Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 012041
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
341 PM CDT SAT AUG 1 2015

.SHORT TERM...
325 PM CDT

THROUGH SUNDAY EVENING...

MAIN FORECAST CONCERNS ARE WITH THE POTENTIAL FOR SEVERE STORMS
LATE SUNDAY INTO SUNDAY EVENING...AS WELL AS STRONG GUSTY WINDS
OUTSIDE OF ANY THUNDERSTORMS.

LATE THIS AFTERNOON INTO EARLY EVENING...WEAK UPPER LEVEL
DISTURBANCE WILL BE OVER THE REGION. SEVERAL MODELS AND HI-RES
NEAR TERM GUIDANCE HAS BEEN INSISTENT ON BREAKING OUT ISOLATED
SHOWERS. HOWEVER...RECENT AIRCRAFT SOUNDINGS SHOW VERY DRY LOW
LEVELS AND CUMULUS FIELD OVER PORTIONS OVER THE REGION DOESNT
APPEAR VERY ROBUST. CANNOT COMPLETELY RULE OUT A VERY ISOLATED
SHOWER OVER NORTHERN IL OR NW IN...BUT HAVE LEFT OUT MENTION OF
ANY PRECIP. ATTENTION WILL THEN TURN TO CONVECTION/POSSIBLE
LOOSELY ORGANIZED MCS FROM NW WI TO EASTERN NE OVERNIGHT AND ITS
EVOLUTION. ASSOCIATED SHORTWAVE WILL APPROACH THE AREA BY EARLY
SUNDAY. WEAKENING STORMS COULD TRY TO SNEAK INTO FAR NORTHERN
ILLINOIS TOWARD DAYBREAK. CORFIDI VECTORS ARE WEST TO EAST...SO
ITS UNCERTAIN HOW EFFECTIVE PROPAGATION INTO CWA WILL BE INTO
SUNDAY MORNING. LOW LEVEL JET WILL INCREASE LATE AND ADVECTION OF
STEEP MIDLEVEL LAPSE RATES WILL INCREASE MUCAPE...THOUGH IT DOES
NOT APPEAR ANYTHING OTHER THAN A STRONG STORM OR TWO IS FAVORED TO
SURVIVE INTO NORTHERN IL. HAVE TAILORED POPS TO MID CHANCE NORTH
OF INTERSTATE 80 AND LOW CHANCE TO SLIGHT MOVING SOUTH FROM THERE.

WILL NEED TO WATCH FOR ANY OUTFLOW BOUNDARIES THAT COULD KICK OFF
ISOLATED STORMS FROM MID DAY INTO THE EARLY-MID
AFTERNOON...ALTHOUGH OVERALL FORCING IS LACKING DURING THIS TIME.
SYNOPTICALLY...EXPANSIVE HIGH PRESSURE WILL SINK SOUTHEAST AND
STREGNTHENING COLD FRONTAL TROUGH WILL BE APPROACHING FROM
NORTHWEST. PRESSURE FALLS/TIGHTENING GRADIENT WILL RESULT IN
STRONG SOUTHWEST WINDS GUSTING UP TO 35 MPH...WITH SOUNDINGS
INDICATING SOME POTENTIAL FOR OCCASIONAL GUSTS TO 40 MPH.
ALOFT...A VERY STRONG EML WILL BE ADVECTED EASTWARD...WITH STEEP
NEARLY DRY ADIABATIC MIDLEVEL LAPSE RATES. ASSUMING A PERIOD OF
NEARLY FULL SUN...THERMAL PROGS SUPPORT HIGHS REACHING AT LEAST
UPPER 80S-LOWER 90S AND GUSTY SOUTHWEST LOW LEVEL FLOW WILL RETURN
DEWPOINTS INTO THE 60S TO AROUND 70. STEEP MIDLEVEL LAPSE RATES
AND WARM/HUMID AIRMASS WILL YIELD MODERATE TO STRONGLY UNSTABLE
AIRMASS BUT EML COULD ALSO YIELD A STRONG CAP...WITH MAIN SHORT
WAVE FORCING STILL OFF TO NORTH CLOSER TO THE COLD FRONT. IF THERE
IS AN UNFORSEEN SUBTLE SHORTWAVE TO BREACH THE CAPPING DURING
AFTERNOON...ANY STORM THAT GOES IN ENVIRONMENT WITH PLENTY OF
INSTABILITY...40-45 KT OF DEEP LAYER SHEAR AND IMPRESSIVE LOWER
LEVEL WIND FIELDS COULD BECOME SEVERE.

