Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 021946
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
246 PM CDT TUE SEP 2 2014

.DISCUSSION...
331 AM CDT

SHORT TERM TODAY AND TONIGHT...

THE AREA DOGGED A BULLET OVERNIGHT...WITH ALL THE HEAVY RAIN
REMAINING SOUTH OF MY CWA ACROSS CENTRAL AND SOUTHERN ILLINOIS AND
SOUTHERN INDIANA. A FEW SHOWERS AND A STORM OR TWO MAY STILL IMPACT
MY FAR SOUTHERN COUNTIES EARLY THIS MORNING BEFORE THIS ACTIVITY
SHIFTS OUT OF THE REGION BY MID MORNING.

ALTHOUGH IT APPEARS MUCH OF THE DAY TODAY WILL BE DRY...WE STILL
HAVE ANOTHER UPPER LEVEL DISTURBANCE EXPECTED TO SHIFT ACROSS
NORTHERN ILLINOIS THIS MORNING...AND THIS COULD PRODUCE A SHORT
PERIOD OF SHOWERS AND THUNDERSTORMS ACROSS FAR NORTHERN ILLINOIS
THIS MORNING. WATER VAPOR IMAGERY INDICATES THAT IS A FAIRLY STOUT
DISTURBANCE...CURRENTLY SHIFTING EASTWARD ACROSS WESTERN SECTIONS OF
IOWA EARLY THIS MORNING. THE IMPULSE HAS BEEN DRIVING A LINE OF
CONVECTION EASTWARD ACROSS IOWA FOR THE LAST SEVERAL HOURS. THE
ACTIVITY HAS EVEN ESTABLISHED A GOOD COLD POOL...WITH OUTFLOW GUSTS
REPORTED IN EXCESS OF 30 KT. THE MAIN QUESTION IS HOW FAR SOUTH INTO
ILLINOIS THIS ACTIVITY WILL DEVELOP. CURRENT RADAR TRENDS SUGGEST
THAT IS MAY ON BUSH FAR NORTHERN ILLINOIS AFTER 13-14 UTC THIS
MORNING. I WILL CONTINUE TO MONITOR TRENDS AND WILL LIKELY MAKE A
LAST MINUTE DECISION ON HOW FAR SOUTH TO PLACE THE CHANCES FOR
STORMS THIS MORNING. AT THIS TIME IT APPEARS THE MAIN THREAT WILL
STAY NORTH OF I-88. THESE STORMS COULD PRODUCE SOME 20 TO 35 KT
WIND GUSTS ACROSS FAR NORTHERN PORTIONS OF THE AREA AS THEY SHIFT
EASTWARD.

THIS DISTURBANCE WILL SHIFT EAST OF THE AREA BY EARLY
AFTERNOON...TAKING THE CHANCES OF PRECIPITATION WITH IT.
THEREFORE...THE AFTERNOON TODAY LOOKS GREAT ACROSS THE REGION...WITH
MOSTLY SUNNY SKIES EXPECTED AND TEMPERATURES IN THE LOW 80S. A LAKE
BREEZE MAY DEVELOP ALONG THE IMMEDIATE LAKE SHORE AREAS AS THE
SYNOPTIC SCALE WINDS REMAIN LIGHT OUT OF THE WEST.

A SURFACE RIDGE OF HIGH PRESSURE WILL SHIFT OVER THE AREA TONIGHT.
LIGHT SURFACE WINDS AND MOSTLY CLEAR SKIES UNDER THIS HIGH COULD
RESULT IN SOME FOG AGAIN TONIGHT INTO WEDNESDAY MORNING...SO I ADDED
SOME PATCHY FOG TO THE FORECAST.

KJB

LONG TERM...

