AREA FORECAST DISCUSSION
NWS CHICAGO IL
AFDCHI 231 PM CST THU JAN 18 2001
THE SURFACE ANALYSIS AT 07 UTC SHOWS A LARGE TEMPERATURE GRADIENT IN
EASTERN IOWA AND CENTRAL WISCONSIN. AT 00 UTC A JET STREAK IS OVER
NORTHEAST ILLINOIS AND NORTHERN WISCONSIN. THE 850 MB ANALYSIS SHOWS
A TEMPERATURE GRADIENT OVER NORTHEAST ILLINOIS. AS THE UPPER AIR
WAVE MOVES THROUGH NORTHEAST ILLINOIS AND NORTHWEST INDIANA COLD
AIR WILL INVADE. THE GRADIENT IN IOWA AND WISCONSIN WILL MOVE
THROUGH BY 00 UTC SATURDAY. THE NGM IS THE FASTEST WHILE THE ETA AND
AVN BRING THE COLD AIR INTO OUR FORECAST AREA BY 12 UTC SATURDAY.
THE WINDS WILL TURN NORTH AND LAKE EFFECT SNOWS ARE A POSSIBILITY.
FOR CHICAGO AND NORTHWEST INDIANA. THE MESOETA SHOWS CONVERGENCE
OVER CHICAGO AND NORTHWEST INDIANA FROM TONIGHT THROUGH SATURDAY
AT 18 UTC THE CONVERGENCE BECOMES FOCUSED AROUND 18 UTC. THE LAPSE
RATES ARE FORECASTED TO BE HIGH AND FORECAST SOUNDINGS SHOW A DEPTH
OF COLD AIR. CURRENT ACARS SOUNDING AND FORECAST SOUNDINGS SHOW
THAT THE LAPSE RATES WILL INCREASE DURING THE NEXT 18 HOURS.
WILL FORECAST SNOW FOR AROUND THE LAKES. THERE WILL BE SOME WEAK
DIVERGENCE AS THE JET MOVES EAST TO ENHANCE LIFTING. THIS DIVERGENCE
WILL MOVE EAST BY FRIDAY AFTERNOON. BY FRIDAY NIGHT THE LOWER LEVEL
OF THE ATMOSPHERE ARE EXPECTED TO DRY OUT.
THE NEXT FRONT IS EXPECTED SUNDAY. THE ETA KEEPS MOST OF THE 500 MB
POSITIVE VORTICITY ADVECTION NORTH OF THE FORECAST AREA. THE AVN HAS
THE POSITIVE VORTICITY ADVECTION JUST NORTH OF CHICAGO.
.CHI...NONE.
$$
WHW
AREA FORECAST DISCUSSION
NWS LOUISVILLE KY
AFDSDF 345 AM EST FRI JAN 19 2001
MANY FORECAST CHALLENGES ABOUND. THE MODELS ARE IN DECENT AGREEMENT
TODAY...AND THAT HELPS...ALTHOUGH THE AVN SEEMS SUSPECT IN MOVING
THE PRECIPITATION OUT TOO FAST. THE FOCUS OF THE HEAVIER RAINFALL
HAS NOW SHIFTED TO OUR EASTERN FORECAST ZONES...AND THIS SHOULD
REMAIN THE RULE TODAY...AS GREATEST MOISTURE REMAINS IN AN AREA FROM
BOWLING GREEN THROUGH CAMPBELLSVILLE AND INTO LEXINGTON. AS THE COLD
AIR CONTINUES TO FILTER IN TODAY...WE SHOULD STILL SEE A CHANGEOVER
OF RAIN TO SNOWFALL FROM NORTHWEST TO SOUTHEAST. LATEST ACARS
SOUNDINGS FROM SDF INDICATE THE THERMAL PROFILE IS BEGINNING TO
COLLAPSE...WITH THE H85 0 DEGREE LINE NEAR THE OHIO RIVER AT THIS
TIME...RIGHT ON SCHEDULE.
THE 06Z MESOETA CONFIRMS THE FORECAST SCENARIO...WITH RAIN CHANGING
TO SNOW IN THE LOUISVILLE AREA THROUGH THE MORNING...AND ACROSS OUR
EASTERN AREAS THIS AFTERNOON. THE SOUTHEASTERN ZONES WILL BE THE
LAST TO CHANGE THIS EVENING. THE POTENTIAL IS THERE FOR SNOWFALL
ACCUMULATION ESPECIALLY FROM COLUMBIA TO LEXINGTON THIS EVENING AS
THE LOW GRADUALLY PULLS EAST AND LINGERING PRECIPITATION FALLS IN
THE FORM OF SNOW.
THEN OVERNIGHT...I THINK THERE WILL BE A DIMINISHING TREND FROM WEST
TO EAST AS THE DEEPEST MEAN RELATIVE HUMIDITY PULLS EAST OF THE
FORECAST AREA. HOWEVER...JUST WHEN THINGS BEGIN TO CALM DOWN...THE
UPPER LOW NOW MOVES FROM THE LOWER MISSISSIPPI RIVER VALLEY INTO THE
TENNESSEE VALLEY ON SATURDAY. THIS WILL ENERGIZE WHAT REMAINING
MOISTURE THERE IS ACROSS THE FORECAST AREA...AND SNOW SHOWERS SHOULD
ENSUE ONCE AGAIN (IF YOU BUY INTO THE ETA/MESO ETA) IN THE COMMA
HEAD IN A REGION OF ENHANCED VERTICAL VELOCITY...ESPECIALLY ALONG
AND JUST NORTH OF THE H5 CIRCULATION ACROSS SOUTHERN KENTUCKY.
THUS...THERE COULD BE ADDITIONAL SNOWFALL ON SATURDAY BEFORE
DIMINISHING AGAIN FROM WEST TO EAST THROUGH THE DAY SATURDAY.
THUS...WILL ISSUE A WINTER WEATHER ADVISORY FOR MUCH OF THE AREA...
SAVE SOUTHERN INDIANA AND A PORTION OF NORTHWEST KENTUCKY...FOR
TOTAL SNOWFALL ACCUMULATIONS THROUGH THE EVENT FROM 1 TO 3 INCHES IN
THE I65 CORRIDOR...AND FROM 2 TO 4 INCHES FOR OUR EASTERN ZONES.
TEMPERATURE TRENDS ARE REPRESENTED WELL IN THE ONGOING FORECAST AND
SEE LITTLE REASON TO DEVIATE.
THANKS TO PAH...BNA...ILN AND JKL FOR THE COORDINATION.
.SDF...WINTER WEATHER ADVISORY FOR MUCH OF CENTRAL KENTUCKY THROUGH
SATURDAY MORNING.
VLD