Area Forecast Discussion
Issued by NWS Chicago, IL

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602
FXUS63 KLOT 210539
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1239 AM CDT THU MAY 21 2015

.UPDATE...
818 PM CDT

THE UPPER LEVEL DISTURBANCE WITH MOST OF THE RAIN AND
INTERMITTENT ICE PELLETS IS MOVING AWAY ACROSS NORTHERN INDIANA
INTO OHIO...BEING USHERED BY AN IMPRESSIVE 100+ KT UPPER JET
MAXIMUM ACROSS THE AREA. CLOUDS REMAIN WITH DIMINISHING BUT COOL
NORTHEAST FLOW HELPING TO KEEP THESE STEADFAST. THE 00Z DVN
SOUNDING INDICATED A NEAR-SATURATED LAYER AROUND 5000 FT
THICK...SO NOT EXPECTING THESE TO DIMINISH TONIGHT...EVEN WITH
SOME DRY ADVECTION IN THAT INDICATED LAYER. ALSO HELPING TO
SOLIDIFY THIS FORECAST IS THAT THE 700MB TROUGH AND LOWER-LEVEL
THERMAL TROUGH ARE STILL LOCATED TO THE WEST OF THE AREA. EVEN
SOME LIGHT SHOWERS HAVE BEEN NOTED AT TIMES ON REGIONAL MOSAIC AS
FAR WEST AS DES MOINES EARLY THIS EVENING...SO CANT EVEN RULE OUT
AN ISOLATED ONE OR TWO OF THOSE THROUGH EARLY OVERNIGHT. STILL DO
THINK CLOUDS WILL SCATTER FROM WEST TO EAST DURING THE MORNING
INTO EARLY AFTERNOON ON THURSDAY AS MORE PRONOUNCED MID-LEVEL
DRYING MOVES IN.

THE 00Z SOUNDINGS FROM ACROSS THE UPPER MISSISSIPPI VALLEY LOOK
MORE OF LATTER MARCH THAN THEY DO LATTER MAY. THE DVN 850MB
TEMPERATURE OF -3C AND 925MB TEMPERATURE OF 1C ARE AT OR BELOW
DAILY AND EVEN LATE MAY RECORDS FOR THEIR 20-YEAR HISTORY...AND
ILX WITH A LONGER PERIOD OF RECORD IS CLOSE AS WELL. ENVISION
TEMPERATURES OOZING DOWN EVER SO SLOWLY TONIGHT...WITH MINIMUMS IN
THE UPPER 30S TO LOWER 40S. IF CLEARING WERE TO OCCUR /AGAIN NOT
EXPECTED/...THEN WOULD BE CONCERNED WITH A FOG THREAT.

MTF

&&

.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...
311 PM CDT

THURSDAY NIGHT THROUGH WEDNESDAY...

STILL LOOKING LIKE A FEW QUIET DAYS TO START THE PERIOD BEFORE WE
ENTER A WARMER AND ACTIVE PATTERN LATER IN THE HOLIDAY WEEKEND
THROUGH MID NEXT WEEK. A TROUGH/BACKDOOR COLD FRONT WILL SWING
SOUTH THURSDAY NIGHT AND TURN WINDS NORTHERLY. WILL NEED TO WATCH
FOR A BRIEF PERIOD WHERE A FEW RAIN SHOWERS COULD BE POSSIBLE IN
NORTHEASTERN 1/4 OF CWA AS UPPER WAVE MOVES OVER LAKE MICHIGAN
WITH SOUTHWARD TRANSLATION OF FRONT/TROUGH. HAVE MAINTAINED A DRY
FORECAST FOR NOW. FRIDAY LOOKS TO BE A BEAUTIFUL DAY WITH HIGH
PRESSURE CENTERED OVER LAKE MICHIGAN. THERMAL PROGS SUPPORT UPPER
60S TO LOWER 70S INLAND...BUT LOCAL CLIMO ALONG THE LAKE FOR
ONSHORE FLOW THROUGHOUT THE DAY INDICATES THE LAKEFRONT WILL BE
LIMITED TO THE MID TO UPPER 50S.

UPPER PATTERN WILL TRANSITION TO WESTERN TROUGHING AND EASTERN
RIDGING OVER THE HOLIDAY WEEKEND INTO NEXT WEEK. A STRONG UPPER
LOW WILL DROP SOUTHEAST FROM HUDSON BAY FRIDAY INTO SATURDAY AND
THEN SLIDE EAST THE REST OF THE WEEKEND. THIS WILL ENABLE BETTER
HEIGHT RISES TO MOVE OVER OUR AREA AND SOUTHWEST TO WEST SOUTHWEST
FLOW TO DEVELOP ALOFT AS DEEP UPPER TROUGH OVER THE WEST
DAMPENS/OPENS UP AND LIFTS VERY SLOWLY NORTHEAST. AT THE
SURFACE...WEAK LOW PRESSURE WILL DEVELOP OVER THE PLAINS WITH HIGH
PRESSURE CENTERED ALONG THE EAST COAST AND RIDGING EXTENDING
WESTWARD. SOUTHERLY FLOW AND MOISTURE RETURN WILL COMMENCE
SATURDAY NIGHT...WITH SEASONABLY HIGH DEWPOINTS AND PWATS LATER
SUNDAY THROUGH MID WEEK NEXT WEEK.

