Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 292008
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
308 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 20Z...

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

BMD

//DISCUSSION...UPDATED 20Z...
BOUNDARY HAS ACCELERATED SOUTH SO HAVE BUMPED UP TIMING A COUPLE
TIMES ALREADY EARLY THIS AFTERNOON. ALSO INCREASED WINDS WITH
UPSTREAM OBS SHOWING A BRIEF WINDOW OF 25-30KT WIND GUSTS
IMMEDIATELY BEHIND THE FRONT. IFR SEEMS MORE CERTAIN SO CHANGED
THE SCT007 TO BKN007.

PREVIOUS DISCUSSION 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 20Z...

* HIGH CONFIDENCE IN TIMING WIND SHIFT.
* HIGH CONFIDENCE IN MVFR/IFR CIGS...LOW-MEDIUM CONFIDENCE IN A
  WINDOW OF 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...
305 PM CDT

A SHARP COLD FRONT IS MARCHING DOWN THE LAKE THIS AFTERNOON WITH A
WIND SHIFT TO NORTH- NORTHEASTERLY BEHIND THE FRONT. THE FRONT HAS
ALREADY PASSED THROUGH MILWAUKEE AS OF 300 PM CDT. OBSERVATIONS BEHIND
THE FRONT HAVE HELD AT 30 KT OR LESS...SO FEEL CONFIDENT THAT
WINDS WILL GENERALLY REMAIN SUB- GALE THROUGH THIS COLD FRONT
PASSAGE AS THE PRESSURE GRADIENT WILL WEAKEN SOMEWHAT AS THE FRONT
PUSHES SOUTH. THERE COULD BE A FEW GUSTS ABOVE 30 KT WITH THE
FRONTAL PASSAGE. EXPECT WAVES TO BUILD THROUGH TUESDAY AS WE
REMAIN UNDER NORTH FLOW THROUGH THE DAY...WITH THE HIGHEST WAVES
ON SOUTHERN PORTIONS OF THE LAKE.

HIGH PRESSURE OVER SOUTHERN ONTARIO WILL WEAKEN AND SLOWLY SHIFT
EAST WHILE THE RIDGE AXIS ASSOCIATED WITH IT WILL SHIFT OVER THE
LAKE TOMORROW AFTERNOON...THEN MOVE EAST OF THE LAKE ON WEDNESDAY.
AS THIS OCCURS ADDITIONAL LOWS IN THE PLAINS WILL LEAD TO A
TIGHTENING PRESSURE GRADIENT LATER WEDNESDAY INTO THURSDAY. AT THIS
POINT THE STRONGER LOW HEADS NORTH THROUGH MANITOBA WHILE ANOTHER
LOW MOVES OVER THE LAKE ON THURSDAY NIGHT. GUIDANCE STILL REMAINS
MIXED AS TO WHETHER THIS LOW WILL BE IN A DEEPENING PHASE AS IT
TRANSITS THE LAKE OR IF IT WILL INTENSIFY MORE SO NORTH AND EAST OF
LAKE MICHIGAN.

A STRONG COLD FRONT WILL THEN MOVE THROUGH LATER THURSDAY NIGHT INTO
FRIDAY BRINGING GUSTY NORTHWESTERLY WINDS. IF THE LOW IS
STRONGER...THERE COULD BE GALES IN THE BETTER MIXED ATMOSPHERE
BEHIND THE LOW FRIDAY. KMD

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...ILLINOIS NEARSHORE WATERS UNTIL 7 PM TUESDAY.

     SMALL CRAFT ADVISORY...LMZ743-LMZ744-LMZ745...8 PM MONDAY TO 7
     PM TUESDAY.

&&

$$

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