Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 250301
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
901 PM CST SAT JAN 24 2015

.UPDATE...
900 PM CST

EVENING UPDATE...

HAVE UPDATED GRIDS TO REFLECT MUCH LOWER SNOWFALL AMOUNTS ON SUNDAY
TO THE NORTH OF INTERSTATE 80 AND ALSO SLOWED ARRIVAL OF
PRECIPITATION MAINLY TO EARLY TO MID SUNDAY MORNING. RADAR...SURFACE
ANALYSIS AND SATELLITE TRENDS SUPPORT THE NEW 00Z GUIDANCE AND
LATEST HIGHER RESOLUTION GUIDANCE ROLLING IN DIGGING THE 500 MB WAVE
FARTHER SOUTHWEST AND ASSOCIATED SURFACE LOW FARTHER SOUTHWEST AND
THEN SOUTH OF THE CWA.  NOW LOOKING AT GENERALLY AROUND A FEW TENTHS
TO UP TO AN INCH NORTH OF INTERSTATE 88...ONE TO TWO INCHES BETWEEN
I-88 AND I-80 AND ONE TO THREE INCHES SOUTH OF INTERSTATE 80. IF THE
FARTHER SOUTH TREND BECOMES MORE AGGRESSIVE AS THE SYSTEM APPROACHES
SUNDAY...THEN EVEN THESE AMOUNTS COULD END UP BEING TOO HIGH.
NAM/WRF AND OTHER HIGHER-RES GUIDANCE CONTINUES TO HINT AT LAKE
ENHANCEMENT DURING THE DAY SUNDAY INTO THE AFTERNOON INTO PARTS OF
NORTHEAST IL AND NW IN...BUT UNLESS FRICTIONAL CONVERGENCE CAN
ENHANCE RATES A BIT...THE THERMODYNAMICS AND INVERSION HEIGHTS ARE
VERY MARGINAL TO ACTUALLY ADD MUCH TO SNOW TOTALS.

REGARDING THE MIXED PRECIPITATION IN THE SOUTHERN CWA...THE
SOUTHWARD TREND AND DELAYED TIMING OF THE SYSTEM SHOULD ALLOW FOR
COLDER AIR TO BLEED SOUTHWARD AND SUPPRESS THE MIXING ZONE SOUTHWARD
FROM THE PREVIOUS FORECAST. ANY WINTRY MIX/RAIN SNOW MIX IN THE FAR
SOUTHERN CWA WILL LIKELY TRANSITION TO SNOW BY MID DAY/EARLY
AFTERNOON.

UPDATED ZONES WILL BE SENT SHORTLY.

RC/KREIN

&&

.SHORT TERM...
250 PM CST

THROUGH SUNDAY...

