197  
FXUS63 KLOT 210533  
AFDLOT  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL  
1233 AM CDT SAT SEP 21 2019  
   
UPDATE
 
 
830 PM CDT  
 
DURING THIS EVENING, WE HAVE CONTINUED TO ASSESS TIMING OF RAIN  
AND POTENTIAL THUNDERSTORMS DURING THE DAY SATURDAY. BOTTOM LINE  
UP FRONT IS THAT THE CHANCES FOR RAIN ARE HIGHEST DURING THE DAY  
ACROSS NORTH CENTRAL ILLINOIS IN THE MORNING, AND THIS INCLUDES  
EMBEDDED THUNDERSTORMS. THE CHANCE FOR SCATTERED SHOWERS AND A  
FEW STORMS CONTINUES THROUGH THE AFTERNOON AND EARLY EVENING  
BEFORE A NOTABLE INCREASE IN THE RAIN FOOTPRINT LATER SATURDAY  
EVENING INTO OVERNIGHT. WHILE CERTAINLY SOME EFFICIENCY TO RAIN  
RATES COULD BE REALIZED WITH ANY SHOWERS DURING THE DAY, THE  
PRIMARY HEAVY RAIN THREAT REMAINS AFTER DARK ON SATURDAY.  
 
SATELLITE IMAGERY, INCLUDING THE WATER VAPOR BAND AND THE AIR MASS  
RGB, INDICATE A MOIST AIR MASS EXPANDING NORTHWARD FROM THE OZARKS  
REGION. THIS WAS SAMPLED AT NEAR 1.8 PRECIPITABLE WATER ON 00Z  
LZK AND SGF SOUNDINGS. AN UPPER JET MEASURED AT 100 KT AT ABQ IS  
PROPOGATING NORTHEASTWARD AND WILL CONTINUE TO RESULT IN A  
POLEWARD FLUX OF THIS MOISTURE TONIGHT. THERE SHOULD BE A GRADUAL  
INCREASE IN SHOWERS AND SOME THUNDERSTORMS ACROSS NORTHERN  
MISSOURI, SOUTHERN IOWA, AND WESTERN ILLINOIS INTO THE OVERNIGHT  
IN RESPONSE TO THE JUXTAPOSE OF UPPER FORCING FOR ASCENT AND  
MOISTURE TRANSPORT WITH THE LOW-LEVEL JET. IN FACT, ALREADY HAVE A  
COUPLE CONVECTIVE CELLS SOUTHEAST OF THE QUAD CITIES.  
 
THE PRIMARY CHANCES FOR NORTH CENTRAL ILLINOIS LOOK TO BE AFTER 4  
A.M. AND PERSISTING THROUGH THE MORNING AS A MID-LEVEL WAVE  
ASSOCIATED WITH THE SPEED MAX MOVES NORTHEAST ACROSS THE REGION.  
THE 00Z DVN SOUNDING ALREADY INDICATED 1000 J/KG WITH MODIFYING  
FOR SATURATING AT 850 MB, OR BASICALLY A PROXY FOR THE LOW-LEVEL  
JET LAYER PROXY. SO THINK SOME THUNDER (E.G. SCATTERED) IS A  
FAIRLY GOOD BET INCLUDING INTO THE A.M. OVER NORTH CENTRAL  
ILLINOIS.  
 
WHILE THE DEEP MOIST PLUME WILL SPREAD ACROSS THE ENTIRE AREA BY  
MIDDAY, ISENTROPIC ASCENT WEAKENS SOME FURTHER EAST, SO SHOWERS  
WILL LIKELY REMAIN SCATTERED OR EVEN JUST ISOLATED INTO NORTHEAST  
ILLINOIS AND NORTHWEST INDIANA LATE MORNING INTO THE AFTERNOON.  
THIS IS WHY THE PRIMARY COVERAGE TIME DURING THE DAY SATURDAY IN  
THE CWA LOOKS LIKE THE A.M. AND ESPECIALLY NORTH CENTRAL ILLINOIS.  
THAT SAID, SUSPECT THERE WILL BE SOME SPOTTY AFTERNOON SHOWERS AND  
POSSIBLY A COUPLE STORMS.  
 
ANY OTHER MODIFICATIONS TO THE FORECAST INTO TOMORROW WERE MINOR.  
 
