Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 290902
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
402 AM CDT WED OCT 29 2014

.DISCUSSION...
315 AM CDT

SHORT TERM...THROUGH WEDNESDAY NIGHT...FOR THE REMAINDER OF THE NIGHT
AND INTO TUESDAY MORNING...THE MAIN FORECAST ISSUE WILL BE
ASSOCIATED WITH WINDS AND CLOUD COVER,

LATEST SATELLITE IMAGERY SHOWS A STRATOCU DECK SPREADING ACROSS NRN
IL...ASSOCIATED WITH A SECONDARY SURGE OF COOLER AIR AROUND THE SWRN
PERIPHERY OF THE DEEP LOW PRESSURE CENTERED NEAR JAMES BAY.  CLEAR
SKIES PREVAILED OVER THE CWA THROUGH MUCH OF THE EVENING ALLOWING
WINDS TO DIMINISH AS RADIATIONAL COOLING ALLOWED A NOCTURNAL
INVERSION TO FORM...DECOUPLING THE LOWER LEVELS FROM STRONGER WINDS
ALOFT.  HOWEVER...AS THE WRAPAROUND CLOUD COVER OVERSPREADS THE
REGION...WINDS WILL PICK UP ONCE AGAIN...AND PER UPSTREAM OBS.
EXPECT THAT THE WEAKER WSWLY WINDS WILL VEER TO MORE WNWLY AND GUST
UP TO ARND 25KT.  THE STRONGER WINDS AND CLOUD COVER SHOULD ONLY
COVER LOCATIONS GENERALLY NORTH OF THE ILLINOIS AND KANKAKEE
RIVERS...PER LATEST MODEL RH GUIDANCE AND TIMING OF THE LEADING EDGE
OF THE CLOUD DECK...SO THE SRN PORTIONS OF THE CWA SHOULD REMAIN
UNDER MOSTLY CLEAR SKIES WITH LIGHTER WINDS.  TEMPERATURES TODAY
SHOULD BE EVEN LOWER THAN YESTERDAY AS THE REGION REMAINS UNDER NWLY
FLOW AT THE SFC AND ALOFT...WITH PERSISTENT COLD ADVECTION.  HIGHS
WEDNESDAY ARE EXPECTED TO ONLY REACH INTO THE UPPER 40S GENERALLY
NORTH OF THE I-88 CORRIDOR WITH LOWER 50S TO THE SOUTH.

HIGH PRESSURE BUILDING OUT OF THE NRN PLAINS IS EXPECTED TO SPREAD
ACROSS THE MISSISSIPPI VALLEY AND INTO THE WRN PORTIONS OF THE CWA
BY EARLY AFTERNOON AND THEN CONTINUE TO SPREAD EAST ACROSS THE
REMAINDER OF THE CWA THROUGH THE AFTERNOON AND INTO THE EVENING.
THIS SHOULD BRING DECREASING CLOUD COVER AS WELL AS DECREASING WINDS
WITH THE DECREASING PRESSURE GRADIENT.  BY EARLY THIS EVENING...
WINDS SHOULD BECM NEAR CALM. THE CLEARING SKIES AND CALM WINDS
SHOULD ALLOW FOR STRONG RADIATIONAL COOLING...AND WITH DEWPOINTS
EXPECTED TO BE IN THE LOWER 30S...TEMPERATURES SHOULD DROP INTO THE
LOWER 30S AS WELL.  EVEN THE CHICAGO URBAN HEAT ISLAND AREA WILL GET
QUITE CHILLY...WITH LOWS IN THE MIDDLE 30S.

KREIN

THURSDAY THROUGH FRIDAY...
WHILE THE WEEK BEGAN ON A VERY MILD NOTE...WITH TEMPERATURES WELL
INTO THE 70S MON...THE WEEK CLOSES OUT AS NEARLY THE INVERSE OF HOW
IT BEGAN. BROAD MID-LVL TROUGHING WILL CONTINUE ACROSS THE BULK OF
THE GREAT LAKES/OHIO VALLEY REGIONS THUR...WITH A SECONDARY
SHORTWAVE POISED TO SWEEP SOUTHEAST ACROSS THE UPPER
MIDWEST/NORTHERN PLAINS EARLY THUR. GUIDANCE HAS CONTINUED TO
SUGGEST WITH THIS FEATURE THAT A WEAK SFC FEATURE WILL DEVELOP NEAR
NORTHEAST KS...WITH WEAK ADVECTION AHEAD OF THIS SYSTEM SOME LGT
PRECIP MAY OVERSPREAD THE FORECAST AREA.