MAIN FORECAST CONCERN WILL BE PASSAGE OF A CLIPPER SYSTEM ACROSS THE
MIDWEST BRINGING ACCUMULATING SNOWFALL AND FOR SOME AREAS A WINTRY
MIX OF PRECIPITATION. CLOUD COVER WILL CONTINUE TO INCREASE THE
REMAINDER OF THE TODAY AS THE LOW DIVES ACROSS THE DAKOTAS INTO
MINNESOTA. MODELS ARE IN DECENT AGREEMENT TRACKING THE SURFACE LOW
ACROSS FAR NE MO LATE TONIGHT INTO DOWNSTATE IL BY SUNDAY AFTERNOON.
1000MB-700MB LAYER MAX WETBULBS ARE UP TO 2C SOUTH OF THE I-80
CORRIDOR THROUGH LATE MORNING/EARLY AFTERNOON SUNDAY...THEN START TO
FALL QUICKLY FROM THE NORTH THROUGH THE AFTERNOON HOURS. WITH THAT
IN MIND...THINK THAT PRECIP WILL BEGIN AS PRIMARILY SNOW NORTH OF
THE I-80 CORRIDOR WITH RAIN/SLEET TO THE SOUTH...EVENTUALLY CHANGING
OVER TO ALL SNOW. MODELS IN GENERAL HAVE THE AXIS OF HIGHEST QPF
SOUTH OF I-80...BUT WITH WARMER TEMPERATURES...IT APPEARS THE
HIGHEST SNOWFALL MAY OCCUR A LITTLE FARTHER NORTH. DID BUMP UP SNOW
TOTALS ACROSS PORTIONS OF THE CHICAGO METRO WHERE MODELS CONTINUE TO
ADVERTISE AN INCREASE IN THE LOW LEVEL LAPSE RATES AS WINDS TURN NE
OFF LAKE MICHIGAN WITH LAKE ENHANCEMENT APPEARING AT LEAST POSSIBLE.
NAM AND REGIONAL GEM GUIDANCE BOTH SHOW A SLIGHT MAX IN THE QPF
VALUES OVER THE AREA AND HAVE BEEN CONSISTENTLY ADVERTISING A BUMP IN
THE LOW LEVEL OMEGA STARTING LATE MORNING INTO THE AFTERNOON.
FORECAST SNOW TOTALS FOR THIS EVENT WILL BE 1-3 INCHES WEST OF A
WAUKEGAN TO STREATOR LINE...AND 2-4 INCHES EAST. WILL HAVE TO
MONITOR THE MESOSCALE BANDING THREAT FOR SOME LOCALLY HIGHER
AMOUNTS. NAM GUIDANCE IN PARTICULAR WAS HINTING AT AN AREA OF LOW
LEVEL FGEN DEVELOPING FOR EAST CENTRAL IL AND NORTHWEST
INDIANA...BUT LAPSE RATES UPSTAIRS ARENT OVERLY IMPRESSIVE IN THE
LATEST MODELS RUNS. ALSO SURFACE LOW SLOWLY FILLING AS IT MOVES
TOWARDS THE OHIO VALLEY WILL ALSO BE A LIMITING FACTOR. WILL ISSUE
AN UPDATED SPS THIS AFTERNOON FOR THE SNOWFALL. AS MENTIONED...STILL
CANNOT RULE OUT SOME LOCALLY HIGHER AMOUNTS THAT MIGHT REQUIRE AN
ADVISORY HOWEVER DONT HAVE MUCH CONFIDENCE WHERE OR EVEN IF THIS
WOULD OCCUR.

WINDS WILL START TO RAMP UP FROM EARLY MORNING ON OUT OF THE
NORTHEAST AS A TIGHT GRADIENT SWEEPS ACROSS THE REGION ON THE BACK
SIDE OF THE LOW. NORTHEAST WINDS ARE EXPECTED TO GUST 30-35 MPH AT
TIMES ACROSS THE AREA. THESE MODERATELY STRONG NORTHEAST WINDS OVER
LAKE MICHIGAN WILL DRIVE 8-11 FOOT WAVES INTO THE NEARSHORE WATERS
AND SOME OF THESE WAVES MAY RESULT IN SOME MINOR LAKE SHORE FLOODING
TO EXPOSED AREAS. IN ADDITION...BLOWING SNOW MAY BECOME A PROBLEM
ESPECIALLY IN OPEN AREAS. WETTER THAN AVERAGE SNOW...PARTICULARLY
SOUTH OF I-80 MAY HELP LIMIT THE BLOWING SNOW SOME.

DEUBELBEISS

&&

.LONG TERM...

SUNDAY NIGHT THROUGH SATURDAY...

250 PM...FORECAST CONCERNS INCLUDE A CHANCE OF LIGHT SNOW MONDAY
AFTERNOON INTO MONDAY NIGHT AND THEN THE POTENTIAL FOR A WINTRY
MIX WEDNESDAY NIGHT.

