Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 290906
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
306 AM CST THU JAN 29 2015

.UPDATE...

901 PM...EVENING UPDATE...WEAK UPPER WAVE MOVING ACROSS THE AREA
LATE THIS EVENING WITH LIGHT RETURNS SHOWING UP ON RADAR. PIA
REPORTED 10 MINUTES OF LIGHT RAIN AND EXPECT THIS ACTIVITY WILL
REMAIN SUFFICIENT TO PRODUCE SOME SPRINKLES OR VERY LIGHT RAIN AND
WILL HAVE SLIGHT CHANCE POPS FOR THE REST OF THIS EVENING. BACK
EDGE OF THE CLOUD COVER NOW PUSHING INTO NORTHWEST IL AND EXPECT A
FEW HOURS OF MOSTLY CLEAR GOING INTO THE EARLY MORNING HOURS.
TEMPS HOLDING STEADY NEAR OR JUST ABOVE FREEZING WITH A FEW SITES
WARMING A DEGREE OR SO IN THE PAST FEW HOURS. CAN/T COMPLETELY
RULE OUT A CHANCE OF LIGHT FREEZING RAIN BUT GIVEN THE EXPECTED
LIGHT/SPOTTY NATURE OF THE PRECIP AND TEMPS AT OR ABOVE
FREEZING...NOT EXPECTING ANY SIGNIFICANT ICING CONCERNS.

SURFACE LOW OVER SOUTH CENTRAL IA WILL MOVE EAST/NORTHEAST TO THE
FAR SOUTHERN END OF LAKE MICHIGAN THURSDAY MORNING. AS IT DOES...
IT WILL TUG LOWER CLOUDS ACROSS SOUTHERN MN AND SOUTHERN WI SOUTH
ACROSS THE AREA...THUS CLEARING SHOULD BE RATHER SHORT LIVED. AS
THESE LOWER CLOUDS ARRIVE...THERE WILL LIKELY BE SOME PATCHY
DRIZZLE OR POTENTIALLY SOME LIGHT RAIN...BUT MODELS CONTINUE TO
DOWNPLAY PRECIP/QPF OVERNIGHT. ASSUMING THIS DRIZZLE OR VERY LIGHT
RAIN DOES MATERIALIZE...WILL NEED TO WATCH TEMP TRENDS ONCE CLOUDS
CLEAR TO SEE IF THEY DROP BACK TO NEAR FREEZING...THUS POTENTIAL
ICING CONCERNS...WHICH WOULD MAINLY BE ACROSS THE NORTHERN HALF OR
THIRD OF THE CWA. AT THIS POINT...WILL BE MAINTAINING AIR TEMPS AT
OR ABOVE FREEZING OVERNIGHT. CMS

&&

.SHORT TERM...
340 PM CST

THROUGH FRIDAY...

MAIN FORECAST CONCERNS ARE WITH PRECIP POTENTIAL AND TEMP TRENDS
TONIGHT...LINGERING LAKE EFFECT SNOW SHOWERS THURSDAY INTO EARLY
FRIDAY...AND POSSIBLE ACCUMULATING SNOW THIS WEEKEND.