MTF  
 

 
   
SHORT TERM
 
 
220 PM CDT  
 
THROUGH SATURDAY...  
 
FOR THE REMAINDER OF THE AFTERNOON, MAIN CONCERNS WILL BE THE  
POTENTIAL FOR ISOLD SHRA/TSRA ACROSS THE REGION AND THE POTENTIAL  
FOR ANY DEVELOPMENT ALONG A LAKE BREEZE BOUNDARY PUSHING INLAND.  
 
THE ENVIRONMENT REMAINS WARM/MOIST/MODESTLY UNSTABLE, BUT THE LACK  
OF ANY SIGNIFICANT SFC FOCUSING MECHANISM WILL KEEP AND SHRA/TSRA  
DEVELOPMENT RELATIVELY ISOLATED. THE EXCEPTION COULD BE THE  
POTENTIAL FOR SHRA/TSRA DEVELOPMENT ALONG THE LAKE BREEZE  
BOUNDARY, WHICH COULD PROVIDE SOME SFC FORCING. HOWEVER, THE  
ENVIRONMENT ACROSS NERN IL IS A LITTLE MORE STABLE AND HAS BEEN  
UNDER MORE EXTENSIVE CLOUD COVER THROUGH MUCH OF THE DAY, SO ANY  
CONVECTIVE DEVELOPMENT ALONG THE LAKE BREEZE SHOULD BE LOW CHANCE  
AS WELL. LATEST RADAR IMAGERY SHOWS SOME ISOLATED SHRA INVOF THE  
KANKAKEE RIVER. SOME OF THE SHRA COULD HAVE THE POTENTIAL TO  
PRODUCE BRIEF HEAVY RAINFALL. THE ISOLATED ACTIVITY WILL ALSO BE  
SOMEWHAT DIURNAL IN NATURE, SO PCPN POTENTIAL SHOULD DIMINISH AT  
SUNSET. HIGHS THIS AFTERNOON SHOULD TOP OUT IN THE LOW TO MIDDLE  
89S FOR MOST AREA, EXCEPT FAR NERN IL WHERE SELY FLOW OFF OF THE  
LAKE WILL LIMIT TEMPS TO THE MIDDLE TO UPPER 70S. CONDITIONS  
SHOULD BE QUIET OVERNIGHT WITH LOWS IN THE MIDDLE TO UPPER 60S.  
 
FOR SATURDAY, DEEP LAYER MOISTURE WILL BE ON THE INCREASE AS SSELY  
FLOW SETS UP BETWEEN AN APPROACHING COLD FRONT/TROUGH FROM THE  
WEST AND HIGH PRESSURE PARKED OVER THE MID-ATLANTIC COAST. LATEST  
GUIDANCE IS CONSISTENT IN BUILDING A PLUME OF PWATS OF 2.0 INCHES  
ACROSS THE LOCAL AREA IN A SUSTAINED FETCH OF MOISTURE FROM THE  
WESTERN GULF OF MEXICO. PCPN CHANCES SHOULD INCREASE THROUGH THE  
DAY WITH STRENGTHENING WARM ADVECTION IN ADVANCE OF THE FRONTAL  
TROUGH WITH STRONG, GUSTY SSWLY WINDS, WITH GUSTS UP TO 30 MPH  
DEVELOPING BY LATE MORNING. WITH THE STRONGEST FOCUS EXPECTED TO  
BE WITH THE SFC COLD FRONT, WHICH SHOULD STILL BE WELL WEST OF THE  
CWA, CONVECTIVE ACTIVITY SHOULD BE MORE SCATTERED, BUT RAMPING UP  
IN COVERAGE FROM WEST TO EAST THROUGH THE AFTERNOON. WHILE  
ISOLATED STRONGER STORMS CANNOT BE RULED OUT, AND THE POTENTIAL  
FOR SEVERE WEATHER STILL APPEARS LOW AT THIS TIME. THE MAIN  
THREAT WILL ONCE AGAIN BE HEAVY DOWNPOURS.  
 

 
   
LONG TERM
 
 
308 PM CDT  
 
SUNDAY THROUGH FRIDAY...  
 