LIGHT SNOW WILL BE ENDING ACROSS THE SOUTHEAST CWA SUNDAY EVENING
AS NORTHEAST WINDS STEADILY DIMINISH SUNDAY EVENING. LAKE EFFECT
CLOUDS AND PERHAPS A FEW FLURRIES ARE POSSIBLE SUNDAY EVENING. LOWS
BY MONDAY MORNING WILL DIP INTO THE TEENS MOST LOCATIONS BUT AS A
WEAK RIDGE PASSES ACROSS THE AREA AND WINDS BECOME LIGHT OR CALM...
LOWS COULD BE SEVERAL DEGREES COLDER DEPENDING ON CLOUD COVER.

ANOTHER WEAK CLIPPER SYSTEM MOVES SOUTHEAST ACROSS THE WESTERN
LAKES MONDAY AFTERNOON INTO MONDAY NIGHT AS IT SLOWLY BEGINS TO
WEAKEN. MODELS VARY SOME ON TIMING AND PRECIP ARRIVAL. A CONSENSUS
WOULD HAVE THE ARRIVAL DURING MONDAY AFTERNOON. INITIALLY THE
SATURATION IS DEEP ENOUGH THAT PRECIP SHOULD BEGIN AS LIGHT SNOW.
BUT INTO THE EVENING...MID LEVELS BEGIN TO DRY OUT SOME AND PRECIP
COULD TRANSITION MORE TO A LIGHT SNOW MIXED WITH OR JUST FREEZING
DRIZZLE. FAIRLY LOW CONFIDENCE AT THIS POINT...SO MAINTAINED JUST
LIGHT SNOW FOR THE ENTIRE DURATION FOR NOW.

PROGRESSIVE PATTERN BRINGS ANOTHER RIDGE OF HIGH PRESSURE ACROSS
THE REGION TUESDAY NIGHT INTO WEDNESDAY AND THEN ANOTHER STORM
SYSTEM WEDNESDAY NIGHT INTO THURSDAY MORNING. MODELS CURRENTLY IN
DECENT AGREEMENT WITH THE TRACK OF THIS LOW GENERALLY RIGHT ACROSS
THE CWA EARLY THURSDAY MORNING. WITH THIS TRACK...TEMPS LIKELY TO
REMAIN FAIRLY STEADY IN THE LOWER/MID 30S WEDNESDAY NIGHT AND
THERMAL PROFILES WOULD SUGGEST PRECIP WOULD BE MAINLY RAIN OR
PERHAPS FREEZING RAIN...IF SURFACE TEMPS REMAINED BELOW FREEZING.
AND ALSO COULD BE SOME SNOW IF THE LOW WERE TO SPEED UP AND/OR
TRACK FURTHER SOUTH. THUS DID NOT GET TOO SPECIFIC WITH PRECIP
TYPE WEDNESDAY NIGHT INTO THURSDAY BUT A WINTRY MIX IS POSSIBLE.
CMS

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 00Z...

* MVFR OR IFR CIGS LIKELY MOVE BACK IN LATER TONIGHT CONTINUE
  THROUGH SUNDAY EVENING
* SNOW DEVELOPS EARLY SUNDAY MORNING THE TAPERS OFF SUNDAY
  AFTERNOON...IFR VSBY LIKELY WITH PERIODIC LIFR POSSIBLE
* STRONG NE WINDS DEVELOP SUNDAY MORNING CONTINUE INTO EARLY
  SUNDAY EVENING...GUSTS TO 30KT AT TIMES SUNDAY LATE MORNING AND
  AFTERNOON

