Area Forecast Discussion
Issued by NWS Chicago, IL

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

000
FXUS63 KLOT 180853
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
353 AM CDT SAT APR 18 2015

.SHORT TERM...
337 AM CDT

THROUGH SUNDAY...

SOME CHANGES ARE IN THE AIR FOR THE NORTHERN HALF OF THE AREA TODAY
AND LOCATIONS CLOSER TO THE LAKESHORE...WHILE SOUTHERN AREAS WILL
ENJOY SUMMER LIKE CONDITIONS THAT PREVAILED ON FRIDAY.

THE CLOSED UPPER LOW THAT WAS OVER THE FOUR CORNERS AS OF THIS TIME
YESTERDAY IS NOW ONLY CENTERED OVER SOUTH CENTRAL COLORADO. THE
CLOUD SHIELD IN THE WARM MOIST ADVECTION SECTOR AHEAD OF THE LOW IS
GETTING CLOSER TO THE AREA...BUT STILL LARGELY SOME MID AND HIGH
CLOUDS DRIFTING THROUGH. VERY MILD CONDITIONS ARE IN PLACE WITH
DEWPOINTS HOLDING IN THE MID 40S NORTH TO LOWER 50S SOUTH KEEPING
TEMPS IN THE 50S OUTLYING AREAS TO NEAR 65 DOWNTOWN IN THE HEAT
ISLAND DUE TO THE MILD SW OFFSHORE FLOW.

TODAY AND TONIGHT...THE FIRST CHANGES COME IN THE FORM OF A SHALLOW
COLD FRONTAL BOUNDARY THAT WILL SINK SOUTHWARD DOWN THE LAKE IN THE
COMING HOURS...WITH NORTHERLY WINDS SHIFTING NE THIS MORNING. EXPECT
A RAPID TEMPERATURE DROP DOWNTOWN JUST AFTER DAYBREAK AS THIS OCCURS
WITH TEMPS HOLDING STEADY IN THE 50S MUCH OF THE DAY. THIS BOUNDARY
WILL CONTINUE TO OOZE SOUTHWARD BUT WILL BE HALTED BY THE INCOMING
SYSTEM AND WARM SSE FLOW.  THEREFORE EXPECT QUITE A CONTRAST IN
TEMPS TODAY AS HIGHS IN THE TIER OF COUNTIES CLOSE TO THE LAKE PEAK
THIS MORNING IN THE 60S THEN COOL...WHILE AREAS SOUTH LIKE PONTIAC
REALIZE FULL MIXING AGAIN AND PUSH THE UPPER 70S. EXPECT MORE CLOUDS
TO FILTER IN THROUGH THE DAY...NOT ONLY ALONG THE FRONT BUT IN WARM
SECTOR AS WELL. SOME HI-RES GUIDANCE TRIES TO BREAK OUT SOME SHOWERS
ALONG THE BOUNDARY AS WELL...BUT HAVE MAINTAINED A DAY FCST AT THIS
TIME WITH THE UPPER LEVEL RIDGE IN PLACE LIKELY KEEPING CLOUDS
SHALLOW IN NATURE. WITH THE SYSTEM CONTINUING ITS SLOW MOVEMENT
TONIGHT..THE SURFACE LOW WELL TO THE SOUTH AND MOISTURE ADVECTION
CONFINED THERE AS WELL...HAVE CONTINUED PREVIOUS SHIFTS TREND TO
SLOW POPS AND GONE DRY OVERNIGHT AS WELL.

