Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 250036
AFDLOT

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

.UPDATE...

736 PM...EVENING UPDATE...BUMPED POPS UP QUITE A BIT FOR THIS
EVENING. UPPER WAVE MOVING ACROSS THE AREA PRODUCING CONVECTIVE
SHOWERS WITH SOME EMBEDDED LIGHTNING NEAR THE QUAD CITIES.
INSTABILITY ALOFT SHOULD WEAKEN AS THIS WAVE SHIFTS EAST...BUT DID
ADD A SLIGHT CHANCE OF THUNDER ACROSS THE SOUTHWEST CWA THRU MID
EVENING. WHILE THE LOW LEVELS REMAIN DRY INITIALLY...RAINFALL IS
SUFFICIENT TO MOISTEN THE LOW LEVELS WITH DEWPOINTS RISING THRU
THE 30S ACROSS NORTHWEST IL AND RFD MEASURED 0.14 OF RAIN THRU
00Z. HOWEVER...THE DRY LOW LEVELS COULD LEAD TO SOME BRIEF/MIXED
SLEET AT ONSET. HRRR HAS BEEN HANDLING THIS WAVE AND ITS
ASSOCIATED PRECIP QUITE WELL AND SHOWS PRECIP SHIFTING EAST OF THE
AREA BY LATE EVENING SO TRENDED POPS BACK LOWER TOWARD MIDNIGHT.
THEN RAIN CHANCES INCREASE IN THE PREDAWN HOURS INTO SATURDAY
MORNING AS THE MAIN LOW ARRIVES AND MADE NO CHANGES TO THIS TIME
PERIOD.

TEMPS HAVE ALREADY DROPPED INTO THE LOWER/MID 40S WHERE RAIN IS
FALLING AND WILL LIKELY NEED TO MAKE SOME DOWNWARD ADJUSTMENTS TO
TEMPS FOR TONIGHT...ESPECIALLY NORTH OF I-80. CMS

&&

.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 00Z...

* EAST-NORTHEAST WINDS 10-15 KT THIS EVENING BECOMING EAST AND
  INCREASINGLY GUSTY LATE TONIGHT AND SATURDAY. WINDS SHIFT
  NORTHEAST SATURDAY EVENING.

* AREA OF LIGHT RAIN/SHOWERS THIS EVENING...HEAVIEST
  NORTH/NORTHWEST OF CHICAGO. SCATTERED/ISOLATED SHOWERS
  OVERNIGHT.

* STEADY RAIN DEVELOPING MID-LATE MORNING SATURDAY WITH CONDITIONS
  DETERIORATING TO IFR BY MIDDAY. IFR LIKELY TO PERSIST THROUGH
  MUCH OF THE DAY...WITH FAIRLY QUICK IMPROVEMENT TO MVFR/VFR
  DURING SATURDAY EVENING.

RATZER

//DISCUSSION...UPDATED 00Z...

LOW PRESSURE DEVELOPING OVER WESTERN KANSAS EARLY THIS EVENING
WILL MOVE EAST ACROSS NORTHERN MISSOURI AND INTO DOWNSTATE
ILLINOIS THROUGH SATURDAY. DRY EASTERLY FLOW FROM RETREATING HIGH
PRESSURE OVER THE GREAT LAKES REGION WILL HELP TO SLOW THE ADVANCE
OF WIDESPREAD PRECIPITATION TONIGHT AS A LEAD MID-LEVEL
DISTURBANCE LIFTS ACROSS THE AREA...THOUGH THE COLUMN IS EXPECTED
TO SATURATE AS THE MAIN LOW AND MID-LEVEL TROUGH MOVE ACROSS THE
REGION SATURDAY. LIGHT RAIN/SHOWERS OVER NORTHWEST AND NORTH
CENTRAL ILLINOIS WILL EXPERIENCE SOME EROSION AS THEY WORK INTO
NORTHEAST IL THIS EVENING...WITH A LULL IN FORCING LEADING TO ONLY
ISOLATED/WIDELY SCATTERED RAIN LATER TONIGHT. STEADY RAIN WILL
OVERSPREAD THE REGION SATURDAY MORNING HOWEVER...WITH CONDITIONS
EXPECTED TO QUICKLY DETERIORATE TO IFR. RAIN/IFR CIGS AND IFR/MVFR
VIS TO LINGER THROUGH THE DAY...BEFORE WINDS SHIFT TO MORE NORTH-
NORTHEASTERLY IN LOW LEVELS SATURDAY EVENING AND PULL DRIER AIR
INTO THE AREA AS THE LOW SLIDES OFF ACROSS THE OHIO VALLEY.

AHEAD AND NORTH OF THE LOW TRACK...CURRENT NORTHEAST WINDS WILL
GRADUALLY INCREASE AND BECOME MORE EASTERLY OVERNIGHT. SUSTAINED
EAST WINDS 15-20 KTS DURING THE DAY SATURDAY WILL LIKELY BE A
LITTLE GUSTY AS WELL...THOUGH SHALLOW NATURE OF COOL SURFACE
MIXED LAYER MAY LIMIT GUST MAGNITUDE. WINDS SHIFT NORTHEAST
SATURDAY EVENING...AND MAY REMAIN GUSTY 20+ KTS INTO THE NIGHT.

RATZER

//ORD AND MDW CONFIDENCE...UPDATED 00Z...

* MEDIUM-HIGH IN MAINLY VFR LIGHT RAIN/SHOWERS THIS EVENING.

* MEDIUM-HIGH IN WIND DETAILS. LEAST CONFIDENT IN GUST SPEEDS
  SATURDAY AFTERNOON/EVENING.

* HIGH IN IFR CONDITIONS IN RAIN SATURDAY. MEDIUM CONFIDENCE IN
  TIMING ONSET AND EVENTUAL IMPROVEMENT.

RATZER

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

SUNDAY...VFR. GUSTY NORTH-NORTHEAST WINDS 20-25 KTS.

MONDAY...VFR. NORTHEAST WINDS.

TUESDAY...VFR. NORTHEAST WINDS BECOMING EAST.

WEDNESDAY...SLGT CHC -RA. BRIEF MVFR POSSIBLE. NORTH-NORTHWEST
WINDS.

THURSDAY...VFR. NORTH-NORTHEAST WINDS.

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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