Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 280759
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
259 AM CDT THU APR 28 2016

.UPDATE...
938 PM CDT

MAIN TWEAK TO FORECAST WAS WITH HOURLY POP/WX TRENDS PER RADAR AND
HIGH RES GUIDANCE TRENDS. THUNDERSTORM ACTIVITY OVER CENTRAL
ILLINOIS LATE THIS AFTERNOON AND EVENING FOCUSED ALONG THE WARM
FRONT...SOME OF IT SEVERE...LOOKS TO FOCUS INTO THE SOUTHEAST CWA
LATER THIS EVENING INTO THE OVERNIGHT. EXPECTING IT TO MAINLY POSE
A THREAT ONLY FOR LIGHTNING AS IT OUTRUNS THE BETTER MUCAPE...BUT
MAY NEED TO MONITOR FOR POTENTIAL IN FAR SOUTHERN CWA FOR BETTER
CORES IN TERMS OF A MARGINAL HAIL THREAT GIVEN DECENT EFFECTIVE
SHEAR.

FOR THE REMAINDER OF THE AREA...A LULL WITH ISOLATED TO
WIDELY SCATTERED SHOWERS IS IN STORE UNTIL LINE OF STORMS CLOSER
TO UPPER TROUGH MAKES ITS WAY EAST AND NORTHEAST OVERNIGHT. ALSO
EXPECTING THIS ACTIVITY TO WEAKEN WITH ONLY ISOLATED EMBEDDED TSRA
AS BEST INSTABILITY AXIS IS OUTRUN. SHOWER COVERAGE SHOULD
INCREASE OVERNIGHT BUT MAY REMAIN MORE SCATTERED IN NATURE AND
THEN PERHAPS DIMINISH WITH LOSS OF INSTABILITY FARTHER
NORTHEAST...A SCENARIO DEPICTED ON RECENT RAP/HRRR RUNS.

OTHERWISE...TWEAKED HOURLY TEMP AND DEWPOINT TRENDS WITH THEM
REMAINING NEARLY STEADY IN THE 40S AND 50S ALONG WITH STEADY
EAST-NORTHEAST WINDS. FAR NORTHEAST ILLINOIS MAY DIP INTO THE
UPPER 30S DURING THE EARLY MORNING ON THURSDAY.

RC

&&

.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 06Z TAFS...

VSBYS AND CIGS ARE THE MAIN QUESTIONS FOR THE REMAINDER OF THE
NIGHT AND INTO THE MORNING...ALTHOUGH A LINE OF STORMS ALSO HAS
BEEN HANGING TOGETHER ACROSS WESTERN IL AND MOVING TOWARD THE
LOCAL TERMINALS. THE OVERALL TREND FOR THIS LINE HAS BEEN TO
WEAKEN AS IT LIFTS NORTHEAST...AND STABILITY PROFILES DO NOT
APPEAR FAVORABLE FOR WIDESPREAD SUPPORT OF TSRA AS THE LINE MOVES
INTO NORTHEAST IL. HAVE THEREFORE ELECTED NOT TO FOCUS TOO
GREATLY ON TS CHANCES BUT MAINTAIN VCSH WITH TEMPO RA...BUT THERE
COULD BE BRIEFER PERIODS OF ISOLATED TSRA AS WELL. ALSO AM NOT
SEEING A LOT OF SUPPORT FOR WIDESPREAD IFR CIGS AND VSBYS IN THE
NEAR TERM AWAY FROM HEAVER SHOWER ACTIVITY...BUT TRENDS TOWARD
MORNING APPEAR TO LOWER.

ANOTHER CONCERN...MAINLY TOWARD RFD OVERNIGHT...IS BRIEF BUT SHARP
CHANGES IN WIND DIRECTION FROM DUE EAST TO DUE WEST WITH OUTFLOW
FROM THE TSRA ACTIVITY. THIS HAS BEEN NOTICED AT A FEW SITES
UPSTREAM. SINCE THE LINE IS WEAKENING...TAFS WILL NOT ATTEMPT TO
REFLECT THIS VARIABILITY BUT IT REMAINS A POSSIBILITY TO BE AWARE
OF AS THE LINE MOVES THROUGH.

AFTER SUNRISE...A BIT OF A WET MURKY DAY IS IN STORE WITH
PERIODIC SHOWERS AND MAYBE SOME PATCHY DRIZZLE. TIMING IS TOUGH TO
PIN DOWN...BUT A BREAK IN THE EARLY MORNING WITH RENEWED ACTIVITY
TOWARD LATE MORNING AND AFTERNOON STILL LOOKS REASONABLE. LATER
UPDATES MAY EXTEND SHRA PAST 21Z AS WELL.

LENNING

&&

.MARINE...
252 AM CDT

A SMALL CRAFT ADVISORY IN THE ILLINOIS NEARSHORE WATERS...MAINLY
FOR WAVE ACTIVITY WITH ONSHORE WINDS TODAY...LOOKS ON TRACK TO
EXPIRE BY MID AFTERNOON WHEN WINDS RELAX AND ALLOW WAVES TO BEGIN
SUBSIDING. ADDITIONAL CHANCES FOR EXCEEDING ADVISORY CRITERIA
APPEAR POSSIBLE SATURDAY AFTERNOON AND AGAIN SUNDAY. OTHERWISE
RELATIVELY LIGHT NORTHERLY TO EASTERLY FLOW SHOULD PERSIST
THROUGH THE WEEKEND AND INTO EARLY NEXT WEEK.

LENNING

&&

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

&&

$$

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