Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 190910
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
410 AM CDT TUE AUG 19 2014

.DISCUSSION...
336 AM CDT

SHORT TERM...THROUGH TUESDAY NIGHT...

FOR THE REMAINDER OF THE OVERNIGHT HOURS...THE MAIN FOCUS WILL BE
THE PROGRESSION OF THUNDERSTORMS ALONG A WEAK COLD FRONT MOVING
THROUGH SRN WI/NERN IA AND PUSHING INTO NRN IL.  THE MAIN SHORTWAVE
ASSOCIATED WITH THE ACTIVITY IS MOVING QUICKLY OVER ERN WI AND CNTRL
LAKE MICHIGAN.  AS THE SHORTWAVE RACES RATHER QUICKLY TO THE
EAST...THE LINE OF THUNDERSTORMS IS TAKING ON A MORE EAST-WEST
ORIENTATION ALONG THE WISCONSIN BORDER...BUT IS SAGGING INTO
WINNEBAGO AND BOONE COUNTIES.  SOME STRONG THUNDERSTORMS ALONG THIS
PORTION OF THE FRONT HAVE THE POTENTIAL TO PRODUCE SOME SMALL HAIL
AND LOCALIZED STRONGER WIND GUSTS.  THERE IS STILL SOME CONCERN
ABOUT HOW FAR SOUTH THE ACTIVITY WILL REACH.  CURRENTLY ANTICIPATE
THAT THE GREATEST THUNDERSTORM COVERAGE SHOULD BE OVER THE NORTHERN
TIER COUNTIES ALONG THE WISCONSIN BORDER.  HOWEVER...THERE IS STILL
SOME SWLY FLOW ALOFT WITH WITH 850MB WINDS ARND 30KT...WHILE SFC
FLOW IS LIGHT AND GENERALLY SOUTHERLY.  THERE IS PLENTY OF LOW
LEVEL MOISTURE IN PLACE AND WITH AT LEAST MODEST ISENTROPIC LIFT IN
THE WARM SECTOR WHICH COULD AT LEAST GENERATE SCT SHRA/TSRA.  THE
LATEST HIGH RES GUIDANCE IS INDICATING A DIMINISHING TREND TO THE
CONVECTION ALONG THE SRN PORTION OF THE FRONT AS IT MOVES THROUGH
THE AREA THIS MORNING...SO HAVE BACKED OFF POPS TO SLIGHT
CHANCE...BUT WITH DAYTIME WARMING...THERE SHOULD BE AN UPTICK IN THE
CONVECTIVE ACTIVITY AD THE FRONT PUSHES THROUGH THE SRN PORTIONS OF
THE CHICAGO METRO AREA AND INTO ECNTRL IL AND NWRN INDIANA. PORTIONS
OF THE CWA.  WITH INCREASING SLWY DEVELOPING WITH THE APPROACH OF
THE COLD FRONT AND VEERING TO WLY FOLLOWING THE FROPA...NO LAKE
BREEZE DEVELOPMENT IS EXPECTED TODAY AND THE WARM AIR SHOULD HAVE NO
TROUBLE REACHING RIGHT UP TO THE LAKEFRONT...WITH TEMPS IN THE LOWER
TO MIDDLE 80S EXPECTED FOR THE ENTIRE CWA.

ANY RESIDUAL PCPN ASSOCIATED WITH THE PASSING COLD FRONT SHOULD PUSH
TO THE EAST OF THE CWA BY LATE THIS EVENING.  WEAK HIGH PRESSURE
SHOULD BUILD IN BEHIND THE FRONT WEDNESDAY EVENING AND OVERNIGHT.
WITH GENERALLY WLY FLOW BEHIND THE FRONT...AND NO SGFNT DRY AIR
UPSTREAM...THE LOW LEVELS SHOULD REMAIN VERY MOIST...SO FOG
DEVELOPMENT LATE TUESDAY NIGHT AND POSSIBLY INTO WEDNESDAY MORNING
IS A DISTINCT POSSIBILITY.

