000 FXUS63 KFSD 080326 AFDFSD AREA FORECAST DISCUSSION FOR SOUTHEAST SD/SOUTHWEST MN/NORTHWEST IA NATIONAL WEATHER SERVICE SIOUX FALLS SD 925 PM CST SAT NOV 7 2009 .DISCUSSION... DVLPG SSWLY INFLOW AOB 700 MB FM NEB NWD INTO DVLPG E/W MID LVL BOUNDARY ACROSS SRN SD ATTM. MID LVL CONVERGENCE AND ELEVATED INSTABILITY SPARKING OFF SPKLS/ISOLD -SHRA AND EVEN AN ISOLD -TSRA IN SCNTRL SD LAST HOUR. THIS MID LVL LIFT AND CONVERGENCE SHOULD DVLP NEWD FM WRN TO THE NRN CWA REST OF NGT...BUT ONLY WENT 20 POP DUE TO DRYNESS OF THE AIR MASS IN LWR LVLS. RIGHT NOW PRETTY GOOD DISCONNECT BTWN MID LVL BOUNDARY AND LWR LVL MOISTURE POOLING ALG AND SOUTH OF SFC WMFNT IN SRN NEB. THIS MOISTURE AND PSBL LWR CLOUDS SHOULD MOVE NWD REST OF NIGHT AND AFFECT AREAS SOUTH OF I90 BY MRNG. IF THIS MOISTURE WERE TO ADVECT NWD ISENTROPICALLY IT COULD THEORETICALLY BE USED AT SOME POINT SUN MRNG...ESP IN SWRN MN WHERE LATER SHIFTS WILL HAVE TO MONITOR FOR SURPRISE SHWRS/ISOLD TSRA. FOR TNGT MID AND HIGH CLOUDS PUSHING ENEWD OVER CWA SHOULD PREVENT MUCH FURTHER TEMP DROP AT SOME POINT...AND FINE TUNED GRIDS FOR TEMPS AND SKY BASED ON CURRENT TRENDS. /RYRHOLM && .AVIATION... VFR THRU 10Z. ISOLD HI BASED SHWRS INCLUDG A TSTM OR TWO WILL BE FEEING OFF GNLY DRY AIR EVEN AT MID LVLS...WILL CONT TO WATCH BUT FOR NOW THINK THEY WILL STAY A LOW HAZARD DUE TO ISOLD NATURE AS THEY MOVE ACRS WRN TO FAR NRN FCST AREA. MVFR CIG HAZARD EXISTS SERN FCST AREA...MAINLY SUX TO SPW...10Z-18Z. DRYNESS OF AIR NEAR SFC SHUD KEEP ANY CIGS NO WORSE THAN 2K FT. AFT 18Z AND THRU 09/06Z EXPECT GNLY VFR CONDS WITH PSL ISOL -SHRA MAINLY ERN PART OF FCST AREA. WITH SFC TROF MOVG UP TO SRN FCST AREA DURG PD AND THEN EWD...THE DRY LO LVL AIR SEEMS TO PRECLUDE MORE WDSPRD LWRG OF CONDS BLO VFR...SUCH AS WUD OTRW OCCUR WITH THIS TYPE OF SYS./WILLIAMS && .PREV DISCUSSION... MAIN FCST PBLM FOR TONIGHT WL BE LOW TEMPS AND CLOUD COVER. CURRENTLY ONLY PATCHY HIGH CLOUDS STREAMING NEWD ACRS CWA ATTM. BOTH GFS AND NAM PROG HIGH LEVEL MOISTURE WILL CONT TO INCREASE AND SPREAD NEWD ACRS CWA TONIGHT...WITH MAIN IMPACT OF HIGH CLOUDS ACRS WRN INTO NRN CWA. BASED ON SAT...THIS LOOKS REASONABLE. SO WL GIVE HIGHEST HIGH CLOUD COVERAGE TO THOSE AREA DURING THE NIGHT. LOOKS LIKE NW IA WL