Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 011629
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1129 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 16Z...

* ISOLATED-SCATTERED SHRA/TSRA TODAY.
* LAKE BREEZE LIKELY BY EARLY/MID AFTERNOON.
* WIND DIRECTION MAY BE COMPLICATED BY TSRA OUTFLOW.

RATZER/KB

//DISCUSSION...UPDATED 16Z...
STILL MONITORING THE POSSIBLE DEVELOPMENT OF THUNDERSTORM ACTIVITY
NEAR THE EASTERN TERMINALS AFTER THE 17-18 UTC HOUR. AMDAR SOUNDINGS
OUT OF MDW OVER THE LAST HOUR INDICATE LITTLE TO NO CAP IN PLACE
NOW THAT TEMPERATURES HAVE WARMED INTO THE LOWER 80S.
SO...CUMULUS CLOUDS SHOULD CONTINUE TO BE ON THE INCREASE OVER
THE NEXT COUPLE HOURS AS FORCING FOR ASCENT CONTINUES OVER THE
AREA IN ASSOCIATION WITH AN UPPER LEVEL DISTURBANCE MOVING OVER
THE AREA. THERE ARE ALSO SIGNS OF A LAKE BREEZE DEVELOPING ALONG
THE SHORES OF LAKE MICHIGAN. THIS LAKE BREEZE WILL LIKELY PUSH
INLAND OVER THE NEXT FEW HOURS...LIKELY REACHING KMDW AND KORD IN
THE 20 TO 21 UTC TIME FRAME. THIS BOUNDARY COULD ALSO ACT AS A BIT
OF A FOCUS FOR THUNDERSTORM DEVELOPMENT AS IT PUSHES INLAND.
THEREFORE...THE BEST TIMING FOR THUNDERSTORMS NEAR THE EASTERN
TERMINALS APPEARS TO BE IN THE 18 TO 20-21 UTC TIME FRAME. ONCE
THE BOUNDARY PUSHES PAST THE TERMINALS THE BETTER CHANCES FOR
STORMS SHOULD SHIFT TO THE WEST AND SOUTH OF THE TERMINALS FOR
LATE AFTERNOON. I STILL DO NOT HAVE ENOUGH CONFIDENCE THAT A GIVEN
THUNDERSTORM WILL DIRECTLY IMPACT THE TERMINAL AT THIS TIME GIVEN
THE FACT THAT THE STORMS COULD REMAIN SCATTERED IN NATURE...SO I WILL
MAINTAIN THE VCTS IN THE FORECAST.


//DISCUSSION...UPDATED 12Z...

WEST-EAST LINE OF SCATTERED THUNDERSTORMS HAS DEVELOPED ROUGHLY
ALONG A MLI-LOT LINE EARLY THIS MORNING...IN AN AREA OF WEAK MOIST
ASCENT AHEAD OF A SLOW MOVING MID-LEVEL WAVE. INDIVIDUAL CELLS
HAVE BEEN DRIFTING EAST 10-15 KT DUE TO WEAK FLOW SURFACE AND
ALOFT. HIGH-RES EXPLICIT CONVECTION MODELS CONTINUE TO INDICATE
THE GREATEST COVERAGE OF SHRA/TSRA WILL LIKELY BE ACROSS SOUTHERN
PORTIONS OF C-90 AIRSPACE IN ADVANCE OF THIS WAVE THROUGH THE
DAY...WITH CONVERGENCE ALONG THE LAKE BREEZE ALSO A POTENTIAL
SOURCE OF TSRA INITIATION FARTHER NORTH THIS AFTERNOON. IN THIS
SCENARIO...HIGHEST CONFIDENCE FOR ENOUGH COVERAGE OF TSRA TO
MENTION IN TAF IS AT GYY...DIMINISHING NORTHWARD TOWARD MDW/ORD.
STORM DEVELOPMENT AND MOTION WILL BE SLOW AND HIGHLY INFLUENCED BY
CONVECTIVE OUTFLOW BOUNDARIES AS WELL. ISOLATED TSRA WILL LIKELY
LINGER INTO THE EARLY EVENING...WITH A DIURNAL DECREASE WITH SUNSET.

WEST-SOUTHWEST WINDS LESS THAN 10 KT ARE EXPECTED OUTSIDE OF
OUTFLOW AND LAKE BREEZE INFLUENCES TODAY. HAVE MOVED LAKE BREEZE
TIMING UP AN HOUR TO AROUND 20Z FOR ORD/MDW PER FASTER TREND IN
HIGH- RES GUIDANCE. WIND DIRECTION DETAILS ARE RELATIVELY LOW
CONFIDENCE HOWEVER WHERE TSRA OUTFLOW BOUNDARIES DEVELOP.
PREVAILING FLOW SHOULD BE GENERALLY LESS THAN 10 KT HOWEVER
OUTSIDE OF ANY TSRA GUSTS.

RATZER

//ORD AND MDW CONFIDENCE...UPDATED 16Z...

* LOW CONFIDENCE IN TSRA DIRECTLY IMPACTING ORD/MDW... THOUGH
  FAIRLY HIGH CONFIDENCE IN SCATTERED TSRA ACROSS ESPECIALLY
  SOUTHERN PARTS C-90 AIRSPACE TODAY.
* MEDIUM-HIGH CONFIDENCE IN LAKE BREEZE DEVELOPING BY EARLY/MID
  AFTERNOON.
* LOW CONFIDENCE IN WIND DIRECTION IF AFFECTED BY TSRA OUTFLOW.

RATZER/KB

//OUTLOOK FOR ORD/MDW FOR 00Z SUNDAY-12Z FRIDAY...UPDATED 12Z...

SATURDAY NIGHT...DRY/VFR.

SUNDAY...SLGT CHC AFTN/EVE TSRA...OTHERWISE VFR.

MONDAY THROUGH THURSDAY...CHANCE OF SHRA/TSRA.

RATZER

&&

.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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