RELATIVELY QUIET CONDITIONS THIS AFTERNOON...WITH INCREASING
MID/HIGH LEVEL CLOUD COVER OUT AHEAD OF APPROACHING SYSTEM OVER
THE CENTRAL PLAINS. SURFACE LOW OVER EASTERN NEBRASKA WILL
CONTINUE TO SHIFT EAST TOWARDS THE AREA THIS EVENING AS MID/UPPER
LEVEL TROUGH SWING EAST THROUGH THE CENTRAL CONUS. AS THIS
OCCURS...STRONG ISENTROPIC ASCENT OUT AHEAD OF IT WILL GRADUALLY
SHIFT EAST ACROSS THE CWA LATER THIS EVENING. INITIAL DRY
CONDITIONS IN PLACE WILL NEED TO BE OVERCOME THIS EVENING BUT
INCREASING ASCENT THIS EVENING SHOULD ALLOW FOR THE COLUMN TO
SATURATE...WITH DEVELOPING PRECIP TO THEN SPREAD OVER THE REGION.
GUIDANCE DOES VARY WITH REGARDS TO WHEN THE DRIER AIR WILL BE
OVERCOME...WITH SOME INDICATING THIS WILL OCCUR AS EARLY AS THIS
EVENING AND WITH OTHERS POINTING AT THE DEVELOPMENT AFTER THE
MIDNIGHT TIME FRAME. HAVE TRENDED TOWARDS THE PREVIOUS FORECAST
WITH THE POSSIBILITY OF PRECIP DEVELOPING BY MID TO LATE
EVENING...STRAYING AWAY FROM THE DRIER GUIDANCE. PERSISTENT
FORCING THROUGH THIS EVENING SHOULD ALLOW FOR AT LEAST LIGHT
DEVELOPMENT ACROSS A GOOD PORTION OF THE CWA. RAIN DOES APPEAR TO
BE THE PREDOMINANT PRECIP TYPE OVERNIGHT TONIGHT AS STRONG WARM
ADVECTION OUT AHEAD OF THIS SYSTEM OCCURS...AND ESPECIALLY AS
SOUTHERLY FLOW CONTINUES TO RAMP UP AHEAD OF THE SURFACE LOW.
HOWEVER...DO ANTICIPATE FOR SOME SLIGHT COOLING OF THE CURRENT MID
30 TEMPS LATE THIS AFTERNOON INTO THE EARLY EVENING BEFORE A
STEADY TEMP RISE IS OBSERVED FOR ALL AREAS. IF THE PRECIP WERE TO
OCCUR SLIGHTLY EARLIER THAN CURRENT THINKING...THEN A BETTER
CHANCE FOR LIGHT FREEZING RAIN COULD OCCUR. THIS TIME FRAME WOULD
BE IN THE 1-4Z PERIOD WHERE LOW TEMPS IN THE LOW 30S WILL BE
OBSERVED...BEFORE A RISE TO THE MID TO UPPER 30S WILL OCCUR. THIS
TEMP RISE SHOULD LIMIT ANY FREEZING RAIN POTENTIAL TO A SMALL
SEVERAL HOUR WINDOW...WITH ONCE AGAIN RAIN THEN EXPECTED EVEN INTO
EARLY THURSDAY MORNING.

EXPECT BEST DEVELOPMENT TO OCCUR MORE TOWARDS EARLY THURSDAY
MORNING INTO MID MORNING AS SURFACE LOW AND SURFACE BOUNDARIES
PUSH THROUGH...WITH THE RAIN BECOMING ALL SNOW THROUGH MID TO LATE
MORNING. PRECIP SHIELD SHOULD BE EXITING DURING THIS TRANSITION
WITH ANY SNOW ACCUMULATION EXPECTED TO NO MORE THAN A COUPLE OF
TENTHS OF AN INCH. LIGHT SNOW COULD LINGER INTO NORTHWEST INDIANA
AS LAKE EFFECT POTENTIAL INCREASES...BUT BETTER FOCUS WILL
INITIALLY BE FURTHER TO THE EAST. AS COLDER AIR SPILLS SOUTH AND
BOUNDARY LAYER FLOW SHIFTS...FOCUS FOR BETTER LAKE EFFECT SNOW
DEVELOPMENT WILL MOVE INTO THE CWA...WITH THE PORTER COUNTY AREA
WITHIN THE CWA LIKELY OBSERVING THIS BEST DEVELOPMENT. BUILDING
HIGH PRESSURE WITH BACKING LOW LEVEL FLOW WILL GRADUALLY HELP THIS
SNOW TO DIMINISH WHILE SHIFTING EAST THROUGHOUT THE DAY ON FRIDAY.

RODRIGUEZ

&&

.LONG TERM...
340 PM CST

FRIDAY NIGHT THROUGH WEDNESDAY...

