Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 270202
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
902 PM CDT TUE APR 26 2016

.UPDATE...
848 PM CDT

FOR EVENING UPDATE...

FOG HAD STARTED TO ROLL INTO LAKE MICHIGAN SHORE AREAS LATE THIS
AFTERNOON...THOUGH WAS SHORT LIVED WITH VISIBILITY AND CLOUD BASES
RISING PER WEB CAMS THIS EVENING. RAP FORECAST SOUNDINGS SEEM TO
HAVE A GOOD HANDLE ON THIS WITH DRYING IN SFC-950 MB LAYER
SEEMINGLY WELL CORRELATED TO OBSERVED TRENDS. THUS HAVE BACKED OFF
FOG MENTION ALONG THE LAKE OVERNIGHT. FARTHER SOUTH...PRECIP
ACROSS CENTRAL IL/IN HAS MOVED OFF TO THE SOUTHEAST AND HAVE BEEN
ABLE TO PULL POPS FROM FAR SOUTH TIER OF COUNTIES FOR THE
REMAINDER OF THE NIGHT.

MOVING INTO WEDNESDAY...HIGH-RES GUIDANCE TRENDS APPEAR TO SUPPORT
A LITTLE SLOWER ARRIVAL OF RAIN ESPECIALLY ACROSS NORTHEAST IL
WHERE DRY EAST-NORTHEAST FLOW WILL PERSIST. ADJUSTED POPS LOWER
ACROSS NORTHEASTERN PARTS OF THE CWA WEDNESDAY MORNING. OTHERWISE
FORECAST ON TRACK AND NO OTHER SIGNIFICANT CHANGES MADE.

RATZER

&&

.SHORT TERM...
303 PM CDT

THROUGH THURSDAY...

THE MAIN FOCUS DURING THE SHORT TERM PERIOD WILL BE WITH THE
TIMING OF SHOWERS AND THUNDERSTORMS WEDNESDAY INTO THURSDAY OVER
THE AREA.

A LARGE AND POTENT UPPER LEVEL STORM SYSTEM...CURRENTLY NOTED IN
WATER VAPOR IMAGERY SPINNING OVER THE CENTRAL ROCKIES...WILL
CONTINUE TO DRIVE/DEVELOP SURFACE LOW PRESSURE OVER KANSAS TODAY
AND TONIGHT. THE STORM SYSTEM WILL ALSO SUPPORT NUMEROUS SEVERE
THUNDERSTORMS INTO TONIGHT OVER THE PLAINS. ON WEDNESDAY THE
SYSTEM IS EXPECTED TO QUICKLY BECOME VERTICALLY STACKED OVER
...THEN IS FORECAST TO GRADUALLY SHIFT EASTWARD OVER NORTHERN
ILLINOIS BY LATE THURSDAY. ADDITIONALLY...DURING THIS PERIOD...A
LARGE SURFACE RIDGE OF HIGH PRESSURE WILL CONTINUE TO DOMINATE
OVER SOUTHERN CANADA AND THE UPPER GREAT LAKES REGION.

THE PRESSURE PATTERNS DISCUSSED ABOVE WILL CONTINUE TO FAVOR A
PERSISTENT EAST-NORTHEASTERLY LOW-LEVEL FLOW OF COOL LAKE
MODIFIED AIR OVER NORTHERN ILLINOIS. AS A RESULT OF THIS...HIGH
TEMPERATURES WILL CONTINUE TO VARY WIDELY OVER THE REGION OVER THE
NEXT COUPLE OF DAYS. THE COOLEST AREAS WILL BE NEAR LAKE
MICHIGAN...WHERE TEMPERATURES WILL LIKELY REMAIN IN THE MID TO
UPPER 40S. HOWEVER...SOUTH OF INTERSTATE 80...TEMPERATURES COULD
WARM WELL INTO THE 60S WEDNESDAY AND THURSDAY.

