Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 302202
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
502 PM CDT WED JUL 30 2014

.DISCUSSION...
254 PM CDT

A HIGH AMPLITUDE LONGWAVE PATTERN THAT HAS BEEN IN PLACE FOR THE
PAST FEW DAYS WILL PERSIST FOR THE NEXT FEW DAYS WITH DEEP LONG WAVE
TROUGHING OVER THE ERN CONUS AND AN UPPER LOW CENTER OVER JAMES
BAY...WITH RIDGING OVER THE WRN CONUS.  THE LOCAL AREA WILL REMAIN
UNDER GENERALLY NWLY FLOW ALOFT UNDER THE SWRN PERIPHERY OF THE
UPPER LOW.  THIS WILL ALLOW FOR A SERIES OF WEAK SHORT WAVES TO
RIPPLE THROUGH THE NWLY FLOW ALOFT OVER THE LOCAL AREA.

FOR THE REMAINDER OF THIS AFTERNOON AND THIS EVENING...THE MAIN
CONCERN WILL BE THE LAKE BREEZE BOUNDARY QUICKLY PUSHING INLAND
THROUGH NERN IL AND NWRN IN.  TEMPERATURES WILL DROP OFF SEVERAL
DEGREES WITH THE PASSAGE OF THE BOUNDARY AND LAKEFRONT LOCATIONS
HAVE ALREADY DROPPED INTO THE MIDDLE 60S.  THE ENVIRONMENT AHEAD OF
THE BOUNDARY IS MODESTLY MOIST AND UNSTABLE.  WITH CYCLONIC FLOW
ALOFT...DIURNAL CU HAS BEEN DEVELOPING OVER THE REGION PER THE LATEST
SATELLITE IMAGERY.  AS OF EARLY THIS AFTERNOON...HAVE NOT SEEN ANY
SHRA/TSRA ACTIVITY OVER NRN IL/NWRN IN...BUT THE REGION REMAINS
MODESTLY UNSTABLE WITH DEWPOINTS IN THE MIDDLE TO UPPER 50S.  THERE
WILL STILL BE A CHANCE FOR SOME SHRA/TSRA FOCUSED ALONG THE LAKE
BREEZE BOUNDARY AS WELL AS SOME ISOLD SHRA OR AN EVEN MORE ISOLD TS
OVER THE AREA THROUGH SUNSET.  DURG THE EVENING AND INTO THE
OVERNIGHT HOURS...ANY DIURNAL PCPN WILL DISSIPATE AND AND RESIDUAL
PCPN WILL BE WELL NORTH OF THE AREA INVOF ON OF THE SEEMINGLY
ENDLESS SERIES OF IMPULSES DROPPING THROUGH THE NORTHWEST FLOW.

LITTLE CHANGE IS EXPECTED FOR TOMORROW THROUGH SATURDAY AS THE
SYNOPTIC SCALE PATTERN SHOULD CHANGE LITTLE AND A WEAK GRADIENT
PATTERN WILL PERSIST AT THE SFC.  UNDER PERSISTENT NWLY FLOW
ALOFT...THE SERIES OF WEAK SHORTWAVES WILL CONTINUE TO BRING
PERIODIC CHANCES FOR SCT SHRA AND ISOLD TSRA...WITH THE BETTER
CHANCES FOR TS DURG THE AFTERNOON HOURS.  THE WEAK GRADIENT PATTERN
AT THE SFC SHOULD ALLOW FOR DAILY LAKE BREEZE DEVELOPMENT...WITH THE
MAIN QUESTION BEING HOW FAR INLAND A LAKE BREEZE WILL PENETRATE.
CERTAINLY...IT IS LIKELY THAT LAKEFRONT LOCATIONS WILL TURN COOLER
IN THE AFTERNOON...BUT IT IS LESS CERTAIN HOW FAR INLAND THE LAKE
COOLING WILL PENETRATE.  SO...WILL KEEP THE MAX TEMP FORECAST A BIT
ON THE CONSERVATIVE SIDE AS FAR AS LAKE INFLUENCE IS CONCERNED AND
LIMIT THE COOLING TO THE IMMEDIATE LAKEFRONT AND ALLOW MAX TEMPS TO
BE MORE REPRESENTATIVE OF THE LARGE SCALE PATTERN.  UNDER THE COOL
NWLY FLOW ALOFT...HIGHS WILL REMAIN A FEW DEGREES BELOW NORMAL INTO
THE WEEKEND WITH HIGHS AROUND 80F OR SO.  TIMING OF PERIODS OF PCPN
WILL BE LESS CERTAIN THAN THE TEMPERATURE TRENDS FOR THE NEXT FEW
DAYS AND WILL GENERALLY BE DEPENDENT ON THE TIMING OF SHORTWAVE
ENERGY DROPPING THROUGH THE NWLY FLOW ALOFT.  SO...WHILE NO DAY IN
THE SHORT TERM FORECAST PERIOD WILL BE A TOTAL WASHOUT...THERE
SHOULD BE PERIODS OF SCT SHRA AND ISOLD TSRA.

