Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 300307
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1007 PM CDT WED OCT 29 2014

.DISCUSSION...
906 PM CDT

FOR EVENING UPDATE...

MINOR CHANGES TO GOING FORECAST TO LOWER MIN TEMPS A LITTLE OVER
NORTH CENTRAL IL...PARTICULARLY THE ROCK RIVER VALLEY...WHERE CLEAR
SKIES AND LIGHT WINDS IN VICINITY OF SFC RIDGE AXIS HAVE ALLOWED
STRONG RADIATIONAL COOLING AND A QUICK DROP IN TEMPS TO THE LOW-MID
30S. GIVEN PROXIMITY OF THE RIDGE AND CLEAR SKIES UNTIL EARLY
MORNING WHEN HIGH CLOUDS APPROACH FROM THE NORTHWEST...THINK A FEW
UPPER 20S LIKELY AT THE COOLER SPOTS IN THAT AREA.

OTHERWISE...MAINLY A FEW TWEAKS TO SKY COVER TO MATCH GOES 11-3.9
MICRON IMAGERY OF CLEARING ACROSS MUCH OF NORTHERN IL. NORTHWEST
INDIANA WILL BE A LITTLE SLOWER TO CLEAR BASED ON SATELLITE IMAGERY.
REMAINDER OF FORECAST LOOKS TO BE IN GOOD SHAPE WITH MOST ATTENTION
FOCUSED ON FRIDAYS IMPRESSIVE STORM SYSTEM.

RATZER

//PREV DISCUSSION...
330 PM CDT

THE MAIN FORECAST FOCUS THIS AFTERNOON CONTINUES TO BE WITH REGARDS
TO THE SIGNIFICANT STORM SYSTEM EXPECTED TO DEVELOP ACROSS THE GREAT
LAKES AND OHIO VALLEY FRIDAY. THIS SYSTEM PROMISES TO PRODUCE
STRONG WINDS FRIDAY...POSSIBLY IN EXCESS OF 50 MPH ACROSS PORTIONS
OF THE REGION. THERE IS ALSO A DECENT CHANCE FOR SOME RAIN AND
SNOW...ESPECIALLY ACROSS EASTERN SECTIONS OF THE REGION FRIDAY INTO
FRIDAY NIGHT.

WATER VAPOR IMAGERY EARLY THIS AFTERNOON...ACROSS NORTH AMERICA...
INDICATES THE PRESENCE OF POTENT STORM SYSTEM DIGGING ACROSS THE
GULF OF ALASKA...WITH THE CENTER OF CIRCULATION NEAR 53N 150W. THIS
SYSTEM WILL PLAY A MAJOR ROLE OVER THE NEXT 24 TO 36 HOURS IN
AMPLIFYING THE UPPER LEVEL FLOW PATTERN ACROSS NORTH AMERICA. MOST
NOTABLE WILL BE THE SHARPENING AND AMPLIFICATION OF AN UPPER LEVEL
RIDGE ACROSS THE INTER MOUNTAIN WEST AND HIGH PLAINS BY LATE
THURSDAY. THIS PARTICULAR PATTERN WILL FAVOR FALLING HEIGHTS AND THE
DEVELOPMENT OF STRONG NORTH-NORTHWEST ALOFT ACROSS THE GREAT LAKES
AND OHIO VALLEY REGIONS. MODEL GUIDANCE CONTINUES TO SUGGEST THAT
THIS EVOLVING LARGE SCALE WEATHER PATTERN WILL ACT TO STEER A
SHORTWAVE DISTURBANCE...CURRENTLY ACROSS NORTHERN SASKATCHEWAN AND
MANITOBA...SOUTHWARD RIGHT ACROSS THE AREA BY FRIDAY MORNING.

