Severe Thunderstorm Warning 171, NWS Sioux Falls, SD

AREA FORECAST DISCUSSION FOR SOUTHEAST SD/SOUTHWEST MN/NORTHWEST IA
NATIONAL WEATHER SERVICE SIOUX FALLS SD
410 AM CDT FRI AUG 18 2006

.DISCUSSION...
MAIN CONCERN TODAY IS EVOLUTION OF CONVECTION OVER THE NEXT 24
HOURS. THERE IS A WEAK WAVE MOVING ACROSS NORTHERN SOUTH DAKOTA
EARLY THIS MORNING. LOTS OF CONVECTION ASSOCIATED WITH THIS WAVE BUT
WITH STABLE AIR IN PLACE OVER THE AREA AND THE DEEPER CONVERGENCE
NORTH AND WEST...CONVECTION HAS BEEN UNABLE TO REACH HURON.
EXPECTATION IS THAT THIS CONVECTION WILL CONTINUE TO STRUGGLE TO
OVERCOME STABLE AIR OVER THE REGION AND HAVE ONLY CHANCE POPS EARLY
THIS MORNING AROUND HURON AND CHAMBERLAIN. MEANWHILE...MORE
CONVECTION HAS DEVELOPED IN EASTERN NEBRASKA AND WESTERN IOWA NEAR
THE LEADING EDGE OF MORE UNSTABLE AIR AND AHEAD OF A WEAKER WAVE
MOVING ON SOUTHERLY EDGE OF WESTERLIES. WITH 500 MB FLOW
BACKING...THIS CONVECTION SHOULD MOVE NORTHEAST AND THEN BEGIN TO
SLOWLY TURN EAST AS IT MOVES TOWARD HIGHWAY 20. WHILE THE HEAVIEST
PRECIP IS EXPECTED TO REMAIN SOUTH OF HIGHWAY 20...A FEW STORMS
COULD FOLLOW THE INCREASING MID LEVEL INSTABILTY NORTH. BASED ON
MOVEMENT OF BOTH WAVE AND CONVECTION...EXPECTATION IS THAT STORMS
WILL REMAIN SOUTHEAST OF A SPENCER TO SIOUX CITY LINE THIS MORNING
AND SHOULD MOVE EAST OF STORM LAKE AROUND NOON. ISOLATED STORMS HAVE
ALSO BEGUN TO DEVELOP NEAR THE NEBRASKA BORDER BUT WITH ONGOING
CONVECTION IN VICINITY OF I80...THESE STORMS ARE EXPECTED TO REMAIN
ISOLATED THIS MORNING.

FOR THE REMAINDER OF THE FORECAST FOLLOWED THE GFS WHICH CONTINUES
TO HAVE A BETTER DEPICTION OF THE MID LEVEL WAVE EVOLUTION. THIS IS
A FASTER AND SOMEWHAT DRIER SOLUTION THAN THE WRF. MUCH STRONGER
WAVE APPROACHES THIS AFTERNOON. THE INVERTED TROUGH ALSO MOVES EAST
OF THE MISSOURI RIVER BY 18Z EXTENDING FROM HURON TOWARD
CHAMBERLAIN. EVEN THOUGH INSTABILITY WILL BE LIMITED DUE TO CLOUDS
WHICH WILL KEEP TEMPS IN THE 70S TO LOWER 80S...NO CAP EXISTS AND
THE INTERACTION OF THE INVERTED TROUGH AND UPPER WAVE SHOULD RESULT
IN CONVECTIVE DEVELOP NORTH OF CHAMBERLAIN BY EARLY AFTERNOON. THESE
STORMS WILL MOVE ALONG AND NORTH OF INTERSTATE 90 THROUGH THE
AFTERNOON REACH INTERSTATE 29 LATE THIS AFTERNOON. FAIRLY CERTAIN
MOST AREAS ALONG AND NORTH OF I90 WILL SEE SOME PRECIP SO HAVE 60 TO
70 POPS. HOWEVER...WITH THE UPPER DYNAMICS MUCH WEAKER SOUTH OF
I90...THERE IS A QUESTION OF HOW WIDESPREAD PRECIP WILL BE. IN
FACT...AS THE AFTERNOON GOES ON EXPECT THAT CONVECTION WILL
REDEVELOP IN NORTHERN KANSAS OR SOUTHERN NEBRASKA. THIS SHOULD
ORGANIZE INTO A FAIRLY LARGE MCS WHICH WILL ACT TO SUPPRESS
CONVECTION FARTHER NORTH THROUGH SUBSIDENCE AND CUTTING OFF OF
MOISTURE. THIS WILL MEAN AS CONVECTION PROGRESSES EAST IT MAY BEGIN
TO WEAKEN ON THE SOUTHERN PERIPHARY BY LATE AFTERNOON OR EARLY
EVENING. IN FACT...THERE IS A CHANCE THAT MOST CONVECTION IN THE
VICINITY OF HIGHWAY 20 COULD COMPLETELY DISSIPATE BY THE TIME IT
MOVES EAST OF THE BIG SIOUX RIVER. THEREFORE KEPT AFTERNOON/EVENING
POPS IN MOST OF NW IA AT 30 OR 40 WITH 40 TO 50 NEAR THE NEBRASKA
AND SOUTH DAKOTA BORDER.

