Area Forecast Discussion
Issued by NWS Chicago, IL

Home | Current Version | Previous Version | Graphics & Text | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

000
FXUS63 KLOT 040139 AAA
AFDLOT

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
839 PM CDT TUE MAY 3 2016

.UPDATE...
838 PM CDT

THERE WERE ONLY A FEW CHANGES MADE TO THE FORECAST TONIGHT AND ALL
FAIRLY MINOR SINCE THE MAIN THEME OF THE FORECAST
CONTINUES...SHOWERS AND SOME STORMS TONIGHT WITH WINDY AND
COOLER CONDITIONS MOVING IN. THE CHANGES INCLUDED 1.) HAVING
INCREASED THE THUNDER LIKELIHOOD OVERNIGHT...2.) BUMPED UP WIND
GUSTS LATE TONIGHT AND WEDNESDAY MORNING ESPECIALLY NEAR THE LAKE
SHORE WHERE SOME GUSTS OF 40 TO 45 MPH ARE POSSIBLE...AND 3.) FOR
WEDNESDAY AFTERNOON WENT WITH MORE COVERAGE WORDING OF SHOWERS AS
CONFIDENCE IN DEVELOPMENT IS FAIRLY HIGH...ESPECIALLY FOR AREAS
EAST OF INTERSTATE 39.

WATER VAPOR IMAGERY THIS EVENING PORTRAYS A REALLY WELL-DEFINED
SHORT WAVE UPPER CIRCULATION AT THE LEADING EDGE OF A SHARPLY
DIGGING BROADER TROUGH. THE 00Z INL SOUNDING SAMPLED A 115 KT DUE
NORTHERLY JET...HIGHLY ANOMALOUS ORIENTATION AND SPEED FOR MAY.
THE SHORT WAVE HAS BEEN ON A STEADY TRACK TOWARD THE AREA WITH ITS
LEADING EDGE PRESENTLY IN CENTRAL WISCONSIN. CONVECTION AND
ASSOCIATED LIGHTNING HAS BEEN STEADY ALONG THIS FRONT EDGE AND
OFTEN WITH THESE COLD CORE WAVES KEEP THEIR LIGHTNING OVERNIGHT.
SO HAVE INCREASED THUNDER CHANCES AS STEEP MID-LEVEL LAPSE RATES
/SAMPLED ON 00Z MPX SOUNDING/ MOVE INTO THE REGION. THE OVERALL
COVERAGE OF SHOWERS AND ANTICIPATED FORCING...AS WELL AS OUTPUT
FROM CONVECTIVE ALLOWING MODELS...IS HIGH ENOUGH TO WARRANT NEAR
100 POPS BETWEEN MAINLY 11 PM AND 4 AM FROM NORTH TO SOUTH.

UPSTREAM OBSERVATIONS AT THE SURFACE ARE INDICATE A STRENGTHENING
ISALLOBARIC COUPLET AS THE LOW OF 1002 MB NEAR LA CROSSE
WISCONSIN CONTINUES TO DEEPEN. SOME UPSTREAM WINDS ARE ALREADY
OUTPERFORMING GUIDANCE AND WOULD EXPECT THAT TO CONTINUE TO BE THE
TREND IN THIS PATTERN. HAVE INCREASED GUSTS WITH TEMPORARY 30 TO
40 MPH GUSTS EXPECTED OVER MUCH OF THE AREA IMMEDIATELY BEHIND THE
FRONT AND THEN 30 TO 35 MPH GUSTS MUCH OF THE DAY WEDNESDAY.
CLOSER TO THE LAKE SHORE GIVEN THE FRICTIONLESS NORTHERLY
DIRECTION THE GUSTS SHOULD PEAK MORE 35 TO 45 MPH DURING WEDNESDAY
MORNING.

WAVES ARE LIKELY TO BUILD TO OR A LITTLE OVER 10 FT ON THE
CHICAGO SHORE BY LATER WEDNESDAY MORNING AND 12 TO 15 FT ALONG
THE INDIANA SHORE BY EARLY AFTERNOON. THIS FORECAST IS BELOW
THRESHOLDS FOR A LAKE FLOOD ADVISORY.

AS FOR FORECAST TEMPERATURES WEDNESDAY...THESE CONTINUE TO LOOK
TO BE ON TRACK WITH MID TO UPPER 40S HIGHS IN THE DOWNSTREAM PART
OF THE LAKE AND LOWER TO MID 50S ELSEWHERE.