IT APPEARS THE THREAT FOR SHOWERS WILL HOLD OFF UNTIL LATE IN THE
DAY ON WEDNESDAY...ESPECIALLY OVER FAR NORTHEASTERN ILLINOIS...BUT
SHOWERS WILL GRADUALLY BECOME LIKELY FROM SOUTHWEST TO NORTHEAST
THROUGH THE DAY AS BETTER MOISTURE TRANSPORT AND WARM AIR
ADVECTION BEGINS TO OCCUR IN RESPONSE TO THE STORM SYSTEM
APPROACHING FROM THE WEST. WEDNESDAY EVENING LOOKS TO BE A RATHER
WET PERIOD OVER THE REGION AS 850 MB WARM FRONT BEGINS TO SHIFT
NORTHWARD OVER THE AREA. IT ALSO APPEARS THERE WILL BE A LOW
THREAT FOR SOME ELEVATED THUNDERSTORMS AS THE 850 MB FRONT MOVES
UP OVER THE REGION. MODEL SOUNDINGS ALSO INDICATE SOME MODEST
STEEPENING MID-LEVEL LAPSE RATES WEDNESDAY NIGHT AS THE MID LEVEL
DRY SLOT BEGINS TO SHIFT OVER THE AREA. WITH THIS IN MIND...WE
HAVE ADDED A CHANCE OF THUNDER TO THE FORECAST. IT APPEARS THE
BEST AREAS WILL BE SOUTH...BUT WITH A WARM AIR ADVECTION SETUP
WEDNESDAY NIGHT...WE ADDED A SMALL CHANCE TO THE ENTIRE FORECAST
AREA.

ON THURSDAY THE UPPER AND LOWER LEVEL LOW ARE EXPECTED TO SHIFT
OVER THE AREA. THIS COULD SET UP SOME ADDITIONAL DIURNAL
THUNDERSTORMS OVER SOUTHERN SECTIONS OF THE AREA IN CLOSE
PROXIMATE TO THE SURFACE BOUNDARY. IT ALSO APPEARS HIGH
TEMPERATURES COULD GET INTO THE LOWER 70S SOUTH OF THE SURFACE
FRONT OVER MY FAR SOUTH. FARTHER NORTH 50S ARE LIKELY...AND EVEN
COLDER...IN THE MID TO UPPER 40S NEAR LAKE MICHGAN.

KJB

&&

.LONG TERM...
320 PM CDT

THURSDAY NIGHT THROUGH TUESDAY...

FRIDAY WE WILL BE BETWEEN STORM SYSTEMS...SO DRY CONDITIONS ARE
STILL EXPECTED. HOWEVER...ATTENTION QUICKLY TURNS TO ANOTHER
SOUTHWESTERN CONUS SYSTEM...WHICH IS FORECAST TO MOVE ACROSS THE
CENTRAL PORTION OF THE COUNTRY THIS WEEKEND. THIS STORM SYSTEM
WILL PRODUCE OUR NEXT CHANCE OF SHOWERS AND SOME THUNDERSTORMS.
HOWEVER...TIMING OF THE PRECIPITATION STILL LOOKS TO BE IN
QUESTION. AS MENTIONED IN THE PREVIOUS DISCUSSION...THERE WILL BE
A PERSISTENT EASTERLY LOW LEVEL FLOW OF DRIER AIR INTO THE REGION
AS HIGH PRESSURE CONTINUES TO DOMINATE TO OUR NORTH. THIS COMBINED
WITH THE POSSIBILITY OF THE SYSTEM ENDING UP SLOWER THAN CURRENT
MODEL FORECASTS SUGGEST COULD RESULT IN A SLOWER ONSET OF SHOWERS
OVER THE AREA THAN THE FORECAST CURRENTLY SUGGESTS. AT THE
PRESENT...I HAVE HELD OFF THE BEST CHANCES OF SHOWERS UNTIL
SATURDAY NIGHT INTO SUNDAY. OTHERWISE COOL WEATHER IS EXPECTED TO
CONTINUE...ESPECIALLY NEAR LAKE MICHIGAN AS A PERSISTENT ONSHORE
FLOW CONTINUES.

KJB

&&

.AVIATION...
FOR THE 00Z TAFS...

MAIN CONCERNS:
* IFR CIGS THROUGH AT LEAST MID EVE AND CIG TRENDS INTO WEDS AM
* MVFR VSBY TRENDS
* OCCASIONALLY GUSTY NORTHEAST WINDS
* SHRA DEVELOPING WEDS AND BECOMING MORE WIDESPREAD LATE
* LOW POTENTIAL FOR EMBEDDED TSRA WEDNESDAY EVENING

