Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 042002
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
302 PM CDT MON MAY 4 2015

.SHORT TERM...
249 PM CDT

TONIGHT AND TOMORROW...

APART FROM A COUPLE OF CELLS JUST SOUTH OF THE FORECAST AREA...A
DIFFUSE FRONTAL ZONE STRETCHING ACROSS NORTHERN ILLINOIS THIS
AFTERNOON HAS SO FAR BEEN HAVING TROUBLE SUPPORTING REDEVELOPMENT
OF CONVECTION AS THE PERIOD OF PEAK HEATING APPROACHES...THOUGH
WIDESPREAD CLOUD DEBRIS LIKELY IS INHIBITING INSOLATION AND LOW
LEVEL DESTABILIZATION. THE FRONT HAS BEEN EFFECTIVE AT ALLOWING
WINDS ALONG THE LAKE TO TURN NORTHEAST AND PRODUCE A SHARP THERMAL
GRADIENT...WITH TEMPERATURES DROPPING INTO THE 50S WHERE THE FRONT
HAS PASSED AND STAYING IN THE UPPER 60S TO MIDDLE 70S ELSEWHERE.
THIS COOL AIR IS ANTICIPATED TO SPREAD SOUTH AND WEST ACROSS THE
FORECAST AREA AS EVENING APPROACHES...EVENTUALLY ALLOWING FOR LOWS
IN THE UPPER 40S NORTH BUT AROUND A RELATIVELY MILD 60 DEGREES
SOUTH.

DESPITE THE SLOW START...CONVECTIVE TRENDS LIKELY WILL INCREASE
INTO EARLY EVENING ALONG AND SOUTH OF THE FRONT...SO MAINLY IN THE
SOUTHERN THIRD OF THE FCST AREA. THUNDERSTORM ACTIVITY MAY WANE
WITH LOSS OF HEATING AFTER SUNSET EVEN AS SHOWER TRENDS
PERSIST...AND THEN INCREASE AGAIN TOWARD MORNING ACROSS THE ENTIRE
AREA AS THE FRONTAL ZONE LIFTS BACK NORTH AND IS ENHANCED BY
INCREASED MOISTURE CONVERGENCE INTO THE REGION.

HIGHS TOMORROW NORTH OF THE FRONT WILL BE ABOUT 10 TO 15 DEGREES
COOLER THAN TODAY...THOUGH AREAS SOUTH OF THE FRONT WILL TOP OUT
AT TEMPERATURES VERY SIMILAR TO TODAY...AND MAYBE A DEGREE OR TWO
WARMER.

LENNING

&&

.LONG TERM...
249 PM CDT

TUESDAY NIGHT THROUGH MONDAY...

A WESTERN LONGWAVE TROUGH WILL PROGRESS ACROSS THE DESERT SOUTHWEST
TUESDAY NIGHT THEN LIFT NORTHEAST ACROSS THE UPPER MISSISSIPPI RIVER
VALLEY THURSDAY NIGHT. THIS WILL FORCE UPPER RIDGING TO PIVOT ACROSS
THE LOCAL AREA WEDNESDAY AND THURSDAY BEFORE IT SHIFTS TO THE EAST
AND BROADENS THURSDAY. THIS WILL ALLOW THE SURFACE WARM FRONT TO
CONTINUE NORTHWARD ACROSS THE AREA TOMORROW EVENING AND PUSH NORTH
OF THE AREA INTO WEDNESDAY MORNING. THE BULK OF ANY
PRECIP/CONVECTION LOOKS TO SHIFT NORTHWARD THROUGH THE EVENING WITH
ONLY MINIMAL CHANCES CONTINUING ACROSS THE FAR NORTHEASTERN CWA
THROUGH THE OVERNIGHT. THE RIDGE AXIS WILL BE OVERHEAD FOR MUCH OF
WEDNESDAY WHICH WILL MAKE IT DIFFICULT FOR MUCH IF ANY NEW
CONVECTION TO DEVELOP DESPITE A WARM AND HUMID AIRMASS OVER THE
AREA. HOWEVER...WINDS WILL BE TURNING SOUTH TO SOUTHEAST AND BE
LIGHT ENOUGH THAT A MORE PRONOUNCED LAKE BREEZE MAY FORM AND PUSH
INLAND INTO NORTHEAST ILLINOIS. THIS MAY BE ENOUGH TO TRIGGER SOME
ISOLATED CONVECTION BUT OTHERWISE WILL NEED TO RELY ON SOME SORT OF
SUBTLE MID/UPPER WAVE TO HELP KICK ANYTHING OFF. WILL MENTION A
SLIGHT CHANCE FOR NORTHEASTERN AREAS TO MAINTAIN CONTINUITY WITH
PREVIOUS FORECAST FOR NOW. HIGH TEMPS WILL WARM INTO THE LOW TO MID
80S ACROSS MOST OF THE AREA WEDNESDAY WITH MUCH OF THE CHICAGO METRO
BEING HELD INTO THE 70S THANKS TO AN EASTERLY WIND WIND
COMPONENT...WITH ILLINOIS LAKEFRONT AREAS ONLY SEEING LOWER 60S IF
NOT A LITTLE COOLER.

