Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 271729
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
1129 AM MDT SAT SEP 27 2014

.SHORT TERM...TODAY THROUGH MONDAY NIGHT

A LARGE CUTOFF LOW WILL BEGIN TO TREK NORTHEAST TODAY AS A PASSING
SHORT WAVE IN SOUTHERN CANADA KICKS DOWN A COLD FRONT EARLY THIS
MORNING...TO REST ALONG THE DIVIDE TODAY. IT WILL BE PRETTY STABLE
BEHIND THE FRONT AS THE COOL AIR UNDERCUTS THE REMAINING MILD AIR
ALOFT. HOWEVER...THE UPPER LOW APPROACHING FROM THE SOUTHWEST WILL
BE PRECEDED BY DIFLUENT FLOW ALOFT AND BROAD LIFT AHEAD OF THE
LOW...THE DYNAMIC LIFT COUPLED WITH MOIST FLOW OPEN ALL THE WAY TO
THE GULF OF CALIFORNIA...AND AFTERNOON HEATING PROVIDING MORE
INSTABILITY ACROSS THE SOUTH AND WEST. STORM MOVEMENT WILL BE PRETTY
FAST...IN THE 30 TO 40 MPH RANGE BUT THEY WILL BE MOVING ALMOST
PERFECTLY NORTHWARD...WHICH COULD LEAD TO SOME TRAINING ACROSS THE
WESTERN MOUNTAINS. GIVEN THE AMOUNT OF MOISTURE AVAILABLE...THIS
COULD LEAD TO SOME FLASH FLOODING AND MUD SLIDES FROM THE WYOMING
AND SALTS NORTHWARD TO YELLOWSTONE...AND ABOUT AS FAR EAST AS
BONDURANT AND TOGWOTEE PASS.

EAST OF THE DIVIDE...NOT MUCH IN THE WAY OF WIDESPREAD RAINFALL OR
THUNDERSTORMS EXPECTED DUE TO THE POSITION OF THE FRONT UNTIL LATER
IN THE EVENING WHEN THE DYNAMICS FROM THE UPPER LOW NEAR. BUT BY
LATE AFTERNOON...ESPECIALLY ALONG/NEAR/SOUTH OF THE FRONT...A FEW
STRONGER STORMS ARE POSSIBLE IN AREAS THAT HAVE CLEAR SKIES EARLIER
IN THE DAY...CAPE AND SHEAR SHOULD BE MAXIMIZED ALONG AND SOUTH OF
THE FRONT....ENCOMPASSING MOST OF SWEETWATER COUNTY AS WELL AS
PORTIONS OF SOUTHEAST FREMONT COUNTY. 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. WINDS WILL ALSO BECOME
QUITE GUSTY OUT OF THE EAST TO NORTHEAST TO NORTH...ALONG AND BEHIND
THE FRONT FROM EASTERN FREMONT...EAST AND NORTH INTO NATRONA AND
JOHNSON COUNTIES.

LATER TONIGHT INTO SUNDAY MORNING...RAIN CONTINUES AND BECOMES MORE
WIDESPREAD AS THE UPPER LOW CONTINUES TO APPROACH...WITH MOUNTAIN
SNOW POSSIBLE ABOVE PASS LEVEL. AT THE SURFACE...ANOTHER COLD FRONT
MOVES THROUGH FROM WEST TO EAST THIS TIME WITH DEEPER COOL AIR AND
MOISTURE WITH IT. POTENTIAL EXISTS FOR SOME RAINFALL TOTALS OVER AN
INCH ACROSS MUCH OF THE FORECAST AREA...OVER 2 INCHES POSSIBLE ALONG
OUR WESTERN BORDER...AS THIS SYSTEM MOVES THROUGH. WHILE IT IS A
COOL SYSTEM IT IS NOT COLD ENOUGH TO BRING SNOW LEVELS DOWN TO AN
IMPACTFUL LEVEL...AT THIS TIME IT LOOKS AS THOUGH ACCUMULATING SNOW
WILL REMAIN ABOVE 9500 FEET IN MOST AREAS...HIGHER IN THE BIGHORNS.

