142  
FXUS63 KLOT 050930  
AFDLOT  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL  
330 AM CST FRI MAR 5 2021  
   
SHORT TERM
 
 
205 AM CST  
 
THROUGH SUNDAY...  
 
METEOROLOGICAL SPRING CONTINUES ITS QUIET START THROUGH THE  
WEEKEND. LAKE BREEZE POTENTIAL AND HOW MUCH OF AN AREA NEAR THE  
LAKE THIS WIND SHIFT KEEPS COOLER IS REALLY THE ONLY ITEM OF  
NOTE. TEMPERATURES WILL BE A TAD ABOVE NORMAL AND START MAKING  
LARGER STRIDES ABOVE EARLY MARCH CLIMATOLOGY ON SUNDAY.  
 
HIGH PRESSURE OF 1030 MB IS CENTERED OVER WISCONSIN AND SPRAWLED  
INTO NORTHWEST AND NORTH CENTRAL ILLINOIS THIS MORNING. MAINTAINED  
TO THE IMMEDIATE EAST OF THIS FEATURE IS BOUNDARY LAYER FLOW FROM  
THE NORTHWEST TODAY. FORECAST PROFILES FROM HIGH-RESOLUTION  
MODELS DO VARY ON THE MIXED BOUNDARY LAYER DEPTH THIS AFTERNOON,  
WITH THE NAM MORE PRONOUNCED AND KEEPING A LAKE BREEZE OUT OF  
NORTHEAST ILLINOIS WHILE THE HRRR IS MUCH LESS SO AND ALLOWS  
INTRUSION TO A LITTLE WEST OF I-294. LAST WARM SEASON THE HRRR  
WAS AT TIMES TOO AGGRESSIVE ON LAKE BREEZES AND EVEN ITS PROFILES  
INDICATE ~10 KT OF FLOW TODAY, WHICH USUALLY RETARDS A LAKE  
BREEZE TO NEAR THE SHORE. SO HAVE KEPT THE TURN OF THE WINDS FOR  
CHICAGO TO LATE AFTERNOON WITH HIGHS MID 40S, WHILE NORTHWEST  
INDIANA WILL HAVE MORE OF AN ONSHORE COMPONENT THROUGH THE DAY  
WITH HIGHS UPPER 30S TO POSSIBLY LOWER 40S. SOME UPPER 40S ARE  
LIKELY IN THE SOUTHERN CWA. PATCHY SHALLOW FOG POTENTIAL MAY BE  
PRESENT AGAIN TONIGHT OVER NORTH CENTRAL ILLINOIS, BUT WITH  
LITTLE OF IT MATERIALIZING THIS MORNING, DON'T SEE TONIGHT BEING  
ANY MORE OF A THREAT.  
 
THROUGH SATURDAY THERE WILL BE LITTLE MOVEMENT IN THE LONG WAVE  
PATTERN THAT IS PRIMARILY HELD UP BY A STOUT SUB-500 DAM LOW AT  
500 MB OVER EASTERN QUEBEC. NORTHWEST FLOW THAT HAS LITTLE ACTING  
ON IT TO BE AGEOSTROPHIC WILL CONTINUE OVER OUR FORECAST AREA AND  
AND SURFACE HIGH PRESSURE UNDERNEATH. WITH A SLIGHT EASTWARD  
INCHING OF THE HIGH ON SATURDAY, THIS WILL ALLOW AN ONSHORE TURN  
OF THE WINDS NEAR THE LAKE WITH HIGHER CONFIDENCE THAN TODAY, AND  
KEEP LAKESIDE AREAS IN THE 30S.  
 
SUNDAY WILL START TO SEE MOVEMENT IN THE MID/UPPER LEVEL PATTERN,  
WITH HEIGHTS BUILDING AND LOWER TROPOSPHERIC WARM ADVECTION. THE  
925 MB TEMPERATURES ARE FORECAST TO WARM FROM -1C TO 3C IN THE  
MORNING TO 5C TO 9C BY LATE DAY. THE SURFACE WIND DIRECTION  
REMAINS MORE SOUTH-SOUTHEAST TO DUE SOUTH, WHICH TYPICALLY DOES  
NOT MEAN TEMPERATURES RACING AWAY THIS TIME OF YEAR (LIKE THEY  
MAY THE FOLLOWING DAYS), BUT AT LEAST JUMPING INTO THE 50S FOR  
MOST LOCATIONS. THE EXCEPTION WILL BE NEAR THE LAKE IN ILLINOIS  
WHERE THE DIRECTION SHOULD PROVIDE SOME DEGREE OF COOLING.  
 
MTF  
 

 
   
LONG TERM
 
 
330 AM CST  
 
SUNDAY NIGHT THROUGH THURSDAY...  
 