AS MENTIONED IN PREVIOUS DISCUSSIONS...SUNDAY AND MONDAY OF THE
HOLIDAY WEEKEND DONT LOOK TO BE TOTAL WASHOUTS...BUT MULTIPLE
DISTURBANCES IN ACTIVE SOUTHWEST FLOW PATTERN LOOK TO PUSH ACROSS
THE AREA...SO WE HAVE LIKELY POPS FOR SHOWERS AND POSSIBLY THUNDER
IN THIS PERIOD. INSTABILITY LOOKS FAIRLY MEAGER INITIALLY DUE TO
POOR LAPSE RATES...SO MAY BE MORE OF A SETUP FOR HEALTHY RAIN
TOTALS. SATURDAY WILL BE THE BEST DAY OF THE WEEKEND WITH HIGHS
LOW-MID 70S EXCEPT PERHAPS ALONG IMMEDIATE ILLINOIS SHORE. EVEN
WARMER AIRMASS SUNDAY THROUGH WEDNESDAY MAY NOT REACH FULL
POTENTIAL DUE TO CLOUDS/OCCASIONAL PRECIP...BUT A FEW OF THE DAYS
COULD REACH OR EXCEED 80 DEGREES.

RC

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 06Z...

* WINDS TURNING NORTHEAST IN THE PREDAWN HOURS FRIDAY.

BMD

//DISCUSSION...UPDATED 06Z...

RIDGE OF HIGH PRESSURE LIES SOUTH OF THE TERMINALS TONIGHT WHILE
A COLD FRONT IS IN PLACE ACROSS THE NORTHERN GREAT LAKES REGION.
RELATIVELY QUIET CONDITIONS WILL BE IN PLACE THROUGH THE TAF
PERIOD UNDER THE INFLUENCE OF THE RIDGE WHICH WILL PROVIDE VFR
CONDITIONS AND LIGHT WEST FLOW. DURING THE DAY THURSDAY...MIXING
WILL ALLOW WINDS TO PICK UP SOME WITH GUSTS INTO THE MID TO HIGH
TEENS. THE COLD FRONT WILL MAKE ITS WAY DOWN LAKE MICHIGAN THROUGH
THE PERIOD AND WILL DROP ACROSS THE TERMINALS LATE THURSDAY
NIGHT/EARLY FRIDAY MORNING TURNING WINDS TO THE NORTHEAST.

BMD


//ORD AND MDW CONFIDENCE...UPDATED 06Z...

* MEDIUM-HIGH ON WINDS THROUGH THURSDAY...BUT MEDIUM THURSDAY
  EVENING/NIGHT.
* HIGH ON REMAINING FORECAST ELEMENTS.

BMD

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

* FRIDAY...DRY/VFR. NORTHEAST WINDS.
* SATURDAY...DRY/VFR. SOUTH WINDS.
* SUNDAY...SHRA LIKELY. CHANCE OF TSRA. MAINLY VFR. SOUTH WINDS.
* MONDAY...SHRA/TSRA LIKELY. MVFR POSSIBLE. GUSTY SOUTHWEST WINDS.
* TUESDAY...CHANCE OF SHRA/TSRA. MAINLY VFR. GUSTY SOUTHWEST WINDS.

&&

.MARINE...
318 PM CDT

GENERALLY 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 LATE THURSDAY NIGHT. COULD BE A BRIEF
PERIOD WHERE NORTHERLY GUSTS APPROACH 25 KT...BUT THEN SPEEDS WILL
QUICKLY DIMINISH BY FRIDAY MORNING. HIGH PRESSURE BUILDS BACK
OVERHEAD ON FRIDAY...SO WINDS NEAR THE SHORE WILL BE CONTROLLED BY
LAKE BREEZE INFLUENCES. SOUTH FLOW WILL THEN RETURN ACROSS THE
LAKE FOR THE WEEKEND AS LOW PRESSURE TAKES SHAPE OVER THE PLAINS
AND HIGH PRESSURE SLIDES EAST. SOUTHWEST GUSTS UP TO 25 KT ARE
POSSIBLE ON THE NORTHERN PORTION OF THE LAKE SATURDAY INTO
SATURDAY NIGHT. THE WARM AND MOIST AIRMASS MOVING OVER THE LAKE
LATER IN THE WEEKEND THROUGH MID NEXT WEEK COULD END UP BEING
SUPPORTIVE OF FOG DEVELOPMENT.

RC

&&

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

$$

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