SUNDAY...SURFACE LOW PRESSURE DOWNSTATE INTENSIFIES AND LIFTS NE
AHEAD OF THE INCOMING COLORADO UPPER LEVEL LOW. MODELS DISAGREE ON
THE STRENGTH OF THE INDIVIDUAL LOWS THAT FORM...BUT MANY DO LIFT THE
FIRST ONE THROUGH EASTERN IL/WESTERN IN. AS THIS OCCURS...A WARM
FRONT WILL LIFT NORTHWARD AND BECOME THE FOCUS FOR SHOWER ACTIVITY
WITH A 30-40 KT LOW LEVEL SE FLOW DRIVING INTO THE FRONT. WITH
PRECIPITABLE WATER VALUES UP AT 1.25 INCHES OR SO...SOME 2 S.D.
ABOVE NORMAL AS NOTED IN THE SPC PRECIPITATION DISCUSSION...ALONG
WITH A DEEP MOIST COLUMN AND HIGH FREEZING LEVELS SHOULD PROMOTE
EFFICIENT RAIN PRODUCERS. THE FOCUS OF THIS ACTIVITY AT THIS TIME
WILL BE ALONG THE EASTERN HALF OF OUR AREA AS IT LIFTS
NORTH...THOUGH THIS A COMPLICATED EVOLUTION WITH SEVERAL SMALL SCALE
DISTURBANCES IN PLACE.  INSTABILITY IS WEAK AND VERY HIGH
BASED...BUT DECENT FORCING AND PARCELS LIKELY UNDERGOING CHARGE
SEPARATION...ISOLATED THUNDER MENTION SEEMS A REASONABLE COURSE OF
ACTION.

KMD

&&

.LONG TERM...
352 AM CDT

SUNDAY NIGHT THROUGH FRIDAY...

PRECIPITATION WILL CONTINUE LIFTING NORTH AND EAST AHEAD OF THIS
FIRST WAVE. WITH A MODEST CYCLONE...EXPECT SOME DEFORMATION
PRECIPITATION TO CONTINUE DURING THE EVENING HOURS.  SOME REMNANT
ENERGY ASSOCIATED WITH THE COLORADO LOW WILL LARGELY DRAG BY TO THE
SOUTH...BUT OUR FOCUS WILL BE ON THE NORTHERN STREAM UPPER LOW THAT
WILL DOMINATE OUR WEATHER PATTERN FOR A FEW DAYS. A COLD FRONT ON
THE BACK SIDE OF WHAT APPEARS TO BE A SECONDARY LOW WILL ALLOW RAIN
SHOWERS TO CONTINUE THROUGH THE NIGHT AND INTO MONDAY MORNING BEFORE
DIMINISHING IN THE AFTERNOON.

THE UPPER LEVEL PATTERN BECOME MORE AMPLIFIED NEXT WEEK AS RIDGING
REBUILDS IN THE WEST AND A STRONG UPPER RIDGE IN THE ATLANTIC WITH
THE UPPER LOW STUCK WITH NOWHERE TO GO.  EXPECT SEVERAL COOL DAYS
WITH SMALL CHANCES FOR RAIN SHOWERS AS WE REMAIN ON THE SOUTH SIDE
OF THE NOW SLOW MOVING NORTHERN STREAM LOW AND ELOGNATED VORT MAXES
PIVOT SE ACROSS NE IL/NW IN. WITH THE GROWING SEASON
UNDERWAY...EXPECT FROST CONDITIONS MAY BECOME A CONCERN IN
OUTLYING AREAS.

KMD

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 09Z...

* LIGHT VRB WINDS GENERALLY SOUTHWEST ARND 4KT...TURNING
  NORTH/NORTHEAST AND INCREASING ARND DAYBREAK. GUSTS DEVELOPING
  UP TO 20KT.

* WINDS TURN EAST LATE IN THE AFTN.

BEACHLER

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

WEAK RIDGING REMAINS OVER NORTHERN IL/NORTHWEST IN AIRFIELDS EARLY
THIS MORNING...ALLOWING VRB WIND DIRECTION AND SPEEDS GENERALLY
ARND 4KT OR LESS. WINDS WILL TURN NORTHWEST BY 10Z THEN QUICKLY
TURN NORTH/NORTHEAST BY DAYBREAK. A FRONTAL BOUNDARY WILL SLIDE
SOUTH ACROSS THE WESTERN GREAT LAKES...ALLOWING AN INCREASING WIND
SPEED AND GUSTS TO DEVELOP BY MID-MORNING THRU LATE AFTN WITH
GUSTS TO 20KT AT TIMES. THE DURATION OF GUSTS DOESN/T APPEAR TO BE
LENGTHY...HOWEVER THE DIRECTION WILL BE TURNING NORTHEAST BETWEEN
020-050 DEG THEN MORE TOWARDS A 090 DIRECTION BY LATE AFTN/EARLY
EVE. CLOUDS SHUD THICKEN...HOWEVER REMAIN VFR WITH BASES ARND
4-6KFT AGL. ANY PRECIP SHUD HOLD OFF UNTIL AFT DAYBREAK
SUN...POSSIBLY UNTIL MIDDAY SUN.