THE SFC LOW THEN DEEPENS ACROSS LOWER MICHIGAN LATE THUR NGT/EARLY
FRI...AS A POTENT CANADIAN HIGH PRESSURE BEGINS TO SLIDE SOUTH INTO
THE NORTHERN PLAINS. THE GRADIENT BETWEEN THE TWO SYSTEMS TIGHTENS
QUICKLY FRI MORNING...AND WILL PRODUCE BREEZY CONDITIONS WITH GUSTS
UP TO 40 MPH POSSIBLE MIDDAY FRI. ACCOMPANYING THE STRONG CANADIAN
HIGH WILL BE MUCH COLDER AIR...AS TEMPS WILL LIKELY STRUGGLE TO WARM
BEYOND THE LOW TO MID 40S. GUIDANCE CONTINUES TO ADVERTISE A SLUG OF
850MB TEMPS DROPPING TO -6 TO -10 DEG C MIDDAY FRI/EARLY
AFTN...WHICH MAY BE STRONG ENOUGH TO KEEP TEMPS COOLER IN THE UPR
30S/LOW 40S FRI. THE COMBINATION OF GUSTY WINDS AND TEMPS IN THE LOW
40S...WILL LIKELY CREATE WIND CHILL VALUES AROUND 30-33 DEGREES FRI
AFTN. PRECIP AT THIS TIME SHUD REMAIN IN LIQUID FORM FRI...HOWEVER
AS COLDER AIR ADVECTS IN FROM THE NORTH/NORTHWEST FRI LATE
AFTN/EARLY EVE...WE SHOULD BEGIN TO SEE A MIX OF RAIN/SNOW. LAKE
EFFECT PRECIP LOOKS FAVORABLE AS WELL...HOWEVER GIVEN THE STRONG
WINDS AND REDUCED RESONANCE TIME...THE ONSET WILL LIKELY START
CELLULAR. THEN AS WINDS DIMINISH LATE FRI NGT A FEW BANDS MAY TRY TO
DEVELOP OVER NORTHWEST IND. TEMPS WILL COOL ENOUGH OVERNIGHT TO
CHANGE ALL PRECIP OVER TO SNOW...BUT NOT EXPECTING ANY ACCUMULATIONS
AT THIS TIME. TEMPS FRI NIGHT OVER LAND WILL LIKELY DIP INTO THE
MID/UPR 20S ACROSS THE WESTERN HALF OF THE LAKE WHERE SKIES SHUD
ERODE EARLIER FRI NGT. FURTHER EAST LAKE EFFECT CLOUDS WILL HELP TO
KEEP TEMPS SLIGHTLY WARMER.

CONFIDENCE...HIGH.

SATURDAY THROUGH EARLY NEXT WEEK...
THE WEEKEND WILL START UNSEASONABLY COLD WITH A DEEP 500MB TROUGH
OVER THE EASTERN HALF OF THE CONUS...AND A HIGH AMPLITUDE RIDGE
STRETCHING NORTH THROUGH THE PLAINS INTO GREENLAND. BY THE SECOND
HALF OF THE WEEKEND THE TROUGH BEGINS TO WEAKEN. SFC RIDGE OVER THE
GREAT LAKES SAT WILL SHIFT EAST SUN. GUIDANCE HAS SLOWED THE
DEPARTURE OF THE SFC RIDGE SLIGHTLY...WHICH WILL RESULT IN HOW
QUICKLY THE SECONDARY TROUGH CAN PUSH INTO THE FOUR-CORNERS REGION.
CURRENT THINKING IS THAT A TROUGH WILL PUSH INTO THE SOUTHWEST CONUS
LATE SUN/EARLY MON...WITH A SFC LOW DEVELOPING EARLY NEXT WEEK
ACROSS THE ROCKIES. THIS WILL LIKELY SET THE STAGE FOR A SLOW
MODERATION IN TEMPS EARLY NEXT WEEK ALONG WITH PRECIP CHANCES
INCREASING MON.

CONFIDENCE...HIGH.

BEACHLER

&&

.AVIATION...

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

* GUSTY WESTERLY WINDS OVERNIGHT AND WEDNESDAY.

KREIN

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

AS THE DEEP LOW PRESSURE SYSTEM CONTINUES TO TRACK NEWD THROUGH
CANADA...A SECONDARY SURGE OF COOLER AIR AND A BKN-OVC STRATOCU
DECK IS PUSHING INTO NWRN IL. AHEAD OF THE STATOCU DECK...WINDS
HAVE BEEN ABLE TO WEAKEN AS THE LOWER LEVELS BECOME DECOUPLED FROM
STRONGER WINDS ALOFT...BUT UNDER THE CLOUD DECK...WINDS REMAIN
STRONGER AND GUSTY DUE TO THE LACK OF RADIATIONAL COOLING TO FORM
A NOCTURNAL INVERSION. SO...AS THE CLOUD DECK MOVES OVER THE
TERMINALS...EXPECT THAT WINDS WILL INCREASE AGAIN...WITH GUSTS TO
ARND 25KT FOR THE REMAINDER OF THE NIGHT AND INTO WEDNESDAY
MORNING. THROUGH THE DAY ON WEDNESDAY...HIGH PRESSURE WILL BUILD
ACROSS THE PLAINS AND SPREAD EAST OF THE MISSISSIPPI RIVER THROUGH
THE AFTERNOON...LEADING TO CLEARING SKIES AND DIMINISHING WINDS.
DURG THE EARLY EVENING HOURS...THE HIGH WILL BE RIGHT ON TOP OF
THE AREA...AND WINDS SHOULD BECOME LGT/VRBL UNDER CLEAR SKIES...
WHICH SHOULD THEN BE THE PREVAILING CONDITION THROUGH THE
REMAINDER OF THE FORECAST PERIOD.

