Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 031953
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
253 PM 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 18Z...

* GUSTY SOUTHWEST WINDS THRU EARLY EVENING.
* CHANCE OF SHRA THIS AFTERNOON.
* CHANCE OF SHRA/TSRA MID EVENING THRU EARLY MORNING.
* SHRA WITH SCATTERED TSRA FROM DAYBREAK THRU LATE MORNING.
* WIND SHIFT TO NORTHEAST LATE MONDAY MORNING/EARLY MONDAY
  AFTERNOON. CMS

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

MULTIPLE FORECAST CONCERNS THIS PERIOD WITH RELATIVELY LOW
CONFIDENCE OVERALL. UPPER WAVE CURRENTLY MOVING ACROSS EASTERN IA
WILL MOVE ACROSS NORTHERN IL THIS AFTERNOON. THUNDERSTORMS ACROSS
FAR NORTHWEST IL AND EASTERN IA HAVE BEEN WEAKENING AS THEY MOVE
EAST INTO MORE STABLE AIR THOUGH SHOWERS HAVE MADE IT AS FAR EAST
AS DPA. THIS AREA OF SHOWERS MAY AFFECT ORD OVER THE NEXT FEW
HOURS. ADDITIONAL SHOWERS ARE POSSIBLE THRU LATE AFTERNOON OR
EARLY EVENING AS THIS WAVE MOVES ACROSS THE AREA. CONFIDENCE
REGARDING COVERAGE IS LOW ENOUGH TO MAINTAIN JUST VICINITY MENTION
FOR NOW. BEST CHANCE OF ANY THUNDER WOULD BE AT RFD AND STILL LOW
ENOUGH TO NOT MENTION AT THIS TIME.

APPEARS THERE WILL BE A BREAK IN PRECIP POTENTIAL EARLY THIS
EVENING BUT THERE REMAINS QUITE A BIT OF DISAGREEMENT ABOUT HOW
MUCH WARM AIR ADVECTION SHRA/TSRA MAY DEVELOP OVER THE AREA LATER
THIS EVENING. SHORT TERM/HI RES GUIDANCE SUGGESTS NEW DEVELOPMENT
WILL OCCUR ON THE BACK SIDE OF THIS WAVE BY MID EVENING...PERHAPS
AS EARLY AS 02Z AND POSSIBLY CONTINUING THRU 07Z-09Z. GIVEN THE
LONGER PERIOD OF POSSIBLE ACTIVITY AND LOW CONFIDENCE IN BOTH
OCCURRENCE AND COVERAGE...OPTED TO STAY DRY IN THE TAFS DURING
THIS TIME PERIOD...BUT THIS WILL NEED TO BE MONITORED CLOSELY
LATER THIS AFTERNOON INTO THIS EVENING.

A COLD FRONT WILL MOVE ACROSS THE WESTERN GREAT LAKES TONIGHT AND
INTO NORTHERN IL BY MID/LATE MONDAY MORNING. ADDITIONAL SHOWERS
AND THUNDERSTORMS WILL DEVELOP AHEAD OF THIS FRONT AND MOVE ACROSS
THE TERMINALS MONDAY MORNING. MEDIUM CONFIDENCE THIS WILL OCCUR SO
MAINTAINED PREVAILING MENTION BUT DURATION AND START/END TIMES
WILL LIKELY REQUIRE REFINEMENT. CIGS WILL EVENTUALLY LOWER INTO
MVFR EITHER WITH THE PRECIP OR ONCE WINDS TURN NORTHEAST OFF THE
LAKE. FAIRLY CONFIDENT WIND SHIFT TO THE NORTHEAST WILL HAPPEN...
BUT LOW CONFIDENCE AGAIN WITH TIMING. CMS

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

* MEDIUM FOR CHANCE OF SHRA THIS AFTERNOON.
* LOW FOR CHANCE OF SHRA/TSRA THIS EVENING/EARLY OVERNIGHT.
* MEDIUM FOR SHRA/SCATTERED TSRA DAYBREAK THRU LATE MONDAY
  MORNING.
* HIGH FOR WIND SHIFT MIDDAY MONDAY...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...
253 PM CDT

BRISK SOUTH-SOUTHWEST WINDS PREVAILING ACROSS LAKE MICHIGAN WILL
EASE THIS EVENING.  A VERY STABLE VERTICAL PROFILE RESULTING FROM
THE WARM MOIST AIRMASS ADVECTING NORTH OVER COLD LAKE WATER ALSO IS
INHIBITING BETTER TRANSFER OF HIGHER MOMENTUM AIR ALOFT TO THE LAKE
SURFACE.

WITH WINDS EASING OFF THIS EVENING WILL ALLOW SMALL CRAFT ADVISORY
TO EXPIRE AS SCHEDULED.  RELAXING PRESSURE GRADIENT OVERNIGHT IN
ADVANCE OF APPROACHING COLD FRONT WILL CAUSE WINDS TO SLACKEN
FURTHER OVERNIGHT.  THE COLD FRONT ITSELF DROPS SOUTHEAST ACROSS
ABOUT THE NORTH HALF OF THE LAKE BY 0700-0800 CDT MONDAY... AND THEN
REACHES THE FAR SOUTH END OF THE LAKE BY MID AFTERNOON MONDAY.

WINDS WILL BECOME NORTHWEST AND THEN QUICKLY NORTH BEHIND THE FRONT
MONDAY... AS THE FRONT THEN STALLS JUST SOUTH OF LAKE MICHIGAN
MONDAY NIGHT.  WINDS ACROSS THE LAKE WILL BECOME NORTHEAST TO EAST
LATER MONDAY NIGHT AND TUESDAY... BEFORE THE FRONT LIFTS BACK NORTH
AS A WARM FRONT TUESDAY NIGHT AND WEDNESDAY.  GIVEN THE PRESENCE OF
A WARM MOIST AIRMASS WOULD EXPECT FOG TO DEVELOP OVER THE LAKE
TUESDAY INTO WEDNESDAY. AS THE FRONT THEN LIFTS BACK NORTH... THIS
WILL SET THE STAGE FOR SOUTHERLY WINDS ON THE LAKE FOR MUCH OF LATE
WEEK.

ED F

&&

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

&&

$$

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