Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 282112
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
412 PM CDT SUN JUN 28 2015

.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 20Z...

* PERIODS OF SHOWERS BEGINNING EARLY THIS EVENING AND CONTINUING
  INTO THE OVERNIGHT HOURS.

* SMALL CHANCE OF A FEW EMBEDDED THUNDERSTORMS...MAINLY MID TO
  LATE THIS EVENING AND TO THE SOUTH OF THE TERMINALS.

* LIGHT SW WINDS GRADUALLY BACKING TO THE SOUTHEAST BY EARLY
  EVENING.

* LOWERING CIGS THIS EVENING/OVERNIGHT...MVFR TO IFR LIKELY BY
  LATE TONIGHT EARLY MONDAY MORNING.

KJB

//DISCUSSION...UPDATED 18Z...

AN AREA OF LOW PRESSURE OVER THE UPPER MIDWEST WILL SHIFT
SOUTHEASTWARD OVER ILLINOIS BY EARLY MONDAY MORNING. THIS SYSTEM
WILL BRING A PERIOD OF SHOWERS AND THUNDERSTORMS TO PORTIONS OF THE
AREA AS IT APPROACHES THE AREA TONIGHT. THE MAIN QUESTION AT THIS
TIME IS WHERE DOES THE HEAVIEST RAIN AND THUNDERSTORM POTENTIAL END
UP. SOME OF THE LATEST FORECAST GUIDANCE AND RECENT OBSERVATIONAL
TRENDS SUGGEST THAT THE SURFACE LOW WILL TRACK A BIT FARTHER SOUTH
THAN PREVIOUSLY THOUGHT. THIS SUGGESTS THAT MUCH OF THE HEAVIER
RAIN AND THUNDERSTORM CHANCES MAY END UP SOUTH OF THE TERMINALS
TONIGHT. DUE TO THE LOW EXPECTED TO PASS TO THE SOUTH OF THE
TERMINALS...AND THE BEST DYNAMICS ALSO LOOKING TO SET UP TO THE
SOUTH...I HAVE OPTED TO PULL THE MENTION OF THE PROB 30 FOR
THUNDERSTORMS THIS EVENING. HOWEVER...SOME EMBEDDED STORMS WILL
LIKELY OCCUR NOT TOO FAR AWAY TO THE SOUTH OF THE TERMINALS.
PERIODS OF SHOWERS ARE STILL EXPECTED AT TIMES STARTING THIS
EVENING. CIGS AND VISIBILITIES WILL ALSO BE ON A DOWNWARD TREND
OVERNIGHT WITH THE APPROACH OF THIS LOW. A PERIOD OF IFR CIGS ARE
LIKELY EARLY MONDAY MORNING BEFORE CONDITIONS SLOWLY IMPROVE
DURING THE MORNING.

KJB

//ORD AND MDW CONFIDENCE...UPDATED 20Z...

* MEDIUM-HIGH WITH SHRA CHANCES THIS EVENING INTO THE OVERNIGHT.
* LOW WITH THE CHANCES OF A THUNDERSTORM IMPACTING THE TERMINALS
  THIS EVENING.
* MEDIUM-HIGH WITH GENERAL WIND TRENDS.
* HIGH WITH MVFR CIGS DEVELOPING LATE THIS EVENING OR
  TONIGHT...MEDIUM IN IFR CIGS OVERNIGHT.

KJB

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

MONDAY NIGHT...CHC SHRA/SLT CHC TSRA. LIGHT WINDS.
TUESDAY...CHC SHRA/SLT CHC TSRA. NW/N WINDS.
WEDNESDAY...DRY. NE WINDS.
THURSDAY...DRY. NE WINDS.
FRIDAY...SLT CHC TSRA. LIGHT WINDS.
SATURDAY...SLT CHC TSRA. SW WINDS.

BMD

&&

.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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