Severe Thunderstorm Warning 151, NWS Chicago, IL

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
635 PM CDT TUE JUL 29 2008

.DISCUSSION...
311 PM CDT

THERE IS A CHANCE OF THUNDERSTORMS TONIGHT AND WEDNESDAY. SOME OF
THE THUNDERSTORMS MAY PRODUCE SMALL HAIL AND HEAVY RAINS. THE HIGH
TEMPERATURES ARE FORECAST TO BE IN THE UPPER 90S SATURDAY AND
SUNDAY. THERE MAY SOME DANGEROUS HEAT INDEX FOR THOSE DAYS.

A SURFACE TROUGH WILL MOVE INTO ILLINOIS THIS EVENING. THIS IS A
WIND SHIFT LINE THAT MAY BE A RESULT OF AGEOSTROPHIC CIRCULATION
FROM THE APPROACHING 500 MB TROUGH OVER KANSAS AND OKLAHOMA. THIS
TROUGH CAN BE SEEN ON THE VISIBLE SATELLITE IMAGERY AS A
CIRCULATION. THE VERTICAL MOTION FROM THIS THE ADJUSTMENT CAN BE
INFERRED BY THE POTENTIAL TEMPERATURE CHANGES AS THE VORTICITY
MAXIMUM MOVES INTO MISSOURI AND ILLINOIS. THERE IS SOME
INSTABILITY AS THE LEVEL OF FREE CONVECTION IS AROUND 6,977 FEET
AS SEEN ON THE FORECAST SOUNDING AT 2 AM CDT. THE LIFTED
CONDENSATION LEVEL IS FORECAST TO BE AROUND 1,858 FEET. THE
INSTABILITY IS RATHER DEEP. WE WILL KEEP THE THUNDERSTORM CHANCES
TONIGHT WITH THE HIGHEST PROBABILITY BEFORE AND DURING WEDNESDAY
MORNING. WE EXPECT SCATTERED THUNDERSTORMS. A COLD FRONT WILL MOVE
SOUTH INTO NORTHERN ILLINOIS BY LATE WEDNESDAY MORNING. WE USED
THE 12 UTC RUN OF LOCAL WRF MODEL AND 12 UTC RUN OF THE NAM
MODELS.

FOR WEDNESDAY THIS FRONT WILL WASH OUT OVER NORTHERN ILLINOIS.
THERE MAY BE ENOUGH LIFT FOR A CHANCE OF THUNDERSTORMS
WEDNESDAY. THE HIGH TEMPERATES ARE EXPECTED TO CLIMB INTO THE
UPPER 80S WEDNESDAY AND THURSDAY.

THE WEEKEND WILL BE SIGNIFICANT WITH VERY WARM AND HUMID AIR
OVER NORTHERN ILLINOIS AND NORTHWEST INDIANA. THE GFS MODEL SHOW
A LARGE 500 MB RIDGE OVER NORTHERN ILLINOIS SATURDAY. THIS LESSER
DENSE AIR INDICATES VERY HIGH TEMPERATURES. WE HAVE FORECAST THE
HIGH TEMPERATURES IN THE UPPER 90S FOR SATURDAY AND SUNDAY. THIS
WILL GIVE HEAT INDEX AROUND 100 IN SOME LOCATIONS. WE WILL MONITOR
THIS FORECAST FOR POSSIBLE HEAT ADVISORIES AND WARNINGS FOR THE
WEEKEND.

&&

.AVIATION...

