Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 061754
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1254 PM CDT WED MAY 6 2015

.UPDATE...
1106 AM CDT

WE HAVE ALLOWED THE DENSE FOG ADVISORY TO EXPIRE ON SCHEDULE.
WEBCAMS SUGGEST THE DENSE FOG IS ISOLATED TO THE IMMEDIATE LAKE
SHORE AREAS. EXPECT THAT WILL REMAIN THE CASE FOR AT LEAST FOR A
FEW MORE HOURS...SO CAUTION IF TRAVELING NEAR THE LAKE...INCLUDING
LAKE SHORE DRIVE.

MEANWHILE...INLAND AREAS ARE WARMING UP QUICKLY...AND EXPECT THAT
TREND TO CONTINUE AS CLOUDS THIN. STILL A CHANCE FOR A FEW POP UP
SHOWERS/ISOLATED T-STORMS THIS AFTERNOON...THOUGH COVERAGE WOULD
BE VERY LOW.

KMD

&&

.SHORT TERM...
329 AM CDT

THROUGH THURSDAY...

NUMEROUS CHALLENGES TO OPEN UP TODAY WITH A WARM FRONTAL BOUNDARY
DRAPED ACROSS CENTRAL ILLINOIS AND DENSE FOG IN PLACES.
OBSERVATIONS AND WEBCAM IMAGERY INDICATE WIDESPREAD 3/4SM OR LESS
VISIBILITY ACROSS THE AREA...WITH SOME LOCATIONS UNDER 1/4SM OF A
MILE. AS OF 330 AM...THIS IS NOT WIDESPREAD ENOUGH TO EXPAND THE
ADVISORY BUT WILL CERTAINLY HIGHLIGHT IN SPS AND GRAPHICAST GOING
INTO THE MORNING COMMUTE. PATCHES OF GOES EXPERIMENTAL CLOUD
THICKNESS DATA WHERE AVAILABLE BETWEEN HIGHER CLOUDS AND AMDAR
DATA INDICATE A CLOUD DEPTH OF AROUND 700-1000 FT...WITH THE
LATTER INDICATING A SHARP INVERSION OF AROUND 15F IN THAT DEPTH.
WITH THE SYNOPTIC AREA NORTH OF THE WARM FRONT FAVORED...DO NOT
ENVISION THE FOG RAPIDLY DISSIPATING NORTH OF I-80 THIS
MORNING. WILL INCLUDE MENTION IN FORECAST GRIDS THROUGH MID TO
LATE MORNING. THE MOST VULNERABLE AREA FOR DENSE FOG THROUGH THE
MORNING WILL BE NEAR THE LAKE SHORE INCLUDING DOWNTOWN CHICAGO.
WILL CONTINUE THE ADVISORY FOR THIS AREA AS IS.

THE CLOUDS ARE EXPECTED TO ERODE/ADVECT NORTH LATE THIS MORNING INTO
EARLY AFTERNOON...BUT ONSHORE FLOW WILL CONTINUE INTO LAKE ADJACENT
COUNTIES. THE SURFACE WARM FRONT WILL ADVANCE NORTHWARD IN RESPONSE
TO THE UPPER LOW LIFTING NORTH ACROSS THE EASTERN ROCKIES REGION. IT
WILL NOT BE ENOUGH TO SHUNT THE LAKE EFFECT COOLING IN IMMEDIATE
NORTHEAST ILLINOIS TODAY...SO A DECENT TEMPERATURE DIFFERENCE WILL
OCCUR AGAIN IN THE AREA. HIGHS POSSIBLY REMAINING BELOW 60 WILL BE
SEEN ALONG THE IMMEDIATE LAKE SHORE...TO MID TO UPPER 70S NORTH OF
I-88...TO MID 80S SOUTH OF I-80. THE MID 80S ARE IMPRESSIVE FOR THIS
EARLY IN MAY...BUT VERIFIED YESTERDAY UPSTREAM WITH 14C AT 850MB AND
21C AT 925MB ON THE 00Z ILX SOUNDING...AND WIDESPREAD OBSERVED MID
80S IN CENTRAL IL.

THE SYNOPTIC WARM FRONT WHILE BECOMING SOMEWHAT LOOSELY DEFINED IN
THE WIND FIELD MAY BE ENOUGH WITH THE LAKE BREEZE TO PROVIDE A
SOURCE OF LIMITED BUT POOLED INSTABILITY AND CONVERGENCE ACROSS
NORTH CENTRAL IL INTO THE NORTHWEST CHICAGO METRO. CONTINUE WITH
THE SLIGHT CHANCE OF AN AFTERNOON SHOWER OR STORM THERE...THOUGH
WILL DEPEND ON JUST HOW QUICKLY THE CLOUDS DEPART. COVERAGE SHOULD
BE LIMITED IF ANY CAN DEVELOP.

TONIGHT WILL POTENTIALLY SEE SOME FOG AGAIN FOR LAKESIDE AREAS AND
FAR NORTHERN IL...THOUGH THE WARM FRONT IS EXPECTED TO SHIFT WINDS
SOUTHERLY BEFORE DAYBREAK THURSDAY. THIS WILL LEAD TO A
LIKELY WARMING OVERNIGHT IN LAKE ADJACENT AREAS.

