Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 130129
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
729 PM CST FRI FEB 12 2016

.UPDATE...
729 PM CST

ONLY SOME MINOR ADJUSTMENTS TO GOING FORECAST...MAINLY TO SLOW THE
TEMPERATURE FALL TONIGHT AS STRONG WINDS ARE SLOWING THE TYPICAL
DIURNAL COOLING AS WE REMAIN WELL MIXED. COOLING THUS FAR THIS
EVENING HAS BEEN LARGELY ADVECTIVE AND THAT SHOULD CONTINUE TO BE
THE CASE THROUGH A GOOD PORTION OF THE NIGHT. TEMPERATURE GRADIENT
IS NOT TERRIBLY STRONG UPSTREAM SO REALLY ONLY LOOKING FOR A
SLOW/STEADY TEMP DROP THROUGH THE NIGHT. THIS DID REQUIRE NUDGING
LOW TEMPERATURES UP A DEGREE OR TWO IN SPOTS AND WITH WINDS LIKELY
TO BEGIN SUBSIDING PRETTY QUICKLY AFTER MIDNIGHT IT IS POSSIBLE
THAT ANY -20F WIND CHILLS WILL BE FEW AND FAR BETWEEN. EVEN IF WE
DO FALL A BIT SHY OF ADVISORY CRITERIA...IT IS STILL GOING TO BE A
BITTERLY COLD NIGHT AND NO CHANGES ARE PLANNED FOR THE WIND CHILL
ADVISORY. OUTSIDE OF THE LAKE EFFECT SNOW BELT SKIES HAVE CLEARED
AND ARE LIKELY TO REMAIN CLEAR SO ADJUSTED SKY GRIDS ACCORDINGLY.
UPDATED GRIDS AND TEXT FORECASTS ALREADY SENT.

IZZI

&&

.SHORT TERM...
239 PM CST

THROUGH MONDAY...

THE MAIN CONCERNS DURING THE PERIOD WILL BE THE COLD WIND
CHILLS TONIGHT INTO SATURDAY...FOLLOWED BY THE POTENTIAL FOR
ADVISORY LEVEL SNOW ACCUMULATIONS ON SUNDAY INTO SUNDAY NIGHT.

AFTER A QUICK HIT OF SNOW EARLIER THIS MORNING ASSOCIATED WITH THE
ARCTIC COLD FRONT...SKIES HAVE BECOME PARTLY SUNNY. TEMPERATURES ARE
MAINLY IN THE LOWER 20S...THOUGH GUSTY NORTHWEST WINDS ARE ALREADY
RESULTING IN WIND CHILLS AROUND 3 TO 6 DEGREES ABOVE ZERO. A MUCH
COLDER AIRMASS...CURRENTLY OVER THE UPPER MIDWEST...WILL SHIFT
SOUTHWARD INTO THE AREA TONIGHT AS A 1042 MB HIGH BUILDS SOUTH OVER
THE MIDWEST. AS A RESULT...TEMPERATURES TONIGHT WILL LIKELY DROP
INTO THE LOWER SINGLE DIGITS ABOVE IN MOST AREAS TONIGHT...WITH EVEN
SOME AREAS DROPPING BELOW ZERO. GIVEN THAT THE NORTHWEST WINDS WILL
REMAIN ELEVATED TONIGHT ALONG THE EASTERN PERIPHERY OF THE SURFACE
HIGH...THIS SHOULD CREATE WIND CHILLS OF 20 TO 25 BELOW ACROSS
NORTHERN ILLINOIS...AND 15 TO 20 BELOW SOUTH OF I-80. THE CURRENT
WIND CHILL ADVISORY HANDLES THIS QUITE WELL...SO NO CHANGES ARE
PLANNED TO HEADLINES AT THIS TIME.

THE SURFACE HIGH WILL BUILD OVER THE AREA SATURDAY AFTERNOON AND
EVENING...SO WIND SPEEDS SHOULD ABATE LATER SATURDAY. TEMPERATURES
DURING THE DAY WILL LIKELY REMAIN WELL BELOW AVERAGE...WITH HIGHS
ONLY EXPECTED INTO THE LOWER TO MIDDLE TEENS. AFTER TEMPERATURES
DROP IN THE EVENING...INCREASING CLOUDS AND SOUTHEASTERLY WINDS
OVERNIGHT SATURDAY WILL LIKELY ALLOW TEMPERATURES  TO STEADY
OUT...OR EVEN RISE A COUPLE OF DEGREES BY SUNDAY MORNING.