THE PRIMARY FOCUS CONTINUES TO RESIDE AROUND THE INCREASING RISK  
FOR HEAVY RAINFALL AND POSSIBLE FLASH FLOODING ACROSS THE AREA,  
PARTICULARLY SATURDAY NIGHT THROUGH SUNDAY EVENING.  
 
A LARGE UPPER TROUGH OVER THE WESTERN CONUS WILL SHIFT EASTWARD  
ACROSS THE ROCKIES AND OVER THE PLAINS STATES BY SATURDAY NIGHT.  
AS THIS OCCURS, STRONG AND DEEP SOUTH-SOUTHWESTERLY FLOW WILL BE  
INDUCED ACROSS MUCH OF THE MISSISSIPPI RIVER VALLEY AND WESTERN  
GREAT LAKES REGION. WHILE THE PRIMARY LARGER SCALE DYNAMICS WITH  
THIS APPROACHING SYSTEM WILL REMAIN TO OUR NORTHWEST SATURDAY  
NIGHT, WE ARE STILL LIKELY TO SEE AN INCREASE IN THE COVERAGE OF  
SHOWERS AND STORMS OVER NORTHERN IL. THIS INITIAL PERIOD OF  
SHOWERS AND THUNDERSTORMS LOOKS LIKE IT MAY FOCUS OVER FAR  
NORTHERN IL SATURDAY NIGHT INTO SUNDAY MORNING, ESPECIALLY ALONG  
AND NORTH OF I-80. AN ADDITIONAL WAVE OF SHOWERS AND  
THUNDERSTORMS ARE THEN EXPECTED SUNDAY AFTERNOON INTO THE EVENING  
IN ASSOCIATION WITH THE APPROACH OF A SURFACE COLD FRONT AND THE  
MAIN UPPER TROUGH.  
 
GIVEN THE FACT THAT THE STEERING LAYER FLOW IS EXPECTED TO REMAIN  
NEARLY PARALLEL TO THE ORIENTATION OF THE FRONTAL ZONE, TRAINING  
SHOWERS AND STORMS WILL BE FAVORED OVER THE AREA AHEAD OF THE  
FRONT INTO SUNDAY EVENING. THIS IS ESPECIALLY CONCERNING GIVEN  
THE PRESENCE OF A PREFRONTAL TROPICAL AIRMASS, WHICH WILL FEATURE  
CLOSE TO 2" PRECIPITABLE WATER VALUES AND DEEP WARM CLOUD DEPTHS.  
VERY EFFICIENT WARM RAINFALL PROCESSES WILL THUS BE FAVORED WITH  
THESE SHOWERS AND STORMS, WHICH SHOULD SUPPORT RAINFALL RATES IN  
EXCESS OF 1.5" PER HOUR SATURDAY NIGHT AND SUNDAY. TOTAL RAINFALL  
AMOUNTS OF 2-3 INCHES LOOKS LIKELY ACROSS MOST OF NORTHERN IL AND  
INTO FAR NORTHWESTERN IN. HOWEVER, I WOULD NOT BE SURPRISED TO  
SEE SOME AREAS END UP WITH 4+ INCHES.  
 
WHILE CONFIDENCE IS INCREASING THE THE AREA WILL EXPERIENCE SOME  
HEAVY RAINFALL, THERE ARE STILL SOME SMALL UNCERTAINTIES ON THE  
EXACT PLACEMENT OF THE HEAVIEST RAINFALL. WITH THIS IN MIND, WE  
HAVE OPTED TO HOLD OFF ON ISSUING A FLASH FLOOD WATCH AT THIS  
TIME. HOWEVER, WE HAVE HIT THE MESSAGING A BIT HARDER, WITH A  
SITUATION REPORT ISSUED. WE WILL ALSO CONTINUE TO HIGHLIGHT THE  
POTENTIAL IN AN ESF (HYDRO OUTLOOK).  
 
SURFACE HIGH PRESSURE IS EXPECTED TO BUILD ACROSS THE AREA MONDAY  
FOLLOWING SUNDAY EVENINGS FRONTAL PASSAGE. THIS SHOULD RESULT IN A  
COUPLE OF QUIET AND PLEASANT WEATHER DAYS EARLY NEXT WEEK BEFORE  
THE POSSIBILITY FOR SHOWERS AND STORMS RETURNS TO THE AREA MID TO  
LATE NEXT WEEK.  
 
KJB  
 

 
   
AVIATION
 
 
FOR THE 06Z TAFS...  
 
