Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 301140
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
640 AM CDT SAT AUG 30 2014

.DISCUSSION...
344 AM CDT

THE MAIN FORECAST CONCERNS WILL CENTER AROUND CONVECTIVE CHANCES AND
TIMING THROUGH THE PERIOD...ALONG WITH THE THREAT FOR STRONG TO
SEVERE CONVECTION...ESPECIALLY MONDAY.

WATER VAPOR IMAGERY EARLY THIS MORNING INDICATES TWO MID/UPPER LEVEL
DISTURBANCES OF INTEREST FOR TODAY AND TONIGHTS WEATHER. THE FIRST IS
LOCATED OVER IOWA AND INTO EXTREME NORTHERN MISSOURI...AND THIS IS
DRIVING AN AREA OF SHOWERS AND SOME EMBEDDED STORMS INTO WESTERN
ILLINOIS AT THIS HOUR. THIS WAVE IS EXPECTED TO LIFT NORTHEASTWARD
ACROSS WISCONSIN LATER THIS MORNING...AND AS IT DOES SO...A FEW
SCATTERED SHOWERS AND A THUNDERSTORM OR TWO CAN BE EXPECTED ACROSS
NORTHERN ILLINOIS. A SECOND DISTURBANCE IS CURRENTLY LOCATED ACROSS
THE EASTERN DAKOTAS SOUTHWARD INTO EASTERN NEBRASKA. THIS WAVE IS
EXPECTED TO SHIFT EASTWARD ACROSS WISCONSIN AND NORTHERN ILLINOIS
DURING THE AFTERNOON TODAY. THESE TWO DISTURBANCES WILL HELP INDUCE
A UPPER LEVEL 60+ KT SPEED MAX ACROSS THE LOWER GREAT LAKES
REGION...ESSENTIALLY SETTING UP PORTIONS OF MY AREA TO LINE UP WITHIN
THE FAVORABLE RIGHT ENTRANCE REGION OF THE UPPER LEVEL JET MAX THIS
AFTERNOON AND INTO TONIGHT. THIS SHOULD HELP TO DRIVE AN AREA OF
SHOWERS AND THUNDERSTORMS ACROSS PORTIONS OF THE AREA ALONG AND
AHEAD OF AN APPROACHING SURFACE COLD FRONT.

IT APPEARS THE BEST CHANCES FOR MORE ROBUST CONVECTIVE DEVELOPMENT
WILL BEGIN BY EARLY TO MID AFTERNOON OVER OR JUST TO THE SOUTH AND
EAST OF THE CHICAGO AREA. HIRES CONVECTIVE ALLOWING MODELS CONTINUE
TO INDICATE THE FAVORABLE LOCATIONS FOR AFTERNOON CONVECTIVE
ACTIVITY WILL PRIMARILY BE ALONG AND SOUTH OF A CHICAGO TO OTTAWA
LINE. ALTHOUGH MORNING SHOWERS AND CLOUD COVER COULD HAMPER
INSOLATION SOME ACROSS THE AREA...IT APPEARS THAT WITH DEW POINTS IN
THE LOWER 70S...MLCAPES SHOULD END UP AROUND 1500 J/KG...ESPECIALLY
ACROSS THE SOUTHEASTERN HALF OF MY AREA. THE INCREASING MID AND AN
UPPER LEVEL WIND FIELD ALSO LOOKS TO INCREASE THE DEEP LAYER SHEAR
UP TO 25 TO 30 KT...WHICH COULD SUPPORT A FEW STRONGER STORMS
CAPABLE OF STRONG WIND GUSTS. THESE STORMS WILL ALSO LIKELY PRODUCE
SOME TORRENTIAL DOWNPOURS GIVEN THE HIGH PWAT AIRMASS IN PLACE.
STORM MOTIONS OF 30 TO 40 KT SHOULD LIMIT THE RESIDENCE TIME OF
THESE STORMS OVER A GIVEN AREA...MAKING EXTREME RAINFALL AMOUNTS IN
A GIVEN AREA UNLIKELY.  THESE STORMS LOOK TO LINGER INTO THE
EVENING HOURS TONIGHT...ESPECIALLY ACROSS MY FAR SOUTHEASTERN
COUNTIES IN NORTHWESTERN INDIANA AND EAST CENTRAL ILLINOIS.

