Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 262202
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
402 PM MDT FRI SEP 26 2014

.SHORT TERM...TONIGHT THROUGH SATURDAY

LARGE RIDGE PASSING TO THE EAST AND NOW OVR THE CNTRL CONUS...WITH
LARGE TROF NOW APPROACHING FROM THE WEST WITH FIRST INDICATIONS
WITHIN WV IMAGERY THAT UPR LVL LIFT WILL BE INCREASING OVR OUR WRN
BORDERS LATER THIS AFTERNOON AS THE WEAKER LEAD PORTION OF A
VIGOROUS JET ASSOCIATED WITH THE WRN TROF RIDES OVERHEAD ACROSS THE
WRN BORDER REGION FROM OUT OF UT. OTHERWISE...SKIES HIGH AND DRY
ATTM FOR MOST AREAS WITH ONLY A FEW MID/UPR CLOUDS ACROSS THE NW FA.
AT THE SFC...DEVELOPING LOW WELL TO THE NORTH OVR S CNTRL CA WITH
TRAILING TROF DROPPING SWD AND REMAINING MOSTLY IN THE LEE OF THE
FRONT RANGE AREAS...WHILE STILL WEAK DEVELOPING FRONT LAY OFF TO THE
NW AND THROUGH WRN MT...S CNTRL ID AND ON INTO NV.

AS MENTIONED ABOVE...THE APPROACH OF THE WRN CONUS TROF AND THE
INCREASED JET LVL WINDS STARTING LATER TODAY WILL INTERACT WITH WARM
LOW TO MID LVL CONDITIONS AND GOOD LAPSE RATES ALONG WITH INCREASING
BUT STILL MODEST MAINLY MID LVL MOISTURE TO PRODUCE ISOLATED TO
WIDELY SCATTERED SHOWERS AND THUNDER THIS AFTERNOON AND EVENING. ML
CAPE WILL INCREASE TO BETWEEN 250 AND 500 J/KG NEAR THE WRN BORDER
WITH DEEP SHEAR PUSHING 30 TO 35 KTS. DCAPE EARLY ON WILL ALSO RANGE
FROM 500 TO 1000 J/KG. WITH THIS IN MIND...AN ISOLATED STORM OR TWO
WILL BE ABLE TO PRODUCE HAIL AND GUSTY WINDS...POSSIBLY ON THE
STRONG SIDE BUT MOSTLY REMAINING LESS THAN SEVERE.

OTHERWISE LARGE TROF WITH CLOSING LOW CENTER INTO/OVER THE GREAT
BASIN WILL CONTINUE TO SLOWLY AFFECT THE WRN FA THROUGH THE ENTIRE
FCST PERIOD...EXPANDING TO THE CNTRL FA BY THE VERY END OF THE FCST.
SHOWERS WILL BECOME EMBEDDED WITH ISOLATED THUNDER ONCE AGAIN BY SAT
AFTERNOON AS A FRONT SLOWLY MOVES S ACROSS THE FA THRU THE MORNING
HRS...SETTING UP MAINLY E OF THE DIVIDE AND AS FAR S AS SOUTH PASS
ON THE WEST...STRETCHING E/ENE THRU ERN FREMONT AND INTO NATRONA
COUNTIES BY THE AFTERNOON. THE RESULTS...WRN WY WILL SEE RATHER FAST
MOVING SHOWER AND EMBEDDED ISOLATED THUNDER MOVING FROM S TO N AT 30
TO 40 MPH...SOME PRODUCING HEAVY RAIN AT TIMES AS THE ATMOSPHERE
BECOMES SATURATED BY MID TO LATE MORNING. WILL HAVE TO WATCH FOR
TRAINING SHRA/TSRA OVR SOME LOCATIONS FOR THE POSSIBILITY OF
ISOLATED FLASH FLOODING. EAST OF THE DIVIDE...NOT MUCH IN THE WAY OF
PRECIP/STORMS EXPECTED UNTIL LATER IN THE DAY. BUT BY LATE
AFTERNOON...ESPECIALLY ALONG/NEAR/SOUTH OF THE FRONT...A FEW
STRONGER STORMS CAN BE EXPECTED UNDER ML CAPE OF 300 TO 700 J/KG AND
SHEAR APPROACHING 40 KTS WHERE SOME SUN CAN MAKE IT TO THE GROUND
EARLY ON. THESE STORMS WILL ALSO BE MOVING SOUTH TO NORTH...BUT
SOMEWHAT SLOWER...25 TO 30 MPH AND CAPABLE OF SOME ORGANIZATION AND
PRODUCING BOTH WIND AND HAIL...SOME APPROACHING MARGINALLY SEVERE
PARAMETERS. SFC WINDS WILL ALSO BECOME QUITE GUSTY OUT OF THE E TO
NE TO N...ALONG AND BEHIND THE FRONT FROM ERN FREMONT...E AN N INTO
NATRONA AND JOHNSON COUNTIES.


