Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 201949
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
249 PM CDT WED MAY 20 2015

.SHORT TERM...
247 PM CDT

THROUGH THURSDAY...

A BAND OF FRONTOGENESIS WITHIN THE 850-700 MB LAYER CONTINUES TO
SUPPORT THE MAIN AREA OF RAIN ACROSS THE NORTHERN TIER OF COUNTIES
IN ILLINOIS EARLY THIS AFTERNOON. ELSEWHERE...ADDITIONAL RAIN
SHOWERS CONTINUE TO TRY TO SHIFT EASTWARD OVER THE AREA WITHIN A
BROAD AREA OF WARM AIR ADVECTION/ISENTROPIC UPGLIDE IN ASSOCIATION
WITH AN APPROACHING MID-LEVEL DISTURBANCE. HOWEVER...THERE CONTINUES
TO BE A LOT OF DRY AIR ACROSS MUCH OF NORTHEASTERN ILLINOIS AND
NORTHWESTERN INDIANA. IN FACT...THE LATEST AMDAR SOUNDINGS OUT OF
KMDW CONTINUE TO INDICATE LARGE DEWPOINT DEPRESSIONS IN EXCESS OF 10
DEGREES CELSIUS BELOW 750 MB. THIS IS MAKING IT HARD FOR THE AREA OF
RAIN TO MAKE MUCH PROGRESS EASTWARD OUTSIDE OF THE STRONGLY FORCED
BAND OF FGEN OVER FAR NORTHERN ILLINOIS. THE DRY AIR HAS ALSO
ALLOWED FREEZING HEIGHTS TO LOWER DUE TO STRONG EVAPORATIVE
COOLING...AND THIS HAS ALLOWED THE PRECIP IN SOME AREAS TO ONSET AS
A RAIN AND ICE PELLET MIXTURE...WITH SURFACE TEMPERATURES FALLING
INTO THE LOWER 40S.

WE ARE STILL THINKING SOME LIGHT RAIN WILL BE POSSIBLE INTO THE
CHICAGO METRO LATE THIS AFTERNOON BEFORE THE ACTIVITY SHIFTS EAST OF
THE REGION. THEREFORE...WE WILL CONTINUE WITH CHANCE POPS INTO EARLY
THIS EVENING FOR MY EASTERN AREAS. ONCE THE RAIN ENDS THIS
EVENING...SKIES MAY BE SLOWER TO CLEAR THAN CURRENTLY FORECAST.
REGIONAL SATELLITE IMAGERY INDICATES MUCH OF THE CENTRAL CONUS IS
CLOUDED OVER AT THIS HOUR. AS SUCH...WE MAY HANG ON TO SOME CLOUD
COVER TONIGHT...AND IF THIS HAPPENS...TEMPERATURES MAY NOT DROP OFF
TOO MUCH FROM THEIR CURRENT VALUES.

WARMER WEATHER IS STILL SET UP TO RETURN BY THURSDAY. THEREFORE
THE CURRENT COLD CONDITIONS WILL BE A DISTANT MEMORY BY THURSDAY
AFTERNOON. HIGH TEMPERATURES LOOK TO TOP OUT IN THE MID TO UPPER
60S IN MOST AREAS.

KJB

&&

.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 20Z...

* NORTHEAST WINDS OVER 10KT THROUGH AT LEAST 00Z.

* LIGHT SHOWERS APPROACHING TERMINALS.

MM

//DISCUSSION...UPDATED 20Z...

RADAR AND SURFACE OBS CONTINUE TO SHOW LIGHT RAIN PROGRESSING ACROSS
NORTHERN IL...IMPACTING CHICAGO TERMINALS MOMENTARILY. EXPECTING
CIGS TO DROP TO NEAR 050 WHILE THE RAIN CROSSES. VSBYS MAY BRIEFLY
DROP TO 5SM IN LIGHT RAIN...BUT FOR THE MOST PART THIS HAS BEEN A
VFR RAIN. AREAS TO THE NORTH...HEAVIER RAIN SET UP AND IMPACTED RFD
DROPPING CIGS TO IFR. THIS WILL LIKELY NOT BE THE CASE FOR MDW/ORD
AS THERE IS STILL A SIGNIFICANT AMOUNT OF DRY AIR TO OVERCOME.
THUNDERSTORMS ARE NOT EXPECTED.

MM

//ORD AND MDW CONFIDENCE...UPDATED 20Z...

* HIGH CONFIDENCE IN RAIN SHOWERS TIMING

* MEDIUM CONFIDENCE IN REMAINING VFR.

* 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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