A HYBRID NORTHERN PACIFIC CLIPPER TYPE SYSTEM IS EXPECTED TO MAIN
PLAYER IN THE WEATHER ACROSS THE AREA ON SUNDAY AND SUNDAY NIGHT.
THIS SYSTEM IS EXPECTED TO TRACK EAST SOUTHEASTWARD OVER THE
NORTHERN PLAINS TO THE LOWER GREAT LAKES BY SUNDAY NIGHT. AS IT DOES
SO...INCREASING SOUTHERLY FLOW OVER THE REGION WILL LOOKS TO RESULT
IN A RATHER LONG DURATION OF ISENTROPIC UPGLIDE (WARM AIR
ADVECTION). GUIDANCE CONTINUES TO ADVERTISE 20 TO 30 KT
WINDS...NEARLY PERPENDICULAR TO THE ISOBARS ON THE 285 K AND 290 K
SURFACES. THIS COMBINED WITH VERY LOW CONDENSATION PRESSURE DEFICITS
(LESS THAN 2 MB)...AND MIXING RATIOS AROUND 2.0 G/KG...ALL SUGGEST
THAT A LONG DURATION LIGHT TO MODERATE SNOW IS LIKELY SUNDAY INTO
SUNDAY NIGHT.

IT APPEARS THAT THE THERMAL PROFILE WILL ALSO BE SUPPORTIVE OF A
HIGHER RATIO SNOW...ESPECIALLY DURING THE DAY ON SUNDAY. MODEL
SOUNDINGS SUGGESTS THAT THE DENDRITIC ZONE COULD BE 12,000 FEET FOR
A PERIOD ON SUNDAY BEFORE SOME WARMER AIR SHIFTS OVER THE AREA LATE
IN THE DAY. THIS DEEP ZONE SUGGESTS THAT THE SNOW COULD BE VERY
FLUFFY AND HAVE RATIOS UP AROUND 18:1. USING 15 TO 18 TO 1 AVERAGE
RATIOS DURING THIS EVENT SUGGESTS THAT SNOW AMOUNTS IN THE 2 TO 5
INCH RANGE ARE POSSIBLE...WHICH COULD MEAN AN ADVISORY MAY BE NEEDED
FOR PORTIONS OF THE AREA IF THESE TRENDS CONTINUE.

IT CURRENTLY APPEARS THE SNOW WILL DEVELOP FROM WEST TO EAST OVER
THE AREA DURING THE MORNING ON SUNDAY...WITH A STEADY SNOW LIKELY BY
LATE MORNING AND THROUGH THE AFTERNOON. LIGHT SNOW WILL ALSO LIKELY
CONTINUE THROUGH SUNDAY EVENING/NIGHT BEFORE ABATING BY LATE SUNDAY
NIGHT OR VERY EARLY MONDAY MORNING. GIVEN THE EXPECTED SNOW COVER
ACROSS THE AREA...I HAVE OPTED TO GO ON THE COLD SIDE OF GUIDANCE
FOR HIGHS ON MONDAY.

KJB

&&

.LONG TERM...

MONDAY NIGHT THROUGH FRIDAY...

222 PM...FORECAST CONCERNS INCLUDE POSSIBLE CLIPPER SYSTEM AND
SNOW ON TUESDAY AND THEN A SIGNIFICANT...BUT POSSIBLY BRIEF...WARM
UP FRIDAY.

ANOTHER CLIPPER SYSTEM WILL DIVE INTO THE UPPER MIDWEST MONDAY
NIGHT BUT MODELS THEN DIVERGE ON ITS TRACK AND SPEED WITH THE GFS
A BIT SLOWER AND BRINGS IT ACROSS THE CWA TUESDAY WITH THE
ECMWF/GEM FASTER AND MUCH FARTHER SOUTH. DIFFICULT TO TRY TO PIN
DOWN SPECIFICS WITH THESE DIFFERENT SOLUTIONS. HAVE TRENDED TOWARD
THE GFS WITH LOW CHANCE POPS FOR SNOW. BUT WITH HIGHS POSSIBLY IN
THE MID/UPPER 30S...POSSIBLE FOR A MIX OF RAIN/SNOW ACROSS
SOUTHERN AREA...WHICH ADDS TO THE UNCERTAINTY FOR SNOW AMOUNTS...
IF PRECIP DOES INDEED OCCUR. ANOTHER PERIOD OF COLDER TEMPS IS
THEN EXPECTED BEHIND THIS SYSTEM WITH HIGHS ON WEDNESDAY LIKELY
BELOW NORMAL.

THERE HAS BEEN A FAIRLY STRONG SIGNAL FOR A WARM UP LATE NEXT
WEEK AND THIS NOW APPEARS TO BEGIN THURSDAY AND STRENGTHENS
THURSDAY NIGHT INTO FRIDAY AS BOTH THE GFS/ECMWF SHOW 850MB TEMPS
ANYWHERE FROM +8C TO +11C WITH HIGHS THURSDAY WELL IN THE
30S...POSSIBLY LOWER 40S SOUTH THEN UPPER 40S TO PERHAPS MID
50S...OR WARMER FRIDAY. ASSUMING THERE IS STILL SOME SNOW COVER
FROM SUNDAY AND POSSIBLY TUESDAY...THIS MAY HOLD TEMPS DOWN...AT
LEAST INITIALLY BUT AS DEWPOINTS RISE THURSDAY NIGHT INTO
FRIDAY...WHATEVER SNOW IS LEFT SHOULD QUICKLY MELT...AND AS THIS
OCCURS FOG WILL PROBABLY BE LIKELY.