THE NEW WORK WEEK WILL BE A CONTINUATION OF THE DRY AND  
INCREASINGLY MILD CONDITIONS, WITH HIGHS IN THE 60S A GOOD BET ON  
MONDAY AND TUESDAY. OUR LONG QUIET STRETCH THEN APPEARS LIKELY TO  
COME TO AN END IN THE WEDNESDAY-THURSDAY TIMEFRAME, WHEN AT LEAST  
ISOLATED THUNDERSTORMS APPEAR POSSIBLE.  
 
THE SYNOPTIC PATTERN IS A GOOD ONE ON MONDAY AND TUESDAY FOR  
UNSEASONABLE WARMTH, WITH STRONG POSITIVE MID-LEVEL HEIGHT  
ANOMALIES AND PERSISTENT WARM ADVECTION. AT THE SURFACE, ELONGATED  
TROUGHING OVER THE HIGH PLAINS AND EXPANSIVE HIGH PRESSURE OVER  
THE EASTERN CONUS WILL KEEP SOUTHWEST TO SOUTHERLY WINDS ELEVATED.  
THE POSITION OF THE SURFACE HIGH CENTER OVER THE SOUTHEAST AND  
RIDGING BACK ACROSS THE GULF STATES WILL BLOCK OFF MOISTURE  
TRAJECTORIES. CLIMO OF PROGGED 925 MB TEMPS OF +8 TO +10C OR A BIT  
HIGHER AT PEAK HEATING ON MONDAY-TUESDAY SUPPORTS HIGHS IN THE  
LOWER TO MID 60S. MONDAY WILL FEATURE BREEZY SOUTHWESTERLY WINDS,  
BECOMING CLOSE TO DUE SOUTH-SOUTHWESTERLY AND INCREASING ON  
TUESDAY AS THE PRESSURE GRADIENT TIGHTENS. IF THERE'S LESS CLOUDS,  
TUESDAY COULD BE A CANDIDATE FOR DEEPER MIXING, MEANING TEMPS  
OVERPERFORM A BIT AND DEW POINTS POSSIBLY UNDERPERFORM. WITH WIND  
GUSTS UP TO OR OVER 30 MPH, EXTENDED DRY PATTERN OVER AREAS THAT  
LOST SNOW COVER EARLIEST COULD YIELD AN ELEVATED FIRE WEATHER  
RISK. STILL A FEW DAYS TO MONITOR TRENDS FOR THIS PERIOD.  
 
THE GUSTY SOUTHERLY WINDS WILL INCREASE THROUGH TUESDAY NIGHT,  
LIKELY KEEPING TEMPS IN THE LOWER 50S, WELL ABOVE NORMAL HIGHS FOR  
THE DATE. BOTH THE GFS AND ECMWF DEPICT A PLUME OF VERY STEEP  
MID-LEVEL LAPSE RATES BEING ADVECTED EASTWARD ACROSS OUR AREA.  
HAVE SOME LOW POPS INDICATED OVERNIGHT, WHICH WOULD BE ELEVATED  
WARM ADVECTION SHOWERS. WITH SUCH STEEP LAPSE RATES, IF PARCELS  
CAN OVERCOME THE VERY WARM EML BASE AT 700 MB, WOULD SPELL A NON-  
ZERO CHANCE FOR ISOLATED THUNDERSTORMS. WE LOOK TO FINALLY TAP  
INTO BETTER BOUNDARY LAYER MOISTURE ON WEDNESDAY, WITH DEW POINTS  
GETTING TO 50F OR HIGHER DRAWN NORTHWARD BY SOUTH-SOUTHWEST WINDS  
GUSTING UP TO 30-40 MPH. DESPITE MOSTLY CLOUDY SKIES, VERY MILD  
START AND STRONG WARM ADVECTION WILL SUPPORT HIGHS AGAIN IN THE  
LOW-MID 60S. WHILE WE LOOK TO BE IN THE WARM SECTOR OF TROUGHING  
OFF TO THE WEST AND NORTHWEST, SOME UPPER JET SUPPORT COULD HELP  
SCATTERED SHOWERS BREAK OUT DURING THE DAY.  
 