635 PM CDT

FOR THE 00Z TAFS...MAIN AVIATION CONCERNS WILL BE CONVECTIVE
POTENTIAL THIS EVENING...AND THEN WINDS. CONVECTIVE COMPLEX WHICH
DEVELOPED EARLIER THIS AFTERNOON ACROSS SOUTHERN IOWA/NORTHERN
MISSOURI HAS DEVELOPED STRONG OUTFLOW ACROSS WEST CENTRAL ILLINOIS.
TAMDAR DATA FROM THIS AREA INDICATING FAIRLY LARGE 850 HPA DEW PT
DEPRESSIONS WHICH SHOULD ACT TO MAINTAIN OUTFLOW STRENGTH OVER THE
NEXT FEW HOURS...BUT IT APPEARS AS THOUGH THE GREATER WIND THREAT
SHOULD REMAIN SOUTH OF THE TERMINALS. HOWEVER...SHOULD SEE PERIOD OF
SHOWERS AND THUNDERSTORMS THIS EVENING IN THE 01Z-04Z TIME PERIOD AS
A SHORT WAVE TROUGH ASSOCIATED WITH REMNANTS OF DOLLY MOVES INTO NRN
ILLINOIS. THIS DISTURBANCE WILL GRADUALLY WEAKEN TONIGHT AS IT LIFTS
NORTHEAST AND AS DIURNAL INSTABILITY WANES EXPECTING THREAT OF
PRECIP TO DIMINISH AFTER 04Z-05Z ONCE CONVECTIVE COMPLEX TO THE
SOUTH EXITS THE AREA. DID MAINTAIN MENTION OF MVFR VSBYS IN LIGHT
FOG/HAZE TONIGHT GIVEN WEAK WIND FIELD IN ADVANCE OF SFC FRONT THAT
WILL MOVE THROUGH NRN IL TOMORROW MORNING AND ABUNDANT LOW LEVEL
MOISTURE. WIND SHIFT TO THE WEST/NORTHWEST STILL LOOKS REASONABLE BY
MID MORNING TOMORROW WITH WIND DIRECTIONS IN THE 270-300 DEGREE
RANGE EXPECTED TOMORROW AFTERNOON AND WIND SPEEDS AROUND 10 KNOTS.
HAVE KEPT TAFS DRY FOR TOMORROW WITH NEXT UPPER LEVEL SHORT WAVE OF
SIGNIFICANCE NOT EXPECTED UNTIL TOMORROW EVENING.

MARSILI

&&

.MARINE...

303 PM CDT

RELATIVELY DEEP LOW PRESSURE ACROSS WESTERN ONTARIO FOR LATE JULY
WILL CONTINUE TO TRACK EASTWARD NORTH OF LAKE SUPERIOR TONIGHT
INTO EARLY WEDNESDAY...AND INTO SOUTHERN QUEBEC WEDNESDAY NIGHT.
IN THE PROCESS...A TRAILING COLD FRONT WILL MOVE SOUTHEAST ACROSS
NORTHERN LAKE MICHIGAN LATE TONIGHT...AND SAG ACROSS SOUTHERN LAKE
MICHIGAN WEDNESDAY. SURFACE PRESSURE FALLS ACROSS THE NORTHERN
LAKES IN ADVANCE OF THIS LOW WILL TIGHTEN THE PRESSURE GRADIENT
ESPECIALLY OVER NORTHERN LAKE MICHIGAN THIS EVENING...RESULTING IN
STRENGTHENING SOUTHERLY WINDS OVERNIGHT. WINDS WILL THEN SHIFT TO
THE WEST AND NORTHWEST WITH THE PASSAGE OF THE COLD FRONT
WEDNESDAY...WITH THE GRADIENT QUICKLY DECREASING AS WEAK HIGH
PRESSURE MOVES INTO THE WESTERN LAKES WEDNESDAY NIGHT AND
THURSDAY. THIS HIGH WILL MOVE SOUTHEAST OF THE LAKE BY
SATURDAY...WITH SOUTHERLY FLOW RETURNING AS LOW PRESSURE DEVELOPS
ACROSS THE NORTHERN PLAINS. THIS LOW WILL WEAKEN BY EARLY SUNDAY
AS ANOTHER LOW PUSHES ACROSS CENTRAL CANADA...AND DRAGS ANOTHER
COLD FRONT SOUTHEAST ACROSS THE WESTERN LAKES.

RATZER

&&

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

<!doctype html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <title>Service Notice</title>
  <link rel="stylesheet" href="index.css">
</head>
<body>
  <main class="wrap">
    <section class="card" role="status" aria-live="polite">
      <div class="status" id="status">Temporary Notice</div>
      <h1 id="headline">This service is currently unavailable.</h1>
      <p id="message1">
        The site you requested is temporarily down for maintenance or an unexpected outage.
      </p>
      <p id="message2">
        Please try again in a few minutes.
      </p>
      <p class="small" id="detail">
        If you are an administrator, restore upstream service and remove the redirect/fallback rule when normal operations resume.
      </p>
    </section>
  </main>
  <script src="index.js"></script>
</body>
</html>