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 172020
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
320 PM CDT WED SEP 17 2014

.DISCUSSION...
319 PM CDT

THE LAKE BREEZE SLOWLY EASING INLAND ACROSS FAR EASTERN COOK AND
LAKE COUNTIES IN ILLINOIS...AND AS SUCH...THIS IS PRODUCING
SLIGHTLY COOLER CONDITIONS LAKESIDE WITH THE ONSHORE FLOW.

ATTENTION IS FOCUSED ON A COLD FRONTAL BOUNDARY...CURRENTLY SHIFTING
SOUTHWARD ACROSS THE U.P. OF MICHIGAN. THIS BOUNDARY IS THE LEADING
EDGE OF A RATHER STOUT BAROCLINIC ZONE. MUCH COLDER AIR...COLD
ENOUGH TO SUPPORT SNOW...IS IN PLACE NORTH OF THE FRONT ACROSS
SOUTHERN CANADA. FORTUNATELY FOR US...THE CORE OF THIS COLD AIRMASS
WILL SLIDE EAST-SOUTHEASTWARD ACROSS SOUTHEASTERN CANADA AND NEW
ENGLAND. HOWEVER...THE SURFACE BOUNDARY IS EXPECTED TO SHIFT OVER
THE AREA AS A BACK DOOR COLD FRONT BY EARLY THURSDAY MORNING. THIS
SHOULD RESULT IN WINDS COMING IN OFF THE LAKE THROUGH THE DAY
TOMORROW...ALONG WITH INCREASED CLOUDINESS. IT APPEARS THAT SOME
LOWER STRATOCU WILL OVERSPREAD PORTIONS OF NORTHERN ILLINOIS BY
AROUND DAYBREAK THURSDAY...POSSIBLY LASTING INTO THE AFTERNOON
BEFORE SLOWLY DISSIPATING. THERE COULD EVEN BE A FEW LIGHT LAKE
EFFECT RAIN SHOWERS/SPRINKLES LATER THURSDAY MORNING. MODELS
SOUNDINGS INDICATE JUST ENOUGH COOL AIR MOVING OVER THE LAKE TO
RESULT IN SOME HIGHER EL`S UP TO AROUND 7500 FEET AGL. THIS COULD BE
ENOUGH TO SQUEEZE OUT A FEW VERY LIGHT SHOWERS. I ADDED A SLIGHT
CHANCE OF SPRINKLES TO COVER THIS MINOR THREAT. OTHERWISE...COOLER
CONDITIONS...IN THE LOWER 60S...WILL BE FAVORED ACROSS NORTHEASTERN
ILLINOIS DUE TO THE ONSHORE FLOW.

THE WARMER AIR WILL MAKE A QUICK RETURN TO THE AREA ON FRIDAY AS
WARM AIR ADVECTION AND UPPER LEVEL HEIGHT RISES RAMP UP AHEAD OF AN
AREA OF LOW PRESSURE ACROSS CANADA. THIS SHOULD RESULT IN
TEMPERATURES WELL INTO THE 70S AREA-WIDE BY FRIDAY...AND INTO THE
LOWER 80S FOR SATURDAY.

THUNDERSTORMS STILL LOOK LIKE A GOOD BET ON SATURDAY INTO SATURDAY
EVENING AS A COLD FRONT APPROACHES THE AREA. VERY WARM AND MOIST
CONDITIONS ARE FORECAST TO SETUP ALONG A CORRIDOR TO THE SOUTH OF THE
APPROACHING COLD FRONT...AND THIS SHOULD YIELD CONDITIONS FAVORABLE
FOR THUNDERSTORMS WITH AT LEAST A BRIEF TORRENTIAL DOWNPOUR THREAT.
GUIDANCE INDICATES PWATS COULD EXCEED 1.6 INCHES WITHIN THE
PREFRONTAL AIRMASS ACROSS NORTHERN ILLINOIS/NORTHWESTERN INDIANA ON
SATURDAY. AS SUCH...THIS COULD LEAD TO SOME EFFICIENT RAIN RATES WITH
ANY STORMS THAT DEVELOP. THERE COULD ALSO BE A FEW STRONG...MORE
ORGANIZED...STORMS SATURDAY AFTERNOON GIVEN THE POTENTIAL OF A MID
LEVEL JET DEVELOPING SOUTHWARD OVER THE AREA.

FOLLOWING THIS SYSTEM TEMPERATURES WILL TREND COOLER AGAIN...AND THE
WEATHER SHOULD BE DRY FOR A COUPLE OF DAYS EARLY NEXT WEEK AS ANOTHER
SURFACE HIGH IS PROGGED TO SHIFT RIGHT OVER THE AREA BY LATE MONDAY
INTO TUESDAY.

KJB

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 20Z...

* LAKE BREEZE THIS AFTERNOON...MOVED TIMING OF BOUNDARY PASSAGE
  AND WIND SHIFT TO ELY UP TO 22Z AND BUMPED UP WINDS TO 8KT.

* COLD FRONT PASSAGE EARLY THURSDAY MORNING WITH MVFR STRATUS
  BEHIND...CHANCE THAT CIGS COULD BRIEFLY DROP TO IFR FOR AN HOUR
  OR SO.

* NELY WINDS TO 10KT THURSDAY AFTERNOON.

KREIN

//DISCUSSION...UPDATED 18Z...

