601
FXUS63 KFSD 090945
AFDFSD
AREA FORECAST DISCUSSION FOR SOUTHEAST SD/SOUTHWEST MN/NORTHWEST IA
NATIONAL WEATHER SERVICE SIOUX FALLS SD
445 AM CDT WED JUL 9 2008
DISCUSSION
CHANGES UNDERWAY IN LARGE SCALE PATTERN THIS MORNING AS WRN RIDGE
NOW APPEARING TO START FLATTENING TREND. WAVE UPSTREAM DIGGING THRU
MN WITH TAIL BACK INTO ERN SD...WHICH HAS BEEN ENHANCING CONVECTION
AT TIMES FROM E CNTRL SD AND SW MN...BACK TOWARD LARGER CLUSTER IN
NERN NEBRASKA...AND THEN BACK TOWARD THE NRN PANHANDLE. UPR AIR
ANLYS INDICATES A STRONG MID LVL THERMAL BNDRY IN PLACE FROM NRN
ROCKIES TO NRN NEBRASKA...WHICH SEEMS TO BE FOCUSING AT LEAST ONE
AREA OF ACC. ANOTHER AREA OF ACC ON SRN FLANK OF UPR WAVE ACRS MN.
THIS WAVE HAS DRIVEN A WEAK SFC BNDRY UP AGAINST THE FAR NWRN
PORTIONS OF CWA. CHALLENGES FOR THE DAY MANY...BUT PRIMARY FOCUS
SURROUNDS LOW PCPN THREAT THRU EARLY THU...AND FRONTAL PASSAGE FRI
AFTN/EVNG. NAM AND GFS ARE REALLY NOWHERE CLOSE ON LOCATIONS OF
VARIOUS WK WAVES...AND WITH BETTER SUPPORT FROM CANADIAN
REGIONAL/GLOBAL AS WELL AS EC...FAVOR GFS TO GUIDE REASONING.
ISOLD TO SCT CONVECTIVE THREAT ONGOING EARLY MORNING. MAIN ELEVATED
THETA E ADVECTION EXPECTED TO SHIFT GRADUALLY TOWARD LOWER MO VLY...
EXPECT THAT AS WAVE SHEARS BY TO NORTHEAST DURING MORNING...WHAT
MEAGER COVERAGE IS GOING EARLY ON TDA SHUD DECREASE FURTHER...BUT
MAY NOT ENTIRELY VANISH FROM FAR SRN CWA ALG BEST MID LVL GRADIENT.
LOW LVL BNDRY WL WANDER INTO CWA...BUT EXPECT TO LOSE DEFINITION AS
BETTER SYNOPTIC FORCED BNDRY TAKES SHAPE TO SW AND BEGINS TO LIFT
NEWRD OVERNIGHT.
WHILE THE OVERALL THREAT SHUD BE MINOR AT BEST...WL HAVE TO MONITOR
STORMS BY LATER AFTN/EVNG WHICH DEVELOP THRU LOWER MO VLY WHERE
INSTABILITY BY LTR AFTN/EVNG ON EDGE OF BETTER LOW LVL MOISTURE
RESIDUAL AND AHEAD OF WARMING ALFT FEEDS 1500-2000 J/KG CAPE WITH
DECENT DEEP LYR SHEAR...MAY END UP WITH SOME MARGINAL SVR WINDS/HAIL.
MOISTURE TO S CONTINUES TO GATHER AND WAVE ACRS NRN ROCKIES WL PUSH
INTO REGION TONIGHT WITH MID LVL CAP SPREADING EWRD AS WELL. THIS WL
SET UP DECENT MOISTURE TRANSPORT AND CHANCE FOR NOCTURNAL CONVECTION
OVERNIGHT. BEST CONVERGENCE OF LLJ IS OFF TO N...BUT STILL ENOUGH
STEEPER LAPSE RATES WITH RETURN MOISTURE TO WARRANT A CHANCE POP
OVERNIGHT...LINGERING THRU ERN AREAS ON THU MORNING. MOST OF THU
SHUD SHUT DOWN ON CONVECTIVE THREAT WITH WARMING TMPS ALFT SURGING
EWRD AS FLATTENING UPR RIDGE SHIFTS ACRS CWA. WL START TO SEE MORE
SIG INCREASE IN SFC DWPTS THU...WITH SLY WINDS INCREASING. MAIN
THERMAL RIDGE WL BE TO W OF AREA THU...SO WL HOLD BACK BEST WARMING
INTO 90S FROM JAMES VLY W...WITH INCREASED LOW LVL MOISTURE AND
INCREASINGLY CAPPED LOW LVLS RESTRAINING WARMING FARTHER TO E.
