Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 241914
AFDLOT

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

.UPDATE...
1115 AM CDT

NOT ANY SIGNIFICANT CHANGES MADE TO AFTERNOON AND EARLY EVENING
FORECAST. A MID-LEVEL SHORT WAVE ACROSS WESTERN/CENTRAL IA IS
HELPING TO FOCUS ASCENT WITHIN THE BROAD WARM ADVECTION ZONE
LEADING TO BANDS OF DBZ INCREASING ACROSS NORTHEAST IA AND
NORTHWEST IL. LITTLE OF THESE EASTERN ECHOES ARE REACHING THE
GROUND THANKS TO IMPRESSIVELY DRY AIR BELOW 6000 FT OR SO. SURFACE
DEW POINTS ARE IN THE LOWER TO MID TEENS ACROSS MOST OF THE AREA
WITH 40 DEGREE PLUS T/TD SPREADS...AND RESULTING RELATIVE HUMIDITY
VALUES UNDER 25 PERCENT. THIS SHOULD WIN OUT DURING THE DAYLIGHT
HOURS KEEPING MOST OF THE AREA DRY AND POTENTIALLY THROUGH EVEN
MID-EVENING. THE HIGH-RES GUIDANCE THAT SUPPORT THIS AND HAVE THIS
AFTERNOONS PRECIP MORE NORTH OF THE WI/IL BORDER HAVE THE MORE
ACCURATE DRIER LOW-LEVEL AIR ANALYZED. SO DO NOT FEEL INCLINED TO
SHIFT FROM CURRENT LOW POPS.

TEMPERATURES ARE GENERALLY ON TRACK. A LAKE BREEZE HAS DEVELOPED
BUT PRESENTLY IS HUGGING THE SHORE. THE DEEP MIXED BOUNDARY LAYER
DEVELOPING OVER LAND MAY HELP TO SLOW THE LAKE BREEZE PUSH INLAND
BUT AT THIS TIME STILL EXPECTED IT TO EASE INWARD BRINGING COOLING
TEMPERATURES IN LAKESIDE COUNTIES AS IT DOES.

MTF/RC

&&

.SHORT TERM...
252 AM CDT

THROUGH SATURDAY...

QUIET WEATHER ON TAP FOR MOST OF TODAY AS RIDGE AXIS CENTERED JUST
TO OUR EAST EARLY THIS MORNING SLOWLY SLIDES EAST ALLOWING RETURN
FLOW TO SPREAD MOISTURE UP ALONG WITH MISSISSIPPI RIVER VALLEY
AND INTO THE FORECAST AREA TONIGHT AHEAD OF LOW PRESSURE OVER THE
CENTRAL PLAINS. IN THE MEANTIME...EXPECT MORNING SUNSHINE TO GIVE
WAY TO INCREASING HIGH AND MID LEVEL CLOUD COVER. LIGHT WINDS IN
PLACE TODAY WILL ALLOW A LAKE BREEZE TO DEVELOP THIS AFTERNOON
ALONG THE INDIANA AND ILLINOIS SHORES. TEMPERATURES WILL WARM INTO
THE UPPER 50S TO AROUND 60 INLAND...BUT TEMPERATURES WILL LIKELY
STAY IN THE MID TO UPPER 40S ALONG THE LAKE FRONT.

CHANCES FOR PRECIP WILL INCREASE STARTING AROUND MID EVENING TONIGHT
AS SURFACE LOW MOVES INTO KS/NE AND LEADING EDGE OF MOISTURE PUSHES
INTO NORTHERN ILLINOIS ALONG WITH 850MB BAROCLINIC ZONE AND BROAD
WARM ADVECTION PROVIDING MODERATE ASCENT. DRY AIR BELOW 800MB WILL
HAVE TO BE OVERCOME BEFORE PRECIP REACHES THE SURFACE MAINLY LENDING
TO SOME UNCERTAINTY IN TIMING ONSET OF PRECIP. BY SATURDAY MORNING
EXPECT THE COLUMN TO BE SATURATED WITH DEEP MOISTURE IN PLACE AND
PWATS AROUND AN INCH. NAM INDICATES VERY MODEST INSTABILITY MAINLY
SOUTH OF A DIXON TO RENSSELAER LINE. VERY SKINNY CAPE PROFILES ARE
NOTED HOWEVER AND IT MAY BE DIFFICULT TO REALIZE THE FULL
POTENTIAL...SO WILL INTRODUCE ONLY A SLIGHT CHANCE OF THUNDER FOR
SOUTHWESTERN/SOUTHERN COUNTIES IN THE FORECAST AREA.

