Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 250556
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
1256 AM CDT MON APR 25 2016

.UPDATE...
843 PM CDT

FOR EVENING UPDATE...

MINOR TWEAKS MADE TO GOING FORECAST...INCLUDING LOWERING MIN TEMPS
AND ADJUSTING HOURLY TEMP TRENDS ESPECIALLY ALONG THE NORTH SHORE
ACCOUNT STRONG LAKE BREEZE...AND SCALING BACK LOW-CHANCE POPS
THROUGH THE OVERNIGHT HOURS.

EARLY EVENING SURFACE ANALYSIS DEPICTS OCCLUDING LOW PRESSURE
OVER SOUTH DAKOTA...WITH A NEARLY STATIONARY FRONT EXTENDING
SOUTHEAST TO SOUTHERN LAKE MICHIGAN. FRONTAL BOUNDARY CUTS ACROSS
FAR NORTHEAST IL...ENHANCED BY EASTERLY LAKE BREEZE FROM THIS
AFTERNOON. THIS HAS LED TO TEMPS FALLING INTO THE LOW 50S ALONG
THE SHORE NORTH OF CHICAGO...WHILE MANY AREAS WEST/SOUTH OF THE
BOUNDARY WERE STILL ENJOYING EVENING TEMPS IN THE LOW 70S AS OF 8
PM. THIS FRONT IS EXPECTED TO LIFT NORTH OF THE IL/WI BORDER LATE
TONIGHT/EARLY MONDAY HOWEVER...AS SURFACE LOW MOVES EAST ACROSS
THE UPPER MISSISSIPPI VALLEY DURING THE DAY AND INTO WI MONDAY
EVENING IN RESPONSE TO FAIRLY VIGOROUS SHORT WAVE ROTATING
NORTHEASTWARD AROUND THE ASSOCIATED CLOSED UPPER LOW OVER THE
DAKOTAS. LOW LEVEL INSTABILITY AT THIS TIME IS LIMITED TO AREAS
AHEAD OF THE SURFACE COLD FRONT ACROSS THE MID AND UPPER MISSOURI
VALLEYS...AND THIS ACTIVITY IS EXPECTED TO WANE OVERNIGHT AS THE
SHORT WAVE TRACKS NORTHEAST INTO WI AND WEAKENS THROUGH EARLY
MONDAY MORNING. GUIDANCE TRENDS CONTINUE TO LIMIT SIGNIFICANT
PRECIP TO AREAS WEST-NORTHWEST AND NORTH OF THE FORECAST AREA
THROUGH THE NIGHT...AND WITH FORECAST SOUNDINGS MAINTAINING WARM
LAYER/CAP BETWEEN 600-700 MB THROUGH 12Z HAVE TRIMMED SLIGHT
CHANCE POPS BACK TO MAINLY JUST NORTH CENTRAL IL DURING THE PRE-
DAWN HOURS.

WHILE SOME SHOWERS AND CLOUD COVER ARE POSSIBLE MONDAY
MORNING/MID-DAY...IT APPEARS THE GREATEST POTENTIAL FOR MORE
ORGANIZED CONVECTIVE ACTIVITY IN THE WARM SECTOR WILL HOLD OFF
UNTIL THE LATE AFTERNOON AND EVENING HOURS WITHIN THE LOW LEVEL
THETA-E AXIS AHEAD OF THE OCCLUDED FRONT.

RATZER

&&

.SHORT TERM...
154 PM CDT

THROUGH MONDAY NIGHT...

VERY MILD DAY HAS SETUP ACROSS MUCH OF THE FORECAST AREA THIS
AFTERNOON...WITH SOUTHERLY FLOW USHERING IN HIGHER DEWPOINTS AND
SATELLITE IMAGERY INDICATING ONLY A CIRRUS DECK SLIDING OVERHEAD
THIS AFTERNOON. THIS HAS ALLOWED SFC TEMPS TO WARM INTO THE
MID/UPR 70S...HOWEVER WITH A SLIGHTLY LIGHTER GRADIENT AT THE SFC
WINDS HAVE REMAINED SLIGHTLY ONSHORE FOR FAR NORTHEAST ILLINOIS
AND TEMPS REMAIN IN THE UPPER 50S/AROUND 60.

GUIDANCE CONTINUES TO LEAN TOWARDS MID-LVL HEIGHTS REMAINING
ELEVATED THROUGH TONIGHT FROM CENTRAL WISC STRETCHING SOUTH
THROUGH NORTHERN IL/NORTHWEST IN...WHICH WILL MAINTAIN A DRY
STRETCH. THE VORT MAX WILL BEGIN TO PUSH EAST FROM THE CENTRAL
PLAINS AROUND 6Z...WITH A WEAKER LEE-SIDE LOBE OF VORTICITY ACROSS
EASTERN KANSAS/MISSOURI LIFTING NORTHEAST. HEIGHTS WILL BEGIN TO
DECLINE AROUND 9-12Z MON...HOWEVER THE LEE-SIDE VORT WILL
GRADUALLY ENCOUNTER THESE ELEVATED HEIGHTS AND LIKELY BE SLOWING
AND WEAKENING. BUT THERE COULD BE SOME LINGERING SHOWERS AND
PERHAPS AN ISOLATED THUNDERSTORM MAINLY IN THE 14-18Z
TIMEFRAME...THEN WEAK DIFFLUENT FLOW RETURNS EARLY MON AFTN.

