Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 180314
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1014 PM CDT WED SEP 17 2014

.DISCUSSION...
819 PM CDT

NO BIG CHANGES TO MADE TO THE FORECAST THROUGH THURSDAY MORNING.
ANOTHER EARLY EVENING OF RAPID HEATING ESCAPE WITH OUTLYING
LOCATIONS DROPPING 10 TO AS MUCH AS 15 DEGREES IN THE PAST TWO
HOURS. LOOKING AT 24-HOUR CHANGE DOES INDICATE TEMPERATURES AND
DEW POINTS ARE AROUND 5 DEGREES HIGHER THAN LAST EVE AT THIS TIME
AT MOST LOCATIONS. DO THINK LOW TEMPERATURES WILL FALL OUT PRETTY
CLOSE TO THAT DEPARTURE FROM THIS MORNINGS LOWS. THIS IS RIGHT IN
THE NEIGHBORHOOD OF THE GOING FORECAST.

THE BACKDOOR COLD FRONT WITH AN IMPRESSIVE BAROCLINIC GRADIENT
BEHIND IT IS WORKING ITS WAY SOUTHWARD DOWN LAKE MICHIGAN. THE
PRIMARY COLD AIR ADVECTION WITH THIS WILL WEAKEN BY MORNING AND
ONLY GIVE A GLANCING SHOT TO THE AREA. WITH SOMEWHAT OF A SUBTLE
PUSH...THE DEGREE OF MORNING CLOUD COVER MAY END UP BEING LESS
THAN FORECAST OVER FAR NORTHEAST ILLINOIS AND FAR NORTHWEST
INDIANA...AND MOISTURE FORECASTS FROM LATEST GUIDANCE ARE TRENDING
LESS. WITH SATELLITE IMAGERY STILL INDICATING PLENTY OF CLOUDS
BEHIND THE FRONT UPSTREAM WILL OPT TO WAIT AT THIS TIME FOR ANY
CHANGES.

MTF

//PREV DISCUSSION...
319 PM CDT

THE LAKE BREEZE SLOWLY EASING INLAND ACROSS FAR EASTERN COOK AND
LAKE COUNTIES IN ILLINOIS...AND AS SUCH...THIS IS PRODUCING
SLIGHTLY COOLER CONDITIONS LAKESIDE WITH THE ONSHORE FLOW.

ATTENTION IS FOCUSED ON A COLD FRONTAL BOUNDARY...CURRENTLY SHIFTING
SOUTHWARD ACROSS THE U.P. OF MICHIGAN. THIS BOUNDARY IS THE LEADING
EDGE OF A RATHER STOUT BAROCLINIC ZONE. MUCH COLDER AIR...COLD
ENOUGH TO SUPPORT SNOW...IS IN PLACE NORTH OF THE FRONT ACROSS
SOUTHERN CANADA. FORTUNATELY FOR US...THE CORE OF THIS COLD AIRMASS
WILL SLIDE EAST-SOUTHEASTWARD ACROSS SOUTHEASTERN CANADA AND NEW
ENGLAND. HOWEVER...THE SURFACE BOUNDARY IS EXPECTED TO SHIFT OVER
THE AREA AS A BACK DOOR COLD FRONT BY EARLY THURSDAY MORNING. THIS
SHOULD RESULT IN WINDS COMING IN OFF THE LAKE THROUGH THE DAY
TOMORROW...ALONG WITH INCREASED CLOUDINESS. IT APPEARS THAT SOME
LOWER STRATOCU WILL OVERSPREAD PORTIONS OF NORTHERN ILLINOIS BY
AROUND DAYBREAK THURSDAY...POSSIBLY LASTING INTO THE AFTERNOON
BEFORE SLOWLY DISSIPATING. THERE COULD EVEN BE A FEW LIGHT LAKE
EFFECT RAIN SHOWERS/SPRINKLES LATER THURSDAY MORNING. MODELS
SOUNDINGS INDICATE JUST ENOUGH COOL AIR MOVING OVER THE LAKE TO
RESULT IN SOME HIGHER EL`S UP TO AROUND 7500 FEET AGL. THIS COULD BE
ENOUGH TO SQUEEZE OUT A FEW VERY LIGHT SHOWERS. I ADDED A SLIGHT
CHANCE OF SPRINKLES TO COVER THIS MINOR THREAT. OTHERWISE...COOLER
CONDITIONS...IN THE LOWER 60S...WILL BE FAVORED ACROSS NORTHEASTERN
ILLINOIS DUE TO THE ONSHORE FLOW.

