Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 270838
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
338 AM CDT FRI MAR 27 2015

.SHORT TERM...
335 AM CDT

THROUGH SATURDAY...

A POSITIVELY TILTED LONG WAVE TROUGH FROM EASTERN CANADA THROUGH
THE MID-MISSISSIPPI VALLEY HAS STEERED HIGHLY ANOMALOUS COLD AIR
INTO THE REGION...WITH -35C OBSERVED AT 500MB AT GRB LAST EVENING.
THE LOW-LEVEL COLD AIR...AS SAMPLED WITH -13C AT 850MB...HAS
HELPED CONTINUE LAKE EFFECT INSTABILITY FOR SNOW SHOWERS EARLY
THIS MORNING. CONVERGENCE ALONG THE WESTERN SHORT OF THE LAKE HAS
FAVORED A FOCUS WITH 25-30DBZ AND REPORTS INDICATING BRIEF HEAVIER
SNOW IN THESE SHOWERS...INCLUDING IN CHICAGO AS OF 330 AM.

UPSTREAM OBSERVATIONS ALONG THE SHORE OF WISCONSIN HAVE INDICATED A
BACKING WIND TO MORE NORTHWEST...A SIGN THE CONVERGENCE SHIFTS BACK
OVER THE LAKE AS THE SYNOPTIC HIGH DROPS SOUTHWARD IN THE UPPER
MIDWEST. THIS ENDING OF THE MORE FOCUSED SNOW SHOWERS IS AS FAR
SOUTH AS MILWAUKEE AS OF 330 AM AND THIS SHOULD CONTINUE TO MOVE
SOUTHWARD...LIKELY ENDING THE TEMPORARY MODERATE/HEAVY SNOW BY
DAYBREAK IN THE CITY OF CHICAGO. THE SNOW IS EXPECTED TO CONTINUE
INTO NORTHWEST INDIANA AFTER DAYBREAK BUT SEE LESS ORGANIZATION
AND A SLOW DIMINISHING AS CONVERGENCE WANES AND DRIER UPSTREAM AIR
IN NORTHERN AND CENTRAL WI ADVECTS SOUTHWARD.

GIVEN REPORTS AND WEBCAMS OF SNOW ON EVEN PAVED SURFACES...HAVE
ISSUED AN SPS WITH POSSIBLY UP TO AN ISOLATED INCH OF
ACCUMULATION. WE DO THINK THE IMPACTS WILL BE MAINLY DURING THE
EARLY PART OF THE RUSH HOUR FOR NORTHEAST ILLINOIS. AS FOR
FAR SOUTHEAST COOK COUNTY AND NORTHWEST INDIANA...THE MAJORITY OF
THE RUSH HOUR WILL PROBABLY SEE SOME LIGHT SNOW WITH TEMPORARY
BURSTS CLOSER TO THE LAKE.

DESPITE AT LEAST TEMPORARY SUNSHINE AWAY FROM THE LAKE EFFECT
CLOUDS...TEMPERATURES TODAY WILL BE NEAR RECORD LOW MAXS. THEY ARE
FORECAST TO FALL JUST A FEW DEGREES SHORT OF THOSE RECORDS /31 IN
1965 IN RFD AND 26 IN 1904 IN CHICAGO/. TONIGHT WILL SEE THE HIGH
PRESSURE IN THE UPPER MIDWEST AS OF THIS MORNING MOVE DIRECTLY
OVERHEAD. WITH THE LOW DEW POINTS AND CLEAR SKY EXPECTED AWAY FROM
THE IMMEDIATE LAKE SHORE...TEMPERATURES SHOULD DROP INTO THE MID
TO UPPER TEENS IN MOST OUTLYING PLACES.

MTF

&&

.LONG TERM...

TO BE ISSUED SHORTLY.

&&

.AVIATION...

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

* OCCASIONAL LAKE EFFECT SNOW SHOWERS UNTIL DAYBREAK...WITH
  TEMPORARY IFR VISIBILITY POSSIBLE...MORE LIKELY AT MDW.
* WINDS NORTH TO NORTHEAST PRIMARILY IN EXCESS OF 10 KT WITH
  OCCASIONAL GUSTS THROUGH TODAY.
* MVFR CIGS AT TIMES THROUGH THIS MORNING...AND THEN POSSIBLE
  AGAIN TONIGHT.

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

IN THE POST-COLD FRONTAL AIR MASS...NORTHERLY WINDS WILL CONTINUE
ACROSS THE AREA THIS MORNING. AN UPPER LEVEL DISTURBANCE PASSING
PRIOR TO DAYBREAK WILL AT LEAST BRING FLURRIES TO MOST TAF
SITES...BUT OF MORE INTEREST WILL BE LAKE EFFECT SNOW SHOWERS NEAR
THE WESTERN SHORE OF LAKE MICHIGAN THROUGH DAYBREAK AND EVEN AFTER
DAYBREAK AT GYY. TEMPORARY IFR VISIBILITY AND MVFR CIGS UNDER 2000
FT ARE CERTAINLY POSSIBLE WITHIN THESE SNOW SHOWERS GIVEN THE
HIGHER DBZ ON RADAR...AND MDW AND GYY SEEM MORE FAVORABLE TO GET
INTO SOME OF THESE GIVEN UPSTREAM PLACEMENT. THE AIR MASS LOOKS TO
DRY OUT CONSIDERABLY THROUGH THE MORNING SO LAKE EFFECT SNOW
SHOWERS SHOULD WANE TO MORE FLURRIES AROUND GYY...BUT LAKE EFFECT
CLOUDS WILL CONTINUE INTO AFTERNOON AND POTENTIALLY TONIGHT. ITS
PROBABLE ORD AND MDW EXPERIENCE SOME SCATTERING TODAY...MAYBE EVEN
EARLY THIS MORNING...BUT AT LEAST TEMPORARY BROKEN CIGS OF THE
2500-5000 FT NATURE ARE PROBABLE.

