Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 182350
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
650 PM CDT MON AUG 18 2014

.DISCUSSION...

650 PM...EARLY EVENING UPDATE...CONVECTION FIRING ACROSS CENTRAL
WI WILL CONTINUE THIS EVENING WITH ADDITIONAL THUNDERSTORMS
RAPIDLY DEVELOPING OVER FAR NORTHERN IA AND SOUTHERN MN. THIS
ACTIVITY WILL SLOWLY SAG SOUTH BUT PROBABLY NOT REACH NORTHERN IL
UNTIL AFTER 04Z-05Z. ACTIVITY WILL THEN SPREAD SOUTHEAST ACROSS
MAINLY THE NORTHERN TWO THIRDS OF THE CWA OVERNIGHT. SHORT TERM/
HI RES GUIDANCE IS IN FAIR AGREEMENT WITH MUCH OF THE AREA NOW
STAYING DRY THROUGH MIDNIGHT. HAVE UPDATED GRIDS/ZFP FOR THESE
TRENDS ALONG WITH MOSTLY CLEAR SKIES OVER MUCH OF THE AREA AS CU
FIELD DISSIPATES...THOUGH HIGH CLOUDS WILL LIKELY QUICKLY SPREAD
ACROSS THE AREA LATER THIS EVENING AS STORMS APPROACH. NO CHANGES
TO LIKELY POPS OVERNIGHT THOUGH POPS MAY NEED TO BE LOWERED ACROSS
THE FAR SOUTH AS TRENDS EMERGE. CMS

&&

.PREV DISCUSSION...
239 PM CDT

SYNOPSIS...A WARMER AND NOTICEABLY MORE HUMID PATTERN WILL BE
DEVELOPING THROUGH MID WEEK. IN ADDITION TO THE WARMTH AN EVOLVING
UPPER AIR PATTERN WILL BRING PERIODIC CHANCES FOR SHOWERS AND
THUNDERSTORMS THROUGH THE WEEKEND. HIGH TEMPERATURES LATER IN THE
WEEK MAY BE DICTATED BY THE CONVECTION.

TONIGHT...
A LOW PRESSURE TROUGH IS OVER MINNESOTA AND IT WILL BECOME A CLOSED
LOW OVER WISCONSIN AND LAKE SUPERIOR TUESDAY EVENING.  AT THE
SURFACE THERE IS A LOW PRESSURE TROUGH OVER MINNESOTA THAT WILL
BECOME A CLOSED LOW OVER WISCONSIN TOMORROW MORNING.  THE LOW WILL
PUSH EAST OF THE LAKE WEDNESDAY.

EXPECTING THE FIRST ROUND OF STORMS TO MOVE INTO THE RFD AREA THIS
EVENING WITH THE MAJORITY OF CONVECTIVE ACTIVITY REMAINING OVER
WISCONSIN CLOSER TO THE LOW.  THE FIRST WAVE WILL WEAKEN AS IT
PUSHES EAST.  A SECOND WAVE FORMS AHEAD OF A VORT STREAMER OVERNIGHT
AND THINKING THIS WILL BE THE MAIN SHOW TONIGHT.  EXPECTING ACTIVITY
TO REMAIN NORTH OF I-80 MOST OF THE NIGHT. ACTIVITY WILL THEN SPREAD
SOUTH AND EAST COVERING AREAS EAST OF I-39 EARLY TUESDAY MORNING.
THIS ACTIVITY WILL WEAKEN THROUGH THE MORNING AND CONTINUE SOUTHEAST
OUT OF THE WARNING AREA.

THE FIRST CONCERN OVERNIGHT IS HEAVY RAIN AS PWAT VALUES WILL BE
BETWEEN 1.5 AND 1.75 INCHES.  CAPE VALUES WILL BE AROUND 1000 J/KG
THIS EVENING AND THEN DIMINISH OVERNIGHT.  SHEAR VALUES ARE 20-30 KT
SO SEVERE WEATHER IS NOT EXPECTED...BUT SMALL HAIL MAY OCCUR WITH
THE STRONGEST STORMS. STEERING LEVEL FLOW IS ALSO WEAK SO STORMS
WILL BE SLOW MOVING WHICH COULD ENHANCE RAINFALL TOTALS.