EXPECT RAIN ACROSS THE CWA MOST OF SATURDAY MORNING...THEN CHANCES
WILL BEGIN TO WIND DOWN THROUGH THE AFTERNOON AS THE SURFACE LOW
TRACKS ACROSS DOWNSTATE ILLINOIS AND BOUNDARY LAYER FLOW LOCALLY
COMES AROUND TO NORTHEASTERLY. PASSAGE OF THE MID LEVEL WAVE
SATURDAY EVENING AND ARRIVAL OF MID-LEVEL HEIGHT RISES SHOULD MARK
THE END OF PRECIP.

DEUBELBEISS

&&

.LONG TERM...
252 AM CDT

SATURDAY NIGHT THROUGH THURSDAY...

GENERALLY DRY CONDITIONS WITH A GRADUAL WARMING TREND IS EXPECTED TO
CLOSE OUT THE WEEKEND AND CONTINUE THROUGH MOST OF THE UPCOMING
WEEK. LOW PRESSURE THAT WILL BRING RAIN TO THE AREA FRIDAY
NIGHT/SATURDAY WILL MOVE TOWARDS THE MID ATLANTIC COAST SUNDAY WITH
HIGH PRESSURE BUILDING DOWN FROM THE CANADIAN PRAIRIES INTO THE
UPPER MIDWEST. NORTH/NORTHEAST FLOW IN THE MID-LEVELS WILL PUSH THE
BAROCLINIC ZONE BACK TO OUR SOUTH SUNDAY WITH H85 TEMPS FALLING TO
AROUND -6C TO -8C...ALBEIT BRIEFLY. THE HIGH WILL GENERALLY REMAIN
IN PLACE THROUGH MIDWEEK THOUGH WEAKENS THROUGH THIS PERIOD WITH THE
AIRMASS BECOMING INCREASINGLY MODIFIED BY LATE APRIL INSOLATION.
THIS SHOULD ALLOW TEMPS ONLY IN MID 50S SUNDAY TO GRADUALLY WARM
THROUGH THE UPCOMING WEEK WITH TEMPS PUSHING THE 70 MARK BY
THURSDAY. THE EXCEPTION THROUGH THE PERIOD WILL BE NEAR THE LAKE
FRONT WHERE ONSHORE FLOW IS EXPECTED THROUGH AT LEAST THE FIRST HALF
OF THE WEEK KEEPING CONDITIONS COOLER.

DEUBELBEISS

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 18Z...

* LAKE BREEZE PASSAGE TURNING WINDS EAST-NORTHEAST AND SPEEDS
  AROUND 10 KT BETWEEN 19Z-21Z.

* CHANCE FOR SCATTERED LIGHT RAIN SHOWERS BETWEEN 22Z AND 05Z.

* STRONGER EASTERLY WINDS ON SATURDAY.

* RAIN SPREADING OVER DURING THE MID TO LATE MORNING 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 18Z...

* MEDIUM-HIGH IN EAST-NORTHEAST WINDS INCREASING UP TO 10 KT THIS
  AFTERNOON AND MEDIUM IN TIMING.

* LOW-MEDIUM IN SCATTERED LIGHT RAIN SHOWERS EARLY TO MID EVENING.
  HIGH THAT IF ANY SHOWERS OCCUR THEY WILL BE LIGHT WITH VFR
  CONDITIONS.

* HIGH IN WINDS FROM TONIGHT THROUGH SATURDAY.

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

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

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

MTF

&&

.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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