Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 021743
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1243 PM CDT WED SEP 2 2015

.UPDATE...
1128 AM CDT

SEVERAL HIGH-RESOLUTION CONVECTIVE ALLOWING MODELS STILL
ATTEMPTING TO BREAK OUT A FEW SHOWERS AND THUNDERSTORMS IN OUR
WESTERN CWA IN THE AFTERNOON TIME FRAME WITH WEAK VORT MAX ON THE
SOUTHERN END OF BETTER HEIGHT FALLS OUR NORTH. THIS VORT WILL
SLIDE THROUGH DURING OUR AFTERNOON PERIOD WITH PEAK HEATING IN
PLACE...BUT NO SURFACE FORCING TO SPEAK OF. HAVE HELD A DRY FCST
FOR TODAY WITH SUBPAR LAPSE RATES AND DRY AMDAR SOUNDINGS THIS
MORNING...BUT WILL NEED TO WATCH GIVEN THE UNSTABLE/MOSTLY
UNCAPPED ENVIRONMENT.

KMD

&&

.SHORT TERM...
328 AM CDT

THROUGH THURSDAY...

PRIMARY CONCERN IN THE NEAR TERM IS CONVECTIVE POTENTIAL TONIGHT.
ANOTHER VERY WARM TO ALMOST HOT DAY EXPECTED TODAY WITH HIGHS ONCE
AGAIN MAKING A RUN AT 90 DEGREES AND PROBABLY ECLIPSING 90 IN MANY
AREAS. SOME DEBRIS CLOUDINESS FROM IOWA CONVECTION COULD FILTER
SUN A BIT IN OUR WESTERN CWA AND PERHAPS KEEP TEMPS A DEGREE OR
TWO COOLER...BUT WITH CONVECTION LIKELY TO WEAKEN NOT ANTICIPATING
A BIG IMPACT.

WELL DEFINED SHORTWAVE ON WATER VAPOR IMAGERY OVER MINNESOTA AND
WESTERN IOWA WILL CONTINUE TO SLOWLY PROGRESS EASTWARD THROUGH
TONIGHT. BRUNT OF THE CONVECTION IS EXPECTED TO REMAIN TO OUR
NORTH OVER WISCONSIN TONIGHT...BUT GLANCING BLOW FROM SOUTHERN
EXTENT OF THE SHORTWAVE SHOULD RESULT IN SOME WEAK HEIGHT FALLS
AND PERHAPS ENOUGH FORCING TO SUPPORT SOME CONVECTION IN OUR CWA.
VARIOUS CONVECTIVE ALLOWING ENSEMBLES ARE NOT OVERLY AGGRESSIVE
WITH CONVECTIVE CHANCES TONIGHT...SO LARGELY MAINTAINED PREVIOUS
POPS WITH JUST SMALL CHANCES OVER MAINLY NORTHERN CWA CLOSER TO
THE TRACK OF THE STRONGEST PORTION OF THE VORT.

A PORTION OF THIS VORT COULD BEGIN TO TURN SOUTHEAST AS IT REACHES
THE EASTERN EXTENT OF THE UPPER RIDGE...LINGERING IN THE AREA
THURSDAY. COMBINATION OF LOWER HEIGHTS AND POTENTIAL FOR LOW LEVEL
CONVERGENCE ALONG OUTFLOW AND/OR LAKE BREEZE SUGGESTS IT WOULD BE
PRUDENT TO ADD SOME LOW CHANCE POPS FOR THURSDAY AS WELL. ASSUMING
CONVECTION AND/OR CONVECTIVE DEBRIS AREN`T TERRIBLY PROMINENT THEN
THURSDAY WILL BE ANOTHER DAY WITH TEMPS MAKING A RUN AT 90
DEGREES. MUCH OF AVAILABLE GUIDANCE SUGGESTS A LAKE BREEZE COULD
FORM IN THE AFTERNOON...WHICH IF IT MATERIALIZES WOULD CAUSE LAKE
ADJACENT AREAS TO SEE TEMPS DROP OFF A BIT IN THE AFTERNOON.

IZZI

&&

.LONG TERM...
338 AM CDT

THURSDAY NIGHT THROUGH TUESDAY...

CONTINUED UNSEASONABLY WARM AND HUMID CONDITIONS FRIDAY ALONG WITH
SLIGHTLY LOWER HEIGHTS WITHIN A BREAK IN THE UPPER RIDGE OVER THE
REGION ARGUES FOR SOME MORE SLIGHT CHANCE POPS FOR FRIDAY AS WELL
WITH LAKE BREEZE ONCE AGAIN A CANDIDATE TO FOCUS DEVELOPMENT.

