Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 270252
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
952 PM CDT TUE MAY 26 2015

.UPDATE...
908 PM CDT

FOR EVENING UPDATE...

HAVE ADJUSTED POPS A BIT BASED ON OBSERVATIONAL TRENDS WITH
SHOWERS/ISOLATED THUNDERSTORMS MOVING BACK ACROSS THE AREA THIS
EVENING...THOUGH NO BIG CHANGES TO GOING FORECAST.

EVENING WATER VAPOR IMAGERY/00Z RAOB ANALYSIS DEPICTS UPPER
TROUGH/MID-LEVEL CLOSED LOW LOCATED OVER WISCONSIN...WITH EVIDENCE
OF A SMALLER MID-LEVEL SHORT WAVE ROTATING ACROSS EASTERN IA INTO
SOUTHWEST WI/NORTHWEST IL...AND ANOTHER UPSTREAM ACROSS WESTERN
IA. REGIONAL RADAR MOSAIC DEPICTS SCATTERED SHOWERS/ISOLATED
THUNDER ALONG/AHEAD OF S/WV ACROSS NORTHWEST IL...WITH TRENDS
APPEARING TO BE FAIRLY WELL HANDLED BY RECENT HRRR RUNS WHICH
CONTINUE TO SPREAD THESE SHOWERS ACROSS NORTHERN IL OVER THE NEXT
SEVERAL HOURS. SPC MESOANALYSIS INDICATES A SLIGHT INCREASE IN
WEAK MUCAPE ACROSS NE MO/EASTERN IA/WESTERN IL...WITH SOME FAIRLY
PERSISTENT CG LIGHTNING STRIKES WITH CELLS NORTH OF MACOMB. THUS
EXPECT AN INCREASE IN ISOLATED-SCATTERED SHOWERS FROM WEST-EAST
ACROSS THE CWA WITH AN OCCASIONAL RUMBLE OF THUNDER POSSIBLE AS
MID-LEVEL WAVE(S) AND SURFACE COLD FRONT MOVE SLOWLY ACROSS THE
AREA THROUGH EARLY WEDNESDAY.

OTHERWISE...LIGHT SOUTHWEST SFC WINDS CONTINUE SOUTH OF MAIN
SURFACE LOW CENTER OVER WI...WITH TEMPS EXPECTED TO HOLD GENERALLY
IN THE 60S OVERNIGHT AHEAD OF THE COLD FRONT.

RATZER

&&

.SHORT TERM...
253 PM CDT

THROUGH WEDNESDAY...

A SURFACE LOW IS OVER NORTHEAST IA AND SW WI WITH A VORT MAX OVER
IOWA. THE LOWS COLD FRONT REMAINS A BIT FURTHER BACK OVER CENTRAL IA
AND EASTERN KS. THE LOW WILL DEEPEN SLIGHTLY AS IT MOVES OVER
CENTRAL WI AND ITS COLD FRONT SWINGS THROUGH WEDNESDAY. THE MAIN
CONCERN RIGHT NOW IS HOW MUCH CONVECTION WILL DEVELOP THIS
AFTERNOON.  THE LATEST RAP ANALYSIS SUGGESTS A RIBBON OF 1000 J/KG
OF CAPE IS OVER I-39...BUT THINKING THAT CAPE IN REALITY IS AROUND
500 J/KG DUE TO LIMITED HEATING. CAPE VALUES COULD RISE TO AROUND
1000 J/KG AS THERE ARE A FEW BREAKS IN THE CLOUDS...BUT THINKING IN
GENERAL THE BREAKS ARE MOVING TOO QUICKLY AND TOO FEW AND FAR BTWN
TO AMOUNT TO MUCH.  SHEAR VALUES REMAIN VERY FAVORABLE FOR ORGANIZED
STORMS WITH VALUES AROUND 35-40 KT.

THEREFORE EXPECTING SCATTERED SHOWER/TS DEVELOPMENT THIS AFTN IN THE
WAKE OF THE LINE OF SHOWERS THAT IS SHIFTING EAST ACROSS INDIANA
RIGHT NOW.  A SECOND SKINNY LINE OF CONVECTION LIES FROM DIXON IL TO
JERSEYVILLE IL WITH A FEW ADDITIONAL CELLS OUT AHEAD OF IT. DUE TO
LIMITED CAPE AS DISCUSSED ABOVE...THE STRONGER CELLS MAY PRODUCE
DAMAGING WINDS. WINDS HAVE VEERED TO THE SW AND HELICITY IS MUCH
LOWER SO NOT AS CONCERNED ABOUT ISOLATED TORNADOES AS I WAS THIS
MORNING.  PWAT VALUES REMAIN VERY IMPRESSIVE AT 1.4-1.7 INCHES SO
ANY OF THE NEW SHOWERS WILL BE CAPABLE OF ADDITIONAL HEAVY DOWNPOURS.

