Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 100513
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1113 PM CST TUE FEB 9 2010

.DISCUSSION...
306 PM CST

SHORT TERM...TONIGHT AND WEDNESDAY...

HAVE DECIDED TO DROP BLIZZARD WATCH ACROSS SOUTHEASTERN CWA DUE TO
CONCERNS ABOUT MEETING STRICT CRITERIA. OTHER THAN MINOR TWEAKS TO
SNOW AMOUNTS HAVE MAINTAINED SHORT TERM FORECAST PRETTY MUCH AS IS
THROUGH WEDNESDAY.

AFTERNOON SATELLITE IMAGERY SHOWS STOUT UPPER LOW NEAR STL...WHICH
WILL CONTINUE TO MOVE EAST/SOUTHEAST ACROSS OHIO RIVER VALLEY
TONIGHT. LIGHT/OCCASIONALLY MODERATE SNOW CONTINUES IN BROAD
CYCLONICALLY CURVED BAND OF ISENTROPIC UPGLIDE FROM SOUTHERN OHIO
VALLEY AND LOWER GREAT LAKES INTO WI/NORTHERN IL/IA. AS MENTIONED IN
MIDDAY AFD UPDATE...AREAS TO WATCH FOR ENHANCED SNOWFALL ARE ACROSS
COUNTIES BORDERING LAKE MICHIGAN WHERE COMBINATION OF LOW LEVEL
CONVERGENCE AND STEEP LAPSE RATES HELPING TO MAINTAIN STRONGER
REFLECTIVITIES AND LOWER OBSERVED VISIBILITIES...AND ACROSS PORTIONS
OF I-80/I-88 CORRIDORS OVER WESTERN COUNTIES WHERE LOW LEVEL
FRONTOGENESIS MAXIMA OCCURRING NORTH OF SHARP LOW LEVEL BAROCLINIC
ZONE ASSOCIATED WITH SFC TROUGH AXIS. RADAR TRENDS APPEAR TO CONFIRM
IDEA OF LAKE ENHANCED SNOWFALL AFFECTING PORTIONS OF NORTHEAST
IL/CHI METRO AREA THROUGH THE REMAINDER OF THE AFTERNOON/EVENING
RUSH HOUR...WITH MODERATE/HEAVY SNOW AT TIMES CURRENTLY OVER
NORTHERN SUBURBS GRADUALLY SHIFTING SOUTH ACROSS THE CITY BY EVENING
AND INTO NORTHWEST INDIANA OVERNIGHT. RECENT REPORT OF 7 INCHES IN
LAKE COUNTY IL SUGGESTS GOING 8-12 INCH STORM TOTAL FOR LAKE
MICHIGAN COUNTIES STILL APPROPRIATE. HAVE MADE ONLY MINOR TWEAKS
ELSEWHERE WITH STRENGTHENING DEFORMATION/FRONTOGENETIC BAND TO
SUPPLY SEVERAL MORE HOURS OF SNOW TO REMAINDER OF FORECAST AREA INTO
THIS EVENING.

EVENING PHASE OF STORM WILL BE ACCOMPANIED BY INCREASING
NORTH/NORTHWEST WINDS TONIGHT AS SFC LOW CONTINUES TO DEEPEN FOR A
TIME OFF TO OUR EAST. SUSTAINED WINDS APPROACHING 20 KTS WITH GUSTS
NEAR/AROUND 30 KTS EXPECTED OVERNIGHT ESPECIALLY ACROSS SOUTHEASTERN
PORTION OF CWA...WITH BLOWING AND DRIFTING OF SNOW EXPECTED TO
BECOME WIDESPREAD ACROSS THE AREA TONIGHT. WHILE CONDITIONS SUGGEST
WE MAY APPROACH BLIZZARD CONDITIONS IN A FEW SPOTS AT TIMES...
CONFIDENCE IN MEETING BLIZZARD WARNING CRITERIA OF 3 HOURS OF 35 MPH
AND 1/4SM VISIBILITY NOT HIGH ENOUGH TO UPGRADE TO BLIZZARD
WARNING...ESPECIALLY WITH SLIGHTLY LOWER SNOW/LIQUID RATIOS EARLIER
IN THE DAY AND SNOW WINDING DOWN AS WINDS REALLY PICK UP TONIGHT.
THEREFORE WILL LET WINTER STORM WARNING RIDE WITH EMPHASIS ON
BLOWING/DRIFTING AND POOR VISIBILITY.

