Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 290529
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1229 AM CDT MON JUN 29 2015

.UPDATE...
909 PM CDT

FOR EVENING UPDATE...

OVERALL...HAVE MADE FEW SIGNIFICANT CHANGES TO TONIGHTS FORECAST.
HAVE MADE SOME MINOR TWEAKS TO HOURLY POP/WEATHER GRIDS TO ACCOUNT
FOR RADAR TRENDS...AND LOWERED POPS A BIT BY EARLY MONDAY MORNING
ACROSS THE NORTHWESTERN PART OF THE FORECAST AREA.

AREAS OF LIGHT RAIN...SHOWERS AND A FEW ISOLATED THUNDERSTORMS
CONTINUE TO WORK THEIR WAY SOUTHEAST ACROSS THE REGION THIS
EVENING IN ASSOCIATION WITH AN UPPER TROUGH DROPPING THROUGH THE
MID AND UPPER MISSISSIPPI VALLEY. MAIN SURFACE REFLECTION WITH
THIS FEATURE IS AN AREA OF LOW PRESSURE ALONG THE MISSISSIPPI NEAR
QUINCY IL... TRACKING SOUTHEAST TOWARD STRONGEST SURFACE PRESSURE
FALLS OVER SOUTH CENTRAL IL. LOW IS PROGGED TO TURN MORE TO THE
EAST AS A SHORT WAVE ROUNDS THE BASE OF THE UPPER TROUGH...AND
CONTINUE EAST INTO INDIANA MONDAY MORNING.

EXPECTATION IS THAT SHOWERS WILL CONTINUE ACROSS THE AREA INTO
THE OVERNIGHT HOURS...AS MID-LEVEL SHORT WAVE TRACKS ACROSS THE
FORECAST AREA. AXIS OF GREATEST INSTABILITY MAINLY SOUTH
TOO...WITH 00Z ILX RAOB INDICATING MUCAPE OF NEARLY 1600 J/KG
COMPARED TO 600 J/KG AT DVN...AND RECENT AMDAR SOUNDINGS FROM
MDW/ORD SHOWING FAIRLY WEAK MID-LEVEL LAPSE RATES. STRONGEST
CONVECTION HAS REMAINED ACROSS WESTERN IL...AND WHILE THE
POTENTIAL WILL CONTINUE TO EXIST FOR A FEW ISOLATED THUNDERSTORMS
ESPECIALLY OVER THE SOUTHERN CWA IT APPEARS THE GREATEST
THUNDERSTORM THREAT WILL REMAIN TO OUR SOUTH. DESPITE RELATIVELY
WEAK CONVECTIVE ENVIRONMENT...BOTH DVN AND ILX RAOBS MEASURED JUST
OVER 1.30" OF PRECIPITABLE WATER. HAD SEEN A FEW AREAS OF
WEAK/SLOW MOVING CONVECTION PRODUCE SOME LOCALLY HEAVY RAINFALL
EARLIER...AND THIS WILL CONTINUE TO BE A RISK ESPECIALLY SOUTH OF
I-80 CORRIDOR INTO THE NIGHT.

RATZER

&&

.SHORT TERM...
228 PM CDT

THIS AFTERNOON THROUGH MONDAY...