KREIN

WEDNESDAY THROUGH THURSDAY...
MAIN FEATURE IN THE MEDIUM TERM WILL BE THE DEPARTING MID-LVL VORT
FROM THE GREAT LAKES...STEADILY WEAKENING AND EJECTING EAST.
MEANWHILE A DEVELOPING 500MB RIDGE WILL TAKE SHAPE ACROSS THE
PLAINS...THEN LIFT NORTHEAST THUR. THE STALLED FRONTAL BOUNDARY WILL
REMAIN SOUTH OF THE FORECAST AREA THRU WED THEN AS THE RIDGE
STEADILY BUILDS AND EXPANDS NORTH WARM AIR ADVECTION WILL LIFT THE
BOUNDARY NORTH. THIS WILL LIKELY FEATURE MOISTENING LLVLS WITH
DECENT ISENTROPIC LIFT ALONG THE BOUNDARY...TO SUPPORT INCREASING
POPS LATE WED FROM SOUTH TO NORTH. WHILE IT APPEARS FAVORABLE FOR A
CHANNEL OF LIKELY POPS LATE WED-EARLY THUR...THERE IS SOME CONCERN
THAT WE COULD QUICKLY BECOME CAPPED WITH A STRONG PUSH OF VERY WARM
AIR ALOFT. SOME GUIDANCE IS SUGGESTING AFT 00Z 850MB THERMAL TEMPS
RISE TO ARND 19-20 DEG C...WHICH COULD BE ENOUGH TO LIMIT
COVERAGE/INTENSITY. HOWEVER IF THIS ENDS UP BEING SLIGHTLY COOLER
ANY PRECIP THAT DOES DEVELOP COULD EASILY PRODUCE LOCALLY HEAVY
RAINFALL...AS PWAT VALUES ARE EXPECTED TO RISE TO NEAR 1.7-2.0"
OVERNIGHT. TEMPS WED WILL PUSH INTO THE MID 80S...HOWEVER IT IS
PLAUSIBLE THAT A WEAK LK BREEZE COULD FORM EARLY IN THE AFTN AND
KEEP THE NEARSHORE AREAS COOLER.

WITH WARM AIR STEADILY ADVECTING NORTH INTO THE FORECAST
AREA...EXPECT TEMPS TO REMAIN MILD OVERNIGHT IN THE MID/UPR 60S. IF
CLOUDS AND PRECIP ARRIVE EARLIER WED EVE...THEN TEMPS MAY NOT DIP
BELOW 70.

THUR WILL FEATURE STEADILY RISING MID-LVL HEIGHTS AS THE ROBUST
RIDGE ESTABLISHES ITSELF ACROSS A BULK OF OF THE REGION. THIS POSES
CHALLENGES WITH PRECIP FOR THUR. GUIDANCE CONTINUES TO FAVOR A
SHORTWAVE LIFTING NORTHEAST INTO THE UPPER MIDWEST WITH BETTER
MID-LVL VORTICITY THEN SLIDING EAST ALONG THE NORTHERN PERIPHERY OF
THE RIDGE...BEFORE DIVING SOUTHEAST TOWARDS THE EASTERN GREAT LAKES.
THIS COUPLED WITH VERY WARM 850MB TEMPS NEARING 20 DEG C MAY RESULT
IN A CAPPED ENVIRONMENT. CONFIDENCE IS NOT YET STRONG ENOUGH TO
FURTHER REDUCE POPS THUR AFTN. DEPENDING ON CLOUDS/PRECIP THUR WILL
STEER HOW WARM SFC TEMPS BECOME. GUIDANCE CONTINUES TO POINT TOWARDS
MID/UPR 80S. DEW PTS WILL BE PUSHING INTO THE LOW 70S...WHICH THE
COMBINATION OF TEMP/DEW PT COULD BE SUPPORTIVE OF HEAT INDEX VALUES
NEARING THE MID 90S.