MTF

&&

.SHORT TERM...
228 PM CDT

THROUGH WEDNESDAY...

BIG CHANGES ARE IN STORE IN THE SHORT TERM. HIGH PRESSURE IS SLOWLY
SHIFTING SOUTH AS LOW PRESSURE AND A COLD FRONT DROP SOUTH OUT OF
ONTARIO.  EXPECTING A PLEASANT AFTERNOON AND EVENING WITH WEST WINDS
AND FAIR WEATHER CUMULUS CLOUDS.  SHOWERS HAVE ALREADY FORMED AHEAD
OF THE COLD FRONT OVER THE U.P. OF MICHIGAN. REALLY LIKING THE RAP/S
TIMING SO LEANED HEAVILY IN ITS DIRECTION...WITH THE NAM BEING A
SLOWER SOLUTION.  HAVE SLIGHT CHANCE TO CHANCE POPS OUT AHEAD OF THE
MAIN FRONTAL PRECIP JUST IN CASE A FEW STRAY SHOWERS FORM THIS
EVENING. KEPT THE PREVIOUS FORECASTS IDEA OF NUMEROUS SHOWERS
WITH A FEW EMBEDDED THUNDERSTORMS. SEVERE STORMS ARE NOT EXPECTED
AS CAPE VALUES WILL BE MINIMAL AT BEST.

THE BIGGER STORY WILL BE WINDS SHIFTING OUT OF THE NORTH AND GUSTING
TO 30 MPH BEHIND THE FRONT. WINDS WILL BE EVEN GUSTIER ALONG THE
SOUTHERN END OF LAKE MICHIGAN ESPECIALLY ALONG THE INDIANA SHORE.
MUCH COOLER TEMPERATURES FOLLOW THE FRONT WITH HIGHS IN THE LOW TO
MID 50S AWAY FROM THE LAKE.  HIGHS ALONG THE LAKE WILL BE IN THE
UPPER 40S TO MAYBE 50.  THE BACK END OF THE UPPER LEVEL LOW ROTATES
THROUGH TOMORROW AND EXPECTING SCATTERED SHOWERS TO FORM.  FORECAST
SOUNDINGS SHOW VERY SHALLOW CAPE...BUT ANY CONVECTION WOULD BE VERY
LOW TOPPED.  FREEZING LEVELS ARE ALSO LOW AROUND 3000 FT SO THINKING
THE LOW CAPE AND STEEP LAPSE RATES WILL LEAD TO GRAUPEL INSTEAD OF
THUNDER.  NOT EXPECTING A TON OF RAIN...MORE ALONG THE LINES OF
DREARY CONDITIONS WITH OCNL LIGHT RAIN AND POSSIBLY GRAUPEL.

JEE

&&

.LONG TERM...
259 PM CDT

WEDNESDAY NIGHT THROUGH TUESDAY...

THE PERIOD WILL BEGIN ON THE CHILLY SIDE WEDNESDAY NIGHT AS THE
LINGERING EFFECTS OF THE UNSEASONABLE STRONG UPPER LOW CONTINUES
TO SHIFT OVER THE LOWER OHIO VALLEY. CLEARING SKIES WEDNESDAY
NIGHT SHOULD ALLOW TEMPERATURES TO RADIATE SOME INTO THE UPPER
30S...ESPECIALLY ACROSS NORTH CENTRAL ILLINOIS AND PORTIONS OF THE
FOX RIVER VALLEY...AND LOWER 40S ELSEWHERE. IT APPEARS THAT A
LIGHT NORTHERLY WIND WILL CONTINUE THROUGH MUCH OF THE NIGHT...SO
THE THREAT OF FROST LOOKS LIKE IT WOULD ONLY BE SPOTTY AND
CONFINED TO WELL PROTECTED AREAS...SO WE WILL LEAVE THIS MENTION
OUT OF THE FORECAST FOR NOW. OTHERWISE...EXPECT A SURFACE RIDGE OF
HIGH PRESSURE TO BUILD OVER THE AREA ON THURSDAY. ONSHORE FLOW
WILL AGAIN RESULT IN NEAR 50 DEGREE TEMPERATURES ALONG THE
LAKEFRONT...BUT FARTHER INLAND TEMPERATURES LOOK TO REBOUND BACK
INTO THE LOWER 60S UNDER MAINLY SUNNY SKIES.