FOR THE REMAINDER OF THE AFTERNOON AND INTO EARLY EVENING...THE MAIN
ISSUE WILL BE HOW MUCH THE COVERAGE OF SHOWERS AND THUNDERSTORMS
WILL INCREASE AS A STRONG SHORTWAVE CROSSES THE REGION.  LATEST
SATELLITE AND RADAR IMAGERY INDICATES SCATTERED SHOWERS AND ISOLATED
THUNDERSTORMS FORMING OVER NCNTRL IL IN ADVANCE OF THE SHORTWAVE
DROPPING THROUGH THE MIDDLE MISSISSIPPI VALLEY.  AS HIGH PRESSURE
SETTLES ACROSS THE OHIO VALLEY AND SERN CONUS...LOW LEVEL MOISTURE
IS ON THE INCREASE...ESPECIALLY ALONG AND WEST OF THE I-39 CORRIDOR
WHERE SFC DEWPOINTS HAVE REBOUNDED INTO THE LOWER 60S.  ACROSS NERN
IL/NWRN IN...DEWPOINTS REMAIN MUCH LOWER AS THE LOW LEVEL MOISTURE
IS SLOW TO SPREAD EASTWARD.  LATEST RADAR TRENDS INDICATE SOME
SHRA/TSRA FORMING OVER THE AREA OF HIGHER DEWPOINTS...BUT ARE
DISSIPATING AS THEY MOVE TO THE EAST.  PART OF THE REASON THAT
MOISTURE IS SLOW TO RETURN TO THE ERN PORTIONS OF THE CWA IS THAT
THE HIGH PRESSURE OVER THE OHIO VALLEY...WHICH IS MERGING WITH THE
SUB-TROPICAL RIDGE EXTENDING ACROSS THE SERN CONUS THROUGH ERN TX IS
FORCING THE MOISTURE TO TAKE THE LONG WAY HOME...WITH THE MAIN FETCH
OF MOISTURE TRANSPORT ACROSS THE SOUTH HIGH PLAINS AND INTO THE
MIDDLE MISSISSIPPI VALLEY.  AS THE SYSTEM PROGRESSES...ESPECIALLY AS
THE ASSOCIATED ASSOCIATED SFC LOW CROSSES THE MISSISSIPPI RIVER INTO
WESTERN IL...LOW LEVEL MOISTURE SHOULD BE ON THE INCREASE AND EXPECT
THAT SHRA/TSRA COVERAGE WILL INCREASE.  THE LATEST MODEL GUIDANCE
INDICATES THAT PWATS ACROSS THE REGION WILL INCREASE BACK CLOSE TO
1.5 INCHES...SO DEEP LAYER MOISTURE SHOULD BE IN PLACE FOR PCPN
COVERAGE INCREASING.  ALSO...WITH SFC DEWPOINTS INCREASING AND THE
SHORTWAVE MOVING OVERHEAD THROUGH THE EVENING...INSTABILITY THROUGH
THE ATMOSPHERIC COLUMN WILL INCREASE DUE TO INCREASING MID LEVEL
LAPSE RATES INVOF THE SHORTWAVE WHILE INCREASING SFC MOISTURE
CONTRIBUTES TO INCREASING SFC BASED CAPE.  ONE LIMITING FACTOR COULD
BE INCREASING CLOUD COVER LIMITING WARMING...WHILE A LAKE BREEZE
BOUNDARY PUSHING INLAND INTO NERN IL ALSO BRINGS SOME COOLER AIR
INLAND.  HOWEVER...CURRENT THINKING IS THAT PCPN COVERAGE SHOULD
INCREASE ENOUGH TO BRING INCREASING PCPN COVERAGE.

THE INCREASING PWATS SHOULD BRING A CHANCE FOR LOCALLY HEAVY
DOWNPOURS TO JUST ABOUT ANYWHERE THUNDERSTORMS FORM THIS AFTERNOON
AND EVENING...BUT WITH THE SHORTWAVE DIVING SEWD TOWARD CNTRL
IL/IN...FEEL THAT THE SRN PORTIONS OF THE CWA SHOULD SEE THE HIGHEST
AREA AVERAGE RAINFALL AMOUNTS.  ANTICIPATE THAT LOCATIONS SOUTH OF
THE I-80 CORRIDOR COULD SEE AREAL AVERAGE STORM TOTAL PCPN AMOUNTS OF
0.75 TO 1.0 INCHES...WHILE LOCATIONS NORTH OF I-80 SEE A HALF INCH
OR LESS FOR THE PERIOD FROM THIS AFTERNOON THROUGH TOMORROW EVENING.
HAD CONSIDERED THE POSSIBILITY OF A FLOOD WATCH...ESPECIALLY FOR
LOCATIONS THAT HAVE RECEIVED HEAVY RAINS OVER THE PAST SEVERAL DAYS
AND WHERE THE GROUND REMAINS NEAR SATURATION AND WHERE RIVERS ARE
STILL ABOVE FLOOD STAGE...BUT FEEL THAT THE PCPN WILL OCCUR OVER A
LONG ENOUGH PERIOD OF TIME THAT WIDESPREAD AREAL FLOODING IS LOWER
THAN IF THE EXPECTED PCPN WERE TO OCCUR OVER A MUCH SHORTER PERIOD
OF TIME.  HOWEVER...GIVEN THE INCREASING PWATS...THERE IS A CHANCE
FOR SOME LOCALLY HEAVY DOWNPOURS WHICH COULD LEAD TO URBAN AND SMALL
STREAM ADVISORY TYPE CONDITIONS.  THE UPPER LEVEL LONG WAVE PATTERN
REMAINS VERY HIGH AMPLITUDE AND SLOWLY PROGRESSIVE WITH UPPER
RIDGING REMAINING IN PLACE OVER THE ROCKIES AND BROAD UPPER
TROUGHING OVER THE ERN CONUS.  THIS WILL KEEP THE REGION IN NWLY TO
ALMOST NLY FLOW ALOFT...SO THE MAIN SHORTWAVE WILL BE SLOW TO PUSH
EAST OF THE REGION...WITH THE LATEST GUIDANCE SUGGESTING THAT THE
SHORTWAVE WILL NOT PUSH EAST OF THE CWA UNTIL LATE TOMORROW
AFTERNOON OR EARLY EVENING.  THIS SHOULD KEEP LINGERING SHOWERS OVER
THE ERN PORTIONS OF THE CWA INTO THE AFTERNOON.  THE NWLY FLOW AND
BROAD TROUGHING ALOFT SHOULD KEEP TEMPERATURES A BIT BELOW SEASONAL
NORMALS...WITH HIGHS TOMORROW GENERALLY IN THE MID TO UPPER 70S.
LOCATIONS ALONG THE LAKE FRONT SHOULD REMAIN A BIT COOLER AS WINDS
TURN MORE NELY THROUGH THE DAY AS THE SFC LOW PASSES CENTER PASSES
THROUGH CENTRAL IL/IN TOMORROW.

