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000
FXUS63 KUNR 171121
AFDUNR

AREA FORECAST DISCUSSION FOR WESTERN SD AND NORTHEASTERN WY
NATIONAL WEATHER SERVICE RAPID CITY SD
521 AM MDT FRI MAY 17 2013

.SHORT TERM...TODAY THROUGH SATURDAY NIGHT
ISSUED AT 238 AM MDT FRI MAY 17 2013

A VERY LONG FRONTAL BOUNDARY...ALMOST REACHING FROM
THE EAST COAST TO THE WEST COAST...IS ANALYZED ON THE 06Z SURFACE
MAP AND COINCIDES WITH THE SOUTHERN BOUNDARY OF THE MAIN BRANCH OF
WESTERLYS ACROSS THE COUNTRY. ACROSS THE CENTRAL/NORTHERN PLAINS THE
FRONT IS SHOWN AS A WARM FRONT JUST SOUTH AND PARALLEL TO THE SOUTH
DAKOTA NEBRASKA BORDER.  TO THE WEST A FULL LATTITUDE TROUGH IS
SLOWLY MOVING ONTO THE WEST COAST AND SPINNING OFF NUMEROUS SHORT
WAVES INTO THE WESTERLYS.  THE PACIFIC TROUGH IS FORECAST TO
CONTINUE INLAND DURING THE DAY TODAY AND WILL PUSH UP A SHARPER
UPPER LEVEL RIDGE OVER THE PLAINS. ON THE SURFACE AN INVERTED LEE
SIDE TROUGH IS FORECAST TO STRENGTHEN UNDER THE SOUTHWEST FLOW
ALOFT..AND MOVE THE WARM FRONT NORTHWARD INTO SOUTHERN AND CENTRAL
SOUTH DAKOTA.  THIS WARM FRONTAL BOUNDARY...ACTING LIKE A DRY
LINE...IS EXPECTED TO BE A FOCUSING FEATURE FOR PM CONVECTION WITH
SOME MOISTURE POOLING ON THE NORTH SIDE OF THE BOUNDARY. IN ADDITION
AN INCREASING EASTERLY COMPONENT TO SURFACE FLOW WILL CONTRIBUTE
SOME UPSLOPE/OROGRAPH LIFT IN THE BLKHLS REGION. THE LATEST MODEL
SOUNDINGS INDICATE THE POTENTIAL FOR SURFACE BASED CAPE IN EXCESS OF
2000 KJ/KG ALONG THE WARM FRONT FROM THE SOUTHERN BLKHLS INTO
CENTRAL SOUTH DAKOTA...AND 1-6KM BULK SHEAR OVER 40 KTS. THESE
FEATURES WOULD SUPPORT ISOLATED SUPERCELL DEVELOPMENT.  IF THERE
WERE A SIGNIFICANT MID OR UPPER LEVEL LIFTING MECHANISM...THE
ACTIVITY COULD BE MORE WIDESPREAD AND MORE INTENSE...BUT THE
STRONGEST UPPER LEVEL VORTICITY REMAINS WEST OF THE FORECAST AREA
UNTIL AFTER MAX HEATING.

SATURDAY...THE LARGE PACIFIC TROUGH IS FORECAST TO CROSS THE CONTDVD
...MAKE ITS WAY SLOWLY INTO THE PLAINS...AND SET UP CONDITIONS FOR
AN EXTENDED PRECIPITATION EVENT FOR THE DAKOTAS AND EASTERN WYOMING.
A STRONG MOISTURE CONVEYOR IS FORECAST TO SET UP BY EARLY SATURDAY
AND A LARGE SHORT WAVE IS SHOWN SPINNING OUT OF THE TROUGH INTO THE
NORTHERN PLAINS.  MODEL SOUNDINGS SHOW HIGH RH THROUGHOUT THE COLUMN
AND CAPE VALUES PUSHING 2500 KJ/KG IN THE PLAINS EAST OF THE BLKHLS
BY EARLY SATURDAY AFTERNOON. MORE DYNAMIC SUPPORT IS AVAILABLE IN
THE UPPER LEVELS ON SATURDAY...RESULTING IN A LARGER AREA OF THREAT
FOR SUPER CELL THUNDERSTORMS COVERING EASTERN AND CENTRAL SOUTH
DAKOTA AND CENTRAL NEBRASKA.

