Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 010135
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
835 PM CDT MON AUG 31 2015

.UPDATE...
835 PM CDT

MADE A FEW TWEAKS TO GOING FORECAST TONIGHT AND TOMORROW.
EXPANSIVE MID AND UPPER RIDGING OVER THE EASTERN 2/3 OF THE CONUS
CAN CLEARLY BE SEEN ON EVENING W/V IMAGERY. THE SWIRL OVER CENTRAL
INDIANA IS AN UPPER LOW/VORTICITY CENTER TRAPPED IN THE WEAK FLOW
UNDER THE RIDGE. AT THE SURFACE...EXPANSIVE HIGH PRESSURE RIDGING
COVERS THE EASTERN HALF OF THE CONUS. LIGHT TO CALM WINDS AND
MOIST BOUNDARY LAYER WITH DIMINISHING T/TD SPREADS AS TEMPS SLOWLY
COOL COULD AGAIN SET THE STAGE FOR SOME PATCHY FOG. A FEW
LOCATIONS HAVE DROPPED TO 5 MILE VISIBILITY IN LIGHT FOG/HAZE. MOS
AND HI-RES GUIDANCE MUCH LESS BULLISH THAN PAST FEW NIGHTS WITH
FOG...SO CONFIDENCE IS LOWER IN THERE BEING ANY DENSE FOG BUT
CANNOT RULE IT OUT TOWARD DAYBREAK.

HAVE REMOVED SLIGHT POPS FROM FAR SOUTHEAST CWA/EAST CENTRAL IL
AND BENTON COUNTY INDIANA PRIOR TO DAYBREAK. RAP IS BULLISH IS
GENERATING CONVECTION FROM MEANDERING UPPER LOW...BUT SUSPECT 700
MB CAPPING WILL HOLD. SEVERAL MODELS INDICATE ISOLATED TO
SCATTERED CONVECTION ON WESTERN PERIPHERY OF UPPER LOW ON TUESDAY
WHERE THERE WILL BE LESS CAPPING. HAVE KEPT POPS IN 20 PERCENT
RANGE...BUT INDICATED ISOLATED TSRA WORDING EAST OF A FAIRBURY TO
MI CITY LINE DURING THE AFTERNOON. OTHER FOCUS WAS ON TEMPS AND
POTENTIAL FOR LAKE COOLING ALONG IL SHORE. ITS UNCERTAIN WHETHER
WILDFIRE SMOKE THAT RESULTED IN ANOTHER SPECTACULAR SUNSET WILL
KEEP TEMPS DOWN SOME TOMORROW...BUT CONSIDERING MANY LOCATIONS
STILL HIT UPPER 80S TODAY...THINKING THAT 24-26 CELSIUS 925 MB
TEMPS SHOULD YIELD UPPER 80S TO LOW 90S INLAND. SIGNIFICANT
THERMAL GRADIENT BETWEEN LAND AND NEARSHORE AND WEAK ENOUGH FLOW
ALOFT THROUGH EARLY-MID AFTERNOON SHOULD ENABLE SOUTH-SOUTHWEST
WINDS TO TURN SOUTHEAST ALONG IL SHORE/CHICAGO AND NORTH...AND
LIMIT HIGHS ALONG IMMEDIATE SHORE TO LOWER-MID 80S. THIS SCENARIO
WAS DEPICTED NICELY BY 18Z RUN OF OUR LOCAL 8KM WRF-ARW.

RC

&&

.SHORT TERM...
232 PM CDT

TONIGHT AND TUESDAY...PERSISTENT SURFACE RIDGING ACROSS THE
EASTERN CONUS GENERALLY IS MATCHED BY THE UPPER LEVEL PATTERN
EXCEPT FOR A WEAK CUTOFF SHORTWAVE CIRCULATION ALONG A TROUGH
RUNNING FROM THE OHIO VALLEY INTO THE WESTERN GULF. THIS UPPER
CIRCULATION IS QUITE EVIDENT IN THE WATER VAPOR SATELLITE CHANNEL
BUT IN TERMS OF CONVECTION IT HAS NOT BEEN VERY ACTIVE SO FAR
TODAY ASIDE FROM A BRIEF THUNDERSTORM CLUSTER ACROSS CENTRAL IL.
YESTERDAY SAW MORE WIDESPREAD ACTIVITY STRETCHING FROM OHIO INTO
CENTRAL TEXAS.

