Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 200820
AFDLOT

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

.DISCUSSION...
243 AM CDT

TODAY...
MAIN FOCUS IN THE NEAR TERM WILL REVOLVE AROUND TIMING OF
PRECIP/THUNDERSTORMS...WHICH GUIDANCE IS STILL LEANING TOWARDS A
MIDDAY/AFTN TIMEFRAME. THE WILDCARD IS THE EARLY MORNING CONVECTION
OVER CENTRAL IOWA THAT STRETCHES NORTHEAST INTO FAR SOUTHWEST
WISCONSIN. DRY AIR CONTINUES TO LINGER ACROSS THE FORECAST
AREA...ALTHOUGH THIS WILL QUICKLY CHANGE AS THE MORNING PROGRESSES.
ABUNDANT MOISTURE EXISTS JUST WEST OF THE CWFA WITH PWAT`S HOVERING
BETWEEN 1.5-1.8" IN MUCH OF IOWA/NORTHWEST MO. THIS MOISTURE RICH
ENVIRONMENT IS PROGGED TO SLIDE EAST BY LATE MORNING...BRINGING DEW
PTS INTO THE MID/UPR 60S AND PERHAPS A FEW SITES HITTING 70 DEG DEW
PTS THIS AFTN...MAINLY SOUTH OF I-80. MID-LVL LAPSE RATES REMAIN
RATHER STEEP EARLY THIS MORNING...SO IF THE CONVECTION ACROSS IOWA
CAN HOLD TOGETHER IT IS POSSIBLE THE ELEVATED CORES COULD STRENGTHEN
FURTHER AND BRING A LOW RISK OF SEVERE STORMS TO THE AREA BY LATE
MORNING. HI-RES GUIDANCE IS POINTING TOWARDS THE EARLY MORNING LINE
DISSIPATING.

BY LATE MORNING A MID-LVL VORT...THAT CONTINUES TO DIVE SOUTHEAST
ACROSS THE NORTHERN PLAINS...SHOULD CATCH-UP WITH THE TROUGH AND
HELP TO REDEVELOP AND STRENGTHEN THUNDERSTORM ACTIVITY SHORTLY AFT
18Z. THE PLACEMENT WILL CONTINUE TO BE A CHALLENGE...AS SOME
GUIDANCE SUGGESTS THAT IT COULD SLIP JUST PAST CHICAGO BEFORE
DEVELOPING. NONETHELESS HAVE HELD ONTO THE LIKELY POPS FROM PONTIAC
TO CHICAGO EARLY AFTN...THEN EXPECT THIS CHANNEL OF LIKELY POPS TO
SLOWLY SLIDE EAST. MID-LVL VORT MAX WILL BE PIVOTING EAST ACROSS THE
CWFA BY 20Z...WHICH SHUD THEN BRING A SLOW END TO THE STRONGER
STORMS FROM WEST TO EAST SHORTLY AFTER THIS. THE FRONTAL BOUNDARY
APPEARS TO BE LINGERING FURTHER WEST...BUT SHORTLY AFT 00Z THIS
BOUNDARY SHUD ARRIVE ACROSS THE NORTHWEST FORECAST AREA.

TEMPS TODAY WILL BE A CHALLENGE. AT FIRST GLANCE IT APPEARS THE
HIGHS FORECAST IN THE LOW 80S LOOK TO BE IN GOOD SHAPE. HOWEVER WITH
THE CLOUD DEBRIS FROM THE EARLY MORNING COMPLEX ARRIVING THIS
MORNING...IT IS PLAUSIBLE THAT ENOUGH CLOUD COVER LINGERS AND HOLDS
TEMPS DOWN IN THE 70S. THE DEEP LAYERED SHEAR IS PROGGED TO STEADILY
INCREASE BY THIS AFTN AS WELL...WHICH COULD SUGGEST SOME OF THE
STRONGEST STORMS COULD PRODUCE A DAMAGING WIND THREAT AND PERHAPS
BECOME BETTER ORGANIZED.

TONIGHT...
MID-LVL VORT WILL QUICKLY PEEL OFF TO THE EAST/NORTHEAST BY
00Z...AND COULD SEE A QUICKER END TO THE CONVECTION JUST BEFORE 00Z.
FOR NOW WILL SHOW A STEADY DECREASING TREND IN THE PRECIP...WITH
MUCH DRIER/COOLER AIR STEADILY ADVECTING IN FROM THE NORTHWEST.
WINDS WILL SLOWLY TURN NORTHWEST OVERNIGHT. WRAP-AROUND MOISTURE
SHUD HOLD OFF UNTIL JUST AFT DAYBREAK SUN...WITH MUCH OF THE
OVERNIGHT HOURS REMAINING DRY. OVERNIGHT LOWS WILL LIKELY HOLD IN
THE LOW 60S ACROSS THE SOUTHEAST CWFA...WHILE OTHER AREAS FALL INTO
THE MID/UPR 50S.

BEACHLER


LONG TERM SUNDAY THROUGH FRIDAY...

A SECONDARY TROUGH OF LOW PRESSURE AND COLD FRONT WILL SHIFT
SOUTHWARD OVER THE AREA SUNDAY MORNING AND EARLY AFTERNOON.
MEANWHILE...IN THE MID AND UPPER LEVELS...A SIGNIFICANT
DISTURBANCE AND SPEED MAX...NOW OVER SOUTHERN SASKATCHEWAN...WILL
DROP SOUTHEASTWARD ACROSS SOUTHERN WISCONSIN AND NORTHERN
ILLINOIS. THIS LOOKS PRODUCE SOME SCATTERED SHOWERS ACROSS THE
AREA...ESPECIALLY DURING THE MID MORNING AND EARLY AFTERNOON
HOURS. THE EXPECTED CLOUD COVER AND MUCH COOLER AIR MASS WILL
LIKELY ONLY ALLOW FOR TEMPERATURES IN THE LOWER TO MIDDLE 60S
ACROSS NORTHERN ILLINOIS SUNDAY. IT ALSO LOOKS TO BE A BREEZY DAY
AS HIGH PRESSURE BUILDS OVER THE DAKOTAS. WEST TO NORTHWESTERLY
WINDS EARLY SUNDAY MORNING WILL SHIFT NORTHERLY BY THE AFTERNOON
IN THE WAKE OF THE FRONT.

SURFACE HIGH PRESSURE WILL BUILD EASTWARD FROM THE NORTHERN
PLAINS...SUNDAY NIGHT AND MOVE OVERHEAD ON MONDAY. THIS WILL
RESULT IN A COUPLE OF CHILLY NIGHTS ACROSS THE AREA SUNDAY NIGHT
AND MONDAY NIGHT...WITH TEMPERATURES POTENTIALLY DROPPING INTO THE
UPPER 30S IN SOME AREAS.

THE REMAINDER OF THE WEEK AFTER SUNDAY LOOKS VERY NICE AND DRY.
TEMPERATURES WILL MODERATE BACK INTO THE 70S BY MID WEEK AS THE
SURFACE HIGH SHIFTS EAST OF THE AREA.


KJB

&&

.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.

     GALE WATCH...LMZ080-LMZ567-LMZ669-LMZ671-LMZ673-LMZ675-LMZ744-
     LMZ745-LMZ777-LMZ779-LMZ868-LMZ870-LMZ872-LMZ874-LMZ876-
     LMZ878...9 AM SUNDAY TO 7 PM SUNDAY.

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

&&

$$

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