THE WARMER AIR WILL MAKE A QUICK RETURN TO THE AREA ON FRIDAY AS
WARM AIR ADVECTION AND UPPER LEVEL HEIGHT RISES RAMP UP AHEAD OF AN
AREA OF LOW PRESSURE ACROSS CANADA. THIS SHOULD RESULT IN
TEMPERATURES WELL INTO THE 70S AREA-WIDE BY FRIDAY...AND INTO THE
LOWER 80S FOR SATURDAY.

THUNDERSTORMS STILL LOOK LIKE A GOOD BET ON SATURDAY INTO SATURDAY
EVENING AS A COLD FRONT APPROACHES THE AREA. VERY WARM AND MOIST
CONDITIONS ARE FORECAST TO SETUP ALONG A CORRIDOR TO THE SOUTH OF THE
APPROACHING COLD FRONT...AND THIS SHOULD YIELD CONDITIONS FAVORABLE
FOR THUNDERSTORMS WITH AT LEAST A BRIEF TORRENTIAL DOWNPOUR THREAT.
GUIDANCE INDICATES PWATS COULD EXCEED 1.6 INCHES WITHIN THE
PREFRONTAL AIRMASS ACROSS NORTHERN ILLINOIS/NORTHWESTERN INDIANA ON
SATURDAY. AS SUCH...THIS COULD LEAD TO SOME EFFICIENT RAIN RATES WITH
ANY STORMS THAT DEVELOP. THERE COULD ALSO BE A FEW STRONG...MORE
ORGANIZED...STORMS SATURDAY AFTERNOON GIVEN THE POTENTIAL OF A MID
LEVEL JET DEVELOPING SOUTHWARD OVER THE AREA.

FOLLOWING THIS SYSTEM TEMPERATURES WILL TREND COOLER AGAIN...AND THE
WEATHER SHOULD BE DRY FOR A COUPLE OF DAYS EARLY NEXT WEEK AS ANOTHER
SURFACE HIGH IS PROGGED TO SHIFT RIGHT OVER THE AREA BY LATE MONDAY
INTO TUESDAY.

KJB

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 03Z...

* LIGHT AND VARIABLE WINDS TONIGHT...THEN INCREASING OUT OF THE
  NORTHEAST THROUGH THURSDAY MORNING WITH FROPA...LIKELY AT 10 KT
  THROUGH THE AFTERNOON.

* MVFR STRATUS POST FROPA THURSDAY MORNING.

RODRIGUEZ

//DISCUSSION...UPDATED 00Z...

