Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 220240
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
940 PM CDT THU AUG 21 2014

.DISCUSSION...
939 PM CDT

HAVE MADE MINOR CHANGES TO FORECAST FOR REMAINDER OF TONIGHT.
PRIMARY CONCERN IS WITH ANOTHER ROUND OF SHOWERS/STORMS THAT
COULD DEVELOP OVERNIGHT. THERE IS CONFLICTING SIGNALS ON LATEST
HI-RES GUIDANCE AND NEW 00Z NAM...BUT SUBTLE WAVE ON W/V IMAGERY
OVER EASTERN IA ALONG WITH CONVERGENCE ON NOSE OF LOW LEVEL JET IN
WAA PATTERN EXPECTED TO RESULT IN MORE WIDESPREAD SHRA/TSRA
DEVELOPMENT AFTER MIDNIGHT. IT APPEARS THAT FOCUS WILL BE ON
SOUTHEASTERN HALF TO THIRD OF CWA AND LESS SO IN FAR NORTHERN
CWA...SO HAVE TAPERED POPS TO CHC FOR OUR NORTHERN TIER. GENERALLY
MAINTAINED LIKELY POPS SOUTH OF THERE...WITH HIGHEST LIKELIES IN
SOUTHEASTERN THIRD OF CWA. WITH PWAT VALUES RANGING FROM 1.75" AT
ILX TO A BIT UPWARDS OF 2" AT DVN ON 00Z RAOB...NEEDLESS TO
SAY...ANY STORM WILL BE CAPABLE OF PRODUCING TORRENTIAL DOWNPOURS
AND LOCALIZED FLOODING.

ADDITIONAL CONCERN IS FOR AREAS OF FOG...LOCALLY DENSE INVOF WARM
FRONT BOUNDARY DRAPED ACROSS FAR NE IL. AS OF THIS WRITING...KUGN
IS REPORTING 1/4 MILE VSBY IN FOG...THOUGH LAKE COUNTY WEB CAMS DO
NOT SHOW WIDESPREAD DENSE FOG. HAVE ADDED AREAS OF FOG INTO GRIDS
IN THE FAR NORTHEAST...AND WILL MONITOR FOR DETERORIATING
VISIBILITY...ESPECIALLY IF SHRA/TSRA DO NOT MATERIALIZE OVERNIGHT.
PATCHY GROUND FOG IS POSSIBLE FOR REMAINDER OF CWA GIVEN VERY
MOIST AIR MASS IN PLACE AND LOW DEW POINT DEPRESSIONS.

OTHERWISE...GOING FORECAST LOWS APPEAR TO BE IN GOOD SHAPE.

RC

&&

.PREV DISCUSSION...
335 PM CDT

MAIN FORECAST CONCERNS ARE WITH SHOWER/THUNDERSTORM TRENDS THIS
AFTERNOON INTO TONIGHT AS WELL AS CONTINUED CHANCES THROUGH THE
WEEKEND...AND WITH CONTINUED WARMING WITHIN A VERY HUMID
ENVIRONMENT THAT WILL POSSIBLY BRING INDICES ABOVE 100 ON FRIDAY.

