Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 061205
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
705 AM CDT WED MAY 6 2015

.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 12Z...

* LIFR/IFR CIGS AND LIFR TO MVFR VSBY THROUGH THE MID MORNING
  THEN IMPROVING TO VFR LATE IN THE MORNING.

* WINDS LESS THAN 10 KT OUT OF THE EAST AT ORD AND EAST TO
  SOUTHEAST AT MDW.

* VFR HAZE AND PERHAPS MVFR FOG THIS EVENING AND TONIGHT.

JEE

//DISCUSSION...UPDATED 12Z...

FOG IS LIKELY AT ITS WORSE RIGHT NOW WITH ALL SITES AT 1/2SM OR
LESS. PUSHED BACK IMPROVEMENT TO MVFR BY AN HOUR TO 15Z AT ALL OF
THE TERMINALS. THINKING THE SUN WILL HELP BURN OFF THE FOG.
GUIDANCE CONTINUES TO KEEP EAST WINDS AT ORD AND WHILE WIND SPEEDS
MAY APPROACH 10 KT...THINKING THEY WILL STAY BELOW 10 KT. DPA AND
RFD SHOULD SEE SE TO SOUTH WINDS...AND RFD MAY SEE A FEW GUSTS UP
TO 20 KT BUT DO NOT HAVE ENOUGH CONFIDENCE IN GUSTS TO PUT THEM IN
THE TAF AT THIS TIME. GYY IS A LITTLE DIFFERENT IN THAT A LAKE
BREEZE SHOULD TURN GYY/S WINDS NORTHEAST EARLY THIS AFTN. ALL
SITES WILL BECOME SOUTHEAST TO SOUTH THIS EVENING AND OVERNIGHT.
VFR HAZE TO MVFR FOG IS PSBL TONIGHT BUT DO NOT HAVE HIGH
CONFIDENCE IN HOW THE FOG WILL BEHAVE TONIGHT.

JEE


//ORD AND MDW CONFIDENCE...UPDATED 12Z...

* MEDIUM-HIGH CONFIDENCE IN WHEN CIGS AND VSBY IMPROVE TO VFR.

* MEDIUM-HIGH CONFIDENCE IN WINDS BEING EAST TO SOUTHEAST ALL DAY
  AND LESS THAN 10 KT.

* MEDIUM-LOW CONFIDENCE IN CIGS AND VSBY THIS EVENING.

* HIGH CONFIDENCE IN ALL OTHER FORECAST ELEMENTS.

JEE

//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...DENSE FOG ADVISORY...ILZ006-ILZ014 UNTIL 11 AM WEDNESDAY.

IN...NONE.
LM...DENSE FOG ADVISORY...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-LMZ745-
     LMZ777-LMZ779 UNTIL 11 PM WEDNESDAY.

&&

$$

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