WITH THIS BEING SAID...HAVE STAYED CONSERVATIVE WITH POPS UNTIL
BETTER FORCING ARRIVES AND MODELS ARE IN DECENT AGREEMENT IN
DEVELOPING A LINEAR MCS ACROSS WISCONSIN AND PROPAGATING IT
SOUTHEASTWARD. MAIN QUESTION IS TIMING OF THIS POTENTIAL MCS AND
WHETHER SOME DIURNAL WEAKENING WILL OCCUR WITH SOUTHEASTWARD
EXTENT. DID BRING LIKELY POPS ALONG NORTHERN TIER OF COUNTIES IN
EARLY EVENING SOUTHWARD TO CHICAGO AND POINTS WEST AND SOUTH
DURING MID EVENING HOURS...THOUGH COLD POOL FORMATION COULD AID IN
QUICKER PROPAGATION AS SHOWN BY SOME HI-RES GUIDANCE SUCH AS 12Z
4KM NAM. 40-50 KT OF 0-3 KM BULK SHEAR AND AFOREMENTIONED
IMPRESSIVE LOWER LEVEL WIND FIELD WOULD YIELD A DAMAGING WIND
THREAT...THOUGH CANNOT RULE OUT LARGE HAIL CONSIDERING STEEP LAPSE
RATES. IF MCS IS SLOWER TO PROPAGATE INTO CWA...THEN ITS
CONCEIVABLE THAT BETTER SEVERE THREAT WOULD FOCUS INTO FAR
NORTHERN ILLINOIS COUNTIES. ALSO...FASTER STORM MOTIONS COULD
PRECLUDE MUCH OF A FLOODING RISK.

RC

&&

.LONG TERM...
248 PM CDT

LATE SUNDAY NIGHT THROUGH SATURDAY...

PERIOD WILL BEGIN WITH SHOWER/THUNDERSTORM ACTIVITY LIKELY
EXITING TO THE EAST WITH EASTWARD MOVING SURFACE TROUGH/FRONTAL
BOUNDARY. DID LEAVE CHANCE POPS ACROSS EAST CENTRAL ILLINOIS AND
NORTHWEST INDIANA WITH SLIGHT CHANCE ACROSS PORTIONS OF NORTHERN
ILLINOIS AFTER THE MIDNIGHT TIME FRAME. HOWEVER...THERE IS THE
POTENTIAL FOR ANY THUNDERSTORM DEVELOPMENT TO DEPART THE AREA
SLIGHTLY SOONER...ESPECIALLY IF A LINE OF CONVECTION WERE TO
DEVELOP. AFTER THIS POTENTIAL FOR PRECIP...EARLY NEXT WEEK APPEARS
TO BE ON THE DRIER SIDE...AND HAVE REMOVED POPS DURING THIS TIME.
NORTHWEST FLOW WILL BE IN PLACE ACROSS THE AREA AS LARGE SCALE
PATTERN WILL BE DICTATED BY LARGE UPPER LEVEL CENTER SLOWLY
DROPPING SOUTHEAST THROUGH SOUTHERN CANADA. WITH ANY STRONGER MID
LEVEL FEATURES REMAINING TO THE NORTH AND TO THE SOUTHWEST OF THE
AREA...PRECIP DEVELOPMENT WILL STAY OUTSIDE OF THE CWA.
HOWEVER...THIS WILL PROVIDE COOLER AND DRIER CONDITIONS WITH
INITIALLY NORMAL HIGH TEMPS IN THE LOW 80S LIKELY ON MONDAY...BUT
WITH A TREND TOWARDS BELOW NORMAL HIGH TEMPS IN THE 70S BY MID
WEEK. GUIDANCE IN FAIR AGREEMENT WITH TAKING THIS UPPER LEVEL LOW
TO THE EAST LATER IN THE WORK WEEK...WITH BETTER POTENTIAL FOR
UPSTREAM ENERGY AND PRECIP TO WORK ITS WAY TOWARDS THE AREA.

RODRIGUEZ

&&

.AVIATION...

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

* WSWLY WINDS WITH SPEEDS ARND 10-12 KT AND GUSTS TO ARND 18KT
  VEERING BACK TO WNWLY THROUGH THE AFTERNOON. GUSTINESS SUBSIDING
  AT SUNSET.

* WINDS TURN SOUTHWEST EARLY SUNDAY MORNING AND INCREASING 15-20KT
  SUSTAINED WITH GUSTS UP TO 25-30KT.

* ANOTHER CHANCE FOR THUNDERSTORMS SUNDAY EVENING.

KREIN

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

STRENGTHENING WLY-WNWLY WINDS ARE INCREASING ACROSS THE REGION AS
DEEP LOW PRESSURE SITS OVER HUDSON BAY AND HIGH PRESSURE SETTLES
INTO THE MIDDLE MISSISSIPPI AND OHIO VALLEYS. A VERY WEAK MID
LEVEL IMPULSE MOVING THROUGH THE FAST NWLY FLOW ALOFT HAS
GENERATED SOME MID CLOUD WITH BASES ARND 8KFT AND A FEW ISOLD
SHRA. HOWEVER...WHAT LITTLE MID LEVEL FORCING THERE IS ASSOCIATED
WITH THE WEAK IMPULSE WILL REMAIN NORTHEAST OF THE TERMINALS AND
NO PCPN IS EXPECTED. THE REMAINDER OF THE NIGHT SHOULD BE QUIET
WITH WINDS DIMINISHING TO ARND 10KT AT SUNSET.

