Area Forecast Discussion
Issued by NWS Chicago, IL

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358
FXUS63 KLOT 301944
AFDLOT

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

.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 18Z TAFS...

MAIN CONCERNS FOR THE REMAINDER OF THE AFTERNOON THROUGH THE
OVERNIGHT HOURS WILL BE ON CIGS AND VSBY REDUCTIONS. CONSIDERABLE
MOISTURE AND RAIN CONTINUES TO FEED NORTH/NORTHEAST ACROSS
NORTHERN IL AND NORTHWEST IN THIS AFTERNOON...WHICH HAS BEEN
STEADILY OVERCOMING THE INITIAL DRY LAYER FROM THIS MORNING. CIGS
HAVE BEEN SLOWLY LOWERING TOWARDS MVFR CONDS...BUT EXPECT CLOSER
TO 00Z CIGS WILL BE FLIRTING WITH IFR CONDS. THE STEADIER PRECIP
SHOULD BE DIMINISHING IN COVERAGE BETWEEN 00-02Z...WITH JUST SOME
LINGERING DZ OR LGT RA THROUGH THIS EVENING. WINDS WILL CONTINUE
FROM THE EAST WITH OCCASIONAL GUSTS BETWEEN 20-24KT THIS
AFTERNOON...THEN DIMINISHING AFTER SUNSET. OVERNIGHT PRECIP
COVERAGE SHOULD END WITH JUST PATCHY DZ REMAINING. THIS IS THE
TIME WHEN GUIDANCE HAS BEEN LEANING HEAVILY TOWARDS CIGS FALLING
FURTHER TOWARDS LIFR CONDS...WITH BASES BETWEEN 200-300FT AGL. IT
IS POSSIBLE A FEW SPOTS COULD GO FURTHER DOWN OVERNIGHT. HELPING
CIGS TO LOWER TONIGHT WILL BE THE LIGHTER WINDS THAT DEVELOP AS
THE LOW PRESSURE APPROACHES THE REGION FROM THE
SOUTHWEST...HOWEVER WIND DIRECTIONS WILL REMAIN NORTHEASTERLY AND
SPEEDS WILL BE BETWEEN 6-12KT.

AFTER DAYBREAK SUNDAY WINDS WILL TURN MORE NORTHERLY...WHICH
SHOULD HELP TO LIFT CIGS SLIGHTLY BACK TOWARDS IFR CONDS. BUT WITH
THE CONTINUED LACK OF STRONG MIXING AND A WEAK PRESSURE GRADIENT
THE CONTINUED IFR/LOW END MVFR CIGS WILL PERSIST WITH A FEW
SHOWERS IN THE AREA.

BEACHLER

&&

.MARINE...
242 AM CDT

AN EXPANSIVE AREA OF HIGH PRESSURE WILL REMAIN NORTH OF THE LAKES
TODAY...AS AN AREA OF LOW PRESSURE OVER THE SOUTHERN PLAINS LIFTS
NORTHEAST INTO THE LOWER MISSOURI VALLEY BY EVENING...AND THEN
TRACKS TO THE EASTERN LAKES/UPPER OHIO VALLEY ON SUNDAY. COOL
NORTHEAST FLOW FROM THE HIGH TO THE NORTH WILL INCREASE ESPECIALLY
ACROSS SOUTHERN LAKE MICHIGAN TODAY AS THE LOW APPROACHES FROM THE
SOUTHWEST AND TIGHTENS UP THE GRADIENT. EAST-NORTHEASTERLY FLOW AT
20-25 KT WILL PRODUCE SMALL CRAFT ADVISORY CONDITIONS ON SOUTHERN
LAKE MICHIGAN...LIKELY LINGERING THROUGH SUNDAY AS WINDS BACK MORE
NORTHERLY AND THE FETCH LENGTHENS. WEAK HIGH PRESSURE WILL SPREAD
ACROSS THE LAKES MONDAY ALLOWING WINDS AND WAVES TO SUBSIDE.
ANOTHER ROUND OF MODESTLY STRONG NORTH WINDS APPEARS LIKELY MID-
WEEK.

RATZER

&&

.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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