SUNDAY STILL APPEARS TO BE THE PICK DAY OF THE WEEKEND...WITH A
TEMPORARY BREAK FROM THE SHOWER AND THUNDERSTORM ACTIVITY...WHICH
WILL RETURN BY LABOR DAY. THE WEATHER PATTERN WILL BECOME ACTIVE
AGAIN EARLY NEXT WEEK AS A SIGNIFICANT UPPER LEVEL DISTURBANCE AND
ASSOCIATED SPEED MAX...EMANATING FROM THE NORTHEASTERN
PACIFIC...DIGS ACROSS THE NORTHERN CONUS. THIS LOOKS TO SUPPORT A
SURFACE LOW...WHICH SHOULD SHIFT NORTHEASTWARD ACROSS THE UPPER
MIDWEST AND INTO ONTARIO BY MONDAY. CONVECTIVE ACTIVITY...POSSIBLY
STRONG TO SEVERE WILL BE FAVORED LATE SUNDAY FROM KANSAS
NORTHEASTWARD ACROSS ACROSS IOWA AND MINNESOTA AHEAD OF AN
APPROACHING COLD FRONT. THIS COLD FRONT IS EXPECTED TO MOVE INTO
NORTHERN ILLINOIS DURING THE DAY MONDAY...AND IT APPEARS THE DIURNAL
TIMING OF THIS FRONT...AND UPPER LEVEL SYSTEM...COULD BE FAVORABLE
FOR STRONG TO SEVERE CONVECTION ACROSS THE AREA MONDAY AFTERNOON AND
EVENING...ESPECIALLY CONSIDERING THE DEEP LAYER SHEAR IS EXPECTED TO
BE MORE THAN ADEQUATE FOR ORGANIZED STRONG CONVECTION.

UNCERTAINTY INCREASES A BIT LATER MONDAY NIGHT INTO TUESDAY...AS THE
VARIOUS MODEL GUIDANCE ARE IN DISAGREEMENT IN HOW FAST THE SURFACE
COLD FRONT WILL PUSH ACROSS MY ENTIRE AREA. THE ECMWF IS SUGGESTING
A SLOWER MOVEMENT OF THE FRONTAL BOUNDARY INTO TUESDAY AS IT
SUGGESTS THAT ANOTHER MID LEVEL IMPULSE WILL SHIFT OVER THE AREA
LEADING TO THE DEVELOPMENT OF A SECONDARY SURFACE LOW OVER THE AREA.
IF THIS SOLUTION WERE TO PAN OUT...IT COULD SET THE STAGE FOR SOME
VERY HEAVY RAINFALL AMOUNTS ACROSS PORTIONS OF THE AREA INTO TUESDAY
MORNING.

DURING THE MID TO LATER PORTION OF THE WEEK IT APPEARS THINGS WILL
BE ON A WARMING TREND...ESPECIALLY BY THURSDAY AS MID LEVEL HEIGHTS
BUILD SUBSTANTIALLY ACROSS THE AREA. THIS COULD LEAD TO A RUN AT
UPPER 80 TO LOWER 90 TEMPERATURES FOR THURSDAY. I CONTINUED WITH
SLIGHT CHANCE POPS DURING THE LATER PERIOD...MAINLY DUE TO
UNCERTAINTIES ON WHETHER OR NOT ANY SMALL SCALE DISTURBANCES WILL TRY
TO SPARK OFF CONVECTION OVER THE AREA. HOWEVER...WITH THE BUILDING
HEIGHTS AND WARMING TEMPERATURES ALOFT...THE AREA COULD END UP
CAPPED TO CONVECTIVE DEVELOPMENT ON WEDNESDAY AND THURSDAY.

KJB

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 12Z...

* CHANCE OF SHRA/TSRA LATE MORNING THROUGH MIDDAY.

* SWLY WINDS GUSTING TO 18KT SATURDAY.

KREIN

//DISCUSSION...UPDATED 12Z...

