Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 011549
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
1049 AM CDT SUN MAY 1 2016

.UPDATE...
1044 AM CDT

UNFORTUNATELY IT APPEARS MOTHER NATURE HAS OPTED TO HIT THE PAUSE
BUTTON...KEEPING MUCH OF NORTHERN ILLINOIS AND NORTHWEST INDIANA
STUCK IN A COOL/CLOUDY PATTERN. AS HAS BEEN THE CASE FOR THE PAST
FEW DAYS...CLOUDS WILL BLOCK MUCH OF THE SUN AGAIN TODAY AND KEEP
TEMPERATURES GENERALLY IN THE 50S ALONG AND SOUTH OF I-80. PERHAPS
A FEW SITES MAY TOUCH 60 DEGREES IN THE FAR SOUTHERN FORECAST
AREA. ELSEWHERE THE CONTINUED NORTHEAST/NORTH FLOW WILL KEEP THE
NORTHERN COUNTIES OF ILLINOIS IN THE UPPER 40S TO LOW 50S...AND
AREAS ADJACENT TO LAKE MICHIGAN WILL HAVE THE COOL MARINE AIR
KEEPING TEMPERATURES IN THE MIDDLE TO UPPER 40S.

AS THE UPPER LEVEL VORT MAX BEGINS TO LIFT NORTHEAST THIS
AFTERNOON FROM SOUTHERN IOWA/NORTHERN MISSOURI...A WEAKER SFC LOW
WILL BE SIMULTANEOUSLY LIFTING NORTHEAST ACROSS CENTRAL IL INTO
CENTRAL IN. THIS WILL HELP TO PROVIDE A MORE FORCED ENVIRONMENT
OVER THE FORECAST AREA LATER THIS AFTERNOON THAT SHOWERS WILL ONCE
AGAIN DEVELOP...WITH A SLIGHT CHANCE FOR A FEW EMBEDDED
THUNDERSTORMS MAINLY ALONG AND SOUTH OF THE KANKAKEE VALLEY.

BEACHLER

&&

.SHORT TERM...
248 AM CDT

THROUGH TONIGHT...

A CLOUDY AND DREARY DAY IS ON TAP TODAY ACROSS NORTHERN ILLINOIS
AND NORTHWEST INDIANA. AN ELONGATED AREA OF LOW PRESSURE IS
DRAPED ACROSS CENTRAL ILLINOIS AND INDIANA WITH LOW
CLOUDS...PATCHY FOG...AND DRIZZLE IN PLACE ACROSS THE LOCAL
FORECAST AREA EARLY THIS MORNING. MEANWHILE...AN UPPER LOW OVER
NEBRASKA EARLY THIS MORNING IS PROGGED TO MOVE EAST ACROSS IOWA
THIS AFTERNOON AND INTO NORTHERN ILLINOIS THIS EVENING. SCATTERED
SHOWERS ARE EXPECTED TO REDEVELOP OUT AHEAD OF THIS LOW MAINLY
DURING THE AFTERNOON AND EVENING HOURS FOR NORTHERN ILLINOIS AND
NORTHWEST INDIANA. MODELS CONTINUE TO INDICATE MODEST ELEVATED
INSTABILITY...SO CANNOT RULE OUT AN ISOLATED THUNDERSTORM MAINLY
SOUTH OF I-80 NEAR THE SURFACE FRONT...BUT THE OVERALL LIKELIHOOD
OF THUNDER IS LOW. THE UPPER LOW WILL EXIT TO OUR EAST AROUND
MIDNIGHT OR SO WITH PRECIP CHANCES QUICKLY TAPERING THEREAFTER.
BUMPED DOWN TEMPERATURES SOME FOR TODAY AND EXPECT FAIRLY SIMILAR
CONDITIONS TO YESTERDAY WITH NO REAL CHANGE IN AIRMASS. LESS
PRECIP TODAY MIGHT HELP US WARM A LITTLE MORE BUT OVERCAST SKIES
AND MORNING FOG WILL INHIBIT INSOLATION AND MODELS HAVE BEEN TOO
WARM THE LAST COUPLE OF DAYS. THE FAR SOUTHERN TIER OF COUNTIES
WOULD HAVE THE BEST CHANCE FOR BREAKING OUT OF THE LOW CLOUDS AND
SEEING A LITTLE SUNSHINE THUS EXCEEDING TEMPERATURE PROJECTIONS
BUT LEANING AGAINST THAT SCENARIO FOR NOW.

DEUBELBEISS

&&

.LONG TERM...
248 AM CDT

MONDAY THROUGH SATURDAY...