LINGERING LAKE EFFECT INTO PORTER COUNTY LATE TONIGHT/WEDNESDAY
MORNING...THOUGH FORECAST SOUNDINGS OVER SOUTHERN LAKE MICHIGAN
INDICATE INVERSION HEIGHTS/EQUILIBRIUM LEVELS DECREASING RAPIDLY
DURING THE MORNING WEDNESDAY. AT THIS TIME HAVE MAINTAINED SNOW
AMOUNTS FROM PREVIOUS FORECAST FOR LAKE SNOW BELT AREAS...WITH 10 TO
15 INCHES ACROSS NORTHEAST PORTER COUNTY BY THE TIME IT TAPERS OFF
WEDNESDAY AFTERNOON.

RATZER

LONG TERM...WEDNESDAY NIGHT THROUGH TUESDAY...

AT 500 MB...A NORTHWEST FLOW WILL CONTINUE OVER NORTHERN ILLINOIS
AND NORTHWEST INDIANA WEDNESDAY NIGHT THROUGH FRIDAY. A 500 MB
TROUGH IS FORECAST TO MOVE INTO MINNESOTA BY FRIDAY EVENING. THIS
TROUGH WILL MOVE THROUGH ILLINOIS FRIDAY AFTERNOON. WE WILL
FORECAST A CHANCE OF SNOW FRIDAY. ANOTHER 500 MB TROUGH IS
FORECAST TO MOVE THROUGH ILLINOIS LATE SATURDAY NIGHT AND SUNDAY
MORNING. THIS TROUGH IS FORECAST TO BE MORE INTENSE THAN THE ONE
THAT OCCURRED FRIDAY. A SURFACE LOW WILL DEVELOP IN IOWA SATURDAY
EVENING AND WILL MOVE ACROSS CENTRAL ILLINOIS SUNDAY. THEN BY
SUNDAY EVENING THE SURFACE LOW WILL BE IN SOUTHERN OHIO. ANOTHER
HIGH WILL MOVE INTO THE MISSISSIPPI VALLEY MONDAY NIGHT. WE USED
THE GFS MODEL FOR GUIDANCE AND FOR TEMPERATURES...WE FOLLOWED MOS
GUIDANCE. FOR THE LAST SEVEN DAYS THE MEAN ABSOLUTE ERRORS WERE
LOWER THAN THE GFS MOS GUIDANCE. THE FORECAST HIGH TEMPERATURES
WILL BE IN THE MIDDLE TO UPPER 20S THURSDAY THROUGH MONDAY. THE
FORECAST LOW TEMPERATURES WILL BE IN THE TEENS SATURDAY THROUGH
TUESDAY.

WW

&&

.AVIATION...
1111 PM CST

0600 UTC TAFS...LAKE EFFECT HAS COMPLETED TRANSITION TO SINGLE
BAND EXTENDING FROM MID CENTRAL PORTION OF LAKE MI SSW TO ALONG
THE SE WI AND NE IL SHORES. FLOW TO CONTINUE TO BACK THROUGH THE
OVERNIGHT RESULTING IN TAIL END OF BAND SHIFTING GRADUALLY
EASTWARD ACROSS THE IND STATE LINE.

WINDS HAVE PICKED UP BUT STRONG GUSTS HAVE YEST TO MAKE IT INTO
THE IMMEDIATE CHI AREA. WHILE SFC LOW OVER FAR WEST END OF LAKE
ERIE WILL CONTINUE TO WEAKEN...THE RIDGE EXTENDING FROM CENTRAL
CANADA S ACROSS THE PLAINS TO EAST TX WILL CONTINUE TO SHIFT EAST
MAINTAINING PRESSURE GRADIENT AT LEAST INTO WED MORNING SO WHILE
LIGHT SYNOPTIC SNOW AND MODERATE TO HEAVY LAKE EFFECT WILL BE
ENDING...SOME EPISODES OF BLOWING SNOW EXPECTED TO PRODUCE
RESTRICTED VISIBILITIES THROUGH FIRST PART OF WED MORNING.

