Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 020248
AFDLOT

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

.UPDATE...
936 PM CDT

ACTIVITY OVER THE PLAINS HAS STRUGGLED TO GET GOING. 0Z SOUNDINGS
TO OUR WEST DEPICT THE ELEVATED MIXED LAYER...BUT SOME DEGREE OF
CAPPING. THE LOW LEVEL JET THAT IS FORECAST TO PICK OVERNIGHT IS
NOT FAVORABLE ORIENTATION TO WIDESPREAD CONVECTIVE DEVELOPMENT TO
OUR WEST...DESPITE FAVORABLE LAPSE RATES. THIS COUPLED WITH DRY
AIR AND NO REAL STRONG UPPER LEVEL SHORTWAVE TO TRIGGER ANYTHING
WIDESPREAD. AT THE MOMENT THESE PIECES OF THE PUZZLE TEND TO
SUGGEST LESS IN THE WAY OF STORM COVERAGE LATE TONIGHT INTO SUNDAY
MORNING. DON`T WANT TO PULL THE PLUG JUST YET AS NORTHWEST FLOW
COUPLED WITH SUBTLE SHORTWAVE ACTIVITY AND DECENT LAPSE RATES
STILL HOLD THE POTENTIAL TO TRIGGER A FEW STORMS IN THE MORNING.

THE WIND POTENTIAL STILL LOOKS FAIRLY GOOD TOMORROW...WITH GUSTS
TO 35 MPH A DECENT BET...40 MPH POSSIBLE...ESPECIALLY IF CLOUD
COVER IS LESS.

THE MORE ORGANIZED SHORTWAVE FEATURE IS MOVING SOUTHEASTWARD INTO
FAR NW MINNESOTA...AND THIS IS FORECAST TO ROLL THROUGH WISCONSIN.
OUTFLOW BOUNDARIES FROM THIS ACTIVITY ARE THE WILD CARD FOR
TOMORROW LATE AFTERNOON AND ESPECIALLY THE EVENING...WHICH COULD
MAINTAIN A WIND CONCERN DURING THE EVENING HOURS AHEAD OF FAIRLY
LOW PRESSURE FOR THE START OF AUGUST.

KMD

&&

.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
STRENGTHENING 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 UNFORESEEN 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 00Z...

* SOUTHWEST TO POSSIBLY SOUTH-SOUTHWEST WINDS ON SUNDAY AFTERNOON
  AT SUSTAINED SPEEDS TO 20 KT AND GUSTS TO 30-35 KT.

* SCATTERED LIKELY WEAKENING SHRA/TSRA IN THE REGION SUNDAY
  MORNING.

* LIKELY TSRA DURING SUNDAY EVENING.

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

A FEW EARLIER CONVECTIVE CELLS IN NORTHWEST IL HAVE WEAKENED AS
CONVERGENCE HAS WANED AND HAVING NOW PASSED PEAK HEATING. GUSTS
WILL CONTINUE TO FALL OFF THROUGH 01Z-02Z. SO A QUIET NIGHT
EXPECTED AT LOCAL TERMINALS OUTSIDE OF 30-40 KT FLOW DEVELOPING
LATE TONIGHT BETWEEN 1000-2000 FT...WITH THE HIGHER END SPEEDS
NEAR RFD POSSIBLY BRINGING LLWS.

SCATTERED STORMS ARE EXPECTED TO DEVELOP ACROSS MN...WI...AND
POSSIBLY PARTS OF IA TONIGHT AND MOVE SOUTHEAST. DUE TO LACK OF A
STRONG UPPER DISTURBANCE AND A MARGINAL FOCUS OF THE LOW-LEVEL
JET...A WELL-DEFINED MESOSCALE CONVECTIVE SYSTEM /MCS/ IS NOT
EXPECTED TO DEVELOP UPSTREAM OF THE AREA. HOWEVER...SCATTERED
SHRA/TSRA...LIKELY IN A WEAKENING STATE...ARE POSSIBLE DURING THE
MORNING HOURS AFTER SUNRISE SO HAVE CONTINUED THAT MENTION IN THE
TAFS.

SUNDAY AFTERNOON CONTINUES TO LOOK LIKE A TIME WITH LIMITED FOCUS
FOR CONVECTIVE REDEVELOPMENT ACROSS NORTHERN IL AND NORTHWEST
IN. THE BOUNDARY LAYER IS PRESENTLY FORECAST TO MIX SUFFICIENTLY
WITHIN A TIGHTENING PRESSURE GRADIENT TO BRING SUSTAINED SPEEDS
AROUND 20 KT AND GUSTS AT OR EXCEEDING 30 KT...POSSIBLY REACHING
35 KT. THE DIRECTION COULD GO AS FAR BACKED AS 210 DEGREES...AT
LEAST SPORADICALLY AT ORD AND MDW.

THE SYSTEM COOL FRONT IS EXPECTED TO PROGRESS SOUTHEAST THROUGH
WISCONSIN LATE IN THE DAY SUNDAY AND THEN ACROSS THE TERMINALS
DURING THE MID-LATE EVENING. THERE SHOULD BE CONVECTIVE
DEVELOPMENT ALONG THIS OVER WI WITH POTENTIAL MCS DEVELOPMENT OR
AT LEAST A CORRIDOR OF CONVECTION MOVING SOUTHEAST. SO HAVE
MENTIONED THUNDER FOR A COUPLE HOUR WINDOW IN THE EXTENDED
PORTION OF THE ORD TAF.

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

* MEDIUM-HIGH IN WIND GUSTS EXCEEDING 30 KT. LOW-MEDIUM IN START
  TIME AND DURATION OF WINDS.

* MEDIUM IN WIND DIRECTION REMAINING 220-230 DURING PEAK WINDS.

* LOW IN TSRA CHANCES IN THE MORNING.

* MEDIUM-HIGH IN TSRA CHANCES DURING THE EVENING. LOW-MEDIUM IN
  TIMING.

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

MONDAY-TUESDAY...VFR. MAINLY NORTHWEST WINDS.
WEDNESDAY...VFR. NORTHEAST WINDS.
THURSDAY-FRIDAY...CHANCE OF TSRA. NORTHEAST WINDS.

MTF

&&

.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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