Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 180817
AFDLOT

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

.SHORT TERM...
315 PM CDT

THROUGH SATURDAY...

MAIN WEATHER FEATURE OF NOTE THIS AFTERNOON ON W/V IMAGERY IS
PROMINENT CLOSED UPPER LOW OVER THE FOUR CORNERS REGION...RESULTING
IN BROAD RIDGING OVER THE TOP OF OUR AREA. PRESENCE OF MIDLEVEL
MOISTURE AND DEEP MIXING OF WARM AIRMASS TODAY ENABLED FAIRLY
EXTENSIVE HIGH BASED CU FIELD TO DEVELOP. MODEL RH FIELDS INDICATE
FAIRLY HIGH MOISTURE IN H9-H8 LAYER THROUGH THE NIGHT...SO STILL
EXPECTING GENERALLY PARTLY CLOUDY SKIES TONIGHT. IF CLEARING DOES
OCCUR...MAY NEED TO ADD PATCHY FOG/HZ IN OUTLYING AREAS AS SHOWN
ON MET MOS GUIDANCE. WEAK LAKE BREEZE BOUNDARY WILL WASH OUT THIS
EVENING...FOLLOWED BY LIGHT/VRB WINDS OVERNIGHT. AIRMASS WILL
REMAIN MILD AT AROUND +10C AT 850 MB AND 925 MB TEMPS IN MID TEENS
CELSIUS...SO EXPECTING OVERNIGHT LOWS TO ONLY DROP TO THE LOWER TO
MID 50S.

COLD FRONT BOUNDARY TRAILING FROM WEAK LOW PRESSURE MOVING INTO
QUEBEC SATURDAY MORNING WILL SERVE AS A BACKDOOR FRONT AND TURN
WINDS TO A GUSTY NORTHEAST TO EAST-NORTHEAST DIRECTION ON SATURDAY
NORTH OF I-80. THIS WILL RESULT IN TEMPERATURES ONLY TOPPING OUT
IN THE 50S ALONG THE LAKEFRONT...AS WELL AS COOLER CONDITIONS FOR
INLAND AREAS CLOSER TO THE LAKE. AIRMASS WILL STILL REMAIN
SIMILARLY MILD UNDER BROAD RIDGING ALOFT...SO AREAS WELL AWAY FROM
THE LAKE EXPECTED TO STILL REACH THE 70S...WITH UPPER 70S SOUTH OF
THE IL AND KANKAKEE RIVERS. MOISTURE MAY POOL AS FRONT SETTLES
INTO SOUTHERN/SOUTHWESTERN CWA...SO FAIRLY EXTENSIVE HIGHER BASED
CU FIELD COULD AGAIN DEVELOP. SOME GUIDANCE...NAMELY THE
GFS...BREAKS OUT SHOWER ACTIVITY DURING THE AFTERNOON CLOSER TO
THE FRONTAL BOUNDARY. HOWEVER...THIS APPEARS TO BE OVERDONE GIVEN
PRESENCE OF MID/UPPER RIDGING AND MEAGER INSTABILITY. CANNOT
COMPLETELY RULE OUT VERY ISOLATED SHRA/SPRINKLES IN SOUTH/SW
CWA...BUT HAVE MAINTAINED DRY FORECAST IN GRIDS.

RC

&&

.LONG TERM...
315 PM CDT

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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