Severe Thunderstorm Warning 3, NWS Cheyenne, WY

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHEYENNE WY
704 PM MDT SUN MAY 9 2010

.UPDATE...CURRENT KCYS RADAR LOOP SHOWS SCATTERED RAIN SHOWERS
NORTH OF INTERSTATE 80 WITH A FEW EMBEDDED TSTORMS ACROSS CONVERSE
AND NIOBRARA COUNTIES THIS EVENING. THERE ARE ALSO A FEW TSTORMS
DEVELOPING SOUTH OF THE AREA ACROSS NORTHEAST COLORADO. WITH
COVERAGE OF PRECIP NOT AS HIGH AS PREVIOUSLY EXPECTED THIS
EVENING...ESPECIALLY ACROSS THE SOUTHERN HALF OF THE FORECAST
AREA...LOWERED POP OVER THE HIGH PLAINS AND VALLEYS THIS EVENING.
WITH MODEL SOUNDINGS AND MOS GUIDANCE SHOWING INCREASING LLVL
MOISTURE WITH INCREASING EAST TO SOUTHEAST WINDS LATE TONIGHT FROM
INTERSTATE 80 FROM THE SRN LARAMIE RANGE EASTWARD TOWARDS
SIDNEY...ADDED PATCHY FOG TO THE FORECAST FOR LARAMIE
COUNTY...KIMBALL AND CHEYENNE COUNTIES NORTHWARD TOWARDS THE NORTH
PLATTE RIVER VALLEY TONIGHT AND EARLY MONDAY MORNING. DENSE FOG IS
NOT EXPECTED AT THIS TIME. PRECIPITATION IS EXPECTED TO INCREASE
IN COVERAGE BY MONDAY...WITH THE BEST CHANCE OF SHOWERS OVER THE
MOUNTAINS AND NORTH OF THE NORTH PLATTE RIVER VALLEY.
&&

PREV DISCUSSION... /ISSUED 224 PM MDT SUN MAY 9 2010/

SHORT TERM...TONIGHT THROUGH TUESDAY...
SHOWERS HAVE BEEN DEVELOPING OVER AND NEAR THE MOUNTAINS SINCE THE
LATE MORNING.  LIFTED INDICES OVER MUCH OF SOUTHEAST WYOMING ARE
CURRENTLY AROUND ZERO...WITH LAPS CAPE VALUES AS HIGH AS 400 J/KG
OVER ALBANY COUNTY.  COVERAGE OF THE SHOWERS HAS BEEN SLOWLY
INCREASING BUT THE ACTIVITY STILL REMAINS WEAK AS OF 1930Z PARTLY
DUE TO THE DRY BOUNDARY LAYER.  SOME THUNDERSTORMS ARE POSSIBLE
THROUGH THE EARLY EVENING AS THE SHOWER COVERAGE INCREASES.  MAIN
THREAT FROM MOST OF THE  STORMS WILL CONTINUE TO BE WIND GUSTS TO 40
MPH...WITH SMALL HAIL AND MODERATE RAINFALL FROM THE STRONGER
STORMS.  SHOWERS SHOULD BECOME WIDESPREAD EARLY TONIGHT AS THE
SHORTWAVE TROUGH APPROACHES AND SURFACE UPSLOPE INCREASES.  BY 12Z
MONDAY THE MAIN CIRCULATION ALOFT WILL BE OVER WESTERN COLORADO AND
EASTERN UTAH AND THE ASSOCIATED SURFACE LOW OVER EASTERN COLORADO.
MODELS ARE FAIRLY SIMILAR BUT HAVE LEANED MORE TOWARDS THE ECMWF AND
GFS AS THEY HAVE HAD GOOD RUN TO RUN CONSISTENCY.  THE WIDESPREAD
SHOWERS WILL CONTINUE OVER THE CWA ON MONDAY...THE DYNAMICS SUPPORTING
THE HEAVIER PRECIPITATION OVER THE NORTH PART OF THE CWA STARTING LATE
TONIGHT AND THEN THROUGH MONDAY.  THE SNOW LEVEL SHOULD LOWER TO
ABOUT 6500 FEET MSL TONIGHT WITH SMALL ACCUMULATIONS OVER THE
MOUNTAINS.  LIGHT ACCUMULATING SNOW WILL CONTINUE OVER THE MOUNTAINS
THROUGH MONDAY.  THE STORM SYSTEM WILL MOVE ONTO THE PLAINS MONDAY
NIGHT WITH A SHORTWAVE RIDGE AXIS OVER THE CWA.  THE RESULT SHOULD BE
A BRIEF BREAK IN THE MORE WIDESPREAD QPF OVER THE CWA.  THE NEXT
PACIFIC STORM SYSTEM WILL MOVE OVER THE CENTRAL ROCKIES BY 00Z
WEDNESDAY.  AT THAT TIME...THE ECMWF PLACES THE UPPER LOW OVER UTAH
WITH THE ASSOCIATED SURFACE LOW OVER COLORADO.  MOISTURE AND SYNOPTIC
LIFT WILL INCREASE OVER MUCH OF THE CWA ON TUESDAY.  FOR NOW HAVE
TRENDED POPS UP AND TEMPERATURES DOWN A BIT FOR TUESDAY.  THE SNOW
LEVEL WILL BE AROUND 7000 FEET MSL ON TUESDAY...WITH 2 TO 4 INCHES OF
NEW SNOW POSSIBLE FOR THE SNOWY RANGE AND NORTHERN LARAMIE RANGE.
AT THIS TIME WILL GO WITH BETWEEN A QUARTER AND A THIRD OF AN INCH OF
LIQUID PRECIPITATION ON TUESDAY OVER SOUTHEAST WYOMING.

