Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 181848
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
148 PM CDT THU SEP 18 2014

.DISCUSSION...
258 AM CDT

FORECAST CONCERNS THIS MORNING INCLUDE CLOUD COVER AND TEMPS
TODAY...FOLLOWED BY BRIEF SURGE OF UNSEASONABLE WARMTH AND T-STORM
CHANCES LATE FRIDAY INTO SATURDAY.

COLD FRONT CURRENTLY DROPPING SOUTH ACROSS THE WESTERN GREAT LAKES
AND IS PICKING UP MOMENTUM DOWN LAKE MICHIGAN WITH THE FRONT ON THE
WESTERN SHORE ALREADY THROUGH MKE AT 07Z. FRONT WILL CONTINUE SOUTH
INTO NORTHERN AND NORTHEASTERN ILLINOIS EARLY THIS MORNING BEFORE
EVENTUALLY WASHING OUT THIS AFTERNOON. WHILE THE FRONT WILL BE
LOSING SOME OF ITS UMPH...STILL SHOULD SEE A MODEST-MODERATE
NORTHEAST WIND OFF THE LAKE RESULTING IN SUBSTANTIAL LAKE COOLING
RIGHT ALONG THE SHORE WHERE TEMPS SHOULD HOLD IN THE UPPER 50S TO
LOWER 60S THIS AFTERNOON WITH STILL MILD TEMPS IN THE 70S WELL
INLAND WEST AND SOUTH OF THE LAKE. LAKE STRATUS HAS SLOWED ITS
SOUTHWARD ADVANCE ALREADY SO NOT THINKING IT WILL MAKE IT AS FAR
SOUTH AS OUR AREA...BUT IF IT DOES SHOULD MAINLY BE INTO NE IL AND
TEND TO CLEAR OUT THIS AFTERNOON WITH SUNNY SKIES EXPECTED
ELSEWHERE.

WARM AIR ADVECTION SHOULD BEGIN TONIGHT...BUT PROBABLY NOT SOON
ENOUGH OR WITH ENOUGH GUSTO TO PREVENT ANOTHER COOL NIGHT ACROSS THE
CWA. WARM ADVECTION SHOULD REALLY BECOME PRONOUNCED FRIDAY WITH H8
TEMPS PROGGED TO WARM 6-8C FROM 12Z FRI TO 00Z SAT. FORECAST
SOUNDINGS SUGGEST SKIES SHOULD BE GENERALLY SUNNY SO BIG QUESTION IS
HOW MUCH OF THE WARMING ALOFT WILL BE REALIZED AT THE SURFACE. THE
PROGGED 18Z 850/925 TEMPS FAVOR MID-UPPER 70S WHILE 00Z TEMPS
SUGGEST 80S...FOR NOW HAVE CONSERVATIVELY MAINTAINED MID-UPPER 70S
WHICH IS GENERALLY A BIT ABOVE OBJECTIVE GUIDANCE. MODERATE
SOUTHERLY WINDS AND CONTINUED LOW LEVEL WARM AIR ADVECTION SHOULD
ALLOW FOR AN UNSEASONABLY MILD NIGHT FRIDAY SETTING THE STAGE FOR A
WARM AND POTENTIALLY VERY WARM SATURDAY. ANY LINGERING CONVECTION
AND DEBRIS CLOUDINESS LOOKS TO REMAIN NORTH OF THE AREA EARLY
SATURDAY...SO IT IS LOOKING LIKE AMPLE SUNSHINE AND STRONG SOUTHWEST
WINDS SHOULD ALLOW TEMPS TO SOAR INTO THE 80S. QUESTION IS HOW FAR
INTO THE 80S? THE FORECAST 850/925 TEMPS ACTUALLY COULD SUPPORT
MID-UPPER 80S...THOUGH GUIDANCE IS ONLY CRANKING OUT TEMPS AROUND 80
SO AM LEARY ABOUT GOING TO CRAZY WITH TEMPS PARTICULARLY WITH TIMING
OF CONVECTION AND CLOUD COVER QUESTIONABLE. MAINTAINED THE GOING
FORECAST HIGHS WHICH ARE ABOVE GUIDANCE BUT SET-UP REALLY LOOKS
FAVORABLE TO ME FOR HIGHS GETTING WELL INTO THE 80S IF CLOUDINESS
DOESNT MUCK THINGS UP.

