Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 190910
AFDLOT

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

.SHORT TERM...
259 AM CST

THROUGH SUNDAY...

THE MAIN CONCERNS DURING THE SHORT TERM WILL BE SOME PERIODIC
CHANCES OF VERY LIGHT SNOW OR DRIZZLE THROUGH THE WEEKEND AS SEVERAL
WEAK...AND FAST MOVING MID LEVEL DISTURBANCES SHIFT ACROSS THE AREA.

SURFACE HIGH PRESSURE IS CENTERED ACROSS MUCH OF THE MID AND UPPER
MISSISSIPPI RIVER VALLEY EARLY THIS MORNING. THIS FEATURE IS EXPECTED
TO REMAIN IN CONTROL ACROSS NORTHERN ILLINOIS DURING THE DAY TODAY.
OVERALL...WHICH WILL RESULT IN A VERY LIGHT AND VARIABLE WIND
FIELD ACROSS THE AREA TODAY. AN AREA OF LAKE EFFECT FLURRIES
CONTINUES ACROSS PORTIONS OF THE CHICAGO AREA...AND INTO
NORTHWESTERN INDIANA NEAR LAKE MICHIGAN. MODEL SOUNDINGS INDICATE
JUST DEEP ENOUGH INVERSION HEIGHTS TO SUPPORT ICE IN THE
CLOUDS...AND GIVEN MDW HAS BEEN REPORTING ONLY -SN FOR MOST OF THE
NIGHT IT APPEARS THE THREAT OF FREEZING DRIZZLE WILL BE LOW. ALL
OF THIS ACTIVITY SHOULD WANE ACROSS THE AREA DURING THE MORNING AS
THE WIND FIELD CONTINUES TO WEAKEN OVER SOUTH LAKE MICHIGAN IN
RESPONSE TO THE APPROACHING SURFACE RIDGE OF HIGH PRESSURE.

SURFACE WINDS WILL BEGIN TO SHIFT OUT OF THE SOUTH BY EARLY SATURDAY
MORNING AS THIS SURFACE HIGH BEGINS TO SHIFT EAST OF THE REGION. AS
THIS OCCURS...GUIDANCE IS INSISTENT WITH BRINGING A WEAK UPPER
DISTURBANCE ACROSS NORTHERN ILLINOIS SATURDAY MORNING. THIS WEAK
WAVE IS NOTED IN THE WATER VAPOR IMAGERY CURRENTLY ACROSS
SOUTHEASTERN ALBERTA. MOISTURE APPEARS TO REMAIN LIMITED WITH THIS
WAVE. HOWEVER...I THINK THIS WAVE...AND SOME WEAK ISENTROPIC ASCENT
COULD SQUEEZE OUT A FEW FLURRIES SATURDAY MORNING...SO I HAVE
INCLUDED A SLIGHT CHANCE FOR SUCH IN THE FORECAST.

WARM AIR ADVECTION/ISENTROPIC ASCENT WILL CONTINUE TO INCREASE
ACROSS MUCH OF THE MIDWEST AND GREAT LAKES REGION INTO SUNDAY IN
RESPONSE TO AN INCREASING SOUTHERLY WIND FIELD AHEAD OF A STORM
SYSTEM...WHICH IS EXPECTED TO DEVELOP ACROSS THE PLAINS. THIS COULD
AGAIN RESULT IN A PERIOD OF LIGHT PRECIP ACROSS MAINLY FAR NORTHERN
ILLINOIS VERY LATE SATURDAY NIGHT AND INTO SUNDAY.
OVERALL...CONFIDENCE IS LOW AS TO HOW FAR SOUTH THIS LIGHT PRECIP
WILL BE ABLE TO DEVELOP...BUT GIVEN THE POTENTIAL THAT IF THIS
PRECIP DOES DEVELOP AS FAR SOUTH AS NORTHERN ILLINOIS...THERE COULD
BE A PERIOD OF FREEZING DRIZZLE INTO EARLY SUNDAY MORNING BEFORE
TEMPERATURES JUMP ABOVE FREEZING...I HAVE OPTED TO INCLUDE A SLIGHT
MENTION LATE SATURDAY NIGHT INTO EARLY SUNDAY...MAINLY FOR AREAS
NORTH OF INTERSTATE 90.