SCT SHRA AND ISOLD TSRA CONTINUE THROUGH THE DAY TODAY AS THE AREA
REMAINS ENTRENCHED IN A WARM/MOIST AIRMASS. A WEAK COLD FRONT IS
EXPECTED TO MOVE ACROSS THE AREA TODAY IN ASSOCIATION WITH A
STRONGER UPPER LEVEL TROUGH. THE UPPER TROUGH AND SFC FRONT SHOULD
PROVIDE SOME FOCUS FOR MORE THUNDERSTORM ACTIVITY. HOWEVER...THE
TIMING OF THE FRONT WILL PLAY A SIGNIFICANT ROLE IN THE TS
COVERAGE AS IT SHOULD MOVE NRN IL BY LATE MORNING...AT A TIME
BEFORE MAX DIURNAL WARMING. THE HIGH RESOLUTION MODEL GUIDANCE
SUGGESTS THAT TS COVERAGE SHOULD INCREASE AS THE FRONT MOVES
THROUGH ECNTRL IL AND NWRN IN DURG THE EARLY AFTERNOON HOURS.
HAVE OPTED TO GO WITH THE TEMPO GROUP FOR TS FOR THE MOST
FAVORABLE TIME FOR CONVECTIVE ACTIVITY OVER THE AREA...BUT THIS IS
STILL A LOW CONFIDENCE FORECAST.

WINDS SHOULD BECOME MORE SWLY WITH GUSTS TO 15-20KT FOR THE LATE
MORNING INTO EARLY EVENING AS SOME BREAKS IN THE OVERCAST ALLOWS
FOR SOME MIXING DOWN OF STRONGER WINDS ALOFT. WINDS SHOULD DROP
OFF AND BECOME LGT/VRBL OVERNIGHT AS HIGH PRESSURE SPREADS ACROSS
THE AREA. WITH CLEARING SKIES...LIGHT WINDS AND AMPLE RESIDUAL LOW
LEVEL MOISTURE IN PLACE...THERE IS AN INCREASING CHANCE FOR FOG
AND STATUS TO DEVELOP DURG THE LATE NIGHT HOURS TONIGHT.

KREIN

//ORD AND MDW CONFIDENCE...UPDATED 12Z...

* LOW CONFIDENCE IN PRECIP TRENDS.

* MEDIUM-HIGH CONFIDENCE IN WINDS.

* MEDIUM CONFIDENCE IN MVFR/IFR CIGS/VIS LATE TONIGHT.

KREIN

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

SUNDAY NIGHT...VFR.

MONDAY...PERIODS OF TSRA LIKELY.

TUESDAY...SLIGHT CHANCE TSRA EARLY.

WEDNESDAY...SLIGHT CHANCE TSRA OVERNIGHT.

THURSDAY AND FRIDAY...CHANCE TSRA.

KREIN

&&

.MARINE...
300 AM CDT

A PERIOD OF ACTIVE WEATHER IS SETTING UP FOR LAKE MICHIGAN FOR
THE NEXT SEVERAL DAYS. A SERIES OF WEAK LOW PRESSURE SYSTEMS WILL
TRACK ACROSS THE UPPER GREAT LAKES REGION INTO CANADA DRAGGING
WEAK COLD FRONTS ACROSS THE LAKE. MODERATE SOUTH TO SOUTHWESTERLY
WINDS WILL PERSIST ACROSS THE LAKE THROUGH MUCH OF SATURDAY UNTIL
THE FIRST OF THE COLD FRONTS SWEEPS ACROSS THE LAKE LATE THIS
AFTERNOON AND THIS EVENING...TURNING WINDS TO NORTHWESTERLY
TONIGHT. A WEAK RIDGE OF HIGH PRESSURE WILL BRIEFLY BUILD IN
BEHIND THE FRONT...ALLOWING WINDS TO DROP OFF AND BECOME MORE
VARIABLE IN DIRECTION EARLY SUNDAY. BY SUNDAY AFTERNOON...ANOTHER
DEVELOPING LOW OVER THE NORTHERN PLAINS WILL LEAD TO STRENGTHENING
SOUTHERLY WINDS FOR MONDAY AND MONDAY EVENING...UP TO 25 KT...BUT
THE NEXT FRONT WILL CROSS THE LAKE MONDAY NIGHT. THIS FRONTAL
PASSAGE WILL NOT BE AS ABRUPT AS THE FIRST WITH WINDS ONLY VEERING
SOUTHWESTERLY. WINDS SHOULD INCREASE AGAIN TO UP TO 25KT
IMMEDIATELY FOLLOWING THE FRONTAL PASSAGE MONDAY NIGHT...BUT SHOULD
THEN WEAKEN SLIGHTLY INTO MIDWEEK WHILE REMAINING GENERALLY
SOUTHWESTERLY.

KREIN

&&

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

$$

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