THE OTHER CONCERN IS FOG TONIGHT. IF WE CLEAR OUT AT ALL EXPECTING
AREAS OF DENSE FOG TO DEVELOP. HAVE MEDIUM CONFIDENCE IN SKY COVER
SO ONLY HAVE PATCHY FOG IN THE FORECAST TONIGHT AFTER MIDNIGHT.

TUESDAY AND TUESDAY NIGHT...
MORE STORMS ARE EXPECTED AS THE LOW SHIFTS EAST OVER WISCONSIN.
EXPECTING FOG/LOW STRATUS/GENERAL CLOUD COVER TO THIN TUESDAY
MORNING. HEATING WILL RESULT IN DECENT CAPE VALUES. THE MOISTURE
RICH NAM FEATURES THE HIGHEST CAPE VALUES AROUND 3000 J/KG BUT THE
MORE CONSERVATIVE GFS ONLY SHOWS LESS THAN 1000 J/KG.  SHEAR VALUES
STILL LOOK MARGINAL AROUND 35 KT.  BUT PWAT VALUES WILL BE A BIT
LOWER AT 1.3 TO 1.5 INCHES.  HEAVY RAIN IS STILL POSSIBLE BUT STORMS
SHOULD BE MOVING AT A BETTER PACE WITH STRONGER STEERING LEVEL
FLOW.  THE FORCING WILL BE A COMBO OF THE LOWS FRONT THAT MOVES
THROUGH IN THE LATE AFTN/EVENING AND VORTICITY STREAMERS THAT ROTATE
AROUND THE LOW.  HAVE MEDIUM CONFIDENCE IN WIDESPREAD TS
COVERAGE...BUT KEPT POPS IN THE CHANCE RANGE DUE TO LOW CONFIDENCE
IN WHEN AND WHERE STORMS WILL DEVELOP. HAIL AND STRONG WINDS ARE
POSSIBLE IN ADDITION TO THE HEAVY DOWNPOURS.

TEMPS REMAIN A CHALLENGE AS WE COULD BE WARMER IF SKY COVER IS ON
THE WEAKER SIDE. HOWEVER...HAVE MEDIUM CONFIDENCE IN WIDESPREAD TS
COVERAGE SO KEPT HIGH TEMPS IN THE LOW TO MID 80S.