THE UPPER TROUGH WILL LIFT ACROSS THE UPPER MISSISSIPPI RIVER VALLEY
WEDNESDAY NIGHT AND CONVECTION ASSOCIATED WITH IT MAY BE ABLE TO
SKIRT THE WESTERN CWA TOWARD THURSDAY MORNING. THE UPPER RIDGE AXIS
WILL SHIFT EASTWARD BRINGING A STRONGER SOUTHWESTERLY FLOW OVER THE
AREA INTO THE WEEKEND. WESTERN AREAS WILL SEE A CHANCE FOR STORMS
THURSDAY BUT OTHERWISE MOST AREAS WILL REMAIN DRY. FROM THEN ON
PERIODIC CHANCES FOR STORMS WILL PERSIST THROUGH THE WEEKEND AS A
SERIES OF UPPER DISTURBANCES MOVE THROUGH. AT THE SURFACE...A WEAK
FRONTAL BOUNDARY WILL DRIFT SOUTHEAST INTO THE AREA FRIDAY NIGHT AND
BETTER ALIGN WITH THE TRACK OF THE UPPER WAVES YIELDING THE BEST
CHANCE FOR ROUNDS OF CONVECTION FROM LATER FRIDAY THROUGH MONDAY.
HIGH TEMPS THURSDAY AND FRIDAY LOOK TO BE IN THE UPPER 70S AND LOWER
80S WITH LAKESHORE AREAS SEEING THE 70S. DEPENDING ON HOW MUCH
PROGRESS THE EXPECTED SURFACE FRONT MAKES INTO SATURDAY AND THE
AMOUNT OF CONVECTION TEMPS WILL LIKELY BE COOLER...IN THE 70S...WITH
LAKESHORE AREAS PROBABLY BEING HELD INTO THE 60S. A SIMILAR TEMP
REGIME IS EXPECTED SUNDAY.

MDB

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 18Z...

* WNW WINDS QUICKLY TURNING NORTHEAST THIS AFTERNOON LIKELY
  INCREASING TO 10 KT OR SLIGHTLY HIGHER THROUGH EARLY AFTERNOON
  BEFORE EASING THIS EVENING.

* A BRIEF PERIOD OF MVFR CIGS AROUND 2500-3500 FT THROUGH MID
  AFTERNOON.

* SMALL CHANCE OF -SHRA THIS AFTERNOON...BUT CHANCES OF
  SHRA/EMBEDDED TS SHARPLY INCREASING BY LATE EVE INTO OVERNIGHT.

* MVFR AND POSSIBLE IFR CONDITIONS WITH EAST WINDS TUESDAY
  MORNING.

KMD

//DISCUSSION...UPDATED 18Z...

VARIBLE NORTHWEST WINDS PREVAIL UNDER A FAIRLY LIGHT PRESSURE
GRADIENT REGIME. MEANWHILE...WINDS AS OF 18Z HAD ALREADY SHIFTED
TO ENE ALONG THE LAKEFRONT. A COMBINATION OF NORTHERLY WINDS FROM
THE SOUTHWARD MOVING FRONT AND LAKE BREEZE WINDS WILL ALLOW WINDS
INLAND TO SHIFT TO NE IN THE COMING HOURS. CONFIDENCE ON SPECIFIC
WIND DIRECTION IS NOT SUPER HIGH...BUT SHOULD BE IN THE ENE-NNE
DIRECTION AND EXPECT IT TO BE 10 KT OR SLIGHTLY HIGHER WITH THE
INITIAL PUSH.

