Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 192016
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
316 PM CDT FRI SEP 19 2014

.DISCUSSION...

303 PM...MAIN FORECAST CONCERN IS THUNDERSTORM COVERAGE/TIMING
AND SEVERE POTENTIAL SATURDAY INTO SATURDAY EVENING.

GRADIENT CONTINUES TO TIGHTEN THIS AFTERNOON BETWEEN DEPARTING
HIGH PRESSURE TO THE EAST AND LOW PRESSURE OVER ONTARIO AND ITS
TRAILING COLD FRONT INTO THE PLAINS. WHILE GUSTS WILL DIMINISH
SOME THIS EVENING...STILL EXPECT BREEZY CONDITIONS ACROSS THE
NORTHERN CWA TONIGHT WHERE THE GRADIENT WILL BE THE TIGHTEST. THIS
WILL HELP KEEP LOW TEMPS IN THE UPPER 50S TO LOWER 60S. WINDS WILL
DIMINISH FURTHER ACROSS THE SOUTHERN CWA ALLOWING LOWS TO LIKELY
DIP INTO THE MID 50S.

THE COLD FRONT NOTED ABOVE WILL MOVE ACROSS THE AREA ON SATURDAY...
THOUGH THE INITIAL WIND SHIFT WILL ONLY BE WESTERLY...WITH ANOTHER
COLD FRONT/WIND SHIFT SATURDAY NIGHT...WHICH WILL SHIFT WINDS TO
THE NORTHWEST. SOUNDINGS THROUGH MIDDAY/EARLY AFTERNOON SUGGEST
FAIRLY STRONG CAPPING WILL BE IN PLACE WHICH WILL LIMIT CONVECTIVE
DEVELOPMENT. BUT AS THE CAP ERODES...AND A TONGUE OF DEWPOINTS IN
THE MID TO PERHAPS UPPER 60S POOLS ALONG THE FRONT...EXPECT
CONVECTIVE DEVELOPMENT BY EARLY/MID AFTERNOON. BEST CHANCES APPEAR
TO BE ALONG/SOUTH OF A CHICAGO TO STERLING LINE. MAINTAINED LIKELY
POPS FROM SATURDAY AFTERNOON THROUGH LATE SATURDAY EVENING BUT AS
THIS TIME PERIOD APPROACHES...POPS CAN BE BETTER REFINED. MAIN
SEVERE THREAT WILL BE DAMAGING WINDS WITH LARGE HAIL ALSO
POSSIBLE.

CONTINUED WITH HIGHS IN THE LOWER 80S SATURDAY BUT CONFIDENCE IS
LOW DEPENDING ON CLOUD COVER AND HOW EARLY THUNDERSTORMS DEVELOP.
POSSIBLE NORTHERN AREAS MAY STAY IN THE 70S WITH MORNING CLOUD
COVER AND POSSIBLY DIMINISHING PRECIP AND A LATER PRECIP
DEVELOPMENT MAY ALLOW TEMPS TO REACH THE MID 80S SOUTH OF I-80.
REGARDLESS...TEMPS SHOULD BE 5-10 DEGREES ABOVE NORMAL MOST AREAS.

AN UPPER WAVE WILL MOVE SOUTHEAST ACROSS THE WESTERN LAKES
SATURDAY NIGHT...WITH THE SECOND COLD FRONT. THIS WILL LIKELY
ALLOW RAIN SHOWERS TO SPREAD ACROSS THE LAKES REGION. MOST OF
THESE SHOULD REMAIN NORTH OF THE CWA BUT SOME LOW CHANCE OF
SHOWERS IS POSSIBLE ACROSS THE FAR NORTH SATURDAY NIGHT.

HIGH PRESSURE WILL SLOWLY MOVE ACROSS THE REGION SUNDAY THROUGH
TUESDAY ALLOWING FOR DRY WEATHER. AS THIS HIGH SHIFTS EAST
MIDWEEK...SOUTHERLY FLOW WILL ALLOW TEMPS TO WARM ABOVE NORMAL...
INTO THE MID/UPPER 70S THURSDAY/FRIDAY. CMS

&&

.AVIATION...

