Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 231127
AFDUNR

AREA FORECAST DISCUSSION FOR WESTERN SD AND NORTHEASTERN WY
NATIONAL WEATHER SERVICE RAPID CITY SD
527 AM MDT WED JUL 23 2014

.SHORT TERM...TODAY THROUGH THURSDAY NIGHT
ISSUED AT 230 AM MDT WED JUL 23 2014

08Z SURFACE ANALYSIS HAD LEE TROUGH FROM CENTRAL MT INTO
SOUTHEAST WY. WEAK OUTFLOW BOUNDARIES NOTED ACROSS SOUTHEAST MT
INTO CENTRAL SD. CLUSTER OF SEVERE CONVECTION HEADING SOUTHEAST
ACROSS SOUTHERN PORTIONS OF THE CWA EARLY THIS MORNING...SPURRED
ON BY WEAK SHORTWAVE OVER NORTHEAST WY PER WATER VAPOUR LOOP AND
30KT LOW LEVEL JET. BEHIND IT...UPPER RIDGE BUILDING OVER MT/WY
WITH UPSTREAM WAVES OVER ID/WESTERN MT. SHORT RANGE GUIDANCE SHOWS
ORGANIZED TSRA EXITING CWA BY 12Z...BUT COULD BE A FEW LINGERING
CELLS EARLY.

TODAY...UPPER RIDGE AMPLIFIES INTO THE NORTHERN PLAINS WITH THERMAL
RIDGE SLOWLY BUILDING. LEE TROUGH SHIFTS EAST A BIT WITH LOW
PRESSURE DEVELOPING OVER CENTRAL WY IN RESPONSE TO SHORTWAVE
ROTATING AROUND RIDGE INTO MT. SURFACE HIGH OVER THE MIDWEST
COMBINED WITH WY LOW RESULTS IN SOMEWHAT TIGHT PRESSURE GRADIENT AND
STRENGTHENING SOUTH/SOUTHEAST FLOW UP TO 25KTS. AHEAD OF LEE
TROUGH...2-4KJ/KG MLCAPE DEVELOPS BY AFTERNOON WITH MLCIN
WEAKENING...ESPECIALLY AFTER 21Z. DIRECT UPSTREAM MID-LEVEL
SHORTWAVES HARD TO DEPICT IN GUIDANCE OR TO SEE UPSTREAM ON WATER
VAPOUR...BUT A COUPLE OF WEAK 700MB SHORTWAVES FORECAST BY MODELS
TO SHIFT INTO EASTERN WY LATE IN THE DAY. THIS SHOULD PROVIDE THE
LAST NUDGE TO ALLOW TSRA TO ERUPT. 0-6KM BULK SHEAR 15-20M/S WITH
RATHER SLOW RIGHT-MOVING SUPERCELL MOVEMENT...IN THE 5-10KT RANGE.
0-6KM MEAN WIND 15-20KTS. WITH SLIGHTLY FASTER MOVEMENT THAN
GUIDANCE SHOWED YESTERDAY...IT APPEARS FLASH FLOOD POTENTIAL NOT
AS CLEARLY DEFINED...ALTHOUGH PRECIPITABLE WATERS DO REACH 150
PERCENT OF NORMAL. LOCAL FLASH FLOOD COMPOSITE PARAMETER HAS
BACKED OFF NOTICEABLY. THUS WILL HOLD OFF ON ANY WATER HEADLINES
AT THIS POINT AND PASS ON CONCERN TO DAY SHIFT. TEMPERATURES WILL
BE NEAR GUIDANCE IN THE WEST AND NEAR BIAS CORRECTED VALUES IN THE
EAST.

TONIGHT...LOW PRESSURE CONSOLIDATES OVER EASTERN MT WITH WARM FRONT
ACROSS NORTHERN SD. ACTIVITY WILL SLOWLY SHIFT EAST AS 30-40KT LOW
LEVEL JET DEVELOPS/INTERSECTS WARM FRONT...TRANSLATING BEST
850-700MB THETA-E ADVECTION EAST OF AREA AFTER MIDNIGHT.
TEMPERATURES WILL STAY UP GIVEN PERSISTENT SOUTHERLY FLOW.

