Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 272008
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
308 PM CDT WED APR 27 2016

.SHORT TERM...
307 PM CDT

THROUGH THURSDAY...

OUR FIRST ROUND OF SHOWERS IS CURRENTLY APPROACHING THE CHICAGO
AREA. WHILE THERE WERE SOME LIGHTENING STRIKES EARLIER WITH THIS
ACTIVITY...THIS AREA OF SHOWERS IS MOVING INTO A MORE STABLE AIR
MASS OVER NORTHEASTERN ILLINOIS. AS A RESULT...IT APPEARS THE
THREAT OF THUNDER FROM THIS INITIAL AREA OF SHOWERS WILL BE LOW
OVER NORTHEASTERN ILLINOIS. OVERALL...EXPECT THESE SHOWERS TO
CONTINUE INTO EARLY THIS EVENING OVER NORTHEASTERN ILLINOIS AND
NORTHWESTERN INDIANA. AS THIS INITIAL AREA OF SHOWERS SHIFTS
NORTHEASTWARD THROUGH EARLY THIS EVENING...THERE COULD END UP
BEING A FEW HOUR BREAK IN THE SHOWERS FROM SOUTHWEST TO NORTHWEST
FROM MID TO LATE THIS EVENING.

CONFIDENCE WITH THE TIMING AND CHANCES OF ADDITIONAL SHOWERS AND
THUNDERSTORMS BEGINS TO FADE A BIT INTO TONIGHT. HOWEVER...WITH
THE 850 MB WARM FRONT...AND ASSOCIATED MOISTURE CONVERGENCE AXIS
EXPECTED TO BE IN PLACE ACROSS NORTHERN ILLINOIS...ALONG WITH
STEEPENING MID LEVEL LAPSE RATES...ADDITIONAL SHOWERS AND STORMS
COULD CONTINUE TO FESTER AT TIMES OVER PORTIONS OF THE AREA
TONIGHT. THE OTHER CONCERN FOR ADDITIONAL THUNDERSTORM ACTIVITY
WILL BE WITH THE REMNANTS OF THE CONVECTION CURRENTLY DEVELOPING
OVER NORTHWESTERN MISSOURI ALONG THE MISSOURI RIVER. THIS ACTIVITY
IS LIKELY TO CONTINUE TO DEVELOP EASTWARD ACROSS SOUTHERN
IOWA/NORTHERN MISSOURI INTO EARLY THIS EVENING. THE REMNANTS OF
THIS ACTIVITY COULD SHIFT ACROSS OUR AREA OVERNIGHT
TONIGHT...BRINGING ANOTHER ROUNDS OF SHOWERS AND
THUNDERSTORMS...ESPECIALLY OVER SOUTHERN SECTIONS OF THE AREA. DUE
TO UNCERTAINTIES ON TIMING ON THIS ACTIVITY...IT WAS REALLY
DIFFICULT TO FOCUS HOURLY POPS DURING ANY PARTICULAR PERIOD
TONIGHT...BUT DID TRY TO SLOW A BRIEF DOWNWARD TREND IN POPS FOR A
PERIOD LATE THIS EVENING. THE EVENING SHIFT SHOULD BE ABLE TO GET
A BETTER HANDLE ON IF THE CONVECTION OVER MISSOURI HOLDS TOGETHER
AND MOVES TOWARDS OUR AREA LATER.

SCATTERED SHOWERS LOOK TO CONTINUE AT TIMES ACROSS THE AREA ON
THURSDAY WITH ANOTHER CLOUDY AND COOL DAY. THE THREAT OF THUNDER
LOOKS LOWER...THOUGH MY SOUTHEASTERN AREAS COULD EXPERIENCE SOME
STORMS IN CLOSER PROXIMITY TO THE SURFACE FRONT AND REMNANT
SURFACE LOW MOVING ACROSS THE AREA. OTHERWISE...EXPECT EAST-
NORTHEASTERLY WINDS AND VERY COOL CONDITIONS ON
THURSDAY...ESPECIALLY NORTH. TEMPERATURES NORTH OF INTERSTATE 80
COULD STRUGGLE TO GET OUT OF THE 40S ON THURSDAY. FARTHER SOUTH
TOWARDS CENTRAL ILLINOIS...SOME 60S COULD BE REACHED IN CLOSER
PROXIMITY TO THE SURFACE FRONT.

KJB

&&

.LONG TERM...
248 PM CDT

THURSDAY NIGHT THROUGH WEDNESDAY...