MUCH WARMER CONDITIONS ARE EXPECTED ON FRIDAY AS A WESTERLY WIND
SHIFT ALLOWS THE THERMAL RIDGE OVER THE PLAINS TO SHIFT OVER THE
WESTERN GREAT LAKES. MODEL GUIDANCE IS ADVERTISING 925 MB
TEMPERATURES AROUND +20C BY MIDDAY FRIDAY. THIS SHOULD ALLOW
TEMPERATURES TO WARM INTO THE LOWER 80S...ESPECIALLY OVER NORTH
CENTRAL ILLINOIS. THERE IS A POSSIBILITY THAT A LAKE BREEZE COULD
DEVELOP FRIDAY AFTERNOON DUE TO RELATIVELY WEAK WESTERLY FLOW AT
THE LAKE FRONT...AND THIS COULD RESULT IN SLIGHTLY COOLER
CONDITIONS THERE IF ONE DOES DEVELOP. IN ADDITION TO THE QUICK
WARM-UP...I CANNOT TOTALLY RULE OUT A FEW ISOLATED OR WIDELY
SCATTERED THUNDERSTORMS LATE THURSDAY NIGHT INTO FRIDAY WITHIN THE
STRONG WARM AIR ADVECTION REGIME. HOWEVER...DUE TO THE RELATIVELY
WEAK SIGNAL...AND THE LIKELY LOW AREAL COVERAGE...WE HAVE OPTED TO
LEAVE THE FORECAST DRY AT THIS TIME.

A POTENT AND FAST EASTWARD MOVING STORM SYSTEM ACROSS SOUTHERN
CANADA FRIDAY INTO SATURDAY LOOKS TO DROP A COLD FRONT SOUTHWARD
OVER THE AREA BY LATE SATURDAY. THIS SHOULD SET THE STAGE FOR OUR
NEXT DECENT THREAT FOR SHOWERS AND THUNDERSTORMS. ALTHOUGH DEEP
LAYER SHEAR IS NOT ADVERTISED TO BE OVERLY IMPRESSIVE OVER THE
AREA...WE CANT RULE OUT A FEW STRONG THUNDERSTORMS WITH THIS
FRONTAL PASSAGE SATURDAY AFTERNOON. ALSO...WITH THE TIMING OF THIS
FRONT LOOKING TO BE DURING THE MIDDAY TO AFTERNOON HOURS...IT
APPEARS HIGHS ON SATURDAY COULD AGAIN REACH INTO THE 70S IN MOST
INLAND AREAS...TO POSSIBLY LOW 80S AGAIN SOUTH PRIOR TO THE ONSET
OF CONVECTION WITH THE APPROACHING FRONTAL BOUNDARY.

SURFACE HIGH PRESSURE LOOKS TO SET UP OVER THE AREA BY SUNDAY...SO
DRY...BUT COOLER CONDITIONS IN THE 60S ARE EXPECTED.
HOWEVER...WARMER CONDITIONS...INTO THE 70S...BEGIN TO RETURN BY
EARLY NEXT WEEK. IT ALSO APPEARS THE THE WEATHER PATTERN COULD BE
CONDUCIVE TO ADDITIONAL PERIODS OF SHOWERS AND THUNDERSTORMS INTO
NEXT WEEK AS MODEL/ENSEMBLE GUIDANCE ALL ARE FORECASTING ANOTHER
STORM SYSTEM TO TAKE SHAPE OVER THE PLAINS AND THEN SHIFT INTO THE
UPPER MIDWEST BY THE MIDDLE OF NEXT WEEK.

KJB

&&

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

MAIN FORECAST CONCERNS ARE WITH SHOWER AND THUNDER CHANCES LATER
TONIGHT...LOW END MVFR AND IFR CEILINGS...AND STRONG NORTHERLY
WINDS WEDNESDAY MORNING.

