Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 290256
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
956 PM CDT TUE JUL 28 2015

.UPDATE...
915 PM CDT

MAIN CONCERN IS WITH CONVECTION AND ASSOCIATED HEAVY RAIN THREAT
LATE THIS EVENING AND OVERNIGHT. DVN 00Z RAOB SAMPLED VERY MOIST
AIRMASS WITH PWAT VALUE OF 2.02 INCHES. SPC MESOANALYSIS IS
HANDLING MOIST AXIS FROM SOUTHERN HALF OF IOWA TO WESTERN IL
CONTAINING PWATS OVER 2 INCHES WELL AND SOUTHWESTERLY 925-850 MB
LOW LEVEL JET WILL TRANSPORT THIS MOISTURE FARTHER EASTWARD.
ASCENT AHEAD OF MIDLEVEL VORT MAX RESPONSIBLE FOR WIDESPREAD
CONVECTION OVER SOUTHERN IOWA AND CONVERGENCE ON NOSE OF LOW LEVEL
JET SHOULD RESULT IN INCREASING COVERAGE OF GENERALLY SCATTERED
THUNDERSTORMS LATE THIS EVENING THROUGH THE OVERNIGHT FROM WEST
NORTHWEST TO SOUTHEAST.

IN ADDITION...THERE WILL BE BETTER LOW LEVEL MOISTURE CONVERGENCE
AS COLD FRONT TRAILING FROM EXCEPTIONALLY DEEP LOW OVER SOUTHERN
MANITOBA APPROACHES TOWARD DAYBREAK. DESPITE DIURNALLY UNFAVORABLE
TIMING...AFFOREMENTIONED SUPPORTING FACTORS MEAN THAT THERE SHOULD
BE AT LEAST SCATTERED COVERAGE. HOWEVER BEST FOCUS FOR MOST
WIDESPREAD SHRA/TSRA COVERAGE COULD BE AT AND SOUTH OF LATITUDE OF
CURRENT IA CONVECTION ALONG AXIS OF HIGHEST INSTABILITY...IN
SOUTHERN HALF OR THIRD OF CWA. HAVE MENTIONED HEAVY RAIN WITH ANY
THUNDERSTORMS LATE THIS EVENING THROUGH THE OVERNIGHT GIVEN VERY
MOIST AIRMASS. HAVE NOT ISSUED FLASH FLOOD WATCH DUE TO SOME
UNCERTAINTY ON EVOLUTION BUT WILL LIKELY NEED TO MONITOR FOR
FLOODING...ESPECIALLY IF ANY CONVECTION CAN TRAIN OVER THE SAME
AREAS REPEATEDLY.

RC

&&

.SHORT TERM...
246 PM CDT

THROUGH TONIGHT...

FOR THE REMAINDER OF THE AFTERNOON...FOCUS WILL BE ON TEMPERATURES
OVER THE ERN PORTIONS OF THE CWA AND PCPN TRENDS FOR THE REMAINDER
OF THE AFTERNOON.  A DECAYING MCS CONTINUES TO PUSH THROUGH
NWRN/NCNTRL IL.  THE ONLY ACTIVE TS ARE APPROACHING WRN LA SALLE AND
LIVINGSTON COUNTIES WITH SOME TRAILING SCT SHRA TO THE NORTH AND
WEST. LATEST RADAR TRENDS INDICATE THAT THE RESIDUAL STRATIFORM RAIN
OVER FAR NWRN IL IS DIMINISHING AND EVEN THE LEADING SHRA/TSRA ARE
ON A WANING TREND AS THE ACTIVITY MOVES TO THE SOUTHEAST THROUGH THE
FAVORABLE AREA OF UPPER DIFFLUENCE...BUT IS AT THE SAME
TIME...MOVING AWAY FROM THE MAIN SHORTWAVE WHICH IS LIFTING NEWD
INTO THE UPPER GREAT LAKES. AT THIS POINT...IT IS UNLIKELY THAT ANY
RESIDUAL SHOWERS WILL REACH THE CHICAGO METRO AREA. TEMPERATURE
TRENDS ARE GENERALLY ON TRACK WITH TEMPERATURES IN THE UPPER 80S
INLAND OF A PESKY LAKE BREEZE NERN IL AND AWAY FROM THE RAIN COOLED
AIR OVER NWRN IL.  SOUTH OF THE ILLINOIS AND KANKAKEE RIVERS...WHERE
TEMPS IN THE UPPER 80S AND DEWPOINTS IN THE MID 70S HAVE COMBINED TO
PRODUCE HEAT INDICES AT OR SLIGHTLY ABOVE 100F.