WHILE ITS A BIT EARLY FOR DETAILS...CURRENTLY FAIRLY STRONG LOW
PRESSURE IS EXPECTED TO TRACK ACROSS THE MIDWEST/NORTHERN GREAT
LAKES LATE NEXT WEEK ASSOCIATED WITH THIS WARM UP. A TRAILING
COLD FRONT WILL LIKELY MOVE ACROSS THE AREA WITH AT LEAST A LOW
CHANCE FOR RAIN SHOWERS FRIDAY AFTERNOON/NIGHT. BREEZY/WINDY
CONDITIONS ALSO LIKELY DURING THIS TIME. CMS

&&

.AVIATION...
FOR THE 00Z TAFS...

STRONG NORTHWEST WINDS SHOULD REMAIN RATHER GUSTY WELL INTO THE
NIGHT BEFORE GUSTINESS BEGINS TO SLOWLY SUBSIDE AFTER MIDNIGHT.
OTHERWISE NO AVIATION CONCERNS THROUGH THIS FORECAST PERIOD.

IZZI

&&

.MARINE...
150 PM CST

MAIN CONCERNS FOR TONIGHT CONTINUE TO BE FOCUSED ON GALES AND
HEAVY FREEZING SPRAY. WINDS HAVE BEEN STEADILY INCREASING FROM
NORTH TO SOUTH OVER LAKE MICHIGAN TODAY...WITH THE FREQUENCY IN
GALES STEADILY INCREASING. WITH NORTHWEST WINDS PERSISTING FOR A
LONGER PERIOD...THIS WILL BUILD THE WAVES ALONG THE SOUTHERN AND
SOUTHEASTERN PORTIONS OF LAKE MICHIGAN. THE VERY COLD AIR WILL
CONTINUE TO FLOW SOUTH FROM CANADA TONIGHT...WHICH COUPLED WITH
THE EXPECTED GALES TO 40KT WILL CREATE HEAVY FREEZING SPRAY
POTENTIAL FOR ANY VESSELS ON LAKE MICHIGAN TONIGHT. STRONG
CANADIAN HIGH PRESSURE WILL SLIDE SOUTH FROM THE UPPER MIDWEST
INTO THE CENTRAL PLAINS...WHICH WILL KEEP A VERY TIGHT PRESSURE
GRADIENT ALONG THE WESTERN GREAT LAKES AND LAKE MICHIGAN THROUGH
MIDNIGHT. THEN HIGH PRESSURE IS EXPECTED TO SLOWLY DRIFT EAST BY
DAYBREAK SATURDAY AND STEADILY DIMINISH GALES TO BETWEEN 20-25KT
BY DAYBREAK SATURDAY OR SHORTLY AFTER.

HIGH PRESSURE WILL THEN SLIDE OVERHEAD SATURDAY NIGHT AND BRING
MUCH LIGHTER WINDS TO LAKE MICHIGAN...HOWEVER IT WILL LIKELY TAKE
A LITTLE LONGER FOR WAVES TO FULLY SUBSIDE IN THE SOUTHERN PORTION
OF THE LAKE DUE TO THE PROLONGED PERIOD OF NORTH/NORTHWEST
FLOW...WHICH COULD RESULT IN CONTINUED HAZARDOUS WAVES TO SMALL
CRAFT SATURDAY AFTN/EVE MAINLY FOR THE NORTHWEST INDIANA
NEARSHORE. HIGH PRESSURE QUICKLY SHIFTS EAST SUNDAY WITH FLOW
TURNING SOUTHERLY AND INCREASING AHEAD OF YET ANOTHER LOW PRESSURE
SYSTEM FOR SUNDAY NIGHT/MONDAY. CURRENT INDICATIONS ARE THAT
SOUTHERLY FLOW WILL RANGE FROM 15 TO 25 KT.

BEACHLER

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...WIND CHILL ADVISORY...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-
     ILZ011-ILZ012-ILZ013-ILZ014...MIDNIGHT SATURDAY TO 9 AM
     SATURDAY.

IN...NONE.
LM...GALE WARNING...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-LMZ745-LMZ777-
     LMZ779 UNTIL 6 AM SATURDAY.

     HEAVY FREEZING SPRAY WARNING...LMZ744-LMZ745-LMZ777-LMZ779 UNTIL
     6 AM SATURDAY.

&&

$$

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