THE MAIN THREAT FOR ACTIVE WEATHER WILL BEGIN THURSDAY NIGHT AND
LAST INTO FRIDAY NIGHT. AS THE AFOREMENTIONED DISTURBANCE ACROSS
NORTHERN CANADA DIGS SOUTHWARD OVER WISCONSIN BY THURSDAY NIGHT...IT
WILL INDUCE AN AREA OF SURFACE LOW PRESSURE ACROSS LOWER MICHIGAN.
THIS SURFACE LOW IS OVERALL EXPECTED TO REMAIN FAIRLY WEAK (IN TERMS
OF CENTRAL PRESSURE) AS IT PROGRESSES SOUTHEASTWARD OVER THE OHIO
VALLEY LATER FRIDAY. AN ASSOCIATED COLD FRONT WILL BE DRIVEN ACROSS
THE AREA INTO EARLY FRIDAY MORNING...WITH STRONG COLD AIR ADVECTION
AND STRONG NORTHERLY WINDS EXPECTED IN ITS WAKE. IN SPITE OF THE
FACT THAT THE SURFACE LOW WILL REMAIN WEAK...THE SURFACE PRESSURE
GRADIENT IS EXPECTED TO BE QUITE STRONG ACROSS THE WESTERN LAKES
REGION AND INTO NORTHERN ILLINOIS/NORTHWESTERN INDIANA ON
FRIDAY...THANKS TO A STRONG 1033 MB HIGH THAT WILL BE RAPIDLY
BUILDING EASTWARD ACROSS THE UPPER MIDWEST. OVERALL...THIS LOOKS TO
SET UP TO STRONG PRESSURE RISES...POSSIBLY IN EXCESS OF 6 MB/6HR
PERIOD...ESPECIALLY ACROSS LAKE MICHIGAN AND ADJACENT AREAS OF
NORTHEASTERN ILLINOIS AND NORTHWESTERN INDIANA. THIS COMBINED WITH
THE STRONG PUSH OF MUCH COLDER AIR...SHOULD DRIVE SOME VERY STRONG
WINDS. MODEL SOUNDINGS INDICATE MEAN LAYER FLOW IN EXCESS OF 45 KT
OVER SOUTHERN LAKE MICHIGAN...WITH AROUND 50 KT OF FLOW AT THE TOP
OF THE MIXING LAYER. IT APPEARS THAT THE STRONGEST WINDS WITH THIS
SYSTEM WILL BE DURING THE LATE MORNING AND AFTERNOON ON FRIDAY
FOLLOWING THE PASSAGE OF THE UPPER LEVEL VORT MAX. THE TRACK OF THIS
SYSTEM ALSO SUGGESTS THAT THE STRONGEST WINDS WILL ALSO BE OVER
APPROXIMATELY THE EASTERN HALF OF MY CWA...ROUGHLY FROM THE CHICAGO
METRO AREA AND POINTS EASTWARD. THESE AREAS COULD REACH WIND
ADVISORY CRITERIA WITH WIND GUSTS UP TO 50 MPH.

THE STRONG NORTHERLY WINDS COMING IN OFF THE LAKE COULD ALLOW WIND
GUSTS TO APPROACH 50KT FOR AREAS NEAR THE LAKE SHORE...ESPECIALLY IN
NORTHWESTERN INDIANA. VERY HIGH CRASHING WAVES IN EXCESS OF 18 FT
ALSO APPEAR LIKELY FOR THE INDIANA AND PORTIONS OF THE ILLINOIS
SHORELINES...WHICH COULD RESULT IN SOME LAKE SHORE FLOODING CONCERNS
FRIDAY.

