Tornado in Wisconsin, August 18, 2001

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
249 PM CDT SAT AUG 18 2001

FORECAST CONCERNS THIS PACKAGE...SHRA/TSRA THIS EVENING...CLOUD
TRENDS TONIGHT AND SUN...TEMPERATURES.

DATA ANALYSIS AT 18Z SHOWED THE SURFACE LOW SYSTEM OCCLUDING OVER
THE AREA EARLY THIS AFTERNOON. ONE CENTER WAS WEAKENING NEAR KEAU...
WHILE OTHERS WERE FORMING OVER EASTERN WI AND CENTRAL IL.
CIRCULATION OF MID/UPPER LOW COULD BE SEEN IN LOOP OF LOCAL WSR-88D
DATA...AND LOOKED TO LINE UP WELL WITH MODELS AT 18Z. BEST
CONVECTION ACROSS THE FORECAST AREA EARLY THIS AFTERNOON OCCURRING
JUST AHEAD OF THE MID/UPPER LEVEL LOW CENTER. CONVECTION IN THE
AREA THIS AFTERNOON LOOKS BE IN PART DIURNALLY DRIVEN. VIS AND WV
IMAGERY SHOWED PINWHEEL OF THE UPPER LOW SITTING OVER THE FORECAST
AREA VERY NICELY. MESO-ETA TOTAL CLOUD PRODUCT HAD A GOOD HANDLE ON
CLOUD FIELD ACROSS THE REGION AT 18Z. COLD CORE ALOFT AND ABUNDANCE
OF CLOUDS WERE HOLDING TEMPS IN THE MID 60S TO NEAR 70 AT MOST
LOCATIONS ACROSS THE AREA.

12Z MODELS ALL LOOKED TO INITIALIZE WELL THIS MORNING...BOTH AT THE
SURFACE AND ALOFT...AND OFFER SIMILAR SOLUTIONS THRU 48-60HRS. 12Z
MODELS DO TREND A BIT DEEPER WITH A TROUGH IN THE EASTERN U.S. IN
THE SUN NIGHT/MON TIMEFRAME. THIS IN TURN SLOWS UP THE PROGRESSION OF
SHORTWAVES OUT OF A DEVELOPING PACIFIC COAST TROUGH AND TOWARD THE
REGION FOR MON NIGHT AND BEYOND. WILL FAVOR ETA SOLUTION FOR THE
SHORT TERM WITH ITS BETTER RESOLUTION.

IN THE SHORT TERM...COLD CORE UPPER LOW CONTINUES TO PINWHEEL AND
MOVE SLOWLY SOUTHEAST AWAY FROM THE AREA TONIGHT...ENDING UP NEAR THE
MI/IN BORDER BY 12Z SUN. LOW LEVEL WARM ADVECTION...Q-G CONVERGENCE
EXIT THE AREA DURING THE EVENING...SO WILL HAVE TO LINGER SOME SHRA
ACROSS THE AREA EARLY TONIGHT. WILL DELINEATE THIS WITH RADAR TRENDS
AT ISSUE TIME. WITH DEEP...COOL CYCLONIC FLOW OVER THE AREA
TONIGHT...CLOUDS SHOULD BE SLOW TO DECREASE...ESPECIALLY OVER THE
SOUTHEAST PART OF THE FORECAST AREA. RIDGING AT THE SURFACE AND ALOFT
BUILDS IN FOR SUN...WITH FLOW BECOMING NEUTRAL/ANTI-CYCLONIC OVER THE
AREA BY LATER IN THE DAY. ALL MODELS LINGER PLENTY OF LOW LEVEL
MOISTURE ACROSS THE AREA. DIURNAL WARMING SHOULD DEVELOP PLENTY OF
CUMULUS ACROSS THE AREA FOR SUN. CENTER OF SURFACE DRIFTS OVER FOR
SUN NIGHT. SUBSIDENCE UNDER THE SURFACE AND UPPER RIDGE SHOULD CLEAR
OUT THE CLOUDS SUN NIGHT. SUN NIGHT LOOKING TO BE FAVORABLE FOR LATE
NIGHT VALLEY FOG...PERSISTING INTO MON MORNING. WILL LEAVE OUT FOR
NOW...BUT ALERT LATER SHIFTS TO THE POTENTIAL. PLENTY OF MOISTURE FOR
DIURNAL CUMULUS AGAIN MON.

NUMERICAL GUIDANCE TEMPS TONIGHT THRU MON QUITE SIMILAR AND LOOK
GOOD AND FOLLOWED. FAVORED HIGHER AVN-MAV POPS FOR TONIGHT AND THESE
RAISED A BIT.

WITH MODELS TRENDING TO A SLOWER PROGRESSION OF FEATURES OUT OF THE
WESTERN TROUGH...LEFT MON NIGHT DRY. MODELS PROG DECENT 850MB WARM
ADVECTION/MOISTURE TRANSPORT INTO THE REGION FOR TUE...THOUGH THEY
DO SHOW SOME DIFFERENCES IN NORTH/SOUTH POSITION. CHANCE SHRA/TSRA
ALREADY IN THE FORECAST FOR TUE. MEDIUM RANGE MODELS TODAY COMING IN
LINE WITH EARLIER ECMWF RUN FROM FRI MORNING. UPPER RIDGE AXIS IS
FURTHER EAST WITH THE AREA UNDER BROAD WEST-SOUTHWEST FLOW FOR
LATER IN THE WEEK. THIS LEAVES THE AREA OPEN TO MOISTURE RETURN THRU
THE PLAINS...SHORTWAVES EJECTING OUT OF A BROAD WESTERN U.S.
TROUGH...AND A CHANCE SHRA/TSRA FOR DAYS 5 TO 7.

.LSE...NONE.

RRS