IN THE NEAR TERM...LATEST RADAR IMAGERY SHOWING SHOWERS AND
THUNDERSTORMS CONTINUING TO WORK EAST THIS AFTERNOON MAINLY ACROSS
NORTHERN ILLINOIS...WITH ISOLATED SHOWER DEVELOPMENT ALONG I-80
CORRIDOR. SUBSTANTIAL CLOUD COVER OVER NORTHERN ILLINOIS HAS
LIMITED HEATING/INSTABILITY WITH ANY THUNDERSTORM DEVELOPMENT
REMAINING ON THE WEAKER SIDE TODAY....BUT WITH VERY MOIST
CONDITIONS IN PLACE STILL ENABLING FOR MODERATE TO AT TIMES HEAVY
RAINFALL. DESPITE SOME DIMINISHING TREND IN COVERAGE WITH THIS
PRECIP SHIELD...EXPECT THIS TO CONTINUE MOVING EAST THROUGH THE
CWA LATE THIS AFTERNOON INTO THE EARLY EVENING...AS SHORTWAVE
TROUGH HELPING TO DRIVE THIS CONVECTION CONTINUES TO MOVE THROUGH
EASTERN IOWA AND NORTHERN ILLINOIS. HAVE MAINTAINED HIGHER POPS
FOR THIS AFTERNOON INTO THE EVENING PRIMARILY FOR NORTHERN
ILLINOIS WITH THIS GENERALLY EASTWARD MOVEMENT. DID MAKE SOME LAST
MINUTE CHANGES TO POPS FOR EARLY THIS EVENING TO MAINTAIN SOME
HIGHER CHANCE TO LIKELY POPS FOR NORTHEAST/EAST CENTRAL ILLINOIS
AND NORTHWEST INDIANA. AS THIS WAVE CONTINUES EAST THIS AFTERNOON
IT WILL MOVE OVER A LINGERING BOUNDARY STRETCHED FROM NORTHEAST
ILLINOIS TO NORTHWEST INDIANA AND FOR AREAS ALONG AND SOUTH OF
INTERSTATE 80 ALONG THIS BOUNDARY...MORE HEATING HAS ALLOWED FOR
WEAK TO MODERATE INSTABILITY WITH LITTLE TO NO CIN TO DEVELOP.
THIS ALL COULD SERVE TO PROVIDE FOCUS FOR AN INCREASE IN COVERAGE
AND INTENSITY OF THESE SHOWERS AND THUNDERSTORMS...ESPECIALLY FOR
AREAS IN EAST CENTRAL ILLINOIS AND NORTHWEST INDIANA. NOT OUT OF
THE QUESTION FOR AN ISOLATED STRONG TO MAYBE EVEN SEVERE STORM TO
DEVELOP LATE THIS AFTERNOON...WITH DECENT BULK SHEAR VALUES OF
30-40KT IN PLACE. LIKELY HAZARDS WILL CONTINUE TO BE HEAVY
RAINFALL...WITH STRONG WIND GUSTS POSSIBLE AS WELL.

FORCING AND THIS PRECIP WILL SHIFT EAST LATER THIS EVENING WITH A
LULL IN ANY WIDESPREAD ACTIVITY LIKELY OCCURRING. PERSISTENT WEAK
FORCING IN THIS VERY MOIST ENVIRONMENT COULD ALLOW FOR A STRAY
SHOWER OR THUNDERSTORM DURING LATE THIS EVENING BUT THINK THIS
WOULD BE THE EXTENT OF ANY PRECIP BEFORE FOCUS BEGINS TO SHIFT
BACK WEST TOWARDS THE MIDNIGHT TIME FRAME. STILL SOME UNCERTAINTY
WITH HOW THE SITUATION WILL PLAY OUT LATE TONIGHT INTO EARLY
FRIDAY MORNING...BUT A LIKELY SCENARIO WILL BE FOR THE DRIER
CONDITIONS OBSERVED LATER THIS EVENING TO BEGIN FILLING BACK IN
WITH SHOWERS/THUNDERSTORMS OVER THE WESTERN CWA. THE DEPARTURE OF
THE WAVE THIS AFTERNOON AND EVENING WILL LEAVE RATHER WEAK LARGE
SCALE LIFT ACROSS THE AREA..BUT WITH AN INCREASING LLJ TO BEGIN
RAMPING UP WAA THROUGHOUT THE REGION. ONE AREA OF FOCUS WILL BE TO
THE NORTH AND NORTHEAST LATER TONIGHT...BUT WITH A SEPARATE AREA
OF STRONGER WAA DEVELOPING OVER WEST CENTRAL ILLINOIS. THIS SHOULD
BE ENOUGH LIFT IN A MODERATELY UNSTABLY ENVIRONMENT FOR DEVELOPING
SHOWERS AND THUNDERSTORMS IN THIS AREA TO SPREAD NORTHEAST OVER
THE REMAINING CWA INTO EARLY FRIDAY MORNING. CONFIDENCE WITH
OVERALL COVERAGE IS LOW BUT WITH FOCUS LIKELY TO BE THROUGHOUT THE
CWA TONIGHT...DID BRING LIKELY POPS OVER THE ENTIRE CWA TOWARDS
EARLY FRIDAY MORNING. VERY HIGH PWATS OF 1.8-2.0 INCHES AND VERY
HIGH SURFACE/805MB DEWPOINTS WILL ONCE AGAIN PROVIDE HEAVY
RAINFALL CONCERNS...WITH THIS LIKELY THE MAIN HAZARD TONIGHT. A
CHALLENGING COMPLEX SCENARIO WITH CONFIDENCE ONCE AGAIN LOW ON THE
OVERALL EVOLUTION OF ANY PRECIP TONIGHT WILL LIMIT ANY FLOOD
HEADLINES THIS AFTERNOON...BUT WITH THIS CONTINUING TO BE
MONITORED.

