Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 261741 AAA
AFDLOT

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1241 PM CDT TUE AUG 26 2014

.MESOSCALE DISCUSSION...
1240 PM CDT

STORMS ACROSS NORTHWEST INDIANA INTO IROQUOIS COUNTY ARE AHEAD OF
THE PROGRESSIVE SHORT WAVE AND ROOTED ON A SHARP INSTABILITY
GRADIENT. THESE STORM HAVE BUILT A MESOSCALE COLD POOL AND CONTINUE
TO BE FORWARD PROPAGATING...THOUGH SOMEWHAT DISORGANIZED AS A WHOLE.
ISOLATED PULSY CORES FOR PRECIPITATION LOADING WIND AND EVEN SOME
HAIL /MOST CURRENT WARNED STORM HAS 60DBZ OVER 30KFT/ MAY BE SEEN
BEFORE THE STORMS EXIT THE FORECAST AREA.

A CHANGE IN AIR MASS HAS OCCURRED AND WILL CONTINUE TO ACROSS THE
FORECAST AREA EARLY THIS AFTERNOON...WITH NORTHWEST WINDS ON THE
SYNOPTIC SCALE ENHANCED IN A NORTHEAST DIRECTION FROM A MESOSCALE
SURFACE REFLECTION ASSOCIATED WITH THE DEPARTING STORMS. DRY AIR
ADVECTION WILL CONTINUE AND NORTHERLY FLOW BELOW 7000 FT PER
PROFILERS SHOULD HELP THE REMAINDER OF THE AFTERNOON TO BE QUIET
BEHIND THE DEPARTING STORMS. HOURLY TEMPERATURE AND POP TRENDS HAVE
BEEN ADJUSTED.

MTF

PREV DISCUSSION...
328 AM CDT

SHOWER AND THUNDERSTORM CHANCES AND TIMING WILL BE THE MAIN FORECAST
CHALLENGE THROUGH THE PERIOD FOLLOWED BY TEMPS AS THEY MAY BE
INFLUENCED BY ROUNDS OF CONVECTION. WATER VAPOR IMAGERY SHOWS THE
UPPER WHICH CONTRIBUTED TO THE BREAKING DOWN OF THE PERSISTENT UPPER
RIDGE OVER THE AREA IS TRACKING NORTHEAST ACROSS JAMES BAY LEAVING
CORRIDOR OF FAIRLY ZONAL FLOW ACROSS THE UPPER MIDWEST AND GREAT
LAKES. ANOTHER UPPER LOW IS SLOWLY SPINNING SOUTHWARD ACROSS
NORTHERN CALIFORNIA AND NEVADA CUTTING OFF FROM A TROUGH AXIS TO THE
NORTH WHICH IS MOVING EAST ACROSS THE CANADIAN PRAIRIES. A RIDGE
AXIS IS PIVOTING EASTWARD OVER WESTERN CANADA AS AN UPPER TROUGH
MOVES EASTWARD ACROSS SOUTHERN ALASKA. AT THE SURFACE...LOW PRESSURE
IS OVER JAMES BAY WITH A COLD FRONT TRAILING IT ACROSS CENTRAL
WISCONSIN AND IOWA. ONE AREA OF HIGH PRESSURE IS STRETCHED UP AND
DOWN THE EASTERN SEABOARD WHILE ANOTHER HIGH IS OVER THE NORTHERN
PLAINS IN THE WAKE OF THE COLD FRONT.

