Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 301917
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
217 PM CDT TUE SEP 30 2014

.DISCUSSION...
1110 AM CDT

HAVE ONLY MADE MINOR ADJUSTMENTS TO GOING FORECAST...MAINLY IN
SLIGHT DOWNWARD ADJUSTMENTS TO HIGH TEMPERATURES IN PORTIONS OF
NORTHERN ILLINOIS...PARTICULARLY NEAR THE WISCONSIN BORDER AND
NEAR LAKE MICHIGAN. LATEST VISIBLE SATELLITE IMAGERY IS SHOWING
HOLES DEVELOPING IN THE STRATUS DECK OVER INLAND AREAS DUE TO
DRIER AIR AND STILL RELATIVELY STRONG END OF SEPTEMBER SUN. THESE
AREAS SHOULD BE ABLE TO WARM INTO THE LOWER TO MID 60S...WITH A
FEW UPPER 60S IN CENTRAL IL. NEAR THE LAKE...DESPITE SOME DRYING
...COOL NORTH-NORTHEAST BOUNDARY LAYER FLOW IS WRINGING OUT
MOISTURE IN THE FORM OF OCCASIONAL SPRINKLES...WHICH GIVEN LITTLE
CHANGE IN CONDITIONS SHOULD PERSIST THROUGH AT LEAST EARLY
AFTERNOON. HAVE UPDATED GRIDS TO HAVE OCCASIONAL DRIZZLE MENTION
IN FAVORED LAKE EFFECT AREAS THROUGH THIS TIME. TEMPERATURES WILL
STRUGGLE TO WARM TO BETWEEN 55 AND 60 DEGREES DUE TO PERSISTENT
OVERCAST. AS ALLUDED TO ABOVE...HAVE ALSO ADJUSTED HIGHS DOWNWARD
SLIGHTLY TO UPPER 50S TO AROUND 60 FOR INLAND AREAS NEAR WI
BORDER...AS VISIBLE SATELLITE SUGGESTS STRATUS IS THICKER THERE
AND SOME AREAS INCLUDING RFD ARE STILL IN UPPER 40S AS OF THIS
WRITING.

RC

//PREV DISCUSSION...
332 AM CDT

MAIN FORECAST CONCERNS/CHALLENGES ARE WITH CLOUD COVER AND TEMPS
TODAY...AND THEN INCREASING CHANCES FOR SHOWERS AND THUNDERSTORMS
LATE WEDNESDAY NIGHT INTO THURSDAY AND THURSDAY NIGHT.

COLD FRONT NOW SOUTH OF THE CWA EARLY THIS MORNING AS LOW STRATUS
HAS MOVED OVER MUCH OF THE REGION...WHILE TEMPS CONTINUE TO FALL
INTO THE 40S AND 50S UNDER A STEADY NORTHEAST WIND. WITH ISOLATED
SHOWERS NOW EAST OF THE CWA EXPECT DRY CONDITIONS TODAY BUT WITH
THIS STRATUS LIKELY HANGING AROUND FOR SOME TIME THIS MORNING.
GUIDANCE APPEARING TO BE TOO QUICK SCATTERING OUT THESE CLOUDS
THIS MORNING AND TRYING TO INDICATE THERE WILL BE HOLES BY 12Z.
WITH THIS NOT A LIKELY SCENARIO HAVE GONE MORE PESSIMISTIC WITH
TRENDS THIS MORNING AND DONT HAVE ANY REAL CLEARING UNTIL LATE
MORNING/MIDDAY. MOST AREAS WILL LIKELY OBSERVE A SCATTERING TREND
INTO THE AFTERNOON...BUT WITH AREAS NEAR THE LAKE HOLDING ONTO
THESE CLOUDS FOR MUCH OF THE DAY. DID LOWER HIGH TEMPS AS WELL AS
SLOWED TEMP TREND THIS MORNING...WITH IT POSSIBLE FOR LOCATIONS IN
FAR NORTHEAST ILLINOIS AND NORTHWEST INDIANA NOT MAKING IT OUT OF
THE 50S TODAY.

QUIET BUT COOL NIGHT TONIGHT AS BOUNDARY LAYER FLOW VEERS AND ANY
LINGERING CLOUD COVER COMING OFF THE LAKE PUSHES NORTH INTO
WISCONSIN LATER THIS EVENING...AND MOST LOCATIONS OBSERVING CLEAR
TO PARTLY CLOUDY SKIES. FAIRLY ENERGETIC VORT MAX CURRENTLY
LIFTING NORTH THROUGH THE CENTRAL PLAINS WILL CONTINUE NORTH INTO
THE NORTHERN PLAINS TODAY INTO TONIGHT. AS THIS OCCURS...UPPER
LEVEL RIDGING WILL BE BRIEF TONIGHT AS RIDGE AXIS SHIFTS TO THE
EAST AND WE QUICKLY TRANSITION TOWARDS A SOUTHWEST FLOW PATTERN.
THIS WILL ALLOW FOR A PERIOD OF WAA GOING INTO WEDNESDAY WITH
TEMPS REBOUNDING BACK INTO THE 70S FOR MOST LOCATIONS. MADE
MINIMAL CHANGES TO GOING FORECAST HIGHS ON WEDNESDAY AS
UNCERTAINTY WITH APPROACHING MID/HIGH LEVEL CLOUD COVER COULD PLAY
INTO THE EXTENT OF WARMING THAT OCCURS.

