THROUGH THE
AFTERNOON.

THE NEXT DIGGING TROUGH ALONG THE WEST COAST WILL SEND OUT A LEADING
SHORT WAVE SET TO ARRIVE IN THE WEST EARLY THIS EVENING. A SERIES OF
DISTURBANCES WILL AFFECT THE WEST THROUGH FRIDAY AFTERNOON WITH NOT
MUCH IMPACT. RAIN AND SNOW WILL EVENTUALLY BLANKET THE WESTERN
MOUNTAINS AND VALLEYS BY FRIDAY EVENING AS THE BROAD LIFT ASSOCIATED
WITH THE APPROACHING LONG WAVE TROUGH ARRIVE. HOWEVER MILD SOUTHWEST
FLOW WILL CONTINUE THROUGH THE NIGHT WITH SNOW LEVELS REMAINING
HIGH. EAST OF THE DIVIDE...DRY AND WARM SOUTHWEST FLOW WILL COMBINE
WITH GUSTY WIND TO CREATE HAZARDOUS FIRE CONDITIONS FOR MUCH OF THE
DAY IN THE LOWER ELEVATIONS AND ESPECIALLY DRIER RIVER BOTTOMS
ACROSS SWEETWATER...LINCOLN...SOUTHERN SUBLETTE...NATRONA...AND
FREMONT COUNTIES.

THE TROUGH DOES NOT PROGRESS TO QUICKLY TO THE EAST FRIDAY AND
SATURDAY AS IT CONCENTRATES ON DIGGING SOUTHWARD. IT EVENTUALLY
MOVES ACROSS THE CONTINENTAL DIVIDE ON SATURDAY AND BECOMES
NEGATIVELY TILTED. THERE IS STILL SOME DISAGREEMENT AS TO EXACTLY
WHERE LEE CYCLOGENESIS WILL BE FOCUSED...BUT IF IT DOES HAPPEN AS
ADVERTISED BY THE GFS THIS SHOULD SET UP FAVORABLE UPSLOPE FLOW EAST
OF THE DIVIDE...BACKING SOME MOISTURE INTO THE CENTRAL BASINS. AT
THE SAME TIME THE COLD FRONT IS PROGRESSING EAST DURING THE
EVENING...LOWERING SNOW LEVELS TO ABOUT 6500FT EAST OF THE DIVIDE
AND TO THE VALLEY FLOORS ACROSS THE WEST. BROAD UPPER LEVEL SUPPORT
COMBINED WITH THE FROPA...NEGATIVELY TILTED TROUGH...STEEP LAPSE
RATES AND THE POTENTIAL FOR CONVECTION...AS WELL AS POTENTIALLY
FAVORABLE SURFACE FLOW LED TO SOME GENEROUS POPS FOR SATURDAY AND
SATURDAY NIGHT BUT RECENT MODEL INCONSISTENCIES ARE LEADING TO LOW
CONFIDENCE IN THE DETAILS OF HOW THIS SYSTEM WILL EVOLVE BUT AT THIS
TIME IT LOOKS LIKE IT HAS POTENTIAL TO DROP SOME MEASURABLE
PRECIPITATION ACROSS MUCH OF THE FORECAST AREA.

.LONG TERM...SUNDAY THROUGH WEDNESDAY

A STRONG...NEARLY VERTICALLY STACKED UPPER LOW IS PROJECTED TO BE
ACROSS THE COLORADO FRONT RANGE BY 12Z SUNDAY. THIS UPPER LOW IS
EXPECTED TO PROGRESS SLOWLY NORTHEAST INTO NORTHWEST KANSAS/WESTERN
NEBRASKA SUNDAY. WRAP AROUND MOISTURE FROM THIS SYSTEM...AND SOME
LOW LEVEL UPSLOPE EAST OF THE DIVIDE WILL GIVE THE FAR EAST A VERY
GOOD SHOT FOR PRECIPITATION SUNDAY...BUT THE ENTIRE AREA WILL HAVE A
CHANCE AT SOME PRECIPITATION. JOHNSON AND NATRONA COUNTIES COULD SEE
SIGNIFICANT PRECIPITATION SUNDAY. HOWEVER STACKED LOWS HAVE THE
TENDENCY TO HAVE A SHARP EDGE TO THE WRAP AROUND PRECIPITATION. THUS
THE EXACT TRACK WILL BE CRITICAL. PRECIPITATION TYPE WILL BE
MOUNTAIN SNOW...VALLEY RAIN WITH SNOW LEVELS GENERALLY ABOUT 7500
FEET...BUT LOWERING WITH TIME SUNDAY INTO SUNDAY NIGHT.

