Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 180838
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
338 AM CDT SAT APR 18 2015

.SHORT TERM...
337 AM CDT

THROUGH SUNDAY...

SOME CHANGES ARE IN THE AIR FOR THE NORTHERN HALF OF THE AREA TODAY
AND LOCATIONS CLOSER TO THE LAKESHORE...WHILE SOUTHERN AREAS WILL
ENJOY SUMMER LIKE CONDITIONS THAT PREVAILED ON FRIDAY.

THE CLOSED UPPER LOW THAT WAS OVER THE FOUR CORNERS AS OF THIS TIME
YESTERDAY IS NOW ONLY CENTERED OVER SOUTH CENTRAL COLORADO. THE
CLOUD SHIELD IN THE WARM MOIST ADVECTION SECTOR AHEAD OF THE LOW IS
GETTING CLOSER TO THE AREA...BUT STILL LARGELY SOME MID AND HIGH
CLOUDS DRIFTING THROUGH. VERY MILD CONDITIONS ARE IN PLACE WITH
DEWPOINTS HOLDING IN THE MID 40S NORTH TO LOWER 50S SOUTH KEEPING
TEMPS IN THE 50S OUTLYING AREAS TO NEAR 65 DOWNTOWN IN THE HEAT
ISLAND DUE TO THE MILD SW OFFSHORE FLOW.

TODAY AND TONIGHT...THE FIRST CHANGES COME IN THE FORM OF A SHALLOW
COLD FRONTAL BOUNDARY THAT WILL SINK SOUTHWARD DOWN THE LAKE IN THE
COMING HOURS...WITH NORTHERLY WINDS SHIFTING NE THIS MORNING. EXPECT
A RAPID TEMPERATURE DROP DOWNTOWN JUST AFTER DAYBREAK AS THIS OCCURS
WITH TEMPS HOLDING STEADY IN THE 50S MUCH OF THE DAY. THIS BOUNDARY
WILL CONTINUE TO OOZE SOUTHWARD BUT WILL BE HALTED BY THE INCOMING
SYSTEM AND WARM SSE FLOW.  THEREFORE EXPECT QUITE A CONTRAST IN
TEMPS TODAY AS HIGHS IN THE TIER OF COUNTIES CLOSE TO THE LAKE PEAK
THIS MORNING IN THE 60S THEN COOL...WHILE AREAS SOUTH LIKE PONTIAC
REALIZE FULL MIXING AGAIN AND PUSH THE UPPER 70S. EXPECT MORE CLOUDS
TO FILTER IN THROUGH THE DAY...NOT ONLY ALONG THE FRONT BUT IN WARM
SECTOR AS WELL. SOME HI-RES GUIDANCE TRIES TO BREAK OUT SOME SHOWERS
ALONG THE BOUNDARY AS WELL...BUT HAVE MAINTAINED A DAY FCST AT THIS
TIME WITH THE UPPER LEVEL RIDGE IN PLACE LIKELY KEEPING CLOUDS
SHALLOW IN NATURE. WITH THE SYSTEM CONTINUING ITS SLOW MOVEMENT
TONIGHT..THE SURFACE LOW WELL TO THE SOUTH AND MOISTURE ADVECTION
CONFINED THERE AS WELL...HAVE CONTINUED PREVIOUS SHIFTS TREND TO
SLOW POPS AND GONE DRY OVERNIGHT AS WELL.

SUNDAY...SURFACE LOW PRESSURE DOWNSTATE INTENSIFIES AND LIFTS NE
AHEAD OF THE INCOMING COLORADO UPPER LEVEL LOW. MODELS DISAGREE ON
THE STRENGTH OF THE INDIVIDUAL LOWS THAT FORM...BUT MANY DO LIFT THE
FIRST ONE THROUGH EASTERN IL/WESTERN IN. AS THIS OCCURS...A WARM
FRONT WILL LIFT NORTHWARD AND BECOME THE FOCUS FOR SHOWER ACTIVITY
WITH A 30-40 KT LOW LEVEL SE FLOW DRIVING INTO THE FRONT. WITH
PRECIPITABLE WATER VALUES UP AT 1.25 INCHES OR SO...SOME 2 S.D.
ABOVE NORMAL AS NOTED IN THE SPC PRECIPITATION DISCUSSION...ALONG
WITH A DEEP MOIST COLUMN AND HIGH FREEZING LEVELS SHOULD PROMOTE
EFFICIENT RAIN PRODUCERS. THE FOCUS OF THIS ACTIVITY AT THIS TIME
WILL BE ALONG THE EASTERN HALF OF OUR AREA AS IT LIFTS
NORTH...THOUGH THIS A COMPLICATED EVOLUTION WITH SEVERAL SMALL SCALE
DISTURBANCES IN PLACE.  INSTABILITY IS WEAK AND VERY HIGH
BASED...BUT DECENT FORCING AND PARCELS LIKELY UNDERGOING CHARGE
SEPARATION...ISOLATED THUNDER MENTION SEEMS A REASONABLE COURSE OF
ACTION.

