Heavy Snow in Illinois, April 10 to 11, 2007

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE QUAD CITIES IA IL
910 PM CDT TUE APR 10 2007

.UPDATE...
BASED ON 00-02 ANALYSIS AND CONTINUED TRENDS OF SOLUTIONS APPEARS
INCREASING CHANCE OF HEAVIER SNOW 20 TO 30 MILES SOUTH OF CURRENT
FORECAST. SINCE VOLATILE SITUATION WILL DEFER TO OVERNIGHT SHIFT
ON ADDING IOWA COUNTIES ALONG I-80 FOR NOW. WILL ADD AN INCH OR
TWO THOUGH TO SNOW AMOUNTS. UPDATED CHIWSWDVN PRODUCT AND FORECAST
TO BE OUT NEXT 30 MINUTES.  ..NICHOLS..

&&

.PREV DISCUSSION... /ISSUED 255 PM CDT TUE APR 10 2007/

SYNOPSIS...
A EARLY SPRING SNOWSTORM WILL MOVE THROUGH THE REGION LATE TONIGHT
THROUGH WEDNESDAY PRODUCING ACCUMULATING SNOW...MAINLY ALONG AND
NORTH OF INTERSTATE 80. AFTER A QUIET THURSDAY AND THURSDAY NIGHT...
YET ANOTHER SIGNIFICANT STORM SYSTEM IS FORECAST TO AFFECT THE
REGION FRIDAY INTO SATURDAY.

SHORT TERM...TONIGHT AND WEDNESDAY...
RAPIDLY EVOLVING SITUATION REGARDING THIS STORM SYSTEM. 12Z UPPER
AIR AND NEW MODEL RUNS HAVE PULLED THE SYSTEM MUCH FURTHER SOUTH
THAT PREVIOUSLY THOUGHT AND REACHING MAXIMUM STRENGTH ON WEDNESDAY
ACROSS ILLINOIS. THEREFORE WILL BE RAISING WARNING AND ADVISORY
HEADLINES FOR AREAS ALONG AND NORTH OF I-80.

INITIALLY DRY ATMOSPHERE IS DOING A SLOW TOP DOWN SATURATION. THIS
SATURATION WILL BE COMPLETED THIS EVENING. ONCE THE RAIN BEGINS
..SHOULD SEE A FAIRLY QUICK DROP IN TEMPERATURES. LOW LEVEL
EASTERLY FLOW TO COMBINE WITH DYNAMIC COOLING TO DROP TEMPERATURES
FURTHER AFTER MIDNIGHT. PRECIP TO CHANGE FROM RAIN TO SNOW LATE
TONIGHT WITH THE CHANGE OVER COMPLETING AS COLD AIR IS PULLED INTO
THE SYSTEM WEDNESDAY AFTERNOON. THIS CHANGE OVER WILL RESULT IN
TEMPERATURES SLOWLY FALLING DURING THE AFTERNOON.

WHEN THE CHANGE OVER OCCURS IS THE KEY REGARDING HOW MUCH SNOW WILL
FALL. THE GROUND HAS WARMED SOME DUE TO THE RECENT WARM WEATHER.
THAT WILL CUT DOWN SOME ON MEASURABLE AMOUNTS. CHANGE OVER SHOULD
BEGIN IN THE 06-09Z TIME FRAME ALONG HIGHWAY 20 TO JUST NORTH OF
HIGHWAY 30...AND 09-12Z FROM HIGHWAY 30 TO I-80. HIGHEST SNOW
AMOUNTS TO BE RIGHT ALONG THE HIGHWAY 20 CORRIDOR WITH AMOUNTS
DROPPING QUICKLY AS ONE MOVES SOUTH. BEST GUESS RIGHT NOW IS 4-6
INCHES IN THE HIGHWAY 20 CORRIDOR WITH 1-4 INCHES FROM I-80 ON
NORTH.   ..08..

LONG TERM...WEDNESDAY NIGHT THROUGH NEXT TUESDAY...
UNSETTLED/ACTIVE PATTERN TO REMAIN IN PLACE WITH MAIN FOCUS ON NEXT
STORM IN HIT PARADE LATE WEEK AND EARLY IN THE WEEKEND. CHALLENGES
ABOUND WITH THIS STORM WITH USUAL TRACK...INTENSITY AND TIMING...
ALL OF WHICH TO HAVE BIG IMPACT ON PCPN TYPE/AMOUNTS. PATTERN AND
ENERGETICS CONDUCIVE FOR MODERATE TO LOCALLY HEAVY QPF. MODEL TRENDS
OF LATE HAVE BEEN TO TAKE SYSTEM FURTHER SOUTH. THIS WOULD INVITE
MORE COLD AIR THUS INCREASING POTENTIAL FOR SNOW OR RAIN/SNOW
MIX...ESPECIALLY NORTH HALF. OVERALL THOUGH... VERY LOW CONFIDENCE
ON DETAILS OF SYSTEM WITH IMPACTS FROM INITIAL SYSTEM TONIGHT-WED...
BUT HARD TO IGNORE SOUTHERN TRENDS AND HAVE STARTED TO TREND COLDER
ON TEMPS AND INTRODUCE RAIN/SNOW MIX TO NORTH. TREND OF LATE HAS
ALSO BEEN TO SLOW DOWN THE STORM WHICH IS COMMON WITH THESE
SOUTHWEST SYSTEMS. HAVE THEREFORE REMOVED POPS UNTIL FRI AFTN... BUT
IF SLOWING TRENDS PERSIST WHICH I BELIEVE WILL BE THE CASE THEN PCPN
MAY NOT ARRIVE UNTIL LATE FRI NIGHT AND ESPECIALLY SAT/SAT NIGHT.
BEYOND WEEKEND MEDIUM RANGE MODELS SUGGEST OMEGA BLOCK DEVELOPING.
WHEN THAT BLOCK BREAKS DOWN WILL DETERMINE NEXT PCPN THREAT.