THE LONGER RANGE GUIDANCE IS IN GENERALLY GOOD AGREEMENT ON THE
UPPER LEVEL PATTERN BECOMING MORE PROGRESSIVE THROUGH THE WEEKEND.
THIS WILL INITIATE A WARMING TREND AS THE UPPER LEVEL RIDGING MOVES
OUT OVER THE WESTERN PLAINS ON SUNDAY WHILE THE UPPER LOW OVER JAMES
BAY KICKS OUT TO THE NORTHEAST...LEAVING WEAKER LONGWAVE TROUGHING
OVER THE ERN CONUS.  SUNDAY SHOULD BE A TRANSITIONAL DAY...WITH
INLAND TEMPS A LITTLE HIGHER...BUT A WEAK GRADIENT PERSISTING AT THE
SURFACE WILL ALLOW FOR ANOTHER LAKE BREEZE DAY AND COOLER CONDITIONS
NEAR THE LAKE. RISING HEIGHTS OVER THE MIDWEST SHOULD ALLOW FOR MAX
TEMPS TO CLIMB INTO THE MIDDLE 80S BY MONDAY.

MORE UNCERTAINTY WILL CREEP INTO THE FORECAST DURING THE EXTENDED
FORECAST PERIOD AS FLOW BECOMES MORE ZONAL OVER THE MIDWEST.  IN THE
INCREASINGLY ZONAL FLOW...SHORT WAVE STRENGTH AND TIMING WILL BECOME
MORE UNCERTAIN...SO...CONFIDENCE IN THE SENSIBLE WEATHER FORECAST
WILL INCREASE WITH TIME THROUGH THE EXTENDED FORECAST PERIOD.
HOWEVER...THE LATEST LONG RANGE GUIDANCE SUGGESTS THAT THE WARMING
TREND COULD BE BRIEF AS A WEAK UPPER LOW HANGS BACK OVER THE LOWER
MISSISSIPPI VALLEY THROUGH THE PERIOD AND THE UPPER RIDGING WILL NOT
BUILD ANY FARTHER EAST THAN THE CENTRAL PLAINS.  ALSO...THE SFC
PATTERN SHOULD REMAIN WEAK...SO DAILY LAKE BREEZE DEVELOPMENT WILL
BE LIKELY AS WELL. THE TREND FOR THE EXTEDNED FORECAST PERIOD IS
SHAPPING UP FOR MORE BELOW NORMAL TEMPERATURES WITH PERIODIC CHANCES
FOR AT LEAST SOME LIGHT PCPN.

KREIN

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 22Z...

* EAST/NORTHEAST WINDS JUST UNDER 10 KTS THRU EARLY EVENING.
* ISOLATED SHRA/TSRA THROUGH MID EVENING. CMS

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

A LIGHT WIND FIELD ACROSS THE AREA IS ALLOWING A LAKE BREEZE TO
MAKE SOME GOOD PROGRESS INLAND ACROSS NORTHEASTERN ILLINOIS EARLY
THIS AFTERNOON. CURRENT TIMING LOOKS TO PUT THE BOUNDARY THROUGH
KMDW AROUND 1930 UTC AND THROUGH KORD IN THE 20 TO 21 UTC TIME
FRAME. WINDS DO NOT APPEAR TO BE OVERLY STRONG BEHIND THE LAKE
BREEZE...WITH GENERALLY ONLY AROUND 10 KT OF EAST-NORTHEASTERLY
FLOW EXPECTED. THE WINDS WILL GRADUALLY SHIFT BACK OUT OF THE
WEST-SOUTHWEST TONIGHT INTO THURSDAY. ANOTHER LAKE BREEZE MAY
DEVELOP ON THURSDAY. HOWEVER...WITH A SLIGHTLY STRONGER WEST-
SOUTHWESTERLY WIND FIELD EXPECTED ON THURSDAY...THE LAKE BREEZE
MAY STRUGGLE TO REACH THE TERMINALS.

THE OTHER CONCERN THIS AFTERNOON WILL...SIMILAR TO YESTERDAY...BE
THE THREAT FOR SOME WIDELY SCATTERED SHOWERS AND PERHAPS A
THUNDERSTORM ACROSS THE AREA. CUMULUS CLOUDS WILL LIKELY BE ON AN
UPTICK OVER THE NEXT HOUR OR SO AS TEMPERATURES CONTINUE TO WARM
THROUGH THE MID AND UPPER 70S. THIS CLOUD COVER COULD EVEN LEAD TO A
PERIOD OF LOWER END VFR CIG AROUND 4000 FEET AGL...ESPECIALLY AHEAD
OF THE LAKE BREEZE BOUNDARY. GIVEN THE LOWER AREAL COVERAGE
EXPECTED...I WILL CONTINUE WITH THE VCSH AND LEAVE THE MENTION OF
THUNDER OUT OF THE TAF AT THIS TIME.

