National Weather Service National Headquarters

• Home
• Site Map
• News
• Organization
• Search for: NWS All NOAA 

000
FXUS63 KLOT 101227
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
627 AM CST WED FEB 10 2010

.DISCUSSION...
335 AM CST

SHORT TERM (TODAY AND TONIGHT)...

JACQUES LE PLUME HAS MOVED INTO INDIANA EARLY THIS MORNING...BUT NOT
BEFORE LAYING DOWN A VERY IMPRESSIVE DUMPING OF SNOW OVER CHICAGO
TUESDAY EVENING WITH SNOWFALL RATES OF 1-3 INCHES PER HOUR. OVER THE
PAST COUPLE HOURS THE BAND HAS BEEN MORE TRANSIENT...HOWEVER BACKING
OF THE LOW LEVEL FLOW SHOULD SLOW AS THE PLUME MOVES INTO PORTER
COUNTY...WHERE THERE COULD STILL BE A HEALTHY DUMPING OF SNOW YET
THIS MORNING BEFORE THE LAKE EFFECT BECOMES IWX`S HEADACHE. HOW MUCH
MORE SNOW FALLS OVER PORTER COUNTY WILL DEPEND ON HOW QUICKLY THE
PLUME SHIFTS EAST...WHICH IS NOT SOMETHING THAT WE HAVE A GREAT DEAL
OF SKILL IN FORECASTING AS A WELL DEVELOPED LAKE EFFECT PLUME CAN
SEEMINGLY HAVE A MIND OF ITS OWN. CERTAINLY REASONABLE THAT PORTIONS
OF NORTHERN PORTER COUNTY COULD PICK UP AN ADDITION 4-8 INCHES OF
SNOW THIS MORNING...GIVEN EXPECTED RESIDENCE TIME OF THE PLUME AND
1-2"+/HOUR SNOWFALL RATES WITH THE MOST INTENSE ELEMENTS.

BESIDE THE LAKE EFFECT...ANOTHER CONCERN IS WITH THE BLOWING AND
DRIFTING SNOW. GLANCE OUT THE WINDOW DOES SHOW SOME BLOWING AND
DRIFTING SNOW...BUT NO SIGNIFICANT VSBY RESTRICTIONS. WINDS HAVE
SUBSIDED EVEN FURTHER OVER WESTERN CWA SO WILL BE CANCELING ALL
HEADLINES EARLY FOR NC IL. THE REMAINDER OF THE AREA WILL BE
DOWNGRADED TO A WINTER WX ADVISORY FOR BLOWING/DRIFTING
SNOW...EXCEPT FOR LAKE/PORTER/JASPER COUNTIES IN INDIANA WHERE
WARNING WILL CONTINUE DUE TO HEAVY LAKE EFFECT SNOW.

AS WINTER STORM WINDS DOWN ATTENTION WILL SHIFT TO CLOUD COVER. NOT
A PARTICULARLY EXPANSIVE AREA OF LOW CLOUDS IN THE WAKE OF THIS
SYSTEM...SO WORRY THAT FORECAST SOUNDINGS MAY BE A BIT TOO
AGGRESSIVE WITH HANGING ONTO THE LOW LEVEL MOISTURE. WRF-NAM AND GFS
BOTH SUGGEST WE SHOULD LARGELY LOSE THE CYCLONIC CURVATURE TO THE
LOW LEVEL FLOW THIS EVENING...ESPECIALLY WESTERN CWA...SO GIVEN
CURRENT SATELLITE TRENDS HAVE GONE WITH A CLEARING TREND IN THE
GRIDS TONIGHT. SHOULD SKIES INDEED CLEAR TONIGHT AS EXPECTED THEN
TEMPS SHOULD REALLY BOTTOM OUT. TEMPS ABOVE THE INVERSION ARE
PROGGED TO MODERATE...HOWEVER EXPECT WINDS TO DECOUPLE AS TEMPS
REALLY TANK UNDER CLEAR SKIES. UPSTREAM OVER WESTERN IOWA TEMPS HAVE
PLUMMETED TO WELL BELOW ZERO...EVEN WITH 5-10KT OF WIND AND SEE NO
REASON THE SAME WONT OCCUR IN OUR WESTERN COUNTIES THURSDAY MORNING
AND HAVE MAJORLY UNDERCUT GUIDANCE TEMPS. IF WE DO RADIATE OUT THEN
WINDS SHOULD HOPEFULLY BE LIGHT ENOUGH TO PRECLUDE THE NEED FOR ANY
WIND CHILL HEADLINES...BUT THURSDAY MORNING SHOULD BE NOTICEABLY
FRIGID.

IZZI

LONG TERM (THURSDAY THROUGH TUESDAY)...

MAIN ISSUES WILL BE RESOLVING SKY COVER AND TEMP TRENDS...ESPECIALLY
GIVEN FRESH NEW SNOWPACK FROM RECENT WINTER STORM THAT WILL PLAY
HAVOC WITH TEMPERATURES.