ON MONDAY THE UPPER LOW TRANSITIONS INTO AN OPEN WAVE AND TAKES ON A
NEGATIVE TILT AS IT LIFTS OUT OF THE AREA. THERE COULD STILL BE SOME
DECENT RAINFALL AMOUNTS AS THIS OCCURS...ESPECIALLY EAST OF THE
DIVIDE AND ALONG THE I25 CORRIDOR. KEPT SOME SLIGHT CHANCE POPS IN
THROUGH THE PERIOD AS SOME WRAP AROUND MOISTURE COULD KEEP SHOWERS
GOING WELL INTO MONDAY NIGHT.

.LONG TERM...TUESDAY THROUGH SATURDAY

SYNOPSIS...THE UPPER LEVEL LOW PRESSURE SYSTEM WILL LIFT NORTH
ACROSS THE NORTHERN HIGH PLAINS ON TUESDAY WITH PRECIPITATION
WRAPPING BACK INTO NORTHEAST WYOMING AND THE BIGHORN RANGE.  A COOL
AND UNSETTLED NORTHWEST FLOW WILL FOLLOW WEDNESDAY AND THURSDAY.  A
RIDGE OF HIGH PRESSURE WILL BUILD IN FRIDAY AND SATURDAY BRINGING
DRY AND WARMER WEATHER.

DISCUSSION...UPPER LOW IS FORECAST TO MOVE OUT OF THE CENTRAL
ROCKIES INTO THE NORTHERN HIGH PLAINS ON TUESDAY DRIFTING ALMOST DUE
NORTH AS IT RUNS INTO STRONG DOWNSTREAM RIDGE.  GEM IS FURTHEST WEST
WITH THE UPPER LOW TRACK...THE GFS SHOWS A MORE STACKED SYSTEM OVER
WESTERN DAKOTAS ON TUESDAY THAN THE ECMWF...AND THUS WRAPS MORE
PRECIPITATION BACK INTO NORTHEAST WYOMING UP TO THE BIGHORN RANGE.
POPS HAVE BEEN BUMPED UP IN THIS AREA TO ACCOUNT FOR THE WRAPAROUND
PRECIP.  EVEN ON THE COLDER BACK SIDE OF THIS SYSTEM...SNOW LEVELS
IN THE BIGHORN RANGE SHOULD BE AT HIGHER PASS LEVEL OR AROUND 9000
FEET AND UP.

THE NEXT UPPER LOW MOVING EAST ACROSS THE GULF OF ALASKA EARLY THIS
MORNING WILL SLIDE DOWN INTO THE NORTHERN ROCKIES LATE TUESDAY.  THE
TREND HAS BEEN TO SPEED THIS NEXT SYSTEM UP CLOSING THE GAP BETWEEN
SYSTEMS ON TUESDAY.  THUS...AS PRECIPITATION TAPERS OFF IN NORTHEAST
WYOMING THROUGH THE DAY TUESDAY...THIS NEXT SYSTEM WILL SPREAD RAIN
AND SNOW SHOWERS THE NORTHWEST THROUGH THE DAY AND INTO NORTHERN AND
CENTRAL WYOMING TUESDAY NIGHT AND WEDNESDAY.  SNOW LEVELS ARE EXPECTED TO
LOWER TO BETWEEN 7000 AND 8000 FEET ACROSS THE NORTH TUESDAY NIGHT
AND WEDNESDAY MORNING.

