Area Forecast Discussion
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000
FXUS63 KLOT 311758
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1258 PM CDT TUE MAR 31 2015

.UPDATE...
1116 AM CDT

MADE ONLY MINOR TWEAKS TO GOING FORECAST MAINLY FOR CLOUD TRENDS
AND RESULTING TEMPERATURE CHANGES.

LOW PRESSURE WHICH PASSED THROUGH THE AREA LAST NIGHT CONTINUES
TO MOVE AWAY TO OUR EAST ACROSS NORTHERN OHIO THIS MORNING...WITH
MODEST COLD ADVECTION ON NORTH WINDS ACROSS THE FORECAST AREA IN
THE WAKE OF THE LOW AND ITS COLD FRONT. AREA OF LOW CLOUDS...AND
FOG OVER AND ALONG THE LAKE...ARE ERODING PER VISIBLE SATELLITE AS
WE DIURNALLY WARM THIS MORNING AND SHOULD CONTINUE TO DISSIPATE TO
PARTLY CLOUDY CONDITIONS THIS AFTERNOON. SOUTH AND SOUTHWEST PARTS
OF THE CWA HAVE WARMED A LITTLE QUICKER WITH FULL SUNSHINE
THERE... AND HAVE BUMPED TEMPS UP A COUPLE OF DEGREES THIS
AFTERNOON IN THOSE AREAS BASED ON LOW LEVEL MODEL THERMAL FIELDS.
WINDS MAY BE THE TRICKIEST PART OF THE FORECAST THIS
AFTERNOON...AS SYNOPTIC HIGH PRESSURE OVER THE PLAINS DRIFTS
TOWARD THE LOWER OHIO VALLEY AND WEAK TROUGHING LINGERS OVER THE
NORTHERN LAKES. LOCALLY...MESOSCALE THERMALLY-INDUCED HIGH IS
EXPECTED TO DEVELOP OVER THE SOUTH END OF LAKE MICHIGAN AS LAKE
BREEZE CIRCULATION DEVELOPS BY MID/LATE AFTERNOON...THOUGH FARTHER
INLAND IT LOOKS LIKE WINDS WILL BACK MORE WEST-NORTHWEST AND
PREVENT THE TRUE LAKE BREEZE FROM PUSHING TOO FAR WEST AT LEAST
UNTIL THIS EVENING. HIGH-RES GUIDANCE REPRESENTS A MESSY WIND
FIELD AMONGST THESE FEATURES...SO HAVE NOT MADE MUCH CHANGE TO
WINDS AND WILL JUST HAVE TO MONITOR FOR LAKE BREEZE TRENDS EARLY
THIS AFTERNOON.

OTHERWISE NO SIGNIFICANT CHANGES TO GOING FORECAST THROUGH THIS
AFTERNOON.

RATZER

&&

.SHORT TERM...
338 AM CDT

THROUGH WEDNESDAY...

LOW PRESSURE ACROSS THE SOUTHERN TIP OF LAKE MICHIGAN AT DAYBREAK
WILL CONTINUE TO RACE EASTWARD THIS MORNING. A TRAILING COLD
FRONT...PARTIALLY HELPED BY THE COOLER AIR OVER THE LAKE...WILL DROP
OVER THE ENTIRE FORECAST AREA BY MID MORNING. AT LEAST TEMPORARY
OVERCAST LOW CLOUDS ARE LIKELY IN NORTHEAST ILLINOIS WITHIN THE
MARINE MODIFIED COOL SECTOR OF THE SYSTEM DURING THIS
MORNING...POSSIBLY EVEN INTO AFTERNOON. SYNOPTICALLY THE WINDS
WILL WANT TO BECOME MORE NORTHWEST BY LATE MORNING INTO
AFTERNOON...HOWEVER THE SPEEDS WILL ALSO BE DIMINISHING AS THE
ADVECTION BEHIND THE LOW EASES. THIS SHOULD ALLOW FOR A LAKE
BREEZE TO DEVELOP...OR AT LEAST FOR CONTINUED ONSHORE FLOW...INTO
PARTS OF NORTHEAST ILLINOIS AND ESPECIALLY NORTHWEST INDIANA. ALL
THIS LEADS TO SOME TEMPERATURE UNDULATIONS IN LAKESIDE
COUNTIES...WITH HIGHS FORECAST IN THE MID 40S TO LOW-MID 50S FROM
NEAR TO FURTHER FROM THE LAKE IN THOSE COUNTIES. IF CLOUDS PERSIST
LONGER THOSE IMMEDIATE LAKESIDE TEMPERATURES MAY END UP BEING IN
THE UPPER 30S. FAR OUTLYING AREAS FROM THE I-39 CORRIDOR AND
DEKALB TO KANKAKEE AND SOUTHWEST FROM THERE SHOULD MODERATE TO
THE UPPER 50S TO LOWER 60S.

