Area Forecast Discussion
Issued by NWS Cheyenne, WY

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000
FXUS65 KCYS 282212
AFDCYS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHEYENNE WY
412 PM MDT SUN SEP 28 2014

.SHORT TERM...(TONIGHT THROUGH TUESDAY NIGHT)
ISSUED AT 311 PM MDT SUN SEP 28 2014

AN ACTIVE WEATHER PATTERN WILL PERSIST THROUGH MOST OF THE SHORT
TERM FCST PERIOD. WATER VAPOR SATELLITE SHOWS AN IMPRESSIVE UPPER
LOW MOVING SLOWLY EASTWARD ACROSS CENTRAL NV THIS AFTN. VERY MOIST
AIR MASS AHEAD OF THIS SYSTEM OVER WESTERN CO IS ADVECTING NORTH
INTO OUR SOUTHWEST ZONES AS OF 21Z WITH SHOWERS AND EMBEDDED TSTMS
INCREASING IN COVERAGE OVER CARBON AND WESTERN ALBANY COUNTIES. IT
STILL APPEARS THAT UPPER-LEVEL DIFFLUENCE WILL BECOME QUITE STRONG
THIS EVE AND OVERNIGHT AS THE AFORMENTIONED SYSTEM ROLLS INTO THE
FOUR CORNERS REGION AND A 100 KT JET STREAK ROUNDS THE BASE OF THE
TROUGH OVER EASTERN UT. MAIN THREAT FROM SHOWERS AND TSTMS TONIGHT
WILL BE LOCALLY HEAVY RAIN AS UPSTREAM AM SOUNDINGS AND NEAR-TERM
MODELS SHOW PWATS WELL IN EXCESS OF 1 INCH AND EXCELLENT MOISTURE
TRANSPORT IN THE LLVLS. DEEP SATURATION ON VARIOUS BUFKIT PROFILES
WOULD ALSO SUPPORT THIS...SO MENTIONED HEAVY RAIN THRU THE NIGHT
WHERE PCPN SHOULD BE MOST NUMEROUS. STILL SOME CONCERN FOR STRONG
TO SEVERE CONVECTION ALONG THE I-80 CORRIDOR EAST OF CHEYENNE IN
ZONE OF STRONGER DIURNAL HEATING...BUT BECOMING LESS LIKELY WITH
LITTLE VERTICAL CU GROWTH AND DYNAMICS LAGGING TO THE W. IF STORMS
CAN GET GOING BY LATE AFTN OR EARLY EVE...AROUND 1000 J/KG MUCAPES
AND 40-50 KTS OF EFFECTIVE BULK SHEAR WOULD SUPPORT ISOLATED LARGE
HAIL. INCREASED LOWS TONIGHT IN ANTICIPATION OF WIDESPREAD CLOUDS
AND PCPN.

COMPACT SHORTWAVE EJECTS NORTHEAST OVER THE FRONT RANGE ON MON AS
ANOTHER MIDLVL SHORTWAVE DIVES INTO THE PAC NW. MOIST LLVL UPSLOPE
CONTINUES OVER THE EASTERN PLAINS...SUPPORTING STRATIFORM RAIN AND
EMBEDDED TSTMS THROUGH THE DAY. GFS/NAM/EC SHOW SIGNIFICANT SFC
CYCLOGENESIS BY LATE MON NIGHT WITH A 990-995 MB LOW THE NORTHEAST
OF OUR CWA. EXCELLENT QG ASCENT ON THE BACK SIDE OF THIS SYSTEM
AND GOOD MOISTURE SHOULD PROVE SUPPORTIVE OF A PERIOD OF VERY
HEAVY RAIN. LATER SHIFTS MAY WANT TO CONSIDER A FLASH FLOOD WATCH
FOR PARTS OF SOUTHEAST WY AND THE PANHANDLE. HOWEVER...700-300 HPA
MOISTURE PROGS DRY OUT RATHER QUICKLY OVERNIGHT WHICH COULD LIMIT
OVERALL QPF AMOUNTS. GFS SHOWS H7 TEMPS FALLING TO BETWEEN -1C AND
1C OVER THE SNOWIES AND SIERRA MADRES DURING THIS TIME...SO COULD
SEE A FEW INCHES OF SUB-ADVISORY SNOWS THERE. MOST OF THE DYNAMICS
WILL BE WELL TO THE EAST BY THEN...SO DO NOT EXPECT MAJOR MOUNTAIN
ACCUMULATIONS. GROWING CONCERNS FOR SEVERE TSTMS ON MON AFTN. THE
LATEST SPC DAY 2 OUTLOOK PLACES MUCH OF THE PANHANDLE IN A SLIGHT
RISK. GFS AND NAM ARE IN GOOD AGREEMENT WITH UP TO 1500 J/KG OF
SBCAPE AND LIS AROUND -5C. SHEAR PROFILES WILL BE RATHER STRONG AS
WELL BENEATH AROUND 40 KTS OF H5 FLOW. MAIN QUESTION RIGHT NOW IS
WHETHER WE WILL HAVE ENOUGH SUNSHINE TO REALIZE INSTABILITY. HELD
OFF ON MENTION OF SEVERE IN THE GRIDS FOR NOW.