PATCHY MVFR STRATOCU UPSTREAM TO NW WI AND SE MN THEN MAINLY SKC.
WITH DEEP NW FLOW BY WED EVE ALONG WITH RIDGE AXIS MOVING TO UPPER
MS AND LOWER MO VALLEYS EXPECT CLOUDS TO CLEAR OUT BY LATE
AFTERNOON-EARLY EVENING. WINDS TO ALSO GRADUALLY DIE OFF AS LOW TO
THE EAST FILLS AND RIDGE NEARS.

TRS

&&

.MARINE...

139 PM...PRIMARY FORECAST CONCERN REMAINS GALES THRU WEDNESDAY
EVENING.

OVERALL...NO MAJOR CHANGES TO GOING FORECAST/GRIDS. A LARGE RIDGE
OF HIGH PRESSURE EXTENDS FROM HUDSON BAY SOUTHWEST ACROSS THE
NORTHERN AND CENTRAL PLAINS. THIS RIDGE WILL WEAKEN AS IT SLOWLY
SHIFTS EAST WEDNESDAY AND WEDNESDAY NIGHT AND BECOMES CENTERED
OVER THE OHIO RIVER VALLEY THURSDAY NIGHT. AHEAD OF THIS RIDGE IS
AN AREA OF LOW PRESSURE LIFTING NORTHEAST ACROSS OHIO. THIS LOW
WILL DEEPEN FURTHER AS IT REACHES LAKE ERIE THIS EVENING...THEN
NEW LOW PRESSURE WILL DEVELOP OFF THE MID ATLANTIC COAST AND
RAPIDLY DEEPEN TONIGHT...ABSORBING THE LOW OVER LAKE ERIE.

AS A RESULT...NORTHEAST WINDS WILL CONTINUE TO INCREASE THRU THIS
EVENING INCREASING TO GALES. WINDS WILL TURN MORE NORTHERLY AFTER
MIDNIGHT THEN TURN NORTHWEST WEDNESDAY MORNING. THE GRADIENT WILL
SLOWLY WEAKEN...ESPECIALLY AFTER THE LOW OVER LAKE ERIE WEAKENS
AND IS ABSORBED. THUS GALES WILL DIMINISH WEDNESDAY AFTERNOON
INTO WEDNESDAY EVENING. BUT SINCE THE HIGH WILL RE-CENTER ITSELF
OVER THE OHIO VALLEY...THE GRADIENT WILL REMAIN TIGHT ACROSS THE
WESTERN LAKES THROUGH THURSDAY MORNING...THUS WINDS WILL REMAIN
STRONG WEDNESDAY NIGHT.

STILL SOME UNCERTAINTY REGARDING THE NEXT SYSTEM TO AFFECT THE
LAKES REGION THIS WEEKEND. LOW PRESSURE IS EXPECTED TO MOVE
ACROSS THE SOUTHERN LAKES REGION WITH ANOTHER STRONG HIGH
BUILDING ACROSS THE PLAINS. HOWEVER...WIND SPEEDS WILL BE
DEPENDENT ON THE STRENGTHEN/LOCATION OF THE LOW AND WITH LOW
CONFIDENCE FROM THIS DISTANCE...WILL KEEP WINDS IN THE 15-25KT
RANGE FOR NOW. CMS

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...WINTER STORM WARNING...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-
     ILZ011-ILZ012-ILZ013-ILZ014-ILZ019-ILZ020-ILZ021-ILZ022-
     ILZ023-ILZ032-ILZ033-ILZ039 UNTIL 9 AM WEDNESDAY.

IN...WINTER STORM WARNING...INZ001-INZ002-INZ011 UNTIL NOON WEDNESDAY.

     WINTER STORM WARNING...INZ010-INZ019 UNTIL 9 AM WEDNESDAY.

LM...GALE WARNING...LMZ080-LMZ669-LMZ671-LMZ673-LMZ675-LMZ743-LMZ744-
     LMZ745-LMZ777-LMZ779-LMZ870-LMZ872-LMZ874-LMZ876-LMZ878
     UNTIL 9 PM WEDNESDAY.

     GALE WARNING...LMZ261-LMZ362-LMZ364-LMZ366-LMZ563-LMZ565-LMZ567-
     LMZ740-LMZ741-LMZ742-LMZ868 UNTIL 4 PM WEDNESDAY.

&&

$$









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  • Page last modified: Nov 10th, 2009 17:50 UTC
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