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 212329
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
629 PM CDT TUE OCT 21 2014

.DISCUSSION...
309 PM CDT

MAIN FORECAST CONCERNS FOCUS ON PRECIPITATION CHANCES AND TIMING
WITH A COUPLE OF SYSTEMS WHICH WILL AFFECT THE REGION THURSDAY AND
NEXT MONDAY. ALSO...IT APPEARS THAT SOME 70 DEGREE TEMPERATURES ARE
POSSIBLE FOR PARTS OF THE FORECAST AREA OVER THE WEEKEND AS WELL AS
ON MONDAY.

TONIGHT THROUGH WEDNESDAY NIGHT...

GOES WATER VAPOR IMAGERY DEPICTS A STRONG MID-LEVEL SHORT WAVE
TROUGH WHICH HAD DUG SOUTH ACROSS THE REGION TODAY IS NOW EXITING
THE AREA ACROSS THE OHIO VALLEY TO THE SOUTHEAST. SUBSIDENCE IN THE
WAKE OF THIS FEATURE WAS EVIDENT BOTH IN RISING SURFACE PRESSURE
ACROSS THE CWA...AND IN VISIBLE SATELLITE IMAGERY AS EXTENSIVE AREA
OF STRATOCU ACROSS THE REGION ERODES FROM THE NORTH ACROSS WI/MI AND
LAKE MICHIGAN. EARLIER LAKE-INDUCED SHOWERS HAVE WEAKENED AND FADED
AS EXPECTED DUE TO LOWERING SUBSIDENCE INVERSION AND ASSOCIATED
WARMING ALOFT. CLOUD COVER IS EXPECTED TO CONTINUE TO DECREASE IN
COVERAGE INTO THE EVENING HOURS...WITH PERHAPS JUST SOME PATCHY THIN
SHALLOW STRATOCU LINGERING OFF THE LAKE BY LATE EVENING. LOOKING AT
THE BIGGER PICTURE...HIGH-AMPLITUDE UPPER RIDGE ANALYZED OVER THE
CENTRAL PORTION OF THE NATION WILL MOVE ACROSS THE MISSISSIPPI
VALLEY AND WESTERN/NORTHERN GREAT LAKES REGION THROUGH WEDNESDAY
MORNING...MAINTAINING SUBSIDENCE WHICH SHOULD FURTHER ERODE
LINGERING CLOUDS AND ALLOW SURFACE HIGH PRESSURE RIDGE TO MOVE
ACROSS THE AREA TONIGHT AND WEDNESDAY. WITH CLEARING SKIES AND
DECREASING WINDS ASSOCIATED WITH THE SURFACE RIDGE AXIS TONIGHT...
TEMPS ARE EXPECTED TO DROP OFF INTO THE 30S IN MANY AREAS AWAY FROM
THE IMMEDIATE LAKE MICHIGAN SHORE AND CORE OF CHICAGO. LATE NIGHT
MINS IN THE MID 30S AWAY FROM THE METRO AREA WILL LIKELY ALLOW AREAS
OF FROST TO DEVELOP WHERE WINDS DECOUPLE BECOME LIGHT/VARIABLE.
UPPER AND LOW LEVEL HIGH PRESSURE RIDGES REMAIN OVER THE AREA
WEDNESDAY ALLOWING PLENTIFUL SUNSHINE...THOUGH WITH ONLY MODEST
WARMING DUE TO EASTERLY WIND COMPONENT OFF LAKE AND COOLER UPSTREAM
AIR MASS. A BLEND OF MOS GUIDANCE APPEARS REASONABLE BASED ON RAW
MODEL LOW LEVEL THERMAL FIELDS...WITH WEDNESDAY AFTERNOON MAXES
EXPECTED TO RANGE FROM THE LOW 50S ALONG THE LAKE TO THE UPPER 50S
ACROSS THE FAR WESTERN/SOUTHERN COUNTIES OF THE CWA. ATTENTION THEN
TURNS BACK TOWARD THE NORTHWEST WEDNESDAY NIGHT...AS A WELL-DEFINED
SHORT WAVE TROUGH TRACKS ACROSS THE SOUTHERN CANADIAN PRAIRIES AND
DAKOTAS BEFORE APPROACHING THE UPPER MISSISSIPPI VALLEY WEDNESDAY
EVENING. HEIGHT/PRESSURE FALLS ACROSS THE UPPER MIDWEST NUDGES THE
SURFACE RIDGE AWAY TO OUR EAST AND INDUCES INCREASING
SOUTH-SOUTHEAST WINDS AHEAD OF THE APPROACHING SURFACE TROUGH
OVERNIGHT. CLEAR-PARTLY CLOUDY CONDITIONS AND LIGHT WINDS WILL
LIKELY ALLOW FOR QUICK DECOUPLING OF BOUNDARY LAYER WINDS EARLY
WEDNESDAY EVENING WITH TEMPS DROPPING INTO THE 30S AGAIN IN OUTLYING
AREAS...THOUGH SOME INCREASE IN HIGH CLOUDS AND THE INCREASE IN
WINDS OVERNIGHT WILL LIKELY LIMIT FROST SOMEWHAT.