A PROGRESSIVE...COMPLICATED UPPER LEVEL PATTERN IS SETTING UP FOR
THE EXTENDED FORECAST PERIOD.  MULTIPLE SYSTEMS ARE EXPECTED TO
IMPACT THE REGION...OR COME CLOSE ENOUGH THAT THERE MAY BE SOME
POTENTIAL IMPACTS SHOULD SOME CHANGES OCCUR DURING THE EXTENDED
FORECAST PERIODS.

BY THURSDAY NIGHT...A SLOW MOVING UPPER LOW TRACKING THROUGH THE
CENTRAL PLAINS WILL MAKE A TURN TO THE ENE...ACROSS SRN WISCONSIN.
THE LONGER RANGE GUIDANCE IS RATHER CONSISTENT ON THE PATH OF THE
UPPER SYSTEM AND WILL REPRESENT THE TAIL END OF THE PCPN EVENT
MOVING INTO THE REGION WEDNESDAY AFTERNOON.  WITH THE MAIN FORCING
TO THE NORTH OF THE CWA...THE HIGHEST POPS SHOULD BE OVER THE NRN
TIER COUNTIES AND SRN WI...BUT THERE SHOULD BE ENOUGH WRAPAROUND
MOISTURE AND WEAK INSTABILITY ON THE BACK SIDE OF THE EXITING
ASSOCIATED SFC LOW FOR THE CHANCE FOR SOME LINGERING SHOWERS THROUGH
THURSDAY NIGHT.  FRIDAY INTO FRIDAY NIGHT SHOULD BE DRY AS HIGH
PRESSURE BUILDS OUT OF CANADA AND ACROSS THE GREAT LAKES REGION.
THIS SHOULD BE A COOL...DRY AIRMASS...WITH TEMPERATURES ONLY
EXPECTED TO BE IN THE UPPER 50S TO LOWER 60S ACROSS AREAS AWAY FROM
LAKE MICHIGAN.  PREVAILING NELY FLOW OFF OF LAKE MICHIGAN WILL KEEP
LAKEFRONT LOCATIONS MUCH COOLER...WITH HIGHS ONLY IN THE UPPER 40S
TO AROUND 50F.  BY SUNDAY...A SLOW MOVING SHORTWAVE LIFTING OUT OF
THE CENTRAL PLAINS WILL BRING THE NEXT CHANCE FOR PCPN ...POSSIBLY
AS EARLY AS SATURDAY MORNING.  BROAD WARM ADVECTION SHOULD BE
CONDUCIVE TO INCREASING MOISTURE...BUT SFC TEMPERATURES WILL HAVE
TROUBLE REBOUNDING MUCH HIGHER THAN THE PREVIOUS FEW DAYS AS STRONG
HIGH PRESSURE BUILDING OUT OF CANADA AND ACROSS THE UPPER GREAT
LAKES WILL LIMIT THE NWD PROGRESSION OF A SFC WARM FRONT.  WHILE
THERE ARE SOME DIFFERENCES AMONG THE LONGER RANGE MODELS IN THE
FINER SCALE DETAILS...THE GENERAL IDEA IS THAT THE WARM FRONT SHOULD
STAY TO THE SOUTH OF THE CWA THROUGH SUNDAY...KEEPING RELATIVELY
COOL FLOW OFF OF THE LAKE IN PLACE.  SO...WHILE INLAND TEMPERATURES
MAKE BE ABLE TO REACH AROUND 60 F ON SUNDAY...THE LAKEFRONT WILL
REMAIN MUCH COOLER WITH HIGHS AGAIN ONLY AROUND 50F...AND POSSIBLY
ONLY UPPER 40S.

