Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 242215
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
515 PM CDT FRI APR 24 2015

.SHORT TERM...
323 PM CDT

THROUGH SATURDAY NIGHT...

SHORT TERM...THROUGH SATURDAY NIGHT...

PRIMARY FORECAST CONCERN IS WITH FAIRLY STRONG LOW PRESSURE
SYSTEM TRACKING SOUTH OF THE AREA ON SATURDAY AND ASSOCIATED RAIN
AND AMOUNTS.

DRY AIR MASS REMAINS OVER THE AREA THIS AFTERNOON...BUT WITH
GRADUAL MOISTENING FROM BONE DRY CONDITIONS EARLIER TODAY THAT
WERE AMONGST DRIEST IN ENTIRE COUNTRY. W/V IMAGERY SHOWS SHORT
WAVE/MIDLEVEL SPEED MAX INTO EASTERN IA THAT IS RESULTING IN
OCCASIONAL VERY LIGHT RAIN/SPRINKLES REACHING THE GROUND DESPITE
LARGE DEWPOINT DEPRESSIONS. SUSPECT THAT HRRR MODEL IS TOO
AGGRESSIVE WITH HIGHER REFLECTIVITY/RAIN REACHING THE GROUND SOUTH
OF I-88 OR SO AND FOCUS FOR MOST ISOLATED TO SCATTERED ACTIVITY
SHOULD BE IN FAR NORTHERN CWA.

LATER THIS EVENING INTO THE OVERNIGHT...AREA WILL BE IN BROAD
WARM ADVECTION ALOFT BUT ALSO LACKING FOCUS FOR PRECIP WITH
MIDLEVEL RIDGING OVERHEAD. SUSPECT THAT THERE WILL BE SEVERAL
MAINLY DRY HOURS WITH PERHAPS A FEW ISOLATED SHOWERS. THEN MAIN
WAVE WILL START TO SPREAD IN FROM SOUTHWEST AFTER MIDNIGHT INTO
SATURDAY MORNING AS CLOSED LOW AND ASSOCIATED SFC LOW MIGRATES
ACROSS LOWER MO VALLEY. PRECIP WILL BE FIGHTING DRY EASTERLY LOW
LEVEL FLOW ESPECIALLY IN NORTHEAST CWA...BUT THEN AS 700 MB FGEN
STRENGTHENS AND TROWAL LIKE FEATURE SPREADS ~1 TO ~1.25 INCH PWATS
ACROSS AREA...EXPECTING MID-LATE SATURDAY AM TO BE ACCOMPANIED BY
PERIODS OF RAIN/RAIN SHOWERS AND STEADY TO EVEN BRIEFLY HEAVY
RATES. VERY MINIMAL ELEVATED INSTABILITY WILL BE PRESENT WELL
SOUTH OF I-80...SO HAVE MAINTAINED SLIGHT EMBEDDED THUNDER CHANCES
THERE. STILL HAVE HIGH END LIKELY TO CAT POPS IN LATE
AFTERNOON...BUT DRIER AIR WORKING IN FROM NORTH SHOULD REDUCE
RATES...WITH PRECIP QUICKLY SHIFTING SOUTH DURING EVENING AND
SKIES CLEARING TOWARD EARLY SUNDAY.

RAIN AMOUNTS FROM MAIN SYSTEM LATE TONIGHT THROUGH SATURDAY
SHOULD RANGE FROM ABOUT 1/2 INCH IN FAR NORTHEAST TO A BIT UPWARDS
OF ONE INCH IN SOUTHWEST 1/4 TO 1/3 OF CWA. CONFIDENCE IN LOW-
MEDIUM IN THESE AMOUNTS GIVEN POTENTIAL FOR BANDING PLACEMENT TO
SHIFT.

