AREA FORECAST DISCUSSION
NWS CHICAGO IL
AFDCHI 230 AM CST SAT JAN 20 2001
FCST CONCERNS THIS PCKG INITIALLY REVOLVE AROUND LAKE SNOW SHOWERS
NE IL/NW IND THIS MORNING...THEN TEMPERATURES AS DRY AIR AND QUIET
WEATHER DOMINATE.
GOES IR LOOP SHOWS SINGLE BAND OF LAKE CLOUDS CONTINUES TO IMPINGE
ON IL SHORE OF LAKE MICHIGAN...WITH ANOTHER MASS OF CLOUDS IN LOW
LEVEL CONVERGENT ZONE OVER SOUTH END OF LAKE. RADAR AND SURFACE OBS
INDICATE THAT THESE CLOUDS CONTINUE TO PRODUCE LIGHT SNOW AND
FLURRIES. MOST RECENT GLSEA ANALYSIS OF LAKE WATER TEMP INDICATES
OPEN WATERS AROUND +3...AND AIRCRAFT SOUNDINGS OUT OF ORD SHOW 850
MB TEMP OF -12 TO -15 DEG C. RESULTING LAKE-850 MB DELTA T'S OF 15
TO 18 DEG. MODEL FORECASTS SHOW WARM ADVECTION FROM THE NORTHWEST BY
12Z...SO DECREASING DELTA T'S...LOWERING INVERSION HEIGHTS AND
BACKING LOW LEVEL WINDS SHOULD BRING AN END TO THE LAKE SNOW THIS
MORNING. WL CONT MENTION OF LIGHT SNOW/FLURRIES NEAR THE LAKE THIS
MORNING BASED ON LAST MINUTE RADAR TRENDS.
CLOSED UPPER LOW OVER SOUTHERN IL THIS AM TO MOVE EAST TODAY...AS
TWO WEAKER SHORT WAVES DROP INTO THE MEAN TROF FROM THE NORTHWEST.
THE FIRST WILL PASS OVER THE UPPER GREAT LAKES TONIGHT...WHILE THE
SECOND DROPS THROUGH THE WESTERN HIGH PLAINS AND CUTS OFF OVER
NORTHERN TEXAS BY EARLY SUNDAY. AIRMASS OVER UPPER MIDWEST IS VERY
DRY...WITH LAYER RH VALUES ON THE ORDER OF ONLY 30-40 PERCENT. BASED
ON THIS...ONLY SOME SCATTERED HIGH CLOUDS EXPECTED...ALONG WITH THE
PASSAGE OF A WEAK SURFACE TROF ON SUNDAY. A BIT OF A WARM UP EARLY
IN THE WEEK AS UPPER FLOW TEMPORARILY FLATTENS...BUT TROF OVER ERN
CANADA SHARPENS AGAIN TUESDAY...WITH NEXT CHANCE OF PRECIP FOR US AS
NEXT ROUND OF COLD AIR DUMPS INTO THE NORTHERN PLAINS. LOOKING AT
PARCEL TRAJECTORIES AND YESTERDAY'S UPSTREAM TEMPS FWC GUIDANCE
LOOKS A BIT WARM TODAY AND SUNDAY...AND WILL BLEND TOWARD MAV
NUMBERS. PREFER COOLER MAV OR MAV/FWC BLEND ON MINS AS WELL.
.CHI...NONE.
RATZER