Area Forecast Discussion
Issued by NWS Cheyenne, WY

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000
FXUS65 KCYS 211513
AFDCYS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHEYENNE WY
913 AM MDT THU AUG 21 2014

.UPDATE...
ISSUED AT 908 AM MDT THU AUG 21 2014

CURRENT FORECAST FOR TODAY LOOKS ON TRACK. MADE SOME MINOR
ADJUSTMENTS TO PRECIP PROBABILITIES AND TSTORM COVERAGE TODAY.
CONVECTION WILL STRUGGLE TO DEVELOP THIS AFTERNOON FURTHER SOUTH
DUE TO SUBSIDENCE AND DRIER AIR ALOFT. HOWEVER...MODEL SOUNDINGS
SHOW THIS WARM/DRY LAYER ALOFT ERODING BY THIS EVENING. CAN NOT
RULE OUT AN ISOLATED SHOWER OR THUNDERSHOWER ALONG I80 LATE THIS
AFTERNOON AND THIS EVENING AS A WEAK DISTURBANCE ALOFT WILL MOVE
ACROSS THE REGION. STILL EXPECT MOST OF THE ACTIVITY TO REMAIN
ACROSS CENTRAL AND NORTHEAST WYOMING AS A FRONTAL BOUNDARY
GRADUALLY DRIFTS SOUTHWARD AND STALLS ACROSS NORTH-CENTRAL WYOMING
TODAY.

&&

.SHORT TERM...(TODAY THROUGH SATURDAY NIGHT)
ISSUED AT 426 AM MDT THU AUG 21 2014

FLOW ALOFT REMAINS SOUTHWEST. ISOLATED TO SCATTERED LATE DAY
SHOWERS AND THUNDERSTORMS ON TAP WEST OF A LARAMIE TO CHADRON LINE
WHERE MOISTURE AND INSTABILITY WILL COINCIDE ALONG WITH BEST UPPER
FORCING.

TONIGHT...COLD FRONT MOVES WEST TO EAST ACROSS THE COUNTIES WITH A
FEW SHOWERS POSSIBLE LATE NORTH OF A LUSK TO SIDNEY LINE.

FRIDAY...NEXT NEGATIVELY TILTED SHORTWAVE TROUGH ALOFT MOVES ACROSS
WESTERN COLORADO...WITH MID LEVEL SHORTWAVE TROUGH OVER NORTHERN
COLORADO COMBINED WITH ADEQUATE LOW AND MID LEVEL MOISTURE TO
PRODUCE SCATTERED LATE DAY SHOWERS AND THUNDERSTORMS...MOST
NUMEROUS EAST OF A CHEYENNE TO LUSK LINE.

FRIDAY NIGHT...SHORTWAVE TROUGH ALOFT ROTATES NORTHEAST ACROSS
EASTERN WYOMING AND WESTERN NEBRASKA...WITH AT LEAST SCATTERED
SHOWERS AND THUNDERSTORMS DEVELOPING...LIKELY MOST NUMEROUS ACROSS
WESTERN NEBRASKA.

SATURDAY...UNSETTLED PATTERN CONTINUES OVER OUR COUNTIES AS THE
DOMINANT UPPER TROUGH PREVAILS OVER MONTANA AND WYOMING...WITH THE
NEXT SHORTWAVE TROUGH ALOFT ROTATING ACROSS WESTERN WYOMING...
PRODUCING AT LEAST SCATTERED SHOWERS AND THUNDERSTORMS NEAR THE LOW
AND MID LEVEL THETA-E RIDGE AXIS.

.LONG TERM...(SUNDAY THROUGH THURSDAY)
ISSUED AT 426 AM MDT THU AUG 21 2014

00Z MEDIUM RANGE DETERMINISTIC MODELS CONTINUE TO PAINT A FAIRLY
SIMILAR PICTURE FOR SUNDAY AND MONDAY...BUT DEVIATE DRAMATICALLY
ON TUESDAY THROUGH THURSDAY. SATURDAYS FRONT WILL BE SOUTH OF THE
CWA FOR SUNDAY...WITH THE ASSOCIATED SHORTWAVE ENERGY LIFTING
NORTHEAST ACROSS THE DAKOTAS. LOOKS LIKE A RELATIVELY COOL AND
STABLE DAY SHAPING UP...WHICH WILL SUPPRESS MOST CONVECTIVE
CHANCES. THE GFS DOES PAINT SOME MOISTURE ALONG THE I80 CORRIDOR
FROM CHEYENNE TO SIDNEY AS THE WARM FRONT LIFTS BACK NORTH LATE
SUNDAY EVENING. SOUTHWEST FLOW WILL PERSIST ON MONDAY AS THE NEXT
SHORTWAVE DIVES SOUTHEAST INTO MONTANA/IDAHO. INCREASING MOISTURE
AND ASCENT WITH THE APPROACH OF THE UPSTREAM TROUGH WILL INCREASE
THUNDERSTORM CHANCES W-E MONDAY AND MONDAY NIGHT. IT AS AT THIS
TIME WHEN MODEL SOLUTIONS BEGIN TO DIVERGE. THE 00Z GFS AND
CANADIAN MAINTAIN A PROGRESSIVE PATTERN FOR THE TUE-THU TIMEFRAME.
MEANWHILE...THE STALWART ECMWF SHOWS THIS SHORTWAVE TROF BEGINNING
TO ELONGATE INTO NRN CALIFORNIA LATE MONDAY...EVENTUALLY CUTTING
OFF OVR THE GREAT BASIN MID-WEEK. THE DOWNSTREAM IMPACTS ARE
MANY...PENDING WHICH SOLUTION IS CORRECT. HAVE LARGELY DISCOUNTED
THE 00Z ECMWF GIVEN ITS DEVIATION RUN-TO-RUN AS WELL AS FROM OTHER
GLOBAL MODELS. OF COURSE...ANY ADDL SUPPORT BOTH RUN-TO-RUN OR
AMONGST OTHER GLOBAL MODELS IN FUTURE OUTPUT WOULD ADD CREDENCE TO
THIS OUTCOME. IN THE NON-ECMWF SOLUTION...ANOTHER FRONT WOULD
SLICE THRU THE CWA ON TUESDAY... USHERING THE COOLEST TEMPS OF THE
WEEK AND ANOTHER GOOD CHANCE FOR PRECIP. PRECIP WILL EXIT QUICKLY
TUE NT...WITH DRIER AND WARMER CONDITIONS EXPECTED FOR WED AND
THU.

&&

.AVIATION...(FOR THE 12Z TAFS THROUGH 12Z FRIDAY MORNING)
ISSUED AT 426 AM MDT THU AUG 21 2014

VFR CONDITIONS WILL PREVAIL THROUGH THE PERIOD...AS THE AREA
REMAINS IN BETWEEN SYSTEMS. TERMINALS CAN EXPECT JUST SOME PASSING
MID/HIGH-LEVEL CLOUDS FROM TIME TO TIME. OTHERWISE...WEST WINDS
WILL GUST TO 15-25 KNOTS AT SE WYOMING SITES 16-24Z.


&&

.FIRE WEATHER...
ISSUED AT 426 AM MDT THU AUG 21 2014

MINIMAL CONCERNS AS THE WINDS AND HUMIDITIES REMAIN BELOW
CRITICAL THRESHOLDS.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...NONE.
NE...NONE.
&&

$$

UPDATE...TJT
SHORT TERM...RUBIN
LONG TERM...HAHN
AVIATION...HAHN
FIRE WEATHER...RUBIN







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