THE BEST WAA/MOIST ADVECTION WILL REMAIN JUST TO THE WEST FOR MOST
OF THE DAY WEDNESDAY BUT THIS WILL CHANGE WEDNESDAY NIGHT AS
MID/UPPER LEVEL TROUGH DIGS ACROSS THE CENTRAL PLAINS...AND
STRENGTHENING LLJ BECOMES ORIENTED TOWARDS NORTHERN ILLINOIS. THIS
COINCIDING WITH FAIRLY ENERGETIC MID LEVEL FLOW...DEVELOPING
PRECIP SHIELD SHOULD BEGIN REACHING MOST LOCATIONS IN NORTHERN
ILLINOIS AFTER THE MIDNIGHT TIME FRAME. DESPITE INCREASING
INSTABILITY WEDNESDAY NIGHT...INSTABILITY AXIS DOES APPEAR TO STAY
JUST TO THE WEST/SOUTHWEST OF THE CWA DURING THIS TIME. ALTHOUGH
THIS MAY BE THE CASE...STRENGTHENING LLJ COULD OFFSET ANY LACKING
INSTABILITY AND SO HAVE MAINTAINED CHANCE THUNDER WORDING OVER THE
ENTIRE CWA...WITH THE FAR WEST AND SOUTHWEST AREAS LIKELY
OBSERVING BEST CHANCES FOR THUNDER. MOISTURE WILL CONTINUE TO
INCREASE DURING THIS TIME AND DO FEEL THERE COULD BE SOME PERIODS
OF MORE MODERATE RAINFALL...BUT THIS WONT LIKELY OCCUR UNTIL VERY
LATE IN THE NIGHT AND TOWARDS EARLY THURSDAY MORNING.

BETTER AGREEMENT OBSERVED WITH VARYING MODELS THIS MORNING
REGARDING APPROACHING MID/UPPER LEVEL TROUGH AND ASSOCIATED
SURFACE FEATURE THURSDAY AND THURSDAY NIGHT. SURFACE LOW SHOULD
LIFT RIGHT ALONG THE MISSISSIPPI RIVER AREA INTO SOUTHWEST
WISCONSIN THURSDAY AND THEN INTO THE GREAT LAKES REGION THURSDAY
NIGHT. CONTINUED LARGE SCALE SUPPORT DURING THIS PERIOD WILL ALLOW
FOR A WET DAY THURSDAY INTO THURSDAY NIGHT AS PERIODS OF SHOWERS
AND THUNDERSTORMS ARE LIKELY. A SLOWER DEPARTING TREND WILL OCCUR
THURSDAY NIGHT...AND DID KEEP LIKELY WORDING LONGER INTO THE
NIGHT. ALTHOUGH MOISTURE AND INSTABILITY WILL CONTINUE TO INCREASE
DURING THIS TIME...IT DOES LOOK LIKE THE BEST INSTABILITY WILL
REMAIN SOUTH OF THE CWA. SO AT THIS TIME...AM NOT OVERLY CONCERNED
WITH ANY SEVERE POTENTIAL BUT AM MORE CONCERNED WITH HEAVY
RAINFALL AND POSSIBLE LOCALIZED FLOODING...ESPECIALLY AS THE BEST
OVERALL MOIST ADVECTION OCCURS INTO THURSDAY NIGHT.

RODRIGUEZ

&&

.AVIATION...

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

* OCCASIONAL SPRINKLES THIS AFTERNOON.
* MVFR CIGS THROUGH MID MORNING WEDNESDAY.

BMD

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

A COLD FRONT LIES ACROSS DOWNSTATE IL AND SOUTHERN IN THIS
AFTERNOON WITH BROAD AREA OF STRATUS BEHIND WHICH HAS BEGUN TO
OVERTURN IN SOME AREAS ALLOWING SKIES TO SCATTER. THE CHICAGO
TERMINALS REMAIN SOCKED IN UNDER MVFR STRATUS HOWEVER WITH
MOISTURE FROM THE LAKE AND LAKE EFFECT CLOUDS/SPRINKLES EXPECTED
TO KEEP MVFR CONDITIONS AT THE CHICAGO TERMINALS THROUGH THE
REMAINDER OF THE DAY. WINDS WILL GRADUALLY BACK THROUGH THE
DAY...FROM N/NNE EARLY THIS AFTERNOON TO NORTHEAST MID/LATE
AFTERNOON...AND EVENTUALLY SOUTHEAST LATE TONIGHT. SOUTHEAST WINDS
WILL CONTINUE THROUGH THE DAY WEDNESDAY. RFD IS EXPECTED TO SEE
SKIES SCATTER OUT FOR A PERIOD THIS AFTERNOON AND EVENING...BUT
MVFR CIGS MAY SPREAD BACK OVERHEAD TONIGHT.

