Ice Storm and Winter Weather in Illinois, February 7 to 8, 2001

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE QUAD CITIES IA IL
320 AM CST WED FEB 7 2001

HIGH PRESSURE RIDGE WAS IN PLACE OVER THE NORTHERN HALF OF THE CWA AT
06Z.  COLD AIR WITH THE HIGH HAS DROPPED SITES WITHOUT STRATUS/FOG IN TO
THE SINGLE DIGITS WHILE STRATUS HAS KEPT OTHER LOCATIONS FROM FALLING
BELOW 20 SO FAR THIS MORNING.  LOW PRESSURE IS TAKING SHAPE OVER
EASTERN COLORADO WITH BEST 3 HOUR PRESSURE FALLS OVER NE CO.  REGIONAL
RADAR COMPOSITE SHOWS AREA -SN OVER MUCH OF S DAKOTA.  SNOW IS BEING
DRIVEN BY STRONG ISENTROPIC LIFT WITH SOME HELP FROM A WEAK S/W TROF.
EVENING UA ANALYSIS SHOWED W COAST TROUGH TAKING SHAPE.  BETTER THAN
100M HEIGHT FALLS AT 850 NOTED OVER THE GREAT BASIN WITH H500 FALLS
OF 180 METERS NOTED.  WATER VAPOR LOOP SHOWS STRONG VORT DIVING SOUTH
ALONG THE CA COAST INTO THE BASE OF THE TROUGH.

AVN/ETA/NGM INITIALIZED THEIR UPPER AIR FIELDS REASONABLY WELL.
HOWEVER...AT THE SFC...MODELS WERE HAVING TROUBLE WITH PRESSURE FIELDS
OVER THE HIGH PLAINS.  AVN/NGM CAPTURED THE GRADIENT OVER PLAINS AND
NORTHERN ROCKIES BETTER THAN THE ETA WHICH HAS SLP ASSOCIATED WITH THE
DEVELOPING SFC LOW OVER CO TO HIGH AND IS TOO LOW WITH THE HIGH
CENTERED OVER SOUTH CENTRAL CANADA.

LOTS OF THINGS TO CONSIDER THIS ROUND.  IN THE NEAR TERM WILL HAVE
TO DEAL WITH DENSE FOG OVER CWA THIS MORNING.  THEN ONSET
OF/TYPE/AMOUNT/ OF PCPN THIS AFTERNOON AND THIS EVENING AND WHETHER A
WINTER WEATHER HEADLINE IS NECESSARY.  DIVERGING MODEL SOLUTIONS IN THE
LATER PERIODS WILL HAVE IMPACT ON LIQUID QPF AND FLOOD POTENTIAL,
TEMPERATURES, AND SNOW FALL AMOUNTS.

ETA CONTINUES TO BE THE COLDEST MODEL AND FAVORS A MORE EASTERN TRACK
AS SW U.S. LOW EJECTS INTO THE MID MISSISSIPPI VALLEY.  AVN ON THE
OTHER HAND BRINGS THE WARMER AIR FURTHER WEST AND EJECTS SFC TO THE NW
OF THE ETA TRACK.  WILL SIDE WITH THE AVN SINCE IT HAS BEEN MORE
CONSISTENT AND THE ETA CONTINUES TO TREND TOWARD THE AVN SOLUTION.
THE FIRST 24 HOURS THE AVN/ETA ARE CLOSE IN DEVELOPING A WARM FRONT
FROM ERN CO THRU SRN KS INTO CENTRAL MO AND LIFTING IT NORTH INTO
EASTERN IA AND NW IL BY THURSDAY MORNING.  MODELS HOLD THE HIGH PRESSURE
RIDGE OVER NERN CWA THROUGH 12Z THEN MOVE IT OFF TO THE EAST.  WITH
LIGHT WIND IN THE VICINITY OF THE RIDGE EXPECT THE FG TO HANG IN FOR
A WHILE THIS MORNING.  EASTERLY FLOW SHOULD PICKUP BY 15Z LIFTING
THE DENSE FOG.  AN DENSE FOG ADVISORY WILL BE ISSUED FOR
APPROXIMATELY THE NRN TWO THIRDS OF THE CWA THROUGH MID MORNING.

