AREA FORECAST DISCUSSION
NWS CHICAGO IL
AFDCHI 900 PM CST TUE MAR 18 2003
DISCUSSION FOR THE 9 PM ZONE UPDATES
USING THE RADAR REFLECTIVITY WILL FORECAST PRECIPITATION
ENTERING PONTIAC 11 PM AND MIDNIGHT AND INTO ROCKFORD
AROUND 1 TO 2 AM CST. THE SURFACE WINDS WILL REMAIN
NORTHEAST. A LOW LEVEL INVERSION UP TO 875 MB IS SEEN
ON ACARS AT ORD AROUND 0200 UTC. THE FORECAST SOUNDINGS
FROM THE RUC40 MODEL KEEP THIS INVERSION THROUGH THE
MORNING HOURS WHILE INCREASING THE MOISTURE ABOVE THE
SOUNDING. ISENTROPIC LIFT AT 300K SURFACE IS FORECASTED
TO INCREASE BY 12 UTC WEDNESDAY MORNING. WILL LOWER THE
FORECAST LOW TEMPERATURES ACROSS NORTHEAST ILLINOIS AND
NORTHWEST INDIANA. THE WINDS REMAINING NORTHEAST AND
AN INVERSION. THE RAIN MAY WARM THE LAYER ABOVE THE
INVERSION A LITTLE WITH RELEASE OF ENTHALPY OF
VAPORIZATION. EXPECT THE WINDS TO REMAIN NORTHEAST
AT 15 TO 28 MPH THROUGH THE NIGHT. THE LOW IN THE
TEXAS PANHANDLE WILL KEEP THE GRADIENT STRONG.
.CHI...NONE
WHW
AVIATION DISCUSSION FOR 00Z TAFS
LOW CLOUDS HAVE SCATTERED OUT THIS PM. EXPECT THIS TO CONTINUE NEXT
FEW HOURS BEFORE LOW MVFR CIGS RETURN AFTER SUNSET. VISIBILITIES AND
CIGS GRADUALLY LOWER TONIGHT. BEFORE PRECIP ARRIVES 08-10Z...MAY SEE
SOME AREAS OF -DZ. NOT SURE HOW LOW VIS AND CIGS WILL GO BY MORNING
ESPECIALLY IF RAIN BECOMES MODERATE OR EVEN HEAVY AT TIMES. BUT
1/4SM PROBABLY TOO LOW. WILL CONTINUE LIFR FORECAST BY MORNING BUT
NOT AS PESSIMISTIC. LIGHT RAIN WILL CONTINUE INTO EARLY AFTERNOON
WITH RAIN BECOMING MORE SHOWERS BY AFTERNOON. CHANCE OF THUNDER BY
END OF THIS PERIOD BUT HAVE DECIDED TO LEAVE OUT THIS CYCLE AND TAKE
A LOOK AT 00Z RUNS FOR HOW FAR NORTH THUNDER WILL GET. NORTHEAST
WINDS STAY 12-15KTS ALL NIGHT WITH GUSTS TO 25KTS THIS EVENING THEN
AGAIN BY MORNING.
CMS