Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 300756
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
256 AM CDT WED JUL 30 2014

.DISCUSSION...
218 PM CDT

FORECAST ISSUES CONTINUE TO FOCUS ON DAILY DIURNAL ISOLATED TO
SCATTERED SHOWER/THUNDERSTORM CHANCES...WITH VERY GRADUAL WARM UP IN
TEMPERATURES THROUGH THE FORECAST PERIOD AS LONG WAVE UPPER TROUGH
OVER EASTERN NORTH AMERICA EFFECTIVELY RETROGRADES THROUGH THE END
OF THE WEEK BEFORE FINALLY WEAKENING AND PROGRESSING OFF TO THE
NORTHEAST BY LATE WEEKEND AND EARLY NEXT WEEK.

WESTERN GREAT LAKES REGION REMAINS IN WESTERN PERIPHERY OF THIS
AFOREMENTIONED TROUGH...WITHIN REGION OF BROAD CYCLONIC CIRCULATION.
SERIES OF RELATIVELY MINOR SHORT WAVES WILL CONTINUE TO RIPPLE
SOUTH-SOUTHEAST THROUGH THE FLOW ACROSS THE AREA THROUGH THE
REMAINDER OF THE WEEK. GOES WATER VAPOR IMAGERY INDICATES A COUPLE
OF WEAK RIPPLES PASSED ACROSS THE REGION THIS MORNING...WITH A
SOMEWHAT MORE ROBUST SHORT WAVE NOTED UPSTREAM PROPAGATING SOUTH
ACROSS WESTERN ONTARIO. THESE SYSTEMS WILL TEND TO MODULATE
MID-LEVEL TEMPS AND INSTABILITY...ESPECIALLY WHEN JUXTAPOSED ATOP
AFTERNOON/EARLY EVENING DIURNAL WARMTH. FORECAST SOUNDINGS CONTINUE
TO INDICATE WEAK/SKINNY MLCAPES OF 200-300 J/KG THIS AFTERNOON...
WITH VISIBLE SATELLITE IMAGERY INDICATING CUMULUS CONGESTUS
DEVELOPMENT ALONG WEAK LOW LEVEL CONVERGENCE AXIS FROM NEAR ROCKFORD
TO THE SOUTHEAST SIDE OF CHICAGO/NORTHWEST INDIANA WHERE THE LAKE
BREEZE BOUNDARY WAS PROVIDING SOME ADDITIONAL CONVERGENCE.
CURRENT WEAK RADAR RETURNS EXPECTED TO CONTINUE TO SLOWLY GROW
THIS AFTERNOON THROUGH PEAK INSTABILITY...RESULTING IN ISOLATED TO
SCATTERED SHOWERS AND PERHAPS A FEW THUNDERSTORMS. WANING
INSTABILITY AFTER SUNSET THIS EVENING WILL ALLOW A DECREASE IN
COVERAGE. SIMILAR TREND EXPECTED TOMORROW...THOUGH ONTARIO SHORT
WAVE LOOKS TO MOVE ACROSS AREA DURING THE MORNING...AS OPPOSED TO
DURING PERIOD OF BETTER INSTABILITY LATER IN THE DAY. THIS MAY
FAVOR SOUTHEAST PARTS OF THE CWA WHERE FORCING WILL DECREASE
LAST...WHILE DAYTIME INSTABILITY DEVELOPS. THESE DIURNAL
SHOWERS/ISOLATED THUNDERSTORMS...TIED TO CONVERGENCE ZONES AS IS
THE CASE TODAY...WILL CONTINUE IN SIMILAR FASHION THURSDAY AS
WELL. OBVIOUSLY...NOT ALL AREAS WILL SEE PRECIP AND THERE WILL BE
MANY DRY HOURS EVEN IN PLACES THAT DO. BY FRIDAY...ADDITIONAL
SHORT WAVE ENERGY WILL DROP INTO THE WESTERN PORTION OF THE TROUGH
FROM THE NORTHERN PLAINS AND WILL EFFECTIVELY RETROGRADE THE LONG
WAVE TROUGH AXIS WESTWARD INTO THE WESTERN LAKES AND
MISSISSIPPI/LOWER OHIO VALLEY AREA. LARGE SCALE HEIGHT FALLS WILL
LEAD TO SLIGHTLY COOLER MID-LEVEL TEMPS WHICH MAY SUPPORT GREATER
COVERAGE IN SHOWERS/THUNDERSTORMS FRIDAY AND SATURDAY BEFORE THE
TROUGH AXIS BECOMES MORE PROGRESSIVE AND MOVES OFF TO THE EAST
SATURDAY NIGHT AND SUNDAY. THUS HAVE MAINTAINED SOMEWHAT HIGHER
CHANCE POPS WITH A GREATER AERIAL COVERAGE FOR THOSE DAYTIME
PERIODS.

SURFACE HIGH PRESSURE THEN BUILDS ACROSS THE AREA SUNDAY...BEFORE
DRIFTING TO THE SOUTHEAST MONDAY AS ANOTHER SHORT WAVE TROUGH
APPROACHES IN NORTHWEST FLOW ALOFT. THIS MID-LEVEL WAVE IS
ACCOMPANIED BY A SURFACE LOW AND COLD FRONT...WHICH RETURNS THE
SHOWER/THUNDERSTORM THREAT FOR MONDAY/TUESDAY OF NEXT WEEK.