TODAY...THE AFOREMENTIONED COLD FRONT TO THE NORTHWEST WILL BE
ENTERING THE CWA IN THE NEXT FEW HOURS BUT LOOKS TO GET HUNG UP NEAR
THE I-80 CORRIDOR FOR A TIME LATER THIS MORNING. CONVECTIVE TRENDS
ARE A CHALLENGE THIS MORNING WITH THE FRONT AND SEVERAL OTHER
FEATURES PRESENT TO KEEP SHOWERS AND THUNDERSTORMS GOING. EVENING
UPPER AIR ANALYSIS SHOWED AN UPPER JET CORE FROM SOUTHWEST MINNESOTA
TO JAMES BAY WHICH PLACES THE RIGHT ENTRANCE REGION OVER IOWA. THE
NOSE OF A LOW LEVEL JET IS AIMED INTO SOUTHEAST NEBRASKA BUT TURNS
WESTERLY ACROSS SOUTHERN IOWA. THIS HAS LED TO A DECENT DIFFERENTIAL
DIVERGENCE SET UP BETWEEN THE LOW LEVEL AND UPPER LEVEL JETS. IN
ADDITION...AN UPPER WAVE APPEARS TO BE TRACKING NORTHEAST ACROSS
NORTH CENTRAL KANSAS. ALL OF THIS IS LEADING TO VARYING DEGREES OF
CONVECTIVE DEVELOPMENT FROM EAST CENTRAL NEBRASKA/WEST CENTRAL
IOWA...WHERE THE COVERAGE IS EXTENSIVE...TO EASTERN IOWA AND NOW
JUST INTO NW ILLINOIS WHERE COVERAGE IS ISOLATED THUS FAR. 850-300
MB THICKNESS TAKES THIS ACTIVITY EASTWARD. HOW WELL THIS SUSTAINS
ITSELF IS IN QUESTION BUT GIVEN THAT THERE ARE SEVERAL LARGE SCALE
FACTORS DRIVING IT AND THERE IS A GOOD POOL OF INSTABILITY NEAR AND
AHEAD OF THE FRONT THERE IS GOOD REASON FOR IT TO CONTINUE. NOT TOO
MENTION THE TREND OF THE PAST SEVERAL DAYS HAS BEEN FOR MORNING
CONVECTION TO FESTER AND/OR SPIN OFF NEW CONVECTION MIDDAY ONWARD.
ISOLATED TO SCATTERED ACTIVITY WILL BEGIN TO AFFECT THE AREA IN THE
NEXT FEW HOURS WHILE THE MORE SOLID ACTIVITY OVER WEST CENTRAL IOWA
AND EAST CENTRAL NEBRASKA OR ACTIVITY SPUN OFF FROM IT WOULD
POTENTIALLY AFFECT THE AREA LATE MORNING/EARLY AFTERNOON. THIS
COMPLEX HAS BEEN ORGANIZING AND ACCELERATING EASTWARD TO SOME DEGREE
IN THE LAST HOUR OR SO. IN ADDITION...WITH THE FRONTAL BOUNDARY
LIKELY STILL IN THE SOUTHERN CWA THIS AFTERNOON IT MAY SERVE AS
FOCUS FOR NEW CONVECTION AS WELL. IT DOES APPEAR THAT POPS WILL
TAPER OFF ACROSS THE NORTH DURING THE LATE AFTERNOON. TORRENTIAL
DOWNPOURS AND LOCALIZED FLOODING WILL BE THE MAIN THREAT WITH ANY
STORMS ONCE AGAIN...WITH GUSTY STRAIGHT LINE WINDS AND ADDITIONAL
THREAT. MID LEVEL AIR WILL STILL BE QUITE WARM WITH H85 TEMPS NEAR
+20C BUT LOW LEVEL WINDS WILL TURN NORTHERLY AND EVEN NORTHEASTERLY
NEAR LAKE MICHIGAN. EXPECT HIGHS IN THE UPPER 80S FAR SOUTH AND NEAR
80 NEAR THE LAKE AND FAR NORTHERN INLAND AREAS...WITH LOW TO MID 80S
IN BETWEEN. CONVECTION COULD ONCE AGAIN HAVE AN IMPACT ON
HIGHS...ESPECIALLY SOUTH IF ACTIVITY CONTINUES INTO THE AFTERNOON
GIVEN THE BETTER PROXIMITY TO THE FRONTAL BOUNDARY.

TONIGHT THROUGH FRIDAY...CONVECTIVE CHANCES REMAIN A CHALLENGE THIS
PERIOD. THE SURFACE FRONT WILL BE TO THE SOUTH OF THE AREA BUT THE
MAIN MID LEVEL THERMAL GRADIENT WILL BE OVERHEAD AND EVEN NORTH OF
THE AREA. THE LOW AMPLITUDE UPPER TROUGH CURRENTLY WORKING INTO THE
CANADIAN PRAIRIES IN THE MAIN NORTHERN STREAM OF FLOW WILL BE
PASSING TO THE NORTH SUPPLYING MODEST HEIGHT FALLS TO THE LOCAL AREA
TONIGHT AND WEDNESDAY MORNING. THE RIGHT ENTRANCE OF THE UPPER JET
WILL ALSO BE PASSING OVER THE AREA. GUIDANCE SUGGESTS THAT THERE
WILL BE A WAVE EJECTING FROM THE CUTOFF UPPER TROUGH ACROSS THE
CENTRAL PLAINS AS WELL. IT APPEARS THAT CONVECTION WILL BE
DEVELOPING TO THE WEST TONIGHT THEN TRACK EASTWARD IN CONJUNCTION
WITH THE SYNOPTIC FEATURES. THIS SHOULD RESULT IN BEST CHANCES FOR
PRECIP LATER TONIGHT AND WEDNESDAY MORNING....THOUGH HAVE AN INKLING
THIS MAY BE A BIT FAST. THIS MAY BE ANOTHER SITUATION WHERE ACTIVITY
WILL BE DECAYING IN THE MORNING AS IS GETS CLOSER TO THE AREA. MUCH
OF THE AREA COULD SEE PRECIP BUT THE BEST FOCUS MAY BE ACROSS THE
SOUTH IN CLOSER PROXIMITY TO THE FRONT. THE CUTOFF UPPER LOW WILL BE
SLOWLY BUT STEADILY MOVING EASTWARD FORCING MODEST RIDGING INTO THE
AREA GOING INTO THURSDAY. THIS WILL BRING ANOTHER PUSH OF WARM
ADVECTION ACROSS THE CENTRAL U.S. WEDNESDAY NIGHT AND DRIVE THE
FRONTAL BOUNDARY NORTH AS A WARM FRONT. AT THIS POINT THE BULK OF
THE CONVECTION LOOKS TO BE TO THE WEST WEDNESDAY NIGHT WITH IT THEN
AFFECTING THE CWA THURSDAY...WITH BEST CHANCES NORTH AS THE WARM
FRONT LIFTS NORTH. THE AREA WILL BE ON THE PERIPHERY OF THE RIDGE
ONCE AGAIN FAVORING MORE CONVECTION...ESPECIALLY NORTH...THROUGH
FRIDAY. HEAVY RAINFALL AND GUSTY WINDS WILL BE THE MAIN THREATS WITH
THE STORMS THROUGH THE END OF THE WEEK. TEMPERATURES WILL RANGE FROM
THE LOWER 70S NEAR THE LAKE WEDNESDAY WITH A STEADY NORTHEAST
WIND...WITH UPPER 70S INLAND NORTH TO LOWER 80S SOUTH. EASTERLY
WINDS WILL KEEP SIMILAR TEMPS IN PLACE THURSDAY WITH WARMER READINGS
FRIDAY ALL THE WAY TO THE LAKE WITH THE FRONT MOVING NORTHWARD.
TEMPS MAY BE AFFECTED BY CONVECTION EACH DAY.