KMD

&&

.LONG TERM...
315 PM CDT (NOTE UPDATED MORNING DISCUSSION COMING SOON)

SATURDAY NIGHT THROUGH FRIDAY...

MAIN UNCERTAINTY IS WITH PROGRESSION OF ENERGY EMANATING FROM
UPPER LOW AS IT OPENS UP AND TRACKS EAST. TYPICAL TREND IS SLOWER
WITH THESE SYSTEMS AND OTHER THAN GEM BEING FAST OUTLIER...THIS IS
HOW MOST GUIDANCE TRENDED TODAY. WEAKENING MIDLEVEL SHORT
WAVE/VORT MAX SHOULD PASS OVER WESTERN CWA SATURDAY NIGHT...BUT
DRIER EAST-SOUTHEAST FLOW SHOULD KEEP MUCH OF THE AREA DRY. ONLY
HAVE LOW CHANCE POPS DURING EVENING IN FAR WESTERN CWA. INCREASING
CLOUDS WILL KEEP MOST LOWS IN 50S....EXCEPT 40S ALONG IL SHORE.

CONFIDENCE IS HIGH IN THERE BEING RAIN SHOWERS ON SUNDAY AS
SURFACE LOW ASSOCIATED WITH UPPER LEVEL ENERGY DEVELOPS AND TRACKS
TOWARD AREA. HAVE KEPT CATEGORICAL POPS CONFINED TO SOUTH OF I-80
THROUGH MID-DAY THEN SPREAD THEM NORTH FOR THE AFTERNOON. MODELS
IN GOOD AGREEMENT ON SYNOPTIC SETUP...BUT EXPECT CHANGES IN TRACK
OF SURFACE LOW...WHICH WILL AFFECT PLACEMENT OF BANDS OF HEAVIER
RAINFALL IN SEASONABLY MOIST AIRMASS ADVECTED NORTHWARD. FAIRLY
COOL TEMPERATURES ALOFT COULD SUPPORT AN ISOLATED TSRA
THREAT...BUT PROGGED MUCAPE AND MIDLAPSE RATES DO NOT SUPPORT MORE
THAN SLIGHT CHANCE TSRA MENTION. CLOUDS WILL KEEP INLAND TEMPS IN
60S...WITH CURRENT FAVORED TRACK OF SFC LOW RESULTING IN STEADY
ONSHORE WINDS ESPECIALLY INTO IL AND HIGHS ONLY AROUND 50.

UPPER LEVEL TROUGHINESS WILL LINGER OVER THE REGION THROUGH MUCH
OF NEXT WEEK THANKS TO REDEVELOPMENT OF HIGHLY AMPLIFIED WESTERN
RIDGING. THEREFORE...THE NICE PATTERN WE HAVE BEEN IN WILL BE
REPLACED BY UNSEASONABLY COOL TEMPS...WITH HIGHS IN 50S AND LOWS
ONLY IN 30S AND 40S. BEST CHANCE FOR RAIN SHOWERS WILL BE ON
MONDAY WITH STRONGER UPPER DISTURBANCE WRAPPING AROUND LARGE UPPER
TROUGH/LOW...WITH CHANCES THE REST OF THE WEEK TIED TO FAST MOVING
TOUGH TO PIN DOWN DISTURBANCES IN WEST-NORTHWEST FLOW. ANY NIGHTS
THAT CLEAR OUT COULD PRESENT A CHANCE FOR AT LEAST PATCHY FROST.

RC

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 09Z...

* LIGHT VRB WINDS GENERALLY SOUTHWEST ARND 4KT...TURNING
  NORTH/NORTHEAST AND INCREASING ARND DAYBREAK. GUSTS DEVELOPING
  UP TO 20KT.

* WINDS TURN EAST LATE IN THE AFTN.

BEACHLER

//DISCUSSION...UPDATED 06Z...