AS FOR TEMPS...MID AND UPPER 70S BEING REACHED THIS AFTERNOON WITH
CONSIDERABLE SUNSHINE. MID-LEVEL TEMPS CONTINUE TO MODERATE VERY
SLOWLY THROUGH THE PERIOD...WITH TO SLOWLY WARMING FROM UPPER 70S TO
AROUND 80 TO LOWER 80S OUTSIDE OF LAKE BREEZE AND SHOWERS INTO THE
WEEKEND. COLD FRONTAL PASSAGE LATE MONDAY WILL ALLOW SUBTLE
COOL-DOWN FROM THE NORTH BY TUESDAY.

RATZER

&&

.AVIATION...

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

* LAKE BREEZE EXPECTED WITH NE WINDS AT 10 KT BEHIND IT.

* SCT SHRA THIS AFTN AND EVENING. A FEW TSRA PSBL.

JEE

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

SHOWERS HAVE PUSHED SOUTH OF THE TERMINALS BUT TODAY IS LOOKING
VERY SIMILAR TO YESTERDAY. A LAKE BREEZE IS EXPECTED ALONG WITH A
WEAKER UPPER LEVEL WAVE. THE LAKE BREEZE WILL PROVIDE ENOUGH
FORCING FOR SCT SHRA THIS AFTN AND EVENING...BUT THINKING WE WILL
SEE LESS ACTIVITY THAN YESTERDAY. A FEW THUNDERSTORMS ARE ALSO
EXPECTED AS WELL BUT THEY SHOULD BE ISOLATED ENOUGH TO LEAVE OUT
OF THE TAF FOR NOW.

WINDS BECOME NE ARND 10 KT BEHIND THE LAKE BREEZE AT MDW AND
ORD....AND NNE AT GYY. HAVE HIGH CONFIDENCE THAT THE LAKE BREEZE
WILL MAKE IT THROUGH MDW AND GYY AND MEDIUM-HIGH CONFIDENCE ABOUT
ORD. WINDS SLOWLY RETURN TO THE SW IN THE EVENING WITH CONTINUED
VFR CONDITIONS.

JEE

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

* HIGH CONFIDENCE IN LAKE BREEZE...MEDIUM CONFIDENCE IN
  TIMING...MEDIUM CONFIDENCE IN HOW FAR INLAND IT TRAVELS.

* MEDIUM-HIGH CONFIDENCE IN SHRA DEVELOPMENT AND COVERAGE...LOW
  CONFIDENCE IN TSRA COVERAGE AND DEVELOPMENT.

* HIGH CONFIDENCE IN ALL OTHER FORECAST ELEMENTS.

JEE

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

THURSDAY...SLIGHT CHANCE TSRA IN AFTN

FRIDAY...SLIGHT CHANCE TSRA IN AFTN

SATURDAY...CHANCE TSRA

SUNDAY...VFR

MONDAY...SLIGHT CHANCE TSRA

DANIELSON/RODRIGUEZ

&&

.MARINE...
255 AM CDT

A WEAK PRESSURE GRADIENT IS OVER THE LAKE WITH A WEAK COLD
FRONT/UPPER LEVEL WAVE MOVING DOWN THE LAKE EARLY THIS MORNING.
WINDS BECOME NORTH AROUND 10 KT BEHIND THE FRONT. HIGH PRESSURE
OVER THE PLAINS ATTEMPTS TO SPREAD EAST TODAY LEADING TO A
CONTINUED VERY WEAK PRESSURE GRADIENT. ONSHORE FLOW IS EXPECTED
ACROSS NEARSHORE AREAS LEADING TO VARIABLE WINDS 10 KT OR LESS AT
TIMES OVER THE OPEN WATERS. WINDS BECOME MORE UNIFORM THIS EVENING
WITH SOUTHWEST WINDS ACROSS THE LAKE BY EARLY THURSDAY MORNING. A
COUPLE MORE WEAK COOL FRONTS ARE EXPECTED BY WEEKS END...AT THIS
TIME THURSDAY NIGHT AND AGAIN FRIDAY NIGHT INTO EARLY SATURDAY.
EACH OF THESE WILL TURN SOUTHWEST WINDS TO THE NORTHWEST/NORTH BUT
SPEEDS PRIMARILY REMAINING BELOW 15 KT.

THE GREAT LAKES REGION WILL LIKELY EXPERIENCE AFTERNOON AND EARLY
EVENING SHOWERS AND SOME STORMS MAINLY DRIVEN BY THE HEATING OF
THE DAY OVER LAND. SOME OF THESE COULD BE ABLE TO MOVE INTO
NEARSHORE AREAS ON THE WESTERN SIDE OF THE LAKE AS SEEN OVER WI/IL
SHORES LAST EVENING...AND PROFILES OVER LAND DO SUPPORT SOME QUICK
RACING OUTFLOWS/GUST FRONTS FROM SHOWERS THE NEXT FEW DAYS...SO
SOMETHING FOR NEARSHORE MARINERS TO KEEP SOME TABS ON DESPITE THE
MAINLY QUIET WIND/WAVE REGIME.

JEE/MTF

&&

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

$$

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