558 FXUS63 KARX 111757 AFDARX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE LA CROSSE WI 1157 AM CST FRI JAN 11 2013 .SHORT TERM... 1157 AM CST FRI JAN 11 2013 VISIBILITY HAS REMAINED LOW AT A QUARTER MILE OR LESS ACROSS THE REGION AND HAS NOT IMPROVED MUCH AT ALL THROUGH THE MORNING WITH TEMPERATURES STICKING AROUND THE MID 30S. THE INCREASING WINDS APPEAR TO HAVE ACTUALLY MADE CONDITIONS WORSE WITH CREATING AN ADVECTION FOG THAT IS ERODING MORE OF THE SNOW PACK. THE WORST VISIBILITIES ARE NORTH OF A WARM FRONT THAT IS CURRENTLY IN SOUTHERN IOWA AND EXPECT THAT TO BE THE CASE THROUGH THIS AFTERNOON AND EVENING. THEREFORE...HAVE EXTENDED THE DENSE FOG ADVISORY THROUGH MIDNIGHT. THAT TIMING COULD END UP BEING TOO LONG...OR TOO SHORT DEPENDING ON WHAT OCCURS THIS EVENING. THE 11.16Z TO 11.19Z RUNS OF THE HRRR HAVE DONE A GOOD JOB OF SHOWING THE LOWER VISIBILITIES...PARTICULARLY WITH THE HINTING AT SLIGHTLY BETTER CONDITIONS IN THE COULEES...AND SHOWS AN IMPROVEMENT FROM THE SOUTH THIS EVENING AS THE WARM FRONT LIFTS NORTH. CURRENT THINKING IS THAT VISIBILITY WILL IMPROVE FROM THE SOUTH TO NORTH BETWEEN 6PM TO MIDNIGHT TONIGHT WHERE WE CAN CANCEL THE ADVISORY EARLY IN THE SOUTH IF THE CONDITIONS DO IN FACT IMPROVE. THE WILD CARD IS THAT ONCE THE WARM FRONT MOVES THROUGH THAT THE STRONGER SOUTHWEST WINDS AND WARMER AIR COULD JUST MAKE THINGS WORSE AGAIN WITH THE ERODING SNOW PACK. FOR NOW...HAVE TRENDED TOWARD THE HRRR SINCE THE OBS ACROSS SOUTHERN IOWA DO IMPROVE SUBSTANTIALLY SOUTH OF THE FRONT. SHOULD THE DENSE FOG CONTINUE INTO THE OVERNIGHT HOURS...IT DEFINITELY WILL COME TO AN END ONCE THE POTENT COLD FRONT COMES THROUGH AND INCREASES WINDS INTO THE 25 TO 40 MPH RANGE. .LONG TERM...SUNDAY NIGHT THROUGH THURSDAY 300 AM CST FRI JAN 11 2013 THE GFS AND 11.00Z ECMWF AGREE THAT THE POLAR VORTEX WILL BE MOVING SOUTH FROM OVER NUNAVUT TO NEAR HUDSONS BAY. THIS WILL PUSH A COLD FRONT ACROSS THE UPPER MIDWEST DURING THE MIDDLE OF NEXT WEEK. THE GFS KEEPS THE SURFACE LOW OFF TO THE NORTHEAST OVER ONTARIO AND IS QUICKER TO BRING THE FRONT THROUGH ON TUESDAY NIGHT. THE ECMWF AND 11.00Z GEM INDICATE THE SURFACE LOW WILL MOVE ACROSS THE UPPER MIDWEST AND GREAT LAKES AND DO NOT BRING THE FRONT THROUGH UNTIL WEDNESDAY OR WEDNESDAY NIGHT. FOR NOW...WILL STAY WITH THE MODEL CONSENSUS AND CARRY A CHANCE OF SNOW FOR TUESDAY NIGHT AND WEDNESDAY. && .AVIATION...THROUGH 18Z SATURDAY 1157 AM CST FRI JAN 11 2013 LOW VISIBILITY AND CEILINGS WILL PERSIST AT RST AND LSE THROUGH TONIGHT AS A WARM AND MOIST AIR MASS SITS ACROSS THE REGION. THE COMBINATION OF THIS AIR MASS WITH A MELTING SNOW PACK HAS LED TO WIDESPREAD DENSE FOG WITH VISIBILITIES DROPPING TO A QUARTER MILE OR LESS THROUGHOUT THE REGION. SOME OF THE RIVER VALLEY LOCATIONS HAVE BEEN SLIGHTLY HIGHER WITH THE VISIBILITY AT TIMES BUT CEILINGS HAVE REMAINED LOW AT A FEW HUNDRED FEET. LITTLE CHANGE IN THESE CONDITIONS ARE EXPECTED THROUGH TONIGHT AS A WARM FRONT MOVES IN FROM THE SOUTH AS A LOW PRESSURE SYSTEM TRACKS THROUGH THE PLAINS ON INTO MINNESOTA. THERE MAY BE SOME IMPROVEMENTS THIS EVENING AFTER THE WARM FRONT MOVES THROUGH...BUT THE WARMER AIR AND STRONGER WINDS MAY JUST LEAD TO MOVE SNOW MELT AND CONTINUED LOW VISIBILITIES. LOW LEVEL WINDS WILL BE STRENGTHENING AHEAD OF A COLD FRONT THAT WILL MOVE THROUGH OVERNIGHT...WITH LOW LEVEL WIND SHEAR OCCURRING FROM SFC WINDS AROUND 15KTS AND 2KFT WINDS STRENGTHENING TO 50-60KTS. AFTER THE COLD FRONT MOVES THROUGH...VISIBILITY AND CEILINGS WILL IMPROVE TO MVFR BUT WINDS WILL RAPIDLY INCREASE TO BEING OUT OF THE WEST- SOUTHWEST AT 20-35KTS. THE BACK EDGE OF THE SYSTEM WILL THEN MOVE THROUGH TOMORROW MORNING WITH SOME SNOW POSSIBLE THAT COULD BRING VISIBILITY BACK DOWN TO IFR. && .ARX WATCHES/WARNINGS/ADVISORIES... 1157 AM CST FRI JAN 11 2013 WI...DENSE FOG ADVISORY UNTIL MIDNIGHT CST TONIGHT FOR WIZ017-029- 032>034-041>044-053>055-061. MN...DENSE FOG ADVISORY UNTIL MIDNIGHT CST TONIGHT FOR MNZ079-086>088- 094>096. IA...DENSE FOG ADVISORY UNTIL MIDNIGHT CST TONIGHT FOR IAZ008>011-018- 019-029-030. && $$ SHORT TERM...HALBACH LONG TERM...04 AVIATION...HALBACH