.LONG TERM...SATURDAY NIGHT THROUGH FRIDAY

LARGE UPPER LOW WILL BE CENTERED OVER NORTH CENTRAL NV TO START THE
PERIOD. THIS WILL BRING A MOIST SOUTHERLY FLOW ACROSS THE
REGION...AND BRING WIDESPREAD SHOWERS THROUGH THE NIGHT...WITH THE
BEST CHANCES COMING ACROSS WESTERN AND CENTRAL AREAS. 700 MB TEMPS
LOOK TO REMAIN ABOVE FREEZING THROUGH SUNDAY MORNING...SO ANY SNOW
CHANCES WILL BE ISOLATED...AND ONLY AT THE HIGHEST ELEVATIONS
THROUGH SUNDAY MORNING. THROUGH THE DAY SUNDAY...UPPER LOW SLOWLY
PROGRESSES EASTWARD...AND WILL THEN BRING WIDESPREAD SHOWERS AND
COOLER TEMPS. A POTENTIAL FOR HIGH QPF AMOUNTS WILL BE SEEN FROM
THIS SYSTEM SUNDAY...AND EVEN AGAIN MONDAY...WITH THE ABUNDANT
MOISTURE ASSOCIATED WITH THIS SYSTEM. IN ADDITION A SURFACE LOW IN
NORTHEASTERN COLORADO COULD PUT AREAS EAST OF THE DIVIDE IN A
FAVORABLE UPSLOPE FLOW...WHICH COULD ALSO AID IN ENHANCING QPF
AMOUNTS FROM THIS SYSTEM. COLD AIR WILL THEN FILTER IN ACROSS
WESTERN PORTIONS SUNDAY NIGHT INTO MONDAY MORNING...AS THE UPPER LOW
BECOMES MORE CENTERED OVER WESTERN ID AND NORTHERN UT. THIS COULD BRING
WIDESPREAD SNOW ABOVE 9500 FEET...ACROSS WESTERN WYOMING...WITH
SNOWFALL AMOUNTS APPROACHING 6 INCHES AT THE HIGHEST ELEVATIONS BY
MONDAY EVENING. MONDAY WILL CONTINUED TO BE COOL WITH WIDESPREAD
SHOWERS FOR THE LOWER ELEVATIONS. THE BEST CHANCES WILL COME ACROSS
WESTERN AND EASTERN PORTIONS...AS A DRY SLOT WILL LOWER CHANCES
ACROSS CENTRAL PORTIONS.

BY LATE TUESDAY MORNING UPPER LOW IS LOCATED ACROSS SE MT. THIS WILL
BRING WRAP AROUND SHOWERS FOR CENTRAL AND EASTERN PORTIONS THROUGH
THE DAY...WITH THE BEST CHANCES COMING ACROSS JOHNSON COUNTY. MOIST
WESTERLY FLOW COULD ALSO CONTINUE A FEW SHOWERS FOR WESTERN AND
SOUTHERN PORTIONS...WITH SOME CONTINUED SNOW ACROSS THE HIGHER
ELEVATIONS.

