000
FXUS63 KLOT 300547
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1247 AM CDT FRI JUL 30 2010

.DISCUSSION...
827 PM CDT

IR IMAGERY SHOWING A NARROW CHANNEL OF CIRRUS SLIDING SE ALONG THE
LEE-SIDE OF THE MID-LVL RIDGE AXIS IOWA...NORTHERN IL...SOUTHERN
WI. ADDITIONALLY A COMPLEX OF TSRA DEVELOPING OVER NORTHCENTRAL
NEBRASKA/SOUTHERN SOUTH DAKOTA WITH DECENT 850MB MOISTURE FLUX
CONVERGENCE AND A BETTER SHEARED ENVIRONMENT. FOR THE GREAT LAKES ANTI-
CYCLONIC FLOW IN THE LLVLS WILL MAINTAIN A DRY ATMOSPHERE
OVERNIGHT. LATEST SREF/NAM12 GUIDANCE HINTS THAT PRECIP ARRIVAL
MAY BE DELAYED UNTIL CLOSER TO 00Z SAT FOR NORTHERN IL. OVERNIGHT
LOWS WILL COOL INTO THE 60S. WINDS WILL REMAIN LGT AND VRB THRU
DAYBREAK FRI.

BEACHLER

//PREV DISCUSSION...
330 PM CDT

THERE WILL BE A CHANCE OF THUNDERSTORMS FRIDAY. WARMER WEATHER
WILL RETURN MONDAY.

TONIGHT...THE SHOWERS AND THUNDERSTORMS OVER SOUTH DAKOTA AND
IOWA WILL CONTINUE TO MOVE EAST. THIS AREA OF CONVECTION IS SEEN
AS A VORTICITY MAXIMUM IN THE FORECAST MODELS. THE LOCAL WRF MODEL
MOVES THIS VORTICITY MAXIMUM INTO SOUTHWEST WISCONSIN BY 06 UTC
FRIDAY. THEN THE MODEL MOVES THE VORTICITY MAXIMUM INTO ILLINOIS
BY 12 UTC. WE WILL PUT A CHANCE OF THUNDERSTORMS IN THE FORECAST
FOR NORTHWEST ILLINOIS AND NORTH CENTRAL ILLINOIS LATE TONIGHT AND
EARLY FRIDAY MORNING. MEANWHILE A SURFACE HIGH WILL MOVE ACROSS
NORTHERN ILLINOIS AND NORTHWEST INDIANA TONIGHT AND EARLY FRIDAY.
THIS ANTICYCLONIC SURFACE FLOW MAY INHIBIT SURFACE CONVERGENCE FOR
THUNDERSTORMS. SO THAT IS ANOTHER REASON WE WILL LEAVE A CHANCE OF
THUNDERSTORMS IN THE FORECAST TONIGHT.

FRIDAY...THE HIGH WILL MOVE EAST.  A LOW WILL DEVELOP OVER IOWA
LATE FRIDAY. THIS LOW WILL MOVE FROM IOWA TO CENTRAL ILLINOIS.
THIS WILL KEEP A CHANCE OF THUNDERSTORMS OVER NORTHERN ILLINOIS
FRIDAY NIGHT. WE USED THE GFS MODEL BECAUSE OF THE TIMING OF THE
500 MB HEIGHT PATTERN WAS CLOSE TO THE MOTION OF THE CLOUDS ON THE
SATELLITE LOOPS.

SATURDAY AND SUNDAY...A HIGH WILL MOVE INTO NORTHERN ILLINOIS.
THE HIGH WILL SPREAD EAST INTO MICHIGAN AND ONTARIO BY SUNDAY
EVENING.

MONDAY THROUGH WEDNESDAY...ANOTHER FRONT IS FORECAST TO MOVE
ACROSS ILLINOIS AND NORTHWEST INDIANA BY TUESDAY. WE WILL KEEP A
CHANCE OF THUNDERSTORMS IN THE FORECAST FOR MONDAY NIGHT AND
TUESDAY. WE EXPECT WEDNESDAY WILL BE DRY WITH HIGH TEMPERATURES IN
THE MIDDLE 80S.

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 06Z...

LAKE BREEZE POTENTIAL FRIDAY AFTERNOON AND TIMING OF PRECIP
ARRIVING FRIDAY NIGHT.