WESTERLY WINDS AND VFR CEILINGS OCCURRING EARLY THIS EVENING WILL
CONTINUE FOR MOST OF THE EVENING UNTIL AN AREA OF LOW PRESSURE AND
FRONTAL BOUNDARY QUICKLY PUSH SOUTH ACROSS THE REGION. DO EXPECT
AROUND A THREE HOUR WINDOW OF PREVAILING SHOWERS WITH SOME
EMBEDDED THUNDER TO OCCUR JUST AFTER THE MIDNIGHT TIME FRAME
TONIGHT. THE MAIN AREA OF PRECIP WILL SWING THROUGH BUT THERE DOES
APPEAR TO BE SOME IMMEDIATE SHOWER DEVELOPMENT STILL POSSIBLE
AFTER. VFR CEILINGS WILL QUICKLY LOWER TO LOW END MVFR AND IFR
DURING THAT TIME AND STAY IN PLACE THROUGH EARLY WEDNESDAY
MORNING. WINDS WILL ALSO QUICKLY SHIFT TO THE NORTH WITH
SPEEDS/GUSTS RAMPING UP...AND LOOKING TO STAY GUSTY OUT OF THE
NORTH FOR MOST OF WEDNESDAY. HAVE CONTINUED MENTION OF ADDITIONAL
SHOWER CHANCES THROUGHOUT THE DAY ON WEDNESDAY AND HAVE CONFIDENCE
OF OCCURRENCE...BUT LOWER CONFIDENCE WITH COVERAGE AND DURATION.

RODRIGUEZ

&&

.MARINE...
333 PM CDT

A COLD FRONT IS EXPECTED TO SHIFT SOUTHWARD DOWN THE LAKE TONIGHT.
THIS WILL SET UP NORTHERLY FLOW LAKE-WIDE BY DAYBREAK WEDNESDAY. IT
ALSO APPEARS THAT GALE FORCE WINDS WILL OCCUR ON THE SOUTHERN HALF
OF THE LAKE WEDNESDAY INTO WEDNESDAY EVENING AS THE SURFACE GRADIENT
TIGHTENS IN RESPONSE TO A DEVELOPING SURFACE LOW OVER LOWER MICHIGAN
AND NORTHERN OHIO. WITH THIS IN MIND...WE HAVE ISSUED A GALE WARNING
FOR THE SOUTHERN HALF OF THE LAKE. AT THE PRESENT TIME...WE HAVE
LEFT THE ILLINOIS NEAR SHORES OUT OF THIS GALE AS IT APPEARS THE
STRONGEST WINDS WILL REMAIN OVER THE OPEN WATERS AND INTO THE
INDIANA NEAR SHORE WATERS. THEREFORE...THE SMALL CRAFT ADVISORY
CONTINUES FOR THE ILLINOIS WATERS. THE WINDS LOOK TO ABATE LATE
WEDNESDAY NIGHT INTO THURSDAY AS A SURFACE RIDGE OF HIGH PRESSURE
BUILDS OVER THE WESTERN GREAT LAKES. THIS SHOULD RESULT IN A QUICK
END TO THE GALES WEDNESDAY NIGHT.

WINDS LOOK TO BECOME SOUTHERLY AGAIN ON FRIDAY AHEAD OF ANOTHER
STORM SYSTEM MOVING INTO WESTERN ONTARIO. THIS STORM SYSTEM IS
EXPECTED TO PUSH ANOTHER COLD FRONT SOUTHWARD DOWN THE LAKE ON
SATURDAY...AND THIS COULD RESULT IN ANOTHER PERIOD OF ENHANCED
NORTHERLY WINDS ON SATURDAY.

KJB

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...GALE WARNING...LMZ740-LMZ741-LMZ742...3 AM WEDNESDAY TO 3 PM
     WEDNESDAY.

     SMALL CRAFT ADVISORY...LMZ740-LMZ741-LMZ742...3 PM WEDNESDAY TO
     10 AM THURSDAY.

     GALE WARNING...LMZ743-LMZ744-LMZ745-LMZ777-LMZ779...3 AM
     WEDNESDAY TO 4 AM THURSDAY.

     SMALL CRAFT ADVISORY...LMZ743-LMZ744-LMZ745...4 AM THURSDAY TO 3
     PM THURSDAY.

&&

$$

VISIT US AT HTTP://WEATHER.GOV/CHICAGO (ALL LOWERCASE)

FOLLOW US ON FACEBOOK...TWITTER...AND YOUTUBE AT:
WWW.FACEBOOK.COM/NWSCHICAGO
WWW.TWITTER.COM/NWSCHICAGO
WWW.YOUTUBE.COM/NWSCHICAGO


USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.