JEE

WEDNESDAY THROUGH SUNDAY...
THE UPPER LOW OVER THE UPPER GREAT LAKES WILL START TO MOVE MORE
QUICKLY TO THE NORTHEAST WEDNESDAY MORNING AS NORTHERN PACIFIC
TROUGHING PUSHES INLAND OVER WESTERN CANADA. THIS HELPS TO PUSH
CUTOFF LOW PRESSURE ONTO THE CALIFORNIA COAST WITH BOTH OF THESE
FEATURES HELPING TO DRIVE UPPER RIDGE DEVELOPMENT OVER THE HIGH
PLAINS. THE LOCAL AREA WILL BE BETWEEN THE DEPARTING UPPER LOW AND
THE APPROACHING RIDGE WEDNESDAY MORNING WITH WEAK LOW LEVEL FLOW
IN PLACE AS WELL. THIS SHOULD RESULT IN DRY WEATHER FOR MUCH OF
WEDNESDAY BEFORE MID LEVEL BAROCLINICITY INCREASES LATE IN THE DAY
AND MORE SO IN THE EVENING AS THE RIDGE APPROACHES. PRECIPITABLE
WATER VALUES WILL INCREASE MARKEDLY WITH THE ARRIVAL OF THE MID
LEVEL WARM ADVECTION LEADING TO MOIST ASCENT WEDNESDAY EVENING AND
WEDNESDAY NIGHT. THIS LIFT ALONE MAY BE ENOUGH TO PRODUCE SHOWERS
AND THUNDERSTORMS AND MAY BE AIDED BY THE PASSAGE OF AN UPPER WAVE
WHICH GUIDANCE HAS BEEN HINTING AT. THIS IS CERTAINLY PLAUSIBLE
GIVEN THE PRESENCE OF THE DEEP TROUGH TO THE WEST AND THE TENDENCY
FOR WAVES TO EMANATE FROM IT. WILL CARRY HIGHEST POPS LATE
WEDNESDAY EVENING THROUGH THURSDAY MORNING. THE RIDGE AXIS WILL
SHIFT EASTWARD ACROSS THE AREA THROUGH FRIDAY. THE AMPLITUDE OF
THE RIDGE AND THEREFORE THE POSITIONING OF THE TRACK OF
SHORTWAVES/THUNDERSTORMS ALONG ITS NORTHERN PERIPHERY WILL BE
DRIVEN BY THE SPEED WITH WHICH THE LARGE WESTERN UPPER TROUGH
EVOLVES EASTWARD. THE GFS CONTINUES TO BE MORE PROGRESSIVE WITH
THE TROUGH AND THEREFORE SHARPER WITH THE RIDGE SUGGESTING THAT
THE MAIN STORM TRACK WILL BE NORTH OF THE LOCAL AREA FRIDAY
WHEREAS THE SLOWER AND LOWER AMPLITUDE ECMWF KEEPS THE FAVORED
TRACK ACROSS THE AREA INTO THE WEEKEND. THE UPPER TROUGH WILL
EVOLVE EASTWARD PUSHING THE RIDGE TO THE EAST AND BRINGING A COOL
FRONT INTO THE REGION LATE IN THE WEEKEND. THE BIGGEST UNCERTAINTY
IS THE TIMING. WITH PRECIPITABLE WATER VALUES OVER 2 INCHES FROM
THURSDAY ONWARD HEAVY RAINFALL WILL BE A CONCERN WITH ANY
STORMS...ESPECIALLY IF MULTIPLE ROUNDS ARE ABLE TO AFFECT THE AREA
THROUGH THE WEEKEND. HOWEVER A SHARPER RIDGE WOULD RESULT IN A
SHIFT IN THE TRACK TO THE NORTH AND STRONGER CAPPING LOCALLY.

TEMPERATURES WILL BE WARMING INTO THE MID 80S WEDNESDAY AND MOSTLY
MID AND UPPER 80S FROM THURSDAY INTO THE WEEKEND. LAKESHORE AREAS
WILL LIKELY BE COOLER MOST DAYS WITH LAKE INFLUENCES EXPECTED DUE TO
A RATHER LIGHT LOW LEVEL FLOW. THUNDERSTORMS AND ANY REMNANT
INFLUENCES MAY NEGATIVELY AFFECT HIGHS DEPENDING ON THEIR TIMING.
CANNOT RULE OUT SOME 90 DEGREE READINGS IN THE WEST AND SOUTH LATE
IN THE WEEK...ESPECIALLY IF THUNDERSTORM ACTIVITY KEEPS AT A
MINIMUM.

MDB

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 00Z...

* CHANCE FOR TSRA BY EARLY OVERNIGHT WITH TSRA LIKELY LATE TONIGHT
  INTO EARLY TUE MORNING.

* POTENTIAL FOR LOWER CIGS LATE TONIGHT AND TUE MORNING.

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

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

A QUIET EVENING IS EXPECTED OVER LOCAL AIRPORTS BEFORE TSRA
CHANCES INCREASE OVERNIGHT. MUCH OF THE CUMULUS IS EXPECTED TO
FADE WITH SOUTH TO SOUTHWEST WINDS. REGENERATING STORMS ACROSS
CENTRAL WISCONSIN WILL LIKELY EVOLVE SLOWLY SOUTHEAST...HOWEVER
THESE ARE NOT ANTICIPATED TO REACH ALL THE WAY DOWN INTO NORTHERN
IL. EVEN IF THEY WERE TO BY LATE THIS EVENING...THEY SHOULD SLOWLY
FADE IN INTENSITY.