REMAIN MOCLR THRU MOST OF NIGHT AND THIS AREA LIKELY TO SEE COOLEST LOWS WITH GOOD RADIATIONAL COOLING. HWVR...BY LATE TONIGHT...LLM INCREASE IN RESPONSE TO BNDRY LIFTING NWD INTO NEB AND IA. TIME SECTIONS OF BOTH NAM AND GFS SHOW POT FOR STRATUS FORMING LATE TONIGHT AS LLM GETS TRAPPED BLO H85 INVERSION. WITH ELY FLOW DEVELOPING CAN NOT RULE OUT STRATUS ACRS FAR SRN CWA LATE TONIGHT AND THUS WL INCREASE CLOUDS ACRS NW IA AND FAR SERN SD AFTER 09Z. BELIEVE LOWS IN MOST LOCATIONS WL BE BEFORE 08Z AND THEN SLOWLY RISE AS CLOUD COVER BECOMES THICKER. TIME SECTIONS ARE STILL FAIRLY CERTAIN THAT AT LEAST SOME STRATUS WL BE AN ISSUE LIFTING NWRD WITH LOW LVL WARM FRONTAL BNDRY THRU NW IA AND SWRN MN...SPREADING AT LEAST TEMPORARILY WWRD THRU PARTS OF FAR SERN SE/NERN NEBRASKA. IN FACT...SOME PRETTY DECENT SHEAR TOWARD TOP OF STABLE LAYER IS PRESENT...AND IF JUST A BIT DRIER ALFT WOULD HAVE TO REALLY CONSIDER SOME AREAS OF DZ/FG...ESPLY FROM SPW/SHL AREAS NWRD THROUGH THE SRN PORTIONS OF THE BUFFALO RIDGE LTR MORNING INTO EARLY AFTN. SEEMS A LOW PROBABILITY ENOUGH THREAT TO NOT MENTION AT THIS TIME IN FCST...BUT SOMETHING TO WATCH. TRACK RECORD ON HANDLING THE LOW LVL RH HAS NOT BEEN GOOD IN THIS ANOMALOUSLY WARM PATTERN OF LATE...REFERENCE TO PAST THU NIGHT/FRIDAY OVERFORECAST. ON THE GRAND SCHEME...DYNAMICS WITH SYSTEM ARE NOT VERY FORMIDABLE DOWN INTO THE NRN PLAINS...FOCUSED PRIMARILY WELL TO NORTH OF AREA. STILL...WITH AT LEAST WK DIV Q SLIDING BY DURING AFTN/EARLY EVNG AND WITH SOME MID BASED MOISTURE AVAILABLE AS MID LVL FRONTAL BNDRY SLIPS SEWRD WITH AT LEAST A LITTLE CLOSER TO NEUTRALITY IN TERMS OF STABILITY...WOULD NOT BE SURPRISED TO GET A PASSING SPRINKLE FOR AREAS E OF I29 AND TOWARD THE IA GREAT LAKES. EVEN IF NOT A MAJOR STRATUS EVENT...SHOULD BE ENOUGH MID TO HIGH CLOUDS TO AFFECT TMPS GIVEN THE DEGREE OF ELY FLOW EXPECTED INTO MIDDAY. WL PLAY QUITE CONSERVATIVE THRU SWRN MN AND NRN PORTIONS OF NW IA...BUT ALLOW A DECENT MIXING THRU FAR SRN CWA...AND ALSO IN WRN AREAS WHERE FLOW SHUD TURN TO A DRIER N/NW DURING THE AFTN. MID LVL FRONTAL BNDRY WL CONTINUE TO WORK SWRD SUN NIGHT INTO MONDAY...AND WL LKLY STALL SOMEWHAT FOR A TIME MONDAY AS ALL MDLS INDICATE ANOTHER WAVE IN THE ZONAL FLOW SLIDING FROM