CONCERN REMAINS FOR THUR NGT ON POPS...BUT MORE IMPORTANTLY ON
POTENTIAL FOR HEAVY RAINFALL. PWAT VALUES CONTINUE TO HOVER ARND 2"
WITH VERY WARM DEW PTS BETWEEN 72-74 DEG. WITH THE LACK OF A STRONG
WIND FIELD...STORMS COULD BE SLOW MOVING AND EASILY PRODUCE LOCALLY
HEAVY RAINFALL. OVERNIGHT LOWS MAY NOT DIP BELOW THE MID/UPR
70S...WHICH SETS THE STAGE FOR A POTENTIALLY HOT/HUMID DAY FRI.

FRIDAY THROUGH EARLY NEXT WEEK...
MID-LVL RIDGING CONTINUES TO BE PROGGED FOR THE FRI-SUN
PERIODS...WHICH SETS THE STAGE FOR POTENTIALLY A VERY WARM STRETCH
ACROSS THE REGION WITH NUMEROUS AREAS REACHING 90 DEG IN THIS
TIMEFRAME. GUIDANCE CONTINUES TO FAVOR A HIGH PROBABILITY THAT DEW
PTS WILL EASILY REMAIN IN THE 72-74 DEG RANGE FRI/SAT...WHICH
COUPLED WITH TEMPS ARND 90 COULD EASILY PRODUCE 100 DEG HEAT INDEX
READINGS. THE CURRENT ECMWF SOLUTION PAINTS AN EVEN WARMER THERMAL
RIDGE AT 850MB FOR SUN AT 23-25 DEG C. CLOUDS/PRECIP WILL LIKELY
STEER HOW WARM SUN BECOMES.

THEN ENSEMBLES CONTINUE TO DEMONSTRATE SOME CONSISTENCY THAT THE
MID-LVL RIDGE WILL WEAKEN EARLY NEXT WEEK WITH A RETURN TO TROUGHING
ACROSS THE MIDDLE OF THE CONUS. CURRENT GUIDANCE SUGGESTS A FRONTAL
BOUNDARY WILL ARRIVE IN THE MON/TUE TIMEFRAME AND BRING TEMPS BACK
TO SEASONAL OR SLIGHTLY SUB-SEASONAL CONDS FOR THE MIDDLE OF NEXT
WEEK.

BEACHLER

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 09Z...

* CHANCE SHRA/TSRA POSSIBLE SOMETIME BETWEEN 08Z-11Z.

* POTENTIAL FOR LOWER CIGS LATE TONIGHT INTO TUE MORNING.

* CHANCE FOR SCATTERED SHRA/TSRA MUCH OF TUE WITH A SLIGHTLY MORE
  FAVORED TIME DURING THE AFTERNOON.

KREIN

//DISCUSSION...UPDATED 09Z...

LATEST RADAR TRENDS INDICATING THAT THE THREAT FOR THUNDERSTORMS
AT THE CHICAGO AREA TERMINALS IS DIMINISHING EARLY THIS MORNING AS
THE WEAK COLD FRONT IS ONLY SKIMMING ALONG THE NRN TIER COUNTIES
ADJACENT TO THE WISCONSIN BORDER.HOWEVER...A WEAK PREFRONTAL
TROUGH IS MOVING THROUGH NRN IL AND IS PROVIDING SOME WEAK SFC
CONVERGENCE...WHICH IN THE PRESENCE OF A MOIST...AND WEAKLY
UNSTABLE AIRMASS IN PLACE...IS GENERATING SOME ISOLD SHRA/TSRA.
CONFIDENCE IS DECREASING IN THE POTENTIAL FOR PREVAILING TS AT THE
CHICAGO AREA TERMINALS...SO HAVE BACKED OFF TO A VCTS FOR 10-12Z.
WHILE THE CHANCES FOR TS DIRECTLY IMPACTING THE CHICAGO AREA
TERMINALS CANNOT BE TOTALLY RULED OUT...ANY ACTIVITY THAT DOES
DEVELOP SHOULD BE BRIEF.