KREIN

&&

.LONG TERM...
309 PM CDT

MONDAY NIGHT THROUGH SUNDAY...

UPPER LEVEL TROUGH WILL REMAIN SITUATED ACROSS THE GREAT LAKES
REGION AND AS THE MORE VIGOROUS SHORT WAVE CONTINUES TO EXIT TO
THE EAST...ANOTHER WAVE IS FORECAST TO DROP SOUTHEAST TOWARDS THE
REGION MONDAY NIGHT. ALTHOUGH THE GREATER ASCENT APPEARS TO BE
WELL NORTH OF THE CWA ACROSS NORTHERN WISCONSIN...WEAKER MID LEVEL
IMPULSES AND FAIR LOW/MID LEVEL CONVERGENCE ASSOCIATED WITH
SURFACE TROUGH SHOULD ALLOW AT LEAST WIDELY SCATTERED DEVELOPMENT
TO OCCUR DURING THIS TIME. SO...SCATTERED MONDAY DAYTIME
DEVELOPMENT WILL LIKELY BE DIMINISHING MONDAY EVENING WITH A DRY
PERIOD LIKELY FOR MOST LOCATIONS...AND SO HAVE LOWERED POPS DURING
THIS TIME AND CONFINED THE HIGHEST ACROSS FAR NORTHERN/NORTHWEST
ILLINOIS. AS LARGE SCALE ASCENT INCREASES LATE IN THE EVENING AND
CLOSER TO THE MIDNIGHT TIME FRAME...THIS WIDELY SCATTERED
DEVELOPMENT WILL BE POSSIBLE ANYWHERE ACROSS NORTHERN ILLINOIS AND
NORTHWEST INDIANA. DUE TO THE LOWER COVERAGE EXPECTED...HAVE ONLY
MAINTAINED LOW CHANCE POPS. WITH WEAK INSTABILITY PRESENT...HAVE
ALSO KEPT SLIGHT CHANCE THUNDER MENTION.

CHANCES FOR SCATTERED SHOWERS/THUNDERSTORMS APPEAR TO REMAIN
PRESENT GOING INTO TUESDAY AS A SERIES OF IMPULSES CONTINUE TO
DROP SOUTHEAST TOWARDS THE AREA. AS DAYTIME MIXING LIKELY OCCURS
QUICKLY IN THE MORNING WITHIN A MOIST/UNSTABLE
ENVIRONMENT...SCATTERED DEVELOPMENT APPEARS POSSIBLE BY LATE
MORNING. GUIDANCE DOES VARY TO THE EXTENT OF THE COVERAGE AND
LOCATION ON TUESDAY...BUT THINK MOST LOCATIONS ACROSS THE CWA HAVE
A CHANCE TO OBSERVE PRECIP ON TUESDAY OWING TO EITHER SOUTHWARD
MOVING SURFACE TROUGH OR DEVELOPMENT ALONG THE LAKE BREEZE. ONCE
AGAIN...WITH THE EXPECTED SCATTERED NATURE OF THESE
SHOWERS/THUNDERSTORMS...HAVE LIMITED POPS TO HIGH CHANCE. WEAKER
FLOW ALOFT SHOULD LIMIT ANY ORGANIZED DEVELOPMENT DURING THIS
TIME...BUT WILL NEED TO MONITOR THE POTENTIAL FOR A STRONGER STORM
OR TWO. TEMPS DURING THIS TIME WILL LIKELY BE BELOW NORMAL...WITH
HIGHS IN THE UPPER 70S ON TUESDAY AND SLIGHTLY LOWER ON WEDNESDAY.
ALSO...EXPECTED PRECIP DEVELOPMENT WILL LIKELY PLAY A FACTOR WITH
HIGH TEMPS...WITH LOCATIONS OUTSIDE OF A LONGER PERIOD OF PRECIP
LIKELY BEING SLIGHTLY WARMER. TO START THE EXTENDED...OUTSIDE OF
LOW CHANCE POPS...IT DOES APPEAR THAT MOST LOCATIONS WILL MORE
THAN LIKELY BE ON THE DRIER SIDE WITH TEMPS ALSO REMAINING BELOW
NORMAL. ALTHOUGH CONFIDENCE IS NOT THE GREATEST THROUGH THE
REMAINDER OF THE FORECAST PERIOD...TOWARDS THE END OF THE WORK
WEEK...IT DOES LOOK LIKE INCREASING MOISTURE/INSTABILITY WILL
ALLOW BETTER CHANCES FOR SHOWERS AND THUNDERSTORMS TO RETURN.