WINDS LOOK TO CONTINUE OUT OF WEST-NORTHWESTERLY DIRECTION INTO
WEDNESDAY. HOWEVER...ANOTHER LAKE BREEZE MAY DEVELOP WEDNESDAY
AFTERNOON...AND DUE TO WEAKER WESTERLY FLOW AROUND 925 MB...THE
BOUNDARY COULD THREATEN THE EASTERN TERMINALS WITH AN EASTERLY WIND
SHIFT LATER IN THE AFTERNOON. CONFIDENCE IS A BIT LOW THIS FAR
OUT...SO I HAVE LEFT THIS MENTION OUT OF THE 30 HOUR ORD TAF.

KJB

//ORD AND MDW CONFIDENCE...UPDATED 22Z...

* HIGH FOR WINDS THROUGH THURSDAY MORNING...MEDIUM THURSDAY
  AFTERNOON.
* LOW FOR ISOLATED SHRA/TSRA THROUGH MID EVENING. CMS

//OUTLOOK FOR ORD/MDW FOR 00Z FRIDAY-12Z WEDNESDAY...UPDATED 12Z...

THURSDAY NIGHT...SCHC TSRA EARLY IN THE EVENING...SCHC SHRA LATE
IN THE EVENING.

FRIDAY...CHC SHRA WITH A CHC OF TSRA IN THE AFTN.

SATURDAY...SCHC SHRA IN THE MORNING...CHC OF TSRA IN THE AFTN.

SUNDAY...VFR.

MONDAY...CHC TSRA OVERNIGHT.

TUESDAY...CHC TSRA.

JEE

&&

.MARINE...
200 PM CDT

A WEAK PRESSURE GRADIENT IS OVER THE LAKE THIS AFTERNOON...THOUGH A
WEAK SURFACE LOW/TROUGH HAS DRIFTED TO THE SOUTHERN TIP OF THE LAKE.
THIS GENERALLY RESULTED IN A SHIFT TO NORTHERLY WINDS DOWN THE LAKE.
MEANWHILE AN UPPER LEVEL LOW CONTINUES TO SPIN NEAR JAMES BAY WITH
WITH SURFACE LOWS/TROUGHS EMANATING FROM THIS FEATURE THAT PROGRESS
SOUTH AND EAST AT TIMES WHILE THE STRONGER PARENT SURFACE LOW
REMAINS CLOSER TO JAMES BAY. MEANWHILE HIGH PRESSURE REMAINS IN
CONTROL OF THE CENTRAL AND NORTHERN PLAINS...RESULTING IN SOUTHERLY
WINDS ACROSS THE NORTHERN HALF OF THE LAKE THIS AFTERNOON.

AFTER ONSHORE FLOW IN THE NEARSHORE WATERS THIS EVENING DUE TO THE
PASSING SURFACE TROUGH...EXPECT WINDS TO BECOME MORE UNIFORM THIS
EVENING WITH SOUTHWEST WINDS ACROSS THE LAKE BY EARLY THURSDAY
MORNING. A COUPLE MORE WEAK TROUGHS/COLD FRONTS WILL PASS THROUGH
THE LAKE AT TIMES THROUGH WEEKS END...WITH ONE THURSDAY NIGHT AND
WITH A BIT MORE PRONOUNCED FRONT FRIDAY NIGHT INTO EARLY SATURDAY.
EACH OF THESE WILL TURN SOUTHWEST WINDS TO THE NORTHWEST/NORTH BUT
SPEEDS PRIMARILY REMAINING BELOW 15 KT AS THE PRESSURE GRADIENT
REMAINS WEAK FOR THE NEAR FUTURE.

THE GREAT LAKES REGION WILL LIKELY EXPERIENCE AFTERNOON AND EARLY
EVENING SHOWERS WITH OCCASIONAL THUNDERSTORMS AT TIMES. WHILE THESE
ARE NOT EXPECTED TO BE WIDESPREAD AT ANY GIVEN TIME...ANY STORMS
THAT FORM COULD CREATE GUSTY OUTFLOW WINDS THAT WOULD CREATE
HAZARDOUS CONDITIONS TO BOATERS...SO CAUTION IS URGED DURING THE
LATE AFTERNOON AND EVENING PERIODS THE NEXT FEW DAYS.

KMD

&&

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

$$

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