THE TRENDS HAVE BEEN FOR A FURTHER NORTH AND WEAKER SHEARED SETUP
FOR MON AFTN...WHICH IS SUPPORTED BY MOST OPERATIONAL SOLUTIONS
WITH THE LACK OF ORGANIZED PRECIP/CONVECTION IN THE 18-21Z PERIOD
MONDAY OVER NORTHERN IL AND NOW BEING PROGGED TO BE IN
CENTRAL/EASTERN WISC. AS THE LATE AFTN/EARLY EVE PROGRESSES...A
FRONTAL BOUNDARY WILL BE SLIDING SOUTH ACROSS THE LAKE WITH THE
500MB VORT FLATTENING. THERE REMAINS SOME MID-LVL CONVERGENCE
ALONG AND AHEAD OF THE FRONTAL BOUNDARY...WHICH COUPLED WITH AN
UNSTABLE 0-6KM LAYER COULD BE ENOUGH TO TRIGGER CONVECTION. SOME
OF THE HI-RES DATASETS SUGGEST A RAGGED LINE OF CONVECTION DIVING
SOUTHEAST FROM SOUTHERN WISC ACROSS FAR NORTHEAST IL INTO NORTHERN
IN AFTER 00Z...BUT QUICKLY ENDING JUST AFT MIDNIGHT MON NIGHT.

WARM THERMAL RIDGE LINGERS OVERHEAD AGAIN MON...AND WOULD SUPPORT
TEMPS YET AGAIN FLIRTING WITH 80 DEGREES IN MANY AREAS. THE
SOUTHERLY/SOUTHWESTERLY FLOW SHOULD BE STRONG ENOUGH MON AFTN TO
PREVENT ANY ONSHORE COMPONENT FOR FAR NE ILLINOIS. DEW POINTS WILL
ALSO BE ADVECTING SLIGHTLY HIGHER INTO THE MID/UPR 50S...WHICH
COULD ALSO SLOW THE WARMING RATES IN THE AFTN MON...AND IF CLOUDS
ARE THICKER THROUGHOUT THE DAY THEN TEMPS MAY STRUGGLE TO MOVE
BEYOND THE MID 70S.

HEIGHTS THEN AGAIN BEGIN TO RISE AFTER MIDNIGHT MON NGT...AS FLOW
QUICKLY TURNS NORTH/NORTHEASTERLY IN THE WAKE OF THE FRONTAL
BOUNDARY. PRECIP WILL QUICKLY COME TO AN END LATE MON NGT...WITH
TEMPS FALLING INTO THE UPR 40S TO 50S.

BEACHLER

&&

.LONG TERM...
242 PM CDT

TUESDAY THROUGH SUNDAY...

WITH THE DEPARTURE OF THE MID/UPPER LEVEL TROUGH AND SURFACE
LOW...FRONT WILL CONTINUE TO SLOWLY SAG SOUTH THROUGH THE CWA LATE
MONDAY NIGHT INTO TUESDAY MORNING. HOWEVER...ITS POSSIBLE THAT
THIS FRONT MAY STILL BE IN AND AROUND THE FAR SOUTHERN CWA TUESDAY
MORNING WITH SOME LINGERING ISOLATED/LIGHT SHOWERS STILL POSSIBLE.
WOULD THINK ANY CHANCE OF AN ADDITIONAL THUNDERSTORM WOULD BE
LOW...WITH THE HIGHER CHANCES SITUATED TO THE SOUTH. THIS BETTER
FOCUS FOR ANY PRECIP SHOULD REMAIN TO THE SOUTH OF THE CWA DURING
THE DAY TUESDAY...AND HAVE KEPT MAINLY DRY CONDITIONS ACROSS THE
CWA. MAX TEMPS WILL LIKELY BE HIGHLY VARIABLE OVER THE CWA WITH
THIS FRONT STILL IN AND AROUND A PORTION OF THE CWA...BUT
DEFINITELY WILL BE VERY DIFFERENT TEMPS FROM THE MILD CONDITIONS
ON MONDAY FOR MOST AREAS.