LONG TERM...TUESDAY NIGHT THROUGH SUNDAY
RELIED ON THE ECMWF FOR THE LONG TERM FORECAST.  VERY COOL AND
UNSETTLED WEATHER WILL OCCUR DURING THE FIRST HALF OF THE EXTENDED
PERIOD.  SW FLOW WILL BE ALOFT TUES NIGHT AS THE UPPER LOW MOVES
OVER THE CENTRAL ROCKIES.  DEEP MOIST UPSLOPE FLOW WILL OCCUR FROM
TUES NIGHT THROUGH THURS AS THE SURFACE LOW MOVES ACROSS THE CENTRAL
PLAINS.  HEAVY RAINS WILL OCCUR ACROSS THE SOUTHEAST PLAINS OF WY
AND INTO THE NEB PANHANDLE DURING THE EVENING...BEFORE CHANGING TO
SNOW IN THE WY PORTION OF THE CWA BY MIDNIGHT.  HEAVY SNOW MAY OCCUR
IN THE SOUTHERN LARAMIE RANGE AT THIS TIME.  ON WED...SNOW WILL
CONTINUE THROUGH THE DAY ABOVE 5000 FEET WITH RAIN ELSEWHERE.  THE
UPPER LOW WILL MOVE OVER WY AT THIS TIME.  SNOW WILL OCCUR ACROSS
THE ENTIRE CWA WED NIGHT AS THE UPPER LOW MOVES INTO THE DAKOTAS.  WRAP
AROUND MOISTURE WILL MOVE OVER THE FORECAST AREA AND COLD AIR
ADVECTION WILL OCCUR AT THIS TIME.  CYCLONIC FLOW WILL CONTINUE
ALOFT ON THUR...WHICH WILL CONTINUE THE CHANCE OF SNOW IN THE
MORNING...WHICH WILL CHANGE TO RAIN BY AFTERNOON WITH THE EXCEPTION
OF THE MOUNTAINS. THERE IS ALSO A CHANCE OF RAIN AND SNOW SHOWERS
THUR NIGHT WITH SW FLOW ALOFT AS THE UPPER TROUGH AXIS APPROACHES
THE AREA.  A SLIGHT CHANCE OF SHOWERS AND AFTERNOON THUNDERSTORMS
CAN BE EXPECTED ACROSS THE WY PORTION OF THE CWA FRI AS THE UPPER
TROUGH AXIS MOVES INTO THE STATE.  THE DISTURBANCE WILL MOVE ACROSS
THE EASTERN PORTION OF THE FORECAST AREA SAT WHICH WILL RESULT IN A
CHANCE OF SHOWERS AND AFTERNOON AND EVENING THUNDERSTORMS FOR THE
NEB PANHANDLE.  SAT NIGHT AND SUN WILL FINALLY BE DRY AS AN UPPER
RIDGE BUILDS TO THE WEST...RESULTING IN NW FLOW ALOFT OVER THE
AREA.  HIGH TEMPERATURES WILL BE FROM 20 TO 25 DEGREES BELOW NORMAL
FOR WED AND THUR...5 TO 10 DEGREES BELOW NORMAL FOR FRI...AND AROUND
NORMAL FOR SAT AND SUN.

AVIATION...00Z TAF ISSUANCE
WIDELY SCATTERED RAIN SHOWERS WILL GRADUALLY INCREASE IN COVERAGE
LATER THIS EVENING...WITH RAIN CHANGING TO SNOW OBSCURING VISIBILITIES
OVER THE SNOWY RANGE LATER TONIGHT. PERIODS OF VFR TO MVFR CEILINGS
AND VISIBILITIES IN PRECIP WILL DEVELOP OVERNIGHT AND CONTINUE
THROUGH MOST OF MONDAY. NORTHWEST WINDS WILL DEVELOP OVER SOUTHEAST
WYOMING AND GUST AS HIGH AS 25 KNOTS MONDAY...WHILE EAST WINDS 10
TO 15 KNOTS WILL PREVAIL OVER THE NEBRASKA PANHANDLE AS THE MAIN
SURFACE LOW PRESSURE SYSTEM TAKES SHAPE OVER EASTERN COLORADO AND
MOVES EAST. VFR CONDITIONS WILL RETURN MONDAY NIGHT AND TUESDAY
MORNING...FOLLOWED BY DETERIORATING CEILINGS AND VISIBILITIES IN
RAIN AND SNOW TUESDAY AFTERNOON AND NIGHT IN ADVANCE OF THE NEXT
STORM SYSTEM.

FIRE WEATHER...
WARM...BREEZY AND DRY CONDITIONS WILL CONTINUE OVER MUCH OF THE
CWA THROUGH THE EARLY EVENING. RELATIVE HUMIDITIES WILL BE BETWEEN
20 AND 30 PERCENT OVER THE LOWER ELEVATIONS DURING THAT TIME. IN
ADDITION...SOME HIGH BASED THUNDERSTORMS WILL BE POSSIBLE THROUGH
THE EARLY EVENING. THE ACTIVITY WILL PRODUCE BRIEF AND LOCAL WIND
GUSTS TO 40 MPH. COOL AND WET WEATHER WILL THEN BE COMMON OVER THE
CWA THROUGH THURSDAY AS TWO STORM SYSTEMS MOVE ACROSS THE CENTRAL
ROCKIES. WARMER AND DRIER WEATHER WILL RETURN TO THE CWA FRIDAY
AND NEXT WEEKEND.

&&

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

&&

SHORT TERM/AVIATION...
LONG TERM...

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