339  
FXUS63 KLOT 042309  
AFDLOT  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL  
609 PM CDT SAT APR 4 2020  
   
SHORT TERM
 
 
239 PM CDT  
 
THROUGH SUNDAY NIGHT...  
 
HIGH PRESSURE HAS BECOME ESTABLISHED ACROSS THE UPPER MIDWEST  
THIS AFTERNOON, WITH INCREASING SURFACE PRESSURE AND DRY ADVECTION  
SLOWLY WORKING TO ERODE STRATUS CLOUDS FROM THE NORTHWEST. AREAS  
IN THE CLEAR HAVE BEEN ABLE TO SNEAK INTO THE 50S, WHILE IT  
REMAINS A SLOW CLIMB ELSEWHERE. EXPECT MOST AREAS TO EXPERIENCE  
SOME CLEARING, THOUGH IN NORTHWEST INDIANA IT MAY BE A RACE  
BEFORE SUNDOWN.  
 
THE SURFACE HIGH WILL SHIFT OVER THE GREAT LAKES TONIGHT INTO  
SUNDAY AND RESULT IN QUIET AND SLIGHTLY WARMER CONDITIONS ACROSS  
THE AREA AS MORE SUNSHINE WILL BE THE RULE. WE WILL HAVE SOME  
MID-LEVEL CLOUDS OVERNIGHT AND INTO EARLY SUNDAY AS A WEAK  
VORTICITY MAXIMUM ALONG THE IOWA/MISSOURI BORDER WILL SLIDE  
THROUGH THE LOCAL AREA. OTHERWISE WITH MORE SUN THAN CLOUDS  
TEMPERATURES WILL REBOUND TO THE 50S AWAY FROM THE LAKESHORE IN  
THE ONSHORE FLOW REGIME.  
 
KMD  
 

 
   
LONG TERM
 
 
320 PM CDT  
 
MONDAY THROUGH SATURDAY...  
 
THE MAIN CONCERN IS THE OCCASIONAL THREAT FOR THUNDERSTORMS MONDAY  
PM THROUGH TUESDAY EVENING AND POSSIBLY AGAIN WEDNESDAY AFTERNOON  
AND EVENING. THIS INCLUDES A RISK FOR STRONG TO SEVERE  
THUNDERSTORMS MID TO LATE TUESDAY AFTERNOON INTO TUESDAY EVENING.  
 
A WARM FRONT WILL BE LOCATED WELL SOUTH OF THE CWA ON MONDAY  
MORNING, ACROSS DOWNSTATE ILLINOIS. TEMPERATURES LOOK TO REACH THE  
60S FOR MANY AREAS, EXCEPT THE ILLINOIS SHORE, WHICH WILL HOLD  
ONTO ONSHORE FLOW UNTIL LATE AFTERNOON. ON MONDAY AFTERNOON AND  
EVENING, LOW AMPLITUDE MID AND UPPER RIDGING OVER THE MIDWEST MAY  
BE TOPPED BY A LEAD SHORT- WAVE IN WEST- NORTHWEST FLOW ALOFT. THE  
GUIDANCE CONTINUES TO INDICATE THAT A PLUME OF VERY STEEP MID-  
LEVEL LAPSE RATES (8+ C/KM IN 700-500 MB LAYER) WILL BE ADVECTED  
EASTWARD, WHICH SHOULD YIELD SUFFICIENT MUCAPE FOR A THREAT FOR  
SCATTERED EMBEDDED THUNDERSTORMS OVER THE REGION, PERHAPS IN A  
LOOSELY ORGANIZED MCS.  
 
