Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 271730
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1230 PM CDT FRI MAR 27 2015

.UPDATE...
715 AM CDT

SNOW SHOWERS ARE CONTINUING TO SHIFT SOUTH WITH THE UPPER WAVE AND
AS CONVERGENCE ALONG THE WESTERN LAKE SHORE WEAKENS. DISORGANIZED
LAKE EFFECT SNOW SHOWERS ARE EXPECTED INTO NORTHWEST INDIANA AND
PARTS OF SOUTHEAST COOK COUNTY THROUGH THE MORNING. WHILE THE SNOW
INTENSITY IS WEAKENING...CLOUD LAYER TEMPERATURES FAVORING LARGE
SNOWFLAKE GROWTH WILL CONTINUE TO AID IN SOME MINOR ACCUMULATION.

SPS AND GRAPHICAL NOW CONTINUE TO ADDRESS IMPACTS TO MORNING
COMMUTE.

MTF

&&

.SHORT TERM...
335 AM CDT

THROUGH TONIGHT...

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...

SATURDAY THROUGH SUNDAY...

MAIN CONCERNS ARE WITH UNSEASONABLY CHILLY TEMPERATURES ON
SATURDAY...FOLLOWED BY LIGHT RAIN AND WINDY CONDITIONS ON SUNDAY.
A STRONG UPPER DISTURBANCE PUSHING INTO THE PAC NW WILL SHUNT THE
IMPRESSIVE WESTERN MID/UPPER RIDGE EAST...BRINGING HEIGHT
RISES...THOUGH COLD AIR MASS WILL BE SLOWER TO DEPART AND HIGH
PRESSURE OVER THE AREA WILL LIMIT MIXING. 850 MB TEMPS REMAINING
BELOW -10 CELSIUS UNTIL THE MID-LATE AFTERNOON SHOULD RESULT IN
HIGHS IN THE MID-UPPER 30S...WITH A FEW SPOTS REACHING 40. ONSHORE
FLOW SHOULD NOT RESULT IN MUCH OF A DIFFERENCE BETWEEN SHORE AND
INLAND TEMPS...AS LATEST LAKE SFC TEMP OBS ARE GENERALLY IN MID-
UPPER 30S OVER THE OPEN WATERS. THE SFC HIGH WILL BE OVERHEAD
SATURDAY NIGHT...ALLOWING TEMPERATURES TO DIP INTO THE 20S
OVERNIGHT. THE HIGH WILL QUICKLY SLIDE SOUTHEAST BY SUNDAY
MORNING...SO SOUTHERLY SURFACE FLOW AND WARM ADVECTION DUE TO
RAPID HEIGHT RISES SHOULD RESULT IN TEMPS RISING TOWARD DAYBREAK.

GUIDANCE IS IN GOOD OVERALL AGREEMENT ON A STRONG SURFACE LOW
MOVING THROUGH NORTHERN ONTARIO ON SUNDAY...WITH TRAILING COLD
FRONT APPROACHING AREA FROM THE WEST. IN ADDITION...A FAIRLY STOUT
BUT SHEARED MID LEVEL VORT MAX WILL DROP IN FROM THE NORTHWEST.
STRONG ISENTROPIC ASCENT/WAA AND AFOREMENTIONED SHORTWAVE WILL
RESULT IN PRECIP BLOSSOMING AHEAD OF COLD FRONT...LIKELY IN THE
FORM OF LIGHT RAIN BY THE TIME IT STARTS PER FORECAST SOUNDINGS.
THERE ARE SOME TIMING DIFFERENCES ON PROGRESSION OF COLD FRONT
THROUGH THE AREA...WITH ECMWF ON SLOWER SLIDE...BUT MOST PRECIP
SHOULD BE OVER BY THE EVENING. HAVE BUMPED UP TO HIGH END LIKELY
POPS...WITH LINGERING LOW CHANCES IN THE EVENING. OTHER CONCERN
ON SUNDAY IS WITH WINDY CONDITIONS DUE TO STRONG ISALLOBARIC
PRESSURE FALLS WITH DEPARTURE OF SFC HIGH AND APPROACH OF COLD
FRONTAL TROUGH. EVEN WITH CLOUD COVER...SOUTH-SOUTHWEST WINDS
COULD GUST TO 35 TO EVEN 40 MPH AT TIMES UNTIL FROPA. EXPECTING
HIGHS TO REACH THE MID TO UPPER 40S...BUT STRONG WINDS AND LIGHT
RAIN WILL MAKE FOR A RAW DAY.

SUNDAY NIGHT THROUGH THURSDAY...

