Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 291959
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
259 PM CDT TUE JUL 29 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 20Z...

* LAKE BREEZE MAY IMPACT MDW LATER THIS AFTN WITH WINDS SHIFTING
  TO NE OR E BEHIND IT. BREEZE LOOKS TO STALL BEFORE REACHING
  ORD.

* ISOL TO SCT SHRA DEVELOP THIS AFTN INTO THE EARLY EVENING. MAY
  SEE A FEW THUNDERSTORMS AS WELL.

* LAKE BREEZE POSSIBLE AGAIN ON WEDNESDAY AFTERNOON.

KJB

//DISCUSSION...UPDATED 18Z...

OBSERVATIONS NEAR THE SHORES OF LAKE MICHIGAN ARE SHOWING SIGNS OF A
LAKE BREEZE DEVELOPING. THIS BOUNDARY WILL LIKELY PUSH INLAND A
LITTLE THIS AFTERNOON...IMPACTING KGYY AND POSSIBLY KMDW LATER IN
THE AFTERNOON. HOWEVER...I HAVE SOME DOUBTS THAT IT WILL MAKE IT TO
MDW GIVEN THE 12-15 KT OF 925 MB WEST-NORTHWESTERLY FLOW THAT IT
WILL BE WORKING AGAINST. FOR NOW...I WILL CONTINUE TO MONITOR THE
TRENDS OF THIS LAKE BREEZE AND MAKE A DECISION LATER OF WHETHER OR
NOT TO REMOVE THE WIND SHIFT FROM THE MDW TAF.

OTHERWISE...THE OTHER CONCERN LATER THIS AFTERNOON AND EVENING IS
THE THREAT FOR SCATTERED SHOWER AND THUNDERSTORM ACTIVITY ACROSS THE
AREA. IT STILL APPEARS THE BEST TIMING FOR ANY ACTIVITY WOULD BE
AFTER 21 UTC THROUGH AROUND 02 UTC THIS EVENING. DUE TO THE
SCATTERED NATURE EXPECTED...I HAVE CONTINUED TO HANDLE THIS CHANCE
WITH A VCSH.

WINDS LOOK TO CONTINUE OUT OF WEST-NORTHWESTERLY DIRECTION INTO
WEDNESDAY. HOWEVER...ANOTHER LAKE BREEZE MAY DEVELOP WEDNESDAY
AFTERNOON...AND DUE TO WEAKER WESTERLY FLOW AROUND 925 MB...THE
BOUNDARY COULD THREATEN THE EASTERN TERMINALS WITH AN EASTERLY WIND
SHIFT LATER IN THE AFTERNOON. CONFIDENCE IS A BIT LOW THIS FAR
OUT...SO I HAVE LEFT THIS MENTION OUT OF THE 30 HOUR ORD TAF.

KJB

//ORD AND MDW CONFIDENCE...UPDATED 20Z...

* HIGH CONFIDENECE WILL LAKE BREEZE GETTING TO MDW BY 21 UTC. LOW
  CONFIDENCE IN LAKE BREEZE IMPACTING ORD THIS AFTERNOON.

* MEDIUM CONFIDENCE IN SCT SHRA DEVELOPMENT ACROSS THE AREA THIS AFTERNOON.

* LOW CONFIDENCE IN ISOLATED TS IMPACTING THE TERMINALS THIS AFTERNOON.

* HIGH CONFIDENCE IN ALL OTHER FORECAST ELEMENTS.

KJB

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

WEDNESDAY NIGHT...VFR.

THURSDAY THROUGH SATURDAY...PREDOMINANTLY VFR...WITH A CHANCE OF
TSRA IN MAINLY THE AFTERNOON & EARLY EVENING.

SUNDAY...VFR.

MONDAY...SCHC TSRA.

JEE

&&

.MARINE...
210 PM CDT

HIGH PRESSURE IN THE NORTHERN AND CENTRAL PLAINS IS EXPECTED TO
WEAKEN SOME OVER THE COMING DAYS BUT GENERALLY REMAIN STATIONARY FOR
MOST OF THE WEEK AS AN UPPER LEVEL LOW PRESSURE SYSTEMS REMAINS
PARKED OVER JAMES BAY THROUGH SATURDAY. IN THIS PATTERN WEAK LOW
PRESSURE SYSTEMS WILL GLIDE SOUTHEAST DOWN THE LAKE AT TIMES...WITH
THE FIRST EXPECTED LATER TONIGHT INTO EARLY WEDNESDAY.

IN THIS STAGNANT PATTERN THE PRESSURE GRADIENT WILL REMAIN ON THE
LIGHTER SIDE AND WILL PROMOTE THE DEVELOPMENT OF LAKE BREEZES FOR
THE NEARSHORE WATERS AND VARIABLE WINDS OVER MUCH OF THE OPEN LAKE
AND WINDS GENERALLY 15 KT OR LESS.  KMD

KMD

&&

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

$$

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