WEAK RIDGING REMAINS OVER NORTHERN IL/NORTHWEST IN AIRFIELDS EARLY
THIS MORNING...ALLOWING VRB WIND DIRECTION AND SPEEDS GENERALLY
ARND 4KT OR LESS. WINDS WILL TURN NORTHWEST BY 10Z THEN QUICKLY
TURN NORTH/NORTHEAST BY DAYBREAK. A FRONTAL BOUNDARY WILL SLIDE
SOUTH ACROSS THE WESTERN GREAT LAKES...ALLOWING AN INCREASING WIND
SPEED AND GUSTS TO DEVELOP BY MID-MORNING THRU LATE AFTN WITH
GUSTS TO 20KT AT TIMES. THE DURATION OF GUSTS DOESN/T APPEAR TO BE
LENGTHY...HOWEVER THE DIRECTION WILL BE TURNING NORTHEAST BETWEEN
020-050 DEG THEN MORE TOWARDS A 090 DIRECTION BY LATE AFTN/EARLY
EVE. CLOUDS SHUD THICKEN...HOWEVER REMAIN VFR WITH BASES ARND
4-6KFT AGL. ANY PRECIP SHUD HOLD OFF UNTIL AFT DAYBREAK
SUN...POSSIBLY UNTIL MIDDAY SUN.

BEACHLER

//ORD AND MDW CONFIDENCE...UPDATED 09Z...

* HIGH CONFIDENCE IN CIGS/VSBY FORECAST.

* HIGH CONFIDENCE IN WINDS REMAINING LGT/VRB THRU
  10Z...MEDIUM/HIGH CONFIDENCE IN TIMING OF WINDS TURNING
  NORTHEAST WITH GUSTS DEVELOPING.

BEACHLER

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

SUNDAY...SHRA/ISOLATED TSRA. MVFR LIKELY/IFR POSSIBLE. EAST WINDS
BECOMING NORTHEAST.

MONDAY...CHANCE OF SHRA/MVFR EARLY...IMPROVING TO VFR. GUSTY WEST
WINDS.

TUESDAY...SLGT CHANCE SHRA...BUT MAINLY VFR. GUSTY WEST WINDS.

WEDNESDAY...SLGT CHANCE SHRA. MAINLY VFR. WEST-NORTHWEST WINDS.

THURSDAY...VFR. NORTH-NORTHEAST WINDS.

RATZER

&&

.MARINE...
310 AM CDT

MAIN CONCERN FOR TODAY IS THE POTENTIAL FOR MARGINAL WINDS FOR
SMALL CRAFT CONDITIONS. HIGH PRESSURE REMAINS OVER THE WESTERN
GREAT LAKES...ALTHOUGH STEADILY PUSHING EAST. THIS WILL ALLOW A
FRONTAL BOUNDARY TO ENERGIZE AND SLIDE SOUTH ALONG THE LAKE TODAY
AND REACH THE SOUTHERN TIP OF THE LAKE BY LATE THIS AFTERNOON.
THIS WILL ALLOW WINDS TO TURN WEST TO NORTH/NORTHEAST AND INCREASE
TO 15 TO 25 KT. IT IS POSSIBLE THAT A TIGHTER GRADIENT WILL OCCUR
ALONG THE CENTRAL/SOUTHERN PORTIONS OF THE LAKE...WITH WINDS MORE
IN THE 20 TO 25 KT RANGE. THIS MAY RESULT IN SMALL CRAFT WINDS FOR
THE ILLINOIS NEARSHORE THRU THIS AFTERNOON. WINDS WILL BE SLIGHTLY
LESS ROBUST FOR THE INDIANA SHORE...WITH WAVES GENERALLY ONLY
BUILDING TO 3 TO 5 FT WITH UP TO 7 FT WAVES AT TIMES. GIVEN THE
AIR IS SLIGHTLY WARMER THAN THE LAKE SURFACE...IT IS POSSIBLE
WAVES MAY NOT MAXIMIZE. BUT THIS IS A LOW CONFIDENCE AT THIS TIME.

THE GRADIENT BRIEFLY RELAXES BEFORE INCREASING AHEAD OF A
DEVELOPING LOW PRESSURE SYSTEM LATE IN THE WEEKEND INTO EARLY MON.
WINDS TURN EAST TO NORTHEAST...WITH SPEEDS INCREASING YET AGAIN TO
15 TO 25 KT. A FEW GUIDANCE MEMBERS INDICATE WINDS COULD BRIEFLY
TOUCH 30 KT AHEAD OF THE LOW PRESSURE SYSTEM. WITH THE LOW NOW
POISED TO REMAIN SOUTH OF THE LAKE...THE BEST GRADIENT MAY BE
ALONG THE SOUTHERN TIP OF THE LAKE SUNDAY NIGHT/MON.

BEACHLER

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...LMZ740-LMZ741-LMZ742...9 AM SATURDAY TO 4
     PM SATURDAY.

&&

$$

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