Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 122012
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
312 PM CDT SAT JUL 12 2014

.DISCUSSION...
311 PM CDT

THIS AFTERNOON...
THE SURFACE LOW IS OVER NORTHERN MANITOBA AND THE UPPER LEVEL LOW IS
STACKED JUST ABOVE IT.  STILL HAVE MOSTLY CLOUD SKIES WITH A FEW
BREAKS OF SUN AND WE ARE SLOWLY RECOVERING FROM THIS MORNING/S
CONVECTION.  THE 18Z DVN SOUNDING FEATURED 1.7 INCHES OF PWAT AND
AROUND 1500 J/KG OF CAPE. THE SOUNDING ALSO HAD A STOUT CAP AT 850MB
WHICH MATCHES WELL WITH THE LATEST AMDAR SOUNDINGS.  LAPSE RATES TO
950 MB WERE SURPRISINGLY STEEP GIVEN THE THICK CLOUD COVER. WITH THE
CAP IN PLACE AND SLOW RECOVERY NOT EXPECTING MUCH CONVECTIVE
ACTIVITY AHEAD OF THE MAIN LINE OF STORMS. IN GENERAL EXPECTING
CLOUDS TO CONTINUE THIN AND TEMPS RISE INTO THE MID 80S BY LATE THIS
AFTN.  IT WILL CONTINUE TO BE MUGGY AS PWAT VALUES INCREASE TO
AROUND 2 INCHES.

THIS EVENING AND TONIGHT...
AN UPPER LEVEL SHORTWAVE CURRENTLY OVER IOWA WILL MOVE OVER THE CWA
THIS EVENING ADDING FORCING FOR STORMS. A SURFACE LOW IS OVER
NORTHERN MANITOBA AND ITS COLD FRONT STRETCHES FROM NORTHWEST
WISCONSIN THROUGH CENTRAL IOWA.  A LINE OF STORMS WILL FORM ALONG
THE FRONT AND SLOWLY PUSH SOUTH THROUGH THE CWA THIS EVENING AND
OVERNIGHT. 850MB STEERING FLOW WILL BE WEST AT 30-40 KT AHEAD OF THE
LINE BUT ONLY AROUND 15 KT FROM THE NORTH BEHIND THE FRONT. TIMING
DIFFERS BETWEEN THE MODELS BUT THINKING A BLEND OF OUR LOCAL ARW AND
THE RAP LOOK REASONABLE. THE NAM LOOKS TOO FAST WHILE THE HRRR LOOKS
TOO SLOW.

THE LINE MAY PRODUCE SEVERE WIND GUSTS IN EXCESS OF 60 MPH...BUT THE
MAIN CONCERN WILL BE FLASH FLOODING. PWAT VALUES REMAIN HIGH AT
1.7-1.8 INCHES AND VALUES WILL INCREASE TO AROUND 2 INCHES BY THIS
EVENING.  HAVE ISSUED A FLASH FLOOD WATCH FOR THE ENTIRE WARNING
AREA FOR THIS EVENING THROUGH TOMORROW MORNING.  TOTAL RAINFALL FROM
THIS STORM WILL BE IN EXCESS OF 1 INCH. THE HIGHEST TOTALS WILL
LIKELY BE ALONG THE IL/WI STATE LINE WHERE RAINFALL TOTALS WILL
EASILY APPROACH 2 INCHES.  IN ADDITION...SOME AREAS THIS MORNING
FROM COOK COUNTY SOUTH THROUGH PONTIAC AND GIBSON CITY SAW MULTIPLE
INCHES OF RAIN OVERNIGHT INTO THIS MORNING. THESE AREAS WILL BE THE
MOST SUSCEPTIBLE TO FLASH FLOODING AS WATER LEVELS ARE ALREADY
HIGH.

TEMPERATURES WILL QUICKLY FALL BEHIND THE LINE OF STORMS AND LOWS
WILL BE IN THE UPPER 60S TO LOWER 70S.

