Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 290707
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
207 AM CDT SAT AUG 29 2015

.UPDATE...
626 PM CDT

HAVE ADJUSTED POPS DOWNWARD THROUGH THE MID EVENING HOURS TO
ACCOUNT FOR RADAR AND NEAR-TERM GUIDANCE TRENDS. LEADING EDGE OF
SHORTWAVE TROUGH AND VORTICITY MAXIMA WILL MOVE EAST FROM
WISCONSIN THIS EVENING...KEEPING BEST FORCING FOR PRECIP NORTH OF
THE STATE LINE. THUS...JUST HAVE GENERALLY ISOLATED
SHRA/INTERMITTENT SPRINKLE MENTION THROUGH MID EVENING IN FAR
NORTHERN ILLINOIS. VERY DRY AIR NOTED AROUND 800 MB ON A RECENT
AMDAR SOUNDING FROM MDW WILL MAKE IT TOUGH FOR ANY STEADY PRECIP
UNTIL DRY AIR GETS ERODED. UPPER TROUGH/SHORTWAVE AXIS AND
SHEARED VORT MAX ALONG TROUGH AXIS WILL SLOWLY PROGRESS FROM IA TO
NORTHERN IL LATE THIS EVENING AND OVERNIGHT...BRINGING GRADUALLY
INCREASING SHRA/RAIN COVERAGE FROM NORTHWEST TO SOUTHEAST...BUT
MANY AREAS ALONG AND EAST OF AN EVANSTON TO PONTIAC LINE COULD
VERY WELL REMAIN DRY THROUGH DAYBREAK. VERY LITTLE IF ANY
INSTABILITY OVER AREA THROUGH THE NIGHT AND POOR MIDLEVEL LAPSE
RATES SUPPORT KEEPING SLIGHT CHANCE TSRA AT BEST. THE REST OF THE
FORECAST LOOKS TO BE ON TRACK.

RC

&&

.SHORT TERM...

TONIGHT AND SATURDAY...

250 PM...FORECAST CONCERNS INCLUDE PRECIP COVERAGE/TIMING AND
RAINFALL AMOUNTS.

LOW PRESSURE OVER WESTERN IOWA WILL SLOWLY MOVE EAST TONIGHT AND
EVENTUALLY DISSIPATE OVER NORTHERN ILLINOIS SATURDAY. FIRST WAVE OF
PRECIP IS GENERALLY LIFTING NORTHEAST INTO SOUTHERN WISCONSIN AND
THE MODELS SEEM TO HAVE THIS HANDLED FAIRLY WELL WITH THE HRRR ALSO
SHOWING JUST ISOLATED SHOWERS/SPRINKLES INTO EARLY THIS EVENING AND
MAINLY ACROSS NORTHWEST AND NORTHERN IL. ADDITIONAL PRECIP IS
EXPECTED TO DEVELOP LATER THIS EVENING AND SPREAD NORTHEAST
OVERNIGHT INTO SATURDAY MORNING. THIS PRECIP WILL ALSO BE FOCUSED
ACROSS NORTHWEST/NORTHERN IL...INTO SOUTHERN WI. MODEL QPF AMOUNTS
NEAR THE IL/WI STATE LINE ARE IN THE 1 TO 1.5 INCH RANGE. THESE
AMOUNTS HAVE BEEN CONSISTENT BETWEEN RUNS AND VARIOUS MODELS SO IT
IS LIKELY SOME AREAS WILL SEE OVER AN INCH OF RAIN. HOWEVER...THESE
AMOUNTS APPEAR TO DROP OFF QUICKLY BETWEEN I-88 AND I-80 WITH MUCH
OF THE SOUTHEAST THIRD OR HALF OF THE CWA POSSIBLY STAYING DRY INTO
SATURDAY MORNING. INSTABILITY IS LIMITED AND LIGHTNING HAS BEEN
CONFINED TO THE STRONGEST STORMS/CONVECTION. THUNDER IS CERTAINLY
POSSIBLE TONIGHT BUT OVERALL COVERAGE SHOULD BE LIMITED AND HELD
SLIGHT CHANCE MENTION OF THUNDER TO HIGHER POP AREAS.