BEACHLER

//ORD AND MDW CONFIDENCE...UPDATED 09Z...

* HIGH CONFIDENCE IN CIGS/VSBY FORECAST.

* HIGH CONFIDENCE IN WINDS REMAINING LGT/VRB THRU
  10Z...MEDIUM/HIGH CONFIDENCE IN TIMING OF WINDS TURNING
  NORTHEAST WITH GUSTS DEVELOPING.

BEACHLER

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

SUNDAY...SHRA/ISOLATED TSRA. MVFR LIKELY/IFR POSSIBLE. EAST WINDS
BECOMING NORTHEAST.

MONDAY...CHANCE OF SHRA/MVFR EARLY...IMPROVING TO VFR. GUSTY WEST
WINDS.

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

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

THURSDAY...VFR. NORTH-NORTHEAST WINDS.

RATZER

&&

.MARINE...
310 AM CDT

MAIN CONCERN FOR TODAY IS THE POTENTIAL FOR MARGINAL WINDS FOR
SMALL CRAFT CONDITIONS. HIGH PRESSURE REMAINS OVER THE WESTERN
GREAT LAKES...ALTHOUGH STEADILY PUSHING EAST. THIS WILL ALLOW A
FRONTAL BOUNDARY TO ENERGIZE AND SLIDE SOUTH ALONG THE LAKE TODAY
AND REACH THE SOUTHERN TIP OF THE LAKE BY LATE THIS AFTERNOON.
THIS WILL ALLOW WINDS TO TURN WEST TO NORTH/NORTHEAST AND INCREASE
TO 15 TO 25 KT. IT IS POSSIBLE THAT A TIGHTER GRADIENT WILL OCCUR
ALONG THE CENTRAL/SOUTHERN PORTIONS OF THE LAKE...WITH WINDS MORE
IN THE 20 TO 25 KT RANGE. THIS MAY RESULT IN SMALL CRAFT WINDS FOR
THE ILLINOIS NEARSHORE THRU THIS AFTERNOON. WINDS WILL BE SLIGHTLY
LESS ROBUST FOR THE INDIANA SHORE...WITH WAVES GENERALLY ONLY
BUILDING TO 3 TO 5 FT WITH UP TO 7 FT WAVES AT TIMES. GIVEN THE
AIR IS SLIGHTLY WARMER THAN THE LAKE SURFACE...IT IS POSSIBLE
WAVES MAY NOT MAXIMIZE. BUT THIS IS A LOW CONFIDENCE AT THIS TIME.

THE GRADIENT BRIEFLY RELAXES BEFORE INCREASING AHEAD OF A
DEVELOPING LOW PRESSURE SYSTEM LATE IN THE WEEKEND INTO EARLY MON.
WINDS TURN EAST TO NORTHEAST...WITH SPEEDS INCREASING YET AGAIN TO
15 TO 25 KT. A FEW GUIDANCE MEMBERS INDICATE WINDS COULD BRIEFLY
TOUCH 30 KT AHEAD OF THE LOW PRESSURE SYSTEM. WITH THE LOW NOW
POISED TO REMAIN SOUTH OF THE LAKE...THE BEST GRADIENT MAY BE
ALONG THE SOUTHERN TIP OF THE LAKE SUNDAY NIGHT/MON.

BEACHLER

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...LMZ740-LMZ741-LMZ742...9 AM SATURDAY TO 4
     PM SATURDAY.

&&

$$

VISIT US AT HTTP://WEATHER.GOV/CHICAGO (ALL LOWERCASE)

FOLLOW US ON FACEBOOK...TWITTER...AND YOUTUBE AT:
WWW.FACEBOOK.COM/NWSCHICAGO
WWW.TWITTER.COM/NWSCHICAGO
WWW.YOUTUBE.COM/NWSCHICAGO


USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.