IT APPEARS MUCH OF THE PRECIPITATION WITH THIS SYSTEM WILL BE
FAVORED ACROSS AREAS NEAR THE LAKE ON FRIDAY AND FRIDAY EVENING.
HOWEVER...AS THE MAIN UPPER IMPULSE MOVES OVER THE AREA THURSDAY
NIGHT INTO EARLY FRIDAY...IT COULD DRIVE A SHORT PERIOD OF SHOWERS
ACROSS MUCH OF THE AREA. HOWEVER...DRIER AIR AND LARGE SCALE FORCING
FOR DECENT SHOULD OVERSPREAD MOST INLAND AREAS ACROSS NORTHERN
ILLINOIS BY FRIDAY AFTERNOON...LIKELY ENDING THE THREAT OF PRECIP.
HOWEVER...AREAS NEAR THE LAKE IN FAR NORTHEASTERN ILLINOIS AND MY
NORTHWESTERN INDIANA COUNTIES WILL LIKELY CONTINUE TO EXPERIENCE
PRECIPITATION FRIDAY AFTERNOON AND INTO THE EVENING AS THERMODYNAMICS
OVER LAKE MICHIGAN WILL BECOME VERY FAVORABLE FOR LAKE EFFECT
PRECIPITATION THROUGH FRIDAY EVENING. LAKE INDUCED EQUILIBRIUM LEVELS
LOOK TO EXCEED 16,000 FEET WITH LAKE SURFACE TEMPERATURE TO 850 MB
TEMPERATURE DIFFERENCES OF 18-19 DEGREES C. THE OVERALL DEPTH OF
THIS LAKE INDUCED INSTABILITY COULD ALSO ALLOW FOR SOME CHARGE
SEPARATION SUPPORTING THUNDER AND LIGHTENING ACROSS SOUTHERN
PORTIONS OF THE LAKE AND POSSIBLY INTO NORTHWESTERN INDIANA FRIDAY.
SO I HAVE INCLUDED A MENTION OF SUCH IN THE FORECAST.

THE MAIN QUESTION WITH THIS EVENT REMAINS AS TO WHEN THE PRECIP TYPE
COULD MIX WITH OR CHANGE TO SNOW. IT APPEARS THAT WITH 850 MB
TEMPERATURES AROUND -8 DEGREES C...THAT A TRANSITION OVER TO LAKE
EFFECT SNOW SHOWERS APPEARS TO BE A DECENT BET BY LATE FRIDAY
AFTERNOON AND INTO THE EVENING. HOWEVER...DUE TO THE POTENTIAL FOR
CONVECTIVELY DRIVEN HEAVY SHOWERS OVER THE LAKE...DYNAMIC COOLING
COULD BECOME STRONG ENOUGH TO RESULT IN AN EARLY TRANSITION TO SNOW
OVER NORTHWESTERN INDIANA. WITH THIS IN MIND CONFIDENCE ON TIMING OF
ANY TRANSITION TO SNOW ON FRIDAY IS LOW...THOUGH THERE IS THE
POTENTIAL FOR SOME HEAVIER RAIN/SNOW SHOWERS WHICH COULD RESULT IN
SOME ACCUMULATIONS ACROSS PORTIONS OF NORTHWESTERN INDIAN IN SPITE
OF WARM GROUND TEMPS. THE STRONG WINDS COULD ALSO ALLOW THESE
SHOWERS TO GET WELL INLAND ACROSS NORTHWESTERN INDIANA. THE THREAT
OF SNOW SHOULD COME TO END OVERNIGHT FRIDAY AS INVERSION HEIGHTS
TANK IN RESPONSE TO BUILDING UPPER LEVEL HEIGHTS AND WARMING MID
LEVEL TEMPERATURES.

HIGH TEMPERATURES AREA-WIDE LOOK TO REMAIN IN THE LOWER 40S AT BEST
ON FRIDAY AND SATURDAY...THEN MODERATING TEMPERATURES ARE EXPECTED
INTO EARLY NEXT WEEK AS THE WESTERN CONUS UPPER RIDGE BUILDS
EASTWARD OVER THE REGION. ANOTHER COLD FRONT LOOKS TO TAKE AIM ON
THE AREA AROUND NEXT TUESDAY...WHICH WILL RESULT IN OUR NEXT CHANCE
OF SHOWERS. OTHERWISE TEMPERATURES LOOK TO REMAIN NEAR NORMAL DURING
THE LATER HALF OF THE WEEK.

KJB

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 03Z...