IZZI

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

HIGH PRESSURE WILL REMAIN IN CONTROL THE NEXT 12-18 HOURS WITH
LIGHT WINDS AND JUST MID-HIGH LEVEL CLOUDINESS BLOWING OFF
SHRA/TSRA WELL WEST OF THE AREA. A LAKE BREEZE IS EXPECTED TO
DEVELOP TODAY...BUT WITH HIGH CLOUDINESS SHOULD KEEP LAKE/LAND
TEMP CONTRAST RELATIVELY SMALL WHICH COULD LIMIT HOW FAR INLAND
A LAKE BREEZE WILL MAKE IT. HAVE OPTED TO INCLUDE A SHIFT TO A
MORE EASTERLY DIRECTION AT BOTH ORD AND MDW BY LATE AFTERNOON BUT
WINDS SHOULD STAY FAIRLY LIGHT (LESS THAN 10KT) BOTH PRE AND POST
ANY LAKE BREEZE PASSAGE.

HARD TO RULE OUT A SPRINKLE OR BRIEF LIGHT SHOWER LATER FRIDAY
AFTERNOON INTO THE EVENING AS DECAYING TSRA`S TRY TO MOVE EAST
INTO THE DRIER MORE STABLE AIRMASS OVER THE REGION. CHANCES OF
THIS OCCURRING LOOK QUITE LOW AND PLAN TO MAINTAIN A DRY FORECAST
IN THE TAF. BY LATER FRIDAY EVENING INTO THE OVERNIGHT HOURS RAIN
CHANCES WILL INCREASE A BIT AS WARM/MOIST ADVECTION INCREASES.
ENOUGH UNCERTAINTIES EXIST THIS FAR OUT TO JUST KEEP PROB30`S
GOING AND CAN LET LATER TAF ISSUANCES BETTER DEFINE TIMING OF
PRECIP ARRIVAL.

IZZI


//ORD AND MDW CONFIDENCE...UPDATED 06Z...

* HIGH CONFIDENCE IN VFR CONDITIONS THROUGH FRIDAY AFTERNOON AND
  EVENING...AND EVEN OUTSIDE OF ANY SHRA LATE FRIDAY NIGHT
* LOW-MEDIUM CONFIDENCE IN LAKE BREEZE MAKING IT THROUGH ORD/MDW
  FRIDAY AFTERNOON
* LOW-MEDIUM CONFIDENCE ON ANY SHRA MAKING IT TO THE TERMINALS
  FRIDAY NIGHT

IZZI

//OUTLOOK FOR ORD/MDW FOR 12Z SATURDAY-12Z THURSDAY...UPDATED 06Z...

SATURDAY...SLIGHT CHANCE OF TSRA...OTHERWISE VFR.
SUNDAY...VFR.
MONDAY...CHANCE OF TSRA...OTHERWISE VFR.
TUESDAY...VFR.
WEDNESDAY...CHANCE OF TSRA...OTHERWISE VFR.

MARSILI/IZZI

&&

.MARINE...
241 PM CDT

WEAK GRADIENT SHOULD PERSIST ACROSS LAKE MICHIGAN THROUGH MUCH OF
THE FORECAST PERIOD WITH THE STRONGEST WINDS OF THE PERIOD POSSIBLE
DURING EARLY PORTIONS OF NEXT WEEK.

SFC HIGH PRESSURE WILL CONTINUE TO BUILD INTO THE WESTERN GREAT
LAKES REGION TONIGHT ALLOWING FOR A WEAK SURFACE PRESSURE GRADIENT
TO BE MAINTAINED. BY LATE TONIGHT INTO FRIDAY MORNING...SHOULD START
TO SEE A SOUTHEAST COMPONENT PREVAIL AS THE SFC HIGH MOVES OFF TO
THE EAST. A WEAK SFC LOW ACROSS THE CENTRAL PLAINS WILL DRIFT
EASTWARD OVER NEXT 36 HOURS AND TEND TO WEAKEN ACROSS THE MID MS
RIVER VALLEY ALLOWING FOR ANOTHER HIGH PRESSURE AREA TO MOVE IN FOR
SATURDAY. A STRONGER LOW PRESSURE AREA WILL TRACK ACROSS SOUTHERN
CANADA MONDAY PERHAPS SETTING UP A STRONGER SOUTHERLY FLOW. STILL
SOME UNCERTAINTY AS TO THE DEPTH OF THIS LOW...BUT COULD SEE SOME
SOUTH GUSTS APPROACHING 20 KNOTS FOR EARLY PARTS OF NEXT WEEK.

MARSILI

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...NONE.
&&

$$

NWS LOT Office Area Forecast Discussion