Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 201655
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1155 AM CDT SAT SEP 20 2014

.DISCUSSION...

1155 AM...LATE MORNING UPDATE...MONITORING CONVECTIVE TRENDS FOR
THUNDERSTORM DEVELOPMENT AND LOCATION THIS AFTERNOON. LOCAL AMDAR
SOUNDINGS SHOWING FAIRLY STRONG CAPPING ACROSS THE REGION
CURRENTLY. FORECAST SOUNDINGS DO SHOW THIS CAP ERODING BY MID/LATE
AFTERNOON WITH CONTINUED SURFACE HEATING AS CIRRUS/DEBRIS CLOUDS
CONTINUE TO THIN. A FEW ISOLATED SHOWERS HAVE DEVELOPED OVER THE
SOUTHERN PART OF THE LAKE AS WELL AS NEAR THE QUAD CITIES.
CONFIDENCE REMAINS SOMEWHAT LOW REGARDING WHEN/WHERE THUNDERSTORMS
DEVELOP THIS AFTERNOON...BUT TRENDS SEEM TO FOCUS ALONG/SOUTH OF
THE I-80 CORRIDOR FROM EARLY/MID AFTERNOON INTO THIS EVENING. THIS
WOULD ALSO BE THE AREAS FOR THE BEST CHANCES FOR SEVERE WEATHER.
CMS

&&

.PREV 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 16Z...

* SOUTHWEST WINDS GUSTING TO AROUND 20 KT THROUGH AFTERNOON.

* SCATTERED SHRA/TSRA EXPECTED EARLY/MID AFTERNOON...PROBABLY
  AFTER ABOUT 19Z.

* PATCHY MVFR CIGS LATE THIS EVENING.

MDB

//DISCUSSION...UPDATED 16Z...

THUNDERSTORM TIMING/COVERAGE STILL A CHALLENGE AND HAVE MADE SOME
REFINEMENTS TO THE TAFS. UPPER LEVEL FORCING CONTINUES TO PRODUCE
SHOWERS AND EMBEDDED TS ACROSS CENTRAL AND NORTHEAST IA. THERE HAS
NOT BEEN MUCH INCREASE IN COVERAGE OR INTENSITY WITH THIS ACTIVITY
AS IT WORKS EAST AND SOUTHEAST. RECENT AIRCRAFT SOUNDINGS FROM
ORD/MDW SHOW A VERY STRONG CAP AND EVEN MODIFYING THOSE FOR
ANTICIPATED SURFACE WARMING DOES NOT YIELD MUCH/ANY INSTABILITY.
PROFILES SHOULD COOL AS THE UPPER FORCING ARRIVES BUT CONFIDENCE
IN EXPANSIVE COVERAGE ACROSS THE TERMINALS IS WANING. ACTIVITY MAY
HAVE A BETTER CHANCE OF TAKING OFF TO SOME DEGREE SOUTH OF THE
TERMINALS LATER THIS AFTERNOON AS THE UPPER FORCING HELPS ERODE
THE CAP TAKING ADVANTAGE OF MAX DAY TIME HEATING.

MDB

FROM 12Z...