ONE MORE MINOR IMPULSE APPROACHING WESTERN SOUTH DAKOTA THIS MORNING
WILL BE TRACKING ACROSS THE WESTERN GREAT LAKES REGION THURSDAY
MORNING WHICH SHOULD BE FOLLOWED BY WEAK INDUCED SHORT WAVE RIDGING
IN ADVANCE OF NEXT UPPER LEVEL DISTURBANCE TO APPROACH THE REGION
FOR FRIDAY. MAIN QUESTION STARTING OUT THURSDAY WILL BE THE EXTENT
OF SKY COVER WHICH WILL HAVE A BIG IMPACT ON TEMPERATURES. FORECAST
SOUNDINGS CONTINUE TO INDICATE LOWERING LOW LEVEL INVERSION WITH
HIGH LOW LEVEL RH. ANY WEAK LIFT WITH THE AFOREMENTIONED UPPER
DISTURBANCE THURSDAY MORNING SHOULD WELL TO THE NORTH ACROSS THE
UPPER LAKES WITH MODEL TIME/HEIGHT SECTIONS INDICATING FAIRLY STRONG
SUBSIDENCE LATE WEDNESDAY NIGHT INTO EARLY THURSDAY MORNING. GIVEN
THESE ABOVE FACTORS AND VERY SHALLOW/THIN MOISTURE PROFILE...DO FEEL
COMFORTABLE FOLLOWING THE PREVIOUS FORECAST IN TERMS OF SKY COVER
WITH MOSTLY CLEAR WEST/PARTLY CLOUDY EAST ON THURSDAY. GIVEN FRESH
NEW SNOW COVER ACROSS MOST OF THE AREA...DID GO AHEAD AND SHAVE A
GOOD 5 TO 8 DEGREES OFF GOING HIGHS THURSDAY DUE TO THE EFFECTS OF
WEAK LOW LEVEL CAA BEHIND A WEAK SFC TROUGH ASSOCIATED WITH THIS
DISTURBANCE AND SNOW COVER. NEW FORECAST HIGHS AROUND 20 MAY EVEN
NEED TO BE TWEAKED DOWNWARD SLIGHTLY MORE IN FUTURE FORECASTS BUT AT
LEAST WANTED TO BEGIN SHOWING A TREND WITH EXPECTED EFFECTS OF SNOW
COVER.

MID/HIGH CLOUDINESS TO INCREASE FOR THURSDAY NIGHT AND FRIDAY
AS ANOTHER DISTURBANCE ROUNDS WESTERN CONUS RIDGE AND APPROACHES MID
MS RIVER VALLEY. AT THIS POINT...BANKING ON THESE CLOUDS PREVENTING
TEMPS FROM DROPPING OFF TOO DRAMATICALLY THURSDAY NIGHT...SO HAVE
KEPT LOWS IN THE SINGLE DIGITS ABOVE ZERO BUT OF COURSE WILL NEED
WATCHING IN CASE ONSET OF CLOUDINESS IS SLOWER THAN EXPECTED. THIS
DISTURBANCE FOR FRIDAY WILL BE THE FIRST OF TWO FAST MOVING SYSTEMS
TO AFFECT THE AREA FRIDAY THROUGH THE WEEKEND. GENERALLY FOLLOWED
IDEA OF GFS/ECMWF CONSENSUS OF A LITTLE MORE PROGRESSION WITH THIS
FEATURE COMPARED TO NAM AND HAVE PAINTED HIGHEST CHANCE POPS FOR
LIGHT SNOW FOR WESTERN AND SOUTHWEST PORTIONS OF FORECAST AREA
FRIDAY/FRIDAY NIGHT BASED ON EXPECTED TRACK OF THIS WAVE. PERHAPS A
MORE POTENT WAVE LOOKS TO AFFECT AREA BY SUNDAY BUT MUCH MORE
UNCERTAINTY IN REGARDS TO THE TRACK OF THIS SYSTEM WITH LARGE SPREAD
IN DETERMINISTIC RUNS AND ENSEMBLES. HAD TO KEEP A BROADBRUSH CHANCE
OF SNOW GOING FOR THE WEEKEND UNTIL DETAILS CAN BECOME BETTER
RESOLVED WITH THIS POTENTIALLY MORE VIGOROUS UPPER LEVEL LOW.

MARSILI

&&

.AVIATION...