NEBRASKA INTO IA. AT THE SAME TIME...GOOD PUSH OF DRIER AIR WL UNDERCUT IN THE LOWER LVLS AS STRONG RIDGE OF SFC HIGH PRESSURE BUILDS INTO ERN SD THRU THE DAY. THIS SHUD KEEP THREAT OF PCPN CLEAR OF AREA...BUT WOULD STILL EXPECT MID/HIGH CLOUDS TO BE QUITE PREVALENT THRU THE AFTN MONDAY IN AT LEAST THE SERN 2/3 OF THE CWA. EVEN IF CLDS END UP SLIPPING A BIT MORE SWRD...THE LOW SUN ANGLE WL MAKE TMPS TO THE NORTH SUFFER. RAPID CLEARING IN ORDER ON MONDAY EVENING...AND WITH LITTLE REASON TO EXPECT AIR MASS TO BE MUCH MORE MOIST THAN CURRENT SFC RIDGE /LOTS OF 20S DWPTS/ WITH POTENTIAL EXCEPTION OF LESS MIXED AND CLOUDY SRN CWA...LIGHT GRADIENT SHUD ALLOW FOR EFFICIENT RADIATIVE TEMP PLUNGE. TUESDAY...INCREASING RETURN FLOW AS RIDGE SHIFTS OFF TO E...BUT SOME CAUTION TO SPEED AS MAY BE DEALING WITH A TEMPORARY PATTERN SLOWDOWN IF REMNANTS OF IDA KICK UP SOME BETTER NEGATIVE PV INTO ERN RIDGING. TMPS FAVOR TOWARD WARMER SIDE OF SHALLOW MIXING...AWAY FROM COOLER GFS POTENTIAL. SOME INCREASING HIGH CLOUDS AGAIN BY LATE DAY THRU W. SMALL WINDOW AGAIN FOR SOME BETTER RADIATIVE COOLING FOR FAR ERN CWA. IN THE EXTENDED RANGE /WED THRU SAT/ PLENTY OF MODEL DISAGREEMENT. OPERATIONAL GFS CONTINUES TO STRUGGLE WITHIN CONFINES OF GREATER ENSEMBLE SET...AND EVEN RUN TO RUN CONTINUITY OF EC/CANADIAN DO NOT LEND MUCH TO LATCH ON TO. FOR MOST PART...DID NOT MAKE TOO MANY SIGNIFICANT CHANGES TO THE HPC NUMBERS...WITH EXCEPTION OF INCREASING POPS SOMEWHAT BY SATURDAY WITH SUPPORT OF DEVIANT ENSEMBLE NUMBERS. EXPECT AMPLIFYING PATTERN WITH TROUGH DIGGING THRU W AND RIDGING SHIFTING INTO THE MS VALLEY THRU WED WL PRODUCE A BREEZY TO WINDY PERIOD WED INTO THU ACRS THE AREA...WITH TEMPS REMAINING WARMER THAN NORMAL. IF NOT FOR THREAT FOR SOME FAIRLY THICK CI...WOULD LKLY HAVE GONE SOMEWHAT WARMER. VARIOUS TIMING/CONFIGURATION OF TROUGHING... RANGING FROM A MASSIVE CLOSED LOW LIFTING THRU WRN PLAINS ON A FEW PRIOR RUNS...TO MORE NRN STREAM DOMINANT 12Z SLNS...SO SETTLED MAINLY FOR LOW CHC FOR PCPN THU THRU SAT...AND ALLOWED FOR TMPS TO SLIDE DOWN WITH MORE CLOUDS/PCPN THREAT. FOR NOW...DID NOT BRING TMPS DOWN ENOUGH TO MENTION MIX WITH SNOW. /CHAPMAN && .FSD WATCHES/WARNINGS/ADVISORIES... SD...NONE. MN...NONE. IA...NONE. NE...NONE. && $$