KREIN

.PREVIOUS DISCUSSION...UPDATED 06Z...

A QUIET EVENING IS EXPECTED OVER LOCAL AIRPORTS BEFORE TSRA
CHANCES INCREASE OVERNIGHT.

AN UPPER LEVEL DISTURBANCE MOVING EAST THROUGH WISCONSIN CONTINUES
TO GENERATE A LINE OF THUNDERSTORMS EXTENDING FROM NEAR OSHKOSH
SOUTHWEST TO NEAR DES MOINES ALONG A WEAK COLD FRONT...WITH AN
ADDITIONAL CLUSTER OF SHOWERS AND EMBEDDED THUNDERSTORMS RIGHT
UNDER THE MAIN UPPER CIRCULATION CENTER. LATEST TRENDS INDICATE
THAT THE UPPER LEVEL SYSTEM IS TRACKING ALMOST DUE EAST WHILE THE
MAIN CONVECTIVE LINE SLOWLY SAGS SLIGHTLY SOUTH OF EAST. HAVE
DECREASING CONFIDENCE IN THE POTENTIAL FOR ANY OF THE TS TO REACH
ANY OF TERMINALS EXCEPT FOR ROCKFORD. ALSO...THE LATEST MODEL
GUIDANCE IS SHOWING THAT STEERING FLOW IS TURNING MORE DUE
WESTERLY AS THE UPPER SYSTEM MOVES EWD...WHICH WOULD THEN CAUSE
THE SOUTHERN END OF THE CONVECTIVE LINE TO STRETCH OUT IN A MORE
EAST-WEST ORIENTATION NEAR THE IL/WI BORDER. HOWEVER...THERE IS
STILL SOME DEVELOPMENT ALONG THE SRN END OF THE LINE OVER NERN IA
INTO CNTRL IA...SO CONFIDENCE IS NOT HIGH ENOUGH TO PULL THE TS
FROM THE TAFS FOR THE LATE NIGHT HOURS AS THERE IS STILL SOME
MOISTURE TRANSPORT AND BROAD SCALE LIFT OVER NRN IL WHERE 850MB
WINDS ARE STILL WSWLY. ANOTHER FACTOR POINTING TO A LOWER TS
POTENTIAL IS THE LACK OF STRONG SFC FORCING AND THAT THE SYSTEM IS
ALMOST ENTIRELY RELYING ON UPPER LEVEL FORCING. SO...TO MAKE A
SHORT STORY LONG...WILL MAINTAIN THE TS IN THE TAFS BTWN
08-11Z...AND REFINE THE FORECAST WHEN CONFIDENCE IS A LITTLE
HIGHER.

WHAT THUNDER THAT DOES MANAGE TO MOVE SOUTH OF THE WISCONSIN
BORDER SHOULD DISSIPATE BEFORE DAYBREAK...BUT WITH A MOIST AIRMASS
IN PLACE AND THE MAIN SYNOPTIC SCALE UPPER LOW MOVING OVER NRN
WISCONSIN BY MID DAY TUESDAY...THERE WILL STILL BE ADEQUATE
CYCLONIC FLOW ALOFT THAT SCT SHRA AND ISOLD TSRA COULD STILL
DEVELOP DURG THE DAYTIME HOURS. CHANCES FOR MORE ORGANIZED TS WILL
INCREASE THROUGH THE AFTERNOON AS ANOTHER MID LEVEL IMPULSE TRACKS
AROUND THE SRN PERIPHERY OF THE MAIN UPPER LOW.

