Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 190844
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
344 AM CDT SUN APR 19 2015

.SHORT TERM...
340 AM CDT

THROUGH MONDAY...

UPPER AIR ANALYSIS EARLY THIS MORNING DEPICTS DEEP UPPER LEVEL LOW
PRESSURE NOW PUSHING INTO WESTERN KANSAS. MULTIPLE VORT MAXES ARE
ROTATING AROUND THE LOW. THE STRONGEST OF THESE EXIST IN TEXAS AND
THROUGH THE CENTRAL PLAINS...WITH SOME SMALLER SCALE WAVES EXTENDING
INTO EASTERN MO/FAR WESTERN IL WERE SOME SCATTERED THUNDERSTORMS
WERE ONGOING. A NORTHERN STREAM SHORTWAVE IS ENTERING THE NORTHERN
PLAINS WITH SOME SHOWERS OUT AHEAD OF IT AS WELL. AT THE
SURFACE...THE COLD FRONT THAT PASSED THROUGH NORTHEAST ILLINOIS AND
NW INDIANA SATURDAY HAS PUSHED INTO CENTRAL ILLINOIS...BUT HAS
LIKELY COMPLETED ITS SOUTHWARD RETREAT AND HAS WASHED OUT OVER THE
PAST FEW HOURS. OVER THE LOCAL AREA...MUCH DRIER NE FLOW EXISTS WITH
DEWPOINTS IN THE MID TO UPPER 20S IN THE CITY TO NEAR 40
SOUTH...THOUGH DEWPOINTS ARE POOLING ALONG THE FRONT IN THE UPPER
40S JUST SOUTH OF THE LOT CWA. THE SURFACE LOW THAT IS EXPECTED TO
DEVELOP FROM THE SOUTHERN STREAM SHORTWAVE IS ALSO TAKING SHAPE OVER
THE SOUTHERN STATES WITH A WARM FRONTAL BOUNDARY EXTENDING FROM
WESTERN ARKANSAS THROUGH THE LOWER TENNESSEE VALLEY. AN EXPANSION OF
LOWER CLOUD TOPS ON IR IMAGERY ALONG AND AHEAD OF THE WARM FRONT IS
OCCURRING.

LOW LEVEL WARM/MOIST ADVECTION WILL CONTINUE ITS SLOW PROGRESSION
NORTHWARD UNDER MODEST SOUTH/SOUTHEAST FLOW. DEEPER MOISTURE/LIFT
REALLY HOLD OFF UNTIL THE AFTERNOON PERIOD ACROSS MOST OF THE AREA.
A SOMEWHAT COMPLICATED EVOLUTION AS A MAIN SURFACE LOW CENTER IS
FORECAST TO STRENGTHEN AS IT LIFTS NORTH THROUGH INDIANA TODAY THEN
FURTHER STRENGTHEN TONIGHT INTO MONDAY WHILE SEVERAL ADDITIONAL
SURFACE LOWS DEVELOP AND EVENTUALLY MERGE WITH THE MAIN LOW CENTER.
THE SURFACE LOW WILL EVENTUALLY BECOME DRIVEN BY A COMBINATION OF
THE SW FLOW AHEAD OF THE SOUTHERN STREAM UPPER LOW AND THE
NORTHERN STREAM SHORTWAVE.

OF NOTE LATER TODAY...EXPECT COVERAGE OF SHOWERS TO INITIALLY BE
SLOW GIVEN THE DRIER AIR PLACE...BUT INCREASING ASCENT ALOFT...LOW
LEVEL SSE ISENTROPIC ASCENT AHEAD OF THE LOW AND WRAPPING AROUND THE
CLOSED CIRCULATION EXTENDING THROUGH 850 AND EVEN 700 MB COMBINED
WITH MODEST LOWER LEVEL FGEN SHOULD ALLOW FOR A BLOSSOMING OF
SHOWERS ACROSS THE AREA LATER THIS AFTERNOON AND EVENING. THE
HIGHEST PWAT AXIS EXPANDS ACROSS INDIANA TODAY BUT SHOULD GET
WRAPPED INTO NE ILLINOIS. RAP MOST UNSTABLE CAPE VALUES RELATIVELY
LOW BUT DO EXPAND FROM SE TO NW THIS AFTERNOON AND THIS COUPLED WITH
THE PUNCH FROM THE DRY SLOT OF THIS SYSTEM MAY ENABLE A FEW T-STORMS
AS WELL...SO SOME OF THE SHOWERS WILL HAVE A MODEST KICK TO THEM IN
THE MODERATE/HEAVY RAINFALL DEPARTMENT. EXPECT THAT NORTH CENTRAL
ILLINOIS MAY BE UNDER A BIT OF SQUEEZE PLAY FROM THESE TWO
SYSTEMS...THUS WHILE SHOWERS ARE STILL EXPECTED...HIGHER CONFIDENCE
EXISTS IN COVERAGE FARTHER EAST.

