Severe Thunderstorm Warning 366, NWS Rapid City, SD

AREA FORECAST DISCUSSION FOR WESTERN SD AND NORTHEASTERN WY
NATIONAL WEATHER SERVICE RAPID CITY SD
300 PM MDT WED AUG 22 2007

.DISCUSSION...MODELS CONTINUE TO SHOW HEAVY RAINFALL EVENT FOR
TONIGHT. SATELLITE IMAGERY TODAY HAS SHOWN A DEVELOPING BAROCLINIC
LEAF SETTING UP ALONG THE VIGOROUS SHORTWAVE ACROSS THE NORTHERN
ROCKIES. THE SHORTWAVE WILL PUSH INTO NE WY THIS EVENING WITH VERY
STRONG FORCING AND SHEAR. STORMS ALREADY FIRING ACROSS WRN AND
CENTRAL WY AND WILL CONTINUE TO PUSH EAST AS MORE CONVECTION
DEVELOPS THIS AFTERNOON. RAIN WILL THEN OVERSPREAD THE WESTERN HALF
OF THE CWA WITH THUNDERSTORMS MORE LIKELY ACROSS NE WY AND SOUTHWEST
SD WHERE THE BEST INSTABILITY LIES. SHEAR PROFILES ARE QUITE STRONG
WITH 0-1 KM SHEAR AT 20-25 KT IN SOME LOCATION ALONG THE
SOUTHWESTERN CWA BORDER. ALONG WITH THE DEVELOPMENT OF
SUPERCELLS...TORNADOES WILL BE A POSSIBILITY. SEVERE WEATHER THREAT
MAY CONTINUE AFTER 06Z BUT TO A LESSER EXTENT. OTHERWISE RAINFALL
WILL SPREAD OVER THE REST OF THE CWA OVERNIGHT. 12Z NAM HAS STARTED
TO SHIFT THE BEST FORCING FARTHER NORTH ACROSS NORTHWEST SOUTH
DAKOTA AND THE NORTHERN HILLS...WHILE THE GFS HAS REMAINED MORE
CONSISTENT. HAVE OPTED FOR A BLEND OF THE TWO AGAIN AS FAR AS TIMING
AND QPF AMOUNTS GO. THE NAM SHOWS PRECIPITABLE WATER VALUES
INCREASING DRAMATICALLY BETWEEN 00Z AND 06Z WITH AN INCREASE FROM 1
INCH TO 1.5 INCHES. THIS...ALONG WITH THIS STRENGTH OF THE
SHORTWAVE...CONTINUES TO SUPPORT THE POSSIBILITY FOR LOCALLY HEAVY
RAINFALL TONIGHT. MODELS CONTINUE TO SHOW STORM MOTIONS AT 10 TO 20
KT WITH SUPERCELL MOTION AT 20 TO 25 KT WHICH WILL LIMIT THE
POTENTIAL FOR LARGE AMOUNTS OF QPF WITHIN A VERY SHORT AMOUNT OF
TIME.

SHORTWAVE WILL PUSH THROUGH THE AREA THURSDAY MORNING WITH
RESIDUAL SHOWERS AND STORMS...THEN ANOTHER SHORTWAVE MOVING THROUGH
THE AREA IN THE AFTERNOON AND EVENING HOURS. WITH MOSTLY CLOUDY
SKIES EXPECTED...HAVE DROPPED THURSDAY HIGHS TO UPPER 60S/LOWER 70S.
CHANCE FOR SHOWERS AND STORMS CONTINUE THURSDAY NIGHT THROUGH FRIDAY
AS ANOTHER TROUGH AXIS APPROACHES THE AREA KEEPING A PARADE OF
SHORTWAVES MOVING THROUGH. CONDITIONS WILL BEGIN DRYING OUT FRIDAY
NIGHT AS THE MAIN AXIS PUSHES THROUGH LEAVING SUBSIDENCE BEHIND IT.

EXTENDED...UPPER TROF MOVES EAST OF THE AREA AS A PASSIVE WEAK UPPER
RIDGE MOVES THROUGH FOR THE WEEKEND. THIS WILL BRING GENERALLY DRIER
WEATHER ALONG WITH A REBOUND IN TEMPERATURES. UPPER FLOW WILL TURN
SOUTHWEST FOR THE BEGINNING OF THE WORK WEEK OPENING THE DOOR FOR A
VARIETY OF WEAK DISTURBANCES TO MOVE THROUGH. THIS WILL BRING A
DAILY CHANCE FOR PRECIPITATION TO VARIOUS PARTS OF THE AREA THROUGH
MID-WEEK.

&&

.AVIATION...SHOWERS AND THUNDERSTORMS WILL DEVELOP OVER NORTHEAST
WYOMING AND FAR WESTERN SOUTH DAKOTA LATE THIS AFTERNOON INTO EARLY
THIS EVENING BEFORE SPREADING EASTERLY. CIGS WILL DIMINISH TO IFR
LEVELS OVER THE BLKHLS AND ERN WYO RTES INCLUDING KGCC LATE THIS
EVENING...WHILE REMAINING AREAS WILL DIMINISH TO MVFR LEVELS. CIGS
WILL START IMPROVING LATE THURSDAY MORNING.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...NONE.
WY...NONE.

&&

CALDERON/SCHILD

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