MODELS HAVE BEEN DRIFTING THIS TROUGH SLOWLY WESTWARD WHICH MATCHES
THE SATELLITE TRENDS AND LIGHTNING ACTIVITY.  EARLY TOMORROW THIS
TROUGH LIKELY REACHES ITS WESTERNMOST POSITION BEFORE BEING PICKED
UP BY THE NORTHERN STREAM FLOW AND SLOWLY CARRIED OFF TO THE EAST.
THE QUESTION IS WHAT EFFECT IT WILL HAVE ON TSRA CHANCES LOCALLY
THROUGH THE NEXT 24 HOURS.  ACTIVITY TODAY SEEMS TO BE MOST FAVORED
NEAR THE CENTER OF THE TROUGH ALONG THE LOW LEVEL INSTABILITY AXIS
AND WHERE UPPER LEVEL SUBSIDENCE AND THE RELATED CAPPING ARE
WEAKEST. FORECAST SOUNDINGS AND MIDLEVEL TEMPERATURES SUGGEST THE
CUTOFF LINE FOR THE BEST TSRA CHANCES TOMORROW MAY LIE NEAR OR OVER
THE LOCAL FORECAST AREA...WITH OUR SOUTHEASTERN COUNTIES ALONG THE
IL/IN LINE BEING MOST FAVORED.  SOUNDINGS TOWARD THE CHICAGO METRO
AREA AND FARTHER NORTHWEST TOWARD RFD MAINTAIN SLIGHTLY STRONGER
SUBSIDENCE AND CAPPING ALOFT.  FOR NOW WILL CARRY A SLIGHT CHANCE OF
TSRA FOR THE SOUTHEAST PORTION OF THE FORECAST AREA...MAINLY FOR THE
PEAK HEATING PERIOD TOMORROW AFTERNOON.

WITH ALL THIS RIDGING AND MOIST RETURN FLOW OUT OF THE
GULF...TEMPERATURES ARE LIKELY TO APPROACH AND IN A FEW LOCATIONS
EXCEED THE 90 DEGREE MARK TOMORROW.  LOW LEVEL MIXING MAY LIMIT THE
HEAT INDEX VALUES TO THE MID 90S TOMORROW AFTERNOON...BUT REGARDLESS
IT WILL STILL BE A WARM AND HUMID FIRST DAY OF METEOROLOGICAL FALL
ABOUT 10 DEGREES ABOVE NORMAL HIGHS BUT STILL 10 DEGREES BELOW THE
RECORD HIGHS.

LENNING

&&

.LONG TERM...
232 PM CDT

TUESDAY NIGHT THROUGH FRIDAY...
GUIDANCE CONTINUES THE THEME OF UNSEASONABLE WARMTH FOR THE
REGION...AND MOST OF THE AREAS EAST OF THE ROCKIES. 500MB RIDGE
WILL BE STEADILY BUILDING OVERHEAD TUE NGT INTO WED. SOME OF THE
OPERATIONAL SOLUTIONS INDICATE THE LINGERING WEAK MID-LVL WAVE
PROGRESSING EAST THROUGH THE OHIO VALLEY THAT COULD GENERATE AN
ISOLATED SHOWER/THUNDERSTORM TUE NGT/WED...HOWEVER THE FORCING
LOOKS TO REMAIN VERY WEAK. SO HAVE MAINTAINED A DRY FORECAST WITH
HEIGHTS RISING OVERHEAD. THIS SHOULD AID IN KEEPING A LID ON
CONVECTIVE ACTIVITY FOR THE CWFA. THERMAL RIDGE WILL REMAIN OVER
THE FORECAST AREA WED/THUR...WITH HIGHS ABOVE CLIMO AT LEAST IN
THE UPR 80S TO PERHAPS ARND 90. LLVL MOISTURE WILL ALSO BE
PRESENT...WITH DEW POINTS IN THE MID/UPR 60S. THIS WILL CREATE
HUMID CONDITIONS WITH HEAT INDEX VALUES IN THE LOW/MID 90S.

OVERNIGHT LOWS WILL EQUALLY REMAIN MILD...WITH LOWS GENERALLY IN THE
MID/UPR 60S. DEPENDING ON OVERNIGHT CLOUD COVER...A FEW LOCATIONS
MAY REMAIN ARND 70. HAVE NOT INCLUDED A MENTION OF FOG IN THE
EXTENDED PERIODS...HOWEVER WITH THE LACK OF A STRONG GRADIENT AND
LIGHT WINDS OVERNIGHT PATCHY FOG OR HAZE MAY OCCUR.

LATER IN THE WEEK HEIGHTS SHOULD REMAIN OVER THE REGION KEEPING A
LOW CHANCE FOR ANY CONVECTION...AND LIKELY ONLY BEING DIURNALLY
DRIVEN. AFTERNOON LAKE BREEZES WILL BE POSSIBLE AS WELL...WHICH
COULD ALLOW SOME WEAK MESOSCALE FORCING TO HELP TOUCH OFF AN
ISOLATED AFTERNOON STORM OR TWO IN THE EXTENDED PERIODS.

