Area Forecast Discussion
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000
FXUS63 KLOT 290958
AFDLOT

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

.SHORT TERM...
358 AM CST

TODAY THROUGH FRIDAY NIGHT...

THE MAIN FORECAST CONCERNS THIS MORNING INVOLVE THE AREA OF LOW
PRESSURE CURRENTLY OVER WEST CENTRAL ILLINOIS...AND INCLUDE PRECIP
TIMING/TYPE...TEMPERATURE TRENDS...GUSTY WINDS...AND EVENTUALLY THE
POTENTIAL FOR SOME LAKE EFFECT SNOW SHOWERS OVER NORTHWEST INDIANA
TONIGHT INTO FRIDAY.

THE SURFACE LOW CENTER WAS JUST SOUTHWEST OF LASALLE/PERU ILLINOIS
AT 3 AM CST...WITH THE ASSOCIATED CYCLONIC WIND CIRCULATION PULLING
RELATIVELY MILD AIR NORTH ACROSS THE EASTERN 2/3 OF THE CWA.
CHALLENGING HOURLY TEMPERATURE FORECAST...AS READINGS MAY REACH 40
DEGREES IN THE SOUTHEAST EARLY THIS MORNING BEFORE THE LOW PASSES
AND SHIFTS WINDS TO THE NORTHWEST AND PULLS IN COLDER AIR. WITH THE
EXCEPTION OF A BAND OF RAIN SHOWERS LATE LAST EVENING...LITTLE
PRECIPITATION HAS BEEN OCCURRING WITH THIS SYSTEM EXCEPT NORTH/EAST
OF THE FORECAST AREA OVERNIGHT. REGIONAL RADAR MOSAIC HAS BEEN
DEPICTING PERSISTENT LIGHT RETURNS BEHIND THE LOW/COLD FRONT HOWEVER
WHERE SOME OCCASIONAL DRIZZLE OR "UNKNOWN" PRECIP HAS BEEN REPORTED.
HIGH-RES GUIDANCE TRENDS CONTINUE TO INDICATE SOME VERY LIGHT
PRECIPITATION IS POSSIBLE ACROSS THE AREA THIS MORNING...WITH
FORECAST SOUNDINGS INDICATING MAINLY DRIZZLE WITH SATURATION MAINLY
IN LOW LEVELS AS COLDER AIR MOVES IN...THOUGH A BRIEF PERIOD OF
MID-LEVEL MOISTURE WITH RAGGED DEFORMATION AREA ALONG WEST/NORTHWEST
SIDE OF THE LOW COULD BRIEFLY INTRODUCE ICE INTO LOW LEVELS IN A
SEEDER-FEEDER SITUATION WHICH COULD RESULT IN A MIX OF DRIZZLE/LIGHT
RAIN AND LIGHT SNOW THIS MORNING...THOUGH NOT EXPECTING ANY REAL
MEASURABLE PRECIPITATION OR SNOW WITH THIS. STRONG DRYING ABOVE THE
BOUNDARY LAYER FROM THE NORTHWEST BY MIDDAY SHOULD ALLOW ANY LIGHT
PRECIP TO END FROM WEST TO EAST THROUGH THE LATE MORNING AND MIDDAY
HOURS.

WINDY CONDITIONS WILL DEVELOP DURING THE MORNING AS COLD ADVECTION
STEEPENS OUR LOW LEVEL LAPSE RATES IN THE TIGHT PRESSURE GRADIENT
WEST OF THE LOW. FORECAST SOUNDINGS INDICATE GUSTS AROUND 35 MPH
WILL BE POSSIBLE FROM MID-MORNING THROUGH THE AFTERNOON BEFORE WINDS
GRADUALLY DIMINISH TONIGHT. A QUICK NOTE OF TEMPERATURES WEST OF THE
LOW THIS MORNING INDICATES READINGS DO NOT DROP OFF TOO DRAMATICALLY
AND WILL LIKELY SEE STEADY TEMPS FOR THE MOST PART UNTIL LATE TODAY.
(EXCEPT IN THE SOUTHEAST WHERE TEMPS APPROACH 40 THIS MORNING...WILL
LIKELY SETTLE BACK TO THE MID 30S THERE).