REGARDING TEMPS...INCREASING CLOUDS WILL LIMIT FALL TONIGHT...SO
GENERALLY IN 40S TONIGHT EXCEPT UPPER 30S NEAR LAKE COUNTY SHORE.
CLOUDS...RAIN AND GUSTY NORTHEAST FLOW WILL MAKE FOR RAW
CONDITIONS ON SATURDAY AND HIGHS ONLY RECOVERING TO THE LOW/MID
40S TO LOWER 50S...COOLEST ON ILLINOIS SHORE AND WARMEST SOUTH OF
I-80. TEMPS WILL THEN DIP BACK TO THE 30S BY EARLY SUNDAY WITH H8
TEMPS FALLING TO JUST BELOW 0 CELSIUS. EXPECTING SOME NORTHERN
INTERIOR LOCATIONS TO DIP TO AROUND FREEZING. OVERALL IT WILL BE
ANOTHER NIGHT TO PROTECT COLD SENSITIVE VEGETATION.

RC

&&

.LONG TERM...
333 PM CDT

SUNDAY THROUGH FRIDAY...

EXTENDED PERIOD THROUGH AT LEAST THURSDAY WILL BE DOMINATED BY
ONSHORE FLOW AND COOLER CONDITIONS NEAR THE LAKE...BUT ALSO
GRADUAL WARMING INLAND. WITH POSSIBLY MINOR EXCEPTION OF
WEDNESDAY...CONDITIONS SHOULD BE PRIMARILY DRY AS WELL. AIRMASS
WILL BE COOLEST SUNDAY THROUGH EARLY MONDAY...WITH ONLY MID-UPPER
40S ALONG SHORE AND MID-UPPER 50S INLAND AND THEN OVERNIGHT LOWS
SUNDAY NIGHT AWAY FROM CHICAGO HEAT ISLAND IN LOWER-MID 30S. AS
HIGH PRESSURE DROPS SOUTHEAST TOWARD EARLY MONDAY...WINDS MAY
DIMINISH ENOUGH TO SUPPORT AT LEAST PATCHY FROST...BUT HAVE NOT
ADDED IT TO GRIDS YET. INLAND HIGHS SHOULD BE INTO THE 60S EACH
DAY MON-THU AND TEMPS COULD APPROACH 70 NEXT FRIDAY.

RC

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 22Z...

* EAST-NORTHEAST WINDS AROUND 10 KT GRADUALLY INCREASING LATER
  TONIGHT AND SATURDAY MORNING.

* AREA OF LIGHT RAIN EXPECTED TO MOVE IN BY 00Z...WITH GREATEST
  COVERAGE FROM ORD NORTH. UPSTREAM CONDITIONS INDICATE VFR CIGS
  WITH OCNL MVFR VIS WITH RA/SHRA. RAIN LINGERING THROUGH 03Z OR
  SO WITH A LULL POSSIBLE AFTER THAT.

* RAIN SPREADING OVER DURING THE MID TO LATE MORNING ON SATURDAY
  WITH IFR CONDITIONS DEVELOPING A FEW HOURS AFTER ONSET.

//DISCUSSION...UPDATED 18Z...

SOUTH-SOUTHEAST WINDS ARE GRADUALLY INCREASING ACROSS THE REGION
AS HIGH PRESSURE DEPARTS. ON THE LOCAL SCALE...A LAKE BREEZE IS
SEEN ON OBSERVATIONAL DATA INCHING WESTWARD ACROSS NORTHEAST
ILLINOIS. WITH THE DEEP MIXING THAT HAS OCCURRED OVER LAND...THAT
WILL LIKELY CAUSE THE LAKE BREEZE TO FLUCTUATE SOME IN SPEED.

MOISTURE WILL SLOWLY RETURN AS LOW PRESSURE CONTINUES TO ORGANIZE
IN THE CENTRAL PLAINS. THE EXTREME LOW-LEVEL DRYNESS WILL
INITIALLY PREVENT RADAR ECHOES FROM MATERIALIZING AS RAIN SHOWERS
THIS AFTERNOON OVER CHICAGO AIRPORTS...THOUGH SOME SPOTTY LIGHT
RAIN WILL LIKELY BE SEEN NEAR OR AT RFD LATER TODAY. AFTER
DARK...AS A WEAK UPPER DISTURBANCE MOVES ACROSS FAR SOUTHERN
WISCONSIN...LIGHT SHOWERS MAY MOVE NEAR/OVER THE CHICAGOLAND TAF
SITES.