SATURDAY THROUGH MONDAY... HEADING INTO THE HOLIDAY WEEKEND
ENSEMBLES CONTINUE THE THEME OF MID-LVL RIDGING OVER THE EASTERN
CONUS STRETCHING INTO EASTERN CANADA...WITH A TROUGH OVER THE
SOUTHWEST CONUS. SPREAD AMONGST ENSEMBLE MEMBERS IS MINIMAL...WHICH
WILL HELP WITH THE CONFIDENCE OF A CONTINUED UNSEASONABLY WARM
PERIOD INTO THE WEEKEND AND PERHAPS EARLY NEXT WEEK. THUS HAVE
HELD ONTO TEMPS IN THE MID/UPR 80S. THERE CONTINUES TO BE PERIODIC
CHANCES FOR SHOWERS AND THUNDERSTORMS.

BEACHLER

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 00Z...

* POTENTIAL FOR FOG DEVELOPMENT OVERNIGHT.

BMD

//DISCUSSION...UPDATED 00Z...

LAKE BREEZE MADE SOME PROGRESS INLAND EARLIER THIS AFTERNOON AND
HAS SINCE RETREATED SLIGHTLY AND STALLED...INCLUDING RIGHT OVER
THE EASTERN PORTION OF ORD AIRFIELD...AND PROBABLY ABOUT A QUARTER
TO HALF A MILE EAST OF MDW. THIS HAS RESULTED IN SOUTHWESTERLY
FLOW HOLDING ON AT THE TERMINALS MUCH LONGER THAN EXPECTED AND
WITH LITTLE MOVEMENT OF THE BOUNDARY AND SUNSET SOON...NO LONGER
CONFIDENT IN SEEING A STRONG PUSH WITH A NOTABLE WINDSHIFT TO THE
NORTHEAST. MORE LIKELY WINDS MIGHT BACK SLIGHTLY TO THE SOUTHEAST
OR SOUTH-SOUTHEAST FOR A FEW HOURS THIS EVENING AS THE BOUNDARY
WASHES OUT...THEN WINDS SHOULD GRADUALLY VEER BACK TO THE SOUTH-
SOUTHWEST OVERNIGHT. WHATEVER THE WIND DIRECTION IS...WIND SPEEDS
WILL STAY BELOW 10 KT...AND LIKELY STAY 5-6 KT OR LESS. FOG WILL
ONCE AGAIN BE A CONCERN OVERNIGHT THOUGH SHOULDNT BE NEARLY AS BAD
AS MONDAY MORNING GIVEN THAT WE HAVE HAD A DRY DAY WITH DECENT
MIXING. SOUTHWEST WINDS APPEAR TO BE JUST STRONG ENOUGH TOMORROW
TO HOLD OFF A LAKE BREEZE PUSHING INLAND.

BMD

//ORD AND MDW CONFIDENCE...UPDATED 00Z...

* LOW CONFIDENCE IN VSBY TRENDS OVERNIGHT.
* MEDIUM CONFIDENCE IN WIND DIRECTION THIS EVENING...MEDIUM-HIGH
  FOR TOMORROW.

BMD

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

WEDNESDAY...VFR. WEST WINDS.
THURSDAY...SLT CHC TSRA. PRIMARILY VFR. SOUTHWEST WINDS BCMG
SOUTHEAST.
FRIDAY...SLT CHC TSRA. PRIMARILY VFR. SOUTH TO SOUTHEAST WINDS.
SATURDAY...VFR. SOUTH TO SOUTHEAST WINDS.
SUNDAY...SLT CHC TSRA. PRIMARILY VFR. SOUTH WINDS.

BMD

&&

.MARINE...
232 PM CDT

WEAK LOW PRESSURE OVER MANITOBA WILL MOVE NORTHEAST
TOWARDS HUDSON BAY AND STRENGTHEN TUESDAY. THIS WILL INCREASE THE
GRADIENT FOR THE NORTHERN PORTIONS OF LAKE MICHIGAN LATE TONIGHT
INTO TUESDAY. OTHERWISE THE SOUTHERN HALF OF THE LAKE WILL CONTINUE
IN A SOUTHWEST TO SOUTH FLOW. THEN LARGE RIDGE OF HIGH PRESSURE WILL
REMAIN NEARLY STATIONARY THROUGH THE REMAINDER OF THE WEEK...WITH
GENERALLY SOUTHERLY WINDS.

BEACHLER

&&

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

$$

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