THIS EVENING THE FIRST MAIN SHORTWAVE WILL BE PASSING NE WHILE THE
MAIN SOUTHERN STREAM ENERGY PASSES EAST THROUGH THE OHIO
VALLEY...AND THE NORTHERN STREAM SHORTWAVE LAGS BEHIND BACK IN
MN/IA. THERE IS ALSO THE LOSS OF DEEPER MOISTURE...THEREFORE EXPECT
SHOWERS TO TAPER/END/TURN TO DRIZZLE POSSIBLY OVERNIGHT FOR A TIME.
THEN MONDAY MORNING WE GET THE COUPLED UPPER LEVEL FORCING FROM THE
NORTHERN STREAM LOW AND SURFACE COLD FRONT WHICH WILL LIKELY BRING
ANOTHER ROUND OF SHOWERS MONDAY MORNING. SHOWERS WILL EASE MONDAY
AFTERNOON. IT WILL BE QUITE BREEZY/WINDY BEHIND THE FRONT ON MONDAY
IN THE COLD ADVECTION REGIME.

KMD

&&

.LONG TERM...
340 AM CDT

MONDAY NIGHT THROUGH SATURDAY...

THE THEME OF THE EXTENDED WILL BE THE UNSEASONABLY COOL WEATHER. A
HIGH AMPLITUDE PATTERN WILL BE IN PLACE WITH STRONG UPPER LEVEL
RIDGING EXTENDING WELL NORTH INTO THE CANADIAN ROCKIES AND SIMILARLY
HIGH AMPLITUDE RIDGING IN THE NORTH ATLANTIC. THAT LEAVES OUR
NORTHERN STREAM SHORTWAVE FROM SUNDAY/MONDAY WITH NO WHERE TO GO.
THIS WILL KEEP MUCH OF THE EASTERN HALF OF THE COUNTY...INCLUDING
OUR REGION...UNDER COOL NORTHWEST FLOW. THIS PATTERN ALSO FAVORED
STRONGLY SHEARED OUT AND ELONGATED VORT MAXES TO PIVOT THROUGH
BRINGING PERIODS OF SHOWERS. WITH PWAT VALUES QUITE LOW IN THIS
AIRMASS...MUCH OF THE PERIOD WILL BE DRY AND EVEN THE SHOWERS SHOULD
NOT BE VERY STRONG. BUT MORE IMPORTANTLY...SEASONAL AVERAGE
HIGHS/LOWS FOR LATE APRIL ARE THE LOWER 60S (MID 60S SOUTH) AND
LOWER 40S RESPECTIVELY...AND MUCH OF THIS PERIOD WILL FEATURE HIGHS
IN THE 50S AND LOWS IN THE MID 30S...POSSIBLY COOLER A NIGHT OR TWO
IF WINDS ARE LIGHTER. TEMPERATURE ANOMALIES ARE NOT THAT EXTREME
THOUGH...SO RECORD COLD IS NOT ANTICIPATED.

KMD

&&

.AVIATION...

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

* EAST WINDS ARND 6 TO 10 KT...OCCASIONAL GUSTS TO 17KT.

* WINDS TURN NORTH LATE TONIGHT...THEN NORTHWEST EARLY MON.

* RAIN OVERSPREAD TAF SITES LATE THIS AFTN THRU TONIGHT.

* POSSIBLE LOW END MVFR CIGS OR PATCHY IFR CIGS DEVELOPING.

