Ice Storm in Iowa, February 24, 2001

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
331 PM CST FRI FEB 23 2001

DATA ANALYSIS SHOWING SNOW...SLEET AND FREEZING RAIN ACROSS THE
CENTRAL PLAINS.  LARGE SCALE DEPICTS THE STRONG DISTURBANCE OVER THE
WEST WITH A TROPICAL CONNECTION INTO THE EASTERN PACIFIC.  STRONG
MOISTURE TRANSPORT IN THE LOWER LAYERS NOTED ON THE 23/12Z UPPER AIR
ANALYSIS.  SURFACE HIGH CURRENTLY POSITIONED OVER EASTERN COLORADO...
WITH HIGH PRESSURE EXITING OVER LAKE MICHIGAN.  AS THE SYSTEM AFFECTS
THE REGION TONIGHT THROUGH SUNDAY...MAIN FORECAST CHALLENGE WILL BE
PRECIPITATION AMOUNTS/TYPES AND TIMING OF SUCH.

MODELS REASONABLE WITH THE INITIALIZATION...BUT ARE STILL TOO WARM
INTO THE LOWER LAYERS TO THE SOUTH.  THIS MANIFESTS ITSELF IN THE
PARTIAL THICKNESS PRECIPITATION TYPE SCHEME...WHICH IS TOO WARM
WHEN COMPARED TO OBSERVED PRECIPITATION TYPES AT 23/18Z.  AVN AND
ETA STILL STICKING WITH THEIR DIFFERENCE IN THE REPRESENTATION OF
THE DERIVED QPF FIELDS.  PRECIPITATION TYPES ACCORDING TO THE
FORECAST SOUNDINGS KEEP SNOW INTO THE MORNING HOURS WHILE BUFKIT
SHOWS A TROUBLING AMOUNT OF FREEZING RAIN ACROSS MUCH OF THE FORECAST
AREA.  WHICH IS CLOSEST TO REALITY???...PROBABLY NONE.

MOISTURE SURGE MOVES UP INTO THE REGION TONIGHT.  EXPECT SNOW TO MOVE
IN FROM THE SOUTH ON THE NOSE OF THE MOISTURE SURGE.  STRONG WARM
ADVECTION SHOULD CAUSE THE PRECIPITATION TO CHANGE OVER TO SLEET
THEN FREEZING RAIN RATHER QUICKLY.  THEN...THE SURFACE TEMPERATURES
BECOME THE CRITICAL ELEMENT.

A COUPLE OF DEGREES IN ERROR EITHER WAY WILL RESULT IN A HUGE ICE
STORM OR RELATIVELY NOTHING.  THINK THAT ICE ACCUMULATIONS WILL BE
MOST PREVALENT TONIGHT AND EARLY SATURDAY MORNING ACROSS THE
NORTHEAST IOWA EXTREME SOUTHWESTERN ZONES.  EXPECT THIS TO MOVE
FORWARD INTO THE SOUTHEAST MINNESOTA/CENTRAL WISCONSIN ZONES
SATURDAY MORNING.  WE THINK THAT A 6 HOUR PERIOD OF ICE ACCUMULATION
LOOKS REASONABLE AT THIS TIME...WITH LONGER PERIODS ACROSS PORTIONS
OF SOUTHEAST MINNESOTA AND CLARK-TAYLOR ZONES.

BY AFTERNOON...A RESPITE FROM THE PRECIPITATION SHOULD OCCUR AS THE
FORCING IS AT A RELATIVE MIN.  SATURDAY NIGHT...MORE PRECIPITATION
SHOULD RETURN...WITH MUCH OF THE FORECAST AREA BEING RAIN.  SOME OF
THE RAIN MAY BE HEAVY ACROSS THE EXTREME SOUTHERN ZONES AS WELL AS
THE POSSIBILITY OF THUNDERSTORMS.  THIS RESIDES IN THE REGION WHERE
THERE IS SOME ELEVATED INSTABILITY.  GIVEN THAT TEMPERATURES SHOULD
BE IN THE 35 TO 40 RANGE...WILL ISSUE A FLOOD WATCH FOR THIS REGION.

AS THE STORM MOVES THROUGH SATURDAY NIGHT...THE PRECIPITATION
SHOULD CHANGE OVER TO SNOW FROM NORTH AND WEST TO SOUTH AND EAST.
SOME ADDITIONAL ACCUMULATIONS ARE POSSIBLE LATE SATURDAY NIGHT AND
SUNDAY.

NEXT SYSTEM MOVES IN DURING THE TUESDAY/WEDNESDAY TIME PERIOD.
ARCTIC AIR MOVES IN BEHIND THIS SYSTEM AS IT LOOKS NOW...WITH MORE
BELOW ZERO READINGS IN STORE FOR THE FORECAST AREA.

COORDINATED WITH MPX/DVN.

 SOUNDINGS...
WHICH DO SEEM A BIT COLD...

.LSE...ICE STORM WARNING TONIGHT AND SATURDAY MORNING...IAZ008>011-
           018- 019-029-030-WIZ054-055-061
       WINTER STORM WARNING TONIGHT AND SATURDAY...MNZ079-086>088-
          094>096-WIZ032>034-041-042-053
       WINTER STORM WARNING SATURDAY...WIZ017-029-043-044
       FLOOD WATCH SATURDAY AND SATURDAY NIGHT...IAZ030-WIZ054-055-
          061

KRC