Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 122211 AAA
AFDLOT

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

.MESOSCALE DISCUSSION...
510 PM CDT

STORMS ARE INITIATING ACROSS SOUTHERN AND EASTERN IA AND HAVE
PROMPTED THE ISSUANCE OF A TORNADO WATCH THAT EXTENDS AS FAR EAST
AS NORTHWEST IL. LATEST HAND SURFACE ANALYSIS INDICATES A POOL OF
75+ DEW POINTS ACROSS SOUTHEAST IA INTO CENTRAL IL ADVANCING
NORTHWARD WITH MANY SURFACE OBS IN THIS AREA INDICATING
GUSTINESS. AT THE LEADING EDGE OF THIS A WARM FRONT WITH STRONG
MOISTURE CONVERGENCE IS INCHING NORTHWARD NEAR INTERSTATE 88
AHEAD OF A 1012MB SFC LOW IN CENTRAL IA. SUCH A SETUP OF A SLOWING
NORTHWARD-MOVING WARM FRONT AHEAD OF A NOT-TOO-STRONG LOW PRESSURE
WITH GOOD REPLENISHING MOISTURE REALLY SPELLS TROUBLE FOR HEAVY
RAINFALL TONIGHT WITHIN THE REGION. MOST FAVORED THIS EVE WOULD BE
ALONG AND IMMEDIATELY NORTH OF THIS WARM FRONTAL AXIS...SO THE
NORTHERN FORECAST AREA INCLUDING ROCKFORD. AS CONVECTION
EVOLVES/EXPANDS EASTWARD THROUGH MID-LATE EVENING...SOME
SOUTHWARD PROGRESSION IS THEN LIKELY INTO OVERNIGHT. SO THE ENTIRE
AREA REMAINS UNDER AN INCREASING RISK FOR HEAVY RAINFALL CAPABLE
OF FLASH FLOODING...WHICH OBSERVATIONAL TRENDS SUPPORT.

SEVERE WEATHER THREAT REMAINS AS MENTIONED IN PREV DISCUSSION.

MTF

&&

.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 22Z...

* LOW CHANCE OF SHRA/TSRA BETWEEN 00Z AND 02Z...SHRA/TSRA EXPECTED
  SOME TIME AROUND MID EVENING INTO EARLY OVERNIGHT. HEAVY
  RAINFALL WITH IFR VISIBILITY AT TIMES BECOMING MORE LIKELY. WILL
  LIKELY INCLUDE TEMPO IN 00Z TAFS FOR VISIBILITY OF 1 SM OR
  POSSIBLY LOWER.

* CHANCE FOR MVFR CIGS TO LINGER INTO SUNDAY MORNING.


MTF/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 22Z...

* MEDIUM IN WINDS...BUT HIGH THEY WILL REMAIN FROM THE SOUTHWEST
  THROUGH AT LEAST 01Z.

* HIGH THAT CHANCE FOR TSRA WILL WAIT UNTIL AFTER 00Z. HIGH THAT
  TSRA WILL OCCUR TONIGHT...MEDIUM IN TIMING. HIGH THAT TEMPORARY
  IFR VISIBILITY WILL OCCUR.

* MEDIUM-HIGH IN CIGS REMAINING VFR UNTIL TSRA.

MTF/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...
423 PM CDT

SOUTHERLY FLOW PERSISTING OVER THE ENTIRE LAKE THIS AFTERNOON AS
HIGH PRESSURE REMAINS SITUATED OVER THE ATLANTIC COAST...AND AS A
TROUGH OF LOW PRESSURE AND ASSOCIATED FRONTAL BOUNDARY ARE JUST
WEST OF THE LAKE. AS THESE SOUTHERLY WINDS BECOME MORE SOUTHWEST
AND THEN EVENTUALLY WEST TONIGHT WITH THE PASSAGE OF THIS
TROUGH...DO EXPECT SPEEDS TO REMAIN IN THE 10 TO 20 KT RANGE.
EXPECTED COMPLEX OF SHOWERS AND THUNDERSTORMS WILL LIKELY PROVIDE
ERRATIC WINDS LATER THIS EVENING AND OVERNIGHT...AS WELL AS THE
POSSIBILITY FOR SLIGHTLY HIGHER SPEEDS. DO EXPECT MORE OF A WEST
NORTHWEST WIND ACROSS THE LAKE ON SUNDAY...AS A SECONDARY
TROUGH/FRONT MOVE SOUTHEAST THROUGH THE GREAT LAKES REGION.

RODRIGUEZ

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...FLASH FLOOD WATCH...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-
     ILZ011-ILZ012-ILZ013-ILZ014-ILZ019-ILZ020-ILZ021-ILZ022-
     ILZ023-ILZ032-ILZ033-ILZ039 UNTIL 1 PM SUNDAY.

IN...FLASH FLOOD WATCH...INZ001-INZ002-INZ010-INZ011-INZ019 UNTIL 1
     PM SUNDAY.

LM...NONE.
&&

$$

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