BEACHLER

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

WITH THE DEPARTING HIGH PRESSURE TO THE EAST...MID-LVL MOISTURE
WAS SLOWLY LIFTING NORTH ACROSS THE NORTHERN IL/NORTHWEST IN
AIRFIELDS. THIS HAS ALLOWED A CIRRUS CLOUD DECK TO THICKEN...AND
ADDITIONAL MOISTURE IS EXPECTED TO LIFT NORTHEAST AS A LOW
PRESSURE SYSTEM NEARS THE REGION LATER TONIGHT INTO EARLY MON.
GUIDANCE HAS BEEN SLOWLY TRENDING LATER WITH THE ARRIVAL OF PRECIP
UNTIL AFT 18Z...AND MAY NOT BE CLOSER TO 23Z. WILL CONTINUE TO
MENTION VCSH BUT PUSH IT TO 20Z...THEN EXPECT THE COVERAGE TO
INCREASE IN PRECIP CLOSER TO 23Z. WITH THE STEADILY THICKEN AND
LOWER CLOUDS...THE CHALLENGE IS ALSO ON WIND SPEEDS. IT IS STILL
POSSIBLE GUSTS WILL NEED TO BE PUT BACK INTO THE TAF...ALTHOUGH
WITH THE ABUNDANT CLOUD COVER...IT IS QUESTIONABLE HOW FREQUENT
GUSTS WILL OCCUR AT THE TAF SITES. SO HAVE REMOVED THE MENTION AT
THIS TIME AND JUST INCREASED THE SUSTAINED WINDS. THEN AT SOME
POINT LATE THIS EVENING CIGS ARE EXPECTED TO QUICKLY DIP TO MVFR
AND POSSIBLY TO IFR CONDS. PRECIP COVERAGE WILL BEGIN TO DIMINISH
EARLY MON...AND MAY TREND TOWARDS PATCHY DRIZZLE WHICH MAY ALSO
FURTHER LOWER CIGS AND VSBYS EARLY MON MORNING.

BEACHLER

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

* HIGH CONFIDENCE IN ALL ELEMENTS THROUGH 18Z...MEDIUM CONFIDENCE
  IN WIND DIR/SPEED AFT 18Z.

* MEDIUM CONFIDENCE IN PRECIP TIMING/COVERAGE.

* HIGH CONFIDENCE IN CIGS DIMINISHING TO AT LEAST MVFR CONDS AFT
  21Z...AND MEDIUM CONFIDENCE IN CIGS BECOMING IFR AFT 3Z MON.

BEACHLER

//OUTLOOK FOR ORD/MDW FOR 12Z MONDAY-12Z SATURDAY...UPDATED 00Z...

MONDAY...CHANCE OF SHRA/MVFR. GUSTY WEST-NORTHWESTERLY WINDS.

TUESDAY...SLGT CHANCE PM SHRA...BUT MAINLY VFR. GUSTY WEST WINDS.

WEDNESDAY...SLGT CHANCE SHRA. MAINLY VFR. WEST-NORTHWEST WINDS.

THURSDAY...VFR. NORTH-NORTHEAST WINDS.

FRIDAY...MAINLY VFR. CHC PM SHRA.

KJB

&&

.MARINE...
257 AM CDT

LOW PRESSURE OVER THE CENTRAL PLAINS WILL SLOWLY LIFT NORTHEAST
TOWARDS THE LOWER GREAT LAKES REGION TONIGHT/EARLY MON. GUIDANCE
CONTINUES TO INDICATE THE GRADIENT WILL TIGHTEN ACROSS THE
LAKE...HOWEVER THIS APPEARS TO BE MORE FOCUSED ALONG THE SOUTHERN
TIP OF THE LAKE. WITH THE CONTINUED EASTERLY WINDS...IT IS
POSSIBLE MARGINAL SMALL CRAFT CONDITIONS MAY DEVELOP FOR THE
ILLINOIS SHORELINE LATER TODAY INTO TONIGHT. THE LOW PRESSURE WILL
THEN APPROACH THE LAKE AND BRIEFLY WEAKEN THE GRADIENT EARLY MON.
THEN AS THE LOW LIFTS NORTHEAST LATER IN THE DAY MON...IT WILL
DEEPEN AND MERGE WITH ANOTHER LOW...WHICH WILL CREATE A TIGHT
GRADIENT OVER THE LAKE AND INCREASING WEST/NORTHWEST TUE. AFTER
TUE THE DOMINANT WIND DIRECTION WILL BE NORTHWEST THROUGH MID-
WEEK...BEFORE RELAXING TO THE WEST. THIS WILL ALLOW HIGHER WAVES
TO BE FOCUSED IN THE SOUTHERN HALF OF LAKE MICHIGAN THROUGH MID-
WEEK.

BEACHLER

&&

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

$$

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