AFTER THE MAIN AREA OF RAIN MOVES NORTHEAST SATURDAY MORNING...
PRECIP ACROSS THE REST OF THE CWA WILL BE MORE SCATTERED...OR
ISOLATED...INTO THE LATE MORNING/EARLY AFTERNOON. APPEARS THERE
COULD BE NEW DEVELOPMENT IN THE EARLY/MID AFTERNOON BUT COVERAGE
POSSIBLY STILL JUST SCATTERED. THUS DIDN/T MAKE TOO MANY CHANGES
FROM CURRENT POPS IN THE 40-50 PERCENT RANGE.

TEMPS WILL ONLY SLOWLY FALL INTO THE LOWER/MID 60S TONIGHT AND HIGHS
ON SATURDAY WILL BE DEPENDENT ON PRECIP COVERAGE AND CLOUD COVER.
CURRENT FORECAST OF MID 70S NORTH TO LOWER 80S SOUTHEAST WILL
NEED SOME SUN TO OCCUR. SOUTHEASTERLY WINDS WILL KEEP THE IL SHORE
A BIT COOLER. CMS

&&

.LONG TERM...
307 PM CDT

SATURDAY NIGHT THROUGH FRIDAY...

THE LATEST LONGER TERM GUIDANCE IS INDICATING SOME CHANGES POSSIBLE
FOR THE LONGER RANGE TRENDS FOR TEMPERATURES AND PCPN...BUT THE
EARLIER PORTION OF THE LONG TERM FORECAST IS RELATIVELY CONSISTENT
WITH PREVIOUS RUNS.

THE GENERAL TREND FOR TEMPERATURES IS STILL FOR A WARMING TREND FOR
NEXT WEEK...BUT TEMPERATURES SHOULD STILL BE SLIGHTLY BELOW NORMAL
THROUGH SUNDAY.  FOR SATURDAY NIGHT AND INTO EARLY SUNDAY...THERE
SHOULD STILL BE A CHANCE OF SOME LINGERING SHOWERS OR A FEW
THUNDERSTORMS...ESPECIALLY FOR NWRN INDIANA AS THE UPPER LEVEL
SHORTWAVE RESPONSIBLE FOR FOCUSING THE CONVECTIVE ACTIVITY LIFTS
OFF TO THE EAST AND WEAKENS. BY SUNDAY...LEE TROUGHING WILL DEVELOP
OVER THE WRN PLAINS AS A NORTHERN STREAM SHORTWAVE AND AN ASSOCIATED
SFC REFLECTION DROPS OUT OF THE CANADIAN ROCKIES AND INTO THE
PRAIRIES.  THE OVERALL PATTERN WILL BECOME NEARLY STATIONARY THROUGH
MUCH OF NEXT WEEK AS THE SFC HIGH REMAINS PARKED OVER THE MID
ATLANTIC REGION WHILE THE TROUGH REMAINS OVER THE PLAINS.  THIS WILL
HELP AID THE WARMING TREND AS SLY WINDS TRY TO DRAW MOISTURE NWD
FROM THE GLFMEX...THOUGH WEAK SFC RIDGING THROUGH THE NRN GULF COAST
MAY CAUSE SOME DELAY IN THE MOISTURE RETURN.

