Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 010051
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
751 PM CDT SAT APR 30 2016

.UPDATE...
746 PM CDT

STEADIER RAIN CONTINUES TO LIFT NORTH OF THE CWA THIS
EVENING...WITH ADDITIONAL SHOWERY DEVELOPMENT OR DRIZZLE STILL
LINGERING OVER MUCH OF THE CWA. WHILE LARGE SCALE ASCENT WILL
REMAIN PRESENT THIS EVENING/OVERNIGHT WITH THE UPPER LEVEL SYSTEM
JUST TO THE WEST...ISENTROPIC ASCENT WILL LIKELY AID IN EITHER
LIGHT RAIN OR DRIZZLE FOR MUCH OF THE CWA. SO HAVE MAINTAINED
CAT/LIKELY POPS THIS EVENING AND THEN LOWER TO CHANCE WORDING
AFTER MIDNIGHT. HAVE MADE LITTLE CHANGES TO LOW TEMPS
TONIGHT...WITH TEMPS EITHER HOLDING NEARLY STEADY OR SHOWING A
SLIGHT WARMING TREND.

RODRIGUEZ

&&

.SHORT TERM...
146 PM CDT

THROUGH SUNDAY...

COOL/CLOUDY/MOIST AIRMASS CONTINUES TO LIFT NORTH ACROSS NORTHERN
IL/NORTHWEST IN THIS AFTERNOON. IR IMAGERY HAD INITIALLY INDICATED
CLOUD TOPS WERE BETWEEN -60 TO -65 DEG C...BUT OVER THE PAST FEW
HOURS CLOUDS HAVE BEEN QUICKLY WARMING WITH TOPS NOW BETWEEN -50
TO -55 DEG C. IN ADDITION A WEDGE OF DRIER AIR CONTINUES TO LIFT
NORTHEAST AS THE AREA OF LOW PRESSURE MOVES INTO SOUTHCENTRAL IOWA
THIS AFTERNOON. THE PRESSURE GRADIENT HAS ARRIVED OVER NORTHERN IL
THIS AFTERNOON WITH EASTERLY WINDS INCREASING TO BETWEEN 10-15KT
AND GUSTS AROUND 20KT. THE LOW WILL BEGIN TO WEAKEN THIS
EVENING...BUT WITH A BROAD RIDGE POSITIONED OVER ONTARIO IT IS
LIKELY THAT THE LOW WILL STALL FROM LIFTING NORTHEAST OVERNIGHT.
THIS WILL SET THE STAGE FOR STEADIER PRECIP TO SHIFT NORTH AND
END LATE THIS AFTERNOON FROM SOUTH TO NORTH...WITH JUST A FEW
LINGERING SHOWERS THROUGH THE OVERNIGHT. THE BAGGY ATMOSPHERE
WILL CONTINUE TO SEE SHALLOW MOISTURE FEED ACROSS THE
REGION...WITH CONSIDERABLY LOWER CEILINGS OVERNIGHT. THE LACK OF A
LIFT WILL KEEP DROPLET SIZE MINIMAL AND LIKELY LEAD TO PATCHY
DRIZZLE/LGT RAIN. TEMPS OVERNIGHT WILL LIKELY NOT CHANGE
MUCH...WITH LOWS IN THE 40S.

WITH THE BROAD SFC RIDGE DISPLACED TO THE NORTH...THIS WILL KEEP
THE WARM FRONTAL BOUNDARY FROM LIFTING TOO FAR NORTH TOWARDS THE
CWFA OVERNIGHT INTO SUNDAY...HOWEVER THE 500MB VORT MAX WILL BE
UPSTREAM ACROSS SOUTHWEST IOWA TO STAR THE DAY SUNDAY. AS THIS
FEATURE PIVOTS NORTHEAST ACROSS NORTHERN IL...FORCING WILL RETURN
WITH INCREASED CHANCES FOR SHOWERS AND PERHAPS AN ISOLATED
THUNDERSTORM SOUTH OF I-80 SUNDAY AFTERNOON. THE NORTHEAST FLOW
OFF OF LAKE MICHIGAN WILL LIKELY KEEP TEMPS AGAIN SUN AFTN IN THE
40S FOR AREAS ADJACENT TO THE LAKE...MEANWHILE FURTHER INLAND
TEMPS SHOULD BE ABLE TO WARM INTO THE LOW/MID 50S. ACROSS THE
SOUTHERN CWFA THE WARM FRONTAL BOUNDARY WILL BE LINGERING NEARBY
WITH TEMPS NEARING THE LOW 60S.