STRONG BLOCKING RIDGE WILL BE IN PLACE ACROSS MUCH OF THE WESTERN
HALF OF THE CONTINENT TO START NEXT WEEK...WHILE A DEEP TROUGH
CONTINUES ACROSS THE MIDWEST AND GREAT LAKES. SOUTHERN STREAM
SHORT WAVES WILL BRING A SLIGHT CHANCE FOR A FEW LIGHT SHOWERS OR
SPRINKLES SOUTH OF I-80 ON MONDAY...FAVORED DIURNALLY IN THE
AFTERNOON AND EARLY EVENING. FORECAST SOUNDINGS SHOW AN
INVERSION/ISOTHERMAL LAYER BASED AROUND 800 MB MONDAY SO THERE
ISNT MUCH OPPORTUNITY FOR DEEP CONVECTIVE GROWTH...AND SUSPECT GFS
IS OVERPRODUCING QPF. WILL MAINTAIN 20 POPS FOR THE FAR SOUTHERN
FORECAST AREA. EXPECT HIGH TEMPS AROUND 60.

ON TUESDAY...WILL SEE A LITTLE MORE SUNSHINE AS TRANSIENT MID
LEVEL RIDGING PUSHES ACROSS THE AREA. FORECAST SOUNDINGS TUESDAY
INDICATE STEEP LOW/MID LEVEL LAPSE RATES DEVELOPING DURING THE
AFTERNOON WHICH SHOULD SUPPORT AT LEAST SOME DIURNAL CU
GROWTH...THOUGH WITH NO SIGNIFICANT FORCING OVER THE AREA WILL NOT
MENTION ANY POPS AT THIS TIME EVEN THOUGH THERMODYNAMICALLY
TUESDAY LOOKS BETTER FOR PRODUCING PRECIP THAN MONDAY. MODELS ARE
BACKING OFF ON TEMPERATURES TUESDAY AFTERNOON...DESPITE DEEP
MIXING STILL EXPECTED. TEMPS ALOFT NOW APPEAR TO ONLY SUPPORT LOW
TO MID 60S SO HAVE TRIMMED BACK TEMPS A FEW DEGREES.

TUESDAY NIGHT INTO WEDNESDAY WILL BRING MORE UNSETTLED AND COOLER
CONDITIONS BACK TO THE REGION. GUIDANCE CONTINUES TO INDICATE A
WAVE TOPPING THE STRONG WEST COAST RIDGE NEAR THE ARCTIC CIRCLE
EARLY IN THE WEEK WILL DISLODGE A COLD AIRMASS SET TO ARRIVE INTO
PORTIONS OF THE GREAT LAKES TUESDAY NIGHT INTO WEDNESDAY. GFS
CONTINUES TO RUN SLIGHTLY COOLER THAN THE ECMWF THOUGH BOTH MODELS
WOULD SUPPORT HIGHS ONLY IN THE LOW TO MID 50S WEDNESDAY AND
THURSDAY...WITH TEMPS NEAR THE LAKE ONLY IN THE 40S. WILL CONTINUE
TO NUDGE DOWNWARD FROM THE STARTING POINT OF OUR GENERAL MODEL
CONSENSUS BLEND WHICH STILL APPEARS TOO WARM...HOWEVER ANTICIPATE
FURTHER DOWNWARD REVISIONS TO TEMPS IF TRENDS OVER THE PAST COUPLE
DAYS HOLD. THERE REMAIN SOME MODEST TIMING AND TRACK
DIFFERENCES...BUT SHOWERS ARE POSSIBLE WITH THE COLD FRONT AND
ADDITIONAL SHOWERS ARE POSSIBLE WEDNESDAY AFTERNOON/EVENING AS
DIURNAL INSTABILITY DEVELOPS ONCE AGAIN.

FOR THOSE HOPING FOR MORE SUMMER-LIKE CONDITIONS TO RETURN...
GUIDANCE CONTINUES TO INDICATE THE WEST COAST RIDGE WILL INCH
EASTWARD THROUGH THE WEEK...AND SHOULD BEGIN TO EXERT ITS
INFLUENCE OVER THE WESTERN GREAT LAKES REGION BY LATE IN THE
WEEK AND INTO NEXT WEEKEND. THIS WILL BRING A WARMING TREND FRIDAY
AND SATURDAY ALONG WITH DRY CONDITIONS AND PLENTY OF SUNSHINE. IF
ECMWF/GFS PROJECTIONS HOLD...WE WILL BE WARMING WELL INTO THE 70S
SATURDAY POSSIBLY PUSHING THE 80 MARK IN SOME AREAS.

