Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 312017
AFDLOT

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

.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...
308 PM CDT

FRIDAY THROUGH TUESDAY...

MAIN CONCERNS DURING THE LONG TERM PERIOD WILL CENTER AROUND
PRECIPITATION CHANCES AND TEMPERATURES AS WE LOOK TO BE HEADED INTO
A FAIRLY ACTIVE PERIOD INTO EARLY NEXT WEEK.

AT THE START OF THE PERIOD...MODEL GUIDANCE IS IN DECENT AGREEMENT
IN THE PROGRESSION OF A NORTHERN PACIFIC SYSTEM SHIFTING OVER
CENTRAL PLAINS STATES AND UP OVER THE LOWER GREAT LAKES AND OHIO
VALLEY REGION BY FRIDAY EVENING. THIS LOOKS TO RESULT IN ANOTHER
PERIOD PRECIPITATION ACROSS THE AREA FRIDAY AND INTO FRIDAY EVENING.
MODEL GUIDANCE IS ALSO IN GOOD AGREEMENT THAT THE SURFACE LOW TRACK
WILL BE TO THE SOUTHWEST OF THE AREA...ROUGHLY FROM SOUTHERN
ILLINOIS NORTHEASTWARD ACROSS CENTRAL INDIANA. THIS TRACK COULD PUT
PARTS OF THE AREA IN LINE FOR A CHANCE OVER TO HEAVY WET SNOW FRIDAY
AFTERNOON.

THE POTENTIAL FOR A STRONG BAND OF FGEN FORCED RAIN/SNOW WITHIN AN
EVOLVING DEFORMATION ZONE COULD RESULT IN ENOUGH DYNAMIC COOLING TO
RESULT IN THIS CHANGE OVER...ESPECIALLY CONSIDERING A COLDER AIR
MASS IS FORECAST TO FILTER IN OVER THE AREA DURING THE DAY.
HOWEVER...THIS SYSTEM IS TOO FOR OUT TO GET A GOOD IDEA ON WHERE
EXACTLY THE STRONG MESOSCALE FORCING WILL OCCUR.  SO...OVERALL...A
LOT CAN CHANGE WITH THIS SYSTEM AS IT IS STILL YET TO EVEN DIG
ONSHORE OVER THE WEST COAST. IT ALSO IS NOT A FAVORABLE DIURNAL TIME
OF DAY FOR ACCUMULATING SNOW. HOWEVER...WITH THE POTENTIAL FOR
STRONG MESOSCALE FORCING...A BAND OF HEAVY WET SNOW CAN NOT BE RULED
OUT...WITH SOME ACCUMULATIONS POSSIBLE LATE FRIDAY INTO FRIDAY
EVENING. CONFIDENCE ON ANY AMOUNTS AT THIS TIME REMAINS LOW DUE TO
THE INHERENT UNCERTAINTIES IN THE ACTUAL THERMAL PROFILE AND AREAS
OF BEST DYNAMIC FORCING.

THINGS LOOK TO WARM UP AND DRY OUT FOR A PERIOD ON SATURDAY UNDER
SOUTHWESTERLY FLOW. HOWEVER...A COLD FRONT IS EXPECTED TO SHIFT DOWN
LAKE MICHIGAN LATE SATURDAY AND SATURDAY NIGHT...AND THIS FRONT
COULD END UP STALLING OUT FOR A PERIOD LATE SUNDAY THROUGH TUESDAY
OF NEXT WEEK OVER NORTHERN ILLINOIS AS THE FLOW ALOFT BEGINS TO
BECOME PARALLEL TO THE SURFACE FRONT. IF THIS DOES OCCUR...THERE
WILL LIKELY BE A SHARP TEMPERATURE GRADIENT ACROSS THE AREA...WITH
THE POTENTIAL FOR LOW TO MID 40S FAR NORTH AND NORTHEAST ILLINOIS
AND INTO 50S AND 60S SOUTH. WITH THE FRONT POSSIBLY IN THE AREA...IT
ALSO IS POSSIBLE FOR EXTENSIVE LOW CLOUD COVER AND PERIODS OF
SHOWERS.

AN AREA OF LOW PRESSURE...DRIVEN BY WHAT COULD END UP BEING A DECENT
MID/UPPER LEVEL DISTURBANCE...COULD MOVE ALONG THIS FRONTAL BOUNDARY
AT THE VERY END OF THE PERIOD TUESDAY NIGHT...AND THIS COULD RESULT
IN MORE ACTIVE SHOWERS AND THUNDERSTORMS OVER THE AREA...WITH EVEN A
THREAT FOR STRONG OR EVEN SEVERE STORMS SOUTH OF THE FRONTAL
BOUNDARY.

KJB

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 20Z...

* STILL THE WINDS. QUITE VARIABLE AT ORD BUT MORE SUSTAINED
  EASTERLY AT MDW. SPEEDS SHOULD BE LOW ENOUGH TO ALLOW FOR
  FLEXIBILITY AT ORD AND LET THEM HANDLE A HIGHER RATE.

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 20Z...

* MEDIUM CONFIDENCE IN WINDS THIS AFTERNOON. HIGHER OVERNIGHT AS
  THEY MAY GO LIGHT AND VARIABLE.

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...
316 PM CDT

WINDS WILL INCREASE OUT OF THE SOUTH OVER THE LAKE LATE WEDNESDAY
AND WEDNESDAY NIGHT IN RESPONSE TO STRONG PRESSURE FALLS OUT AREA OF
A STOUT AREA OF LOW PRESSURE EXPECTED TO SHIFT EASTWARD OVER ONTARIO
INTO THURSDAY MORNING. WINDS OF 25 TO 30 KT ARE EXPECTED DURING THIS
PERIOD. FOLLOWING THE SYSTEMS COLD FRONTAL PASSAGE...WINDS ARE
EXPECTED TO INCREASE OUT OF THE NORTH FOR A PERIOD ON FRIDAY AND
FRIDAY EVENING AS ANOTHER AREA OF LOW PRESSURE DEVELOPS OVER THE
SOUTHERN PLAINS AND SHIFTS NORTHEASTWARD OVER THE OHIO VALLEY. WIND
SPEEDS WITH THIS COULD ALSO REACH UP AROUND 30 KT...WITH SOME LOW
END GALE FORCE WINDS NOT OUT OF THE QUESTION OVER SOUTHERN LAKE
MICHIGAN LATE FRIDAY AND FRIDAY EVENING.

LATER IN THE PERIOD A PROLONGED PERIOD OF EAST-NORTHEASTERLY FLOW IS
EXPECTED TO SET UP OVER LAKE MICHIGAN. ALTHOUGH THESE WINDS WILL NOT
BE ALL THAT STRONG IN MAGNITUDE...THE PROLONGED NATURE OF THEM COULD
RESULT IN SOME 4 + FOOT WAVES ALONG THE WESTERN SHORES OF THE LAKE
INTO EARLY NEXT WEEK.

KJB

&&

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

$$

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