Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 031607
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1107 AM CDT SUN MAY 3 2015

.UPDATE...
1002 AM CDT

WELL DEFINED MCV OVER EASTERN IOWA IS MOVE STEADILY EASTWARD AND
STARTING TO SPREAD SOME SHOWERS EAST INTO OUR WESTERN CWA AND ALSO
BRINGING WITH IT A GOOD DEAL OF CLOUD COVER. GIVEN THE LACK OF
ORGANIZED DEEP CONVECTION AND DOWNSTREAM AIR MASS BEING SLIGHTLY
DRIER AND A BIT MORE STABLE...WOULD ANTICIPATE SOME WEAKENING OF
THE VORT AS IT MOVES EAST INTO NORTHERN ILLINOIS THIS AFTERNOON.
DESPITE THIS...THE MCV LOOKS VERY WOUND UP AND IMPRESSIVE ON
SATELLITE AND RADAR IMAGERY THIS MORNING AND SEEMS UNLIKELY THAT
IT WOULD COMPLETELY DISSIPATE BEFORE BRINGING AT LEAST A THREAT OF
SHOWERS AND PERHAPS ISOLATED THUNDERSTORMS THIS AFTERNOON TO THE
AREA. HAVE MADE SOME ADJUSTMENTS TO FIRST PERIOD GRIDS TO BEEF UP
POPS AND CLOUD COVER. EXCEPT WHERE POCKETS OF SUNSHINE ALLOW TEMPS
TO WARM...IT WOULD SEEM THAT OUR GOING HIGH TEMPS ARE TOO WARM AND
NEED TO BE LOWERED. NORTHWESTERN CWA SEEMS MOST PRONE TO BEING
SOCKED IN WITH CLOUD COVER AND SHOWERS...SO FOR NOW JUST GOING TO
LOWER TEMPS THERE BUT MAY NEED TO EVENTUALLY FOLLOW SUIT
ELSEWHERE. UPDATED GRIDS HAVE BEEN SENT AND TEXT PRODUCTS WILL BE
UPDATED SHORTLY.

IZZI

&&

.SHORT TERM...
327 AM CDT

THROUGH MONDAY NIGHT...

A POTENT SHORTWAVE IS OVER NORTHWEST IL THIS MORNING AND IT WILL
MOVE OVER CENTRAL INDIANA THIS AFTERNOON. SCATTERED SHOWERS ARE
ALIGNED WITH THE WAVE...WHILE A COUPLE OF CONVECTIVE COMPLEXES LAG
BEHIND THE WAVE OVER IOWA.  THINKING ISOLATED SHOWERS ARE POSSIBLE
THROUGH 7AM ACROSS THE CWA.  NEAR TERM GUIDANCE AGREES THAT THE BEST
FORCING FOR ADDITIONAL ISOLATED SHOWERS WILL BE ALONG AND WEST OF
THE I-55 CORRIDOR THROUGH NOON.

EARLY MORNING CLOUD COVER SHOULD DISSIPATE THIS AFTERNOON WHICH
WOULD ALLOW TEMPS TO CLIMB INTO THE LOW 80S. HAVE MEDIUM-HIGH
CONFIDENCE IN HIGHS TODAY AS A CHANGE IN CLOUD COVER WILL LEAD TO
EITHER WARMER OR COOLER TEMPERATURES.  WIND GUSTS WILL ALSO BE
DEPENDENT ON CLOUD COVER AND MIXING. BACKED OFF ON GUSTS SLIGHTLY
BUT STILL EXPECTING A FEW GUSTS UP TO 30 MPH THIS AFTERNOON.

THE SURFACE LOW THAT WILL BRING THUNDERSTORMS TO THE REGION IS
MOVING OVER HUDSON BAY THIS MORNING. ITS COLD FRONT WILL SLIDE
THROUGH THE CWA MONDAY MORNING BUT THUNDERSTORMS AHEAD OF IT WILL
INFILTRATE THE CWA FROM NORTH TO SOUTH BEGINNING LATE THIS EVENING.
HAVE MEDIUM CONFIDENCE IN THE TIMING OF THUNDERSTORMS AS GUIDANCE
TYPICALLY IS A BIT FAST WITH COLD FRONTS.  HAVE HIGH CONFIDENCE IN
THUNDERSTORMS AS THERE IS ENOUGH CAPE FOR THUNDERSTORMS AND STEEP
LAPSE RATES. SEVERE WEATHER IS NOT EXPECTED AS THE OTHER PARAMETERS
ARE NOT SUPPORTIVE OF SEVERE STORMS.  HEAVY DOWNPOURS ARE POSSIBLE
WITH THE STORMS HOWEVER AS PWAT VALUES WILL INCREASE TO 1-1.2 INCHES
OVERNIGHT.

