Area Forecast Discussion
Issued by NWS Cheyenne, WY

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000
FXUS65 KCYS 212130
AFDCYS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHEYENNE WY
330 PM MDT SUN SEP 21 2014

.SHORT TERM...(THIS EVENING THROUGH TUESDAY NIGHT)
ISSUED AT 330 PM MDT SUN SEP 21 2014

AFTERNOON SATELLITE IMAGES SHOW WELL-DEFINED LOW PRESURE AREA
LIFTING NORTH/NORTHEAST ACROSS THE GREAT BASIN...WITH ARM OF
MOISTURE ASSOCIATED WITH WEAK SHORTWAVE EXTENDING TO ITS N AND E
ADVECTING INTO THE INTERMOUNTAIN WEST. SHOWERS WERE OBSERVED AT
KCAG EARLIER THIS AFTERNOON...AND KCYS/KGJX 88D RETURNS SUGGEST
THIS MOISTURE HAS MADE IT INTO THE SIERRA MADRES AND ADJACENT WRN
FOOTHILL LOCATIONS. 20Z SFC ANALYSIS SHOWS 1026MB ACROSS THE
DAKOTAS AND NEBRASKA WITH RETURN FLOW ALONG THE FRONT RANGE AND A
WELL DEFINED SFC TROF BANKED UP ALONG THE EAST SLOPES OF THE SNOWY
RANGE. LLVL CONVERGENCE ALONG THIS SFC TROF HAS COMBINED WITH 250+
J/KG OF SBCAPE AS ANALYZED FROM SPC MESOANALYSIS TO SPARK A COUPLE
OF POCKETS OF DEEPER CONVECTION IN THE VICINITY OF CENTENNIAL/
WOODS LANDING/ALBANY OVR THE PAST FEW HOURS. LOW CENTER WILL REACH
IDAHO BY LATE TONIGHT...WITH ARM OF MOISTURE ADVECTION CONTINUING
FROM SW TO NE INTO THE CWFA. HIGH CLOUDS HAVE ALREADY BEEN ON THE
INCREASE ACROSS THE AREA...BUT SHOULD SEE THESE LOWER AND THICKEN
OVERNIGHT. ASCENT ASSOCIATED WITH THE UPPER-LOW WILL INCREASE
TONIGHT...SO WILL SEE SHOWERS AND STORMS INCREASE ESPECIALLY ALONG
AND WEST OF THE LARAMIE RANGE AS WELL. HAVE ALSO ADDED SOME FOG TO
THE SRN LARAMIE RANGE PER LATEST HRRR OUTPUT. LIGHTNING WAS EVIDENT
ACROSS SW COLORADO LAST NIGHT...SO IT MAKES SENSE TO KEEP IT
TONIGHT UP HERE TONIGHT. 12Z MODEL OUTPUT HAS TRENDED SLOWER IN
SPREADING RAIN CHANCES OUT ONTO THE ERN PLAINS...WHICH WILL LIKELY
NOW HOLD OFF NOW UNTIL SOMETIME MONDAY MORNING.

UPPER LOW WOBBLES THRU THE COWBOY STATE ON MONDAY...WHICH WILL
EFFECTIVELY SHARPEN AND ENHANCE AFTERNOON CONVECTION ALONG THE
PREFRONTAL SFC TROF/PACIFIC FRONT. BEST CONVECTIVE CHANCES ON
MONDAY WILL BE TIED CLOSELY TO THE LOCATION OF THIS TROF...WHICH
WILL BE WEST OF THE LARAMIE RANGE IN THE MORNING BEFORE PUSHING
EAST OF THE LARAMIE RANGE MONDAY. PROGD INSTABILITY PARAMETERS
LOOK FAIRLY MEAGER...WITH SBCAPES TOPPING OUT AROUND 500 J/KG.
HOWEVER...WITH THE PROXIMITY OF THE UPPER WAVE COMBINED WITH
DAYTIME DESTABILIZATION...WOULD EXPECT PERHAPS SOME MORE ORGANIZED
CONVECTION FOR LOCATIONS EAST OF THE MOUNTAINS. SHOWER AND
THUNDERSTORM CHANCES WILL DECREASE SOME FROM WEST TO EAST MONDAY
NIGHT.