IZZI

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

LOW END MVFR/SPOTTY IFR CIGS SHOULD CLEAR EASTERN TERMINALS SOON
WITH IT LOOKING MORE LIKELY THAT VFR CONDITIONS WILL BE THE RULE
THIS EVENING. OVERNIGHT WINDS WILL TURN NORTHEAST AND SHOULD ALLOW
LOW END MVFR/HIGH END IFR CIGS TO SLOSH BACK SOUTH INTO THE AREA
OVERNIGHT WITH THE WIND SHIFT. SNOW IS STILL EXPECTED TO DEVELOP
AROUND OR JUST PRIOR TO SUNRISE AND CONTINUE THROUGH SUNDAY
MORNING BEFORE TAPERING OFF TO LIGHT NON-ACCUMULATING SNOW OR
FLURRIES DURING THE EARLY/MID AFTERNOON. AS SNOW ENDS CIGS SHOULD
LIFT AND COULD EVEN SCATTER OUT SUNDAY EVENING AS THE SYSTEM PULLS
QUICKLY AWAY FROM THE REGION. OTHER ISSUE IS EXPECTED TO BE THE
STRONG NORTHEAST WINDS THAT WILL DEVELOP SUNDAY MORNING AND
CONTINUE INTO THE AFTERNOON WITH GUSTS OF 25-30KT LOOKING QUITE
LIKELY FROM MID MORNING THROUGH LATE AFTERNOON SUNDAY.

IZZI

//ORD AND MDW CONFIDENCE...UPDATED 00Z...

* HIGH CONFIDENCE IN GENERAL CIG TRENDS THROUGH THE PERIOD BUT
  LOWER CONFIDENCE ON PRECISE TIMING OF CATEGORY CHANGES
* MEDIUM-HIGH CONFIDENCE IN WINDS
* MEDIUM CONFIDENCE IN SNOW AND RELATED VSBY TRENDS IN TAFS

IZZI

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

MONDAY...SLIGHT CHANCE -SN IN THE AFTERNOON. MVFR POSSIBLE.

TUESDAY...MVFR CIGS POSSIBLE.

WEDNESDAY...MAINLY VFR DURING THE DAY THEN CHANCE OF -RASN AND
MVFR/IFR AT NIGHT.

THURSDAY...CHANCE OF SNOW. MVFR/IFR POSSIBLE.

FRIDAY...CHANCE OF MVFR CIGS.

IZZI

&&

.MARINE...
127 PM CST

A COLD FRONT WILL BEGIN TO SHIFT SOUTHWARD DOWN LAKE MICHIGAN LATE
THIS AFTERNOON...WITH WINDS EXPECTED TO PICK UP OUT OF THE NORTH
INTO TONIGHT. WINDS OVER THE LAKE WILL THEN CONTINUE TO INCREASE
INTO SUNDAY AS A QUICK MOVING CLIPPER SYSTEM TRACKS SOUTHEASTWARD
ACROSS SOUTHERN ILLINOIS. THIS WILL RESULT IN NORTHEASTERLY WINDS UP
TO 30 KT BY EARLY SUNDAY MORNING ACROSS MOST OF THE LAKE...WITH THE
STRONGEST WINDS EXPECTED ACROSS THE SOUTHERN END OF THE LAKE...WHERE
A FEW GALE FORCE GUSTS TO 35 KT ARE POSSIBLE ON SUNDAY. THIS WILL
RESULT IN WAVES UP TO 10 FEET INTO THE NORTHEAST ILLINOIS SHORES
THROUGH SUNDAY EVENING. THE WINDS WILL QUICKLY ABATE ACROSS THE LAKE
SUNDAY NIGHT AS THE LOW SHIFTS TOWARDS THE EASTERN SEABOARD.

ANOTHER WEAKENING LOW WILL SHIFT SOUTHWARD OVER OR JUST WEST OF THE
LAKE LATE MONDAY AND MONDAY NIGHT. THIS WILL RESULT IN A PERIOD OF
LIGHT SOUTHERLY WINDS OVER THE LAKE DURING THIS PERIOD. LATER IN THE
WEEK ANOTHER LOW IS EXPECTED TO IMPACT THE GREAT LAKES REGION. THIS
SYSTEM STILL HAS THE POTENTIAL TO PRODUCE A PERIOD OF STRONG
SOUTHERLY WINDS WEDNESDAY NIGHT...FOLLOWED BY STRONG NORTHERLY WINDS
THURSDAY INTO FRIDAY.

KJB

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-
     LMZ745...6 AM SUNDAY TO 9 AM MONDAY.

&&

$$

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