AS FOR LAKE EFFECT TONIGHT...LOW LEVEL WINDS FROM THE
NORTH-NORTHWEST (AROUND 340 DEG) SUPPORT AN INITIAL CONVERGENCE ZONE
ALONG THE SOUTHWEST MICHIGAN SHORE THIS AFTERNOON AND EARLY THIS
EVENING...THOUGH OUR LOCALLY RUN ARW MODEL DOES VEER WINDS LATER
TONIGHT AND BRINGS CONVERGENT BAND INTO PORTER COUNTY LATE TONIGHT
INTO EARLY FRIDAY. LARGER SCALE FLOW BACKS MORE WESTERLY FRIDAY
AFTERNOON WHICH SHOULD PUSH ANY LINGERING SNOW SHOWERS BACK EAST OF
PORTER COUNTY BY MID/LATE FRIDAY AFTERNOON. THERMODYNAMICS ARE NOT
SPECTACULAR...THOUGH VARIOUS MODEL FORECAST SOUNDINGS DEVELOP
LAKE-850 HPA DELTA-T AROUND 17C EARLY FRIDAY WITH INVERSION HEIGHTS
5000-6000 FT BEFORE CONDITIONS GRADUALLY WANE LATER FRIDAY MORNING.
DEPENDING UPON HOW LONG THE BAND BACKS UP INTO PORTER/EASTERN LAKE
COUNTIES WILL MODULATE THE SNOWFALL ACCUMULATION POTENTIAL...THOUGH
A FEW INCHES ARE CERTAINLY POSSIBLE IF THE BAND DOES HANG UP THERE
FOR A WHILE LATE TONIGHT/EARLY FRIDAY...BEFORE EXPANSIVE SURFACE
HIGH BUILDS ACROSS THE AREA THROUGH FRIDAY NIGHT.

RATZER

&&

.LONG TERM...
358 AM CST

SATURDAY THROUGH WEDNESDAY...

THE MAIN CONCERN IN THE EXTENDED PERIOD CONTINUES TO BE THE
POTENTIAL FOR SOME ACCUMULATING SNOW OVER THE WEEKEND...ESPECIALLY
ACROSS THE SOUTHERN PART OF THE CWA LATE SATURDAY NIGHT AND SUNDAY.
GLOBAL RUNS CONTINUE TO SHOW AGREEMENT IN FIRST SAGGING A COLD FRONT
SOUTH ACROSS THE AREA SATURDAY...THEN WITH THE AMPLIFICATION OF AN
UPPER TROUGH OVER THE REGION WITH SUBSEQUENT SURFACE LOW DEVELOPMENT
OVER THE LOWER MISSISSIPPI VALLEY. WHILE THE TRACK OF THE SURFACE
LOW HAS TRENDED SOUTH OVER THE PAST FEW RUNS (NOW FROM THE LOWER
MISS VALLEY TO THE MID-ATLANTIC OR CAROLINA COAST BY MONDAY
MORNING)...NORTHERN PERIPHERY OF DEFORMATION BAND SNOW ACROSS
CENTRAL ILLINOIS/INDIANA WOULD STILL AFFECT SOUTHERN COUNTIES OF THE
LOT CWA AT THIS POINT. IN ADDITION...INCREASING NORTH-NORTHEAST
WINDS NORTH OF THE LOW TRACK COMBINED WITH ARCTIC AIR INTRUSION
BEHIND SATURDAY COLD FRONT COULD VERY WELL LEAD TO LAKE
EFFECT/ENHANCED SNOW SHOWERS INTO THE CHICAGO METRO AREA. SO...
WHILE MANY DETAILS REMAIN TO BE WORKED OUT HAVE MAINTAINED DECENT
CHANCE POPS FOR MEASURABLE SNOW ACROSS THE FORECAST AREA FOR THE
WEEKEND. STILL LOOKS COLD BEYOND THE WEEKEND INTO NEXT WEEK WITH
DEEP TROUGH ACROSS THE EAST...THOUGH PATTERN CONTINUES TO BE
PROGRESSIVE IN LONGER RANGE GUIDANCE SOLUTIONS WHICH KEEPS IT FROM
STICKING AROUND TOO LONG.

RATZER

&&

.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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