MAIN CONCERN TO START THE PERIOD WILL BE FOR THE POTENTIAL FOR
ACCUMULATING SNOW SATURDAY NIGHT INTO SUNDAY. THERE STILL APPEARS
TO BE THE POTENTIAL FOR SNOW DURING THIS PERIOD...BUT WITH
CONFIDENCE OF THIS POTENTIAL FOR THE CWA LOWERING TODAY. PREVIOUS
CONSISTENCY AMONG VARYING GUIDANCE IS NOT PRESENT TODAY...WITH
GUIDANCE RANGING FROM LIGHT ACCUMULATIONS TO VERY LITTLE TO NONE.
AM NOT TOTALLY CONVINCED IN THE SHIFT WITH THE STORM TRACK AND
POTENTIAL PRECIP ADVERTISED TODAY AND DID NOT CHANGE POPS TOO MUCH
DURING THIS PERIOD...AS GUIDANCE COULD SHIFT BACK ONCE AGAIN.
WOULD LIKE TO SEE CONSISTENCY OF THIS POTENTIAL BEFORE POPS ARE
LOWERED. CURRENTLY HAVE THE HIGHEST POPS FOR THE SOUTHERN HALF OF
THE CWA...WITH A MORE FAVORED SOUTHERLY TRACK OF THIS SYSTEM.

RODRIGUEZ

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 09Z...

* SOUTH-SOUTHEAST WINDS TURNING NORTHWEST BY 13Z WITH A BRIEF
  PERIOD OF LIGHT AND VARIABLE WINDS POSSIBLE FROM 10Z TO 13Z.
  NORTHWEST GUSTS OF 30-35 KT FROM MID MORNING THROUGH THE
  AFTERNOON.

* LOW MVFR CIGS AND POSSIBLY MVFR VISIBILITY ARRIVING NEAR OR
  SHORTLY AFTER DAYBREAK. IFR CIGS POSSIBLE IFR FOR A FEW HOURS
  THIS MORNING.

* DRIZZLE POSSIBLE 12Z-14Z...THEN A BRIEF PERIOD OF LIGHT
  RAIN/SNOW SHOWERS LIKELY MID-MORNING.


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

LOW PRESSURE ACROSS SOUTHEAST IOWA AT 530Z WILL MOVE CLOSE TO MDW
AND GYY BY 12Z AND THEN ONTO LOWER MICHIGAN THROUGH THE REST OF
THURSDAY MORNING. BECAUSE OF THIS PATH...A TEMPORARY PERIOD OF
LIGHTER VARIABLE WINDS IS BECOMING MORE PROBABLE AT CHICAGO AREA
TAF SITES BETWEEN 10Z-13Z OR SO. BEHIND THE LOW PASSAGE ON
THURSDAY MORNING...THE WINDS WILL TURN TO THE NORTH-NORTHWEST AND
THEN NORTHWEST WITH SPEEDS QUICKLY INCREASING. OCCASIONAL GUSTS OF
30 TO 35 KT LOOK PROBABLE FOR DURING THE DAY THURSDAY.

SPOTTY LIGHT RAIN SHOWERS OR DRIZZLE WILL BE SEEN UNTIL THE STRONG
COLD ADVECTION ARRIVES. CIGS WILL ALSO DROP LOWER AS THE SYSTEM
CENTER PASSES. BASED ON UPSTREAM OBSERVATIONS...1000-1500 FT CIGS
ARE HIGHLY LIKELY WITH POTENTIAL FOR A PERIOD OF 600-900 FT CIGS
DURING THE MORNING. IN ADDITION...AS THE SYSTEM BEGINS TO
INTENSITY OVER LOW MICHIGAN...A BACKSIDE DEVELOPING AREA OF LIGHT
RAIN AND/OR SNOW IS EXPECTED TO MOVE CLOSE TO OR OVER THE TAF
SITES. THIS SHOULD LIKELY LAST NO LONGER THAN 90 MINUTES AT ANY
ONE SITE AND NO ACCUMULATION IS EXPECTED. MVFR CIGS ARE LIKELY TO
LAST INTO THURSDAY EVENING.


//ORD AND MDW CONFIDENCE...UPDATED 09Z...

* WHILE LOW ON SPECIFIC WIND DIRECTION 10Z-13Z...HIGH CONFIDENCE
  THAT SPEEDS WILL BE LESS THAN 10 KT IN THAT PERIOD. HIGH
  CONFIDENCE IN WINDS AFTER 13Z.