WINDS WILL FLUCTUATE SOME FROM 340 TO 020 THROUGH DAYBREAK BUT
SHOULD WANT TO BECOME MORE CONFINED TO SLIGHTLY EAST OF NORTH BY
MID-MORNING. SPEEDS WILL BE IN EXCESS OF 10 KTS FOR THE MAJORITY
OF THE TIME. DURING THE AFTERNOON...A LAKE BREEZE MAY FURTHER VEER
THE WINDS TO NORTHEAST AT ORD AND MDW WITH SPEEDS REMAINING ABOVE
10 KT THROUGH SUNSET AND POSSIBLY AN HOUR OR TWO AFTER.

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

* HIGH IN TEMPORARY SNOW PRIOR TO DAYBREAK. LOW CHANCE BETWEEN
  12Z-15Z.
* MEDIUM IN WINDS THROUGH 15Z. CONFIDENCE IS HIGHEST IN THE WIND
  DIRECTION BEING EAST OF NORTH AFTER 15Z.
* LOW IN DURATION OF MVFR CIGS TODAY. HIGH THAT CIGS WILL REMAIN
  ABOVE 1000 FT.

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

* SATURDAY...DRY/VFR. POSSIBLE LAKE BREEZE.
* SUNDAY...RAIN LIKELY. MVFR LIKELY. STRONG/GUSTY SOUTHWEST WINDS.
* MONDAY...DRY/VFR. WEST WINDS.
* TUESDAY...DRY/VFR. POSSIBLE LAKE BREEZE.
* WEDNESDAY...CHANCE OF RAIN. MVFR POSSIBLE. SOUTH WINDS.

MTF/CMS

&&

.MARINE...
203 PM CDT

MAIN MARINE FORECAST CONCERNS FOCUS ON A COUPLE OF PERIODS OF
STRONGER WINDS...THE FIRST FROM THE NORTH LATE TONIGHT THROUGH
FRIDAY EVENING AND THE SECOND A STRONGER PERIOD OF SOUTH WINDS AND
POTENTIAL GALES SATURDAY NIGHT THROUGH SUNDAY AND THEN BECOMING
WEST AND VERY SLOWLY DIMINISHING SUNDAY NIGHT INTO MONDAY.

LOW PRESSURE WAS LIFTING NORTHEAST THROUGH THE OHIO VALLEY THIS
AFTERNOON...WHILE HIGH PRESSURE WAS PUSHING SOUTHWARD FROM CENTRAL
CANADA. IN ADDITION...A WEAK SURFACE TROUGH WAS PUSHING SOUTH
ACROSS THE LAKE...WITH COLDER AIR AND A TIGHTENING GRADIENT AHEAD
OF THE APPROACHING HIGH STARTING TO SHIFT WINDS MORE TO THE NORTH-
NORTHWEST WITH GUSTS NEAR 30 KT ON THE NORTH END OF THE LAKE. THIS
TROUGH WILL CONTINUE TO MOVE SOUTH DOWN THE LAKE THROUGH THIS
EVENING...WITH WINDS BECOMING NORTHERLY AND INCREASING TO 30 KT
TONIGHT AND SLOWLY EASING TO 25 KT OR SO THROUGH LATE FRIDAY. THIS
NORTHERLY FETCH WILL LEAD TO WAVES BUILDING INTO THE 8 FOOT
RANGE...AND WILL HAVE A SMALL CRAFT ADVISORY IN EFFECT AFTER 06Z
FRI/1 AM CDT FOR THE IL/IN NEARSHORE WATERS. WINDS WILL CONTINUE
TO DIMINISH MORE QUICKLY LATER FRIDAY NIGHT AND SATURDAY...THOUGH
WILL HAVE THE SCA RUN THROUGH 21Z SAT/4 PM CDT AS WAVES SUBSIDE
MORE SLOWLY.

THE SURFACE HIGH WILL CONTINUE TO SPREAD SOUTHEAST ACROSS THE
WESTERN LAKES AND INTO THE OHIO VALLEY BY SATURDAY EVENING...
ALLOWING WINDS TO SHIFT BACK TO THE SOUTH. A DEEPENING AREA OF LOW
PRESSURE IS EXPECTED TO DEVELOP AND MOVE EAST THROUGH MANITOBA AND
INTO ONTARIO SATURDAY NIGHT INTO SUNDAY...TIGHTENING THE GRADIENT
AND INCREASING SOUTH WINDS TO 35-40 KT GALES BY SUNDAY MORNING.
WINDS LOOK TO EAST A BIT LATE SUNDAY AND SHIFT WEST AS THE
TRAILING COLD FRONT MOVES ACROSS THE LAKE. WESTERLY WINDS THEN
CONTINUE TO DIMINISH MONDAY.

SOUTH GALES APPEAR TO BE FAIRLY LIKELY FOR DURING THE DAY
SUNDAY... THOUGH ON DAY 4 STILL A LITTLE BEYOND THE TIME PERIOD
FOR A GALE WATCH. IF FORECAST GUIDANCE CONTINUES TO DEPICT A
SIMILAR SCENARIO WILL LIKELY NEED A GALE WATCH IN THE NEXT DAY OR
SO.

RATZER

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...NEARSHORE WATERS
     UNTIL 4 PM SATURDAY.

&&

$$

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