BY EARLY NEXT WEEK...SIGNIFICANT UNCERTAINTY ENTERS INTO THE
FORECAST...WITH THE GFS ADVERTISING AN UPPER RIDGE BUILDING ACROSS
THE CENTRAL PLAINS...TO THE WESTERN GREAT LAKES TUESDAY
NIGHT...WHILE UPPER LEVEL NWLY FLOW AND DEEP LAYER COLD ADVECTION IS
SHUNTED EAST TO THE NERN CONUS. THIS WOULD SUGGEST A WARMING TREND
FOR THE LOCAL AREA.  HOWEVER...THE ECMWF IS SIGNIFICANTLY
DIFFERENT...WITH A NRN STREAM SHORTWAVE DROPPING OUT OF CANADA AND
SWINGING THROUGH THE UPPER MISSISSIPPI VALLEY AND UPPER GREAT
LAKES...ALONG WITH A LONGWAVE TROUGH EXTENDING SOUTH THROUGH THE
MIDDLE  MISSISSIPPI VALLEY.  AN ASSOCIATED COLD FRONT WOULD PUSH
THROUGH THE REGION ON TUESDAY LEADING TO COOLER CONDITIONS FOR
TUESDAY AND WEDNESDAY.  THE GEMNH LENDS SOME SUPPORT TO THE ECMWF
SOLUTION...INDICATING A NORTHERN STREAM SHORTWAVE DROPPING THROUGH
THE WESTERN GREAT LAKES BY TUESDAY MORNING...BUT THAT IS THE END OF
THE AVAILABLE GUIDANCE FROM THAT PARTICULAR MODEL.  GIVEN THE
SIGNIFICANT FORECAST UNCERTAINTY...HAVE OPTED TO MAINTAIN LITTLE
SIGNIFICANT CHANGE TO THE GOING FORECAST...WHICH WOULD SUGGEST
SEASONABLE TEMPERATURES AND LOW PCPN CHANCES.

KREIN

&&

.AVIATION...
FOR THE 18Z TAFS...

A STORM SYSTEM TO OUR WEST WILL GRADUALLY SPREAD SHOWERS
NORTHEASTWARD ACROSS THE TERMINALS LATE THIS AFTERNOON THROUGH EARLY
THIS EVENING. MAIN CHANGE TO THE TAFS WAS TO REMOVE THE VCTS
MENTION...AS IT APPEARS MOST OF THE THUNDERSTORM ACTIVITY MAY REMAIN
SOUTH OF THE TERMINALS. THERE IS STILL A SMALL CHANCE OF STORM AT
THE TERMINALS THIS EVENING...BUT THE PROBABILITY AND DURATION OF IT
IS TO LOW TO CONTINUE A MENTION IN THE TAFS AT THIS TIME. SHOWER
ACTIVITY SHOULD MOVE INTO KRFD BY EARLY THIS AFTERNOON...AND ACROSS
THE EASTERN TERMINALS AFTER 21-22 UTC. THESE SHOWERS WILL CONTINUE
INTO THIS EVENING BEFORE ENDING BY LATE EVENING. ADDITIONAL SHOWERS
ARE POSSIBLE ACROSS THE AREA LATER TONIGHT INTO THURSDAY...BUT DUE
TO UNCERTAINTIES ON THE EXACT TIMING...WITH JUST LEAVE A VCSH IN THE
FORECAST.

EXPECT WINDS TO CONTINUE TO BE GUSTY AT TIMES OUT OF THE EAST-
NORTHEAST THROUGH THE PERIOD. CIGS WILL IMPROVE SOME THIS
AFTERNOON...BUT LOWER MVFR CIGS AROUND...OR JUST BELOW 1,000 FT AGL
ARE LIKELY AGAIN BY THURSDAY MORNING.

KJB

&&

.MARINE...
250 AM CDT

HEADLINES...THE SMALL CRAFT ADVISORIES LOOK GOOD AS IS.

LOW PRESSURE OVER KANSAS WEAKENS AND PASSES OVER CENTRAL IL
THURSDAY. MEANWHILE HIGH PRESSURE OVER CENTRAL CANADA REMAINS
STATIONARY THROUGH THE END OF THE WEEK AND THEN SHIFTS EAST OVER
QUEBEC EARLY NEXT WEEK.  NORTH TO EAST WINDS ARE EXPECTED OVER THE
LAKE THROUGH MONDAY. THE ONSHORE FLOW RESULTS IN HAZARDOUS WAVES FOR
SMALL CRAFT INTO THURSDAY FOR THE IL NSH WATERS.  ANOTHER WEAK LOW
PASSES SOUTH OF THE LAKE SUNDAY...AND THEN HIGH PRESSURE SPREADS
OVER THE LAKE MONDAY. A LOW PRESSURE SYSTEM MOVES ACROSS SOUTHEAST
CANADA MONDAY NIGHT INTO TUESDAY...AND WINDS ARE FORECAST TO BECOME
SOUTH OR SOUTHWEST AS THE HIGH SHIFTS EAST OF THE LAKE TUESDAY.

JEE

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...ILLINOIS NEARSHORE WATERS UNTIL 4 PM THURSDAY.

&&

$$

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