KJB

&&

.LONG TERM...
259 AM CST

SUNDAY NIGHT THROUGH THURSDAY...

THE LONG TERM PERIOD WILL BE A PERIOD OF A SIGNIFICANT WEATHER
PATTERN SHIFT ACROSS NORTH AMERICA AND AS SUCH...IT APPEARS LIKELY
THAT THE CENTRAL CONUS WILL BECOME MUCH MORE ACTIVE INTO THE
HOLIDAY WEEK. THE MAIN CONCERNS LOCALLY WITH THIS PATTERN
SHIFT...WILL BE PRECIP TYPES...AND THE ACTUAL EVOLUTION
OF...POTENTIALLY A SIGNIFICANT STORM SYSTEM ACROSS PORTIONS OF THE
GREAT LAKES REGION CHRISTMAS EVE.

UNFORTUNATELY...THE DETAILS OF THIS STORM SYSTEM THIS FAR OUT ARE
STILL LARGELY UNCERTAIN...AS LARGE SCALE PATTERN SHIFTS SUCH AS THIS
TYPICAL RESULT IN LOWER PREDICTABILITY...ESPECIALLY WITH REGARDS TO
THE TRACK AND ACTUAL STRENGTH OF A DEVELOPING STORM SYSTEM. WHAT
APPEARS TO BE LIKELY...HOWEVER...IS FOR A PERIOD OF RAIN ACROSS THE
AREA LATE MONDAY AND MONDAY NIGHT AS WHAT APPEARS TO BE THE INITIAL
STORM SYSTEM...BEGINS TO SHIFT A WARM FRONT ACROSS NORTHERN
ILLINOIS. GIVEN THE WARM SECTOR PLACEMENT OF MY AREA...PRECIP TYPE
LOOKS TO BE LIQUID...WITH SURFACE TEMPERATURES LIKELY TO REMAIN
ABOVE FREEZING MONDAY NIGHT WITH CONTINUED SOUTHERN FLOW.

THE MAIN UNCERTAINTIES COME INTO PLAY BY CHRISTMAS EVE...AS MODEL
GUIDANCE CONTINUES TO ADVERTISE A SIGNIFICANT AMPLIFICATION TO THE
UPPER LEVEL PATTERN ACROSS THE EASTERN PACIFIC AND NORTH AMERICA. A
STRONG PACIFIC JET (NEAR 40 DEGREES N)...INITIALIZED AT OVER 180 KT
IN THIS EVENINGS MODEL OUTPUT IS FORECAST TO RESULT IN THE BUILDING
OF A POTENT UPPER RIDGE JUST OFF THE CALIFORNIA COAST BY EARLY NEXT
WEEK. TELECONNECTIONS RELATED TO SUCH PATTERN SHIFT CERTAINLY SUPPORT
THE DIGGING OF A SIGNIFICANT TROUGH...AND A GOOD POSSIBILITY OF AN
STORM SYSTEM ACROSS THE CENTRAL CONUS DURING THE UPCOMING HOLIDAY
WEEK. HOWEVER...AS PREVIOUSLY MENTIONED CONFIDENCE IS LOW WITH THE
DETAILS OF HOW AND WHERE EXACTLY THIS STORM MAY TRACK. THIS IS A
PERIOD WORTH WATCHING THOUGH...AS AT LEAST SOME SNOW AND POSSIBLY
STRONG NORTHWEST WINDS COULD BE IN STORE.

KJB

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 09Z...

* PERIOD OF LIGHT SNOW DURING PRE-DAWN HOURS. BRIEF MVFR VIS AND
  CIGS 1200-1500 FT POSSIBLE.

* 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 09Z...

* 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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