THIS CONVECTION SHOULD MOVE EAST OF THE AREA AFTER MIDNIGHT WITH
SKIES SLOWLY CLEARING. NO PRECIPITATION IS THEN FORESEEN THROUGH
EARLY NEXT WEEK. MUCH DRIER AND COOLER AIR WILL MOVE IN OVERNIGHT
AND INTO SATURDAY. WITH SOME RESIDUAL MOISTURE EXPECTED...ESPECIALLY
EAST OF I29...AND RELATIVELY COLD 850 TEMPS...EXPECT THAT SUNSHINE
WILL BE SOMEWHAT SELF DESTRUCTIVE SATURDAY AFTERNOON AND KEPT SKIES
PARTLY CLOUDY.  THIS WILL ALSO MEAN HIGH TEMPERATURES WILL GENERALLY
BE IN THE 70S.  THESE CLOUDS WILL RAPIDLY DISSIPATE ON SATURDAY
EVENING SETTING UP A GOOD RADIATION NIGHT FOR MUCH OF THE AREA SO
LOWS SHOULD FALL INTO THE 50S. ON SUNDAY...THE COLDEST AIR MOVES
EAST AND WITH LITTLE MOISTURE...SKIES SHOULD BE MOSTLY SUNNY. TEMPS
WILL BE A FEW DEGREES WARMER THAN SATURDAY.  MORE WIND AND A SLOW
INCREASE IN MOISTURE ON SUNDAY NIGHT WILL HELP KEEP MIN TEMPS IN THE
MID 50S TO LOWER 60S.

GFS DOES SHOW A FRONT MOVING THROUGH THE AREA MONDAY NIGHT AND
TUESDAY. DID NOT ADJUST FORECAST AT THIS TIME AS THERE IS
UNCERTAINTY IF THIS FRONT WILL MOVE SOUTHWARD THIS QUICKLY.

&&

.AVIATION...
LOW CEILINGS WILL ONCE AGAIN BE A PROBLEM THIS MORNING...WITH
WIDESPREAD IFR CONDITIONS THROUGH THE MORNING HOURS...LINGERING INTO
THE AFTERNOON OVER MUCH OF THE AREA. ALSO AREAS OF LIFR ARE EXPECTED
THROUGH 14Z BEFORE CEILINGS BEGIN TO INCREASE. VISIBILITIES WILL
ALSO BE REDUCED DOWN TO AROUND 2 SM ACROSS MUCH OF THE AREA...AND
WILL BE AS LOW AS 1/4 SM IN PARTS OF NW IA EARLY THIS MORNING.
THUNDERSTORMS WILL ALSO BE POSSIBLE IN NW IA DURING THE MORNING AND
EARLY AFTERNOON HOURS. A COLD FRONT WILL MOVE ACROSS ERN SD THIS
EVENING BRINGING A CHANCE OF THUNDERSTORMS...WITH THE BE CHANCE IN
NE SD.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...

SD...NONE.
MN...NONE.
IA...NONE.
NE...NONE.