WEDNESDAY AND WEDNESDAY NIGHT...
AS HIGH PRESSURE DEPARTS EASTWARD ACROSS THE OHIO VALLEY IN THE
MORNING...RETURN SOUTHERLY FLOW WITH GRADUALLY MOISTENING LOW-LEVELS
IS EXPECTED DURING THE DAY. WHILE HEIGHTS WILL OVERALL BE
BUILDING...THE ZONAL FLOW ACROSS THE MIDWEST WILL LIKELY HAVE SMALL
SCALE PERTURBATIONS WITHIN AND IS ADVERTISED BY BOTH GLOBAL AND
HIGH-RES MODEL GUIDANCE. DRY LOW-LEVEL AIR IS EXPECTED TO OFFSET ANY
SUCH FEATURES THROUGH THE DAY THOUGH MID TO HIGH CLOUD SHOULD BE ON
THE INCREASE BY AFTERNOON. BY NIGHT...A POSITIVELY TILTED BUT AGAIN
PROGRESSIVE UPPER TROUGH WILL BE ADVANCING ACROSS MANITOBA INTO THE
UPPER U.S. ROCKIES. IN RESPONSE...WARM AIR ADVECTION AND A STOUT
35-45 KT LOW-LEVEL JET IS FORECAST BY THE SUITE OF GUIDANCE TO BE
POINTING INTO THE UPPER MISSISSIPPI VALLEY/CORN BELT REGIONS WITH
LIKELY SOME DEVELOPMENT OF CONVECTION. THIS LOOKS MOST CONCENTRATED
TO OUR NORTHWEST /SEE SPC DAY 2 OUTLOOK/ WHERE THERE IS GREATER
925-700MB MOISTURE FLUX...BUT SOME STORM CHANCES REMAIN OVER THE
AREA IN JUST THE INCREASING MOIST RETURN AND THICKNESS DIFFLUENCE.

THURSDAY...
THIS CONTINUES TO BE A WARM AND POTENTIALLY HOT DAY WITH THE AREA
BEING ENVELOPED IN A WARM SECTOR OF A 998MB LOW ADVANCING NORTHEAST
FROM ND DURING THE DAY THURSDAY. MORNING CLOUDS REMAIN A
WILDCARD...ESPECIALLY IN NORTHEAST IL AND FAR NORTHWEST IN. GUIDANCE
IS IN GOOD AGREEMENT WITH SUNNY CONDITIONS ENVELOPING MUCH OF THE
AREA LATE IN THE MORNING. LOOKING BACK AT THE PAST DECADE OF 90+
DAYS IN CHICAGO IN THE FIRST 15 DAYS OF SEPTEMBER /WHICH THERE WAS
13/...NARR REANALYSIS INDICATE SEVERAL SIMILAR MASS FIELD PLACEMENTS
AND MAGNITUDE...INCLUDING 850MB TEMPERATURES /AROUND 21C/ AND
SURFACE LOW PRESSURE POSITION AND QUICK MOVEMENT. THE FORECAST
CONTINUES TO BE RIGHT AROUND 90 WITH LOWER 90S IN THE SOUTHWEST
FORECAST AREA. DEW POINTS LOOK TO TOP 70 AT THIS POINT WHICH WOULD
BRING HEAT INDEX READINGS POTENTIALLY TO 95 TO 105.

THURSDAY NIGHT AND BEYOND...
A CAP SHOULD REMAIN IN PLACE MUCH OF THURSDAY INCLUDING INTO THE
EVENING WITH INCREASING CONFIDENCE. THE SYSTEM COLD FRONT HAS THE
LOOK OF A LATE-SUMMER ONE WITH LIMITED INSTABILITY AND MORE
FORCING FOR PRECIPITATION POSSIBLY FROM MID-LEVEL FRONTOGENESIS
ALONG AND BEHIND THE BOUNDARY. WE CONTINUE THE CHANCE DURING THE
DAY AND LINGER CHANCES INTO FRIDAY NIGHT FOR THE MID-LEVEL
FORCING WHICH BOTH THE GFS AND EC AGREE ON TO BE STRONG ENOUGH FOR
PRECIPITATION...PROBABLY MORE SO SHOWERS THAN THUNDER. NORTHERLY
WINDS WILL ADVECT DRIER AIR INTO THE REGION BY SATURDAY WITH HIGHS
IN THE LOWER 70S ON BOTH WEEKEND DAYS. THE FLOW DOWN LAKE MICHIGAN
LOOKS TO BE STRONG ENOUGH TO PRODUCE 3-6 FT WAVES ON
SATURDAY...WHICH WOULD CREATE A RISK FOR RIP CURRENTS IMPACTING
BEACH-GOERS ON ONE OF OUR FINAL BEACH-GOING WEEKENDS.

MTF

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 20Z...

* NONE.