ON THAT NOTE, THERE IS VARIANCE IN THE MAGNITUDE OF MUCAPE  
REALIZED OVER THE CWA. WITH THE EXTREMELY STEEP MID-LEVEL LAPSE  
RATES BUT POSSIBLY MORE LIMITED MOISTURE ALOFT WITH  
EAST/NORTHEASTWARD EXTENT, THERE MAY BE A SHARP MUCAPE GRADIENT  
ACROSS THE CWA. WILL NEED TO WATCH FOR POTENTIAL THAT THE  
CONVECTION RIDES THIS GRADIENT, WITH BEST CHANCE FOR STORMS IN  
WEST/SOUTHWEST CWA AND POINTS SOUTHWEST, ROUGHLY AS DEPICTED ON  
12Z ECMWF. GIVEN THE UNCERTAINTY, DID CARRY CHANCE THUNDER MENTION  
ACROSS THE CWA, AND LIKELY POPS FROM THE NBM. IF HIGHER MUCAPE  
WITH LITTLE/NO MUCIN ARE REALIZED OVER PORTIONS OF THE CWA,  
EFFECTIVE DEEP LAYER SHEAR COULD TICK UP TO 30-40 KT, WHICH MAY  
SUPPORT AN ISOLATED SEVERE HAIL RISK IN STRONGEST STORMS.  
 
LOOKING AHEAD TO TUESDAY, SURFACE WARM FRONT WILL BE LIFTING  
NORTH, CARRYING DEW POINTS INTO THE LOWER 60S FOR MANY AREAS BY  
MID DAY. ANY EARLY MORNING ELEVATED CONVECTION SHOULD HAVE EXITED  
OFF TO THE EAST AND SOUTHEAST. A VERY STRONG EML WITH MID-LEVEL  
LAPSE RATES AT OR ABOVE 8C/KM WILL STILL BE IN PLACE OVER THE  
AREA, WHICH WOULD LIKELY PRESENT A CAP TO THUNDERSTORM DEVELOPMENT  
FOR PART OF THE DAYTIME AS THINGS STAND NOW. A SURFACE LOW  
PRESSURE SYSTEM WILL TRACK FROM THE NORTHERN PLAINS TO THE  
CENTRAL LAKES, TIED TO A MID-LEVEL SHORT-WAVE AND SPEED MAX. THIS  
WAVE WOULD THEN POTENTIALLY APPROACH TOWARD EARLY EVENING,  
PROVIDING HEIGHT FALLS AND COOLING ALOFT TO ERODE THE CAP, WITH  
LOW LEVEL CONVERGENCE FROM SURFACE COLD FRONT. ASSUMING WE ARE  
CAPPED AND DRY OUT FOR A PORTION OF THE DAY WITH SOME SUN, TEMPS  
WILL LIKELY WARM WELL INTO THE 70S PER 850 MB TEMPS IN THE MID  
TEENS CELSIUS.  
 
AFOREMENTIONED SET-UP WITH TEMPS IN 70S AND AROUND 60/LOWER 60S  
DEW POINTS COULD YIELD MODERATE INSTABILITY OF 1000-2000 J/KG  
MLCAPE. THIS IS CONCERNING AS STRENGTHENING NORTHWEST FLOW ALOFT  
WITH APPROACHING WAVE LATER IN THE DAY/EARLY EVENING MAY SUPPORT  
40-50+ KT OF DEEP LAYER BULK SHEAR AND A STRONGLY VEERING PROFILE.  
THIS PERIOD WILL BE ONE TO WATCH FOR NORTHWEST FLOW STRONG TO  
SEVERE THUNDERSTORM RISK TUESDAY PM. THE ABOVE DESCRIBED WIND  
PROFILES WOULD POTENTIALLY BE SUPPORTIVE OF SUPERCELLS WITH A  
LARGE TO VERY LARGE HAIL RISK GIVEN STEEP LAPSE RATES AND LARGE  
CAPE IN HAIL GROWTH ZONE. STEEP LOW LEVEL LAPSE RATES MAY ALSO  
DEVELOP, POSING A THREAT FOR DAMAGING DOWNBURST WINDS. LOOMING  
UNCERTAINTY WITH THIS THREAT PERTAINS TO THE POTENTIAL FOR EML  
CAPPING TO HOLD IF THERE IS LESS INSOLATION AND/OR FORCING AND  
FRONTAL CONVERGENCE IS NOT ENOUGH TO ERODE THE MLCINH. ECMWF HAS  
BEEN MOST CONSISTENT IN FAVORING CONVECTIVE INITIATION, WITH GFS  
MORE CAPPED, THOUGH ECMWF DOES HAVE SOLID ENSEMBLE SUPPORT  
INCLUDING FROM GEFS. CAPPED POPS IN MID TO HIGH CHANCE RANGE DUE  
TO UNCERTAINTY, AND DID ADD SOME 3-HOURLY TEMPORAL DETAIL TO POPS  
TUESDAY AFTERNOON.  
 