GLOBAL MODELS SHOW THIS SYSTEM MAKING VERY SLOW PROGRESS EASTWARD
MONDAY AND TUESDAY...WITH A REX BLOCK OVER CENTRAL CANADA/U.S.
DISTURBANCES ROTATING AROUND THE CORE UPPER LOW...AS WELL AS
DISTURBANCES SLIDING SOUTHEAST ON THE BACKSIDE OF THE MEAN TROUGH
WILL KEEP CHANCES OF SHOWERS MONDAY AND TUESDAY. MEDIUM RANGE MODELS
ARE IN GOOD AGREEMENT WITH A STRONG UPPER RIDGE DEVELOPING OVER THE
WEST COAST TUESDAY. HOWEVER THE MAIN DIFFERENCE FROM LAST NIGHTS RUN
IS THAT GFS HAS TRENDED TOWARD THE ECMWF KEEPING THE STRONG RIDGE
OVER THE WEST COAST...OR PERHAPS FOLDING OVER INTO SOUTHERN CANADA
WEDNESDAY AND THURSDAY. THUS THE TREND IS FOR COOLER CONDITIONS AND
POSSIBLY SOME SHOWER ACTIVITY. HAVING SAID THAT WILL TREND POPS DOWN
TUESDAY INTO WEDNESDAY...WITH ONLY SLIGHT MOUNTAIN POPS FOR
WEDNESDAY AND THURSDAY FOR NOW.

TEMPERATURES IN THE EXTENDED WILL BE BELOW AVERAGE...WITH WELL BELOW
AVERAGE SUNDAY INTO MONDAY EAST OF THE DIVIDE.

ON A HYDROLOGY NOTE...UNLESS THE SYSTEM COMES IN WARMER THAN
FORECAST...NOT EXPECTING RAIN ON THE MOUNTAIN SNOWPACK. THERE
COULD EASILY BE MINOR RISES ON SMALL CREEKS/STREAMS FROM PREVIOUS WARM
DAYS...AND FROM THE LOW ELEVATION RAIN EXPECTED SUNDAY INTO
POSSIBLY MONDAY. HOWEVER THE PROBABILITIES FOR SMALL STREAM/CREEK
FLOODING AT THIS TIME IS VERY LOW THROUGH MIDDLE OF NEXT
WEEK...UNLESS THIS SYSTEM IS WARMER THAN FORECAST AND WE GET RAIN
ON THE MOUNTAIN SNOWPACK.

&&

.AVIATION.../12Z ISSUANCE/

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

VFR CONDITIONS WILL OCCUR THROUGH 12Z FRIDAY. ISOLATED RAIN OR SNOW
SHOWERS ARE POSSIBLE IN THE NORTHERN BIGHORN MOUNTAINS FROM 21Z
UNTIL 03Z.

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

ISOLATED SNOW SHOWERS WILL OCCUR IN THE NW WY MOUNTAINS THIS
MORNING. COVERAGE OF SNOW SHOWERS WILL INCREASE IN THE NW WY
MOUNTAINS AFTER 18Z AND THIS ACTIVITY WILL CONTINUE THROUGH 06Z
FRIDAY. THE KJAC TERMINAL SITE WILL SEE SNOW SHOWERS IN THE VCNTY
AFTER 18Z AND THEN A PERIOD OF SNOW SHOWER ACTIVITY FROM 22Z UNTIL
02Z. EXPECT MVFR TO IFR CONDITIONS IN NW WY AFTER 18Z THROUGH 06Z
FRIDAY. OTHERWISE...VFR CONDITIONS WILL OCCUR AT THE KBPI/KRKS/KPNA
TERMINAL SITES THROUGH 12Z FRIDAY.

&&

.FIRE WEATHER...

MILD AND BREEZY AGAIN TODAY ESPECIALLY ACROSS THE RAWLINS DISPATCH
AREA...WITH MIN RH`S IN THE TEENS AND 20S ACROSS THE LOWER
ELEVATIONS...WITH 40 TO 60S IN THE MOUNTAINS. HOWEVER...RAIN SHOWERS
WILL RETURN TO TETON DISPATCH THIS AFTERNOON AS A QUICK MOVING UPPER
LEVEL DISTURBANCE MOVES THROUGH THE REGION. WARM AND DRY AGAIN ON
FRIDAY EAST OF THE DIVIDE...WITH LOW HUMIDITY AND BREEZY 20 TO 35
MPH SOUTHWEST WIND GUSTS ACROSS RAWLINS AND SOUTHERN CASPER DISPATCH
ZONES. RAIN SHOWERS AND A CHANCE FOR AN ISOLATED THUNDERSTORM WILL
CONTINUE TO BE A POSSIBILITY ACROSS THE WEST ONCE AGAIN. THIS
WEEKEND WILL COOL BACK OFF QUITE A BIT AS YET ANOTHER SPRING SYSTEM
MOVES THROUGH...WITH EVERYONE GETTING SOME CHANCE FOR WETTING RAINS
AND/OR SNOW...ALONG WITH INCREASED WINDS...AND A CHANCE FOR A
THUNDERSTORM ON SATURDAY AFTERNOON.

&&

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

&&

$$

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







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