SUNDAY AND MONDAY...
THE UPPER LEVEL LOW SPLITS SUNDAY NIGHT AND THE FIRST TROUGH AXIS
MOVES THROUGH MONDAY NIGHT.  AT THE SURFACE A SECOND COLD FRONT
MOVES THROUGH SUNDAY AND THEN STALLS OUT JUST SOUTH OF THE CWA
MONDAY MORNING. THE HIGHEST INSTABILITY VALUES WILL BE SOUTH OF I-80
SO ONLY HAVE A CHANCE OF STORMS SOUTH OF I-80. NOT EXPECTING MUCH IN
THE WAY OF PRECIP AT THIS TIME.

A BAND OF SHOWERS AND STORMS MAY FORM SUNDAY NIGHT FIRST ALONG THE
IL/WI STATE LINE AND THEN SPREAD SOUTH AS A VORTICITY STREAMER FROM
THE UPPER LEVEL LOW PUSHES SOUTH THROUGH THE REGION.  THE SHOWERS
MAY PERSIST INTO LATE SUNDAY NIGHT AND BY THEN ONLY EXPECTING
SHOWERS.  CONFIDENCE IS MEDIUM WITH THE STORMS DEVELOPING AND
COVERAGE.

YET ANOTHER COLD FRONT COMES THROUGH MONDAY AND HAVE A CHANCE OF
SHOWERS AND STORMS MONDAY AFTERNOON AND EVENING.  THE BIGGER STORY
WILL BE THE COOL AIR THAT FOLLOWS THE COLD FRONT.  HIGHS ON SUNDAY
WILL BE IN THE LOWER 80S WHILE MONDAY/S HIGHS WILL BE IN THE MID TO
UPPER 70S.  MONDAY NIGHT LOOKS CRISP WITH LOWS IN THE LOWER 50S.

EXTENDED...TUESDAY THROUGH FRIDAY...
THE UPPER LEVEL TROUGH SLOWLY LIFTS NE THROUGH THURSDAY WHILE A
SURFACE HIGH BUILDS OVER THE REGION TUESDAY NIGHT. THE EXTENDED
LOOKS DRY WITH BELOW NORMAL TEMPERATURES.

JEE

&&

.AVIATION...

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

* BREEZY SOUTHWEST WINDS GUSTING 20-25 KT THROUGH AFTERNOON.

* LOW CHANCE OF SHRA/TSRA LATE AFTERNOON INTO EARLY
  EVENING...SHRA/TSRA EXPECTED SOME TIME AROUND MID EVENING. GUSTY
  WINDS AND HEAVY RAINFALL POSSIBLE.

* MVFR CIGS EXPECTED TO SCATTER TO VFR MID AFTERNOON.

MDB

//DISCUSSION...UPDATED 20Z...

NO MAJOR CHANGES IN THINKING AT THIS POINT. CUMULUS IS STARTING TO
DEVELOP ALONG A NARROW LINE FROM SOUTHWEST IOWA NORTHEASTWARD UP
TOWARDS DBQ ALONG THE FRONTAL BOUNDARY THAT WILL IMPACT THE
TERMINALS THROUGH THE EVENING. EXPECT THAT THIS WILL EVOLVE INTO
THUNDERSTORMS OVER THE NEXT FEW HOURS WITH ACTIVITY POTENTIALLY
DEVELOPING INTO FAR NORTHWEST ILLINOIS...AND POSSIBLY RFD IN A FEW
HOURS. OVERALL TIMING SPECIFICS AT THE TERMINALS IS STILL SOMEWHAT
UNCERTAIN BUT DO NOT SEE MUCH TO SUPPORT ANY IMMEDIATE CHANGES.
OTHERWISE...MVFR CIGS HAVE BEEN SCATTERING THOUGH SOME BROKEN PATCH
MAY REMAIN ACROSS NORTH CENTRAL ILLINOIS.

FROM 18Z...