AN UPPER LEVEL DISTURBANCE OVER MN WILL ROTATE ITS WAY EAST-
SOUTHEAST WITH THE SOUTHERN AREA OF FORCING MOVING OVER NORTHERN
IL AND NORTHWEST IN LATE TONIGHT. COMPLIMENTING THIS WILL BE A
MOIST LOW-LEVEL AIR MASS THAT WILL FURTHER BE MOISTENING OVERNIGHT
AND COULD BLOSSOM MVFR CIGS OVERNIGHT. STORMS ARE EXPECTED TO BE
SCATTERED TO NUMEROUS IN COVERAGE AS THEY PROGRESS EASTWARD OVER
THE AREA. BY DAYBREAK WHAT IS LEFT OF THESE SHOULD BE MAINLY EAST
OF THE TERMINALS...HOWEVER REDEVELOPING SHOWERS AND POSSIBILITY A
FEW STORMS COULD OCCUR TUESDAY MORNING. AS THE SYSTEM COLD FRONT
MOVES THROUGH DURING THE AFTERNOON...SCATTERED REDEVELOPMENT OF
STORMS IS PROBABLE...BUT PLACEMENT OF THAT IS DIFFICULT TO TELL AT
THIS POINT. SOUTHWEST WINDS WILL TURN WESTERLY WITH THIS FRONTAL
PASSAGE WITH SOME GUSTS POSSIBLE BOTH AHEAD AND BEHIND THE FRONT.

//ORD AND MDW CONFIDENCE...UPDATED 00Z...

* MEDIUM IN TSRA OCCURRING LATE TONIGHT AND IN TIMING/DURATION.

* HIGH IN WIND DIRECTION AND MEDIUM IN WIND SPEED THROUGH MUCH OF
  THE TAF. THE EXCEPTION WOULD BE DURING AND IMMEDIATELY AFTER ANY
  STORMS...BUT ANY SHIFT FROM SOUTHWEST SHOULD BE TEMPORARY.

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

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

//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...
239 PM CDT

LOW PRESSURE OVER MINNESOTA WILL MOVE OVER WISCONSIN TOMORROW AND
THEN OVER THE LAKE TOMORROW EVENING AND NIGHT.  WINDS WILL BECOME
SOUTH ACROSS THE LAKE AND INCREASE TO 15 TO 25 KT OVERNIGHT.  AS THE
LOW MOVES OVER THE LAKE TOMORROW...WINDS WILL DIMINISH SLOWLY OVER
THE SOUTH HALF AND BECOME SW. ON THE NORTH HALF...WINDS WILL ALSO
DIMINISH AND BECOME SE TO E AHEAD OF THE LOW.  THE LOW SHIFTS EAST
OF THE LAKE TUESDAY NIGHT AND WINDS BECOME NW 10-15 KT.

THE LOW SHIFTING TO THE EAST WILL LEAVE A WEAK PRESSURE GRADIENT
IN ITS WAKE. THIS WILL RESULT IN LIGHT ONSHORE OR VARIABLE WINDS
WEDNESDAY WITH BEFORE A SOMEWHAT BETTER ORGANIZED PRESSURE
GRADIENT SPREADS IN WITH LOW PRESSURE FROM THE CENTRAL HIGH PLAINS
TO NORTHERN MANITOBA. THIS WILL BRING MAINLY SOUTH TO SOUTHEAST
WINDS ACROSS THE LAKE THURSDAY...WITH OCCASIONAL EAST WINDS OVER
THE FAR NORTH...BUT EVEN THEN THE FLOW MAY BE WEAK ENOUGH TO
SUPPORT A SHIFT TO ONSHORE WINDS THURSDAY AFTERNOON. SOUTH TO
SOUTHEAST WINDS ARE FAVORED THROUGH THE WEEKEND BUT THERE MAY BE
QUITE A BIT OF VARIABILITY IF THE LOW PRESSURE TO THE WEST EVOLVES
MORE SLOWLY AS SOME FORECAST GUIDANCE INDICATES. THE UPPER FLOW
PATTERN DOES LOOK TO SHIFT SUNDAY OR MONDAY BRINGING A COOL FRONT
ACROSS THE LAKE WITH A SHIFT TO NORTHERLY WINDS WHICH MAY BE
RATHER BREEZY.

JEE/MDB

&&

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

$$

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