MVFR CIGS WILL ALSO HOLD FOR AT LEAST THE NEXT FEW HOURS UNTIL THE
COLD FRONT ARRIVES. A FEW HIGH RESOLUTION MODELS SUGGEST A SHOWER
OR TWO IS POSSIBLE ACROSS NE IL THIS AFTERNOON...BUT LOW
CONFIDENCE IN THIS SO HELD IT OUT OF THE TAFS. THEN THE EVENING
PERIOD INITIALLY LOOKS ON THE QUIETER SIDE BEFORE SUBTLE SHORTWAVE
IN WSW FLOW ALOFT ALLOWS THE SOUTHWARD SHIFTING COLD FRONT TO MOVE
BACK NORTHWARD. EXPECT INCREASING COVERAGE OF SHOWERS AFTER 4Z-6Z
FROM SOUTHWEST TO NORTHEAST. LOCALLY HEAVY DOWNPOURS SEEM THE MOST
LIKELY HAZARD...WITH SOUNDINGS SUGGESTING AT LEAST WEAK ELEVATED
CAPE SUPPORTING EMBEDDED TS. CONFIDENCE ON TS COVERAGE IS LOW BUT
HIGH ON SHOWERS DURING THE 6Z-12Z PERIOD. THE BOUNDARY STALLS OUT
ALLOWING -SHRA TO BE POSSIBLE DURING THE MORNING HOURS AS
ADDITIONAL WEAK SW FLOW ENERGY PROPOGATES THROUGH. INCREASED
DEWPOINTS IN THE COOLER AIR MASS NORTH OF THE FRONT WILL PROMOTE
LOW MVFR TO POSSIBLY IFR CONDITIONS TUESDAY MORNING.

KMD

//ORD AND MDW CONFIDENCE...UPDATED 18Z...

* MEDIUM-HIGH IN NORTHEAST WINDS THIS AFTERNOON BUT MEDIUM ON
  TIMING AND WHEN SPEEDS WILL INCREASE OVER 10 KT.

* MEDIUM-HIGH IN CIGS HOLDING AROUND 2500-3500 FT THROUGH MID-LATE AFTERNOON.

* MEDIUM-HIGH ON TIMING OF SHRA TONIGHT AND LOW-MEDIUM ON THUNDER
  COVERAGE.

* HIGH IN EAST WINDS TUESDAY MORNING AND MEDIUM-HIGH IN MVFR
  CONDITIONS. LOW-MEDIUM IN WHETHER IFR WILL OCCUR.

KMD

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

TUESDAY NIGHT-WEDNESDAY...CHANCE OF STORMS. LIGHT EAST WINDS.
THURSDAY...CHANCE OF STORMS DURING THE NIGHT. SOUTH WINDS.
FRIDAY...CHANCE OF STORMS. SOUTH WINDS.
SATURDAY-SUNDAY...CHANCE OF STORMS. WINDS BECOMING NORTHEAST.

MTF

&&

.MARINE...
257 PM CDT

A COLD FRONT THAT DROPPED SOUTH ACROSS THE LAKE TODAY HAS TURNED
WINDS TO THE NORTH-NORTHEAST AND BROUGHT CONSIDERABLY COOLER AIR
TO THE SOUTHERN END. THIS COOLER AIR COMBINED WITH THE HIGH
MOISTURE ALONG IT HAS PROMPTED SOME REPORTS OF FOG THIS AFTERNOON.
WINDS CONTINUE TO TURN EAST TONIGHT AND TOMORROW THEN BACK TO THE
SOUTHEAST AND SOUTH TOMORROW NIGHT INTO WEDNESDAY AS THE FRONT
LIFTS BACK TO THE NORTH. WINDS GENERALLY STAY LIGHT IN THE WEAK
GRADIENT UNTIL LATE IN THE WEEK WHEN GUSTS MAY CLIMB AGAIN ABOVE
20 KTS.

LENNING

&&

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

$$

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