Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 301747
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1247 PM CDT SAT AUG 30 2014

.DISCUSSION...
1235 PM CDT

MODEL FORECAST DATA...SATELLITE IMAGERY...ACARS AIRCRAFT
SOUNDINGS...AND RECENT RADAR TRENDS ALL SUPPORT THE IDEA OF
INCREASED CONVECTIVE DEVELOPMENT THIS AFTERNOON...PRIMARILY
FOCUSED SOUTHEAST OF THE I-55 CORRIDOR. SO FAR THIS MORNING INTO
EARLY AFTERNOON THERE HAS BEEN LITTLE LIGHTNING WITH THE FEW CELLS
THAT HAVE DEVELOPED...BUT WOULD NOT BE SURPRISED TO SEE MORE
ACTIVE CONVECTION AS WE APPROACH PEAK DIURNAL HEATING. LESS
PRONOUNCED MOISTURE TRANSPORT NORTHWEST OF THIS CORRIDOR ALONG
WITH RELATIVELY WARM AIR ALOFT ARE THE FACTORS CONTRIBUTING TO
LESS CONFIDENCE OF WIDESPREAD AFTERNOON CONVECTION ACROSS THE
NORTHWEST HALF OF THE AREA.

LENNING

&&

.PREV 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 18Z...

* PRIMARY CONCERN IS TIMING...EXTENT...AND DURATION OF TSRA THIS
  AFTERNOON.

LENNING

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

MODEL FORECAST DATA...SATELLITE IMAGERY...ACARS AIRCRAFT
SOUNDINGS...AND RECENT RADAR TRENDS ALL SUPPORT THE IDEA OF
INCREASED CONVECTIVE DEVELOPMENT THIS AFTERNOON...PRIMARILY
FOCUSED SOUTHEAST OF THE ORD/MDW TERMINALS. SO FAR THIS MORNING
INTO EARLY AFTERNOON THERE HAS BEEN LITTLE LIGHTNING WITH THE FEW
CELLS THAT HAVE DEVELOPED...BUT WOULD NOT BE SURPRISED TO SEE MORE
ACTIVE CONVECTION AS WE APPROACH PEAK DIURNAL HEATING. LESS
PRONOUNCED MOISTURE TRANSPORT NORTHWEST OF THIS CORRIDOR ALONG
WITH RELATIVELY WARM AIR ALOFT ARE THE FACTORS CONTRIBUTING TO
LESS CONFIDENCE OF WIDESPREAD AFTERNOON CONVECTION ACROSS THE
NORTHWEST HALF OF THE AREA...INCLUDING ORD/MDW AS WELL AS DPA AND
RFD. THE GYY TERMINAL WOULD BE ALONG THE NORTHWEST EDGE OF THE
AREA EXPECTED TO HAVE THE MORE EXTENSIVE COVERAGE.

LENNING


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

* MEDIUM CONFIDENCE THAT WIDESPREAD TSRA REMAINS SOUTHEAST OF
  TERMINALS...BUT LOW CONFIDENCE OF NO TSRA AT ALL.

* MEDIUM TO HIGH CONFIDENCE ON WIND TRENDS.

* MEDIUM CONFIDENCE ON CEILING TRENDS.

LENNING

//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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