PCPN OVER THE DAKOTAS WILL EXPAND E AS ISENTROPIC LIFT INCREASES ACROSS
THE DEVELOPING WARM FRONT. PCPN WILL HAVE TO OVER COME SOME DRY AIR...
BUT PCPN SHOULD BE OVER MOST OF THE CWA BY LATE THIS AFTERNOON. BEST
CHANCE FOR PCPN SHOULD BE ACROSS THE N WHERE IT SHOULD BEGIN AS -SN.
TO THE SOUTH WILL GO WITH A LOW POP.  WILL GO WITH A LOW POP OF RN OR
FREEEZING RN SOUTH WHERE 850 TEMPS WILL WARM ABOVE ZERO...BUT SFC TEMPS
MAY STILL BE AT OR JUST BELOW FREEZING.  EXPECT SNOW TO CHANGE TO
FREEZING RAIN OR RAIN ACROSS THE NORTH AND ALL RAIN IN THE SOUTH OVER
NIGHT AS WARM FRONT PIVOTS TO THE NORTH TO A LAMONI CLINTON LINE BY 12Z.
FEEL THAT QPFS WILL BE ON THE LIGHT SIDE OVERNIGHT AND WITH MIX BAG
ACROSS THE NORTH I DON'T THINK THERE IS A NEED FOR A HEADLINE AT THIS
POINT. BY THURSDAY MORNING THE LOW LEVELS SHOULD BE WARM ENOUGH FOR THE
PCPN TO BE ALL RAIN.  QPF AND FLOODING POTENTIAL WILL THEN BE THE MAIN
CONCERN FOR THURSDAY INTO FRIDAY AS UPPER SYSTEM LIFTS OUT OF THE
SOUTHWEST U.S. AND SFC WAVE MOVES NE ALONG THE WARM FRONT AND DEEPENS.
SHOULD BE PLENTY OF MOISTURE FROM THE GULF TO WORK WITH AND THERE IS A
POTENTIAL FOR CONVECTION TO ENHANCE THE RAINFALL. LOOKS LIKE GREATEST
RAINFALL WILL OCCUR THURSDAY NIGHT.  EVENING SHIFT ISSUED A FLOOD
POTENTIAL OUTLOOK AND WILL UPDATE THIS FOR THE MORNING ISSUANCE. COLDER
AIR WILL RUSH BACK EAST FRIDAY AS SURFACE SYSTEM SHIFTS EAST. SHOULD SEE
RN CHANGING BACK TO SN DURING THE DAY.  AVN SHOWING DEF ZONE SETTING UP
OVER ERN IA/NW IL DURING THE AFTERNOON...SO THERE COULD BE SOME SNOW
ACCUMULATION BY FRIDAY EVENING.

THE TEMP FORECAST WILL BE TRICKY...ESPECIALLY FOR THURSDAY.  DEPENDING
ON WHERE THE WARM FRONT ENDS UP SOME AREAS COULD BE AROUND 40 OR IN
THE MIDDLE 50S.  OVERALL MOS NUMBERS ARE SIMILAR.  WILL PROBABLY GO
LITTLE BELOW MOS THOUGH SINCE THE SNOW COVER MAY HOLD BACK TEMPS SOME.

WILL MAKE SOME ADJUSTMENTS FOR FRIDAY IN THE EXTENDED BY WARMING HIGHS
SOME AND INCLUDING RAIN TO SNOW WORDING.

.DVN...
IA...DENSE FOG ADVISORY UNTIL 9 AM NORTH OF A SIGOURNEY BURLINGTON LINE.
IL...DENSE FOG ADVISORY UNTIL 9 AM NORTH OF BURLINGTON MONMOUTH LINE.
MtO...NONE.

DLF