Area Forecast Discussion
Issued by NWS Chicago, IL

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458
FXUS63 KLOT 201626
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1126 AM CDT WED MAY 20 2015

.UPDATE...
1125 AM CDT

THE MAIN CHANGES TO TODAYS FORECAST WAS TO SHIFT HIGHER POPS FARTHER
NORTH ACROSS NORTH CENTRAL ILLINOIS...WHERE A GOOD BAND OF 850-700
MB FGEN IS CURRENTLY DRIVING AN AREA OF PRECIPITATION. RAPID
EVAPORATIVE COOLING IN THIS AREA IN THE LOWER LEVEL DRIER AIR HAS
EVEN ALLOWED FREEZING LEVELS TO GET LOW ENOUGH TO ALLOW FOR SOME ICE
PELLETS TO MIX IN WITH THE RAIN WITHIN THIS ENHANCED BAND. THE SLEET
WILL LIKELY BE SHORT LIVED AND CONFINED TO AREAS WITHIN THE HEAVIER
BANDING ACROSS FAR NORTHERN ILLINOIS. IT APPEARS AREAS SOUTH AND
EAST OF THIS ENHANCED AREA OF PRECIPITATION WILL REMAIN DRY FOR A
COUPLE MORE HOURS YET AS THE DRIER EASTERLY LOW LEVEL FLOW WINS OUT.
HOWEVER...WITH TIME IT DOES APPEAR THE MOST OF MY EASTERN AREAS WILL
GET AT LEAST SOME VERY LIGHT RAIN BY MID TO LATE THIS AFTERNOON.

KJB

&&

.SHORT TERM...
250 AM CDT

THROUGH THURSDAY...

PRIMARY FORECAST CONCERN IN SHORT TERM IS TEMPS/RAIN CHANCES TODAY.

CLOUDINESS HAS OVERSPREAD THE REGION AND SHOULD THICKEN AND LOWER
DURING THE DAY. COMBINATION OF OVERCAST SKIES...RAIN OVER MAINLY
SOUTHERN PORTIONS OF CWA...AND PROGGED RECORD/NEAR RECORD
850MB/925MB TEMPS ALL SUGGEST IT WILL BE AN UNSEASONABLY COLD DAY
MUCH MORE REMINISCENT OF NOVEMBER THAN LATE MAY. USING THE EXCELLENT
SPC SOUNDING CLIMO PAGE HTTP://SPC.NOAA.GOV/EXPER/SOUNDINGCLIMO (ALL
LOWER CASE)...IT APPEARS THE RECORD LOW 925MB TEMP FOR ILX FOR 00Z
MAY 21 IS 3.2C WHICH IS RIGHT IN THE BALL PARK OF WHAT GFS/NAM HAVE
FOR 925MB TEMPS THIS AFTERNOON. GIVEN THE HIGHER LIKELIHOOD OF RAIN
IN OUR SOUTHERN CWA ACTUALLY HAVE TEMPS PEAKING NEAR 50 THIS MORNING
THEN FALLING INTO THE 40S THIS AFTERNOON WITH THE RAIN/EVAPORATIVE
COOLING. WINDS OFF THE LAKE SHOULD KEEP NE IL...ESPECIALLY NEAR THE
LAKE IN THE 40S AS WELL. NORTHERN EXTENT OF THE RAIN LOOKS TO END UP
SOMEWHERE NOT FAR FROM I-80 WITH SHARP CUT OFF TO THE NORTH. IF
CHICAGO OFFICIALLY DOESN`T REACH 50F TODAY (WHICH IS A DISTINCT
POSSIBILITY)...IT WOULD BE ONLY THE 8TH TIME THIS WILL HAVE OCCURRED
ON OR AFTER MAY 20TH SINCE OBSERVATIONS WERE MOVED AWAY FROM THE
LAKEFRONT BACK IN THE 1930S.

SKIES SHOULD CLEAR FROM THE NORTHWEST TONIGHT...BUT THINKING THAT
CLOUDS WILL STICK AROUND LONG ENOUGH AND THERE SHOULD BE ENOUGH WIND
TO PREVENT SIGNIFICANT FROST THREAT IN THE CWA. HAVING SAID
THAT...SOME OF THE TYPICALLY COLDER HOLES COULD VERY WELL DIP INTO
THE FOR LOWS 30S THURSDAY MORNING. METEOROLOGICAL CALENDAR WILL FLIP
FROM NOVEMBER TODAY BACK TO MORE TYPICAL MAY TOMORROW WITH DRY AIR
MASS AND WEST-NORTHWEST WINDS ADVECTING A COMPRESSIONALLY WARMED
HIGH PLAINS AIR MASS INTO THE AREA IN THE AFTERNOON. FULL
SUNSHINE AND DRY AIR SHOULD ALLOW FOR GOOD MIXING AND ALLOW HIGHS
TO REBOUND INTO THE UPPER 60S TO AROUND 70F.

IZZI

&&

.LONG TERM...
250 AM CDT

THURSDAY NIGHT THROUGH TUESDAY...

