Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 270751
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
251 AM CDT WED APR 27 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 06Z TAFS...

MVFR STRATUS EXTENDS WEST TO ABOUT THE MISSISSIPPI RIVER AND HAS
NOT MOVED MUCH IN THE PAST FEW HOURS. SATELLITE TRENDS AND
GUIDANCE HAVE GIVEN ME ENOUGH CONFIDENCE TO KEEP MVFR STRATUS IN
THE FORECAST INTO THIS AFTERNOON. NORTHEAST TO EAST WINDS WILL
REMAIN ARND 15 KT THIS MORNING WITH GUSTS TO 25 KT BEGINNING LATE
THIS MORNING. HAVE MEDIUM CONFIDENCE THAT THE MVFR CIGS WILL
SCATTER OUT THIS AFTN AHEAD OF THE ADVANCING LOW AND ASSOCIATED
PRECIP. HAVE HIGH CONFIDENCE IN RAIN OCCURRING TONIGHT...BUT ONLY
MEDIUM CONFIDENCE IN TIMING. GUIDANCE VARIES ON TIMING BUT MOST
MODELS FEATURE THE MAIN SWATH OF RAIN ROTATING THROUGH EARLY THIS
EVENING FOR RFD AND THEN MID EVENING FOR THE EASTERN TERMINALS.
REMOVED THE VCSH AND PUSHED THE TIMING OF PRECIP BACK BY AN HOUR.
THINKING MVFR CIGS WILL ACCOMPANY THE SHOWERS AND THERE IS A
CHANCE IFR CIGS MAY ALSO CREEP OVER THE TERMINALS. DID NOT HAVE A
LOT OF CONFIDENCE IN IFR CIGS SO KEPT THEM AS A SCT LAYER FOR NOW.
A RUMBLE OF THUNDER IS PSBL...BUT THINKING THE VAST MAJORITY WILL
BE SOUTH OF THE TERMINALS SO DID NOT INCLUDE THUNDER IN THE TAFS.

JEE

&&

.MARINE...
250 AM CDT

HEADLINES...THE SMALL CRAFT ADVISORIES LOOK GOOD AS IS.

LOW PRESSURE OVER KANSAS WEAKENS AND PASSES OVER CENTRAL IL
THURSDAY. MEANWHILE HIGH PRESSURE OVER CENTRAL CANADA REMAINS
STATIONARY THROUGH THE END OF THE WEEK AND THEN SHIFTS EAST OVER
QUEBEC EARLY NEXT WEEK.  NORTH TO EAST WINDS ARE EXPECTED OVER THE
LAKE THROUGH MONDAY. THE ONSHORE FLOW RESULTS IN HAZARDOUS WAVES FOR
SMALL CRAFT INTO THURSDAY FOR THE IL NSH WATERS.  ANOTHER WEAK LOW
PASSES SOUTH OF THE LAKE SUNDAY...AND THEN HIGH PRESSURE SPREADS
OVER THE LAKE MONDAY. A LOW PRESSURE SYSTEM MOVES ACROSS SOUTHEAST
CANADA MONDAY NIGHT INTO TUESDAY...AND WINDS ARE FORECAST TO BECOME
SOUTH OR SOUTHWEST AS THE HIGH SHIFTS EAST OF THE LAKE TUESDAY.

JEE

&&

.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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