FORCING/PRECIP WILL SHIFT EAST THROUGH FRIDAY MORNING AS LLJ
CONTINUES TO VEER. THIS WILL BRING LOWERING CHANCES FOR PRECIP
THROUGH MID DAY AND EARLY AFTERNOON. A COLD FRONT WILL BE DROPPING
DOWN TOWARDS THE CWA ON FRIDAY THOUGH...PUSHING THROUGH THE CWA IN
THE AFTERNOON. THIS COULD BE A FOCUS FOR ADDITIONAL THUNDERSTORM
DEVELOPMENT OVER THE CENTRAL CWA DURING THIS TIME...ESPECIALLY
WITH HIGH DEWPOINT AIR IN PLACE. CONTINUED WARMING ALOFT WILL
PROVIDE A DECENT CAP ON FRIDAY AND HELP LIMIT DEVELOPMENT.
DEVELOPMENT WILL LIKELY OCCUR ON FRIDAY BUT DO THINK IT WILL BE IN
LIMITED COVERAGE...AND SO DID NOT MAKE ANY CHANGES TO THE LOW
CHANCE POPS IN PLACE. WILL NEED TO MONITOR CLEARING SKIES ON
FRIDAY WITH A VERY WARM AIR MASS IN PLACE POSSIBLY BRINGING 90
DEGREE WARMTH TO THE CWA. WITH GUIDANCE ADVERTISING MID TO EVEN
UPPER 70 DEG DEWPOINTS ON FRIDAY...HEAT INDICES COULD BE IN EXCESS
OF 100. CLOUD COVER AND PRECIP CHANCES ON FRIDAY COULD BE A
LIMITING FACTOR...BUT WITH THIS HEATING A DEFINITE POSSIBILITY FOR
SOME OF THE CWA...ESPECIALLY OVER THE SOUTHERN HALF OF THE CWA.

RODRIGUEZ

LONG TERM...SUNDAY NIGHT THROUGH THURSDAY...

THE MID SUMMER PATTERN WILL CONTINUE THROUGH AT LEAST WEDNESDAY NEXT
WEEK BEFORE A STRONG UPPER TROUGH DIGS INTO THE GREAT LAKES REGION
AND PUTS AN END TO THE EXTENDED STRETCH OF HEAT AND HUMIDITY.

SUNDAY THROUGH TUESDAY...
ENSEMBLES ARE EXHIBITING HIGH CONFIDENCE
IN A 590+ DAM H5 RIDGE CENTERED OVER THE LOWER AND MID MS VALLEY
SUNDAY NIGHT BEFORE BEING FLATTENED/SHUNTED EAST MONDAY INTO TUESDAY
BY DISTURBANCES EMANATING FROM UPPER TROUGH FROM THE CANADIAN
PRAIRIES TO THE NORTHERN ROCKIES. 850/925 MB CLIMO ON MONDAY-TUESDAY
SUPPORTS HIGHS OF AT LEAST AROUND 90/LOW 90S...BUT MAIN UNCERTAINTY
IS EXTENT OF CONVECTIVE ACTIVITY. SYNOPTIC SET-UP WITH FLATTENING
RIDGE FAVORS MAINTAINING LOW TO MID CHANCE POPS ON MON-TUE AS SW
FLOW DISTURBANCES COULD KICK OFF SHRA/TSRA IN PRESENCE OF MINIMAL
CAPPING PER SOUNDINGS AND CONTINUED LOW-MID 70S DEW POINTS. ANY TSRA
COULD PRODUCE HEAVY DOWNPOURS GIVEN PROGGED PWAT VALUES IN 1.5 TO
1.75 INCH RANGE.