THE NEXT CONCERN WILL BE THE POTENTIAL FOR THUNDERSTORMS OVER THE
AREA TOMORROW. THERE IS STILL SOME SPREAD IN THE FORECAST MODEL
SOLUTIONS FOR THE EXACT TIMING OF TS OVER NRN IL/IN.  HOWEVER...
THE GENERAL IDEA IS FOR A STRONGER MID LEVEL IMPULSE TO CROSS NRN
IL/IN TOMORROW MORNING WHICH SHOULD GENERATE SOME SHOWERS AND
THUNDERSTORMS. THE MAIN ISSUE IS TIMING. SOME GUIDANCE IS
SUGGESTING THAT THE PREFERRED TIMING WILL BE DURG THE EARLY
MORNING HOURS WHILE OTHER MODELS FAVOR A SLOWER SOLUTION WITH THE
HIGHER TS POTENTIAL DURG THE LATE MORNING HOURS. GIVEN THE TIMING
UNCERTAINTY AT THIS POINT...WILL MAINTAIN THE TIMING OF THE GOING
FORECAST. THE REMAINDER OF SUNDAY ALSO HAS SOME UNCERTAINTY FOR
THE POTENTIAL FOR ADDITIONAL TS CHANCES THROUGH THE REMAINDER OF
THE DAY. STRONG SWLY FLOW AT 5KFT AND INCREASING SFC HEAT AND
HUMIDITY WILL BE CONDUCIVE TO TS DEVELOPMENT AHEAD OF A COLD
FRONT DROPPING SOUTH THROUGH THE UPPER MISSISSIPPI VALLEY AND
WESTERN GREAT LAKES. LATEST MODEL SOUNDINGS INDICATE THAT THERE
SHOULD BE AN ELEVATED INVERSION...WHICH COULD DELAY ANY TS UNTIL
THE FRONT ACTUALLY CROSSES THE AREA TOMORROW EVENING.

KREIN

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

* HIGH CONFIDENCE IN ALL FORECAST ELEMENTS THROUGH 06Z SUNDAY.

* LOW CONFIDENCE IN TIMING/COVERAGE/DURATION OF TS IMPACTING THE
  TERMINALS SUNDAY MORNING.

KREIN

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

SUNDAY NIGHT...MAINLY VFR. CHANCE OF SHRA/TSRA EARLY IN THE
EVENING.

MONDAY...MAINLY VFR. NORTHWEST WINDS.

TUESDAY...SLIGHT CHANCE OF SHRA. NORTHWEST WINDS BECOMING NORTHEAST.

WEDNESDAY...CHANCE OF SHRA/SLIGHT CHANCE OF TSRA. NORTHEAST
            WINDS.
THURSDAY AND FRIDAY...CHANCE OF SHRA. EAST WINDS.

BEACHLER

&&

.MARINE...
248 PM CDT

WESTERLY WINDS IN PLACE TODAY AS LOW PRESSURE REMAINS TO THE
NORTH AND WHILE HIGH PRESSURE IS SITUATED OVER THE SOUTHERN PLAINS
TO THE MID MISSISSIPPI VALLEY. EXPECT WINDS TO BECOME MORE
SOUTHWEST THROUGH THE AFTERNOON AND INTO THE EVENING...WITH SPEEDS
THEN EXPECTED TO INCREASE THROUGH THE NIGHT. SOUTHWEST WINDS WILL
THEN REALLY INCREASE ON SUNDAY WITH THE APPROACH OF A SEPARATE
SYSTEM AND WITH THE HIGHEST WINDS/GUSTS LIKELY FROM LATE SUNDAY
MORNING THROUGH EARLY SUNDAY EVENING. HAVE INCREASED WINDS DURING
THIS TIME FRAME TO 30KT FOR BOTH THE NEARSHORE AND OPEN
WATERS...BUT PRIMARILY FOR THE SOUTHERN HALF. THERE IS SOME
POTENTIAL FOR OCCASIONAL GALE FORCE GUSTS IN THE AFTERNOON...BUT
THIS WILL BE MAINLY FOR THE NEARSHORE WATERS. WINDS WILL DIMINISH
SUNDAY NIGHT INTO MONDAY...WHILE THEY TURN MORE WEST/NORTHWEST.

RODRIGUEZ

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-
     LMZ745...9 AM SUNDAY TO 10 PM SUNDAY.

&&

$$

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