KREIN

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

* HIGH CONFIDENCE IN ALL FORECAST ELEMENTS.

KREIN

//OUTLOOK FOR ORD/MDW FOR 12Z THURSDAY-12Z TUESDAY...UPDATED 00Z...

* THURSDAY...SLIGHT CHANCE OF SHRA. LIGHT WEST WINDS.
* FRIDAY...SLIGHT CHANCE OF SHRA. STRONG/GUSTY NORTH WINDS.
* SATURDAY...DRY/VFR. NORTHEAST WINDS.
* SUNDAY...SLIGHT CHANCE OF SHRA. SOUTH WINDS.
* MONDAY...CHANCE OF SHRA. GUSTY SOUTH WINDS.

&&

.MARINE...
230 AM CDT

DEEPENING LOW OVER JAMES BAY WILL CONTINUE TO LIFT NORTHEAST AWAY
FROM THE GREAT LAKES...AS HIGH PRESSURE SLOWLY SLIDES SOUTHEAST FROM
THE NORTHERN PLAINS INTO THE MID-MISSOURI VALLEY. THIS HIGH PRESSURE
WILL DIMINISH WINDS ACROSS LAKE MICHIGAN TONIGHT/THUR MORNING. THEN
A WEAK LOW WILL DEVELOP ACROSS THE CENTRAL PLAINS EARLY THUR AND
QUICKLY SLIDE EAST TOWARDS LAKE MICHIGAN...BEFORE DEEPENING TO A
STRONGER LOW PRESSURE OVER LOWER MICHIGAN THUR NGT/EARLY FRI.

THEN THE TIME PERIOD OF GREATEST CONCERN WILL BE FRI WITH A VERY
POTENT CANADIAN HIGH PRESSURE REGION POISED TO SLIDE SOUTH INTO THE
NORTHERN PLAINS FRI MORNING. THE GRADIENT BETWEEN THE LOW/HIGH
CONTINUES TO BE SUPPORTIVE OF GENERATING GALES TO 40KT AND POSSIBLY
45KT AROUND 9Z FRI MORNING ACROSS THE NORTHERN HALF OF THE
LAKE...AND LIKELY DEVELOP FURTHER SOUTH AROUND DAYBREAK FRI OR
SHORTLY AFTER DAYBREAK FRI.

GUIDANCE HAS REMAINED CONSISTENT WITH HIGH CONFIDENCE IN NORTH GALES
40-45KTS DEVELOPING FRI...SO HAVE OPTED TO JUST JUMP TO A GALE
WARNING RATHER THAN A WATCH. THERE IS A SMALL WINDOW MIDDAY FRI
WHERE A FEW STORM FORCE GUSTS COULD OCCUR...BUT THIS IS A LOW
CONFIDENCE FOR ANY DURATION OF STORM FORCE WINDS. WITH THE NORTH
GALES...AND FOR A DECENT DURATION...IT IS POSSIBLE THAT WAVES COULD
BE SUBSTANTIAL ENOUGH TO PRODUCE THE NEED FOR BEACH HAZARDS ALONG
THE INDIANA SHORELINE FRI AS WELL.

HIGH PRESSURE WILL THEN BUILD ACROSS THE LAKE LATE FRI NGT AND
STEADILY DIMINISH THE WINDS. WINDS/WAVES WILL FURTHER DIMINISH
SAT/SUN AS HIGH PRESSURE REMAINS OVER THE LAKE.

BEACHLER

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...NEARSHORE WATERS
     UNTIL 10 AM WEDNESDAY.

     GALE WARNING...LMZ080-LMZ669-LMZ671-LMZ673-LMZ675-LMZ740-LMZ741-
     LMZ742-LMZ743-LMZ744-LMZ745-LMZ777-LMZ779-LMZ870-LMZ872-
     LMZ874-LMZ876-LMZ878...7 AM FRIDAY TO 4 AM SATURDAY.

     GALE WARNING...LMZ261-LMZ362-LMZ364-LMZ366-LMZ563-LMZ565-LMZ567-
     LMZ868...4 AM FRIDAY TO 10 PM FRIDAY.

&&

$$

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