Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 292331
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
631 PM CDT FRI APR 29 2016

.SHORT TERM...
204 PM CDT

THROUGH SATURDAY...

COOL CLOUDY CONDITIONS HAVE PERSISTED AGAIN THIS AFTERNOON...WITH
THE LACK OF FORCING TO SCOUR THE CLOUDS FROM THE WEAK MID-LVL
HEIGHT RISES AND LACK OF PRESSURE GRADIENT. THIS HAS KEPT CLOUD
IMPROVEMENTS TO A MINIMUM AS WELL...AND THE NORTH/NORTHEAST SFC
FLOW HELD TEMPS ALONG THE IMMEDIATE LAKESHORE IN THE MID/UPR 40S.
FURTHER INLAND TEMPS HAVE SLOWLY WARMED INTO THE LOW/MID 50S.

THROUGH THIS EVENING AND MUCH OF THE OVERNIGHT HOURS MID-LVL
HEIGHTS WILL REMAIN SLIGHTLY ELEVATED...WHICH SHOULD MAINTAIN THE
DRY FORECAST THROUGH AT LEAST 9Z. LOW LEVEL MOISTURE WILL BE
STEADILY ADVECTING NORTH AS A FRONTAL BOUNDARY DRIFTS NORTH AHEAD
OF THE SHORTWAVE PRESENTLY POSITIONED OVER THE CENTRAL PLAINS. THE
CONTINUED NORTHEAST/EAST SFC WINDS WILL HELP TO DELAY THE ONSET
OF PRECIP ALONG AND NORTH OF I-80 UNTIL JUST AFTER DAYBREAK...BUT
EXPECT STEADY MOISTENING THROUGH THE COLUMN TO OCCUR WITH POPS
APPROACHING CATEGORICAL BY EARLY AFTERNOON.

VERY MOIST SAT AFTN IS IN STORE...AS PWAT VALUES ARE PROGGED TO
APPROACH 1 INCH AND PERHAPS ALONG THE SOUTHERN HALF OF THE
FORECAST AREA NEARLY 1.2 INCHES. WITH THE MOIST AXIS OVERHEAD SAT
AFTN AND ISENTROPIC ASCENT TO PARCELS COUPLED WITH A LOW LEVEL JET
AROUND 30-40KT...THE POTENTIAL FOR MODERATE RAIN TO OCCUR
THROUGHOUT THE AFTERNOON APPEARS FAVORABLE...ALONG WITH SOME
EMBEDDED THUNDER FROM MODEST INSTABILITY IN THE SOUTHERN CWFA.
TEMPS SAT WILL SLOWLY WARM INTO THE LOW/MID 50S...HOWEVER FOR
AREAS ADJACENT TO LAKE MICHIGAN IN FAR NORTHEAST IL THE NORTHEAST
WINDS WILL LIKELY KEEP TEMPS IN THE UPR 40S.

BEACHLER

&&

.LONG TERM...
256 PM CDT

SATURDAY NIGHT THROUGH FRIDAY...

MAIN CONCERN GOING INTO THE LONGER TERM FORECAST PERIOD WILL BE WITH
THE CONTINUATION OF WIDESPREAD PCPN INTO SATURDAY EVENING.  THE
VARIOUS MODELS ARE IN GOOD AGREEMENT ON TRACKING A BAND OF MODERATE
RAIN THROUGH THE CWA INTO THE EVENING HOURS AS A WEAK NRN STREAM
SHORTWAVE PHASES WITH THE MAIN SYNOPTIC WAVE TRACKING ACROSS THE
UPPER MISSISSIPPI VALLEY.  THE EXTENDED PERIOD OF WARM ADVECTION
THROUGH A DEEP LAYER AHEAD OF THE SYNOPTIC UPPER LOW...REFLECTED IN
FORECAST PWATS IN EXCESS OF 1 INCH THROUGH THE EVENING BACKS UP THE
SCENARIO FOR MODERATE RAIN.  INSTABILITY WILL LARGELY BE CONFINED TO
THE SRN PORTIONS OF THE CWA...SO...THE GREATEST CHANCES FOR THUNDER
SHOULD STILL BE SOUTH OF THE I-80 CORRIDOR.  INSTABILITY IS EXPECTED
TO BE RELATIVELY WEAK...OFF-SETTING MODERATE MID AND UPPER LEVEL
FORCING.  THEREFORE THE SEVERE THREAT IS MINIMAL AND TS COVERAGE
WILL LIKELY BE MORE ISOLD/EMBEDDED.  PCPN SHOULD BEGIN TO DIMINISH
THROUGH THE OVERNIGHT HOURS AS THE UPPER TROUGH CONTINUES TO DIG TO
THE SOUTH AND EAST...CUTTING OFF THE ISENTROPIC LIFT.  THE
PERSISTENT CANADIAN HIGH PARKED NORTH OF THE GREAT LAKES WILL
GRADUALLY BEGIN TO MOVE TO THE EAST...WITH RIDGING EXTENDING INTO
THE NRN/CNTRL PLAINS.  UNSEASONABLY LOW TEMPS WILL PERSIST THROUGH
THE WEEKEND AND INTO EARLY NEXT WEEK AS NELY FLOW CONTINUES TO DRAW
COOLER AIR FROM LAKE MICHIGAN.