* HIGH IN MVFR CIGS AND TIMING. MEDIUM IN CIG BASES AND THAT A
  BRIEF PERIOD OF IFR WILL OCCUR.

* MEDIUM IN PERIOD OF MVFR VISIBILITY THIS MORNING.

* MEDIUM IN CHANCE FOR A BRIEF PERIOD OF MIXED PRECIP THIS
  MORNING. HIGH IN NO SNOW ACCUMULATION OCCURRING.


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

FRIDAY...VFR. NORTHWEST WINDS.
SATURDAY...CHANCE OF SNOW AND IFR DURING THE NIGHT. SOUTHWEST
           WINDS.
SUNDAY...SNOW AND IFR LIKELY. NORTHEAST WINDS.
MONDAY...CHANCE OF SNOW WITH MVFR LIKELY. IFR POSSIBLE. NORTHWEST
         WINDS.
TUESDAY...SLIGHT CHANCE OF SNOW DURING THE NIGHT. SOUTHWEST WINDS.

MTF/MDB

&&

.MARINE...
205 AM CST

A PROGRESSIVE ACTIVE PATTERN WILL CONTINUE ACROSS THE GREAT LAKES
REGION INTO THE BEGINNING OF FEBRUARY. MOVING EASTWARD EARLY THIS
MORNING IS AN ELONGATED TROUGH WITH TWO LOW PRESSURE CENTERS
COMPOSING IT...ONE PASSING JUST SOUTH OF LAKE MICHIGAN AND ANOTHER
OVER EASTERN LAKE SUPERIOR. TOGETHER THESE WILL CONSOLIDATE LATER
TODAY AND DRIVE MORE FOCUSED AND STRENGTHENING COLD ADVECTION
OVER LAKE MICHIGAN. THIS WITHIN THE STRONG AND ADVECTING PRESSURE
GRADIENT WILL SUPPORT NORTHWEST TO NORTH GALES...EVEN 45 KT GUST
POTENTIAL ACROSS PARTS OF THE NORTH. THE ILLINOIS AND ESPECIALLY
THE INDIANA NEAR SHORES SHOULD AT LEAST EXPERIENCE PERIODIC GALE
GUSTS INTO THIS EVENING. COLDER TEMPERATURES WILL BE WORKING
SOUTHWARD ACROSS THE LAKE LATER TODAY AND TONIGHT. SUPPORT FOR
HEAVY FREEZING SPRAY REMAINS OVER MAINLY THE NORTH AND CONTINUE
THE WARNING THERE. WAVES ARE LIKELY TO BUILD TO 12-15 FT TONIGHT
AT DOWNSTREAM FETCH LOCATIONS ON THE EASTERN HALF OF THE LAKE.
WINDS WILL GRADUALLY DIMINISH OVERNIGHT INTO FRIDAY MORNING AND
MORE QUICKLY AFTERWARD AS HIGH PRESSURE MOVES OVER THE LAKE.

BEYOND...THERE LOOKS LIKE OPPORTUNITIES FOR AT LEAST 30 KT WINDS
NEXT WEEK. ONE OF THESE IS BEHIND A COLD FRONT SATURDAY AFTERNOON
AND MORE SO SATURDAY NIGHT. MORE 30 KT WIND POTENTIAL WOULD COME
POSSIBLY TUESDAY AS RETURN SOUTHWEST FLOW SPREADS BACK OVER THE
LAKE.

MTF

&&

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

     GALE WARNING...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-LMZ745...10 AM
     THURSDAY TO 3 AM FRIDAY.

     GALE WARNING...LMZ080-LMZ261-LMZ362-LMZ364-LMZ366-LMZ563-LMZ565-
     LMZ567-LMZ669-LMZ671-LMZ673-LMZ675-LMZ777-LMZ779-LMZ868-
     LMZ870-LMZ872-LMZ874-LMZ876-LMZ878...9 AM THURSDAY TO 3 AM
     FRIDAY.

     HEAVY FREEZING SPRAY WARNING...LMZ261-LMZ362-LMZ364-LMZ366-
     LMZ563-LMZ565-LMZ567-LMZ868...2 PM THURSDAY TO 3 AM FRIDAY.

&&

$$

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