Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 190756
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
156 AM CST FRI DEC 19 2014

.UPDATE...

WEAK LAKE EFFECT SNOW SHOWERS REMAIN LARGELY OFFSHORE...BUT HAVE
HAD REPORTS OF FLURRIES OR A LIGHT MIST IN LAKE AND COOK COUNTIES OF
ILLINOIS. EXPECT FLURRIES/PATCHY FREEZING DRIZZLE TO REMAIN
POSSIBLE FOR THE EVENING...DIMINISHING QUICKLY OVERNIGHT. NO OTHER
CHANGES TO THE GOING FORECAST FOR THE EVENING.

KMD

&&

.SHORT TERM...
249 PM CST

MAINLY CLOUDY CONDITIONS EXPECTED TO PERSIST THROUGH FRIDAY AS
DIRTY HIGH PRESSURE MOVES ACROSS THE REGION. HIGH RES GUIDANCE IS
SHOWING HINTS OF LAKE EFFECT PRECIP DEVELOPING TONIGHT...THOUGH
FORECAST SOUNDINGS SUGGEST THAT LOW INVERSION HEIGHTS WOULD KEEP
THE THREAT OF ANY MEASURABLE LAKE EFFECT PRECIP VERY LOW. STILL
PLAUSIBLE THAT THERE COULD BE SOME SNOW GRAINS/FLURRIES INTO
NORTHEAST PORTER COUNTY TONIGHT AS LOW LEVEL FLOW VEERS MORE
NORTHERLY.

IZZI

&&

.LONG TERM...
249 PM CST

GOING INTO THE WEEKEND FAIRLY QUIET CONDITIONS EXPECTED. WEAK
NORTHERN STREAM SHORTWAVE WILL PASS MAINLY JUST NORTH OF THE REGION
SATURDAY...BUT MOISTURE/FORCING LOOK TOO LIMITED FOR ANY
PRECIP...THOUGH WAA CLOUDINESS LOOKS TO BE A GOOD BET. ANOTHER WEAK
SHORTWAVE PASSES JUST TO OUR NORTH ON SUNDAY WITH GFS SQUEEZING OUT
SOME LIGHT QPF. ECMWF/WRF-NAM AND PARALLEL GFS KEEP LIGHT PRECIP TO
OUR NORTH...SO HAVE OPTED TO HANG ONTO A DRY FORECAST FOR NOW FOR
SUNDAY BUT WITH PLENTY OF CLOUDINESS OF COURSE.

NEXT WEEK A SIGNIFICANT PATTERN CHANGE IS FORECAST TO TAKE PLACE
ACROSS THE NORTHWESTERN HEMISPHERE WITH SIGNIFICANT LONG WAVE TROUGH
SETTING UP OVER THE UPPER MISSISSIPPI VALLEY AND EVENTUALLY THE
EASTERN CONUS. THE SYSTEM THAT WILL EVENTUALLY CARVE OUT THE DEEP
TROUGH OVER THE REGION IS ACTUALLY THE SAME SIGNIFICANT STORM THAT
JUST AFFECTED JAPAN...SO IT HAS A LONG FAIRLY AND DATA SPARSE REGION
OF THE PACIFIC TO TRAVERSE BEFORE ARRIVING IN OUR AREA. AS IS
TYPICAL WITH LARGE SCALE PATTERN CHANGES...THERE HAVE BEEN
SIGNIFICANT RUN TO RUN MODEL VARIATIONS IN THE HANDLING OF THE
DETAILS OF THIS CYCLONE/DEVELOPING TROUGH AND WOULD ANTICIPATE
FURTHER CHANGES IN LATER MODEL RUNS.

AT THIS POINT...FAIRLY HIGH CONFIDENCE THAT A STORMIER AND
EVENTUALLY COLDER PATTERN WILL SETTLE IN NEXT WEEK. INITIALLY LOOKS
TO BE MORE OF A RAIN/SHOWER EVENT EARLY IN THE WEEK TRANSITIONING TO
MORE FLURRIES/SNOW SHOWERS AS THE UPPER LOW SETTLES IN OVER THE
REGION. LATEST RUNS REALLY ARENT LOOKING TOO FOREBODING FOR
SIGNIFICANT SNOWFALL...BUT AT THIS POINT WOULD REALLY CAUTION ABOUT
LATCHING ONTO ANY ONE SOLUTION AS THERE WILL LIKELY BE MORE MODEL
VARIABILITY IN THE COMING DAYS. CERTAINLY SOME POTENTIAL EXISTS FOR
TRAVEL IMPACTS HEADING INTO THE MIDDLE OF NEXT WEEK.

IZZI

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 06Z...

* BRIEF PERIOD OF FLURRIES/VERY LIGHT SNOW PRE-DAWN. MDW MAY SEE
  VIS REDUCED BRIEFLY TO MVFR.

* MVFR CEILINGS TO PERSIST MUCH OF PERIOD...LIFTING NEAR VFR OR
  POSSIBLY SCATTERING OUT FOR A TIME MID-MORNING THROUGH THIS
  AFTERNOON.

* LIGHT WINDS 5 KT OR LESS. STARTING WEST-NORTHWEST EARLY THIS
  MORNING...LIKELY VARIABLE OR CALM LATER TODAY/THIS EVENING.

RATZER

//DISCUSSION...UPDATED 06Z...

