Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 251704
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1104 AM CST WED FEB 25 2015

.UPDATE...

1104 AM...LATE MORNING UPDATE...A LARGE AREA OF LIGHT SNOW
EXTENDS FROM EASTERN NEBRASKA THROUGH MUCH OF IOWA LATE THIS
MORNING WITH SNOW NOW BEING REPORTED AS FAR EAST AS DBQ. EXPECT
SNOW TO ARRIVE A BIT EARLIER THAN PREVIOUSLY EXPECTED AND BUMPED
POPS UP SOME... MAINLY ACROSS THE NORTHWEST CWA. BUT WILL NEED TO
MONITOR TRENDS AS THIS COULD STILL BE A LITTLE SLOW. THE HEAVIEST
OF THE SNOW IS EXPECTED TO FALL ACROSS EASTERN IOWA INTO WEST
CENTRAL IL INCLUDING PARTS OF THE SOUTHWEST CWA AND ITS POSSIBLE
AMOUNTS COULD REACH ADVISORY CRITERIA IN THE FAR SOUTHWEST
COUNTIES...BUT NOT PLANNING ANY HEADLINES AT THIS TIME. CURRENT
AMOUNTS LOOK OK FOR THE REST OF THE CWA AND WILL ISSUE AN SPS
SHORTLY.

OTHERWISE...TEMPERATURES HAVE WARMED INTO THE LOWER/MID 20S IN THE
CITY AND I-80 CORRIDOR TO THE UPPER 20S SOUTHERN AREAS AND HAVE
ADJUSTED TEMPS ACCORDINGLY. NOT EXPECTING TEMPS TO RISE MUCH
FURTHER WITH THE INCREASING CLOUDS AND SLOWLY STRENGTHENING
NORTHEAST WINDS. CMS

&&

.SHORT TERM...
345 AM CST

THROUGH THURSDAY...

SHORT TERM FOCUS IS ON THE INCOMING CLIPPER SYSTEM THAT WILL FRESHEN
THE SNOWPACK ACROSS MUCH OF THE AREA LATER TODAY AND TONIGHT.

FAIRLY QUIET ACROSS THE AREA EARLY THIS MORNING WITH A WIDE RANGE IN
TEMPERATURES AS SURFACE HIGH PRESSURE HOLDS AND MOSTLY CLEAR SKIES
HAS ALLOWED SOME OUTLYING AREAS TO DIP INTO THE SINGLE
DIGITS...WHILE MOST LOCATIONS SIT IN THE TEENS. NORTHERLY FLOW IS IN
PLACE THIS MORNING AS OUR FORMER SURFACE TROUGH MOVES EASTWARD...AND
STRONG HIGH PRESSURE PUSHES SOUTH JUST EAST OF THE NORTHERN CANADIAN
ROCKIES. CLOUD COVER IS QUICKLY SPREADING EAST THROUGH IOWA AND WILL
THICKEN UP FAIRLY QUICKLY ACROSS THE AREA NOT LONG AFTER DAYBREAK
FROM WEST TO EAST. THEREFORE EXPECT TEMPERATURES TO GENERALLY HOLD
IN THE TEENS THIS MORNING...BEFORE SOME INCREASE THIS AFTERNOON
AHEAD OF THE CLIPPER.

THE SURFACE LOW LOOKS TO TRACK SOUTHEAST TODAY FROM THE CENTRAL
PLAINS TO THE MID MISSOURI VALLEY THIS EVENING. THERE IS A FAIRLY
STOUT BAROCLINIC ZONE ON THE NORTH SIDE OF THE LOW DUE TO THE ARCTIC
HIGH WHICH WILL CONTINUE SOUTHWARD AS WELL INTO THE NORTHERN PLAINS.
THE STRONGEST FRONOTOGENESIS WITH THIS SYSTEM WILL REMAIN TO OUR
SOUTH AND WEST...BUT LATER THIS AFTERNOON AND EVENING. THERE IS
ENOUGH FRONTOGENETICAL FORCING ACCOMPANYING THE SOMEWHAT MODEST
HEIGHT FALLS...PRODUCING MODEST OMEGA BUT THROUGH A VERY DEEP
DENDRITIC GROWTH ZONE THAT WE SHOULD SEE SOME PRETTY STEADY THOUGH
LIGHTER SNOW BEGINNING IN THE ROCKFORD AREA MID AFTERNOON AND
SPREADING THROUGH THE REST OF THE AREA TONIGHT. THE STRONGER FORCING
SHIFTS EVEN FARTHER SOUTH TONIGHT...BUT THE UPPER WAVE IS STILL
PASSING THROUGH SO SOME CONTINUED LIGHT SNOW...BUT FAR NW AREA COULD
SEE THINGS TAPER.