AS FOR TEMPS... GENERALLY UNDERCUT MEX GUIDANCE THROUGHOUT. POTENTIAL TO BE
COLDER ESPECIALLY THURS-FRI WITH NEGATIVE FEEDBACK FROM FRESH SNOW
COVER NORTH AND WET GROUND ELSEWHERE...AND WITH SFC RIDGE BUILDING IN.
   ..05..

&&

.DVN WATCHES/WARNINGS/ADVISORIES...
IA...WINTER WEATHER ADVISORY FROM 4 AM TO 7 PM CDT WEDNESDAY FOR
     CEDAR-IOWA-JOHNSON-MUSCATINE-SCOTT.

     WINTER STORM WARNING FROM 1 AM TO 7 PM CDT WEDNESDAY FOR BENTON-
     BUCHANAN-CLINTON-DELAWARE-DUBUQUE-JACKSON-JONES-LINN.

IL...WINTER WEATHER ADVISORY FROM 4 AM TO 7 PM CDT WEDNESDAY FOR
     BUREAU-HENRY IL-PUTNAM-ROCK ISLAND.

     WINTER STORM WARNING FROM 1 AM TO 7 PM CDT WEDNESDAY FOR CARROLL-
     JO DAVIESS-STEPHENSON-WHITESIDE.

MO...NONE.
&&

NICHOLS

HAZARDOUS WEATHER OUTLOOK
NATIONAL WEATHER SERVICE QUAD CITIES IA IL
509 AM CDT TUE APR 10 2007

IAZ040>042-051>054-063>068-076>078-087>089-098-099-ILZ001-002-007-
009-015>018-024>026-034-035-MOZ009-010-111015-
BUCHANAN-DELAWARE-DUBUQUE-BENTON-LINN-JONES-JACKSON-IOWA-JOHNSON-
CEDAR-CLINTON-MUSCATINE-SCOTT-KEOKUK-WASHINGTON-LOUISA-JEFFERSON-
HENRY IA-DES MOINES-VAN BUREN-LEE-JO DAVIESS-STEPHENSON-CARROLL-
WHITESIDE-ROCK ISLAND-HENRY IL-BUREAU-PUTNAM-MERCER-HENDERSON-
WARREN-HANCOCK-MCDONOUGH-SCOTLAND-CLARK-
509 AM CDT TUE APR 10 2007

THIS HAZARDOUS WEATHER OUTLOOK IS FOR PORTIONS OF NORTH CENTRAL
ILLINOIS...NORTHWEST ILLINOIS...WEST CENTRAL ILLINOIS...EAST
CENTRAL IOWA...NORTHEAST IOWA...SOUTHEAST IOWA AND NORTHEAST
MISSOURI.

.DAY ONE...TODAY AND TONIGHT

A STRONG SPRING STORM SYSTEM WILL BRING WIDESPREAD RAIN AND
MEASURABLE SNOW TO PORTIONS OF THE AREA LATE TONIGHT. A MIX OF
RAIN AND SNOW THIS EVENING WILL CHANGE TO ALL SNOW NORTH OF
HIGHWAY 30 AFTER MIDNIGHT...WHERE SNOW ACCUMULATIONS MAY BE AS
HIGH AS 1 TO 2 INCHES BY MORNING. THIS MAY RESULT IN SLICK TRAVEL
CONDITIONS.

ONGOING HIGH WATER LEVELS AND MINOR TO MODERATE FLOODING WILL
CONTINUE ON THE MISSISSIPPI AND AREA TRIBUTARY RIVERS. MORE
DETAILED INFORMATION CAN BE FOUND IN THE LATEST FLOOD ADVISORIES
AND STATEMENTS.

.DAYS TWO THROUGH SEVEN...WEDNESDAY THROUGH MONDAY

ON WEDNESDAY...A STRENGTHENING STORM SYSTEM PASSING THROUGH THE
REGION WILL CONTINUE TO PRODUCE MEASURABLE SNOW ACCUMULATIONS...
MAINLY NORTH OF INTERSTATE 80. SNOWFALL AMOUNTS ARE EXPECTED TO BE
GREATEST ALONG THE HIGHWAY 20 CORRIDOR...WHERE TOTALS MAY REACH 1
TO 3 INCHES BY AFTERNOON. LOCALLY HIGHER AMOUNTS WILL BE
POSSIBLE.

ANOTHER STRONG STORM SYSTEM MAY IMPACT THE AREA FRIDAY AND
SATURDAY. THERE IS THE POTENTIAL FOR THUNDERSTORMS AND LOCALLY
HEAVY RAINFALL FRIDAY AND FRIDAY NIGHT...THEN MEASURABLE SNOW ON
SATURDAY. HOWEVER...THERE IS STILL CONSIDERABLE UNCERTAINTY IN THE
TIMING AND TRACK OF THIS SYSTEM AND MORE SPECIFIC INFORMATION WILL
BE AVAILABLE IN LATER FORECASTS AND OUTLOOKS.

THE ONGOING HIGH WATER LEVELS AND FLOODING ON AREA RIVERS MAY BE
AGGRAVATED BY PRECIPITATION FROM THESE STORM SYSTEMS.

.SPOTTER INFORMATION STATEMENT...

STORM SPOTTER ACTIVATION WILL NOT BE NEEDED TODAY...HOWEVER...REPORTS
OF SNOWFALL AMOUNTS WILL BE GREATLY APPRECIATED TONIGHT AND
WEDNESDAY.

DLS