Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 010844
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
344 AM CDT FRI AUG 1 2014

.DISCUSSION...
340 AM CDT

MAIN FORECAST CONCERNS/CHALLENGES ARE WITH PRECIP TRENDS TODAY
THROUGH THIS WEEKEND...WITH SCATTERED SHOWERS/THUNDERSTORMS
POSSIBLE EACH DAY.

OVERALL QUIET CONDITIONS IN PLACE THIS MORNING WITH THE CWA
CURRENTLY DRY...WHILE SHOWERS SLOWLY APPROACH FROM THE
IOWA/ILLINOIS BORDER. PATCHY FOG HAS DEVELOPED
OVERNIGHT...PERIODICALLY DROPPING TO AROUND ONE MILE IN SOME
LOCATIONS. THE LOWEST VIS REMAINS ISOLATED WITH THESE LOWER
OBSERVATIONS ONLY BEING REPORTED BRIEFLY. THIS WILL CONTINUE TO BE
LIKELY THROUGH THE MORNING HOURS ESPECIALLY AS LATEST SATELLITE
IMAGERY DEPICTING MID/HIGH LEVEL CLOUD COVER PUSHING INTO THE CWA.
SHORT WAVE TROUGH SEEN SHIFTING EAST TOWARDS WESTERN ILLINOIS THIS
MORNING WILL CONTINUE TOWARDS THE CWA...PRIMARILY BRINGING AN
INCREASE IN CLOUD COVER WITH THE SOUTHERN CWA OBSERVING THE
THICKER CLOUD COVER. ASSOCIATED SHOWER ACTIVITY WILL LIKELY HOLD
IN PLACE AS THE BETTER LARGE SCALE ASCENT REACHES THE CWA BY DAY
BREAK. THESE SCATTERED SHOWERS WILL MAINLY AFFECT AREAS SOUTH OF
INTERSTATE 80 THROUGH MID MORNING WITH PARTLY CLOUDY TO PARTLY
SUNNY SKIES IN PLACE.

AS CYCLONIC FLOW PERSISTS TODAY...SO WILL A STEADY STREAM OF WAVES
OF ENERGY TRAVERSING THIS FLOW. AS THIS ENERGY AND INCREASING
LARGE SCALE ASCENT MOVE OVERHEAD TODAY...IT WILL DO SO AS LOW
LEVEL LAPSE RATES STEEPEN. FORECAST SOUNDINGS ARE SHOWING THAT
THE ENVIRONMENT OVER NORTHERN ILLINOIS AND NORTHWEST INDIANA WILL
BE WELL MIXED AS EARLY AS 15-16Z. THIS COINCIDING WITH SLIGHTLY
INCREASING MID LEVEL LAPSE RATES AND DEWPOINTS AROUND 60...WILL
YIELD WEAK TO MODERATE INSTABILITY AND NO CAP AS SOON AS MID DAY.
ALTHOUGH COVERAGE STILL APPEARS TO BE SCATTERED WITH MOST AREAS
LIKELY OBSERVING MORE PERIODS OF DRIER CONDITIONS...MOST GUIDANCE
IN FAIR IN AGREEMENT WITH BETTER COVERAGE THAN HAS BEEN SEEN OVER
THE LAST COUPLE OF DAYS. DO HAVE INCREASING POPS INTO THE
AFTERNOON WITH LOW CHANCE POPS IN PLACE BY 18Z. THE ENTIRE CWA
WILL OBSERVE THIS SCATTERED DEVELOPMENT TODAY BUT DO FEEL THAT THE
SOUTHERN THIRD OF THE CWA...ALONG AND SOUTH OF INTERSTATE
80...COULD OBSERVE BETTER COVERAGE. THIS WILL BE DUE TO BETTER
LARGE SCALE ASCENT WITH THE PASSING MID LEVEL FEATURES. WIDESPREAD
ORGANIZED/STRONG DEVELOPMENT IS NOT ANTICIPATED TODAY BUT ISOLATED
STRONGER WIND GUSTS AND SMALL HAIL COULD BE OBSERVED WITH THE
STRONGER THUNDERSTORMS. THE STEEP LOW LEVEL LAPSE RATES COULD
SUPPORT BETTER DOWNWARD MOMENTUM OF HIGHER WIND GUSTS DESPITE
GENERALLY WEAK FLOW WITHIN THE COLUMN...AS AXIS OF HIGHER PWATS
SHIFTING THROUGH THE CWA COULD POSSIBLY SUPPLEMENT THIS LACK OF
FLOW WITH PRECIP LOADING. EXPECT COVERAGE/INTENSITY TO COME DOWN
LATER THIS EVENING...BUT WITH SOME POSSIBLE LINGERING SHOWERS
ACROSS NORTHERN ILLINOIS AS ANOTHER SHORT WAVE TROUGH APPROACHES
FROM THE NORTHWEST.

EXPECT LOW 80S ONCE AGAIN TODAY...BUT POSSIBLY NOT QUITE AS HIGH
AS THURSDAY DUE TO INCREASING CLOUD COVER/PRECIP. SATURDAY AND
SUNDAY WILL OBSERVE SIMILAR TEMP TRENDS...WITH SCATTERED
SHOWER/THUNDERSTORM ACTIVITY POSSIBLE ONCE AGAIN SATURDAY
AFTERNOON.

RODRIGUEZ

&&

.AVIATION...

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

* CHANCE OF SHRA/TSRA TODAY.
* LAKE BREEZE MID/LATE AFTERNOON.