THU NIGHT...BNDRY LIFTED WELL N AND CAPPED...BOTH IN FAVOR OF
KEEPING CLEAR OF PCPN CHC. WL GET SOME PRETTY DECENT WINDS GOING
ELEVATED AREAS LTR THU NIGHT WITH LLJ...WITH SOME LOWS ABOVE 70.
THERMAL RIDGE AHEAD OF APPROACHING CDFNT ON FRI SHUD LEAD TO WARMEST
DAY OF YEAR THUSFAR FOR SOME...ESPLY LOWER MO VLY...WHERE SOME MID
90S ARE LKLY...AS WELL AS DOWNSTREAM OF SW MN RIDGE. VERY MOIST
DWPTS AS WELL...LEADING TO HIGH DEGREE OF POTENTIAL INSTABILITY IN
AFTN. HOWEVER...WL ALSO BE UNDER THE 12-16C 700 HPA PLUME. CAP PRIOR
TO PCPN AT 100-150 J/KG...WHICH VANISHES ENTIRELY PRIOR TO
DEVELOPMENT OF PCPN IN 3H TIMEFRAME MID TO LATE AFTN...WITH MODEL
INDICATING EXTREME LOW LVL LIFT. CANNOT HELP BUT INTERPRET THIS AS
THE SAME OLD DRY CONVECTIVE PROCESSES THAT OFTEN ERRONEOUSLY ERODE
STRONG CAPS. THEREFORE...HESISTANT TO CARRY ANYTHING BEYOND THE
LOWEST CHC ANY FARTHER WWRD THAN SW MN ALG FRONTAL BNDRY BY LATE
AFTN. ALMOST CERTAIN THAT IF SOMETHING DOES DEVELOP...IT WOULD BE
SEVERE WITH LARGE HAIL AND ESPLY DAMAGING WINDS...WITH SHEAR MORE
THAN SUFFICIENT FOR ROTATING STORMS AT MORE DISCRETE ONSET.
BEYOND THE FRONTAL PASSAGE...LOOKS LIKE A QUIET WEEKEND AHEAD...AT
LEAST PRIOR TO ONSET OF DEVELOPMENT OF LLJ SUN NIGHT. ON SAT...A
NICE MIXING DAY FOR BREEZY CONDITIONS...AND EASILY REACHING THE
1.5-2KM POTENTIAL FOR TMPS. TRAILING UPR ENERGY GLANCES THRU THE MO
VLY...BUT ATMOSPHERE IS A LARGE STEP TOWARD BEING DESICCATED...SO DO
NOT EXPECT THE ABILITY TO RESPOND TO THE WEAK FORCING. SFC RIDGE
BUILDS OVER THE AREA SAT NIGHT AND EARLY SUN...ALLOWING MAX
POTENTIAL FOR DIURNAL RANGE.
SOMEWHAT COOLER TMPS EXPECTED FOR SUNDAY AS COLDEST AIR PUSHES SWRD
AS AMPLIFYING TROUGH DIGS INTO GRTLKS. TREND TOWARD THIS SLN IN
SEVERAL MEDIUM RANGE MODELS PERHAPS A RESULT OF HURRICANE
INTERACTION WITH FLOW UPSTREAM IN THE WESTERN ATLANTIC...SLOWING
PROGRESSION SOMEWHAT OVER THE WEEKEND.
AVIATION
VFR WILL CONTINUE EXCPT FOR ISOLD TSTMS IN THE AREA...MAINLY SOUTH
OF I90 TDA INTO TNGT.
FSD WATCHES/WARNINGS/ADVISORIES
SD...NONE.
MN...NONE.
IA...NONE.
NE...NONE.
CHAPMAN/RYRHOLM
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