INCREASING UPPER HEIGHTS WILL BE SEEN ACROSS THE AREA DURING THE
DAY WEDNESDAY. STRENGTHENING SOUTHERLY FLOW ESPECIALLY BY
AFTERNOON WILL BRING AN IMPINGING LOW-LEVEL THERMAL NOSE INTO THE
AREA. WITH 925MB TEMPERATURES AROUND 10C THIS SHOULD ALLOW FOR
UPPER 60S TO LOWER OR EVEN MID 70S ACROSS MUCH OF THE FORECAST
AREA. ONE MAJOR CHALLENGE THOUGH IS THAT AS PRESSURE FALLS FIRST
START ADVANCING IN...THE WINDS LOOK A LITTLE MORE BACKED TO THE
SOUTHEAST NEAR THE ILLINOIS SHORE...AND THE GRADIENT COULD BE
LIGHT ENOUGH TO ALLOW A LAKE BREEZE TO CREEP INLAND. THIS MAY NOT
MAKE MUCH PROGRESS INWARD AND PROBABLY RETREAT LATER IN THE
DAY...BUT COULD PRESENT A SIGNIFICANT DIFFERENCE IN TEMPERATURES
FOR MUCH OF THE DAY FROM WHAT A SOUTHWEST WIND WOULD YIELD. HAVE
LEANED TOWARD THE LAKE BREEZE...ALBEIT NOT AS CHILLY AS IT COULD
BE...AND GONE MORE 50S IN NORTHEAST ILLINOIS WITHIN 10-20 MILES
OF THE SHORE...BUT CONFIDENCE ON SPECIFICS 36 HOURS OUT WITH THIS
PHENOMENA IS LOW.

MTF

&&

.LONG TERM...
338 AM CDT

WEDNESDAY NIGHT THROUGH MONDAY...

A NORTHERN STREAM TROUGH WILL MOVE OVER THE INTERNATIONAL BORDER OF
THE NORTHERN PLAINS WEDNESDAY NIGHT TO THE GREAT LAKES BY THURSDAY
EVENING. A SUB 990MB LOW IS EXPECTED WITH THIS SYSTEM ACROSS
ONTARIO WITH MOISTURE ADVECTED NORTHWARD AHEAD OF ITS COLD FRONT.
THE SOUTHERLY WINDS WILL MAKE FOR A MILD NIGHT WEDNESDAY NIGHT WITH
LOWS IN THE 50S. CONVECTION IS LIKELY TO TRIGGER ACROSS THE MISSOURI
RIVER VALLEY INTO SOUTHERN MN WEDNESDAY EVENING AND EVOLVE
EAST WITHIN INCREASING UPPER FORCING AND REPLENISHING MOISTURE
TRANSPORT. SHOWERS AND LIKELY SOME THUNDERSTORMS SHOULD TRANSLATE
INTO OUR REGION BY THURSDAY MORNING. HAVE CONTINUED TO FOCUS THE
POPS TEMPORALLY...GUIDING CLOSER TO THE SLOWER 00Z EC AND NCEP NAM
AND SREF MODELS.

THE MORNING CLOUDS AND SHOWERS AND STORMS WILL MAKE FOR A CHALLENGE
WITH TEMPERATURES AND ABILITY TO DESTABILIZE OR REVAMP ANY
ONGOING CONVECTION THURSDAY AFTERNOON...IN ADDITION TO SOME
DISCREPANCIES IN TIMING ALREADY FROM GUIDANCE. BOUNDARY LAYER DEW
POINTS ARE EXPECTED TO BE INTO THE MID-UPPER 50S USING THE BETTER
INITIALIZED EC ACROSS THE CURRENT HIGH DEW POINT AIR OF THE
SOUTHERN PLAINS. SO IF SCATTERING AND SOME RE-WARMING CAN
OCCUR...THIS WILL LIKELY RESULT IN ENOUGH INSTABILITY FOR
CONVECTION TO REDEVELOP NEAR THE FRONTAL ZONE. GIVEN MID-LEVEL
WINDS OF 45-60 KT THIS COULD SUPPORT SOME ORGANIZED STRONGER
STORMS...FOR OUR AREA MAINLY ALONG/SOUTH OF I-80 GIVEN THE CURRENT
EC AND NAM.

HAVE NOT MADE ANY MAJOR CHANGES BEYOND THIS TIME PERIOD. FRIDAY
IS FORECAST BY GUIDANCE TO HAVE A SHORT WAVE...POSSIBLY
STRONG...TRANSLATE ALONG THE FRONTAL BOUNDARY TO OUR SOUTH
RESULTING IN SOME RAIN IMMEDIATELY NORTH OF ITS PATH. BUT AGAIN
SOME SHIFTS IN THIS ARE CERTAINLY PLAUSIBLE. THE BOUNDARY LOOKS
TO RETURN AS A WARM FRONT LATER THIS WEEKEND AND THE GFS IS THE
QUICKEST WITH THIS. THE MORE DEFINED THIS BOUNDARY BECOMES THE
GREATER THE CHALLENGE WITH BOTH TEMPERATURES AND POSSIBLY EARLIER
SHOWER/STORM CHANCES. SUNDAY ESPECIALLY COULD HAVE HIGHS AREAWIDE
IN THE 60S/LOWER 70S...OR 40S ESPECIALLY IN NORTHEAST ILLINOIS
WITH A MORE EASTERLY WIND.