INITIAL COLD FRONT WILL KNOCK TEMPS DOWN ONLY SLIGHTLY FOR  
WEDNESDAY INTO THE 60S TO AROUND 70. A SECONDARY COLD FRONT  
WEDNESDAY EVENING/NIGHT WILL BRING A HEALTHIER SURGE OF COOLER  
AIR. THE MODELS AND ENSEMBLES HAVE TRENDED TOWARD A STRONGER  
SIGNAL FOR RAIN SHOWERS WITH THIS SECONDARY FRONT, SO HAVE  
INCREASED POPS INTO CHANCE RANGE. CAN'T RULE OUT ISOLATED  
THUNDERSTORMS WITH THIS ACTIVITY, BUT HAVE REFRAINED FROM MENTION  
IN THE GRIDS. THURSDAY WILL HAVE BREEZY-WEST NORTHWEST WINDS AND  
TEMPS ONLY IN IN THE UPPER 40S TO AROUND 50, FOLLOWED BY YET  
ANOTHER REINFORCING SHOT OF COOL AIR FOR FRIDAY, WHICH MAY STAY IN  
THE 40S CWA WIDE ALONG WITH CONTINUED BREEZY NORTHWEST WINDS.  
AFTER A LIKELY UNSEASONABLY CHILLY FRIDAY NIGHT, TEMPERATURES ON  
SATURDAY WILL MODERATE BACK TO THE 50S.  
 
CASTRO  
 

 
   
AVIATION
 
 
FOR THE 00Z TAFS...  
 
MVFR CIGS CONTINUE TO PUSH SOUTH OF THE REGION, BUT ANTICIPATE A  
QUICKLY-ADVANCING MID-LEVEL (VFR) DECK WILL OVERSPREAD THE  
TERMINALS THROUGH THE MID TO LATE EVENING HOURS. VERY DRY AIR IN  
THE LOWEST 10 KFT SHOULD PRECLUDE ANY PRECIPITATION WITH THIS  
INCOMING DISTURBANCE. THERE ARE SOME SIGNS OF SOME PATCHY BR  
DEVELOPMENT LATE THIS EVENING AND OVERNIGHT, BUT THE CURRENT  
THINKING IS THAT THE EXPANSIVE DECK OF MID-LEVEL CLOUD COVER WILL  
CURTAIL THIS POTENTIAL WITH NO FORMAL MENTION IN THE TAFS AT THIS  
TIME. RFD MAY CLEAR OUT A BIT QUICKER LATE TONIGHT, SO WE MAY NEED  
KEEP AN EYE ON VISIBILITY TRENDS THERE.  
 
OTHERWISE, NORTHEAST WINDS SHOULD BECOME A BIT MORE NORTHERLY OR  
EVEN NORTH-NORTHWESTERLY TONIGHT AT ORD/MDW, BUT SPEEDS ARE  
EXPECTED TO REMAIN UNDER 5 KTS. SYNOPTIC HIGH PRESSURE BUILDING  
DOWN THE LAKE ON SUNDAY WILL ENHANCE A LAKE BREEZE PUSH THROUGH  
THE MID-LATE MORNING HOURS WITH A REINFORCEMENT OF NORTHEASTERLY  
WINDS.  
 
CARLAW  
 

 
   
LOT WATCHES/WARNINGS/ADVISORIES
 
 
IL...NONE.  
IN...NONE.  
LM...NONE.  

 
 

 
 
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