THURSDAY THROUGH FRIDAY...

THE UPPER MIDWEST SHORT WAVE CONTINUES TO MOVE EAST-SOUTHEAST INTO
THE GREAT LAKES REGION THURSDAY...WITH THE BEST ORGANIZED DEEP LIFT
GENERALLY NORTH OF THE WFO CHICAGO CWA. THE MID-LEVEL TROUGH AXIS
DOES SHIFT ACROSS THE AREA LATER IN THE DAY AND OVERNIGHT HOWEVER...
THOUGH WITH MOISTURE RETURN FROM THE GULF INITIALLY BLOCKED BY THE
SURFACE HIGH PRESSURE RIDGE WHICH EXTENDS SOUTHWEST ACROSS THE
ARKLATEX REGION FOR MUCH OF THE PERIOD. THUS MOISTURE RETURN IS NOT
OVERLY IMPRESSIVE WITH THIS SYSTEM...THOUGH A PERIOD OF SHOWERS DOES
LOOK FAIRLY LIKELY THURSDAY INTO THURSDAY NIGHT. PRECIP POTENTIAL
LOOKS TO INCREASE AHEAD OF THE MID-LEVEL TROUGH AS IT MOVES INTO
NORTH CENTRAL IL DURING THE MID-LATE MORNING HOURS AND SPREADING
INTO NORTHEAST IL DURING THE AFTERNOON. FORCING WEAKENS THURSDAY
EVENING AS THE TROUGH RUNS INTO THE WEAKENING UPPER RIDGE...WITH
PRECIP EXPECTED TO BECOME MORE SPOTTY THURSDAY NIGHT AS IT MOVES
ACROSS THE EASTERN HALF OF THE FORECAST AREA. MID-LEVEL TROUGH AND
WEAKENING SURFACE TROUGH/COLD FRONT OF PACIFIC ORIGIN PASS EAST OF
THE AREA FRIDAY MORNING...WITH WEAK HIGH PRESSURE BUILDING IN DURING
THE DAY. TEMPERATURES ACTUALLY WARM IN THE WAKE OF THIS SYSTEM AS
THE SURFACE RIDGE DRIFTS EAST DURING THE DAY AND WESTERLY FLOW ALOFT
AROUND REBUILDING UPPER RIDGE OVER THE PLAINS ALLOWS WARM ADVECTION
BY AFTERNOON. MODEL THERMAL PROGS INDICATE SOME LOW-MID 60S ACROSS
NORTHERN IL FRIDAY AFTERNOON.

FRIDAY NIGHT THROUGH TUESDAY...

LONGER RANGE MODEL GUIDANCE IN GOOD AGREEMENT IN SPREADING BROAD
UPPER RIDGE EASTWARD FROM THE PLAINS AND ACROSS THE MIDWEST AND
GREAT LAKES REGION THROUGH THE WEEKEND...WHILE DEEP UPPER TROUGH
MOVES INTO THE WESTERN CONUS. MODELS DEPICT STRONG SOUTHERLY FLOW
AND WARM ADVECTION DEVELOPING AS SURFACE LOW PRESSURE DEVELOPS AND
LIFTS NORTHEAST ACROSS THE PLAINS AND THE UPPER MIDWEST SUNDAY...
THOUGH AGAIN WITH LIMITED MOISTURE RETURN FOR THE MAGNITUDE OF THE
FLOW. THIS ALLOWS US TO WARM UP SIGNIFICANTLY HOWEVER...WITH HIGHS
LIKELY GETTING ABOVE 70 DEGREES SATURDAY AND SUNDAY ACROSS PORTIONS
OF THE CWA...AND OVER MOST OF IT MONDAY. SURFACE TROUGH/COLD FRONT
APPROACHES MONDAY WITH AN ATTENDANT CHANCE OF RAIN AND INCREASED
CLOUD COVER. A SECOND LOW IS DEPICTED LIFTING ACROSS THE AREA MONDAY
NIGHT-TUESDAY MAINTAINING RAIN THREAT...AND ALSO LEADING TO MUCH
COOLER WEATHER AGAIN TUESDAY BEHIND THE SYSTEM COLD FRONT.

RATZER

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 00Z...

* NORTH/NORTHEAST WINDS GRADUALLY DROPPING BELOW 10 KT EARLY THIS
  EVENING.

* MVFR CEILINGS HOLDING ON POSSIBLY THROUGH MID EVENING.

KJB

//DISCUSSION...UPDATED 00Z...

