Area Forecast Discussion
Issued by NWS Chicago, IL

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588
FXUS63 KLOT 202150 AAA
AFDLOT

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

.UPDATE...
450 PM CDT

NUMEROUS REPORTS OF ICE PELLETS HAVE BEEN RECEIVED OVER THE PAST
HOUR AND A HALF...INCLUDING OBSERVED HERE AT THE WFO. THIS
INCLUDES PAST ONSET AS THE COLUMN APPEARS COOL ENOUGH WITH
SUFFICIENT FORCING IN A FEW CORRIDORS/BANDS TO ALLOW FOR SOME ICE
PELLETS EVEN AFTER ONSET. DUAL-POL DATA ALSO IS CONFIRMING THIS.
ICE PELLETS HAVE BEEN ADDED INTO THE LATE AFTERNOON/EARLY EVENING
FORECAST AS INTERMITTENT...ALONG WITH SOME ADJUSTMENTS TO THE
TIMING OF RAIN ENDING PER OBSERVATIONAL TRENDS.

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 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...
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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