RODRIGUEZ

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 06Z...

* LOWERING CIGS OVERNIGHT WITH MVFR/IFR POSSIBLE BY EARLY MONDAY
  MORNING...AND THEN AGAIN EARLY TUESDAY MORNING.
* PERIODIC CHANCES FOR SHOWERS MUCH OF THE TAF PERIOD.

BMD

//DISCUSSION...UPDATED 06Z...

FIRST AREA OF LOW PRESSURE IS CENTERED NEAR WEST CENTRAL ILLINOIS
TONIGHT WITH SYNOPTICALLY E/SE FLOW ACROSS NORTHERN ILLINOIS AND
NORTHWEST INDIANA. A FEW LIGHTS SHOWERS PERSIST ACROSS THE AREA
THOUGH COVERAGE IS LOW ENOUGH WILL JUST CONTINUE TO CARRY VCSH AT
THE TERMINALS. THE SURFACE LOW HAS TRACKED A LITTLE FARTHER SOUTH
THAN EXPECTED WHICH LOWERS THE PROSPECTS OF LOW CIGS ACROSS THE
TERMINALS. WHILE IFR IS POSSIBLE...CONFIDENCE IN WANING. THE BEST
CHANCE WILL BE AT RFD NEAR THE EDGE OF THE CLOUD SHIELD WHERE
LOWER STRATUS AND FOG ARE FORMING JUST TO THE WEST. SURFACE LOW
WILL TRACK EAST ACROSS INDIANA MONDAY WITH WINDS CONTINUING TO
BACK TO THE NORTHEAST. SHOWERS REMAIN POSSIBLE THROUGH MUCH OF THE
DAY BUT ESPECIALLY IN THE AFTERNOON AS INSTABILITY BUILDS
DIURNALLY. WEAK/SUBTLE FORCING WILL MAKE IT DIFFICULT TO PIN DOWN
TIMING DETAILS HOWEVER. ANOTHER WEAK SURFACE LOW IS PROGGED TO
DROP INTO NORTHERN ILLINOIS LATE MONDAY NIGHT INTO TUESDAY MORNING
ONCE AGAIN RESULTING IN INCREASED PRECIP CHANCES AND LOWERING
CIGS.

BMD

//ORD AND MDW CONFIDENCE...UPDATED 06Z...

* MEDIUM-HIGH CONFIDENCE IN SCATTERED SHOWERS AROUND THE
  TERMINALS MUCH OF THE TAF PERIOD...BUT LOW CONFIDENCE ON
  SPECIFIC TIMING OF IMPACTS AT THE TERMINALS.
* LOW CONFIDENCE IN IFR TONIGHT...MEDIUM CONFIDENCE IN IFR MONDAY
  NIGHT/TUESDAY MORNING.

BMD

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

* TUESDAY...CHANCE OF TSRA. NORTHWEST WINDS.
* WEDNESDAY...DRY/VFR. NORTHEAST WINDS.
* THURSDAY...SLIGHT CHANCE OF SHRA. NORTHEAST WINDS.
* FRIDAY...CHANCE OF TSRA. NORTHEAST WINDS.
* SATURDAY...CHANCE OF SHRA/TSRA. WESTERLY WINDS.

&&

.MARINE...
412 PM CDT

HIGH PRESSURE SPANNING THE GREAT LAKES REGION IS ALLOWING FOR A
WEAKER SURFACE PATTERN ACROSS THE LAKE THIS AFTERNOON...WITH
SOUTHERLY AND GENERALLY LIGHT FLOW IN PLACE. AS THIS HIGH SHIFTS
TO THE EAST TONIGHT INTO MONDAY...LOW PRESSURE TO THE WEST WILL
SLIDE SOUTHEAST THROUGH THE MID MISSISSIPPI AND OHIO VALLEYS.
HOWEVER...DONT ANTICIPATE SPEEDS TO BE AFFECTED AS THIS LOW
REMAINS WELL SOUTH OF THE LAKE...WITH LIGHTER SPEEDS PREVAILING
ONCE AGAIN ON MONDAY. ALTHOUGH THE PATTERN APPEARS TO REMAIN
ACTIVE WITH SYSTEMS MOVING THROUGHOUT THE REGION...SURFACE PATTERN
OVER THE LAKE WILL ALLOW FOR CONTINUED LIGHTER SPEEDS THROUGH MUCH
OF THIS WEEK.

RODRIGUEZ

&&

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

$$

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