FOR TONIGHT...CONCERN WILL SHIFT TO THUNDERSTORMS EXPECTED TO
DEVELOP IN ASSOCIATION WITH A COLD FRONT APPROACHING FROM THE WEST.
AS OF ARND 2PM CDT...THE COLD FRONT EXTENDED FROM DEEP LOW PRESSURE
OVER SRN SASKATCHEWAN...THROUGH THE UPPER MISSOURI VALLEY AND THEN
BACK INTO THE TX PANHANDLE.  A QUASI-STATIONARY FRONT WAVING ACROSS
CNTRL IOWA...THROUGH CNTRL IL/IN IS ALSO ACTING AS A LIFTING
MECHANISM FOR AN EAST-WEST ORIENTED BAND OF TS OVER CNTRL IOWA...AND
SUSPECT THAT THIS BOUNDARY IS HELPING TO MAINTAIN SOME TS OVER CNTRL
IL...IN THE BLOOMINGTON AREA.  WHILE THE ENVIRONMENT OVER ERN
IA/NWRN IL IS CURRENTLY WORKED OVER FROM EARLIER CONVECTION...EXPECT
THAT THE ENVIRONMENT WILL BECOME MORE CONDUCIVE TO TS AGAIN THROUGH
THE EVENING AS THE COLD FRONT BEGINS TO MOVE FROM NW TO SE ACROSS
THE CWA.  WHILE THE REGION REMAINS IN THE WARM SECTOR AHEAD OF THE
COLD FRONT...THE REGION WILL REMAIN UNDER A VERY MOIST
AIRMASS...WITH DEWPOINTS REMAINING IN THE LOWER TO MIDDLE 70S.  THE
QUASI-STATIONARY SHOULD BE THE INITIAL FOCUSING MECHANISM FOR SOME
ELEVATED TS...BUT THE COLD FRONT SHOULD QUICKLY TAKE OVER AS THE
MAIN FOCUSING MECHANISM FOR MORE SFC BASED CONVECTION.  THE WILD
CARD IN THE DECK IS THE QUASI-STATIONARY FRONT OVER CNTRL IL.  THE
INTERACTION OF THIS BOUNDARY WITH THE COLD FRONT COULD ADD SOME
ENHANCED SFC FOCUSING TO THE CONVERGENCE ASSOCIATED WITH THE COLD
FRONT...LEADING TO THE POTENTIAL FOR SOME STRONGER...AND POSSIBLE
SEVERE THUNDERSTORMS OVER THE FAR SOUTHERN PORTIONS OF THE CWA.  THE
OTHER THREAT WILL BE SOME LOCALLY HEAVY RAINFALL WITH THE POTENTIAL
FOR ECHO TRAINING ALONG THE EAST-WEST ORIENTED BOUNDARY.  CONFIDENCE
IS NOT VERY HIGH ON THE LOCATION OF ANY POTENTIAL ECHO
TRAINING...BUT SHOULD IT DEVELOP...THE POTENTIAL WILL EXIST FOR SOME
LOCALIZED FLOODING OR FLASH FLOODING.

KREIN

&&

.LONG TERM...
252 PM CDT

WEDNESDAY THROUGH TUESDAY...

AS THE DEEP SUMMERTIME LOW PROGRESSES ACROSS ONTARIO ON WEDNESDAY
THE FRONT WILL ADVANCE THROUGH THE FORECAST AREA IN THE MORNING. AT
LEAST ONGOING SCATTERED SHOWERS AND POSSIBLY SOME STORMS ARE
EXPECTED AT DAYBREAK WITHIN THE PRE-TROUGH DEEP MOIST PLUME AND
GLANCING UPPER FORCING. THAT SAID...THE TREND LAST NIGHT WAS FOR
REGIONAL CONVECTION TO BE FURTHER SOUTH THAN HIGH-RES GUIDANCE HAD
INDICATED AND ALREADY SEEING THAT TREND TODAY...SO IT IS CERTAINLY
POSSIBLE THAN ANY MORNING ACTIVITY IS MAINLY SOUTH OF I-80.
CONFIDENCE REMAINS HIGH THAT THE BOUNDARY WILL BE SOUTH OF THE AREA
BY PEAK HEATING AND CONVECTIVE REGENERATION TIME ON WEDNESDAY
AFTERNOON...SO CONTINUE TO DROP POPS OFF FAIRLY QUICKLY DURING THE
LATE MORNING...KEEPING ONLY A SMALL CHANCE IN THE FAR SOUTHEAST IN
THE AFTERNOON.

WITH THE PARENT LOW WELL TO THE NORTH ON WEDNESDAY...THE 850-925MB
THERMAL RIDGE BECOMES ELONGATED OVER THE AREA AND NOT SHUNTED
SOUTHWARD AT AS GREAT OF SPEED AS THE SURFACE FRONT. GIVEN CLEARING
BY EARLY WEDNESDAY AFTERNOON AND VERY MILD STARTING POINT TO
TEMPERATURES TO START THE DAY...HIGHS SHOULD REACH MID TO UPPER 80S
WITH 90 POSSIBLE...ESPECIALLY IF DEW POINTS CAN DROP SOME.