ANOTHER CLIPPER IS EXPECTED TO DIVE SE INTO NW MONTANA THURSDAY
MORNING.  THE ECMWF GIVES NORTHERN WYOMING A GLANCING BLOW WITH THIS
SYSTEM...TAKING A FURTHER NORTHWARD TRACK ACROSS MONTANA INTO THE
DAKOTAS.  GFS KEEPS IT TRACKING DOWN THE EAST SLOPES OF THE DIVIDE
ACROSS WYOMING THURSDAY...INTO THE CENTRAL HIGH PLAINS THURSDAY
NIGHT.  CONTINUITY HAS BEEN POOR WITH BOTH MODELS WITH THIS SYSTEM
AND FORECAST STEERS AWAY FROM DETERMINISTIC GUIDANCE HERE...USING
GEFS MEAN POPS WHICH KEEPS THE HIGHER 30 PCT POPS ALONG AND NORTH OF
A COD-WRL-CPR LINE THURSDAY.  FLAT UPPER RIDGE BUILDS IN FRIDAY AND
SATURDAY WITH STORM TRACK MIGRATING FURTHER NORTH ALONG AND NORTH OF
THE U.S./CANADIAN BORDER.  WE SHOULD SEE MAINLY DRY CONDITIONS AND A
RETURN TO NEAR OR ABOVE NORMAL TEMPERATURES THROUGH NEXT WEEKEND
AFTER A COOL AND WET START TO OCTOBER.

.AVIATION.../18Z ISSUANCE/

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

A COLD FRONT HAS PUSHED UP TO THE CONTINENTAL DIVIDE THIS MORNING.
BREEZY NORTH TO NORTHEAST WINDS WILL CONTINUE BEHIND THE COLD FRONT.
CLOUD COVER WILL INCREASE THROUGH 03Z WITH SUBTROPICAL MOISTURE
MOVING NORTH OVER THE COLDER AIR. VFR CONDITIONS ARE EXPECTED
THROUGH 03Z...THEN AREAS OF MVFR TO IFR CONDITIONS WILL BECOME
WIDESPREAD OBSCURING THE MOUNTAINS ALONG AND EAST OF THE
CONTINENTAL DIVIDE SPREADING NORTH AND EAST THROUGH 18Z
THREATENING KLND...KCOD AND KDUB BY 03Z AND THE REMAINDER OF THE
AIRPORTS BY 06Z.

ALTHOUGH SOME THUNDERSTORM ACTIVITY WILL BE POSSIBLE BETWEEN 21Z
AND 03Z...IT SHOULD BE MAINLY CONFINED TO JUST ALONG AND NORTH OF
THE DIVIDE AS STORMS MOVING OVER THE DIVIDE WILL ENCOUNTER A MORE
STABLE ATMOSPHERE.


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

THE UPPER LEVEL LOW PRESSURE CENTER OVER THE GREAT BASIN WILL
SPREAD SIGNIFICANT SUBTROPICAL MOISTURE AND INSTABILITY TO
SOUTHERN AND WESTERN WYOMING WITH WIDESPREAD MVFR TO IFR CEILINGS
AND VISIBILITIES IN RAIN...MIXED WITH SNOW IN THE HIGHER MOUNTAINS
THROUGH 18Z...THE LOWER ELEVATIONS WILL SEE MVFR TO IFR CEILINGS
BECOME PREVAILING 00Z TO 03Z AND REMAIN DOWN THROUGH 18Z. EMBEDDED
THUNDERSTORMS WILL PRODUCE STRONG WIND GUSTS AND SMALL HAIL.

.FIRE WEATHER...

FIRE WEATHER WILL BE TAME THROUGH THE WEEKEND AND DANGER WILL REMAIN
LOW THROUGH MOST OF NEXT WEEK AS A LARGE LOW PRESSURE SYSTEM AND A
COUPLE OF WET SURFACE COLD FRONTS MOVE THROUGH THE FORECAST
AREA...BRINGING COOLING TEMPERATURES AND INCREASES IN RELATIVE
HUMIDITY ALONG WITH A GOOD CHANCE OF WETTING RAIN.

&&

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

$$

SHORT TERM...ALLEN
LONG TERM...AEM
AVIATION...C.BAKER
FIRE WEATHER...ALLEN







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