BEACHLER

&&

.LONG TERM...
243 PM CDT

SUNDAY NIGHT THROUGH SATURDAY...

MAIN CONCERN GOING INTO THE LONGER TERM FORECAST PERIOD WILL BE THE
TEMPERATURE TREND THROUGH THE PERIOD.  BY SUNDAY NIGHT...ANY
LINGERING PCPN ASSOCIATED WITH THE SYSTEM CURRENTLY BRINGING RAIN TO
NRN IL/NWRN IN WILL COME TO AN END FROM WEST TO EAST AS THE MAIN
SHORTWAVE SHIFTS TO THE ERN GREAT LAKES.  A SERIES OF WEAK
SHORTWAVES IS EXPECTED TO DROP OUT OF THE NRN PLAINS AND ACROSS THE
UPPER MIDWEST EARLY NEXT WEAK...BUT AS HIGH PRESSURE BUILDS FROM THE
CENTRAL PLAINS THROUGH THE GREAT LAKES...AN EXTENDED PERIOD OF
GENERALLY NELY FLOW WILL KEEP MOISTURE ACROSS THE REGION VERY
LIMITED AND BY THE TIME A WEAK ASSOCIATED SFC FRONT MOVES THROUGH
THE AREA...ANY CHANCES FOR PCPN WILL BE MINIMAL AND...FOR NOW...WILL
ONLY CARRY SOME SLIGHT CHANCE POPS FOR THE FAR SERN PORTIONS OF THE
CWA ON MONDAY.  ALSO BY EARLY NEXT WEEK...THE UPPER LEVEL PATTERN IS
EXPECTED TO UNDERGO SIGNIFICANT AMPLIFICATION...WITH STRONG RIDGING
BUILDING OVER THE WRN CONUS AND DEEP LONG WAVE TROUGHING DEVELOPING
OVER THE ERN CONUS.  WHILE THE LONGER RANGE GUIDANCE IS RELATIVELY
CONSISTENT ON THE GENERAL SCENARIO...THERE ARE SIGNIFICANT ENOUGH
DIFFERENCES IN THE FINER SCALE DETAILS...IN PARTICULAR THE SPEED OF
PROGRESSION OF THE LONG WAVE PATTERN.  CONSEQUENTLY...THE TRACK OF
SHORT WAVE ENERGY DROPPING SEWD DOWN THE ERN PERIPHERY OF THE UPPER
RIDGE IS DIFFERENT ENOUGH THAT THE DEGREE OF CONFIDENCE IN THE
LONGER TERM FORECAST RAPIDLY DETERIORATES WITH TIME FROM TUESDAY
THROUGH THE END OF THE WEEK.  SO...IN GENERAL...EXPECT A PERIOD OF
CHANGEABLE WEATHER FOR THE MIDDLE AND LATTER PORTIONS OF NEXT
WEAK...ESPECIALLY TEMPERATURES.  WHILE THERE SHOULD BE PERIODS OF
SEASONABLE AND BELOW NORMAL TEMPERATURES THROUGH THE WEEK...
CONFIDENCE IN THE TIMING IS LOW.

KREIN

&&

.AVIATION...
FOR THE 00Z TAFS...

CONCERNS FOR THE 0Z TAF CYCLE INCLUDE
-VERY LOW CEILINGS AND TIMING LOWERING/RISING TIMES
-IFR VISIBILITY AT TIMES...ESPECIALLY TONIGHT INTO EARLY SUNDAY
-TIMING OF THE END OF DRIZZLE AND RETURN OF SHOWERS ON SUNDAY

