Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 292021
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
221 PM 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 20Z...

* NORTHWEST WINDS GUSTING TO AROUND 25 KT OR SO THROUGH AT LEAST
  MIDNIGHT...TAPERING SOME OVERNIGHT.

* MVFR CIGS PERSISTING INTO THE EVENING...LIFTING TO VFR EITHER
  OVERNIGHT OR INTO FRIDAY MORNING.

* FLURRIES THIS EVENING REDUCING VSBY TO MVFR.

KMD

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

MAIN AVIATION CONCERN IS WITH GUSTY NW WINDS THIS AFTERNOON AND
TONIGHT. WE HAVE HAD A HARD TIME REALIZING DEEPER MIXING WITH THE
SOLID MVFR CLOUD DECK IN PLACE. WE HAVE SEEN OCCASIONAL 30 KT
GUSTS ON 1-MINUTE ASOS OBSERVATIONS...BUT IN GENERAL SUSTAINED
WINDS HAVE BEEN IN THE MID TO UPPER TEENS WITH GUSTS INTO THE MID
20S. EXPECT THESE WINDS TO REMAIN IN PLACE THIS AFTERNOON AND
EVENING AS A SECOND WAVE OF COLD AIR ADVECTION ENSUES TONIGHT.

NEXT CONCERN IS AROUND MVFR CIGS AND POSSIBLE FLURRIES TONIGHT.
CIGS SHOULD GENERALLY REMAIN ABOVE 1500 FT WITH THE GUSTY WINDS
AND MIXED VERY LOW LEVELS. A WEAKER TROUGH AXIS WILL MOVE THROUGH
THE TERMINALS THIS AFTERNOON...FOLLOWED BY A SHARPER VORT MAX THIS
EVENING. UPSTREAM OBSEVATIONS IN NORTHERN MINNESOTA AHEAD OF THIS
WAVE HAVE BEEN REPORTING SOME FLURRIES AND POSSIBLY EVEN SOME
FREEZING DRIZZLE. AS THIS WAVE MOVES THROUGH THIS EVENING...LOWER
LEVEL TEMPERATURES COOL TO THAT POINT TO SUPPORT SOME FLURRIES
AND POSSIBLE VISIBILITY REDUCTION IF THIS ACTIVITY. CONFIDENCE ON
THIS IS LOW ON SPECIFICALLY IMPACTING ANY PARTICULAR TERMINAL...SO
IT HAS NOT BEEN MENTIONED IN THE TAFS AT THE MOMENT. BUT
CONFIDENCE IS PRETTY HIGH THAT THE MVFR CIGS LINGER THROUGH MUCH
OF THE NIGHT. HIGH PRESSURE BUILDS IN TOMORROW WHICH RESULTS IN
LIGHTER WINDS AND LIKELY SOME SCATTERING OUT OF THE LOW CLOUD
DECK...BUT CONFIDENCE TIMING THE SCATTERING IS NOT VERY HIGH.

KMD

//ORD AND MDW CONFIDENCE...UPDATED 20Z...

* HIGH IN WINDS...MEDIUM ON GUSTS.

* HIGH IN MVFR CIGS ABOVE 1800 FT THROUGH AT LEAST MIDNIGHT. LOW-
  MEDIUM ON SPECIFIC SCATTERING TIME...WHICH WOULD COULD BE
  BETWEEN 6Z AND 9Z BUT MOST LIKELY BE SOME TIME AFTER 12Z.

* LOW ON FLURRIES AND ANY ASSOCIATED VSBY REDUCTION THIS EVENING.

KMD

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

FRIDAY NIGHT...VFR. SOUTHWEST WINDS.
SATURDAY...CHANCE OF SNOW AND IFR DURING THE NIGHT. WEST WINDS.
SUNDAY...CHANCE OF SNOW AND IFR. AT LEAST MVFR CIGS LIKELY.
         NORTHEAST WINDS.
MONDAY NIGHT-WEDNESDAY...CHANCES OF MAINLY LIGHT SNOW. WEST WINDS.