SOUTHWEST WINDS ARE STARTING TO INCREASE IN SPEED WITH GUSTS IN
THE 20-25 KT RANGE. CIGS ARE STARTING TO IMPROVE WITH MAINLY MVFR
IN PLACE. A LOOK OUT THE WINDOW AT THE OFFICE SHOWS STEADY
SCATTERING OF THE LOWEST CLOUDS WHICH SUPPORTS THE THINKING THAT
IMPROVEMENT TO VFR WILL OCCUR OVER THE NEXT FEW HOURS. FOCUS TURNS
TO TIMING OF SHRA/TSRA INTO THE EVENING. NOT MUCH IN THE WAY OF
FOCUS FOR DEVELOPMENT THROUGH MID AFTERNOON BUT FORCING WILL
INCREASE FAR NORTHWEST LATE THIS AFTERNOON. SOME SCATTERED
ACTIVITY IS POSSIBLE UP TOWARDS RFD BY THEN BUT THE FOCUS FOR THE
MOST WIDESPREAD DEVELOPMENT WILL BE ALONG A FRONTAL BOUNDARY FROM
SOUTHERN WISCONSIN WEST-SOUTHWEST INTO EASTERN IOWA. THIS ACTIVITY
WILL SAG SOUTHWARD THIS EVENING. TIMING IS STILL IN QUESTION SO
FURTHER ADJUSTMENTS MAY BE NEEDED BUT AT THIS POINT EARLY EVENING
IS FAVORED AT RFD WITH MID EVENING IN THE CHICAGO AREA AND
SLIGHTLY LATER AT GYY AS THINGS SAG SOUTHWARD. THE FRONT WILL BE
FAIRLY SLOW TO MOVE SO TRAINING OF STORMS ALONG THE FRONT IS
EXPECTED LEADING TO A FAIRLY LONG DURATION OF THUNDER. TORRENTIAL
RAINFALL MAY BRING VSBY DOWN BELOW 1SM BUT WILL HOLD OFF ON THIS
DETAIL IN THE TAF UNTIL IT BECOMES MORE CLEAR. WINDS MAY BE
ERRATIC OVERNIGHT BEHIND THE FRONT BUT TREND TO WEST THEN
NORTHWEST INTO SUNDAY MORNING. MVFR MAY LINGER BEHIND THE FRONT AS
WELL BUT VFR SHOULD BUILD IN.

MDB

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

* HIGH CONFIDENCE IN WINDS.

* HIGH CONFIDENCE THAT CHANCE FOR SHRA/TSRA IS LOW LATER THIS
  AFTERNOON AND SHRA/TSRA IS LIKELY THIS EVENING. MEDIUM
  CONFIDENCE IN TIMING THIS EVENING.

* MEDIUM-HIGH CONFIDENCE IN MVFR IMPROVING TO VFR THIS AFTERNOON.
  LOW-MEDIUM CONFIDENCE IN TIMING.

MDB

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

SUNDAY-MONDAY...LOW CHANCES OF TSRA.
TUESDAY...SLIGHT CHANCE OF SHRA.
WEDNESDAY-THURSDAY...VFR LIKELY.

KMD

&&

.MARINE...
244 AM CDT

STILL SEEING GUSTS TO NEAR 20 KT ON THE NORTHERN HALF OF THE
LAKE...BUT DO EXPECT THE PRESSURE GRADIENT TO RELAX TONIGHT INTO
SATURDAY AS HIGH PRESSURE ALONG THE ATLANTIC COAST CONTINUES EAST.
A TROUGH OF LOW PRESS SURE AND FRONTAL BOUNDARY WILL CROSS THE LAKE
ON SATURDAY NIGHT. CONFIDENCE IS LOWER WITH REGARDS TO OVERALL
WIND DIRECTION AND SPEEDS SATURDAY EVENING INTO SATURDAY NIGHT AS
PERIODS OF SHOWERS AND THUNDERSTORMS ARE EXPECTED ALONG THIS
FRONTAL BOUNDARY. A SERIES OF WEAK LOW PRESSURE SYSTEMS AND
ASSOCIATED COLD FRONTS WILL THEN PASS THROUGH THE LAKE THROUGH
MONDAY WHILE HIGH PRESSURE BUILDS INTO THE NORTHERN CENTRAL
PLAINS. WINDS SHIFT TO WEST- NORTHWEST BEHIND THE STRONGER COLD
FRONTAL PUSH MONDAY NIGHT AND REACH 20 KT OR POSSIBLY HIGHER
TUESDAY.

KMD

&&

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

$$

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