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

* SOUTH WINDS GUSTING AROUND 20 KT THIS AFTERNOON.
* SOUTHWEST WINDS NEAR 40 KT AROUND 1000 FT AGL OVERNIGHT. SOME
  SURFACE GUSTINESS WILL NEGATE TRUE LLWS.
* SOUTHWEST WINDS GUSTING AROUND 20 KT AGAIN SATURDAY.
* POTENTIAL FOR SHRA/TSRA DURING THE DAY SAT...MAINLY AFTERNOON.

RATZER

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

SOUTH WINDS STRENGTHENING ACROSS THE REGION THIS AFTERNOON...IN
RESPONSE TO SURFACE PRESSURE FALLS ASSOCIATED WITH LOW DEVELOPING
NEAR THE CANADIAN BORDER. FORECAST SOUNDINGS DEPICT MIXING TO
AROUND 3500-4000 FT AGL AND TAPPING WINDS AROUND 20 KT FOR GUSTS.
SUNSET SHOULD BRING SOME WEAK DECOUPLING OF THE BOUNDARY LAYER
WHICH SHOULD ALLOW GUSTS TO LESSEN...THOUGH SPEEDS SHOULD REMAIN
12-15 KTS THROUGH THE NIGHT. WITH WEAK INVERSION...WINDS 22040KT
EXPECTED IN THE 1000-2000 FT AGL LAYER WHICH MAY PRODUCE LLWS AT
TIMES...THOUGH EXPECTATION IS THAT SOME MECHANICAL CONTINUED MIXING
AND SPORADIC GUSTINESS AT THE SURFACE WILL PREVENT PERSISTENT WIND
SHEAR LAYER. WINDS ARE EXPECTED TO BECOME MORE WEST-SOUTHWEST
DURING THE SATURDAY...AGAIN WITH GUSTS AROUND 20 KT AS BOUNDARY
LAYER MIXES AFTER SUNRISE.

SURFACE LOW PRESSURE WILL PAS NORTH OF LAKE SUPERIOR LATE TONIGHT
AND SATURDAY...TRAILING A COLD FRONT ACROSS THE MIDWEST WHICH WILL
APPROACH THE TERMINALS DURING THE AFTERNOON AND PASS SATURDAY
EVENING. THE COMBINATION OF LOW LEVEL MOISTURE POOLING AHEAD OF
THE FRONT AND A PAIR OF UPPER DISTURBANCES MOVING ACROSS THE AREA
WILL PROVIDE THE POTENTIAL FOR A PERIOD OF TWO OF SHRA/TSRA FROM
SATURDAY MORNING INTO SATURDAY EVENING. FORECAST SOUNDINGS FROM
SEVERAL MODELS DEPICT A CAPPING INVERSION IN PLACE DURING MUCH OF
THE DAY...WITH THE STRONGEST FORCING AND GREATEST POTENTIAL FOR
ERODING THE CAP AND PRODUCING TSRA JUST AHEAD OF THE COLD FRONT
SATURDAY AFTERNOON. SOME HIGH-RES GUIDANCE DEVELOPS SPOTTY SHOWERS
DURING THE MORNING AS WELL...WHILE OTHER RUNS SUCH AS THE WPC 4 KM
WRF SUGGEST LITTLE IN THE WAY OF COVERAGE THROUGH THE DAY. WITHIN
THIS SCENARIO THE GREATER FORCING AND DIURNAL INSTABILITY MAXIMA
DURING THE MID/LATE AFTERNOON HOURS FAVORS THAT PERIOD FOR
INCLUSION OF A PROB30 TOWARD THE END OF THE ORD 30 HOUR FORECAST.
OTHERWISE CONFIDENCE IS TOO LOW FOR INCLUSION OF SHRA/TSRA THROUGH
SATURDAY MORNING FOR THE OTHER TERMINALS.