MTF

&&

.MARINE...
153 PM CST

ACTIVE PATTERN LOOKS TO PERSIST THROUGH THE WEEKEND WITH MUCH
COLDER AIR ARRIVING EARLY NEXT WEEK. WITH THE SLOW AND STEADY
DEPARTURE OF TWO LOW PRESSURES OVER THE WESTERN GREAT LAKES THIS
MORNING...THE TWO LOWS ARE NOW CONSOLIDATING OVER NORTHEAST LAKE
HURON. ALONG THE BACKEDGE OF THESE LOW PRESSURES...THE PRESSURE
GRADIENT HAS TIGHTEN AND WAS PRODUCING GALE FORCE WINDS TO 35 KT
FROM THE NORTHWEST. MOST OF THE AFTERNOON WINDS HAVE BEEN HELD TO
AROUND LOW END GALES OR JUST UNDER GALE FORCE...HOWEVER AS THE
AFTERNOON PROGRESSED MIXING HAS BEEN ABLE TO BRING TO THE SURFACE
MUCH STRONGER WINDS. AS A RESULT WILL MAINTAIN THE CURRENT GALE
WARNING THROUGH THIS EVENING. WITH THE STRONGEST GRADIENT OVER THE
NORTHERN HALF OF THE LAKE...PRODUCING GALES TO 40KT AT TIMES...THIS WILL
LIKELY PRODUCE HEAVY FREEZING SPRAY. FURTHER SOUTH THE GALES WILL
LIKELY REMAIN AROUND 35KT AND THE FREEZING SPRAY SHOULD ONLY BE
ALONG THE MODERATE LEVEL AND NOT REQUIRE A HEADLINE.

WAVES WILL CONTINUE TO BUILD EARLY THIS EVENING...WITH THE HIGHEST
WAVES BETWEEN 12-15 FT TONIGHT. HIGH PRESSURE THEN QUICKLY ARRIVES
AND BRINGS LIGHTER WINDS TO THE LAKE BY MIDDAY FRIDAY. THEN HIGH
PRESSURE QUICKLY SLIDES EAST/SOUTHEAST...WITH WINDS TURNING
SOUTHERLY AHEAD OF YET ANOTHER LOW PRESSURE SYSTEM OVER THE
WEEKEND. A FRONTAL BOUNDARY WILL SLIDE SOUTH FROM CANADA
SATURDAY...AND COULD DEVELOP A TIGHT GRADIENT OVER THE LAKE FOR A
PERIOD SATURDAY NIGHT INTO SUNDAY. AT THIS TIME IT LOOKS LIKE
NORTH/NORTHEAST WINDS WILL TOP OUT AROUND 30KT...BUT COULD SEE A
FEW GALE FORCE GUSTS FOR A FEW HOURS EARLY SUNDAY. THEN HIGH
PRESSURE PUSHES OVER THE WESTERN GREAT LAKES SUNDAY NIGHT INTO
MONDAY. UNFORTUNATELY ANOTHER ACTIVE PATTERN LOOKS FAVORABLE FOR
EARLY NEXT WEEK.

BEACHLER

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...GALE WARNING...LMZ080-LMZ261-LMZ362-LMZ364-LMZ366-LMZ563-LMZ565-
     LMZ567-LMZ669-LMZ671-LMZ673-LMZ675-LMZ740-LMZ741-LMZ742-
     LMZ743-LMZ744-LMZ745-LMZ777-LMZ779-LMZ868-LMZ870-LMZ872-
     LMZ874-LMZ876-LMZ878 UNTIL 3 AM FRIDAY.

     HEAVY FREEZING SPRAY WARNING...LMZ261-LMZ362-LMZ364-LMZ366-
     LMZ563-LMZ565-LMZ567-LMZ868 UNTIL 3 AM FRIDAY.

&&

$$

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