THEN WE SHIFT GEARS QUICKLY TO LAKE EFFECT SNOW POTENTIAL ON
THURSDAY. LAKE INDUCED EQUILIBRIUM LEVELS ARE STILL A LITTLE LOWER
OVERNIGHT SO CURRENT THINKING IS ANY LAKE ENHANCEMENT OVERNIGHT
WOULD INITIALLY BE MINOR...THAT AND THE WIND FIELD IS A BIT STRONGER
AND THERE ARE SOME UNCERTAINTIES WITH LAKE ICE COVERAGE AS TO HOW
MUCH HEAT/MOISTURE FLUX WILL BE TRANSFERRED TO THE LOWER ATMOSPHERE.
BUT THERE STILL LOOK TO BE SOME LAKE EFFECT SHOWERS AFTER MIDNIGHT.
THERE IS A STILL A MODEST SIGNAL FOR LAKE EFFECT SNOW ON THURSDAY IN
NE ILLINOIS...BUT THE LOW LEVEL FLOW DOES LOOK PROGRESSIVE ENOUGH
THAT ANY BANDING THAT FORMS WOULD NOT IMPACT ONE AREA TOO LONG AND
WINDS STILL LOOK TO REMAIN UP LIMITING STRONG CONVERGENCE IN ANY ONE
SPOT. WITH ALL THAT SAID...SNOWFALL TOTALS THROUGH THURSDAY LOOK TO
BE HIGHEST IN THE SW WHERE 2-3 INCHES IS POSSIBLE...WITH 1-2 INCHES
ACROSS MUCH OF THE REMAINDER OF THE AREA...LIGHTEST ALONG THE
WISCONSIN BORDER...AND POSSIBLY HIGHER NEAR THE LAKE.

KMD

&&

.LONG TERM...
345 AM CST

THURSDAY NIGHT THROUGH TUESDAY...

SOME LAKE EFFECT SNOW SHOWERS ARE LIKELY TO PERSIST INTO NORTHWEST
INDIANA ON THURSDAY NIGHT ALTHOUGH SO WILL THE SAME CONCERNS IN THE
SHORT TERM ON HOW ORGANIZED OR NOT THESE WILL BE AND HOW MUCH THE
ICE WILL HAMPER THE HEAT FLUX AND INSTABILITY GENERATION. THERE
ARE INDICATIONS THAT AS THE WIND FIELD EASES SOME AND THE FLOW
BECOMES MORE NORTH-TO-SOUTH ORIENTED...THUS PARALLEL TO THE OPEN
WATER AREA...THAT A MORE ORGANIZED BAND MAY TEMPORARILY BE FAVORED
OVER THE LAKE AND POSSIBLY REACHING LAKE AND PORTER COUNTIES
THURSDAY EVENING.

A FITTING END TO THIS NEAR TO POSSIBLY RECORD COLD FEBRUARY IS IN
STORE WITH TEMPERATURES FLIRTING WITH DAILY RECORD TERRITORY /SEE
MORE IN THE CLIMO DISCUSSION BELOW/. MODEL GUIDANCE IS IN AGREEMENT
WITH 925MB TEMPERATURES OF -15 TO -19 THURSDAY NIGHT INTO FRIDAY
THEN MODIFYING GRADUALLY ON SATURDAY. THESE ARE IN THE LOWEST ONE
PERCENTILE OF 925MB REANALYSIS FROM THE PAST 35 YEARS FOR THE AREA
DURING THIS TIME OF YEAR. WITH SOME LIGHT FRESH SNOW COVER THIS
THE AIR MASS SHOULD REMAIN CHILLED WITH BELOW ZERO READINGS AT
NIGHT APART FROM DOWNTOWN CHICAGO. AS THE HIGH MOVES OVER THE
AREA ON FRIDAY NIGHT...LOW TEMPERATURES COULD END UP EVEN COOLER
AND COOLER THAN FORECAST...WITH SOME NEGATIVE DOUBLE DIGITS
CONCEIVABLE IN THE USUAL COOL SPOTS. WITH SUN EXPECTED DURING THE
DAY FRIDAY AND AT LEAST EARLY SATURDAY...WOULD EXPECT THE LOWER
ALBEDO AREAS SUCH AS THE CITY OF CHICAGO TO MODERATE A LITTLE MORE
THAN OUTLYING LOCALES...SOMETHING COMMON AS WE GET INTO THIS TIME
OF YEAR WITH INCREASING SUN ANGLE YET SNOW COVER STILL PRESENT.