HIGH PRESSURE FROM THE NORTHERN PLAINS THROUGH THE LOWER OHIO VALLEY
ON THURSDAY SHOULD PROVIDE QUIET WEATHER WITH A WEST-NORTHWEST WIND
CONTINUING TO NOT ALLOW MUCH OF OR ANY LAKE BREEZE. A WEAK NORTHERN
STREAM COOL FRONT IS FORECAST BY THE SUITE OF AVAILABLE GUIDANCE TO
INCH SOUTHWARD THROUGH THE GREAT LAKES REGION ON FRIDAY...WITH A
SLIGHTLY BETTER DEFINED FRONTAL BOUNDARY LATER IN THE WEEKEND OR
EARLY NEXT WEEK. DEW POINTS SHOULD GRADUALLY CLIMB THROUGH THE 60S
INTO THE WEEKEND AND MAY ALLOW THAT SECOND BOUNDARY A BETTER CHANCE
TO GENERATE SOME STORMS. TEMPERATURES LOOK TO REMAIN SLIGHTLY ABOVE
NORMAL LATE THIS WEEK AND MORE AT NORMAL THIS WEEKEND.

MTF

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 03Z...

* CHANCES OF SHOWERS/STORMS MAINLY LATE TONIGHT INTO VERY EARLY
  WED...BUT QUESTIONS ON THE EXTENT OF COVERAGE OF TS OVER THE
  TERMINALS.

* WINDS WILL TURN WEST-NORTHWEST BY DAYBREAK WED...WITH GUSTS
  DEVELOPING WED LATE MORNING AFTERNOON TO 20KT.

KJB

//DISCUSSION...UPDATED 00Z...

A LAKE BREEZE PLAYED HAVOC WITH WINDS EARLIER AS IT BRIEFLY MOVED
OVER THE TERMINALS THEN SHIFTED BACK TO THE EAST. SINCE THEN...WINDS
HAVE RETURNED TO A SOUTHWESTERLY DIRECTION...AND WILL LIKELY REMAIN
SO UNTIL EARLY WEDNESDAY MORNING WHEN A COLD FRONT SHIFTS OVER THE
AREA. THE MAIN CONCERNS THROUGH THE PERIOD WILL BE THE CHANCES AND
ACTUAL COVERAGE OF THUNDERSTORMS TONIGHT IN ASSOCIATION WITH THE
APPROACHING COLD FRONT. IT APPEARS THAT OUR BEST CHANCES OF
THUNDERSTORMS AT THE TERMINALS WILL COME OVERNIGHT TONIGHT THROUGH
VERY EARLY WEDNESDAY MORNING. HOWEVER...COVERAGE COULD REMAIN ON THE
LOWER SIDE ADDING TO UNCERTAINTY ON HOW MUCH A GIVEN AIRFIELD GETS
IMPACTED BY ANY STORMS. THEREFORE...I HAVE ELECTED TO KEEP THE
MENTION OF A PROB 30 GROUP FOR THUNDERSTORMS WITHIN A PREVAILING
VCTS. HOWEVER...I DID PUSH OFF THE TIMING UNTIL AFTER 07-08 UTC AT
KRFD...AND AFTER 09-10 UTC AT THE EASTERN TAF SITES.

IT IS POSSIBLY THAT SOME MVFR CIGS COULD BRIEFLY DEVELOP OVER THE
AREA EARLY ON WEDNESDAY MORNING AS THE COLD FRONT ARRIVES.
HOWEVER...ANY LOWER CLOUDS SHOULD IMPROVE BY MID TO LATE MORNING.
WINDS IN THE WAKE OF THE FRONT WILL BECOME WEST-NORTHWESTERLY WITH
SOME GUSTS UP TO 20 KT AT TIMES DURING THE AFTERNOON.

KJB

//ORD AND MDW CONFIDENCE...UPDATED 03Z...

* MEDIUM WITH BEST TIMING OF TSRA LATE TONIGHT.
* LOW TO MEDIUM WITH ACTUAL COVERAGE OF STORMS.
* LOW ON POTENTIAL FOR SHORT PERIOD OF MVFR CIGS EARLY WED AM.
* HIGH WITH WIND TRENDS.

KJB

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

* THURSDAY THROUGH SATURDAY...DRY/VFR. WESTERLY WINDS.
* SUNDAY...CHANCE OF TSRA. SOUTHWEST WINDS.
* MONDAY...CHANCE OF TSRA. LAKE BREEZE POSSIBLE.

KJB

&&

.MARINE...
255 PM CDT

A MAINLY QUIET SUMMERTIME PATTERN WILL PERSIST OVER LAKE
MICHIGAN...THOUGH A LITTLE MORE CONSISTENT FLOW TO 20 KT WILL BE
SEEN THROUGH MID TO LATE WEEK WITH A DEEP LOW OVER WESTERN ONTARIO
WEDNESDAY MORNING SLOWLY MOVING NORTHEAST. A WEAK COOL FRONT IS
EXPECTED TO MOVE SOUTH DOWN THE LAKE ON FRIDAY AND FRIDAY
EVENING...WITH ANOTHER SLIGHTLY BETTER DEFINED ONE LATE IN THE
WEEKEND.

MTF

&&

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

$$

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