* NONE. CMS

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

BKN/OVC 4-5KFT CIGS WILL CONTINUE MOVING SOUTHEAST AND SCATTER OUT
THIS EVENING BUT WILL LIKELY TAKE A FEW MORE HOURS. ONCE THESE
CLOUDS SHIFT EAST...INCREASING HIGH CLOUDS OVERNIGHT TOWARD
MORNING WILL LOWER TO A MID DECK IN THE AFTERNOON AND THEN TO A
4-6KFT THURSDAY EVENING. A WEAK UPPER WAVE WILL MOVE ACROSS THE
REGION THURSDAY BRINGING A CHANCE OF SHOWERS THAT APPEAR TO STAY
JUST SOUTH OF THE TERMINALS...BUT SOME SPRINKLES OR AN ISOLATED
SHOWER IS POSSIBLE...ESPECIALLY BY THURSDAY EVENING.

WEST/NORTHWEST WINDS UNDER 10KTS WILL DIMINISH TO 5KT OR LESS THIS
EVENING AND TURN MORE SOUTHWEST OVERNIGHT...POSSIBLY BECOMING
LIGHT AND VARIABLE OR CALM IN SOME LOCATIONS. WINDS MAY TURN A BIT
MORE SOUTHERLY AFTER SUNRISE THEN SLOWLY TURN BACK WESTERLY BY
AFTERNOON AND NORTHWESTERLY THURSDAY EVENING...BUT SHOULD REMAIN
BELOW 10KT. CMS

//ORD AND MDW CONFIDENCE...UPDATED 03Z...

* HIGH FOR WINDS THRU THE PERIOD. CMS

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

* FRIDAY...STRONG NORTH WINDS GUSTING TO 35 KT. CHANCE OF SHRA OR
  SHSN.
* SATURDAY...DRY/VFR. NORTHEAST WINDS.
* SUNDAY...DRY/VFR. SOUTH WINDS.
* MONDAY...CHANCE OF SHRA. GUSTY SOUTHERLY WINDS.
* TUESDAY...CHANCE OF SHRA. WESTERLY WINDS.

&&

.MARINE...
141 PM CDT

HIGH PRESSURE WILL MOVE ACROSS THE REGION TONIGHT RESULTING IN
EASING WINDS AND WAVES TONIGHT AND THURSDAY. SHARP UPPER TROUGH WILL
DIG INTO THE GREAT LAKES REGION THURSDAY NIGHT WITH SURFACE LOW
DEVELOPING OVER THE EASTERN GREAT LAKES. THE DEVELOPING LOW AND A
STRONG 1038MB HIGH OVER THE NORTHERN PLAINS WILL RESULT IN A
DRAMATIC INCREASE IN THE PRESSURE GRADIENT OVER THE LAKE LATE
THURSDAY NIGHT AND ESPECIALLY FRIDAY. IN ADDITION TO THE STRONG
GRADIENT...VERY COLD AIR MOVING OVER THE LAKE WILL PRODUCE STRONG TO
EXTREME INSTABILITY RESULTING IN DEEP AND EFFICIENT MIXING OF THE
HIGHER WINDS TO THE SURFACE. FORECAST SOUNDINGS SUGGEST THAT A
SEVERAL HOUR WINDOW OF STORM FORCE WINDS ARE POSSIBLE
FRIDAY...ESPECIALLY SOUTHERN PORTIONS OF THE LAKE...HOWEVER ENOUGH
MODEL UNCERTAINTY EXISTS TO HOLD OFF ON A STORM WATCH OR WARNING AT
THIS TIME BUT WILL ADD A FEW STORM FORCE GUSTS TO GLF AND GRIDS.
WINDS WILL SUBSIDE SATURDAY AND THEN BACK WITH FAIRLY STRONG
SOUTHERLY WINDS DEVELOPING LATE SUNDAY AND CONTINUING THROUGH
TUESDAY.

IZZI

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...GALE WARNING...LMZ261-LMZ362-LMZ364-LMZ366-LMZ563-LMZ565-LMZ567-
     LMZ868...1 AM FRIDAY TO 10 PM FRIDAY.

     GALE WARNING...LMZ080-LMZ669-LMZ671-LMZ673-LMZ675-LMZ740-LMZ741-
     LMZ742-LMZ743-LMZ744-LMZ745-LMZ777-LMZ779-LMZ870-LMZ872-
     LMZ874-LMZ876-LMZ878...4 AM FRIDAY TO 4 AM SATURDAY.

&&

$$

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