KLOT/TORD AND TMDW RADARS INDICATING SOME WEAK RETURNS OVER
SOUTHERN LAKE MICHIGAN AND THE IMMEDIATE METRO AREAS AT 0530Z.
THESE RETURNS ASSOCIATED WITH LIGHT LAKE-EFFECT PRECIP IN THE
FORM OF LIGHT SNOW...OR FREEZING DRIZZLE. WEB-CAMS FROM DOWNTOWN
INDICATE SOME DROP IN VISIBILITY WITH PRECIP...WHICH RADAR
INDICATES WILL LIKELY AFFECT METRO TERMINALS FOR A TIME EARLY THIS
MORNING. MID-LEVEL DISTURBANCE PASSING 12-15Z SHOULD ALLOW
INVERSION HEIGHTS OVER THE LAKE TO LOWER...WHICH ALONG WITH
WEAKENING LOW LEVEL WIND FIELD SHOULD END PRECIP.

OTHERWISE...EXTENSIVE AREA OF MVFR CIGS REMAINS TRAPPED BENEATH
SHALLOW INVERSION ASSOCIATED WITH WEAK SURFACE HIGH PRESSURE
ACROSS THE REGION...THOUGH SATELLITE IMAGERY DOES SHOW SOME
EROSION/CLEARING OVER PARTS OF EAST CENTRAL AND NORTHEAST WI AS
WELL AS A CORRIDOR ALONG A MLI-PNT-DNV-LAF LINE. SOME INDICATIONS
EXIST THAT THIS CLEARING OVER PARTS OF WI WILL CONTINUE TO WORK
SOUTH-SOUTHWEST ALONG THE WESTERN SHORE OF LAKE MICHIGAN LATER
THIS MORNING AND COULD SCATTER OUT MVFR CLOUDS FOR A TIME LATE
THIS MORNING INTO THIS AFTERNOON. MODEL GUIDANCE IS A LITTLE MORE
PESSIMISTIC BUT DOES ALSO HINT AT RAISING CEILINGS OR SCATTERING
LOW CLOUDS AS WELL.

LIGHT NORTHWEST OR WEST-NORTHWEST WINDS 5 KT OR LESS THIS MORNING
SHOULD BECOME VARIABLE OR CALM LATER TODAY INTO THIS EVENING AS
THE CENTER OF THE WEAK AND DIFFUSE SURFACE HIGH DRIFTS EAST ACROSS
THE TERMINALS. WEAK SOUTH-SOUTHWEST FLOW DEVELOPS JUST ABOVE THE
SURFACE THIS EVENING...FROM WHICH GUIDANCE SUPPORTS A REDEVELOPMENT
OF MVFR CIGS WITH SLOW LOW-LEVEL MOISTENING.

RATZER

//ORD AND MDW CONFIDENCE...UPDATED 06Z...

* MEDIUM-HIGH IN CEILING TRENDS...AND BRIEF LIGHT SNOW EARLY THIS
  AM.

* HIGH IN WINDS.

RATZER

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

* SATURDAY AND SUNDAY...DRY/VFR. LIGHT SOUTH WINDS.
* MONDAY...CHANCE OF RAIN/SNOW. MVFR LIKELY. GUSTY SOUTHERLY WINDS.
* TUESDAY...CHANCE OF RAIN/SNOW. IFR LIKELY. SOUTHWEST WINDS.
* WEDNESDAY...CHANCE OF SNOW. MVFR LIKELY. STRONG/GUST WEST WINDS.

BEACHLER

&&

.MARINE...
156 AM CST

MAIN MARINE CONCERNS THIS MORNING ARE A PERIOD OF STRENGTHENING
SOUTH-SOUTHEAST WINDS LATE SUNDAY THROUGH MONDAY NIGHT...THEN THE
POTENTIAL FOR WEST GALES LATE WEDNESDAY INTO THURSDAY.

FAIRLY QUIET STRETCH OF WEATHER ACROSS LAKE MICHIGAN TODAY THROUGH
SATURDAY AS A LARGE AREA OF WEAK HIGH PRESSURE DRIFTS SLOWLY EAST
ACROSS THE REGION. LIGHT NORTH WINDS TODAY WILL BECOME SOMEWHAT
LIGHT/VARIABLE...BEFORE BECOMING LIGHT SOUTH ON SATURDAY AS THE
SURFACE RIDGE AXIS PASSES TO THE EAST. SOUTHERLY WINDS WILL THEN
BEGIN TO INCREASE SATURDAY NIGHT AND MORE SO SUNDAY...AS LOW
PRESSURE DEVELOPS IN THE LEE OF THE NORTHERN ROCKIES AND THE
GRADIENT BEGINS TO TIGHTEN ACROSS THE PLAINS AND UPPER MIDWEST.
MEDIUM RANGE FORECAST MODELS ARE IN GENERAL AGREEMENT IN DEVELOPING
A SUB-1000 MB SURFACE LOW ACROSS IOWA OR SOUTHERN MINNESOTA BY
MIDDAY MONDAY...WHICH WOULD LEAD TO SOUTH-SOUTHEAST WINDS IN THE
20-25 KT RANGE THROUGH MONDAY BEFORE OCCLUDING AND WEAKENING AS THE
LOW MOVES ACROSS WISCONSIN AND LAKE MICHIGAN ON TUESDAY.

HOWEVER...VARIOUS MODEL AND MODEL ENSEMBLE MEMBERS THEN DEVELOP A
SECOND LOW SOUTH OF THE LAKES ALONG THE TRAILING COLD FRONT TUESDAY
NIGHT...WHICH WOULD LIFT NORTH AND COMBINE WITH THE FILLING GREAT
LAKES LOW...DEEPENING RAPIDLY WEDNESDAY AND WEDNESDAY NIGHT OVER THE
EASTERN LAKES. THIS SCENARIO WOULD LEAD TO QUICKLY INCREASING
WESTERLY WINDS ACROSS LAKE MICHIGAN...QUITE POSSIBLY WEST GALES...
LATE WEDNESDAY INTO THURSDAY/CHRISTMAS DAY AS THE LOW PULLS AWAY TO
THE NORTHEAST.

RATZER

&&

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

$$

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