WEST WINDS ARE SET TO INCREASE CONSIDERABLY BY TUE AM AS THE LOW
LIFTS OUT. GFS SHOWS A POCKET OF 750 MILLIBAR WINDS AS HIGH AS 70
KTS AT 12Z TUE FROM THE LARAMIE RANGE EASTWARD INTO CYS. BEST
SUBSIDENCE MAY REMAIN TO THE SOUTH...AND WINDS ALOFT ARE FCST TO
DECREASE PRIOR TO AFTN MIXING. EVEN SO...WE COULD SEE OUR FIRST
HIGH WIND EVENT OF THE SEASON AS THE H85 CAG-CPR GRADIENT
INCREASES TO AROUND 60 METERS. LATER SHIFTS WILL NEED TO MONITOR
FOR A POSSIBLE HIGH WIND WATCH FOR AT LEAST THE WIND PRONE AREAS
AS WE GET CLOSER TO THE EVENT. LOOK FOR PCPN TO SLOWLY DECREASE IN
COVERAGE FROM SOUTHWEST TO NORTHEAST ON TUE.

.LONG TERM...(WEDNESDAY THROUGH SUNDAY)
ISSUED AT 311 PM MDT SUN SEP 28 2014

ACTIVE NORTHWEST FLOW WILL CONTINUE WED-THU WITH DRIER CONDITIONS
BENEATH BUILDING HEIGHTS FOR FRI-SUN. INITIALLY...POSITIVELY TILTED
SHORTWAVE ENERGY WILL BE ACROSS MONTANA/IDAHO. THIS SHORTWAVE WILL
SLIDE SOUTHEAST INTO THE CWFA LATE WEDNESDAY AND WEDNESDAY NIGHT
PROVIDING THE BEST OPPORTUNITY FOR PRECIPITATION THROUGH THE PERIOD.
PROGD FREEZING LEVELS HOVER AROUND 10 KFT...WHICH WOULD SUGGEST
ACTUALLY SNOW-LEVELS SHOULD END UP BEING AROUND 9500 FEET. A COUPLE
OF INCHES OF SNOW ACCUMULATIONS WILL CERTAINLY BE A POSSIBILITY
ABOVE THIS LEVEL. TEMPERATURES WILL AGAIN BE BELOW AVERAGE ON
WEDNESDAY WITH H7 TEMPERATURES PROGD TO BE AROUND 2C. HIGH
TEMPERATURES WILL ONLY REACH THE 50S...WITH PERHAPS A FEW LOW 60S IN
LOWEST ELEVATIONS. 12Z MODELS HAVE TRENDED TOWARDS THE MORE
PROGRESSIVE 00Z ECMWF WITH THURSDAYS CLIPPER SYSTEM. ASIDE FROM A
BRIEF REINFORCING SHOT OF COOLER AIR...EXPECT LITTLE IMPACT FROM
THIS SYSTEM. DESPITE THE BRIEF LLVL CAA...BETTER INSOLATION ON BOTH
THU-FRI SHOULD ALLOW HIGHS TO CLIMB BACK INTO THE 60S IN ALL LOWER
ELEVATIONS. BOTH THE ECMWF/GFS MAINTAIN THE DRY AND WARMING TREND
INTO SAT-SUN WITH H7 PROGS CLIMBING BACK TO 7-8C. HIGH TEMPERATURES
BOTH DAY SHOULD BE CLOSE TO 70F WEST OF THE LARAMIE RANGE...WITH 70S
EAST OF THE LARAMIE RANGE.

&&

.AVIATION...(FOR THE 18Z TAFS THROUGH 18Z MONDAY MORNING)
ISSUED AT 1200 PM MDT SUN SEP 28 2014

CHALLENGES ARE MANY FOR THIS TAF FORECAST...INITIALLY TRYING TO
TIME A LINE OF CONVECTION LIFTING NORTH AND EAST ACROSS SE WYOMING
AND THE WRN NEBRASKA PANHANDLE...THEN TRYING TO TIME ITS DEPARTURE
AND RESIDUAL LOW CLOUD/FOG POTENTIAL LATE TONIGHT AND EARLY
MONDAY. BAND OF PRECIP IS JUST NOW BEGINNING TO LIFT NORTH ACROSS
CARBON COUNTY...WITH NO IMPACT AT KRWL JUST YET. THIS WILL CHANGE
THRU 20Z HOWEVER...AS DETERIORATING CONDITIONS ARE EXPECTED THERE.
THIS PRECIP BAND WILL THEN ACCELERATE EAST IMPACTING KLAR AFTER
21Z AND KCYS MOSTLY AFTER 00Z. AREAS EAST INTO THE PANHANDLE WILL
HAVE TO WAIT UNTIL THIS EVENING TO BE IMPACTED BY THIS AREA OF
CONVECTION. THIS AREA OF SHOWERS WILL SPREAD EAST THROUGH MONDAY
MORNING...GRADUALLY BRINGING AN END TO PRECIPITATION CHANCES FROM
WEST TO EAST. HAVE GENERALLY BOTTOMED CONDITIONS OUT IN MOST
PLACES AT MVFR/IFR. MOUNTAINS WILL BE OBSCURED AS WELL THRU MUCH
OF THE PERIOD IMPACTING ROUTES ACROSS SE WYOMING.

&&

.FIRE WEATHER...
ISSUED AT 311 PM MDT SUN SEP 28 2014

NO FIRE WEATHER CONCERNS. A COOLER AND UNSETTLED WEATHER PATTERN
WILL RESULT IN HUMIDITIES WELL ABOVE CRITICAL THRESHOLDS. SHOWER
AND THUNDERSTORM ACTIVITY WILL BE WIDESPREAD OVER THE NEXT 24 TO
36 HOURS...WITH UP TO A HALF INCH OF RAIN POSSIBLE FOR MANY AREAS
ACROSS SOUTHEAST WY AND THE WESTERN NE PANHANDLE. WINDY CONDITIONS
ARE LIKELY FOR TUESDAY.

&&

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

$$

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





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