THE MAIN CHANGE TO THE EXTENDED FORECAST IS RELATED TO HOW THE
MODELS NOW HANDLE THE REMNANTS OF TS ERIKA.  PREVIOUS RUNS OF THE
LONGER RANGE GUIDANCE HAD PREVIOUSLY LIMITED THE WESTWARD
PROGRESSION OF THE TROPICAL SYSTEM...WHILE THE LATEST RUNS ARE
HINTING THAT ERIKA COULD TRACK FARTHER WEST.  SO...WHILE THERE IS
STILL A FAIR AMOUNT OF UNCERTAINTY IN HOW FAR WEST THE PERIPHERAL
EFFECT OF THE TROPICAL SYSTEM WILL EXTEND...AND WE ARE NOT TALKING
ABOUT THE TROPICAL SYSTEM ACTUALLY MOVING ANYWHERE CLOSE TO NRN
IL/IN...RATHER THE IDEA IS THAT IF THE TROPICAL SYSTEM CAN TRACK A
LITTLE FURTHER WEST...THEN THERE WILL BE GREATER MOISTURE TRANSPORT
INTO THE MIDWEST...AND AS A RESULT...GREATER CHANCES FOR SHRA/TSRA
AND MORE CLOUD COVER THAN PREVIOUSLY ANTICIPATED.  SO HAVE BACKED
OFF A COUPLE DEGREES ON MAX TEMPS THROUGH THE EXTENDED PERIOD AS
WELL AS INTRODUCING PERIODIC SLIGHT CHANCE POPS AND A BIT MORE CLOUD
COVER.

KREIN

&&

.AVIATION...

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

* PERIOD OF RAIN OVERNIGHT INTO SATURDAY MORNING
* MVFR OR LOWER CIGS AND VSBY POSSIBLE SAT NIGHT

IZZI

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

WEAKENING STORM SYSTEM WILL BRING A PERIOD OF RAIN WITH POSSIBLE
PERIODIC MVFR VSBY LATE TONIGHT INTO SAT MORNING BEFORE RAIN
TAPERS OFF AND ENDS BY MIDDAY. SMALL CHANCE EXISTS FOR SOME ISOLD
SHRA OR PERHAPS A TSRA DURING THE LATE AFTERNOON AS WHAT IS LEFT
OF THE STORM SYSTEM FESTERS...BUT CHANCES LOOK TOO LOW TO INCLUDE
PRECIP IN TAFS AT THIS TIME. LIGHT EAST TO SOUTHEAST WINDS COULD
TURN MORE SOUTHERLY SAT MORNING BEFORE PROBABLY TURNING FAIRLY
LIGHT AND VARIABLE DURING THE AFTERNOON. SHOULD SEE A TENDENCY FOR
WINDS TO BECOME LIGHT NORTHEAST SAT NIGHT AND WITH AMPLE LOW LEVEL
MOISTURE AND RECENT RAINS COULD SEE SOME LIGHT FOG AND/OR STRATUS
DEVELOP.

IZZI

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

* MEDIUM-HIGH CONFIDENCE IN RAIN LATE TONIGHT/SAT MORNING
* HIGH CONFIDENCE IN WINDS BELOW 10KT...BUT LOW CONFIDENCE IN
  DIRECTION.
* LOW CONFIDENCE IN CIGS/VSBY SAT NIGHT

IZZI

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

SUNDAY...VFR. EAST WIND.
MONDAY...VFR. SOUTHEAST WIND.
TUESDAY...VFR. SOUTH WIND.
WEDNESDAY...SLT CHC TSRA. PRIMARILY VFR. SOUTH WIND.
THURSDAY...VFR. SOUTH WIND.

BMD

&&

.MARINE...
207 AM CDT

NARROW RIDGE OF HIGH PRESSURE STRETCHING FROM NEAR DETROIT TO
GREEN BAY COMBINED WITH WEAKENING LOW PRESSURE OVER IOWA WILL
RESULT IN A NARROW RIBBON OF STRONGER EASTERLY WINDS EARLY THIS
MORNING OVER PORTIONS OF CENTRAL LAKE MICHIGAN BEFORE BACKING MORE
SOUTHEAST LATER THIS MORNING THEN ULTIMATELY EASING THIS
AFTERNOON. WEAK LOW OVER IOWA WILL DISSIPATE TONIGHT AND REMAINDER
OF THE UPCOMING WEEK WILL SEE WEATHER OVER THE LAKE DOMINATED BY
HIGH PRESSURE ANCHORED OVER THE MID ATLANTIC AND NORTHEAST.

IZZI

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...NONE.
&&

$$

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