BRIEF VISIT FROM THE NE NOAM TROUGH THAT HAS BEEN A COMMON FEATURE
THE PAST COUPLE YEAR LOOKS TO BE BRIEF WITH MEDIUM RANGE GUIDANCE
CONTINUING TO ADVERTISE THIS FEATURE WEAKENING AND UPPER RIDGING
DEVELOPING OVER THE CENTRAL CONUS AHEAD OF WESTERN CONUS CUT OFF LOW
THIS WEEKEND. THE STRONG WESTERN TROUGH IS PROGGED TO DAMPEN OUT AS
IT MOVES INTO THE PLAINS AS WE TRANSITION TO AN INCREASINGLY ZONAL
FLOW HEADING INTO NEXT WEEK. BEFORE THIS HAPPEN...SOUTHERLY WINDS
SHOULD DRAW GULF MOISTURE NORTHWARD INTO THE REGION LATER THIS
WEEKEND WITH DECENT CHANCE OF SOME SHOWERS AND THUNDERSTORMS IN THE
REGION LATER SUNDAY INTO MONDAY. NOT LOOKING LIKE A WASH
OUT...THOUGH MEDIUM RANGE MODELS CONTINUE TO SUGGEST A GOOD SHOT OF
AT LEAST SOME RAIN DURING THE PERIOD. COULD REMAIN A SOMEWHAT
UNSETTLED PATTERN HEADING INTO NEXT WEEK AS BAROCLINIC ZONE
POTENTIALLY SETS UP SOMEWHERE IN THE MIDWEST WITH LOW AMPLITUDE
WAVES IN THE ZONAL FLOW POTENTIALLY SETTING A MORE ACTIVE MCS
PATTERN FROM THE CENTRAL PLAINS INTO THE LOWER LAKES OR OHIO VALLEY
REGION.

IZZI

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 16Z...

* EAST WINDS AT 11KT THROUGH 01Z

* LIGHT SHOWERS OR SPRINKLES APPROACHING AFTER 18Z.

MM

//DISCUSSION...UPDATED 16Z...

LATEST RADAR IMAGERY AND SURFACE OBS INDICATE SHIELD OF LIGHT VFR
RAIN APPROACHING RFD. WHILE THERE IS CONSIDERABLE DRY AIR AT THE
SURFACE TO OVERCOME...HIGH CONFIDENCE THAT RFD WILL SEE -SHRA
THROUGH THE NEXT 3 HOURS. EAST-SOUTHEAST OF RFD...THE BAND OF RAIN
WILL LIKELY DISSIPATE CONSIDERABLY SO JUST A TEMPO MENTION OF
-SHRA AT ALL OTHER TAF SITES. THUNDERSTORMS ARE NOT EXPECTED.

MM

PREVIOUS DISCUSSION FROM 12Z...

LOW PRESSURE IS MOVING EAST ACROSS THE PLAINS WITH RAIN ACROSS
PORTIONS OF IOWA AND MISSOURI. THIS RAIN WILL SPREAD EAST ACROSS
NORTHERN ILLINOIS LATE THIS MORNING AND AFTERNOON. MODELS HAVE
BEEN TRENDING A LITTLE FARTHER NORTH WITH MEASURABLE
PRECIPITATION...HOWEVER FORECAST SOUNDING CONTINUE TO SHOW A DEEP
LAYER OR DRY AIR AT THE TERMINALS THAT WILL HELP TO EVAPORATE AT
LEAST SOME OF THE FALLING RAIN. LIGHT RAIN OR SPRINKLES APPEAR
POSSIBLE AT THE TERMINALS FROM EARLY TO MID AFTERNOON TODAY
HOWEVER EXPECT CONDITIONS TO REMAIN VFR. WINDS TODAY WILL BE OUT
OF THE NORTHEAST...BECOMING VARIABLE THIS EVENING AND
OVERNIGHT...THEN SHOULD BE WEST OR NORTHWEST TOMORROW.

BMD

//ORD AND MDW CONFIDENCE...UPDATED 16Z...

* MEDIUM CONFIDENCE IN RAIN SHOWERS TIMING.

* HIGH CONFIDENCE IN NO THUNDERSTORMS.

* HIGH CONFIDENCE IN ALL OTHER FORECAST ELEMENTS.

MM

//OUTLOOK FOR ORD/MDW FOR 00Z FRIDAY-12Z WEDNESDAY...UPDATED 12Z...

THURSDAY NIGHT THROUGH SATURDAY...VFR.
SATURDAY NIGHT THROUGH TUESDAY...PERIODIC CHANCES FOR
THUNDERSTORMS.

BMD

&&

.MARINE...
244 AM CDT

HIGH PRESSURE WILL BUILD ACROSS THE REGION TODAY ALLOWING LIGHT
FLOW AND AFTERNOON LAKE BREEZES ACROSS THE SOUTH END OF THE
LAKE...WHILE SOUTHWEST WINDS OF 15 TO 20 KT ARE IN PLACE ACROSS
THE NORTH. WEST WINDS WILL BE IN PLACE ACROSS THE LAKE ON THURSDAY
AS A SLOW MOVING TROUGH GRADUALLY SLIDES SOUTH ACROSS THE LAKE.
WINDS TURN NORTH/NORTHEAST BEHIND THE TROUGH WHICH SHOULD CLEAR
THE SOUTH END OF THE LAKE BY EARLY ON FRIDAY. HIGH PRESSURE BUILDS
BACK OVERHEAD ON FRIDAY...THEN SOUTH FLOW RETURNS ACROSS THE LAKE
FOR THE WEEKEND.

BMD

&&

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

$$

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