AN UPPER LEVEL CIRCULATION...IF NOT CLOSED THEN NEAR IT...LOOKS
TO DEVELOP ACROSS THE SOUTHWEST U.S. BY SATURDAY NIGHT INTO SUNDAY.
BUILDING HEIGHTS AND RETURN MOISTURE AND ISENTROPIC ASCENT ARE
FORECAST OVER THE MIDDLE OF THE COUNTRY DURING THAT PERIOD. A
COUPLE OF SHORT WAVES EMANATING FROM THE PARENT LOW ARE FORECAST
BY THE 02.25 EC AND GFS TO RIDE OVER THE AREA WHILE THE GEM
MAINLY KEEPS THESE SOUTH. LEANING MORE BY THE FORMER GLOBAL
MODELS...SNOW WOULD BE LIKELY WITHIN A 15-24 HOUR WINDOW OF
SATURDAY NIGHT INTO/THROUGH SUNDAY. MIXING RATIOS OF AROUND 2.5 TO
3 G/KG ARE FORECAST TO RETURN ON GENERALLY CONTINUOUS ISENTROPIC
ASCENT. A BIT TOUGH TO TELL IF ANY MESOSCALE ENHANCEMENT WOULD BE
PRESENT WITHOUT MUCH OF A 850/700MB WAVE FORECAST...BUT AT THIS
POINT LOOKS LIKE THIS COULD BE A SEVERAL-INCHES-OF-SNOW-TYPE EVENT
OVER THAT PERIOD. THE GEM...AS WELL AS A SOUTHWARD TREND IN THE
25.00 GEFS...REMIND US THAT THE SOUTHWARD SOLUTION IS STILL
CERTAINLY A POSSIBILITY YET...AS SOME ADDED ADDITIONAL LIGHT SNOW
COVER THE NEXT 24-36 HOURS AND THE STRONG HIGH IN PLACE STILL ON
FRIDAY NIGHT MAY KEEP THE MID-LEVEL BAROCLINICITY A LITTLE FURTHER
SOUTH AND THUS THE SNOW THIS WEEKEND. SO OVERALL MEDIUM-HIGH
CONFIDENCE IN THE NORTHERN AND CENTRAL IL/IN REGION SEEING SOME
SNOW SATURDAY NIGHT AND SUNDAY...BUT LOW ON EXACTLY HOW FAR NORTH.

EARLY NEXT WEEK THE CONFIDENCE REALLY PLUMMETS AS ENERGY CONTINUES
TO BUCKLE INTO THE SOUTHWEST U.S. AND ROUND THE WEST-SOUTHWEST
FLOW INTO THE MIDDLE OF THE COUNTRY. THERE COULD BE SOME
MODERATION IN TEMPERATURES JUST GIVEN THIS PATTERN...BUT HOW MUCH
AND IF THE PATTERN WILL BE AS ACTIVE AS WHAT SOME OF THE LONG
RANGE MODELS INDICATE IS JUST TOO DIFFICULT TO SAY AT THIS TIME.

MTF

&&

.CLIMATE...

TEMPERATURES ARE FORECAST ONCE AGAIN TO BE CLOSE OR BREAK THE
FOLLOWING RECORDS LATER THIS WEEK...INCLUDING ONE IN ROCKFORD SET
JUST LAST WINTER:

ROCKFORD...

THURSDAY FEB 26TH LOW MAXIMUM: 11 IN 1920
FRIDAY FEB 27TH LOW MAXIMUM: 15 IN 2014
SATURDAY FEB 28TH LOW: -7 IN 1962
SATURDAY FEB 28TH LOW MAXIMUM: 17 IN 1962

CHICAGO...

FRIDAY FEB 27TH LOW: -6 IN 1897
SATURDAY FEB 28TH LOW: 0 IN 1884

MTF

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 16Z...

* NORTHEAST...LIKELY UNDER 10 KT THROUGH MUCH OF THE DAY.
* SNOW THIS EVENING PRODUCING LIGHT ACCUMULATIONS THROUGH THURSDAY. IFR
  VSBY LIKELY WITH MVFR CIGS.

KJB

//DISCUSSION...UPDATED 12Z...