AS THE UPPER LOW MOVES INTO THE NORTHERN PLAINS...A SECONDARY
DISTURBANCE...AND ITS ASSOCIATED COLD FRONT...WILL SWING DOWN FROM
CANADA ON WEDNESDAY. THIS WILL BRING THE NEXT CHANCE FOR PRECIP FROM
NORTH TO SOUTH. TIMING WILL HAVE TO BE WATCHED WITH THIS SYSTEM AS A
SECOND SHOT OF COLD AIR WILL BE POSSIBLE. RIGHT NOW MODELS INDICATE
THIS COOL AIR WILL COME THROUGH WEDNESDAY AFTERNOON...KEEPING THE
PRECIP MOSTLY RAIN...WITH SOME HIGHER ELEVATION SNOW ACROSS THE
NORTHERN AREAS. ECMWF IS MORE AGGRESSIVE WITH THE QPF FOR THIS FRONT
THAN THE GFS...SO TRIED TO FIND A MIDDLE GROUND BETWEEN THE TWO.

AFTER WEDNESDAY MODELS TREND IN DIFFERENT DIRECTIONS. ECMWF HAS
ANOTHER COLD FRONT BLAST THROUGH ON THURSDAY...BRINGING WIDESPREAD
SHOWERS. TEMPS ARE COLD ENOUGH WITH THIS SYSTEM WHERE A RAIN/SNOW
MIXTURE ACROSS THE LOWER ELEVATIONS COLD BE POSSIBLE. HOWEVER...GFS
DEVELOPS A RIDGE OF HIGH PRESSURE EARLIER THAN THE ECMWF AND MOSTLY
HAS NORTHWEST FLOW...AND A WARMING TREND FOR THURSDAY INTO FRIDAY.
PUT IN SLIGHT POPS EAST...AND UP TO CHANCE WEST...FOR
THURSDAY...WITH NOT AS COOL OF TEMPS AS THE ECMWF MAY SUGGEST. BOTH
MODELS THEN AGREE HIGH PRESSURE WILL BUILD OVER THE REGION
FRIDAY...DRYING THINGS OUT...AND BRINGING THE WARMEST DAY OF THE
PERIOD...WITH TEMPS EXPECTED TO BE BACK TO NEAR NORMAL READINGS.


&&

.AVIATION.../00Z ISSUANCE/

VFR CONDITIONS WILL PREVAIL THROUGH THE FORECAST PERIOD AT ALL
LOCATIONS EXCEPT KJAC WHICH SEE MVFR CONDITIONS PREVAIL AFTER 12Z
WITH LOWERING CIGS. A FEW  THUNDERSTORMS WILL CONTINUE NEAR THE
WESTERN FORECAST AREA THROUGH 06Z TONIGHT BEFORE BECOMING MAINLY
SHOWERS THE REST OF NIGHT. OTHERWISE...A LARGE DEEPENING TROUGH OF
LOW PRESSURE OVER THE GREAT BASIN WILL SLOWLY BEGIN TO APPROACH THE
FORECAST AREA TONIGHT...MOVING THROUGH FROM WEST TO EAST SATURDAY.
MOISTURE WILL ALSO INCREASE SLOWLY FROM WEST TO EAST OVER THE NEXT
24 HOURS...PROVIDING GOOD CHANCES FOR INCREASING CLOUD COVER AND
PRECIPITATION WEST OF THE DIVIDE...WHILE EAST OF THE DIVIDE...SKIES
WILL REMAIN MOSTLY CLEAR THROUGH SATURDAY AFTERNOON...BEFORE
SCATTERED VFR CIGS BEGIN.

&&

.FIRE WEATHER...

WARM AND DRY AGAIN TODAY...WILL SLIGHTLY ENHANCED FIRE DANGER
CONDITIONS TODAY...BUT LARGELY REMAINING BELOW CRITICAL LEVELS AS
WINDS DECREASE ACROSS THOSE AREAS OF CONCERN THAT WERE AROUND
YESTERDAY. FIRE WEATHER DANGER CONCERNS WILL CONTINUE TO FALL OFF
THROUGH THE WEEKEND AND REMAIN LOW THROUGH MOST OF NEXT WEEK AS A
LARGE UPPER LEVEL TROUGH OF LOW PRESSURE AND A COUPLE OF SURFACE
COLD FRONTS MOVE THROUGH THE FORECAST AREA...BRINGING COOLING
TEMPERATURES AND GOOD INCREASES IN RELATIVE HUMIDITY ALONG WITH
PRECIPITATION.

&&

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

$$

SHORT TERM...BRAUN
LONG TERM...ANGLIN
AVIATION...BRAUN
FIRE WEATHER...BRAUN







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