SHOWERS AND STORMS CONTINUE THROUGHOUT THE DAY MONDAY AS THE FRONT
STALLS AT OR SOUTH OF I-80. MODELS SHOW BETTER CONSISTENCY
TODAY...BUT STILL HAVE SUBTLE DIFFERENCES AS TO WHERE THE FRONT WILL
STALL OUT.  CONVECTION WILL FIRE ALONG THE FRONT MONDAY AND THE ONLY
CONCERN I SEE IS POSSIBLE FLOODING.  PWAT VALUES INCREASE UP TO 1.5
INCHES MONDAY EVENING AND NIGHT WHICH COULD LEAD TO SOME LOCATIONS
RECEIVING OVER A HALF INCH OF RAIN IN A 24 HOUR PERIOD.

HIGHS MONDAY WILL VARY FROM THE LOW TO MID 70S NORTH OF THE
FRONT...TO THE 60S ALONG THE LAKE...TO THE MID 70S SOUTH OF THE
FRONT.

JEE

&&

.LONG TERM...
327 AM CDT

TUESDAY THROUGH SATURDAY...

THE FRONT REMAINS SOUTH OF I-80 TUESDAY AND LOOKING AT ANOTHER DAY
WITH SCATTERED TO NUMEROUS THUNDERSTORMS.  GUIDANCE SUGGESTS WE MAY
SEE A FEW DRY PERIODS BUT HAVE LOW CONFIDENCE IN WHEN AND WHERE
THOSE WOULD OCCUR SO KEPT AT LEAST A CHANCE OF THUNDER ACROSS THE
CWA.  TEMPS ON TUESDAY WILL ONCE AGAIN VARY DEPENDING ON WHICH SIDE
OF THE FRONT YOU ARE ON. AREAS NORTH OF I-80 WILL BE IN THE 60S WITH
MID TO UPPER 50S ALONG THE LAKE THANKS TO ONSHORE FLOW. SOUTH OF I-
80 AND THE FRONT...MID TO UPPER 70S.

THE FRONT LIFTS NORTH AS A WARM FRONT TUESDAY NIGHT INTO
WEDNESDAY MORNING PUSHING THE CONVECTION NORTH WITH IT LEADING TO
WHAT COULD BE A DRY WEDNESDAY. GUIDANCE SUGGESTS ISOLATED TO
SCATTERED WARM SECTOR CONVECTION IS POSSIBLE WEDNESDAY. MODEL
SOUNDINGS FEATURE A FEW HUNDRED J/KG OF CAPE WITH NO CAP SO WENT
WITH A SLIGHT CHANCE TO CHANCE OF THUNDER.  SHEAR LOOKS WEAK AT BEST
SO NOT EXPECTING SEVERE STORMS AT THIS TIME.  WARMER CONDITIONS ARE
EXPECTED SINCE WE WILL BE IN THE WARM SECTOR. CLOUD COVER WILL ONCE
AGAIN POSSIBLY BUST THE TEMP FORECAST SO WENT WITH CONSERVATIVE
NUMBERS. LOOKING AT MID 70S TO AROUND 80 AWAY FROM THE LAKE.

THE NEXT SURFACE LOW PASSES OVER ONTARIO THURSDAY NIGHT THROUGH
FRIDAY MORNING AND ITS COLD FRONT STALLS OUT NEAR THE IL/WI STATE
LINE SATURDAY MORNING.  ADDITIONAL THUNDERSTORMS ARE POSSIBLE FROM
FRIDAY ONWARD AS THE FRONT HANGS AROUND THE REGION. ONLY HAVE MEDIUM
CONFIDENCE IN THUNDERSTORM COVERAGE AT THIS TIME.  TEMPS LATE THIS
WEEK LOOK TO BE AROUND 80.

JEE

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 16Z...

* GUSTY SOUTHWEST WINDS THRU EARLY THIS EVENING.
* CHANCE OF SHRA THIS AFTERNOON.
* SHRA/TSRA DEVELOPING MID/LATE EVENING AND CONTINUING INTO MONDAY
  MORNING.
* WIND SHIFT TO THE NORTH/NORTHEAST MONDAY AFTERNOON. CMS

//DISCUSSION...UPDATED 12Z...