AREA RESIDES IN BETWEEN EXPANSIVE HIGH PRESSURE NORTH OF THE GREAT
LAKES AND GENERAL LOW PRESSURE FROM THE PLAINS TO THE OHIO VALLEY.
LARGE AREA OF STRATUS IS TRAPPED UNDERNEATH AN INVERSION AND CIGS
HAVE TRENDED LOWER THE PAST FEW HRS TO IFR IN NORTHEAST IL AND NW
IN. CIG TRENDS ARE A BIT UNCLEAR...AS CLEARING OVER LOWER MI IS
ONLY VERY SLOWLY MOVING SOUTHWEST. NEAR-TERM GUIDANCE INDICATES
POTENTIAL FOR AT LEAST IMPROVEMENT TO MVFR BY MID/LATE
EVENING...WITH SOME SHOWING VFR FOR A TIME BEFORE POSSIBLE
DETERIORATION TOWARD DAYBREAK. WONDER IF IMPROVEMENT IS
OVERDONE...WHICH LOWERS CONFIDENCE...BUT DID INDICATE MVFR AT 04Z
IN TAFS. HAVE HIGHER CONFIDENCE IN IFR THROUGH THAT TIME...THOUGH
BASES MAY START TO IMPROVE BASED ON TRENDS AT LAKE SHORE WEBCAMS.
TRENDS WITH LOWER VSBY ARE ALSO A BIT LOWER CONFIDENCE...AS
ORD/MDW/GYY HAVE IMPROVED SLIGHTLY AFTER HAVING DROPPED TO LOW
MVFR AND IFR LATE THIS AFTERNOON.

ON WEDNESDAY...A FEW UPPER LEVEL DISTURBANCES ASSOCIATED WITH
SURFACE LOW MOVING SLOWLY NORTHWARD FROM THE PLAINS WILL MOVE
ACROSS THE AREA. FIRST WAVE COULD SPREAD LIGHT SHRA ACTIVITY
TOWARD AREA LATE MORNING/MID DAY BUT EXPECTING LOW LEVEL DRY AIR
TO PREVENT IT FROM REACHING TERMINALS. NEXT WAVE IN LATE AFTERNOON
SHOULD FOCUS INTO RFD AREA...WITH DRY AIR POSSIBLY PREVENTING MUCH
IN WAY OF PRECIP FOR EASTERN TERMINALS...SO BACKED OFF TO VCSH
THERE. FINALLY...STRONGEST WAVE WILL ARRIVE DURING THE EVENING AND
EXPECTING ANY DRY AIR TO HAVE ERODED...SO BROUGHT IN PREVAILING
SHRA TO THE ORD 30-HR TAF. EMBEDDED TSRA IS POSSIBLE DURING THIS
TIME...BUT CONFIDENCE IS LOW AND LEFT OUT OF ORD TAF. WINDS WILL
SHIFT FROM NORTHEAST THIS EVENING TO EAST-NORTHEAST AND GUSTY ON
WEDNESDAY AS LOW PRESSURE LIFTS NORTH.

RC

&&

.MARINE...
207 PM CDT

NOW THAT THE COLD FRONT HAS CLEARED THE LAKE...WINDS WILL LARGELY
REMAIN NORTHEASTERLY FOR MUCH OF THE PERIOD. HIGH PRESSURE IS
CENTERED OVER WESTERN HUDSON BAY...WITH LOW PRESSURE OVER THE
PLAINS. THIS LOW WILL DEEPEN SOME TONIGHT THEN SLOWLY WEAKEN AS IT
SPREADS FROM THE CENTRAL PLAINS WEDNESDAY TO CENTRAL ILLINOIS
THURSDAY. ONCE THIS LOW MOVES EAST...BROAD HIGH PRESSURE EXTENDING
SOUTH FROM HUDSON BAY WILL REMAIN IN PLACE WHILE ANOTHER LOW FORMS
IN THE PLAINS LATE IN THE WEEK. WINDS OVER THE LAKE NEVER GET TOO
STRONG AS THE GRADIENT IS NOT SUPER STRONG BETWEEN THESE
FEATURES...BUT SPEEDS WILL CERTAINLY SUPPORT OCCASIONAL SMALL
CRAFT ADVISORY LEVEL WAVES >=4FT AND POSSIBLY STRONGER WINDS AT
TIMES. THE GRADIENT WILL BE STRONGEST ACROSS THE SOUTH HALF. WE
HAVE EXTENDED THE CURRENT SMALL CRAFT ADVISORY FOR THE ILLINOIS
NEARSHORE THROUGH THURSDAY AFTERNOON AS THE ENE FETCH WILL
MAINTAIN WAVES GENERALLY AT OR ABOVE 4 FT FOR MOST OF THIS PERIOD
EXCEPT FOR POSSIBLY A BRIEF PERIOD WEDNESDAY EVENING. WE HAVE
MAINTAINED THE INDIANA NEARSHORE EXPIRATION TIME JUST FOR MORE OF
AN EAST WIND CUTTING DOWN WAVES FOR A LONGER PERIOD OF TIME
THOUGH...THOUGH IT IS LIKELY THERE WILL BE ANOTHER PERIOD OF
HIGHER WAVES ON THE INDIANA SHORE THURSDAY.

KMD

&&

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

     SMALL CRAFT ADVISORY...ILLINOIS NEARSHORE WATERS UNTIL 4 PM THURSDAY.

&&

$$

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