SATURDAY AND BEYOND...ACTIVE WEATHER CONTINUES THROUGH EARLY NEXT
WEEK AS THE CUTOFF TROUGH MOVES ACROSS THE AREA OVER THE WEEKEND
BRINGING CONTINUED SHOWER AND THUNDERSTORM ACTIVITY. ANOTHER UPPER
LOW IS FORECAST TO TRACK ACROSS SOUTHERN CANADA MONDAY/TUESDAY
BRINGING A COLD FRONT ACROSS THE AREA ALONG WITH MORE STORM CHANCES.
HIGHS SHOULD CONTINUE IN THE UPPER 70S AND LOWER 80S.

MDB

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 18Z...

* E TO ESE WINDS ARND 10 KT DIMINISH TO LESS THAN 10 KT THIS AFTN.

* EAST WINDS AROUND 10 KT EXPECTED AGAIN TOMORROW.

JEE

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

THE FRONT AND ASSOCIATED STORMS HAVE PUSHED EAST OF THE TERMINALS.
ADDITIONAL CONVECTION IS POSSIBLE WELL SOUTH OF THE
TERMINALS...SOUTH OF A GARY TO PONTIAC LINE. NO ADDITIONAL
CONVECTION IS EXPECTED AT THE TERMINALS. WINDS ARE E TO ESE BEHIND
THE CONVECTION WITH SPEEDS AT 10-15 KT. BASED ON UPSTREAM
OBS...EXPECTING WINDS TO RETURN TO NE ARND 7 KT BY MID AFTN. A
LAKE BREEZE MAY FORM BUT THINKING IT WILL ONLY SHIFT ALREADY NE
WINDS TO EAST LESS THAN 10 KT. GYY AND MDW WILL BE THE MOST
SUSCEPTIBLE TO THE LAKE BREEZE.

HIGH PRESSURE MOVES OVER THE REGION TONIGHT AND EXPECTING VFR
CONDITIONS WITH LIGHT NE WINDS. AS MIXING DEVELOPS MID TOMORROW
MORNING...THINKING NE WINDS WILL BE ARND 10 KT.

JEE


//ORD AND MDW CONFIDENCE...UPDATED 18Z...

* LOW CONFIDENCE IN ANY PRECIP OVERNIGHT INTO TOMORROW MORNING.

* HIGH CONFIDENCE IN ALL OTHER FORECAST ELEMENTS.

JEE

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

* WEDNESDAY NIGHT...SLIGHT CHANCE OF TSRA.
* THURSDAY THROUGH MONDAY...CHANCE OF TSRA.

&&

.MARINE...

246 AM...LOW PRESSURE NEAR JAMES BAY WILL CONTINUE EAST ACROSS
NORTHERN QUEBEC TODAY AS A TRAILING COLD FRONT MOVES SOUTH ACROSS
LAKE MICHIGAN THIS MORNING. WEST/SOUTHWEST WINDS WILL SHIFT TO THE
NORTH BUT SPEEDS SHOULD REMAIN IN THE 10-15KT RANGE. HIGH PRESSURE
OVER THE DAKOTAS WILL MOVE EAST ACROSS THE LAKES REGION WEDNESDAY
INTO WEDNESDAY NIGHT. AS IT DOES...THE GRADIENT WILL TIGHTEN SOME
ON THE FAR SOUTH END OF THE LAKE...ALLOWING NORTH TO NORTHEAST
WINDS TO INCREASE INTO THE 10-20KT RANGE. THIS HIGH WILL MOVE EAST
THURSDAY WITH A WARM FRONT LIFTING BACK NORTH ACROSS THE LAKE
THURSDAY NIGHT...SHIFTING WINDS BACK SOUTHERLY. CMS

&&

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

$$

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