000 FXUS63 KARX 132053 AFDARX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE LA CROSSE WI 350 PM CDT FRI JUL 13 2007 .SHORT TERM...TODAY THROUGH MONDAY NIGHT FORECAST EMPHASIS WAS SPENT ON ASSESSING FRONTAL PASSAGES TONIGHT AND MONDAY. NCEP DETERMINISTIC MODELS /GFS/NAM/ IN GOOD AGREEMENT THROUGH SUNDAY BUT THEN DEPART GREATLY ON TIMING OF MONDAY WEATHER. GOES WATER VAPOR SHOWING A VERY IMPRESSIVE SHORTWAVE TROUGH RIPPING THROUGH THE NORTHWEST FLOW EAST OF LAKE WINNIPEG AT 19Z. MODELS SUGGEST A SECONDARY SHORTWAVE TROUGH AND TROPOPAUSE FOLD FURTHER NORTH NEAR REINDEER LAKE /LARGE LAKE NORTH OF L WINNIPEG/. GOES CANNOT VIEW THIS VERY WELL AS IT IS ON THE EDGE OF ITS COVERAGE. MODIS WATER VAPOR IMAGE DOES SHOW THIS A BIT BETTER IN THE 1716Z PASS. COLD FRONTAL BOUNDARY WAS APPROACHING THE U.S. CANADIAN BORDER WITH SOME SCATTERED SHOWERS ALONG IT PER 88D RADARS. AHEAD THE FRONT..THE SPC ANALYSIS IS SHOWING LITTLE CAPE THROUGH MN FOR CONVECTIVE INITIATION...BUT OVER 2K J/KG IN THE DAKOTAS AND SRN CANADA FURTHER WEST. USED THE NAM FOR SOME OF THE DETAILS THROUGH SUNDAY AS THE 12Z GFS/NAM DIDNT DIFFER MUCH. SHORTWAVE TROUGH MOVES THROUGH OVERNIGHT WITH THE MAIN WEATHER AFFECTS COMING FROM 06-12Z ACROSS THE AREA. MOST OF THE MID-UPPER LEVEL QG CONVERGENCE IS OVER NORTHERN WISCONSIN WITH MAINLY LOW-LEVEL QG FORCING AND 0-2KM THETA-E CONVERGENCE TO THE SOUTHWEST. THIS FAVORS DEEPER ASCENT FURTHER TOWARD THE SHORTWAVE TRACK OVER LAKE SUPERIOR...ALTHOUGH THE SECOND SHORTWAVE TROUGH DROPS A BIT FURTHER SOUTHWEST TO CLIP THE NORTH-CENTRAL COUNTIES OF WI. 18Z RUC AND 12Z MODELS SUGGEST SOME ELEVATED CAPES /3- 500J/KG/ IN THE PRE-FRONTAL AIR MASS OVERNIGHT FROM THE MOISTURE STREAM EMANATING FROM NEBRASKA OVER THE FORECAST AREA. THIS MAINLY LOOKS TO BE STEEP 2-4KM LAPSE RATES. EVEN WITH POOR DIURNAL TIMING...CONVERGENCE ALONG THE FRONT WARRANTS INCREASING THE CHANCE OF TSRA FURTHER SOUTHWEST...MAINLY SW TOWARD THE MISS RIVER. CONFIDENCE IN TSRA/SHRA COVERAGE IS NOT HIGH ONCE WE GET WEST OF THE RIVER...THIS MAY NEED TO BE INCREASED BASED ON LATE EVENING TRENDS. SHEAR IS IMPRESSIVE BUT ONLY IN THE 0-3KM LAYER AND THE PARCELS WILL LIKELY NOT BE SURFACE BASED OVERNIGHT TO USE IT. THUS...SEVERE WEATHER IS NOT ANTICIPATED. SHORTWAVE QUICKLY EXITS AFTER SUNRISE AND THE DAY SHOULD BE FAIRLY SUNNY SATURDAY. UNDER RIDGING THROUGH SUNDAY...OUR FINAL NORTHWEST SHORTWAVE TROUGH LOOKS TO MOVE THROUGH MONDAY. THE 12Z GFS IS VERY FAST WITH THIS SHORTWAVE AND THE 09Z SREF MEAN AND 12Z NAM ARE SLOWER BY 12 HOURS. PREFER THE LARGER POOL OF PROBABILITIES SUGGESTING SLOWER TIMING WITH THE FRONT. THE GULF OF MEXICO WILL BE OPEN FOR THIS SYSTEM WITH SOUTHWEST WINDS BRINGING OVER 65F DEWPOINTS INTO THE PRE- FRONTAL ENVIRONMENT PER SREF MEAN. THIS MEANS ABOUT 1500J/KG MLCAPE AND THE FRONT SEEMS TO HAVE MODERATE 0-1KM THETA CONVERGENCE ALONG IT FOR A SEVERE WEATHER THREAT. THE PROBLEM IS THE CONFIDENCE IN THE TIMING WITH THE LAST THREE GFS RUNS JUMPING FAST/SLOW/FAST. WOULD THINK A DECENT CHANCE OF CONVECTIVE COVERAGE ON MONDAY EXISTS BUT THE EXACT LOCATIONS ARE HARD