MVFR CIGS CONTINUE TO MOVE IN OFF THE LAKE EARLY THIS EVENING VIA A
NORTH-NORTHEASTERLY LOWER LEVEL WIND TRAJECTORY ASSOCIATED WITH A
SURFACE RIDGE OF HIGH PRESSURE BUILDING EASTWARD ACROSS THE UPPER
GREAT LAKES AND OVER EASTERN ONTARIO. IT APPEARS THESE LOW CLOUDS
WILL TRY TO GRADUALLY SCATTERED OUT OVER THE NEXT 2 TO 3 HOURS.
HOWEVER...SOME OF THESE CLOUDS COULD BE A LITTLE SLOWER TO SCATTER
OUT ACROSS THE EASTERN TERMINALS NEAR THE LAKE AS CONDITIONS LOOK
TO REMAIN FAVORABLE FOR SOME CLOUD COVER OVER SOUTHERN LAKE
MICHIGAN. THEREFORE...IT IS POSSIBLE THAT AS THE LOWER LEVEL WINDS
VEER MORE NORTHEASTERLY THIS EVENING...SOME CLOUDS WILL CONTINUE
TO STREAM OFF THE LAKE AND OVER NORTHEASTERN ILLINOIS AND
NORTHWESTERN INDIANA DURING THE EVENING HOURS. I WILL CONTINUE TO
WATCH TRENDS WITH THIS AS THE EXACT TIMING OF SCATTERING IS A BIT
LOWER THAN PREVIOUSLY THOUGHT. THE GOOD NEWS IS THAT CIG HEIGHTS
SHOULD GENERALLY REMAIN AT OR ABOVE 2500 FEET AGL...SO AVIATION
IMPACTS SHOULD BE MINIMAL.

OTHERWISE...THE REMAINDER OF THE PERIOD LOOKS TO BE MAINLY VFR WITH
LIGHT EASTERLY WINDS.

KJB


//ORD AND MDW CONFIDENCE...UPDATED 00Z...

* MEDIUM-HIGH CONFIDENCE WITH WIND SPEED/DIRECTION.

* LOW-MEDIUM CONFIDENCE WITH EXACT TIMING OF IMPROVING CEILINGS
  THIS EVENING.

KJB

//OUTLOOK FOR ORD/MDW FOR 12Z THURSDAY-12Z TUESDAY...UPDATED 00Z...

* THURSDAY...CHANCE OF AFTERNOON SHRA. SOUTH WINDS.
* FRIDAY AND SATURDAY...DRY/VFR. WEST/SOUTHWEST WINDS.
* SUNDAY...DRY/VFR. SOUTH WINDS.
* MONDAY...SLIGHT CHANCE OF SHRA. GUSTY SOUTHWEST WINDS.

KJB

&&

.MARINE...

215 PM...LOW PRESSURE OVER WESTERN NEW YORK WILL MERGE WITH ANOTHER
LOW OVER NEAR THE DELMARVA AREA TONIGHT...AND RAPIDLY DEEPEN
WEDNESDAY NIGHT AND THURSDAY JUST SOUTH OF CAPE COD. THIS LOW WILL
THEN TRACK NORTHEAST TO NOVA SCOTIA BY SATURDAY MORNING.
MEANWHILE...HIGH PRESSURE CENTERED OVER THE SOUTHERN TIP OF JAMES
BAY...WITH HIGH PRESSURE RIDGING EXTENDING THROUGH THE MIDDLE
MISSISSIPPI VALLEY...WILL SLOWLY BUILD EAST. THE BRISK
NORTHEASTERLY WINDS CURRENTLY OVER THE LAKE WILL DIMINISH AND
BECOME LIGHT AND VARIABLE LATE TONIGHT AND EARLY WEDNESDAY AS THE
RIDGE AXIS CROSSES THE LAKE. WHILE WINDS SHOULD DROP BELOW SMALL
CRAFT ADVISORY LEVELS DURG THE AFTERNOON AND EARLY THIS
EVENING...RESIDUAL WAVE ACTION WILL BE SLOWER TO DAMPEN AND STILL
EXPECT THAT WAVES WILL REMAIN...ABOVE SMALL CRAFT ADVISORY LEVELS
THROUGH MUCH OF THE NIGHT. AS THE RIDGE MOVES TO THE EAST OF THE
LAKE WEDNESDAY NIGHT...A TROUGH OF LOW PRESSURE WILL CROSS THE
CENTRAL PLAINS...REACHING THE UPPER MISSISSIPPI VALLEY BY THURSDAY
MORNING. THIS WILL SET UP A STRENGTHENING SOUTHERLY GRADIENT OVER
THE LAKE WITH WINDS INCREASING TO ARND 20-25KT THURSDAY AFTERNOON.
WINDS WILL REMAIN SOUTHERLY UNTIL AN ASSOCIATED WEAK COLD FRONT
PUSHES ACROSS THE LAKE THURSDAY NIGHT.

KREIN

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...NEARSHORE WATERS
     UNTIL 7 AM WEDNESDAY.

&&

$$

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.