AT THIS TIME...IT WOULD APPEAR THAT MONDAY HAS A BETTER CHANCE TO
HAVE LESS IN THE WAY OF CONVECTION AND CLOUDS. IF THIS
OCCURS...FORECAST HIGH TEMPS COULD EVEN BE A BIT CONSERVATIVE...WITH
GUIDANCE INDICATING 850 MB TEMPS IN THE LOW 20SC AND 925 MB TEMPS IN
THE MID 20SC. BOTH DAYS COULD HAVE SOME LAKE COOLING GIVEN FAIRLY
WEAK GRADIENT AND FLOW ALOFT. IF CLOUDS/CONVECTION DO NOT LIMIT
WARMING POTENTIAL TOO MUCH...THEN BOTH DAYS ARE LIKELY TO HAVE HEAT
INDEX VALUES APPROACHING OR EXCEEDING 100 DEGREES FOR AT LEAST
PORTIONS OF THE CWA.

TUESDAY NIGHT THROUGH THURSDAY...
SPREAD INCREASES ON THE ENSEMBLES AND OPERATIONAL MODELS WITH
RESPECT TO SPEED AT WHICH DEEP UPPER TROUGH AND ASSOCIATED SURFACE
LOW DIG INTO GREAT LAKES REGION. HAVE CONCERN IN THIS PERIOD EVEN
THIS FAR OUT FOR HEAVY RAIN/FLOODING POTENTIAL...AS POSITION OF CWA
BETWEEN DEEP UPSTREAM TROUGH AND STRONG DOWNSTREAM RIDGE IS A
SYNOPTICALLY FAVORABLE SET-UP FOR STRONG LLJ/MOISTURE TRANSPORT.
PWAT VALUES COULD NEAR 2 INCHES. INCREASED FLOW ALOFT WILL ALSO
RESULT IN INCREASED DEEP LAYER SHEAR SUPPORTIVE OF STRONG/SEVERE
STORMS PENDING MESOSCALE COMING TOGETHER. 12Z ECMWF IS ACTUALLY ON
FASTER END OF SPECTRUM COMPARED TO GFS AND ENSEMBLE MEAN WITH
PROGRESSION OF SYSTEM...SO GENERALLY MAINTAINED CHANCE POPS FROM
BLENDED GUIDANCE DUE TO UNCERTAINTY. TEMPERATURES WEDNESDAY WILL BE
DICTATED BY TIMING...AS UPSTREAM AIR MASS IS MUCH
COOLER...ESPECIALLY ON LATEST ECMWF. THURSDAY TEMPS COULD RANGE FROM
NEAR TO SLIGHTLY BELOW SEASONABLE TO WELL BELOW NORMAL...BUT DID NOT
STRAY AWAY FROM SMART BLEND.

RC

&&

.AVIATION...

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

* ANOTHER ROUND OF STORMS OVERNIGHT WITH HEAVY RAINFALL POSSIBLE.

* EAST-SOUTHEAST WINDS INTO THIS EVENING...LIKELY UNDER 10 KT.

* POTENTIAL EAST WINDS FRIDAY AFTERNOON BUT GENERALLY UNDER 10 KT.

* ANOTHER CHANCE FOR SOME SCT STORMS FRIDAY AFTERNOON.

KJB

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

THE MAIN AVIATION WEATHER CONCERNS TONIGHT AND INTO FRIDAY MORNING
WILL CONTINUE TO FOCUS ON THE THREAT FOR ANOTHER ROUND OF CONVECTIVE
ACTIVITY.