DEUBELBEISS

&&

.AVIATION...
FOR THE 12Z TAFS...

PRIMARY CONCERNS...

-LIFR CIGS/VSBYS EARLY THIS AM WITH DRIZZLE/FOG.
-TIMING OF MODEST IMPROVEMENTS THROUGH MID-DAY.
-PERIOD OF SHRA EXPECTED LATE AFTN/EVE.
-CIGS LOWERING TO IFR/LIFR AGAIN TONIGHT.

EXTENSIVE AREA OF LIFR CIGS AND IFR/LIFR VIS HAS DEVELOPED ACROSS
NORTHERN IL/NORTHWEST IN OVERNIGHT...IN COOL/MOIST LOW LEVEL
NORTHEAST FLOW NORTH OF STATIONARY FRONT AND WEAK SURFACE LOW
TRACK. EXPECT LIFR CONDITIONS TO IMPACT TERMINALS EARLY THIS
MORNING...WITH A GRADUAL IMPROVEMENT TO IFR BY LATE MORNING AND
PERHAPS LOW MVFR FOR A BIT EARLY THIS AFTERNOON.

MID-LEVEL WAVE OVER THE MID-MISSOURI VALLEY EARLY THIS MORNING
WILL PROPAGATE EASTWARD TODAY...MOVING ACROSS THE FORECAST AREA
EARLY THIS EVENING. SHOWERS ARE ANTICIPATED WITH THIS FEATURE
BEGINNING LATE THIS AFTERNOON AND CONTINUING THROUGH THE EARLY
EVENING. WEAK ELEVATED INSTABILITY IS PRESENT THOUGH PRIMARILY
SOUTH OF TERMINALS...WHICH MAY PRODUCE SOME ISOLATED EMBEDDED
THUNDER SOUTH OF THE TAF SITES. PRECIPITATION WILL MOISTEN COOL
LOW LEVEL AIR IN PERSISTENT NORTHEAST FLOW OFF LAKE MICHIGAN AND
RESULT IN A DETERIORATION OF CIGS AGAIN TO IFR/LIFR. SURFACE HIGH
PRESSURE RIDGE THEN BEGINS TO BUILD IN FROM THE NORTH LATE
TONIGHT...WITH SUBSTANTIAL DRYING OF LOW LEVELS EXPECTED TO
SCATTER OUT LOW CIGS BY SUNRISE MONDAY. NORTHEAST WINDS GENERALLY
10-15 KTS EXPECTED THROUGH THE PERIOD.

RATZER

&&

.MARINE...
253 AM CDT

A PERIOD OF MODERATELY STRONG NORTHEAST WINDS GUSTING 20-25 KT
WILL PERSIST ACROSS LAKE MICHIGAN TODAY...EVENTUALLY BACKING MORE
NORTHERLY AND DIMINISHING AS LOW PRESSURE MOVES EAST ACROSS
ILLINOIS AND INDIANA AND REACHES THE UPPER OHIO VALLEY EARLY
MONDAY. WEAK HIGH PRESSURE RIDGING WILL THEN SETTLE SOUTH ACROSS
THE GREAT LAKES REGION MONDAY AND MONDAY EVENING...BEFORE AN AREA
OF LOW PRESSURE APPROACHES FROM ONTARIO. THIS LOW...AND ITS
ACCOMPANYING COLD FRONTAL TROUGH... WILL PUSH SOUTHEAST ACROSS
THE LAKE TUESDAY AND TUESDAY NIGHT. LIGHT WINDS WILL SHIFT TO THE
SOUTHWEST AND INCREASE TO 15-25 KTS INITIALLY AS THE TROUGH AND
LOW APPROACH TUESDAY...SHIFTING NORTH-NORTHWEST BEHIND THE FRONT
EARLY WEDNESDAY WITH SPEEDS 25-30 KT AND PERSISTING INTO THURSDAY
BEFORE DIMINISHING THURSDAY NIGHT/EARLY FRIDAY AS HIGH PRESSURE
RIDGING BUILDS IN FROM THE NORTHWEST. ON THE SOUTH END OF THE
LAKE...MARGINAL SMALL CRAFT ADVISORY CONDITIONS ARE EXPECTED TO
CONTINUE THROUGH THE DAY. WILL MONITOR AS WE MAY BE ABLE TO CANCEL
EARLIER THAN CURRENT EXPIRATION THROUGH THIS EVENING.

RATZER

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...NEARSHORE WINTHROP HARBOR TO GARY IN UNTIL 1 AM
     MONDAY.

&&

$$

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