Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 200559
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1259 AM CDT SAT SEP 20 2014

.DISCUSSION...
914 PM CDT

UPDATES TO POP/WX VERY LATE TONIGHT...AS WELL AS LOWERED MIN TEMPS
IN THE TYPICAL COOL SPOTS.

RELATIVELY QUIET CONDITIONS THIS EVENING WILL LIKELY REMAIN IN
PLACE FOR MOST OF THE NIGHT WHILE TEMPS COOL TO THE UPPER 50S IN
MOST LOCATIONS. DESPITE RAPID DROPS IN TEMPS EARLIER THIS EVENING
DIDNT MAKE TOO MANY ADJUSTMENTS TO GOING MIN TEMPS TONIGHT AS THIS
FALL WILL LIKELY MODERATE THROUGH THE NIGHT AS BOUNDARY LAYER FLOW
CONTINUES TO INCREASE. HAVE INCLUDED LOW CHANCE POPS FOR THE
NORTHWEST AND NORTHERN TIER OF COUNTIES VERY LATE TONIGHT INTO
EARLY SUNDAY MORNING AS LATEST GUIDANCE CONTINUES TO INDICATE
DEVELOPING PRECIP TO THE WEST WILL APPROACH SOONER. THIS UPSTREAM
CONVECTION WILL BE DRIVEN BY AN INCREASING LLJ OUT AHEAD OF
SEVERAL SHORTWAVES/SPEED NOTED ON WATER VAPOR...WITH GUIDANCE
CONSISTENT IN DROPPING A SPEED MAX TOWARDS THE CWA BY MID MORNING
SATURDAY. WITH THIS PROBABLE SOLUTION...DID MAKE ADJUSTMENTS AND
INCREASE SATURDAY MORNING POPS AS WELL. CONFIDENCE IS LOW WITH
REGARDS TO THE INTENSITY OF ANY APPROACHING SHOWERS/THUNDERSTORMS
DURING THIS PERIOD AND A LIKELY SCENARIO WILL BE FOR THIS PRECIP
TO BE DECAYING AS DRY AND STABLE CONDITIONS REMAIN OVER THE CWA
AT THIS HOUR. GUIDANCE DOES RAPIDLY RETURN MID/UPPER 60S DEWPOINTS
AND MODERATE INSTABILITY ACROSS THE CWA BY LATE MORNING/MIDDAY AND
CANT COMPLETELY RULE OUT THIS SCENARIO...AS THESE CONDITIONS ARE
ALREADY PRESENT JUST TO THE WEST IN IOWA. WITH THE PRESENCE OF
LINGERING STEEP MID LEVEL LAPSE RATES...THE POSSIBLE RETURN OF
BETTER MOISTURE/INSTABILITY AND STRONGER LARGE SCALE ASCENT WITH
APPROACHING SPEED MAX...WILL NEED TO MONITOR THE POTENTIAL FOR AN
ISOLATED STRONGER STORM ESPECIALLY OVER NORTH CENTRAL AND
NORTHEAST ILLINOIS DURING THIS TIME. CONFIDENCE LOWERS EVEN MORE
SO WITH THUNDERSTORM EVOLUTION SATURDAY AFTERNOON AND
EVENING...AND WILL LIKELY BE DICTATED BY WHAT OCCURS SATURDAY
MORNING.

RODRIGUEZ

//PREV 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 06Z...

* S/SW WINDS WITH SPEEDS ARND 10-12KT...WILL SEE GUSTS REDEVELOP
  BY DAYBREAK TO NEAR 20KT.

* PSBL ISOLATED SHRA/TSRA WEST OF ORD/MDW BEFORE 14Z...THEN BETTER
  DEVELOPMENT TO TSRA AFT 17Z THRU 21Z.

* PATCHY MVFR CIGS WITH SHRA/TSRA THRU 02Z. WINDS TURN
  WEST/NORTHWEST BY DAYBREAK SUN.

BEACHLER

//DISCUSSION...UPDATED 06Z...

IR IMAGERY EARLY THIS MORNING SHOWS MINIMAL CLOUD COVER OVER
NORTHERN IL/NORTHWEST IN...HOWEVER A COMPLEX OF
SHOWERS/THUNDERSTORMS ACROSS CENTRAL IOWA/SOUTHERN MINNESOTA WILL
CONTINUE TO SLIDE SOUTHEAST TOWARDS THE AREA. TIMING WOULD SUGGEST
THE HIGH CLOUDS WILL ARRIVE ARND DAYBREAK. GUIDANCE HAS SHOWN THAT
SOME SPOTTY SHRA PSBLY AN ISO TSRA COULD BE GOING ON AT DAYBREAK
IN THE WEST NEAR RFD. THEN THERE IS SOME BETTER AGREEMENT THAN
INCREASED COVERAGE OF TSRA WILL TAKE PLACE MIDDAY THRU THE AFTN
AND IMPACT ORD/MDW. MAIN TIMING APPEARS TO BE 18-21Z. WITH THE
STRONGER STORMS SOME PATCHY MVFR CIGS MAY OCCUR. WINDS WILL ALSO
BE A CHALLENGE AS CLOUDS WILL LIKELY KEEP THE GUSTS DOWN ARND
18-22KT. BUT IF CLOUDS CAN THIN OR ERODE ANY MIDDAY...GUSTS COULD
BE HIGHER OR NEAR 25KT. FRONTAL BOUNDARY IS SLOW TO PUSH THRU...SO
SOME LINGER SHRA COULD OCCUR AFT 21Z BEFORE MUCH DRIER AIR ARRIVES
FROM THE NORTHWEST. SOUTHWEST WINDS WILL THEN TURN WEST/NORTHWEST
LATE TONIGHT INTO EARLY SUNDAY MORNING.

BEACHLER


//ORD AND MDW CONFIDENCE...UPDATED 06Z...

* HIGH CONFIDENCE IN ALL ELEMENTS THRU DAYBREAK.

* MEDIUM CONFIDENCE IN TIMING/INTENSITY OF SHRA/TSRA AFT 18Z.

* HIGH CONFIDENCE IN WINDS TURNING SOUTHWEST TO WEST/NORTHWEST
  LATE.

BEACHLER

//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...NEARSHORE WATERS
     UNTIL 4 PM SATURDAY.

&&

$$

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