WINDS WILL CONTINUE OUT OF THE NORTH-NORTHEAST THROUGH THE DAY
TODAY. HOWEVER...WIND SPEEDS WILL LIKELY REMAIN BELOW 10 KT
THROUGH MUCH OF THE DAY TODAY...BEFORE INCREASING SOME THIS
EVENING. A WEATHER DISTURBANCE WILL APPROACH THE AREA FROM THE
WEST LATER TODAY AND INTO THIS EVENING. THIS SYSTEM WILL LIKELY
BEGIN TO SPREED SOME SNOW ACROSS THE TERMINALS BY VERY LATE THIS
AFTERNOON AFTER 21-22 UTC AT KRFD...OR MORE LIKELY AROUND OR
SHORTLY AFTER THE 00 UTC HOUR THIS EVENING...ESPECIALLY ACROSS THE
EASTERN TERMINALS. IT APPEARS THE HEAVIEST SNOW WILL REMAIN TO THE
SOUTHWEST OF MOST OF THE TERMINALS. HOWEVER...WITH THE INCREASING
POTENTIAL OF LAKE ENHANCEMENT/EFFECT SNOWS OVER NORTHEASTERN
ILLINOIS LATE TONIGHT INTO THURSDAY MORNING ACCUMULATING SNOW OF A
COUPLE INCHES CERTAINLY SEEMS TO BE A GOOD BET FOR MOST TERMINALS
ACROSS NORTHEASTERN ILLINOIS. MVFR CIGS AND IFR TO LIFR
VISIBILITIES ARE LIKELY DURING THE HEAVIER PERIODS OF SNOW.
CURRENTLY...THE MID TO LATE EVENING HOURS APPEARS TO BE THE BEST
TIMING FOR THIS ACTIVITY...AND THIS CONTINUES TO BE HIGHLIGHTED AS
A TEMPO GROUP FOR 1 AND HALF MILE VIS IN -SN. -SN WILL LIKELY
LINGER ACROSS THE EASTERN TERMINALS INTO THURSDAY AS SOME LAKE
EFFECT SNOW SHOWERS CONTINUE TO MOVE ONSHORE ACROSS THE AREA. IT
APPEARS THE THREAT FOR LAKE EFFECT SNOW SHOWERS WILL COME TO AN
END THURSDAY AFTERNOON AT ALL BUT KGYY.

KB/KMD

//ORD AND MDW CONFIDENCE...UPDATED 16Z...

* MEDIUM-HIGH WITH WIND TRENDS THROUGH TODAY...LOW- MEDIUM
  THEREAFTER.
* MEDIUM IN SNOW TIMING
* MEDIUM-HIGH IN MVFR CIGS AND IFR VIS OCCURRENCE...LOW ON HOW
  LONG IFR WILL LAST.

KJB

//OUTLOOK FOR ORD/MDW FOR 00Z FRIDAY-12Z WEDNESDAY...UPDATED 12Z...

THURSDAY NIGHT...MVFR CIGS POSSIBLE.
FRIDAY THROUGH SATURDAY...VFR.
SUNDAY...SNOW...ACCUMULATIONS POSSIBLE.
MONDAY...SLT CHC SNOW LATE.
TUESDAY...CHANCE OF RAIN OR SNOW.

KB

&&

.MARINE...
205 AM CST

A SLIGHTLY LESS ACTIVE PATTERN OVER THE LAKE IS IN STORE FOR THE
REMAINDER OF THE WEEK AS LOW PRESSURES PASS WELL TO THE SOUTH AND
NORTH. AS ONE LOW DROPS FROM THE CENTRAL PLAINS TO THE MID-
MISSISSIPPI VALLEY TONIGHT INTO THURSDAY MORNING...NORTHEAST TO
NORTH WINDS WILL INCREASE ACROSS MAINLY THE SOUTHERN AND CENTRAL
PARTS OF THE LAKE. IF A CONVERGENT LAKE EFFECT SNOW BAND
DEVELOPS FROM THE OPEN WATERS OF THE LAKE TOWARD NORTHEAST
ILLINOIS THURSDAY MORNING AND NORTHWEST INDIANA LATER THURSDAY AND
THURSDAY EVENING...THEN THERE COULD BE SOME GUSTS TO TOP 25 KT.
WINDS WILL TURN BACK TO THE WEST ON FRIDAY AND SOUTHWEST INTO
SATURDAY. AS HIGH PRESSURE PUSHES IN FOR EARLY NEXT WEEK...A COLD
FRONT IS EXPECTED TO DEVELOP AND CROSS THE LAKE LATE IN THE
WEEKEND. THERE REMAINS A HIGH AMOUNT OF UNCERTAINTY FOR NEXT
WEEK...WITH SOME MODELS INDICATING THE STORM TRACK TO AT LEAST
BRIEFLY SHIFT NORTH CLOSER TO LAKE MICHIGAN WHICH WOULD BRING WIND
SPEEDS BACK UP IN EXCESS OF 25 KT ON OR AROUND TUESDAY.

LATE DAY VISIBLE SATELLITE IMAGERY ON TUESDAY APPEARED TO SHOW
SOME OPEN WATER IMMEDIATELY ALONG THE ILLINOIS SHORE SINCE THE ICE
HAD BEEN PUSHED OFFSHORE WITH SOUTHWEST WINDS DURING TUESDAY.
HOWEVER...FOR THE MOST PART ICE APPEARED TO BE 3 MILES OR SO OUT
TO ABOUT 20 MILES FROM THE ILLINOIS AND WISCONSIN SHORES...AND
OUT TO AROUND 30 TO 35 MILES FROM THE INDIANA SHORE. IT APPEARED
MUCH THICKER ALONG THE INDIANA SHORE.

MTF

&&

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

$$

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