Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 291930
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
230 PM CDT MON SEP 29 2014

.DISCUSSION...
230 PM CDT

OUR STRETCH OF ONE WEEK OF ABOVE NORMAL TEMPERATURES IS ABOUT TO GET
A TOUCH OF EARLY AUTUMN REALITY...AND A MUCH RUDER PUNCH LATE IN THE
WEEK. FORECAST CONCERNS WERE ON THE TEMPERATURE DROP LATE THIS
AFTERNOON INTO THE EVE AND SUBSEQUENT LIGHT RAIN/DRIZZLE
CHANCES...THEN CLOUD COVER TUESDAY WHICH COULD GREATLY HINDER
WARMING...AND FINALLY THE STORM SYSTEM WEDNESDAY NIGHT THROUGH
FRIDAY BRINGING WIDESPREAD SHOWER AND STORM CHANCES.

LATE THIS AFTERNOON INTO TONIGHT...
OBSERVATIONS...SATELLITE...AND MKE RADAR ALL HAVE A GOOD HANDLE ON
THE SHARP COLD FRONT DROPPING SOUTHWARD ACROSS THE GREAT LAKES
REGION THIS AFTERNOON. OBSERVATIONS IN EAST CENTRAL WI HAVE DROPPED
BY AS MUCH AS 23 DEGREES IN AN HOUR EARLY THIS AFTERNOON WITH THE
SYNOPTIC AND LAKE-ENHANCED COOL SURGE. WHILE PRESSURE RISES WILL
EASE BEHIND THE FRONT DUE TO THE PARENT LOW WELL OFF TO THE
NORTHEAST...THIS FRONT SHOULD STILL COME BARRELING INTO NORTHEAST IL
BY 530-630 PM WITH SOME GUSTS TO NEAR 30 MPH AND A 20-25 DEGREE
TEMPERATURE DROP IN TWO-THREE HOURS. THIS FRONT WILL THEN CONTINUE
TO MOVE SOUTHWARD THROUGH THE FORECAST AREA EASING SOME IN
BAROCLINICITY.

ANAFRONTAL FORCING IS SUPPORTING SCATTERED SHOWERS ACROSS CENTRAL
WI. THIS MID-LEVEL FRONTOGENESIS AND OMEGA WILL DIMINISH SOME AS
THIS FRONT SLIDES INTO OUR AREA AND SOUNDINGS SUPPORT MORE OF LIGHT
RAIN OR DRIZZLE IF PRECIPITATION CAN OCCUR. THIS SEEMS MOST FAVORED
IN THE MID-LATE EVE OVER NORTHERN IL AND POSSIBLY REACHING INTO FAR
NORTHWEST INDIANA. CLOUDS SHOULD BECOME WIDESPREAD AS VISIBLE
SATELLITE SHOWS UPSTREAM AND THE SLOWING BOUNDARY TRAPS THE MOISTURE
BENEATH A STRENGTHENING INVERSION.

TUESDAY...
SURFACE RIDGING WILL POKE INTO THE WESTERN GREAT LAKES DURING THIS
TIME...BUT THROUGH EARLY AFTERNOON NORTHEAST LOW-LEVEL FLOW SHOULD
CONTINUE OVER THE AREA. CONCEPTUALLY IN A STRONG POST-FRONTAL AIR
MASS WITH A STRONG INVERSION IN PLACE...THINK STRATUS WILL CONTINUE
THROUGH AT LEAST EARLY AFTERNOON IF NOT THE ENTIRE DAY ESPECIALLY
FOR LAKE ADJACENT AREAS. HAVE EXTENDED THE DURATION OF FORECAST
CLOUD COVER CONSIDERABLY AND HAVE CORRELATED TEMPERATURES
DOWNWARD...WITH THE MERCURY ONLY RISING A HANDFUL OF DEGREES IN
NEAR-LAKE AREAS INCLUDING CHICAGO. FURTHER SOUTH AND WEST...MID 60S
MAY BE REALIZED. INCREASING SUBSIDENCE IN THE AFTERNOON SHOULD HELP
TO OPEN SOME HOLES IN THE CLOUDS.

LATTER HALF OF THE WEEK AND NEXT WEEKEND...
NORTHERN HEMISPHERIC WATER VAPOR SHOWS AN ACTIVE PATTERN OVER THE
WESTERN U.S. INTO THE NORTHERN PACIFIC AND THIS WILL EVOLVE OVER OUR
REGION DURING THIS PERIOD. PHASING OF THE CURRENT WESTERN U.S. UPPER
LOW AND A TROUGH MOVING ONSHORE OF WESTERN CANADA IS LIKELY ONE
CULPRIT TO FAIRLY SUBSTANTIAL MODEL DIFFERENCES OVER OUR REGION.
HAVE LEANED TOWARD THE MORE CONSISTENT GFS OUTPUT FOR THIS TIME WITH
CONFIDENCE BEING ESPECIALLY LOW SATURDAY-MONDAY.