MTF

&&

.AVIATION...

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

* WINDS. LAKE BREEZE MOVED THROUGH BOTH TERMINALS...AND EARLIER
  THAN EXPECTED.

* CEILINGS HOVERING AROUND UPPER END OF MVFR THIS AFTERNOON.

* POSSIBLE MVFR/IFR VSBYS LATE TONIGHT.

LENNING

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

AS MENTIONED IN THE 16Z DISCUSSION...WINDS DID TURN AROUND TO THE
EAST BUT EARLIER THAN EXPECTED AND AT ORD AS WELL AS MDW. DO NOT
SEE THESE FLIPPING BACK TO THE NNW EVEN THOUGH OBS AROUND THE ORD
FIELD CONTINUE TO INDICATE MULTIPLE DIRECTIONS ARE STILL PRESENT
IN THE WIND FIELD. LATER THIS AFTERNOON THE DIRECTION LIKELY VEERS
SOUTHEAST AS MAGNITUDES DROP...EVENTUALLY VEERING ALL THE WAY
THROUGH THE COMPASS AND AROUND TO THE NORTHEAST AGAIN BY MORNING.
THIS IS A TYPICAL CHALLENGING FORECAST FOR WHAT WOULD OTHERWISE BE
A PRETTY NICE DAY.

THE OTHER QUESTION...THOUGH OF LESS IMPACT...IS THE EVOLUTION OF
THE CLOUD FIELD THIS AFTERNOON. TRENDS HAVE BEEN TOWARD VFR AND
SCATTERING...THOUGH CIGS COULD HANG IN THERE FOR A WHILE LONGER
NEAR THE LAKE.

ALSO CONSIDERED ADDING SOME REDUCED VISIBILITY FOR THE OVERNIGHT
HOURS AS WINDS DROP OFF AND THE AIRMASS REMAINS SIMILAR TO EARLIER
THIS MORNING...BUT CONFIDENCE IS NOT YET HIGH ENOUGH TO DETERMINE
LOCATION AND TIMING.

LENNING


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

* MEDIUM CONFIDENCE THAT ORD/MDW WINDS REMAIN EAST TODAY.

* MEDIUM TO HIGH CONFIDENCE IN CEILING TRENDS.

* LOW CONFIDENCE IN FOG POSSIBILITY OVERNIGHT.

LENNING

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

WEDNESDAY NIGHT...SLIGHT CHANCE OF TSRA LATE.

THURSDAY...SHRA/SOME TSRA LIKELY. PERIODIC IFR POSSIBLE.

FRIDAY...CHANCE OF MVFR CIGS. NE WINDS 10KT OR GREATER POSSIBLE.

SATURDAY AND SUNDAY...PRIMARILY VFR.

MONDAY...CHANCE OF TSRA.

RC

&&

.MARINE...
403 AM CDT

A FAST MOVING CLIPPER SYSTEM WILL QUICKLY MOVE EAST OF THE
SOUTHERN TIP OF THE LAKE THIS MORNING. GIVEN STRONG SOUTHWEST
WINDS THAT OCCURRED AHEAD OF THE LOW OVERNIGHT...A BRIEF BURST OF
15 TO 25 KT NORTH WINDS APPEARS LIKELY ON THE SOUTHERN TIP OF THE
EARLY THIS MORNING. HAVE NOT ISSUED A SMALL CRAFT ADVISORY BUT
WILL MONITOR TRENDS AND ISSUE A SHORT DURATION ADVISORY IF
NECESSARY. LIGHTER WINDS TONIGHT INTO WEDNESDAY MORNING WILL
SHIFT SOUTH AND INCREASE DURING THE DAY WEDNESDAY AND ESPECIALLY
WEDNESDAY NIGHT AS A FRONTAL TROUGH APPROACHES THE LAKE. WARM AIR
MASS AHEAD OF THE TROUGH WILL LIMIT MIXING OVER THE COLD
LAKE...BUT EXPECTING SPEEDS TO 30 KT GIVEN HIGH SUSTAINED WINDS.
WILL LIKELY NEED A SMALL CRAFT ADVISORY FOR THE NEARSHORE DURING
THIS TIME. A PERIOD OF STRONGER NORTH WINDS IS POSSIBLE ON FRIDAY
AS LOW PRESSURE MOVES UP THE OHIO VALLEY. SPEEDS WILL DEPEND ON
THE TRACK AND STRENGTH OF THE LOW PRESSURE AREA.

RC

&&

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

$$

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