FROM TUESDAY AND THROUGH THE REMAINDER OF THE EXTENDED PERIOD...
FORECAST CONFIDENCE DIMINISHES AS THE LONGER RANGE MODELS CONTINUE TO
HAVE DIFFICULTY IN AGREEING ON A SOLUTION IN HANDLING THE NRN STREAM
OF THE UPPER LEVEL PATTERN.  THERE IS SOME CONTINUITY ON KEEPING THE
SRN STREAM JET SHUNTED WELL TO THE SOUTH...BUT THE GFS/ECMWF DIVERGE
QUICKLY ON THEIR DEPICTION OF THE NRN STREAM...PRACTICALLY BECOMING
180 DEGREES OUT OF PHASE.  SO...HAVE TRENDED CLOSE TO A BLENDED
SOLUTION WITH LITTLE CHANGE TO THE GOING FORECAST. ULTIMATELY...THIS
TRENDS THE FORECAST TO NEAR NORMAL TEMPERATURES WITH PERIODIC
CHANCES FOR PCPN.  GIVEN THAT PCPN CHANCES WILL BE TIED TO NRN
STREAM SHORTWAVE ENERGY...WILL KEEP POPS GENERALLY IN THE SLIGHT
CHANCE RANGE.

KREIN

&&

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

PEAK OF DAYTIME HEATING DID ALLOW FOR SOME BRIEF EROSION OF THE
STRATUS LAYER THIS AFTERNOON. THE THOUGHT WAS THAT THIS EROSION
WOULD PERSIST INTO TONIGHT...BUT LOW LEVEL ADVECTION HAS ALLOWED A
SECOND PUSH OF MVFR STRATUS TO MOVE ACROSS THE TAF SITES. OTHER
LOCATIONS ACROSS THE GREAT LAKES HAVE SEEN THINNING OF THIS
STRATUS...SO IT IS POSSIBLE THAT THE MVFR CIGS SCATTER OUT
BRIEFLY...LEAVING A FL250 CIRRUS DECK IN PLACE. AT THIS POINT IT
SEEMS MORE LIKELY THAT CIGS HOVER AROUND 025-035 THROUGH THE
NIGHT.

INTO SATURDAY...MOISTURE WILL INCREASE IN THE LATE MORNING HOURS
WITH LIGHT SHOWERS LEADING THE PRECIPITATION SHIELD. ONCE THE
COLUMN FURTHER SATURATES BY 18Z...RAIN WILL INCREASE /POSSIBLY TO
MODERATE INTENSITY/ WITH CIGS AND VSBYS DROPPING TO MVFR OR IFR.
WINDS REMAIN FROM THE EAST FOR THE ENTIRE EVENT WITH A FEW GUSTS
NEAR 22KT. IFR CIGS ARE EXPECTED AFTER THE RAIN SUBSIDES AS THE
CENTER OF LOW PRESSURE PASSES VERY NEAR CHICAGO.

MM

&&

.MARINE...
256 PM CDT

WITH STRONG HIGH PRESSURE PARKED NORTH OF THE GREAT LAKES AND LOW
PRESSURE DEVELOPING OVER THE SRN PLAINS...GENERALLY NELY FLOW WILL
CONTINUE OVER LAKE MICHIGAN.  INITIALLY...THE PRESSURE GRADIENT IS
RELATIVELY WEEK...BUT AS THE SFC LOW TRACKS NORTH INTO THE MISSOURI
VALLEY BY SATURDAY EVENING...THE GRADIENT ACROSS THE STRENGTHEN WITH
PREVAILING WIND DIRECTION BECOMING MORE ENELY AT ARND 20-25 KT. THIS
FETCH AND WIND SPEED SHOULD SET UP SMALL CRAFT ADVISORY CONDITIONS
FOR THE ILLINOIS NEARSHORE WATERS BY LATE SATURDAY MORNING OR EARLY
SATURDAY AFTERNOON AND INTO SATURDAY EVENING.  THE LATEST GUIDANCE
SUGGESTS THAT THERE COULD BE A BRIEF LULL IN WIND SPEEDS SATURDAY
NIGHT...BUT THERE IS A CHANCE THAT THE SMALL CRAFT ADVISORY MAY NEED
TO BE EXTENDED INTO SUNDAY AS THE ENELY FETCH COULD KEEP WAVES
AROUND 4 FEET INTO SUNDAY AFTERNOON.  AS THE SFC LOW MOVES OFF TO
THE EAST TO THE MID ATLANTIC REGION SUNDAY NIGHT OR MONDAY
MORNING...HIGH PRESSURE WILL BUILD SOUTH ACROSS THE GREAT LAKES WITH
A RELATIVELY WEAK GRADIENT SETTING UP MONDAY AND PERSISTING THROUGH
MIDWEEK.

KREIN

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...LMZ740-LMZ741-LMZ742...10 AM SATURDAY TO
     10 PM SATURDAY.

&&

$$

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