COLD FRONT IS SLATED TO DROP SOUTHEASTWARD TOWARD THE AREA DURING
THE LATE AFTERNOON/EVENING. MODERATE INSTABILITY IS PROGGED TO
DEVELOP IN ADVANCE OF THE FRONT...THOUGH DEEPER MIXING THAT FORECAST
COULD CAUSE DEWPOINTS TO MIX OUT AND RESULT IN WEAKER INSTABILITY.
NONE THE LESS...MODEST CAPE AND MODERATE-STRONG DEEP LAYER SHEAR
SHOULD STILL SUPPORT A SEVERE THREAT WITH STORM THAT DO DEVELOP.
SYNOPTICALLY SPEAKING THIS ISNT A TYPICAL BIG SEVERE WEATHER AND
WIDESPREAD CONVECTIVE SET-UP AND COVERAGE OF STORMS MAY NOT BE AS
SIGNIFICANT AS THE LOWER RES GLOBAL MODEL QPF WOULD SUGGEST...SO
HAVE MAINTAINED ONLY LOW END LIKELY POPS AND THOSE COULD BE A BIT
OVERDONE.

STILL LOOKS LIKE A MODERATELY HEALTHY SHOT OF COLDER AIR ARRIVES
LATE SUNDAY INTO EARLY NEXT WEEK. SUNDAY THROUGH MUCH OF NEXT WEEK
LOOKS LIKE ANOTHER STRETCH OF DRY WEATHER WITH AMPLE SUNSHINE AND
MODERATING TEMPS FROM CONSIDERABLY BELOW AVERAGE MONDAY REBOUNDING
BACK CLOSER TO AND PERHAPS A BIT ABOVE AVERAGE BY MID-LATE WEEK.

IZZI

&&

.AVIATION...

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

* WIND SPEEDS THIS AFTERNOON.
* FOG DEVELOPMENT OVERNIGHT.
DZ

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

IFR/LIFR STRATUS DECK OVER PORTIONS OF FAR NORTHEAST IL IS
THINNING AND BREAKING RESULTING IN SHALLOW CONVECTIVE CUMULUS
CLOUDS. CLOUD DECK EXPECTED TO ERODE FURTHER AS AFTERNOON SUN
HELPS TO MIX DRIER AIR TOWARD THE SURFACE. AS THIS OCCURS..WIND SPEED
MAY INCREASE AT TIMES FOR AN HOUR OR TWO.

WITH HIGH PRESSURE OVERHEAD WIND TO DECREASE RAPIDLY BY EARLY EVENING.
THE STABLE ATMOSPHERIC CONDITIONS AND RESIDUAL MOISTURE MAY LEAD
TO MVFR/IFR FOG DEVELOPMENT OVERNIGHT ESPECIALLY IN OUTLYING
AREAS.

WINDS TO GRADUALLY TURN CLOCKWISE THROUGH THE DAY BECOMING SOUTHEAST
TONIGHT AND SOUTH TOMORROW. DZ

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

* MEDIUM-HIGH CONFIDENCE IN WIND SPEEDS.
* LOW CONFIDENCE IN FOG DEVELOPMENT AND TIMING.

DZ

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

FRIDAY NIGHT...VFR.
SATURDAY...THUNDERSTORMS.
SUNDAY THROUGH WEDNESDAY...VFR.