BMD

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

* HIGH CONFIDENCE IN GENERAL WIND DIRECTION TRENDS. MEDIUM
  CONFIDENCE IN WINDS STAYING NEAR 10 KT THIS AFTERNOON AND
  EVENING.
* MEDIUM-HIGH CONFIDENCE THAT MVFR WILL CONTINUE THROUGH
  TONIGHT...THOUGH LOW CONFIDENCE IN END TIME FOR MVFR WEDNESDAY.

BMD

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

WEDNESDAY NIGHT...INCREASING CHANCE FOR SHRA/TSRA. MVFR POSSIBLE.
SOUTHEAST WINDS.

THURSDAY...TSRA AND MVFR LIKELY. SOUTH WINDS.

FRIDAY...LOW CHANCE SHRA. MVFR POSSIBLE EARLY. GUSTY WEST WINDS.

SATURDAY...VFR. WEST WINDS.

SUNDAY...VFR. SLIGHT CHANCE SHRA. SOUTHWEST WINDS.

MONDAY...VFR. WEST WINDS.

MDB

&&

.MARINE...
215 PM CDT

HIGH PRESSURE STRETCHING THROUGH SOUTHERN ONTARIO AND QUEBEC WILL
CONTINUE DRIFTING EAST TONIGHT AS LOW PRESSURE NEAR THE EASTERN
GREAT LAKES WEAKENS. WHILE WINDS ARE ABATING SOMEWHAT ACROSS LAKE
MICHIGAN THIS AFTERNOON AS THE PRESSURE GRADIENT RELAXES...THE
NORTHERLY FETCH IS HOLDING WAVES ALONG THE SOUTHERN THIRD OF THE
LAKE AT 4-6 FT OR HIGHER. TONIGHT THE RIDGE AXIS ASSOCIATED WITH
THE DEPARTING HIGH WILL PASS OVER THE LAKE THEN PUSH TO THE
EASTERN LAKES THURSDAY. EXPECT A MORE SIGNIFICANT EASING OF THE
WAVES AS THIS OCCURS.

MEANWHILE...SEVERAL AREAS OF LOW PRESSURE IN THE PLAINS WILL ALLOW
WINDS TO VEER TO SOUTHEASTLERLY. THE FIRST LOW IN WESTERN NEBRASKA
WILL LIFT INTO MANITOBA WEDS THEN DEEPEN FURTHER AS IT CONTINUES
TOWARD HUDSON BAY. A SECONDARY LOW WILL DEVELOP ACROSS THE
SOUTHERN/CENTRAL PLAINS WEDNESDAY NIGHT. IT IS THIS LOW THAT WILL
EJECT NORTHWESTWARD TOWARD THE WESTERN GREAT LAKES LATER IN THE
WEEK. THIS LOW IS EXPECTED TO DEEPEN AS IT PASSES THROUGH WISCONSIN
FRIDAY THAN NORTH INTO ONTARIO ON FRIDAY. EXPECT SOUTHEAST WINDS TO
INCREASE IN RESPONSE TO THE STRENGTHENING LOW...WITH WINDS TO 30 KT
POSSIBLE.  AT THIS TIME IT APPEARS THAT THE STRONGEST WINDS ALOFT
AHEAD OF THE LOW ON THURSDAY WILL HAVE A HARDER TIME SURFACING
DUE TO THIS WESTERLY TRACK...BUT AS THE LOW LIFTS NORTH THERE MAY
BE A BRIEF PERIOD OF SOUTHERLY GALES THURSDAY NIGHT. WHILE THE
WINDS ALOFT SHOW SUPPORT FOR GALES...THERE IS STILL QUITE A BIT OF
UNCERTAINTY WITH HOW DEEP THE LOW WILL BE AND HOW FAR EAST IT WILL
BE DURING THIS PERIOD.

ON THE BACK SIDE OF THE LOW WHICH SHOULD STILL BE STRENGTHENING
NORTH OF THE LAKE...A SHARP COLD FRONT WILL BRING ANOTHER CHANCE FOR
GALES...THIS TIME WITH WEST WINDS. THE WIND SUPPORT ALOFT WOULD
SUGGEST JUST BORDERLINE GALE FORCE GUSTS...BUT IT WILL BE A PRETTY
STRONG FRONT AND UNSTABLE CONDITIONS SHOULD ALLOW FOR BETTER
MIXING. FOLLOWING THE FRONT A MODEST GRADIENT REMAINS IN PLACE
THROUGH THE WEEKEND AHEAD OF A CLIPPER-LIKE LOW FOR MONDAY. KMD


&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...NEARSHORE WATERS
     UNTIL 7 PM TUESDAY.

&&

$$

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