UNCERTAINTY INCREASES IN HOW THINGS WILL EVOLVE WEDNESDAY EVENING-  
THURSDAY, BUT IN A BROAD SENSE WE SHOULD SEE SURFACE COLD FRONT  
MOVE INTO THE AREA AND THEN POSSIBLY STALL OUT DURING THURSDAY AS  
WAVES OF ENERGY INTERACT WITH THE FRONT. PWATS OF 150-250% OF  
NORMAL COULD SET THE STAGE FOR HEAVY RAINFALL, ESPECIALLY  
ASSOCIATED WITH ANY THUNDERSTORMS, WHICH APPEAR QUITE POSSIBLE.  
WITH THIS PERIOD 6+ DAYS OUT AND PLENTY OF TIME FOR CHANGES,  
INTRODUCED A SLIGHT CHANCE FOR THUNDERSTORMS WEDNESDAY NIGHT-  
THURSDAY (I-80 AND SOUTH FOR NOW ON THURSDAY CLOSER TO BOUNDARY).  
TEMPERATURES WILL COOL DOWN TO THE NORTH OF THE BOUNDARY ON  
THURSDAY, IN PARTICULAR NEAR THE LAKE IF WINDS FLIP TO OUT OF THE  
NORTH AND NORTHEAST. THE ACTIVE PERIOD SHOULD CONTINUE THROUGH  
THURSDAY NIGHT AND EARLY FRIDAY, FOLLOWED BY MUCH COOLER  
CONDITIONS NEXT WEEKEND.  
 
CASTRO  
 

 
   
AVIATION
 
 
FOR THE 06Z TAFS...  
 
AVIATION FORECAST CONCERNS:  
 
* SOME POTENTIAL FOR A LAKE BREEZE TO SHIFT WINDS NORTHEAST AT  
KORD AND KMDW LATE FRIDAY AFTERNOON.  
 
SURFACE HIGH PRESSURE RIDGE CONTINUES TO DRIFT SLOWLY SOUTH ACROSS  
WISCONSIN LATE THIS EVENING. ALONG THE SOUTHERN PERIPHERY OF THE  
HIGH, NORTHEAST WINDS CONTINUE TO DIMINISH ACROSS THE TERMINALS.  
THIS TREND WILL CONTINUE OVERNIGHT, WITH WINDS EXPECTED TO SHIFT  
LIGHT NORTH-NORTHWEST DURING THE PRE-DAWN HOURS AS THE RIDGE  
SETTLES INTO NORTHWEST ILLINOIS.  
 
SURFACE HIGH PRESSURE WILL REMAIN JUST WEST OF THE TERMINALS  
FRIDAY, RESULTING IN RELATIVELY LIGHT NORTHWEST WINDS NEAR 10 KTS  
PERSISTING DURING THE DAY. SOME HIGH-RES MODEL GUIDANCE CONTINUES  
TO INDICATE THAT A LAKE BREEZE WILL DEVELOP AND BRING WINDS  
NORTHEAST AT KGYY BY MIDDAY/AFTERNOON, THOUGH MODELS CONTINUE TO  
SUGGEST THAT THE BOUNDARY MAY NOT MOVE THROUGH KMDW/KORD. THIS  
APPEARS TO BE DUE TO 10-15 KT SUSTAINED NNW WINDS AT/ABOVE THE TOP  
OF THE MIXED LAYER DURING THE AFTERNOON, WHICH COULD BE JUST  
ENOUGH TO LIMIT THE INLAND PUSH OF THE LAKE BREEZE FROM THE IL  
SHORE. HAVE RELATIVELY LOW CONFIDENCE IN THIS AT THIS TIME, AS ANY  
WEAKER FLOW ALOFT WILL LIKELY ALLOW THE BOUNDARY TO MOVE INTO THE  
KMDW/KORD TERMINALS AFTER 22-23Z, BUT WITH MOST GUIDANCE KEEPING  
IT EAST OF THESE AIRFIELDS WILL LEAVE IT OUT OF THEIR TAFS FOR NOW.  
 
PROVIDED THE LAKE BREEZE DOES NOT MOVE THROUGH KMDW/KORD LATE  
FRIDAY, LIGHT NORTHWEST WINDS SHOULD CONTINUE FRIDAY NIGHT, WITH  
FLOW EVENTUALLY BECOMING MORE NORTHERLY TOWARD SATURDAY MORNING.  
 
FORECAST SOUNDINGS HINT AT SOME SHALLOW FOG POTENTIAL ACROSS  
PARTS OF NORTH CENTRAL IL LATE TONIGHT INTO EARLY FRIDAY MORNING.  
NORTHEAST SURFACE WINDS HAVE BROUGHT DRIER AIR INTO KRFD OVER THE  
LAST SEVERAL HOURS, WITH 10 DEGREE SURFACE DEW POINT DEPRESSIONS  
NOT INDICATIVE OF FOG DEVELOPING ANY TIME SOON. BETTER CHANCES  
APPEAR TO BE FARTHER SOUTH/SOUTHWEST OF KRFD. OTHERWISE, VFR  
CONDITIONS ARE EXPECTED TO PREVAIL ACROSS THE AREA THROUGH THE  
PERIOD.  
 
RATZER  
 

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

 
 

 
 
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