RATZER

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

* HIGH IN WIND TRENDS.
* MEDIUM THAT CONTINUED NIGHTTIME SFC WIND GUSTS WILL PREVENT
  PROLONGED LLWS CONDITIONS.
* MEDIUM-LOW IN COVERAGE OF SHRA/TSRA ACROSS TERMINALS SATURDAY.

RATZER

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

SATURDAY NIGHT...THUNDERSTORMS LIKELY. POSSIBLY IFR CONDS WITH
STRONGER STORMS.

SUNDAY THROUGH THURSDAY...VFR.

BEACHLER

&&

.MARINE...
315 PM CDT

PRIMARY CONCERNS ARE SOUTH TO SOUTHWEST GALE FORCE WINDS ON THE
NORTHERN 2/3 OF THE LAKE LATE TONIGHT INTO SATURDAY
MORNING...FOLLOWED BY STRONG NORTH-NORTHWEST WINDS ON SUNDAY INTO
SUNDAY NIGHT. WARM AIR MASS IS LEADING TO STABLE CONDITIONS OVER
THE LAKE BUT VERY TIGHT PRESSURE GRADIENT BETWEEN STRONG LOW OVER
HUDSON BAY AND EXPANSIVE HIGH CENTERED OVER NEW ENGLAND. THIS WILL
SUPPORT HIGH SUSTAINED SPEEDS NEAR OR AT TIMES OVER 30 KT...AND
FREQUENT GALE FORCE GUSTS. OCCASIONAL GALE FORCE GUSTS ARE
POSSIBLE ON THE SOUTHERN 1/3 OF THE LAKE THIS EVENING. ANOTHER LOW
WILL FORM IN THE FRONTAL TROUGH OF THE COLD FRONT TRAILING FROM
THE LOW OVER CANADA AND LIFT NORTHEAST ON SATURDAY. SOUTHWEST
WINDS WILL GRADUALLY EASE THROUGH THE AFTERNOON THEN YET ANOTHER
LOW WILL FORM IN THE FRONTAL TROUGH SATURDAY NIGHT AND MOVE INTO
THE EASTERN LAKES ON SUNDAY AS HIGH PRESSURE SPREADS EAST FROM THE
PLAINS. MUCH COOLER AIR MASS AND FAIRLY TIGHT PRESSURE GRADIENT
WILL RESULT IN NORTHERLY WINDS TO 30 KT ON SUNDAY...ESPECIALLY
OVER THE SOUTH HALF OPEN WATERS. THERE IS SOME POTENTIAL FOR GALES
SO WILL NEED TO MONITOR THIS PERIOD. FAIRLY QUIET CONDITIONS ARE
THEN EXPECTED EARLY TO MID WORK WEEK AS HIGH PRESSURE MOVES
GRADUALLY EAST ACROSS THE OHIO VALLEY.

FOR THE NEARSHORE WATERS...SOUTHERLY WINDS ARE STILL EXPECTED TO
INCREASE TO SMALL CRAFT ADVISORY CRITERIA FOR THE IL ZONES LATE THIS
AFTERNOON AND THEN THE INDIANA ZONES THIS EVENING...WITH CRITERIA
WINDS UNTIL MID SATURDAY AFTERNOON. SMALL CRAFT ADVISORY CONDITIONS
ARE THEN POSSIBLE SUNDAY INTO MONDAY MORNING...ESPECIALLY FOR THE
INDIANA ZONES WHERE THE LARGEST WAVES ARE ANTICIPATED.

RC

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...GALE WARNING...LMZ261-LMZ362-LMZ364-LMZ366-LMZ563-LMZ565-LMZ567-
     LMZ669-LMZ868-LMZ870 UNTIL 10 AM SATURDAY.

     SMALL CRAFT ADVISORY...ILLINOIS NEARSHORE WATERS UNTIL 4 PM SATURDAY.

     SMALL CRAFT ADVISORY...LMZ743-LMZ744-LMZ745...10 PM FRIDAY TO 4
     PM SATURDAY.

&&

$$

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