WITH LAKE BREEZE PASSAGE...WINDS HAVE SHIFTED TO MORE OF AN
EASTERLY DIRECTION AT 7-9 KT WHILE VFR CONDITIONS PREVAIL. CURRENT
SPEEDS ARE AT THE HIGHEST WITH SPEEDS EXPECTED TO RAPIDLY DIMINISH
OVER THE NEXT COUPLE OF HOURS. MOST OF TONIGHT WILL REMAIN QUIET
AS FRONT TO THE NORTH DROPS SOUTH TOWARDS THE TERMINALS. TIMING OF
THIS FRONT DOES APPEAR AS IF IT WILL BE SLIGHTLY SOONER EARLY
THURSDAY MORNING...SO HAVE PUSHED UP TIMING OF FROPA. HAVE LOWER
CONFIDENCE WITH CEILINGS/TRENDS THURSDAY MORNING BEHIND THE FRONT.
THERE ARE CURRENTLY MVFR CEILINGS BEHIND THE FRONT ACROSS FAR
NORTHERN LAKE MICHIGAN AND DO EXPECT THESE TO PUSH DOWN THE LAKE
WITH THE FRONT TONIGHT ALTHOUGH...GUIDANCE DOES VARY WITH THE
EXTENT OF THIS SOUTHWARD PUSH. GUIDANCE DOES TRY TO KEEP THE BULK
OF THE LOWER CLOUDS TO THE NORTHWEST AS LOW LEVEL FLOW QUICKLY
VEERS BUT WITH A STRONG NORTHERLY PUSH...CANT RULE OUT THESE LOWER
CLOUDS STILL REACHING THE TERMINALS. SO HAVE MAINTAINED MVFR
CEILINGS IN THE TAFS...BUT DID MAKE SOME MINOR CHANGES TO
HEIGHT/DURATION. DONT THINK CEILINGS SHOULD BE IN PLACE ALL
DAY...WITH SKIES CLEARING IN THE AFTERNOON.

RODRIGUEZ

//ORD AND MDW CONFIDENCE...UPDATED 03Z...

* HIGH CONFIDENCE WITH WIND SPEED/DIRECTION.

* LOW MEDIUM CONFIDENCE WITH MVFR CEILINGS/TIMING/DURATION.

RODRIGUEZ

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

FRIDAY...VFR.

SATURDAY...CHANCE OF THUNDERSTORMS.

SUNDAY...VFR.

MONDAY...VFR.

TUESDAY...VFR.

RODRIGUEZ

&&

.MARINE...
128 PM CDT

AN AREA OF LOW PRESSURE OVER THE EASTERN UPPER PENINSULA OF MICHIGAN
EARLY THIS AFTERNOON WILL CONTINUE TO MOVE EAST TONIGHT...TRAILING A
COLD FRONT WHICH WILL PUSH SOUTH DOWN THE LENGTH OF LAKE MICHIGAN
THROUGH TONIGHT AND EARLY THURSDAY MORNING. AS THE FRONT BEGINS TO
DROP SOUTH OVER THE NORTHERN END OF LAKE MICHIGAN BY EARLY THIS
EVENING...WINDS WILL VEER TO THE NORTH IN THE 20-25 KT RANGE. WINDS
WILL DIMINISH LATE TONIGHT AND THURSDAY AS THE FRONT PUSHES FARTHER
DOWN THE LAKE...AND LOSES SOME OF ITS PUNCH. HIGH PRESSURE WILL THEN
BUILD SOUTHEAST ACROSS THE NORTHERN LAKES AND EVENTUALLY ACROSS NEW
ENGLAND BY FRIDAY...WITH WINDS ON LAKE MICHIGAN SHIFTING BACK TO THE
SOUTH AND INCREASING LATE THURSDAY NIGHT. SOUTH WINDS TO 30 KT ARE
ANTICIPATED ACROSS THE LAKE FRIDAY AS THE GRADIENT TIGHTENS IN
RESPONSE TO LOW PRESSURE MOVING NORTH OF THE LAKES...AND A PERIOD OF
GALES OR AT LEAST SOME GALE FORCE GUSTS ARE POSSIBLE ESPECIALLY
ACROSS NORTHERN LAKE MICHIGAN LATE FRIDAY INTO EARLY SATURDAY.
FORECAST SOUNDINGS DEPICT 35-40 KT ABOVE THE SHALLOW MARINE BOUNDARY
LAYER DURING THIS TIME...WITH THE DRIVING FACTOR BEING THE DEPTH OF
MIXING IN THE SOUTHERLY WARM ADVECTION FLOW. THE LOW WILL PASS NORTH
OF THE LAKE LATE SATURDAY INTO SUNDAY...WITH A COLD FRONT PUSHING
SOUTHEAST SATURDAY NIGHT RESULTING IN A SHIFT TO NORTHWEST WINDS FOR
SUNDAY AND MONDAY.

RATZER

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...NONE.
&&

$$

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