TURN ATTENTION TOWARDS ANOTHER LARGE UPPER LEVEL SYSTEM EXPECTED
TO PUSH THROUGH THE CENTRAL CONUS BY THE MIDDLE OF THE WEEK...WITH
PRECIP CHANCES INCREASING TUESDAY NIGHT THROUGH WEDNESDAY NIGHT.
MODELS VARY TO THE EXTENT OF THE PRECIP PUSH ACROSS THE CWA...WITH
MOST LOCATIONS POSSIBLY STAYING ON THE DRY SIDE TUESDAY NIGHT.
GIVEN THE ADVERTISED CLOSE NATURE OF THIS SYSTEM...TRENDED TOWARDS
THE DRIER MODEL SOLUTIONS WITH THE BULK OF ANY PRECIP LIKELY
STAYING JUST TO THE WEST OF THE CWA. THIS SYSTEM SHOULD BEGIN TO
APPROACH THE REGION DURING THE DAY WEDNESDAY AND WITH THIS
INCREASE IN LARGE SCALE ASCENT...MOST LOCATIONS WILL LIKELY SEE
MORE WIDESPREAD PRECIP. THIS WILL BE MORE EVIDENT ACROSS THE
SOUTHWEST/SOUTH THIRD OF THE CWA AND ESPECIALLY IN THE AFTERNOON.
SOMEWHAT LOWER CONFIDENT WITH OVERALL CONVECTIVE TRENDS ON
WEDNESDAY...AS GUIDANCE STILL SHOWING THAT THE CWA WILL STAY ON
THE COOL SIDE OF A BOUNDARY. HOWEVER...GIVEN THE CLOSE PROXIMITY
TO THIS BOUNDARY AND MOISTURE RICH AIR AND NEGATIVELY TILTED
TROUGH IN PLACE...THUNDER WILL STILL BE POSSIBLE FOR MAINLY THE
SOUTHERN HALF OF THE CWA. THE STRONGEST CONVECTION WILL LIKELY
STAY TO THE SOUTH OF THE CWA ON WEDNESDAY...WITH STEADY RAIN AND
ISOLATED THUNDER THEN EXPECTED INTO WEDNESDAY NIGHT. THIS SYSTEM
WILL AT LEAST DAMPEN WHILE STILL MOVING ACROSS THE REGION ON
THURSDAY WITH LINGERING CHANCES FOR SHOWERS...AND WHILE COOL
CONDITIONS STILL REMAIN IN PLACE. THE END OF THE WORK WEEK IS
TRENDING DRIER...BUT A MORE ACTIVE PATTERN IS APPEARING TO QUICKLY
RETURN BY THE WEEKEND.

RODRIGUEZ

&&

.AVIATION...
FOR THE 06Z TAFS...

SOUTH WINDS BECOME SOUTHWEST AND GUST TO 20-25 KT AT THE EASTERN
TERMINALS. GUSTS OF 25-28 KT ARE POSSIBLE AT RFD THIS AFTN. THE
CONVECTION CURRENTLY OVER CENTRAL IOWA SHOULD DISSIPATE BEFORE
REACHING THE TERMINALS. THE COLD FRONT CURRENTLY OVER THE LAKE
DRIFTS SOUTH LATE THIS AFTERNOON INTO THIS EVENING. GUIDANCE
CONTINUES TO SLOW THE ARRIVAL OF THE FRONT AND A LATER ARRIVAL
WOULD LIKELY RESULT IN LESS THUNDER COVERAGE. KEPT THE LATE
AFTERNOON/EARLY TIMING OF TSRA AT ALL OF THE TERMINALS BUT HAVE
MEDIUM CONFIDENCE IN THUNDERSTORMS IMPACTING THE TERMINALS AND
MEDIUM-LOW CONFIDENCE IN TIMING. COULD SEE A SCENARIO WHERE THERE
ARE ONLY A FEW THUNDERSTORMS SCATTERED ABOUT NORTHERN IL AND NW
INDIANA...AKA A VCTS SITUATION. FOR NOW A TEMPO FOR TSRA SEEMED
APPROPRIATE. GUSTS DIMINISH BEHIND THE FRONT AND WINDS BECOME
NORTHWEST EARLY TUESDAY MORNING. THERE MAY BE A PERIOD OF
SCATTERED VFR CLOUDS BEHIND THE FRONT.

JEE

&&

.MARINE...
326 PM CDT

HIGH PRESSURE STILL SHIFTING TO THE EAST OF THE GREAT LAKES REGION
THIS AFTERNOON...WHILE LOW PRESSURE IS SITUATED OVER THE PLAINS.
PREVIOUSLY MENTIONED FRONTAL BOUNDARY STILL EXPECTED TO BISECT THE
LAKE TONIGHT INTO MONDAY WITH MORE EASTERLY WINDS EXPECTED OVER
THE NORTH HALF...AND SOUTHERLY WINDS OVER THE SOUTH HALF. CONCERNS
REGARDING POTENTIAL GALES OVER THE NORTH HALF REMAIN...WITH THIS
SIGNAL STILL APPARENT TODAY. ALTHOUGH NOT COMPLETELY CONFIDENT
WITH PREVAILING GALES...CONFIDENCE IS HIGH ENOUGH TO WARRANT A
GALE WATCH. HAVE ISSUED THIS BEGINNING LATE MONDAY AFTERNOON AND
CONTINUE IT THROUGH EARLY TUESDAY MORNING. TIMING MAY STILL NEED
TO BE ADJUSTED IF THESE GALES BECOME MORE PROBABLE...BUT INITIAL
STABILITY ISSUES LOOK TO BECOME LESS OF AN ISSUE MORE INTO MONDAY
EVENING. STRONG NORTHERLY WINDS WILL THEN BE OBSERVED OVER THE
ENTIRE LAKE ON TUESDAY...WITH BUILDING WAVES LIKELY INTO THE
AFTERNOON.

RODRIGUEZ

&&

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

$$

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