AVIATION WEATHER CONCERNS ARE MINIMAL THROUGH ABOUT DAYBREAK THIS  
MORNING, BUT NUMEROUS WEATHER IMPACTS DUE TO A COMBINATION OF LOW  
CIGS AND SHOWERS/THUNDERSTORMS ARE POSSIBLE LATER THIS MORNING,  
AGAIN LATER THIS AFTERNOON, AND THEN LATE TONIGHT AND INTO SUNDAY.  
 
VFR CONDITIONS ARE EXPECTED TO PREVAIL TONIGHT AS BETTER LOW AND  
MID-LEVEL MOISTURE BEGINS TO GET PUMPED NORTHWARD AHEAD OF A  
VIGOROUS STORM SYSTEM TRANSLATING EASTWARD INTO THE CENTRAL AND  
NORTHERN GREAT PLAINS. HIGH AND THEN MID-LEVEL CLOUD COVER WILL  
THICKEN UP EARLY THIS MORNING WITH INCREASING CHANCES FOR  
PRECIPITATION IN AN INITIAL BAND OF WARM ADVECTION. FORECAST  
SOUNDINGS THROUGH LATE MORNING REVEAL VERY LITTLE IN THE WAY OF  
USABLE INSTABILITY WITH PROFILES QUICKLY TRENDING TOWARDS MOIST  
ADIABATIC. AS A RESULT, OPTED TO DROP THE THUNDER WORDING AND GO  
WITH VCSH AND TEMPO SHRA WORDING FOR THIS INITIAL WAVE OF ASCENT.  
CAN'T RULE OUT A COUPLE STRIKES, BUT THINK THE CHANCES FOR TS ARE  
JUST TOO LOW TO CARRY A MENTION IN THE TAFS AT THIS TIME.  
 
THIS INITIAL WAVE OF ACTIVITY MAY DECREASE DURING THE EARLY-  
AFTERNOON HOURS, BUT ANOTHER WAVE OF LIFT MAY ENCROACH ON THE  
REGION DURING THE MID-LATE AFTERNOON AND EARLY-EVENING HOURS.  
THERE MAY BE A SLIGHTLY HIGHER POTENTIAL FOR ELECTRIFICATION WITH  
THIS ACTIVITY, BUT STILL AM NOT CONFIDENT ENOUGH TO MOVE -TSRA OUT  
OF A PROB30 GROUP. THIS SECONDARY WAVE IS LOW CONFIDENCE OVERALL  
IN TIMING AND SPATIAL ORIENTATION/EXTENT.  
 
WINDS MAY ALSO BECOME GUSTY TODAY, FAVORING A 210-230 DIRECTION AS  
MOMENTUM TRANSFER OF 35-40 KTS OF FLOW ALOFT OCCURS. WOULDN'T BE  
SURPRISED TO SEE SOME OCCASIONAL GUSTS NORTH OF 30 KTS INTO THE  
CHICAGO-AREA TERMINALS AS A RESULT.  
 
AS THIS SECOND WAVE EXITS, WE MAY EXPERIENCE A BRIEF LULL IN  
ACTIVITY INTO THE LATE-EVENING BEFORE THE MAIN ROUND OF FORCING  
ARRIVES. HAVE INSERTED A MENTION OF PREVAILING SHOWERS AND  
VICINITY THUNDER AFTER 22/10Z TO ACCOUNT FOR THIS.  
 
CARLAW  
 

 
   
LOT WATCHES/WARNINGS/ADVISORIES
 
 
IL...NONE.  
IN...NONE.  
LM...SMALL CRAFT ADVISORY...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-  
LMZ745...10 AM SATURDAY TO 1 PM SUNDAY.  
 

 
 

 
 
VISIT US AT HTTP://WEATHER.GOV/CHICAGO (ALL LOWERCASE)  
 
FOLLOW US ON FACEBOOK...TWITTER...AND YOUTUBE AT:  
WWW.FACEBOOK.COM/NWSCHICAGO  
WWW.TWITTER.COM/NWSCHICAGO  
WWW.YOUTUBE.COM/NWSCHICAGO  
 
CLICK HERE TO GO TO PREVIOUS BULLETINS.

The Nexlab IL Page
The Nexlab IN Page Main Text Page