SOUTHERLY FLOW WILL DEVELOP BACK INTO THE AREA WEDNESDAY AND
THURSDAY IN RESPONSE TO MULTIPLE ROUNDS OF CYCLOGENESIS. THIS LOOKS
TO YIELD HIGHS IN THE 70S THOUGH WITH HIGHS ON THURSDAY DEPENDENT ON
RAIN COVERAGE. THE RAIN CHANCES LOOK TO INCREASE LATER WEDNESDAY
NIGHT AND THROUGH THURSDAY AS DEEP LAYER MOISTURE CONVERGENCE
INCREASES WITHIN BETTER SUPPORT AND FOCUS FOR ASCENT. WHILE
SPECIFICS ON EXACT PRECIP EVOLUTION REMAIN UNCERTAIN ALONG WITH
WHICH PERIOD WITHIN 24-36H WINDOW WILL HAVE THE MOST WIDESPREAD
COVERAGE...CONTINUE THE BROAD LIKELY POPS MENTION. WOULD EXPECT TO
SEE SOME THUNDER WITH SUCH A DEEPENING SYSTEM ON THURSDAY AFTERNOON
INTO THE NIGHT AND A RIBBON OF INSTABILITY AHEAD OF THE INCOMING
COLD FRONT. WHILE THERE IS AT LEAST AN ISOLATED SEVERE WEATHER
POTENTIAL IN THE OVERALL REGION...IT WOULD SEEM TO BE BETTER OVER
THE MID-MISSISSIPPI VALLEY TO OUR SOUTH WITH MOISTURE
TRANSPORT/DEEPER INSTABILITY POTENTIALLY INHIBITED TO REACH THIS FAR
NORTH FROM CONVECTION OVER THE AREA.

MODELS SHOW WIDESPREAD DIFFERENCES BEHIND THIS SYSTEM IN AN
UNSETTLED NORTHWEST FLOW. THE GFS IS NOT AS PRONOUNCED WITH THE COLD
AIR INCOMING ON FRIDAY/FRIDAY NIGHT AND LEANING THAT ROUTE STILL
PROVIDES LOWS IN THE UPPER 30S-MID 40S ACROSS MOST OF THE AREA
FRIDAY/SATURDAY/SUNDAY NIGHTS...ALTHOUGH THERE WILL CERTAINLY BE
SOME DEPENDENCY ON CLOUD COVER. FRIDAY LOOKS WINDY THOUGH MAINLY DRY
WITH SUBSIDENCE BEHIND THE AFOREMENTIONED SYSTEM. WEEKEND CHANCES OF
RAIN ARE TOUGH TO SAY AND DEPENDENT ON WHEN A CLEARER PICTURE IS
PAINTED WITH NORTHWEST FLOW DISTURBANCES.

MTF

&&

.AVIATION...

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

* COLD FRONT SHIFTING WINDS TO THE NORTHEAST EARLY THIS EVENING.
* MVFR AND POSSIBLY IFR CIGS OVERSPREADING TERMINALS BEHIND THE
  FRONT...WITH A WINDOW OF LIGHT RAIN AND DRIZZLE EXPECTED.

BMD

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

A COLD FRONT CONTINUES TO SLIP SOUTH ACROSS WISCONSIN AND LAKE
MICHIGAN THIS AFTERNOON BUT IN THE MEANTIME...VFR CONDITIONS AND
LIGHT WESTERLY FLOW WILL PREVAIL AT THE TERMINALS. THE FRONT WILL
ARRIVE BY EARLY THIS EVENING TURNING WINDS SHARPLY TO THE NORTHEAST
WITH GUSTS UP TO AROUND 20 KT AT TIMES BEHIND THE FRONT. LOWER CIGS
ARE ALSO EXPECTED TO OVERSPREAD THE TERMINALS BEHIND THE
FRONT...WITH HIGH CONFIDENCE IN LOWER END MVFR CIGS AND A DECENT
SHOT AT IFR CIGS. MOISTURE BEHIND THE FRONT IS SOMEWHAT SHALLOW BUT
THERE IS MODEST POST FRONTAL FORCING THAT COULD HELP SQUEEZE OUT
SOME LIGHT RAIN OR DRIZZLE THIS EVENING INTO THE OVERNIGHT HOURS. BY
THE PREDAWN HOURS TUESDAY FORCING DISSIPATES WHILE LOWER MOISTURE
LINGERS. THERE IS EXPECTED TO BE A SLIGHT DIURNAL LIFT OF THE CLOUD
BASES DURING THE DAY TUESDAY...HOWEVER MVFR CIGS ARE EXPECTED TO
LINGER THROUGH THE DAY.