THE PRIMARY PERIOD OF RAIN CONTINUES TO LOOK LIKE SATURDAY
THOUGH HAVE SLOWED THE ONSET TIME IN THE FORECAST UNTIL THE
PROJECTED STRONGER FORCING ARRIVES. THIS LOOKS TO BE MID TO LATE
MORNING. IFR CIGS AND AT LEAST TEMPORARY VISIBILITY OF 3SM OR
LOWER CAN BE EXPECTED DURING THE MODERATE RAIN. EASTERLY WINDS
WILL PREVAIL THROUGH THIS TIME GIVEN LOW PRESSURE TO THE SOUTH OF
THE AREA.

//ORD AND MDW CONFIDENCE...UPDATED 22Z...

* MEDIUM-HIGH IN WINDS.

* MEDIUM-HIGH IN PERIOD OF LIGHT RAIN/SHOWERS STARTING AROUND 00Z
  ESPECIALLY FROM ORD NORTH AND WEST. MEDIUM HIGH IN VFR CIGS AND
  VFR/MVFR VIS IN HEAVIER SHOWERS.

* HIGH IN IFR CONDITIONS IN RAIN ON SATURDAY. MEDIUM IN TIMING.

//OUTLOOK FOR ORD/MDW FOR 00Z SUNDAY-12Z FRIDAY...UPDATED 12Z...

* SATURDAY NIGHT THROUGH THURSDAY...VFR WITH PRIMARILY NORTHEASTERLY
  WINDS.

MTF/RATZER

&&

.MARINE...
215 PM CDT

HIGH PRESSURE MOVING ACROSS THE CENTRAL GREAT LAKES REGION HAS SET
UP GENERALLY LIGHT AND VARIABLE WINDS OVER LAKE MICHIGAN. THE
LIGHT WINDS REGIME AND LAND-LAKE DIFFERENTIAL HEATING HAS SET UP A
LAKE BREEZE WITH NEARSHORE WINDS TAKING AN ONSHORE ORIENTATION
THIS AFTERNOON. AS THE HIGH MOVES OFF TO THE EAST TONIGHT...LOW
PRESSURE OVER THE CENTRAL PLAINS WILL DEEPEN AND MOVE EAST TO NWRN
MO OVERNIGHT TONIGHT. A DEVELOPING WARM FRONT WILL EXTEND EAST
FROM THE LOW ACROSS CNTRL IL/IN...WITH WINDS BECOMING EASTERLY BY
LATE TONIGHT. THE WARM FRONT IS EXPECTED TO REMAIN NEARLY
STATIONARY AS THE LOW TRACKS EWD...KEEPING WINDS OVER THE LAKE
ELY.  AS THE LOW PASSES SOUTH OF THE LAKE AND A RIDGE OF HIGH
PRESSURE BUILDS FROM HUDSON BAY THROUGH THE NRN GREAT LAKES...WIND
SPEEDS WILL INCREASE WITH THE STRONGEST WINDS OF 25-30 KT OVER THE
SRN PORTIONS OF THE LAKE. AS THE LOW PASSES EAST THROUGH THE OHIO
VALLEY SATURDAY NIGHT INTO SUNDAY MORNING...WINDS WILL BACK TO
NORTHERLY. WHILE GALES ARE NOT LIKELY...WINDS OVER THE SRN
PORTIONS OF THE LAKE WILL BE STRONG ENOUGH TO REQUIRE A SMALL
CRAFT ADVISORY AND EVEN THOUGH WINDS SHOULD WEAKEN BELOW SMALL
CRAFT ADVISORY LEVELS WITH THE SHIFT TO NORTHERLY...THE LONG FETCH
SHOULD BE SUFFICIENT TO KEEP SMALL CRAFT ADVISORY LEVEL WAVES INTO
MONDAY MORNING. HIGH PRESSURE WILL BUILD ACROSS THE LAKE BY
TUESDAY MORNING AND REMAIN THE DOMINANT FEATURE THROUGH MUCH OF
NEXT WEEK...SETTING A PERIOD OF RELATIVELY QUIET CONDITIONS OVER
THE LAKE FROM TUESDAY THROUGH THE END OF THE FORECAST PERIOD.

KREIN

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...NONE.
&&

$$

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