THURSDAY WILL SEE THE LOW-LEVEL THERMAL RIDGE ESTABLISHED OVER THE
AREA AND STRONGER LOW-LEVEL FLOW AS PRESSURE FALLS SPREAD EASTWARD
IN THE UPPER MIDWEST. THE 850MB TEMPERATURES OF 14C WITH DEEP
ENOUGH MIXING TO THAT LEVEL ARE EXPECTED ACROSS THE ENTIRE FORECAST
AREA WITH NO LAKE ADJACENT COOLING. COMPARED TO 24 HOURS AGO...THERE
IS A STRONGER SIGNAL FOR SOME CLOUDS...BOTH CIRRUS AND SOME
DIURNAL CUMULUS. HOWEVER...AT LEAST LOWER 80S STILL LOOK LIKE A
SOLID BET...WITH MID 80S FAVORED IF CLOUDS ARE NOT TOO THICK OR
PERSISTENT.

MTF

&&

.LONG TERM...
345 AM CDT

THURSDAY NIGHT THROUGH TUESDAY...

A DIGGING OF THE NORTHERN STREAM JET ALONG WITH AN INTENSIFYING
SUBTROPICAL JET INTO THE SOUTHWEST U.S. WILL CARVE A BROAD
LONGWAVE TROUGH WITH LIKELY EMBEDDED UPPER LOW ACROSS THE WESTERN
PART OF THE COUNTRY. THIS WILL ESTABLISH SOUTHWEST FLOW ALOFT AND
AN INCREASING SUBTROPICAL MOISTURE PLUME INTO THE MIDWEST/CORN
BELT REGION GOING INTO FRIDAY AND THE WEEKEND. SEVERAL MID-LEVEL
IMPULSES ARE EVIDENT IN MODEL GUIDANCE AND WOULD BE EXPECTED IN
SUCH A PATTERN. UNFORTUNATELY THAT PROVIDES LOW PREDICTABILITY OF
SHOWER AND STORM TIMING...EVEN WITHIN A GIVEN 12-HR PERIOD.

LOOKING AT RUN-TO-RUN CONSISTENCY IN MODEL GUIDANCE...THERE IS
GOOD AGREEMENT IN MOISTURE TRANSPORT AND FOCUSED WARM ADVECTION
ALOFT THURSDAY NIGHT TO THE POINT WHERE SOME STORMS IN THE REGION
ARE PROBABLE. FRIDAY AFTERNOON/EVENING IS ALSO A PRONOUNCED
OVERLAP IN GUIDANCE OF A SHORT WAVE TRAVERSING THE EASTERN EDGE
OF THE LONG WAVE TROUGH. HAVE LIKELY POPS IN THAT TIME PERIOD.

THE TEMPERATURE FORECAST IS ALSO A NOTEWORTHY CHALLENGE GIVEN THE
PROBABLE SHOWERS AT TIMES...BUT ALSO A NORTHERN STREAM SURFACE
TROUGH INCHING INTO THE AREA. THIS LOOKS TO ALMOST MIRROR THE
SURFACE SETUP WE ARE IN NOW...WITH THE BOUNDARY DRAPING INTO THE
AREA OR JUST SOUTH DURING THE WEEKEND. IF THIS PANS OUT THIS WILL
COOL NORTHERN AREAS CONSIDERABLY...ESPECIALLY NEAR THE LAKE SHORE.
CONTINUE TO LEAN THAT ROUTE. CONFIDENCE IS REALLY LOW ON
TEMPERATURES GOING INTO MONDAY AS THE FRONTAL PLACEMENT VARIES
GREATLY ON LONG RANGE GUIDANCE. THERE WILL ALSO ONCE AGAIN BE
POTENTIAL FOR FOG IN SUCH A SETUP.

THE CHANCES FOR SEVERE STORMS MAY SLOWLY INCREASE IN THE LONG
RANGE PART OF THE FORECAST BUT LIKELY VERY SLOWLY. IT WOULD
APPEAR A MORE FAVORED SYNOPTIC PATTERN IS PRESENT SUNDAY NIGHT
INTO MONDAY WHEN SHARPER HEIGHT FALLS ARE AGREED UPON IN CURRENT
GLOBAL GUIDANCE TO OVERSPREAD THE AREA. VERY LOW CONFIDENCE THOUGH
AT THIS TIME.

MTF

&&

.AVIATION...

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

* LAKE BREEZE TURNING WINDS FROM SELY TO ELY EARLY AFTERNOON.

* WIND SPEED LESS THAN 8 KT BEFORE AND AFTER LAKE BREEZE PASSAGE.

* CHANCE MVFR HZ/BR THIS EVENING.