BROAD WARM ADVECTION CONTINUES OVER THE REGION THIS MORNING WITH A
SPECKLING OF SHOWERS. INSTABILITY IS STILL QUITE MINIMAL EAST OF
THE MISSISSIPPI RIVER...SO THESE HAVE MAINLY BEEN LIGHT ACROSS
NORTHERN IL. AS AN UPPER DISTURBANCE MOVES FROM SOUTH CENTRAL IL
EARLY THIS MORNING ACROSS NORTHERN IL/FAR SOUTHERN WI THIS
AFTERNOON IT COULD HELP TO FOCUS AN UPTICK IN SHOWER ACTIVITY.
CONFIDENCE ON THIS IS LOW THOUGH AS THIS FEATURE IS CONVECTIVELY-
ENHANCED FROM STORMS LAST NIGHT. SLIGHTLY BETTER CHANCES FOR
SHOWER ACTIVITY WILL BE SEEN AT RFD...BUT THESE COULD REACH
CHICAGO AREA AIRPORTS AS WELL.

WINDS WILL BE ON THE INCREASE FROM THE SOUTH-SOUTHWEST TODAY AS
THE PRESSURE GRADIENT TIGHTENS. HIGH CONVECTIVE CLOUD
DEBRIS...ALONG WITH POSSIBLY MORE CLOUD DEVELOPMENT...COULD LIMIT
THE GUST MAGNITUDE SOME FROM WHAT IS IN CURRENT TAFS. LATER THIS
EVENING...A LOW-LEVEL JET WILL DEVELOP OVER THE AREA WITH 35-45 KT
FLOW ABOVE 1000 FT. WITH SURFACE WINDS REMAINING AROUND 12KT OR SO
TONIGHT...TRUE LOW-LEVEL WIND SHEAR IS NOT FORECAST AT THIS TIME.

WITH THE DEVELOPMENT OF THE LOW-LEVEL JET WILL COME THE
POSSIBILITY OF SCATTERED SHRA/TSRA OVERNIGHT ACROSS NORTHERN IL
AND NORTHWEST IN. AS A COLD FRONT APPROACHES FROM THE NORTHWEST
AND A SERIES OF UPPER DISTURBANCES PASS OVER THE AREA MONDAY
MORNING...THE RAIN COVERAGE IS EXPECTED TO BE HIGHER. EMBEDDED
THUNDER IS LIKELY AND TSRA VERY WELL MAY BE ADDED INTO FUTURE TAFS
FOR PART OF THAT TIME PERIOD. WHEN THE COLD FRONT PASSES EARLY
MONDAY AFTERNOON OR SO...WINDS WILL ABRUPTLY SHIFT TO THE NORTH-
NORTHEAST.

//ORD AND MDW CONFIDENCE...UPDATED 16Z...

* MEDIUM FOR WINDS THRU THE PERIOD.
* LOW FOR SHRA THIS AFTERNOON.
* MEDIUM FOR SHRA/TSRA THIS EVENING THRU MONDAY MORNING.
* HIGH FOR WIND SHIFT MONDAY AFTERNOON...MEDIUM FOR TIMING. CMS

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

MONDAY NIGHT-TUESDAY...SHRA WITH PERIODS OF TSRA LIKELY. MVFR
                       LIKELY AT TIMES. EAST-NORTHEAST WINDS.
WEDNESDAY...CHANCE OF TSRA. SOUTHEAST WINDS.
THURSDAY...VFR. SOUTH WINDS.
FRIDAY-SATURDAY...CHANCE OF TSRA. SOUTH WINDS.

MTF

&&

.MARINE...
135 AM CDT

SOUTH-SOUTHWEST WINDS WILL INCREASE ACROSS THE GREAT LAKES REGION
TODAY AS THE PRESSURE GRADIENT TIGHTENS IN ADVANCE OF AN
APPROACHING COLD FRONT. THE OFFSHORE COMPONENT ALONG THE WESTERN
SHORE OF THE LAKE WILL BE ENOUGH TO WARRANT A SMALL CRAFT
ADVISORY FROM LATE MORNING THROUGH THE AFTERNOON.

LOW PRESSURE FAR TO THE NORTH OF THE GREAT LAKES WILL USHER THE
COLD FRONT INTO THE WESTERN LAKES REGION LATER TONIGHT. THIS FRONT
WILL EASE ITS WAY SOUTHEASTWARD ACROSS LAKE MICHIGAN BY EARLY
AFTERNOON MONDAY TURNING WINDS TO THE NORTH IN ITS WAKE. WITH THE
FRONT FORECAST TO STALL JUST SOUTH OF LAKE MICHIGAN...WINDS ACROSS
THE LAKE WILL TURN EASTERLY LATER MONDAY NIGHT AND THROUGH
TUESDAY. THE FRONT WILL THEN RETURN NORTHWARD AS A WARM FRONT
TUESDAY NIGHT AND WEDNESDAY. SOUTHERLY WINDS ARE THEN EXPECTED FOR
MUCH OF THE REST OF THE WEEK.

MTF

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...ILLINOIS NEARSHORE WATERS UNTIL 7 PM
     SUNDAY.

&&

$$

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