THURSDAY...UPPER RIDGE FLOPS OVER IN RESPONSE TO SHORTWAVE/UPPER LOW
MOVING ACROSS SOUTHERN BC/AB. LOW PRESSURE OVER EASTERN MT WILL MOVE
INTO CENTRAL SD. AHEAD OF IT...STRONG THERMAL RIDGE PEAKS OVER CWA
WITH DRY LINE SURGING INTO SOUTHWEST SD FROM WY. TRAILING COOL FRONT
BEHIND LOW WILL DROP INTO CWA IN THE AFTERNOON. 1-2KJ/KG POST
FRONTAL MLCAPE DEVELOPS OVER THE NORTHERN HALF OF CWA. 2-3KJ/KG
MLCAPE IN WARM SECTOR LIKELY...BUT MLCIN WILL BE MUCH STRONGER
THERE. 0-6KM SHEAR SUFFICIENT FOR SUPERCELLS POST FRONTAL IN THE
NORTH...WITH MARGINAL VALUES IN THE SOUTH...BUT DEEP MIXING MAY
PROMOTE SEVERE WIND GUSTS WITH ANY CONVECTION THAT DEVELOPS OVER
SOUTHWEST SD. EXPECT SEVERE STORMS LATER THURSDAY AFTERNOON IN THE
FAR NORTH. TEMPERATURES WILL BE SLIGHTLY BELOW GUIDANCE POST FRONTAL
AND HAVE ADJUSTED NEAR/ABOVE GUIDANCE SOUTH OF DRY LINE.

THURSDAY NIGHT...COOL FRONT SURGES THROUGH CWA WITH BREEZY SPOTS
BEHIND IT AND CONVECTION MOVING EAST OF THE CWA OVERNIGHT.
TEMPERATURES NEAR GUIDANCE.

&&

.LONG TERM...FRIDAY THROUGH TUESDAY
ISSUED AT 230 AM MDT WED JUL 23 2014

UPPER-LEVEL RIDGE BREAKS DOWN BY THE END OF THE WEEK AS A STRONG
SHORTWAVE TROF SLIDES SOUTHEAST ACROSS SOUTH CENTRAL CANADA. THIS
WILL BRING ANOTHER CHANCE OF SHOWERS AND STORMS TO THE AREA ON
FRIDAY INTO SATURDAY.

AS THE SHORTWAVE TROF SLIDES EAST...THE UPPER RIDGE WILL RETROGRADE
WEST...BRINGING PREDOMINANTLY NORTHWEST UPPER FLOW TO THE NORTHERN
PLAINS. THIS WILL RESULT IN COOLER TEMPERATURES FOR THE REST OF THE
EXTENDED PERIOD...ALONG WITH MAINLY DRY CONDITIONS.

&&

.AVIATION...(FOR THE 12Z TAFS THROUGH 12Z THURSDAY MORNING)
ISSUED AT 522 AM MDT WED JUL 23 2014

ISOLATED TO SCATTERED THUNDERSTORMS WILL CONTINUE JUST EAST OF
KRAP THROUGH SOUTHWEST AND SOUTH CENTRAL SOUTH DAKOTA THROUGH 15Z.
MVFR VIS AND GUSTY WINDS ARE EXPECTED IN THE VICINITY OF THESE
STORMS. VFR CONDITIONS WILL PREVAIL ELSEWHERE THROUGH MUCH OF
THE DAY. ISOLATED TO SCATTERED THUNDERSTORMS WILL RE-DEVELOP OVER
NORTHEASTERN WYOMING AND WESTERN SOUTH DAKOTA LATE THIS AFTERNOON
AND PERSIST INTO THE EVENING. SOME SEVERE STORMS WILL BE POSSIBLE
WITH HEAVY RAIN.

&&

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

$$

SHORT TERM...HELGESON
LONG TERM...15
AVIATION...15





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