THE BACK EDGE OF THE MAIN RAIN SHIELD WILL CROSS THE TERMINALS
AROUND OR EVEN BEFORE THE 0Z HOUR...BUT VERY LIGHT RAIN/DRIZZLE
WILL CONTINUE THIS EVENING. IN SPITE OF THE CLEARING SEEN ON
VISIBLE SATELLITE...NE ILLINOIS AND NW INDIANA WILL LARGELY REMAIN
ON THE COOL SIDE OF A LOW LEVEL WARM FRONT. THERE CONTINUED WEAKER
LIFT OVER THIS BOUNDARY INTO A MOIST LOW LEVEL ENVIRONMENT WILL
MAINTAIN LIGHT RAIN/DRIZZLE AT LEAST INTO THE EARLY MORNING HOURS.
THIS PATTERN WILL ALSO TEND TO ALLOW THE LOW CLOUDS TO PERSIST AND
POSSIBLY EVEN LOWER SOME OVERNIGHT. THE DRIZZLE SEEMS FAVORED TO
END BEFORE DAYBREAK ON SUNDAY...WHICH MAY HELP VISIBILITY VALUES
TO START SUNDAY...BUT LOW CIGS LOOK TO CONTINUE. GUIDANCE
CONTINUES TO SUGGEST LIFR TO DOMINATE TONIGHT INTO SUNDAY...WITH
VERY SLOW IMPROVEMENT EXPECTED INTO THE AFTERNOON AS IFR THEN
POSSIBLY MVFR CONDITIONS LATE IN THE DAY...THOUGH WHETHER THIS
IMPROVEMENT OCCURS AND THE TIMING OF THE TRANSITION IS OF LOW
CONFIDENCE. MOST GUIDANCE HOLDS AT 300 FT MOST OF THE NIGHT...AND
TRIED TO SHOW SOME IMPROVEMENT ABOVE THIS THRESHOLD THROUGH THE
MORNING...BUT AGAIN TIMING CONFIDENCE IS LOW.

THERE COULD BE SOME LIGHTER PRECIP AROUND IN THE MORNING...BUT THE
BETTER CHANCE FOR ORGANIZED RA/SHRA RETURNS SUNDAY
AFTERNOON/EVENING AS THE UPPER LEVEL LOW CENTER ROTATES THROUGH.
AT THIS POINT ANY LIMITED THUNDER THREAT LOOKS TO REMAIN
SOUTH/EAST OF THE TERMINALS.

KMD

&&

.MARINE...
255 PM CDT

A LARGE AREA OF HIGH PRESSURE CENTERED OVER CENTRAL CANADA WILL
SLOWLY MOVE TO THE EAST OVER THE NEXT COUPLE DAYS WHILE LOW
PRESSURE OVER THE UPPER MISSOURI VALLEY TRACKS TO THE EAST ACROSS
CENTRAL IL/IN LATE TONIGHT AND TOMORROW. THIS WILL KEEP PERSISTENT
ELY-NELY FLOW OVER THE LAKE. WINDS AND WAVES HAZARDOUS TO SMALL
CRAFT SHOULD CONTINUE OVER THE IL NEARSHORE WATERS THROUGH SUNDAY
NIGHT AS A MODERATE ELY-NELY PRESSURE GRADIENT REMAINS IN PLACE.
HIGH PRESSURE WILL THEN SETTLE ACROSS THE NORTHERN AND CENTRAL
GREAT LAKES REGION ON MONDAY...SETTING UP A PERIOD OF RELATIVELY
QUIET CONDITIONS OVER LAKE MICHIGAN. THE HIGH PRESSURE RIDGE WILL
WEAKEN ON TUESDAY WHILE PUSHING SOUTH INTO THE OHIO VALLEY AND
DEEPENING LOW PRESSURE MOVES SOUTHEAST ACROSS ONTARIO INTO
THE UPPER GREAT LAKES TUESDAY NIGHT WITH A TRAILING COLD FRONT
PUSHING SOUTH ACROSS THE WESTERN LAKES. ANOTHER AREA OF HIGH
PRESSURE BUILDING OVER THE CANADIAN PRAIRIES BY MIDWEEK AS THE LOW
MOVES OFF TO THE EAST SHOULD BRING STRENGTHENING NWLY-NLY FLOW TO
THE LAKE FOR THE MIDDLE AND LATTER PORTIONS OF NEXT WEEK. AT THIS
POINT...EXPECT WINDS TO REMAIN BELOW GALE FORCE...BUT IF THE COLD
ADVECTION AND PRESSURE RISES BEHIND THE EXITING COLD FRONT ARE
STRONG ENOUGH...THE POTENTIAL FOR GALES CANNOT BE RULED OUT BY THE
END OF THE WEEK.

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...NEARSHORE WINTHROP HARBOR TO GARY IN UNTIL 1 AM
     MONDAY.

&&

$$

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