BMD

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

* HIGH CONFIDENCE IN TIMING WIND SHIFT TO WITHIN AN HOUR.
* HIGH CONFIDENCE IN MVFR CIGS...LOW-MEDIUM CONFIDENCE IN A WINDOW
  OF IFR CIGS AND LIGHT RAIN/DRIZZLE.

BMD

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

TUESDAY NIGHT...VFR. EAST WINDS.

WEDNESDAY...VFR. SOUTHEAST WINDS.

THURSDAY...TSRA LIKELY...MVFR POSSIBLE. SOUTH WINDS TURNING
SOUTHWEST LATE.

FRIDAY...SLIGHT CHANCE SHRA...MVFR POSSIBLE. BREEZY WEST WINDS.

SATURDAY...SLIGHT CHANCE SHRA. VFR. WEST WINDS.

SUNDAY...VFR. SOUTHWEST WINDS.

MDB

&&

.MARINE...
257 AM CDT

A STRONG COLD FRONT WILL MOVE SOUTHWARD DOWN LAKE MICHIGAN TODAY
BRINGING A SHIFT TO NORTH AND NORTHEAST WINDS WITH A SHARP INCREASE
IN SPEEDS. HIGH PRESSURE WHICH HAS BEEN ACROSS THE REGION FOR
SEVERAL DAYS IS WEAKENING AND PUSHING SOUTHWARD ACROSS THE OHIO
VALLEY AND SOUTHEAST U.S. WEAK SURFACE LOW PRESSURE HAS DEVELOPED
AND IS MOVING EASTWARD ACROSS NORTHERN LAKE MICHIGAN EARLY THIS
MORNING DRAGGING THE COLD FRONT WITH IT. STRONG HIGH PRESSURE IS
CENTERED OVER NORTHERN ONTARIO AND WILL MOVE SOUTHWARD BUT WEAKEN AS
IT DOES SO. THE FRONT WILL REACH THE NORTHERN SHORE OF THE LAKE
TOWARD OR SOON AFTER DAYBREAK AND PUSH SOUTHWARD REACHING THE
SOUTHERN TIP EARLY THIS EVENING. SPEEDS WILL INCREASE INTO THE 20-25
KT RANGE WITH GUSTS PUSHING 30 KT BEHIND THE FRONT...ESPECIALLY
ACROSS NORTHERN SECTIONS OF THE LAKE. STRONGER GUSTS HAVE OCCURRED
ACROSS LAKE SUPERIOR EARLY THIS MORNING BUT THE PRESSURE GRADIENT
WILL LIKELY NOT BE QUITE AS STRONG AS IT MOVES ACROSS LAKE MICHIGAN.
CANNOT RULE OUT A SHORT PERIOD OF GUSTS TOPPING 30 KT NORTH THIS
MORNING HOWEVER. EXPECT THE 25-30 KT GUSTS TO SPREAD ACROSS THE
SOUTH END OF THE LAKE THIS AFTERNOON AS WELL AND WILL HOIST A SMALL
CRAFT ADVISORY FOR THE IL/IN NEARSHORE WATERS FROM FIRST THING THIS
EVENING CONTINUING UNTIL LATE TUESDAY AFTERNOON. WINDS WILL STEADILY
DIMINISH TUESDAY AS HIGH PRESSURE RIDGING BUILDS IN BUT WAVES WILL
REMAIN ELEVATED THANKS TO THE LONG FETCH DOWN THE LAKE.

THE RIDGE WILL SHIFT EAST INTO WEDNESDAY TURNING WINDS
SOUTHEASTERLY. LOW PRESSURE WILL TRACK ACROSS THE NORTHERN PLAINS
INTO MANITOBA WEDNESDAY AND WEDNESDAY NIGHT SO THE PRESSURE GRADIENT
WILL BE TIGHTENING SO A BREEZY SOUTH TO SOUTHEAST WIND WILL DEVELOP.
AN UPPER TROUGH WILL BE MOVING EASTWARD ACROSS THE CENTRAL U.S.
SPAWNING ANOTHER SURFACE LOW ACROSS THE SOUTHERN HIGH PLAINS WHICH
WILL THEN TRACK NORTHEASTWARD. GUIDANCE HAS BEEN MORE CONSISTENT
TAKING THE LOW ACROSS THE LAKE LATER THURSDAY INTO FRIDAY BUT IS
HAVING DIFFICULTY LATCHING ONTO THE STRENGTH. THERE WILL LIKELY BE
FURTHER FLUCTUATIONS WITH STRENGTH AND THEREFORE THE INTENSITY OF
WIND SPEEDS. WILL CONTINUE TO MONITOR THE POTENTIAL FOR A PERIOD OF
STRONG WINDS WITH THIS LOW THOUGH LATEST GUIDANCE HAS BACKED OFF
SIGNIFICANTLY ON THE STRENGTH.

MDB

&&

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

     SMALL CRAFT ADVISORY...LMZ743-LMZ744-LMZ745...8 PM MONDAY TO 4
     PM TUESDAY.

&&

$$

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