RATZER

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

MID-LEVEL TROUGH WILL PROPAGATE SLOWLY ACROSS THE REGION TODAY.
FORCING FOR ORGANIZED CONVECTION APPEARS RATHER WEAK...THOUGH
SLIGHTLY COOLER TEMPERATURES ALOFT WILL RESULT IN UNSTABLE
CONDITIONS DEVELOPING BY MID/LATE MORNING AS SURFACE TEMPERATURES
RISE. AREA OF SHOWERS CURRENTLY OVER SOUTHEAST IOWA APPEAR TO BE
ASSOCIATED WITH MID-LEVEL CIRCULATION WITHIN MID-LEVEL TROUGH...
AND CONVECTION-ALLOWING MODEL GUIDANCE INDICATES THESE WILL
CONTINUE TO DRIFT EAST AND PERSIST SOUTH OF THE TERMINALS. IT IS
AHEAD OF THIS FEATURE INTO EAST CENTRAL ILLINOIS AND NORTHWEST
INDIANA WHERE SHRA/TSRA LIKELY TO INITIALLY EXPAND WITH INCREASING
DAYTIME INSTABILITY...WITH GENERALLY LOWER COVERAGE OF SHRA/TSRA
EXPECTED NORTH OF A BDF-SBN LINE. HOWEVER...LIGHT WINDS BOTH
SURFACE AND ALOFT ARE EXPECTED TO ALLOW LAKE BREEZE DEVELOPMENT BY
EARLY AFTERNOON...AND THE LAKE BREEZE BOUNDARY MAY BECOME A FOCUS
FOR ADDITIONAL SHRA/TSRA DEVELOPMENT WITH POTENTIAL IMPACTS FOR
ESPECIALLY THE ORD/MDW/DPA AND GYY TERMINALS. WEAK WIND FIELD WILL
RESULT IN SHRA/TSRA OUTFLOW BOUNDARIES PROVIDING FOCUS FOR NEW
DEVELOPMENT...AS WELL AS FAIRLY SLOW STORM MOTION, THUS WHILE
COVERAGE OF POTENTIAL SHRA/TSRA IS OF SOMEWHAT LOW CONFIDENCE...
ANY SHRA/TSRA WHICH FORM COULD HAVE SUBSTANTIAL IMPACT ON LOCAL
AIRSPACE GIVEN LIKELY SLOW MOVEMENT AND VARIABLE WIND DIRECTION
FROM OUTFLOW.

AS FOR WINDS...GENERALLY LIGHT WEST-SOUTHWEST PREVAILING WIND LESS
THAN 10 KT SHOULD DEVELOP AFTER SUNRISE. LAKE BREEZE WILL LIKELY
DEVELOP BY LATE MORNING...PUSHING INTO ORD/MDW/GYY DURING THE
AFTERNOON. SOME EASTERLY COMPONENT GENERALLY LESS THAN 10 KT
EXPECTED BEHIND LAKE BREEZE...WHICH WOULD EVENTUALLY BECOME
SOUTH-SOUTHEAST DURING THE EVENING. AS INDICATED ABOVE HOWEVER...
ANY SHRA/TSRA OUTFLOW MAY RESULT IN HIGHLY VARIABLE WIND
DIRECTION.

RATZER

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

* LOW CONFIDENCE FOR SHRA/TSRA DIRECTLY OVER TERMINALS.
* MEDIUM CONFIDENCE IN LAKE BREEZE BY MID/LATE AFTERNOON.

RATZER

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

* SATURDAY...CHANCE OF SHRA/TSRA. AFTERNOON LAKE BREEZE.
* SUNDAY...DRY/VFR.
* MONDAY THROUGH WEDNESDAY...CHANCE OF SHRA/TSRA.

&&

.MARINE...
230 AM CDT

NO MAJOR MARINE FORECAST CONCERNS THIS MORNING AS WEAK SURFACE
PRESSURE GRADIENT DOMINATES THE REGION. AN AREA OF LOW PRESSURE JUST
SOUTHEAST OF JAMES BAY WILL LIFT NORTHEAST TODAY...WITH A WEAK
SURFACE TROUGH WASHING OUT ACROSS NORTHERN LAKE MICHIGAN. WEAK HIGH
PRESSURE RIDGING WILL DEVELOP AS THE TROUGH DISSIPATES...COMBINING
WITH EXISTING HIGH PRESSURE OVER THE OHIO VALLEY TO MAINTAIN WEAK
PRESSURE GRADIENT AND GENERALLY LIGHT WIND CONDITIONS ACROSS THE
LAKE WITH DAILY LAKE BREEZE DEVELOPMENT INTO SUNDAY. AN AREA OF LOW
PRESSURE WILL PASS NORTH OF THE LAKES SUNDAY AND SUNDAY NIGHT AND
WILL TRAIL A WEAK COLD FRONT ACROSS THE NORTHERN AND CENTRAL PARTS
OF THE LAKE. THE GRADIENT WILL TIGHTEN A BIT SUNDAY INTO SUNDAY
EVENING ACROSS THE NORTHERN PART OF THE LAKE IN ASSOCIATION WITH THE
PASSAGE OF THIS LOW AND WEAK FRONT...WITH SOUTH WINDS INCREASING
TO A MORE SOLID 15 KT OR SO. THE FRONT WILL THEN DISSIPATE WITH THE
GRADIENT AGAIN BECOMING RATHER WEAK SUNDAY NIGHT INTO MONDAY.
LONGER RANGE GUIDANCE INDICATES ANOTHER SOMEWHAT STRONGER LOW
DEVELOPING OVER THE PLAINS AND APPROACHING THE WESTERN LAKES
REGION LATE NEXT WEEK.

RATZER

&&

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

$$

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