MUCH MILDER CONDITIONS ARE IN STORE AS WE TRANSITION INTO MORE OF
A ZONAL FLOW PATTERN WITH THE PRIMARY STORM TRACK ALONG THE
NORTHERN TIER AND SOUTHERN CANADA. HAVE MAINLY LOW/SLIGHT CHC
POPS WITH A POSSIBLE SYSTEM NEARBY ON MONDAY NIGHT. MOST
GUIDANCE/ENSEMBLES ARE NOT TOO EXCITED ABOUT THIS SYSTEM WITH
EXCEPTION OF 00Z CANADIAN...WHICH SEEMS TO BE AN OUTLIER AT THIS
TIME. NEXT SYSTEM/FRONT LOOKS TO BE LATER WEDNESDAY INTO WEDNESDAY
NIGHT. IF DEWPOINTS CAN SURGE INTO 50S AS SHOWN ON OPERATIONAL
GFS...MAY NEED TO ADD THUNDER TO FORECAST...BUT FOR NOW HAVE JUST
RAIN SHOWER CHANCES IN THIS TIME PERIOD. TEMPERATURES WILL WARM
FROM THE 50S ON MONDAY TO PRIMARILY 60S ON TUESDAY THROUGH
THURSDAY...THOUGH ONSHORE FLOW IS LIKELY LATER TUESDAY AND THEN
ONTO IL SHORE ON WEDNESDAY...KEEPING SHORE TEMPS COOLER.

RC

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 18Z...

* NORTH-NORTHEAST WINDS GUSTING NEAR 20 KT GRADUALLY DIMINISHING
  LATE THIS AFTERNOON/EARLY EVENING.

RATZER

//DISCUSSION...UPDATED 18Z...

LAKE EFFECT SNOW SHOWERS HAVE MOVED EAST OF THE TERMINALS THIS
MIDDAY...THOUGH BROKEN MAINLY VFR CIGS STILL POSSIBLE AT ORD/MDW
AND GYY THROUGH THIS AFTERNOON. WINDS FROM THE NORTH-NORTHEAST
WILL BE GUSTY TO AROUND 20 KT EXCEPT AT GARY...WHERE FETCH RIGHT
OFF OF THE LAKE WILL ALLOW GUSTS CLOSER TO 25-30 KT EARLY.

HIGH PRESSURE BUILDING IN FROM THE NORTHWEST WILL ALLOW WINDS TO
DECREASE AND BECOME NORTHEAST TONIGHT...WITH A FEW/SCT VFR CLOUDS
STILL MOVING INLAND OFF THE LAKE. LIGHT MAINLY NORTHEAST WINDS
WILL LIKELY BECOME MORE EASTERLY AT ORD/MDW SATURDAY AFTERNOON
WITH LAKE BREEZE INFLUENCE IN LIGHT GRADIENT.

RATZER


//ORD AND MDW CONFIDENCE...UPDATED 18Z...

* MEDIUM-HIGH IN WIND TRENDS.
* HIGH ALL OTHER ELEMENTS.

RATZER

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

SATURDAY NIGHT...VFR. SOUTHEAST WINDS.
SUNDAY...MVFR WITH RAIN. IFR POSSIBLE. SOUTHWEST WINDS.
MONDAY...SLIGHT CHANCE OF RAIN. WEST WINDS.
TUESDAY...VFR. NORTH WINDS.
WEDNESDAY...CHANCE OF RAIN SHOWERS. SOUTHEAST WINDS.
THURSDAY...SLIGHT CHANCE OF RAIN. WEST WINDS.

MTF

&&

.MARINE...
412 AM CDT

PRIMARY MARINE CONCERN IS WITH HIGHER END SOUTH-SOUTHWEST GALES ON
SUNDAY. A SMALL CRAFT ADVISORY REMAINS IN EFFECT FOR THE
NEARSHORE THROUGH SATURDAY MORNING...FOR CRITERIA WINDS AND WAVES
TODAY...AND FOR LINGERING WAVES INTO SATURDAY.

ON SUNDAY...A STRONG LOW PRESSURE AREA WILL MOVE NORTH OF LAKE
SUPERIOR AS EXPANSIVE HIGH PRESSURE SLIDES TO THE SOUTHEAST U.S.
THIS WILL BE A GOOD SETUP FOR SOUTHERLY GALE FORCE WINDS AHEAD OF
THE COLD FRONT TRAILING FROM THE LOW. WINDS WILL LIKELY BE
STRONGEST AND UP TO 45 KT ON THE NORTHERN HALF OF THE LAKE. THERE
IS A FEW HOUR WINDOW ON SUNDAY WHERE GUSTS COULD EVEN GET CLOSE TO
STORM FORCE ON THE NORTH HALF. A GALE WATCH AND THEN WARNING WILL
LIKELY BE NEEDED FOR THE ENTIRE OPEN LAKE DURING THIS
PERIOD...WITH NEAR LAKE FORECAST SOUNDINGS OVER THE LAND ALSO
POTENTIALLY SUPPORTIVE OF LOWER END GALES IN THE NEARSHORE WATERS.
WESTERLY WINDS BEHIND THE FRONT ON SUNDAY EVENING/NIGHT ARE
EXPECTED TO TOP OUT AROUND 30 KT AND THEN DIMINISH...AS THE
PRESSURE GRADIENT WILL WEAKEN FAIRLY QUICKLY.

A FAST MOVING LOW PRESSURE SYSTEM COULD MOVE OVER THE LAKE MONDAY
NIGHT INTO TUESDAY AND BRING A BRIEF PERIOD OF STRONGER WINDS FOR
A PORTION OF THE LAKE DEPENDING ON THE TRACK OF THE LOW.

RC

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...NEARSHORE WATERS
     UNTIL 10 AM SATURDAY.

&&

$$

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