IR IMAGERY SHOWS CONSIDERABLE CLOUD COVER STEADILY SLIDING
EAST/SOUTHEAST ACROSS NORTHERN IL/NORTHWEST IN. SOME OF THE CLOUDS
HAVE ERODED...BUT EXPECT SOME DEGREE OF CIRRUS TO PERSIST THRU
THIS MORNING. IN ADDITION THE REGIONAL RADAR MOSAIC INDICATES A
A BROKEN LINE OF SHOWERS/STORMS EXTENDING FROM CENTRAL WISC
STRETCHING SOUTHWEST THROUGH SOUTHCENTRAL IOWA. GUIDANCE CONTINUES
TO POINT TOWARDS THIS AREA OF PRECIPITATION FALLING APART AS IT
SLIDES EAST INTO NORTHWEST IL THIS MORNING. WINDS WILL REMAIN
SOUTHERLY...AND WITH SOME SUNSHINE THIS MORNING MIXING WILL
INCREASE AS WILL GUSTS. EXPECT BY 14-15Z SOUTHWEST WINDS WILL NEAR
10-14KT WITH GUSTS POSSIBLY REACHING 20-24KT. THEN THE ATMOSPHERE
WILL BECOME UNSTABLE BY MIDDAY WITH AN UPPER LEVEL SYSTEM ARRIVING
AND HELPING TO KICK-OFF SHOWERS/THUNDERSTORMS BY 18Z. THE BEST
LOCATION FOR DEVELOPMENT APPEARS TO BE ALONG A PONTIAC TO CHICAGO
LINE. THEN THUNDERSTORMS WILL BE SLIDING EAST/SOUTHEAST THRU THE
AFTN/EARLY EVE. THE FRONTAL BOUNDARY WILL BE TRAILING WHICH WILL
KEEP WINDS SOUTHWEST/WEST THEN BEGIN TURNING WEST/NORTHWEST LATE
THIS EVENING. PRECIP SHUD COME TO AN END ARND 00Z FOR MOST TAF
SITES...THEN IT APPEARS POSSIBLE SOME WRAP-AROUND MOISTURE WILL
ARRIVE ARND DAYBREAK SUN WITH MVFR CIGS RETURNING.

BEACHLER

//ORD AND MDW CONFIDENCE...UPDATED 16Z...

* HIGH CONFIDENCE IN AT LEAST OCCASIONAL WIND GUSTS. MEDIUM
  CONFIDENCE IN TIMING OF WINDS TURNING WEST/NORTHWEST.

* MEDIUM CONFIDENCE IN AT LEAST SCATTERED SHRA/TSRA ACTIVITY
  EARLY/MID AFTERNOON WITH MEDIUM CONFIDENCE IN TIMING DETAILS.

* MEDIUM CONFIDENCE IN PATCHY MVFR CIGS THIS EVENING.

MDB

//OUTLOOK FOR ORD/MDW FOR 00Z MONDAY-12Z SATURDAY...UPDATED 12Z...

SUNDAY NIGHT...MAINLY VFR. PATCHY MVFR CIGS EARLY IN THE EVENING.
BREEZY NORTHWEST WINDS.

MONDAY...VFR. LIGHT WEST WINDS.

TUESDAY...VFR. NORTHEAST WINDS.

WEDNESDAY THROUGH FRIDAY...VFR. GENERALLY SOUTHEAST TO SOUTH WINDS.

BEACHLER

&&

.MARINE...
320 AM CDT

SOUTHWESTERLY GALES WILL CONTINUE TO EASE THIS MORNING...THEN THE
WINDS WILL SHIFT WEST-NORTHWESTERLY BY THIS EVENING AS A COLD
FRONT PUSHES OVER THE LAKE. A SECOND AND STRONGER COLD FRONT WILL
PUSH DOWN THE SOUTHERN HALF OF THE LAKE ON SUNDAY MORNING AS
ANOTHER TROUGH OF SURFACE LOW PRESSURE DEVELOPS AND STRENGTHENS
OVER LOWER MICHIGAN STATE. ONCE THIS SECONDARY COLD FRONT MOVES
THROUGH...NORTHERLY WINDS ACROSS THE SOUTHERN HALF OF THE LAKE
COULD INCREASE TO GALES FOR AT LEAST A FEW HOURS AS THE PRESSURE
GRADIENT STRENGTHENS. GIVEN THIS POTENTIAL...I HAVE ISSUED A GALE
WATCH FOR SOUTHERN LAKE MICHIGAN ON SUNDAY. WIND SPEEDS WILL EASE
SUNDAY EVENING AS THE GRADIENT WEAKENS. SURFACE HIGH PRESSURE WILL
MOVE EAST JUST SOUTH OF LAKE MICHIGAN EARLY NEXT WEEK. THIS WILL
RESULT IN A PERIOD OF WESTERLY WINDS OVER THE LAKE EARLY NEXT
WEEK.

KJB

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...NEARSHORE WATERS UNTIL 4 PM 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.

&&

$$

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