A LARGE AREA OF HIGH PRESSURE COVERS THE REGION. THE WEAK PRESSURE
GRADIENT SHOULD ALLOW FOR A LAKE BREEZE TO DEVELOP LATE THIS
AFTERNOON. WITH LAND AND WATER TEMPERATURES SOMEWHAT
SIMILAR...THERE WILL NOT BE MUCH PUSH BEHIND THE BOUNDARY...SO
EXPECT THAT EXPECTED ELY WINDS SHOULD ONLY BE 5-7 KT.

WITH SUNSET...THE LAKE BREEZE SHOULD WASH OUT AND WINDS WILL
BECOME LGT/VRBL. THE LIGHT WINDS AND MOSTLY CLEAR SKIES SHOULD
ALLOW SOME PATCHY SHALLOW GROUND FOG TO DEVELOP...MAINLY IN RURAL
AREAS.

A COLD FRONT IS EXPECTED TO DROP SOUTH OUT OF WISCONSIN AND DOWN
THE LENGTH OF LAKE MICHIGAN...SETTING UP NELY WINDS BY THE LATE
MORNING HOURS. WITH A STRONGER COLD PUSH BEHIND THIS
BOUNDARY...ANTICIPATE THAT THE NELY WINDS SHOULD APPROACH 10KT
DURG THE AFTERNOON HOURS. THERE IS A CHANCE OF SOME LOWER STRATUS
BEHIND THE COLD FRONT...ESPECIALLY CLOSER TO THE LAKE FRONT...BUT
THE MODEL GUIDANCE IS STILL SHOWING A FAIR AMOUNT OF VARIABILITY
TO THE LOW LEVEL MOISTURE PROFILES...SO WILL KEEP THE GOING
FORECAST FOR VFR SKIES AND MONITOR CONDITIONS AS THE COLD FRONT
WORKS DOWN THE LAKE LATER THIS EVENING AND TONIGHT.

KREIN

//ORD AND MDW CONFIDENCE...UPDATED 20Z...

* MEDIUM-HIGH CONFIDENCE IN LAKE BREEZE TIMING.

* MEDIUM TO HIGH CONFIDENCE IN MVFR CIGS THURSDAY MORNING...LOWER
  CONFIDENCE IN CHANCES FOR IFR CIGS.

* MEDIUM CONFIDENCE THAT WINDS COULD REACH OR SLIGHTLY EXCEED 10
  KT TOMORROW AFTERNOON.

KREIN

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

THURSDAY NIGHT THROUGH FRIDAY...VFR
SATURDAY...CHANCE OF THUNDERSTORMS.
SUNDAY THROUGH TUESDAY...VFR.

BMD

&&

.MARINE...
128 PM CDT

AN AREA OF LOW PRESSURE OVER THE EASTERN UPPER PENINSULA OF MICHIGAN
EARLY THIS AFTERNOON WILL CONTINUE TO MOVE EAST TONIGHT...TRAILING A
COLD FRONT WHICH WILL PUSH SOUTH DOWN THE LENGTH OF LAKE MICHIGAN
THROUGH TONIGHT AND EARLY THURSDAY MORNING. AS THE FRONT BEGINS TO
DROP SOUTH OVER THE NORTHERN END OF LAKE MICHIGAN BY EARLY THIS
EVENING...WINDS WILL VEER TO THE NORTH IN THE 20-25 KT RANGE. WINDS
WILL DIMINISH LATE TONIGHT AND THURSDAY AS THE FRONT PUSHES FARTHER
DOWN THE LAKE...AND LOSES SOME OF ITS PUNCH. HIGH PRESSURE WILL THEN
BUILD SOUTHEAST ACROSS THE NORTHERN LAKES AND EVENTUALLY ACROSS NEW
ENGLAND BY FRIDAY...WITH WINDS ON LAKE MICHIGAN SHIFTING BACK TO THE
SOUTH AND INCREASING LATE THURSDAY NIGHT. SOUTH WINDS TO 30 KT ARE
ANTICIPATED ACROSS THE LAKE FRIDAY AS THE GRADIENT TIGHTENS IN
RESPONSE TO LOW PRESSURE MOVING NORTH OF THE LAKES...AND A PERIOD OF
GALES OR AT LEAST SOME GALE FORCE GUSTS ARE POSSIBLE ESPECIALLY
ACROSS NORTHERN LAKE MICHIGAN LATE FRIDAY INTO EARLY SATURDAY.
FORECAST SOUNDINGS DEPICT 35-40 KT ABOVE THE SHALLOW MARINE BOUNDARY
LAYER DURING THIS TIME...WITH THE DRIVING FACTOR BEING THE DEPTH OF
MIXING IN THE SOUTHERLY WARM ADVECTION FLOW. THE LOW WILL PASS NORTH
OF THE LAKE LATE SATURDAY INTO SUNDAY...WITH A COLD FRONT PUSHING
SOUTHEAST SATURDAY NIGHT RESULTING IN A SHIFT TO NORTHWEST WINDS FOR
SUNDAY AND MONDAY.

RATZER

&&

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

$$

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

FOLLOW US ON FACEBOOK...TWITTER...AND YOUTUBE AT:
WWW.FACEBOOK.COM/US.NATIONALWEATHERSERVICE.CHICAGO.GOV
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.