HEADING INTO THE WEEKEND GFS AND ECMWF VARY ON LOW LEVEL WIND FLOW
AND POTENTIAL FOR LAKE BREEZES. ECMWF WOULD SUGGEST LITTLE OR NO
LAKE BREEZE WHILE GFS MORE PROMINENTLY DEVELOPS HEALTHY LAKE
BREEZES BOTH DAYS. AWAY FROM THE LAKE THE FORECAST IS MORE
STRAIGHT FORWARD...JUST CONTINUED UNSEASONABLY WARM AND RATHER
HUMID. IF GFS ADVERTISED LAKE BREEZE MATERIALIZES THEN TEMPS WOULD
LIKELY HOLD IN THE 70S ALONG THE SHORE OVER THE WEEKEND...WHILE
ECMWF WOULD SUGGEST THE UPPER 80S/NEAR 90 DEGREE HEAT WOULD MAKE
IT RIGHT UP TO THE SHORELINE.

MONDAY COULD BE ONE OF THE HOTTEST DAYS OF THE NEXT SEVEN
DEPENDING ON DEBRIS CLOUDS AND TIMING OF THE FRONT. FOR NOW KEPT
HIGHS IN THE UPPER 80S/NEAR 90...BUT WITH SUNSHINE AND CURRENT
TIMING OF FRONT PANNING OUT HIGHS COULD REACH LOW/MID 90S. COLD
FRONT LOOKS TO APPROACH REGION LATE MONDAY AND COULD SLOW OR STALL
OUT NEARBY MONDAY NIGHT INTO TUESDAY BRINGING A CONTINUED THREAT
OF SHOWERS AND THUNDERSTORMS.

IZZI

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 18Z...

* CHANCE FOR TS AFTER 09Z TONIGHT.

* WINDS TURNING SOUTH OR SOUTH-SOUTHEAST AFTER 20Z THURSDAY.

MM

//DISCUSSION...UPDATED 18Z...

HIGH PRESSURE RETREATS TO THE EAST WHILE LARGE LOW PRESSURE AREA
ADVANCES TOWARDS THE GREAT LAKES. SOUTHWEST WINDS CONTINUE TO PUMP
IN WARM AND MOIST AIR...WHICH KEEPS UNSTABLE CONDITIONS IN PLACE.
THE LACK OF A FORCING MECHANISM HAS LEFT THE TAF SITES DRY THUS
FAR. THE NEXT CHANCE FOR FORCING APPEARS TONIGHT AS AN UPPER
LEVEL DISTURBANCE FROM LAKE SUPERIOR TO NORTHERN MISSOURI MOVES
TOWARDS NORTHEASTERN ILLINOIS. MODEL GUIDANCE HAS A BROAD RANGE OF
TS CHANCES FOR THE AREA...BUT TIMING THE UPPER LEVEL SUPPORT LENDS
TO THE 09-13Z TIMEFRAME PRESENTED IN TAFS.

LATE THURSDAY A WEAK BOUNDARY MAY PRESENT A WIND SHIFT TO THE
SOUTH-SOUTHEAST. AGAIN...MODELS ARE NOT CONSISTENT IN THIS
FEATURE...SO WIND DIRECTION MAY VARY BY 90 DEGREES. SPEEDS SHOULD
REMAIN WELL UNDER 10KT OUTSIDE OF BRIEF TS OCCURRENCE THOUGH.

MM

//ORD AND MDW CONFIDENCE...UPDATED 18Z...

* MEDIUM LOW FOR TS OCCURRING OR IMPACTING ORD/MDW.

* MEDIUM FOR WIND DIRECTION THURSDAY AFTERNOON.

* HIGH ALL OTHER ELEMENTS.

MM

//OUTLOOK FOR ORD/MDW FOR 00Z FRIDAY-12Z WEDNESDAY...UPDATED 12Z...

THURSDAY NIGHT...SLIGHT CHANCE OF TSRA.

FRIDAY...SLIGHT CHANCE OF TSRA.

SATURDAY...SLIGHT CHANCE OF TSRA.

SUNDAY...VFR.

MONDAY...CHANCE OF TSRA.

TUESDAY...CHANCE OF TSRA.

RODRIGUEZ

&&

.MARINE...
429 AM CDT

LARGE HIGH PRESSURE WILL CONTINUE TO DOMINATE THE EASTERN THIRD OF
THE COUNTRY THIS WEEK WITH ANY ADDITIONAL LOW PRESSURE SYSTEMS
STAYING NORTHWEST OF THE REGION. WHAT THIS WILL MEAN FOR THE LAKE
IS GENERALLY LIGHTER WINDS THIS WEEK...WITH DIRECTIONS LIKELY
VARYING FROM AN EASTERLY OR SOUTHERLY DIRECTION. FOG LIKELY OVER
THE LAKE AGAIN THIS MORNING...BUT NOT AS DENSE AS THE LAST COUPLE
OF DAYS. HOWEVER...PATCHY DENSE FOG WILL STILL BE POSSIBLE IN
SOME LOCATIONS TODAY.

RODRIGUEZ

&&

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

$$

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