KREIN

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

HIGH PRESSURE EXTENDS FROM THE UPPER GREAT LAKES THROUGH NEW
ENGLAND WHILE LONG WAVE TROUGHING DEVELOPS OVER THE WESTERN
PLAINS. A WEAK WARM FRONT IS SLOWLY MOVING NWD THROUGH IL/IN AND
IS CURRENTLY ALONG THE ILLINOIS/KANKAKEE RIVERS. NORTH OF THE
FRONT...THE MORNING FOG AND STRATUS HAS DISSIPATED AND LIGHT SELY
WINDS HAVE SET UP. A WEAK LAKE BREEZE IS BEGINNING TO MOVE INLAND
AND SHOULD PUSH THROUGH THE ORD/MDW AREA BY AROUND 20Z.
HOWEVER...THE BOUNDARY IS VERY FAINT ON RADAR SO TIMING IS LOW
CONFIDENCE RIGHT NOW. CONFIDENCE IS HIGHER THAT WIND SPEED SHOULD
REMAIN LESS THAN 8KT BEFORE AND AFTER THE LAKE BREEZE PASSAGE.

THE WARM FRONT IS EXPECTED TO BE SOUTH OF THE TERMINALS THROUGH
THE EVENING HOURS...SO WITH SOME RESIDUAL LOW LEVEL MOISTURE AND
LIGHT SELY WINDS OFF OF LAKE MICHIGAN...THERE IS A CHANCE FOR SOME
VIS RESTRICTION THIS EVENING. THIS IS ALSO A LOW CONFIDENCE
FORECAST...BUT IF ANY HZ/BR DOES DEVELOP...IT VIS SHOULD ONLY DROP
TO MVFR LEVELS.

LATEST MODEL GUIDANCE SUGGESTS THAT THE WARM FRONT SHOULD PUSH
NORTH OF THE TERMINALS BY ARND 06Z WITH WINDS SHIFTING TO
SOUTHERLY. INITIALLY WIND SPEED SHOULD BE LESS THAN 10KT FOR THE
OVERNIGHT AND EARLY MORNING HOURS...BUT BY LATE MORNING...DEEP
LAYER MIXING SHOULD BE ABLE TO TAP THE STRONGER WINDS ALOFT WITH
SLY GUSTS OF 20-25KT FROM LATE MORNING THROUGH THE AFTERNOON.

KREIN

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

* HIGH CONFIDENCE IN WIND SPEED WILL BE LESS THAN 10 KT FROM THIS
  AFTERNOON THROUGH EARLY TOMORROW MORNING.

* MEDIUM-HIGH CONFIDENCE IN SOUTHERLY WINDS GUSTING TO 20-25KT
  LATE TOMORROW MORNING THROUGH THE AFTERNOON.

* MEDIUM CONFIDENCE IN MVFR VIS THIS EVENING.

* HIGH CONFIDENCE IN ALL OTHER FORECAST ELEMENTS.

KREIN

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

THURSDAY NIGHT...CHC TSRA. VFR. SOUTH WINDS.

FRIDAY...SHRA LIKELY...CHC TSRA. MVFR PSBL OVERNIGHT. SOUTHWEST
WINDS.

SATURDAY...CHC SHRA. MVFR PSBL. NORTH WINDS BCMG NORTHEAST.

SUNDAY...CHC SHRA/TSRA. MVFR PSBL. NORTHEAST TO EAST WINDS.

MONDAY...TSRA LIKELY. MVFR PSBL. SOUTH WINDS BCMG NORTHWEST
OVERNIGHT.

TUESDAY...SCHC SHRA. MVFR PSBL. NORTHWEST WINDS.

JEE

&&

.MARINE...
223 AM CDT

THE STATIONARY FRONT IS OVER CENTRAL ILLINOIS AND IT WILL LIFT AS A
WARM FRONT THROUGH TONIGHT. HAVE MEDIUM CONFIDENCE IN HOW QUICKLY IT
WILL ADVANCE AND HAVE MEDIUM-HIGH CONFIDENCE THAT THE DENSE FOG WILL
STAY PUT UNTIL THE WARM FRONT MOVES THROUGH. THEREFORE EXTENDED THE
DENSE FOG ADVSY THROUGH 10PM CDT THIS EVENING. IF THE FRONT IS
SLOWER THAN FORECAST...THE DENSE FOG COULD ALSO PERSIST LONGER. EAST
WINDS BECOME SOUTH BEHIND THE FRONT TONIGHT AND INCREASE TO 15-25 KT
OVER THE NORTHERN HALF OF THE LAKE THURSDAY NIGHT.  A LOW OVER
NORTHEAST CANADA DRAGS A COLD FRONT ACROSS THE LAKE FRIDAY NIGHT
INTO SATURDAY. WINDS BECOME NORTH TO 20 KT BEHIND THE FRONT.  THE
NEXT LOW PRESSURE SYSTEM MOVES OVER THE LAKE EARLY NEXT WEEK BUT
GUIDANCE DIFFERS ON IF THE LOW WILL MOVE OVER THE LAKE OR SOUTH OF
THE LAKE.

JEE

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...DENSE FOG ADVISORY...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-LMZ745-
     LMZ777-LMZ779 UNTIL 11 PM WEDNESDAY.

&&

$$

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