Severe Thunderstorm Warning 50, NWS Lincoln, IL

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LINCOLN IL
200 PM CDT TUE APR 20 1999

FCST CHALLENGES THIS PD WL BE PLACEMENT OF SFC/850 BNDRY...TEMPS AND
TIMING OF PRECIP.

18Z SFC ANALYSIS INDICATING WMFNT ACRS NE KS INTO WRN MO WITH
STRONGEST PRES FALLS ALINGNED ALONG AND JUST NORTH OF FNT. TEMP
ANALYSIS ALSO INDICATING THERMAL TROF ALIGNED NICELY WITH PRECIP
AREA WHICH IS CROSSING CNTRL IL THIS AFTERNOON...WHICH MAY POSE
A PROBLEM IN FUTURE PLACEMENT OF WMFNT...ESP LATER TNT INTO WED
WHEN ONGOING CONVECTION MAY KEEP BNDRY FUTHER SOUTH THAN MODELS
SUGGEST. MODELS CONTINUE TO ADVERTISE A GRADUAL AMPLIFICATION OF THE
PATTERN ACRS THE US OVER THE NEXT SVL DAYS. 12Z RUN CONTINUES THE
TREND OF PAST RUNS IN WHICH WRN TROF A BIT SLOWER IN EVOLVING THAN
EARLIER FCST. ETA AND AVN SEEM TO BE HANDLING THIS CHANGE THE BEST
WITH THE ETA MODEL PREFERRED DURING THIS FCST PD.

WATER VAPOR LOOP INDICATING LEAD SHORTWAVE TROF ACRS CNTRL IA THIS
AFTERNOON WHICH HAS CAUSED THE SCT -SHRA OVR FCST AREA. SOME WEAK
ISENT LIFT NOTED AS WELL ON THE 305K PRES SFC WITH LIFT FCST TO
INCREASE TNT...ESP ACRS MO INTO SW IL WHERE MIXING RATIOS INCREASE
TO 4 TO 6 G/KG. 50 TO 60 KT LOW LVL FLOW POINTED INTO LOW LVL BNDRY
TNT INTO WED MRNG SO DECENT SET UP FOR NOCTURNAL MCS ACRS MO INTO
PARTS OF WRN AND SRN IL. FEEL BEST THREAT OF SIG RAINFALL FROM ANY
CONVECTIVE SYSTEM THAT DEVELOPS TNT WOULD BE ACRS OUR SRN CWFA AS
BNDRY WL BE SLOW TO SHIFT NORTH. FCST SNDGS INDICATING LOTS OF MID
LEVEL MOISTURE AROUND BUT LOW LVLS SLOW TO SATURATE THRU 30 HRS WITH
EVAP COOLING/MESOSCALE BNDRYS PLAYING A BIG ROLE IN JUST HOW QUICKLY
WMFNT CAN SHIFT NORTH ACRS FCST AREA. MAY TEND TO UNDERCUT GUID
NUMBERS TNT DUE TO ABOVE REASONING. MOST AREAS IN STATE WELL SHORT
OF FCST HIGHS THIS AFTERNOON SO SEE NO REASON NOT TO CUT THE LOWS.

WRN US TROF GRDLY SHIFTS SE INTO THE SRN PLAINS BY THU WITH SFC LOW
FCST TO DEEPEN ACRS CNTRL OK. AGAIN THE MAIN PROBLEM IS WITH ANY
CONVECTIVE COMPLEXES THAT DO DEVELOP TNT AND AGAIN WED NIGHT AND
THEIR AFFECT ON THE POSITION OF THE WMFNT. ETA SUGGESTS BNDRY SHOULD
SHIFT INTO NRN IL BY EARLY THU WITH BEST BAROCLINICITY (1000-500 AND
850 MB) SHIFT NORTH AS WELL. H7-H3 QVECTOR CONVERGENCE AND 850-500
QG FORCING SHIFTS WELL NORTH OF STATE AS WELL ON THU...SO WE MAY SEE
A BREAK IN THE CONVECTION DURING THE DAY. MAY HANDLE IT WITH -TSRA
ENDING THU MRNG WITH ADDL DEVELOPMENT AGAIN LATE IN THE DAY OR AT
NIGHT AS MAIN ENERGY PULLS OUT OF SRN PLAINS. MAY TEND TO STAY WITH
THE COOLER FAN NUMBERS ON WED WITH CLD COVER AND PRECIP THREAT OVR THE
FCST AREA.

PRELIMINARY CCF FOR COORDINATION...

SPI 054/072 056/071 59  44776
PIA 051/068 055/066 57  44776
DEC 054/072 056/071 59  44776
CMI 052/072 056/071 59  44776
MTO 052/073 056/073 59  44776
LWV 054/075 058/076 60  44776

SMITH