&&

SCHUMACHER/LIEBL

HAZARDOUS WEATHER OUTLOOK
NATIONAL WEATHER SERVICE SIOUX FALLS SD
725 AM CDT FRI AUG 18 2006

SDZ038>040-050-052>071-MNZ071-072-080-081-089-090-097-098-
IAZ001>003-012>014-020>022-031-032-NEZ013-014-191200-
AURORA-BEADLE-BON HOMME-BROOKINGS-BRULE-BUENA VISTA IA-
CHARLES MIX-CHEROKEE IA-CLAY-CLAY IA-COTTONWOOD MN-DAKOTA NE-
DAVISON-DICKINSON IA-DIXON NE-DOUGLAS-GREGORY-HANSON-HUTCHINSON-
IDA IA-JACKSON MN-JERAULD-KINGSBURY-LAKE-LINCOLN-LINCOLN MN-
LYON IA-LYON MN-MCCOOK-MINER-MINNEHAHA-MOODY-MURRAY MN-NOBLES MN-
OBRIEN IA-OSCEOLA IA-PIPESTONE MN-PLYMOUTH IA-ROCK MN-SANBORN-
SIOUX IA-TURNER-UNION-WOODBURY IA-YANKTON-
725 AM CDT FRI AUG 18 2006

THIS HAZARDOUS WEATHER OUTLOOK IS FOR SOUTHEAST SOUTH DAKOTA...
SOUTHWEST MINNESOTA...NORTHWEST IOWA...AND NORTHEAST NEBRASKA.

.DAY ONE...TODAY AND TONIGHT

THUNDERSTORMS...
SCATTERED THUNDERSTORMS WILL MOVE INTO NORTHWEST IOWA THIS
MORNING BUT ARE NOT EXPECTED TO BE SEVERE. TO THE WEST...A LINE
OF THUNDERSTORMS COULD HOLD TOGETHER AND AFFECT AN AREA FROM
AROUND CHAMBERLAIN...TO MITCHELL TO HURON SOUTH DAKOTA THIS
MORNING. THESE STORMS HAVE HAD SOME SEVERE WIND WITH THEM IN
CENTRAL SOUTH DAKOTA...BUT ARE MOVING INTO A MORE STABLE AIRMASS
FURTHER EAST AND ARE LIKELY TO WEAKEN. THEREFORE THE SEVERE
WEATHER THREAT THIS MORNING IN THIS AFOREMENTIONED AREA IS SLIGHT
AT BEST.

SCATTERED THUNDERSTORMS ARE THEN EXPECTED TO MOVE OUT OF CENTRAL
SOUTH DAKOTA THIS AFTERNOON. THESE STORMS WILL MOVE ACROSS
SOUTHEAST SOUTH DAKOTA AND INTO SOUTHWEST MINNESOTA...NORTHWEST
IOWA AND NORTHEAST NEBRASKA. THESE STORMS ARE NOT EXPECTED TO BE
SEVERE.

WIND AND FOG...
AREAS OF DENSE FOG WILL BE SEEN OVER MUCH OF SOUTHWEST MINNESOTA
AND NORTHWEST IOWA THROUGH MID MORNING...WITH LOCAL VISIBILITIES
AT OR LESS THAN A QUARTER MILE.

FIRE DANGER...
STRONG SOUTHERLY WINDS COMBINED WITH HIGH TEMPERATURES AROUND 90
WITH RESULT IN A VERY HIGH FIRE DANGER ACROSS
JERAULD...BRULE...AND GREGORY COUNTIES.

.DAYS TWO THROUGH SEVEN...SATURDAY THROUGH THURSDAY

THUNDERSTORMS ARE EXPECTED TO MOVE ACROSS THE AREA FRIDAY AND
FRIDAY NIGHT.  WHILE A MAJORITY OF STORMS WILL NOT BE SEVERE...
THERE WILL BE A GREATER POTENTIAL FOR LOCALLY HEAVY RAINFALL
BY LATE AFTERNOON AND EVENING ALONG AN AXIS FROM MARSHALL TO
BROOKINGS TO TYNDALL.  A FEW STORMS ACROSS SOUTHEAST SOUTH
DAKOTA WILL ALSO BE CAPABLE OF PRODUCING DAMAGING WIND GUSTS...
AND PERHAPS AN ISOLATED TORNADIC SPINUP EARLY IN THE THUNDERSTORM
LIFE CYCLE.

.SPOTTER INFORMATION STATEMENT...

SPOTTER ACTIVATION WILL LIKELY NOT BE NEEDED TODAY AND TONIGHT.

&&

FOR MORE INFORMATION PLEASE VISIT OUR WEBSITE AT
WWW.CRH.NOAA.GOV/FSD. THIS INCLUDES THE GRAPHICAL HAZARDOUS
WEATHER OUTLOOK.