1200 UTC TAFS...MAIN ISSUES THIS MORNING ARE WITH LINGERING LOW
LEVEL CEILINGS ACROSS THE REGION.

LAKE EFFECT SNOW HAS FINALLY SHIFTED TO THE EAST INTO NORTHWEST
INDIANA THIS MORNING AS BOUNDARY LAYER FLOW CONTINUED TO BACK. IN
ITS ABSENCE...VISIBILITY AND CLOUD COVER HAS SIGNIFICANTLY COME
UP OVER THE LAST COUPLE OF HOURS. RECENT OBSERVATIONS SHOW THAT
CEILINGS IN AND AROUND THE TERMINALS HAVE RISEN TO VFR CONDITIONS.
ALTHOUGH THERE ARE STILL SOME SCATTERED LOW MVFR CEILINGS STILL
MOVING THROUGH NORTHEAST ILLINOIS...WHICH WILL PROVIDE SOME BRIEF
PERIODS OF LOWER CEILINGS EARLY THIS MORNING. NOT CONFIDENT AT
THIS POINT WHAT WILL HAPPEN WITH LOW CEILINGS SLOWLY MOVING
THROUGH WESTERN WISCONSIN. AS UPPER LEVEL SYSTEM CONTINUES TO
SHIFT TO THE EAST...MID LEVEL MOISTURE AND THIS LOW LEVEL MOISTURE
WILL ALSO CONTINUE TO PROGRESS EAST/SOUTHEAST TODAY. MODEL
GUIDANCE TRIES TO HOLD ONTO THIS LOW LEVEL MOISTURE...BUT FEEL
THIS IS A LITTLE TOO AGGRESSIVE. LATEST SATELLITE IMAGERY
INDICATING THE LEADING EDGE OF THIS LOW LEVEL MOISTURE BEGINNING
TO ERODE. SO OPTED TO GO WITH THIS TREND AND ONLY KEEP A SCATTERED
DECK LATER TODAY...WITH MID LEVEL MOISTURE EXITING.
ALTHOUGH...WILL CONTINUE TO MONITOR TRENDS AND ADJUST ACCORDINGLY
WITH LATER FORECAST.

WINDS TODAY WILL HOLD ONTO A NORTHWEST COMPONENT FOR MUCH OF THE
PERIOD AS LOW EXITS TO THE EAST. WINDS WILL BEGIN TO COME DOWN
TODAY...BUT WITH GUSTS CONTINUING AS A DECENT PRESSURE GRADIENT
WILL REMAIN WITH HIGH BUILDING IN FROM THE WEST.

RODRIGUEZ

&&

.MARINE...
430 AM CST

SFC LOW ACROSS WESTERN PENNSYLVANIA WILL CONTINUE TO MOVE OFF TO
THE EAST TODAY WHICH WILL EVENTUALLY SLACKEN OFF SFC PRESSURE
GRADIENT LATER TODAY. IN THE MEANTIME...DEEP MIXING UP TO AROUND
850 HPA SHOULD ALLOW SOME MARGINAL GALE GUSTS TO CONTINUE THROUGH
AT LEAST THIS MORNING...AND POSSIBLY INTO THIS AFTERNOON ACROSS
SOUTHERN LAKE MICHIGAN. IT DOES APPEAR THAT BY LATE AFTERNOON
POTENTIAL OF GALE GUSTS SHOULD HAVE DIMINISHED ACROSS ALL OF LAKE
MICHIGAN SO DID MOVE UP ENDING TIME OF GALES TO 00Z THIS
EVENING FOR SOUTH PART OF LAKE AND TO 22Z FOR NORTHERN LAKE
MICHIGAN...AND MAY NEED TO CONSIDER MOVING UP THIS TIMING EVEN
MORE CONSIDERING GUSTS FOR NEARSHORE OBSERVATION SITES HAVE ONLY
BEEN MARGINALLY UP TO GALES THIS MORNING. A GALE WARNING IS ALSO
IN EFFECT FOR THE NEARSHORE WATERS UNTIL 18Z TODAY...WHICH WILL
NEED TO BE REPLACED BY SMALL CRAFT ADVISORY THIS AFTERNOON. WINDS
AND WAVES WILL SLOWLY SUBSIDE INTO THURSDAY AS A RIDGE AXIS
APPROACHES THE WESTERN GREAT LAKES.

MARSILI

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...WINTER WEATHER ADVISORY...ILZ005-ILZ006-ILZ012-ILZ013-ILZ014-
     ILZ020-ILZ021-ILZ022-ILZ023-ILZ032-ILZ033-ILZ039 UNTIL 9 AM
     WEDNESDAY.

IN...WINTER STORM WARNING...INZ001-INZ011 UNTIL 9 AM WEDNESDAY.

     WINTER STORM WARNING...INZ002 UNTIL NOON WEDNESDAY.

     WINTER WEATHER ADVISORY...INZ010-INZ019 UNTIL 9 AM WEDNESDAY.

LM...GALE WARNING...LMZ080-LMZ669-LMZ671-LMZ673-LMZ675-LMZ777-LMZ779-
     LMZ870-LMZ872-LMZ874-LMZ876-LMZ878 UNTIL 6 PM WEDNESDAY.

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

     GALE WARNING...NEARSHORE WATERS UNTIL
     NOON WEDNESDAY.

&&

$$