AS FOR WINDS...LIGHT SWLY WINDS WILL CONTINUE IN ADVANCE OF THE
WEAK COLD FRONT...AND INCREASE THROUGH THE MORNING AS THE SFC
PRESSURE GRADIENT STRENGTHENS. BY LATE MORNING...SWLY WINDS SHOULD
INCREASE TO 10-15KT WITH OCNL GUSTS OF 15-20KT PERSISTING THROUGH
EARLY EVENING WHILE GRADUALLY VEERING TO WESTERLY BY LATE
AFTERNOON.

//ORD AND MDW CONFIDENCE...UPDATED 09Z...

* LOW CONFIDENCE IN THE CHANCES FOR TS DIRECTLY IMPACTING THE
  TERMINALS...THOUGH SOME ISOLD SHRA/TSRA IS POSSIBLE INVOF
  ORD/MDW.

* MEDIUM TO HIGH CONFIDENCE IN WIND SPEED AND DIRECTION...THOUGH
  THE WIND FIELD COULD BE LOCALLY DISRUPTED BY ANY TS THAT DO
  DEVELOP.

* LOW IN MVFR CIGS OCCURRING LATE TONIGHT INTO TUE MORNING.

* LOW IN WHETHER TSRA OCCURS TUESDAY AND IN SPECIFIC TIMING.

KREIN

//OUTLOOK FOR ORD/MDW FOR 12Z WEDNESDAY-12Z MONDAY...UPDATED 00Z...

WEDNESDAY...CHANCE OF TSRA AND LOWER CIGS DURING THE NIGHT.
THURSDAY-SATURDAY...PERIODS OF TSRA POSSIBLE.
SUNDAY...SLIGHT CHANCE OF TSRA.

MTF

&&

.MARINE...
316 AM CDT

AN AREA OF LOW PRESSURE OVER CENTRAL WISC WILL CONTINUE TO DRIFT
NORTHEAST ACROSS THE NORTHERN PORTION OF THE LAKE MIDDAY...THEN
CONTINUE TO SLIDE EAST AND WEAKEN WEDNESDAY AFTERNOON ACROSS
SOUTHEAST ONTARIO. THE GRADIENT IS EXPECTED TO TIGHTEN SLIGHTLY
FROM THE SOUTH...WITH WINDS INCREASING THIS MORNING TO ARND 15 TO
25KT. THEN AS THE LOW SLIDES EAST WINDS WILL BECOME WEST AND
DIMINISH TO ARND 10 TO 20 KT LATE TONIGHT. WAVES SHUD BE ABLE TO
BRIEFLY BUILD ACROSS THE LAKE TO ARND 2-4FT...THEN WITH A LIGHTER
GRADIENT ARRIVING WED WAVES WILL SUBSIDE TO ARND 2 FT OR LESS. IN
THE DEPARTING SFC LOW WILL BE A MUCH WEAKER GRADIENT...WITH WINDS
GENERALLY LIGHT FROM THE NORTH ARND 5 TO 10 KT ACROSS THE NORTHERN
HALF OF THE LAKE AND AROUND 10 KT FROM THE SOUTH FOR THE SOUTHERN
HALF OF THE LAKE. A WARM FRONTAL BOUNDARY WILL LIFT NORTH ACROSS
THE SOUTHERN PORTION OF THE LAKE EARLY THUR...WITH MUCH WARMER AIR
LIFTING OVER THE LAKE. AT THIS TIME FEEL THAT FOG MAY NOT
DEVELOP...BUT COULD SEE SOME HAZY CONDITIONS DEVELOP LATE IN THE
WEEK. SOUTH/SOUTHEAST WINDS FOR THE WEEKEND. THE UPPER FLOW
PATTERN DOES LOOK TO SHIFT SUNDAY OR MONDAY BRINGING A COLD FRONT
ACROSS THE LAKE WITH A SHIFT TO NORTHERLY WINDS WHICH MAY BE
RATHER BREEZY EARLY NEXT WEEK.

BEACHLER

&&

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

$$

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