KREIN

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

THE SHOWERS AND THUNDERSTORMS ASSOCIATED WITH A OLD MID LEVEL
IMPULSE HAVE DISSIPATED AND NO PCPN IS NOW EXPECTED INVOF THE
TERMINALS. LATEST RADAR IMAGERY SHOWS AN OUTFLOW BOUNDARY
EXTENDING FROM KARR TO KVYS AND TRACKING TO THE SOUTHEAST. SOME
NWLY WIND GUSTS FOLLOW THE PASSAGE OF THE OUTFLOW BOUNDARY...BUT
DO NOT EXPECT MUCH...IF ANY...IMPACT AT THE TERMINALS. ALSO...A
LAKE BREEZE BOUNDARY IS FORMING ALONG THE LAKE FRONT. WITH SFC
WINDS APPROACHING 10KT AND WINDS JUST OFF THE DECK IN EXCESS OF
10KT...DO NOT EXPECT THE LAKE BREEZE BOUNDARY TO PUSH INLAND MUCH
AND SHOULD REMAIN EAST OF ORD/MDW. UNDER GENERALLY CLEAR
SKIES...EXPECT THE SFC LAYER TO DECOUPLE FROM WINDS ALOFT AND
WINDS SHOULD BECM LGT/VRBL OVERNIGHT. THERE IS A CHANCE FOR SOME
PATCHY GROUND FOG OVERNIGHT TONIGHT. THERE IS A CHANCE IT COULD
IMPACT DPA/RFD...BUT IS UNLIKELY AT ORD/MDW/GYY. WILL LEAVE
MENTION OUT OF THE TAFS FOR NOW SINCE IT SHOULD BE CONFINED TO
SHELTERED...LOW LYING AREAS.

KREIN

//ORD AND MDW CONFIDENCE...UPDATED 20Z...

* MEDIUM TO HIGH CONFIDENCE IN ORD/MDW REMAINING DRY THIS
  AFTERNOON.

* HIGH CONFIDENCE IN ALL OTHER FORECAST ELEMENTS.

KREIN

//OUTLOOK FOR ORD/MDW FOR 00Z THURSDAY-12Z TUESDAY...UPDATED 12Z...

WEDNESDAY NIGHT AND THURSDAY...PREDOMINANTLY VFR...THOUGH THERE
IS A SLIGHT CHANCE OF A THUNDERSTORM.

FRIDAY...PRETTY GOOD CHANCE OF A PERIOD OR TWO OF TSRA.

SATURDAY THROUGH MONDAY...VFR. NORTHEAST WINDS LIKELY.

KJB

&&

.MARINE...
220 AM CDT

A COLD FRONT WHICH CLEARED THE LAKE EARLY THIS MORNING HAS SHIFTED
WINDS WESTERLY. THE WINDS WILL RETURN TO SOUTHERLY BY AFTERNOON AS
HIGH PRESSURE TRANSLATES EAST ACROSS THE OHIO RIVER VALLEY. WIND
SPEEDS IN NEARSHORE AREAS OF ILLINOIS AND INDIANA ARE
EXPECTED TO BE LIGHT ENOUGH THIS AFTERNOON TO ALLOW FOR AT LEAST
SOME TEMPORARY ONSHORE COMPONENT OF WIND.

THE NEXT FEATURE OF INTEREST FOR THE LAKE IS STRONG...ALMOST
EARLY-AUTUMN-LIKE...LOW PRESSURE DEVELOPING ACROSS THE HIGH PLAINS
REGION ON THURSDAY MORNING. THIS LOW WILL TRANSLATE TO NORTH OF THE
GREAT LAKES BY LATE THURSDAY NIGHT. AHEAD OF THIS FEATURE WILL BE
GUSTY SOUTHERLY WINDS OVER LAND. THE OFFSHORE COMPONENT OF THIS WILL
LIKELY ALLOW FOR A WINDOW OF SMALL CRAFT ADVISORY WINDS IN THE
ILLINOIS AND POSSIBLY THE INDIANA NEARSHORE AREAS. AN ASSOCIATED
STRONG COLD FRONT TO THE LOW WILL MOVE DOWN THE LAKE SOMETIME DURING
THE DAY FRIDAY...TURNING WINDS NORTHERLY. THIS WILL BUILD
WAVES...PRESENTLY FORECAST OVER THE SOUTHERLY PART OF THE LAKE TO
REACH 4 TO 6 FEET FRIDAY NIGHT INTO THE DAY SATURDAY. IF THIS
FORECAST WERE TO HOLD...IT WOULD LEAD TO SMALL CRAFT ADVISORIES
AND BEACH HAZARD STATEMENT ISSUANCES FOR ILLINOIS AND INDIANA
SHORE AREAS.

MTF

&&

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

$$

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