UPPER-LOW WILL DEPART DURING THE DAY ON TUESDAY...BRINGING
PRECIPITATION CHANCES TO AN END FROM WEST TO EAST. OTHERWISE...
TUESDAY WILL LIKELY TURN A BIT BREEZY AND WINDS TURN WEST BEHIND
THE DEPARTING PREFRONTAL SFC TROF/PACIFIC FRONT. WIND GUSTS OF 20
TO 35 MPH CAN BE EXPECTED DURING THE AFTERNOON. MONDAY WILL BE THE
COOLEST DAY OF THE WEEK AS CLOUD COVER PROHIBITS THE AMOUNT OF
INSOLATION. EVEN SO...EXECT HIGH TEMPERATURES IN THE 70S/LOW 80S.
HIGHS WILL WARM ABOUT 5 DEGREES OR SO ON TUESDAY...YIELDING MID
70S TO MID 80S.

.LONG TERM...(WEDNESDAY THROUGH SUNDAY)
ISSUED AT 330 PM MDT SUN SEP 21 2014

NOT A WHOLE LOT OF CHANGE IN THE MODEL GUIDANCE THROUGH THE FIRST
HALF OF THE EXTENDED PERIOD. SUBSTANTIAL RIDGE BUILDS OVER THE
INTERMOUNTAIN WEST ON WED AND THIS WILL CAUSE TEMPS TO RISE TO
AROUND 10 DEGREES ABOVE NORMAL ACROSS THE CWA. WINDS WILL BE LIGHT
ON WED WITH WEAK SFC GRADIENTS AND FLOW ALOFT. NOT MUCH CHANGE FOR
THURS EXCEPT THAT SOUTH-TO-SOUTHEAST WINDS WILL BE BREEZY DURING
THE AFTN ACROSS THE PLAINS DUE TO PRESSURE FALLS OVER MONTANA. THE
MODELS CONTINUE TO EXHIBIT DIFFERENCES AS WE HEAD INTO NEXT
WEEKEND. THE GFS IS YET AGAIN THE MOST PROGRESSIVE WITH THE
WESTERN UPPER TROUGH...MOVING ITS AXIS TO NEAR UTAH BY SAT.
WIDESPREAD PRECIP IS PROGGED BEHIND THE COLD FRONTAL PASSAGE ON
SAT NIGHT...SOME OF WHICH COULD BE SNOW OVER THE MTNS. THE ECMWF
SHOWS A CUTOFF OVER CALIFORNIA ON SAT WHICH WOULD SIGNIFICANTLY
DELAY A COOLDOWN AND IS A MUCH DRIER SOLN. WITH SUCH BIG
DIFFERENCES ALMOST A WEEK OUT DECIDED TO INCREASE CLOUD COVER ON
SAT BUT KEEP ONLY SLIGHT CHANCE POPS ON SAT NIGHT.

&&

.AVIATION...(FOR THE 18Z TAFS THROUGH 18Z MONDAY MORNING)
ISSUED AT 1143 AM MDT SUN SEP 21 2014

VFR CONDITIONS THROUGH THE TAF PERIOD. MID LEVEL CLOUD COVER WILL
BE ON THE INCREASE THIS AFTERNOON AND OVERNIGHT AS MOISTURE
STREAMS INTO THE REGION FROM THE SOUTHWEST. THE BEST CHANCE OF
SCATTERED SHOWERS AFTER AROUND 00Z WILL BE TO THE WEST OF THE
LARAMIE RANGE...ESPECIALLY AT RWL. SOUTHEAST WINDS WILL PREVAIL
WITH SPEEDS GENERALLY LESS THAN 15 KTS EXCEPT AT LAR WHERE A FEW
GUSTS TO 25 KTS WILL BE POSSIBLE THIS AFTERNOON.

&&

.FIRE WEATHER...
ISSUED AT 330 PM MDT SUN SEP 21 2014

SHOWER AND THUNDERSTORM CHANCES TO INCREASE TONIGHT AND MONDAY.
THIS WILL KEEP FIRE WEATHER CONCERNS MINIMAL THROUGH AT LEAST
TUESDAY. AN AREA OF LOW PRESSURE WILL BRING RELATIVELY GOOD CHANCES
OF WETTING RAINS TO THE DISTRICTS TONIGHT THROUGH MONDAY EVENING.
DO NOT ANTICIPATE SIGNIFICANT RAINFALL TOTALS...HOWEVER A FEW
TENTHS OF AN INCH WILL BE A POSSIBILITY IN THE HEAVIEST
SHOWERS/STORMS. DRIER AND WARMER CONDITIONS WILL RETURN TUE-FRI.
EVEN SO MINIMUM RELATIVE HUMIDITIES ARE STILL ONLY EXPECTED TO
BOTTOM OUT NEAR 20 PERCENT.

&&

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

&&

$$

SHORT TERM...HAHN
LONG TERM...FINCH
AVIATION...FINCH
FIRE WEATHER...HAHN




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