Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 200008
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
708 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 00Z...

* SOUTHWEST WINDS NEAR 40 KT AROUND 1000 FT AGL OVERNIGHT. SOME
  SURFACE GUSTINESS SHOULD NEGATE TRUE LLWS.

* SOUTHWEST WINDS GUSTING AROUND 20 KT AGAIN SATURDAY.

* POTENTIAL FOR SHRA/TSRA DURING THE DAY SATURDAY...POSSIBLY BY
  LATE MORNING.

MDB

//DISCUSSION...UPDATED 00Z...

SOUTHERLY WIND GUSTS HAVE BECOME LESS FREQUENT OVER THE LAST HOUR
OR SO BUT AM EXPECTING THAT GUSTS BECOME MORE FREQUENT INTO THE
OVERNIGHT AS THE PRESSURE GRADIENT INCREASES AND LOW LEVEL WINDS
ABOVE THE SURFACE DO THE SAME. WILL CONTINUE TO KEEP LLWS MENTION
OUT OF THE TAFS WITH THE EXPECTATION OF GUSTS...WITH PARAMETERS
MARGINAL EVEN IF WINDS ARE LOWER THAN FORECAST. WINDS WILL TURN A
LITTLE MORE SOUTHWESTERLY INTO SATURDAY MORNING WITH SPEEDS AND
GUSTS COMING UP. WINDS LOOK TO TURN WESTERLY DURING THE EVENING AS
A COOL FRONTAL BOUNDARY APPROACHES/CROSSES THE AREA BUT DO NOT
HAVE A GREAT DEAL OF CONFIDENCE IN THE TIMING OF THIS.

CONVECTION WILL BECOME MORE OF A CONCERN LATER TONIGHT THROUGH THE
DAY SATURDAY. ACTIVITY ALONG THE FRONT TO THE NORTHWEST OF THE
AREA TONIGHT WILL MOVE SOUTHEASTWARD BUT IT IS UNCLEAR HOW
ORGANIZED IT WILL REMAIN BY DAYBREAK. WHATEVER ACTIVITY REMAINS
WOULD LIKELY REACH NORTH CENTRAL IL AROUND DAYBREAK. THIS MAY
RESULT IN THE NEED FOR SHRA/TSRA MENTION OF SOME SORT AT RFD BY
12Z. EVOLUTION OF THIS ACTIVITY WILL ALSO COMPLICATE THE FORECAST
FOR THE REMAINING TERMINALS. REMNANT CONVECTION COULD MOVE ACROSS
THROUGH THE MORNING...WHICH ALSO SUGGESTS THE POTENTIAL FOR
LOWER...PERHAPS EVEN MVFR...CIGS DURING THE MORNING. FOR NOW WILL
TRY TO TIME THE LARGER SCALE UPPER TROUGH WHICH SHOULD ARRIVE BY
MIDDAY SO HAVE SPED UP THE PROB30 MENTION OF TSRA BY A FEW HOURS
AT ORD/MDW/DPA/GYY. SHOULD MORNING ACTIVITY HOLD TOGETHER TO AT
LEAST SOME MINIMAL EXTENT THEN NEW DEVELOPMENT COULD BE FOCUSED
MORE TO THE SOUTHEAST OF THE TERMINALS DURING THE AFTERNOON. WITH
SEVERAL POSSIBLE SCENARIOS HAVE FAIRLY LOW CONFIDENCE IN THE
DETAILS BUT FEEL THE BEST COURSE WAS TO STICK CLOSER TO THE TIMING
OF THE UPPER TROUGH WHICH IS REFLECTED IN THE CURRENT TAFS.

MDB


//ORD AND MDW CONFIDENCE...UPDATED 00Z...

* HIGH CONFIDENCE IN OVERALL WIND TRENDS...MEDIUM CONFIDENCE IN
  SPEEDS THROUGH THE EVENING.

* MEDIUM THAT CONTINUED NIGHTTIME SFC WIND GUSTS WILL PREVENT
  PROLONGED LLWS CONDITIONS.

* LOW-MEDIUM IN COVERAGE/TIMING OF SHRA/TSRA ACROSS TERMINALS
  SATURDAY.

MDB

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

SUNDAY...VFR. BREEZY NORTHWEST WINDS.

MONDAY...VFR. LIGHT WESTERLY WINDS.

TUESDAY...VFR. NORTHEASTERLY WINDS.

WEDNESDAY AND THURSDAY...VFR. SOUTHEASTERLY WINDS.

MDB

&&

.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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