CONVECTION IS ONGOING TO THE SOUTHEAST OF THE TERMINALS EARLY THIS
EVENING. THIS ACTIVITY WILL CONTINUE TO SHIFT EAST-SOUTHEASTWARD
OVER THE NEXT COUPLE HOURS. HOWEVER...ADDITIONAL CONVECTIVE ACTIVITY
WILL LIKELY DEVELOP ACROSS THE AREA TONIGHT. CONFIDENCE ON THE EXACT
TIMING OF THIS ACTIVITY IS SOMEWHAT LOW AT THIS TIME...BUT IT DOES
APPEAR CHANCES SHOULD INCREASE AFTER  04-05 UTC THIS EVENING AND
CONTINUE OVERNIGHT. PERIODS OF HEAVY RAINFALL COULD LEAD TO LOCALLY
LOWER MVFR CIGS AND LOW VISIBILITIES DOWN TO A MILE OR TWO.

CONVECTIVE ACTIVITY SHOULD WANE ACROSS THE AREA DURING THE MORNING.
HOWEVER...ADDITIONAL SCATTERED STORMS ARE AGAIN POSSIBLE IN THE
AFTERNOON AS A COLD FRONT AND LAKE BREEZE BOUNDARY MOVES IN ACROSS
NORTHERN ILLINOIS. I ADDED A PROB30 GROUP IN FROM 20 TO 22 UTC TO
COVER THIS THREAT. HOWEVER...THIS TIMING MAY NEED TO BE TWEAKED IN
FUTURE UPDATES.

A NORTHEASTERLY WIND SHIFT APPEARS LIKELY BY MID TO LATE AFTERNOON
ACROSS THE EASTERN TERMINALS IN THE WAKE OF THE LAKE BREEZE
BOUNDARY. WIND SPEEDS LOOK TO GENERALLY REMAIN AT OR UNDER 10 KT.


KJB

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

* MEDIUM IN THUNDER CHANCES LATER TONIGHT...BUT LOW ON EXACT TIMING.

* MEDIUM IN LAKE BREEZE DEVELOPMENT FRIDAY AFTERNOON. MEDIUM IN
  SPEEDS REMAINING UNDER 10 KT.

* LOW-MEDIUM IN SCT STORMS AGAIN FRIDAY AFTERNOON...BUT LOW WITH
  THE EXACT TIMING.

KJB

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

* SATURDAY THROUGH WEDNESDAY...PERIODIC CHANCES FOR TSRA/SHRA WITH
  OCNL MVFR CIGS/VIS.

KJB

&&

.MARINE...
205 PM CDT

THE WEATHER PATTERN WILL NOT CHANGE MUCH OVER LAKE MICHIGAN
THROUGH THIS WEEKEND WITH LOW PRESSURE ACROSS THE CENTRAL PLAINS AND
HIGH PRESSURE OVER ONTARIO. A FRONT EXTENDING EAST FROM THE LOW IS
PRESENTLY DRAPED ACROSS THE FAR SOUTHERN PART OF THE LAKE AND THIS
WILL MOVE LITTLE THROUGH EARLY FRIDAY BEFORE SHIFTING BACK
SOUTH OF THE LAKE ON FRIDAY NIGHT. SOME THUNDERSTORMS ACROSS THE
SOUTHERN PART OF THE LAKE LATE THIS AFTERNOON MAY CAUSE SOME
VARIABILITY AND SLIGHTLY ENHANCED GUSTS IN THE WINDS...BUT ANY OF
THIS SHOULD BE TEMPORARY. THE WINDS WILL THEN TURN SOUTHERLY
ACROSS THE ENTIRE LAKE BY SUNDAY WITH THE LOW PRESSURE LIFTING
NORTH INTO SOUTHERN CANADA BY SUNDAY NIGHT. SPEEDS WILL INCREASE
DURING THE MIDDLE OF NEXT WEEK BEFORE A SURFACE TROUGH / COOL
FRONT PASSES.

MTF

&&

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

$$

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