&&

.LONG TERM...SUNDAY THROUGH THURSDAY
ISSUED AT 238 AM MDT FRI MAY 17 2013

MEDIUM RANGE MODELS REMAIN IN GENERAL AGREEMENT WITH PROGRESSION AND
TRACK OF UPPER TROF/LOW LATER THIS WEEKEND AND EARLY NEXT
WEEK...WITH MODELS CONTINUING TO TREND THE SYSTEM A BIT FURTHER
SOUTH ACROSS THE REGION. THE CONSENSUS OF THE MODELS AT 00Z WAS TO
MOVE THE TROF ACROSS THE NORTHERN ROCKIES AND INTO THE NORTHERN
PLAINS SUNDAY INTO SUNDAY NIGHT. UPPER LOW IS THEN PROGGED TO
DEVELOP ACROSS SD SUNDAY NIGHT...WITH THE SYSTEM CUTTING OFF AND
STALLING SOMEWHERE ACROSS EASTERN SD MONDAY INTO TUESDAY. THE LATEST
CONSENSUS WOULD BRING A SIGNIFICANT AMOUNT OF RAINFALL TO MUCH OF
THE FORECAST AREA...ESPECIALLY FROM THE BLACK HILLS AREA EASTWARD
ACROSS MUCH OF SD. 1 TO 3 INCHES OF RAINFALL WOULD GENERALLY BE
EXPECTED ACROSS THESE AREAS FROM SUNDAY INTO TUESDAY...ESPECIALLY IF
THE SYSTEM STALLS OUT OVER EASTERN SD FOR AS LONG AS IS PROJECTED.
ANY DEVIATIONS TO THE TRACK AND SPEED WOULD ALTER THIS
POTENTIAL...BUT THIS IS LOOKING MORE AND MORE LIKE A SIGNIFICANT
RAINFALL EVENT FOR MUCH OF THE AREA.

HAVE RAISED POPS FOR MOST AREAS FROM SUNDAY TO TUESDAY...WITH LIKELY
POPS ACROSS MUCH OF THE AREA SUNDAY THROUGH MONDAY NIGHT. SOME
THUNDERSTORMS ARE EXPECTED ACROSS THE AREA ON SUNDAY...BUT THIS
SHOULD BECOME MORE OF A GENERAL RAIN/SHOWER EVENT SUNDAY NIGHT AND
EARLY NEXT WEEK AS LOW DEVELOPS EAST OF THE AREA AND INSTABILITY
DIMINISHES. TEMPS COULD BECOME COLD ENOUGH LATE SUNDAY NIGHT AND
MONDAY NIGHT FOR SOME SNOW ACROSS THE HIGHEST ELEVATIONS OF THE
BLACK HILLS...BUT WILL LEAVE OUT POSSIBILITY FOR NOW UNTIL THINGS
BECOME A BIT CLEARER WITH REGARD TO THE TRACK. HIGHS SUNDAY ARE
EXPECTED TO BE IN THE 60S TO MID 70S...WARMEST TOWARD SOUTH CENTRAL
SD. COOL AND BREEZY/WINDY CONDITIONS ARE THEN EXPECTED MONDAY AND
TUESDAY.

AT THIS POINT...IT LOOKS THE SYSTEM WILL EJECT SLOWLY EASTWARD OUT
OF THE REGION TUESDAY AND TUESDAY NIGHT...WITH GRADUALLY IMPROVING
CONDITIONS FROM WEST TO EAST. FORECAST UNCERTAINTY INCREASES FOR THE
REST OF THE FORECAST...AS IT WILL BE DETERMINED BY THE PROGRESSION
OF THIS SYSTEM. THERE SHOULD BE A GENERAL WARMING TREND FOR THE
MIDDLE AND LATTER PART OF THE WEEK...ESPECIALLY ACROSS WESTERN
AREAS...BUT POSSIBLY NOT AS SIGNIFCANT AS HAS BEEN EXPECTED. WILL GO
WITH A MAINLY DRY FORECAST FOR WEDNESDAY AND WEDNESDAY NIGHT AS WEAK
RIDGING IS PROGGED TO PASS THROUGH THE REGION BEHIND EXITING SYSTEM.
SOUTHWEST FLOW ALOFT COULD DEVELOP AGAIN BY THE END OF THE
WEEK...WHICH WOULD BRING BACK CHANCES FOR SHOWERS AND STORMS.

&&

.AVIATION...FOR THE 12Z TAFS THROUGH 12Z SATURDAY MORNING
ISSUED AT 520 AM MDT FRI MAY 17 2013

EASTERLY LOW LEVEL FLOW WILL LEAD TO SOME STRATUS/FOG OVER PARTS OF
THE SD PLAINS THIS MORNING...WITH AREAS OF IFR/LIFR CONDITIONS.
ADDITIONAL SHOWER AND THUNDERSTORM ACTIVITY IS EXPECTED
TODAY...MAINLY DURING THE AFTERNOON AND EVENING HOURS. THE BEST
CHANCE FOR THUNDERSTORMS WILL BE ACROSS FAR NORTHEASTERN WY...THE
BLACK HILLS...AND NORTHWESTERN SD...WHERE THERE IS ALSO A SLIGHT
RISK FOR SEVERE STORMS.

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

$$

SHORT TERM...CARPENTER
LONG TERM...26
AVIATION...26