BMD

&&

.MARINE...
148 PM CDT

THE COLD FRONT HAS PUSHED SOUTH OF THE LAKE WITH HIGH PRESSURE
EXTENDING ALONG THE WEST SHORE OF THE LAKE AND OVER WISCONSIN AND
INTO NRN IL...WHILE AN INVERTED TROUGH HAS DEVELOPED OVER THE EAST
SIDE OF THE LAKE AND WESTERN LOWER MICHIGAN. THIS HAS SET UP A
MESOLOW OVER THE SOUTH END OF THE LAKE WITH A LINE OF CONVERGENCE
CLOUD OVER THE SRN PORTION OF THE LAKE. THERE COULD BE SOME ISOLD
SHRA ALONG THE BOUNDARY...BUT OTHERWISE...CONDITIONS SHOULD REMAIN
QUIET THROUGH THE EVENING AND OVERNIGHT. AS HIGH PRESSURE PUSHES
OFF TO THE EAST AND A TROUGH OF LOW PRESSURE AND AN ASSOCIATED
COLD FRONT DEVELOP OVER THE CENTRAL PLAINS...WINDS WILL BECOME
SOUTHERLY BY FRIDAY MORNING AND STRENGTHEN AS THE PARENT LOW OF
THE COLD FRONT DEEPENS OVER HUDSON BAY. SINCE THE SOUTHERLY WINDS
WILL BRING MUCH WARMER AIR ACROSS LAKE MICHIGAN...THERE REMAINS A
QUESTION OF HOW SOON THE AIRMASS WILL MODIFY OVER THE LAKE TO THE
POINT WHERE THE STRONGER WINDS ALOFT WILL BE ABLE TO MIX DOWN TO
THE SURFACE. THE FORECAST MODELS ARE ADVERTISING WINDS IN EXCESS
OF 45KT AT ARND 1500 FT ABOVE THE SURFACE...BUT THE MARINE LAYER
SHOULD PROHIBIT THE STRONGEST WINDS FROM MIXING ALL THE WAY DOWN
TO THE SURFACE...BUT AT THIS POINT...FEEL THAT GALE FORCE WINDS
ARE POSSIBLE FOR THE NORTHERN 2/3 OF THE LAKE...WHILE THE SURFACE
LAYER OF THE SOUTHERN PORTIONS OF THE LAKE SHOULD REMAIN TOO
STABLE FOR GALE FORCE WINDS TO MIX DOWN TO THE SURFACE. SO...WILL
HOIST A GALE WATCH FOR THE PERIOD FROM FRIDAY AFTERNOON THROUGH
LATE SATURDAY MORNING AS THE PERIOD MOST LIKELY TO SEE GALES OVER
THE LAKE. WHILE GALES ARE CURRENTLY NOT ANTICIPATED FOR THE
SOUTHERN 1/3 OF THE LAKE...WINDS WILL LIKELY REACH SMALL CRAFT
ADVISORY LEVELS. WITH SOUTHERLY FLOW...WAVES SHOULD REMAIN UNDER
CRITERIA...BUT A SMALL CRAFT ADVISORY MAY BE NEEDED FOR BOTH THE
ILLINOIS AND INDIANA NEARSHORE WATERS. FOLLOWING THE FRONTAL
PASSAGE SATURDAY...WINDS SHOULD TURN NWLY FOR SATURDAY AFTERNOON
AND INTO SUNDAY...BUT THERE IS NOT A STRONG PUSH OF COLD AIR
BEHIND THE FRONT...SO ONLY EXPECT A SHORT PERIOD OF BRISK NWLY
WINDS. ANTICIPATE THAT CONDITIONS COULD BE CLOSE TO SMALL CRAFT
ADVISORY LEVELS FOR A SHORT PERIOD...ESPECIALLY FOR THE INDIANA
NEARSHORE WATERS SATURDAY NIGHT. HIGH PRESSURE WILL THEN
OVERSPREAD THE MIDWEST AND LOWER GREAT LAKES REGION WHILE LOW
PRESSURE DROPS OUT OF NORTHERN CANADA AND SOUTH OF JAMES BAY BY
MONDAY NIGHT AND THEN HIGH PRESSURE WILL QUICKLY BUILD IN BEHIND
THIS SYSTEM...SO...WINDS ACROSS THE LAKE WILL REMAIN RELATIVELY
LIGHT FROM SUNDAY THROUGH MIDWEEK.

KREIN

&&

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

$$

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