TO PIN DOWN. IT APPEARS THE NAM/SREF SLOWER SOLUTION IS MORE PROBABLE AND THUS HAVE NOT USED THE 12Z GFS AS TIMING GUIDANCE. THUS...THE OUTCOME LOOKS TO BE A RAIN CHANCE IN LATER SUNDAY NIGHT WITH FROPA MONDAY ACROSS THE FORECAST AREA. DEEP SHEAR COULD BE SUPERCELLULAR THAT DAY AS WE HAVE INSTABILITY AND ARE IN THE NORTHWEST FLOW REGIME. SPC OUTLOOK HAS LOW PROBABILITY BUT THIS WILL BE MONITORED...COULD BE MORE POTENTIAL THERE. SHOULD THE FRONT BE SLOWER...MONDAY COULD BE A WARM DAY. THIS FRONT THEN SHIFTS THROUGH AND STALLS TO THE SOUTH OF THE FORECAST AREA IN THE LONG TERM. .LONG TERM...TUESDAY THROUGH FRIDAY THIS PERIOD IS MARKED BY A SUBSTANTIAL CHANGE IN THE LARGE SCALE WITH PROGRESSION AND AMPLIFICATION OF THE CURRENT FLOW REGIME OVER NORTH AMERICA. A VERY ANOMALOUS RIDGE BUILDS INTO THE CENTRAL U.S. AND SOUTHCENTRAL CANADA. THIS IS A LARGE AND HOT RIDGE AND WILL BAKE AREAS NEXT WEEK AND BE A BIG WEATHER STORY. THE QUESTION IS HOW FAST WILL IT SHIFT EAST. THE 00Z/06Z GFS AND 13.00 ECMWF ARE IN EXCELLENT AGREEMENT IN THIS PERIOD AND THUS THE CONFIDENCE IS HIGHER THAN NORMAL FOR A LONGER RANGE FORECAST. THE ANOMALIES IN THE 850 MB TEMPERATURE FIELD ARE 2.0 TO 2.5 STANDARD DEVIATIONS ABOVE NORMAL WHICH IS NOT RECORD HEAT BUT TOWARD IT. THIS ANOMALY BEGINS TO SHIFT IN AS THE FRONTAL BOUNDARY LIFTS NORTHWARD AGAIN AFTER BEING STALLED ACROSS N ILLINOIS AND IOWA...THURSDAY. THUS...STARTING THURSDAY IT APPEARS THE HEAT IS ON. WITH THE FRONTAL PASSAGE MONDAY...THE AREA REMAINS MORE NEAR NORMAL THAN PREVIOUS FORECASTS...SO TUE AND WED WERE COOLED A BIT. ALSO...CHANCES FOR RAIN WERE DRAWN TOWARD NERN IOWA AND SOUTHERN WI TUE-THU NEARER TO THE FRONT AND CLEARED FROM CENTRAL WI. THE 12Z GFS HAS COME IN TO DISRUPT THE PROBABILITY POOL SUGGESTING THIS FORECAST CHANGE WAS NOT OKAY...DEVELOPING AN MCS WEDNESDAY OVER THE FORECAST AREA. THIS SEEMS LIKE AN OUTLIER AND POSSIBLY A CONVECTIVE FEEDBACK ISSUE IN THE GFS BECAUSE OF ITS LONG LIVED COMPLEX FROM THE DAKOTAS AND ASSOCIATED SURFACE LOW. FOR THIS FORECAST SET...IT WAS DISCOUNTED. && .AVIATION... CU SHOULD DISSIPATE WITH LOSS OF DAYTIME HEATING...HOWEVER BKN DECK OF MID CLOUD WILL MOVE INTO BOTH KRST AND KLSE THIS EVENING AHEAD OF A FRONT MOVING SOUTH TOWARDS THE AREA. SHOWERS AND THUNDERSTORMS WILL BE POSSIBLE AHEAD OF THE FRONT...MAINLY ALONG THE MISSISSIPPI RIVER AND EAST. ALTHOUGH THUNDERSTORMS ARE POSSIBLE...THOUGHTS ARE THAT THEY WILL BE SCATTERED AND BEST CHANCES WILL BE EAST OF KLSE...SO OPTED TO STAY WITH VCSH AT KLSE. THE PRECIP COULD MAKE IT FURTHER WEST...BUT CONFIDENCE WASN/T TOO HIGH SO LEFT OUT OF KRST TAF. THIS PRECIPITATION SHOULD MOVE OUT OF THE AREA IN THE EARLY MORNING HOURS. SOUTHWEST WINDS WILL INCREASE THIS EVENING...AND SHOULD STAY UP THROUGH THE NIGHTTIME HOURS. FRONTAL PASSAGE THROUGH THE TAF SITES WILL BE EARLY SATURDAY MORNING...WITH WINDS SHIFTING TO THE NORTH-NORTHWEST. && .ARX WATCHES/WARNINGS/ADVISORIES... WI...NONE. MN...NONE. IA...NONE. && $$ SHORT/LONG TERM...BAUMGARDT AVIATION...BROOKS ----------------------------------------------------------------------