THE AREA OF SCATTERED SHOWERS AND STORMS SHIFTS EAST OF THE CWA THIS
EVENING...BUT WE ARE NOT DONE YET. ANOTHER ROUND OF SHOWERS AND
PERHAPS OF THUNDERSTORMS MAY FORM THIS EVENING AS THE VORT MAX
APPROACHES. THINKING THE COVERAGE WILL BE BEST ALONG AND WEST OF
GARY INDIANA TO PAXTON IL LINE. HAVE LOW CONFIDENCE IN COVERAGE AND
TIMING OF THIS ROUND OF SHOWERS...BUT NOT EXPECTING MUCH THUNDER AND
NO SEVERE WEATHER.  DEWPOINTS REMAIN IN THE 60S TONIGHT SO THINKING
WE WILL NOT COOL MUCH...SO KEPT LOWS IN THE LOW TO MID 60S.

THE PATTERN REMAINS MESSY TOMORROW BUT NOT AS ACTIVE.  A PAIR OF
VORT STREAMERS AND THE LOWS COLD FRONT ROTATE THROUGH AND SHOWERS
AND STORMS MAY FORM ALONG THEM.  THE FIRST IS ASSOCIATED WITH THE
UPPER LEVEL LOW SHIFTING NE INTO ONTARIO TOMORROW. SHOWERS AND
STORMS WITH THIS VORT STREAMER WOULD CLIP THE NORTHERN CWA...NORTH
OF I-88.  PRECIP MAY SINK SOUTH ACROSS DUPAGE AND COOK COUNTIES IN
THE AFTN...BUT HAVE LOW CONFIDENCE IN HOW FAR SOUTH THE PRECIP WILL
GET.

CLOUD COVER SHIFTS EAST THROUGH THE AFTERNOON WITH AREAS WEST OF I-
55 BECOMING MOSTLY SUNNY BY MID AFTN. DID NOT MAKE TOO MANY CHANGES
TO HIGH TEMPS TOMORROW AS CLOUD COVER WILL PLAY A ROLE.  HAVE TEMPS
IN THE UPPER 70S TO LOW 80S.  A LAKE BREEZE IS POSSIBLE LATE IN THE
AFTN AND EVENING SO EXPECTING LAKE COOLING ESPECIALLY AT WAUKEGAN.

JEE

&&

.LONG TERM...
330 PM CDT

WEDNESDAY NIGHT THROUGH TUESDAY...

LOCAL AREA WILL START OUT FIRMLY UNDER MID/UPPER RIDGING ON
WEDNESDAY NIGHT AND THURSDAY...RESULTING IN A DRY AND WARM PERIOD
AS 925 MB TEMPS CLIMB FROM THE UPPER TEENS CELSISUS BY EARLY
THURSDAY TO THE LOWER 20S CELSISUS EARLY THURSDAY EVENING. INLAND
TEMPERATURES WILL WARM TO THE LOWER-MID 80S ON THURSDAY...BUT
DESPITE SOUTHERLY SURFACE FLOW...GRADIENT WILL BE WEAK ENOUGH TO
ENABLE A LAKE BREEZE TO DEVELOP AND PUSH INLAND FAIRLY EARLY. THIS
WILL KEEP LAKESHORE TEMPS (ESPECIALLY IL SHORE) IN THE UPPER 60S.

CHANGES WILL START TO TAKE PLACE THURSDAY NIGHT WITH UPPER RIDGE
AXIS GETTING SHUNTED EAST AND ALLOWS SHORTWAVE ENERGY TO LIFT
EAST-NORTHEAST ACROSS THE AREA AFTER MIDNIGHT INTO FRIDAY ALONG
WITH STRENGTHENING AND VEERING LOW LEVEL JET. HAVE FOCUSED MID
CHANCE POPS INTO NORTHWEST CWA LATE THURSDAY NIGHT AND THEN SPREAD
LIKELY POPS FOR MUCH OF THE CWA FRIDAY MORNING AND EARLY
AFTERNOON. IT APPEARS THAT BETTER MOISTURE CONVERGENCE AND UPPER
FORCING WILL LIFT NORTH AND REFOCUS INTO NORTHWEST CWA FRIDAY
AFTERNOON AND TAILORED POPS AS SUCH...BUT THEN A STRONGER WAVE
WILL MOVE ACROSS THE AREA SATURDAY NIGHT. CONFIDENCE IS A BIT
LOWER IN HIGH TEMPS FRIDAY DUE TO UNCERTAINTY OVER IMPACT OF
CLOUDS/PRECIP...BUT VERY MILD START TO DAY AND WARM AIRMASS IN
PLACE SHOULD STILL SUPPORT HIGHS INTO THE LOWER-MID 80S...WARMEST
IN SOUTHEAST HALF OF CWA. THE WARM TEMPS...DEWPOINTS WELL INTO THE
60S AND MODEST MIDLEVEL LAPSE RATES WILL YIELD MODEST CAPE
VALUES...SO CANNOT RULE OUT AN ISOLATED SEVERE THREAT FOR DAMAGING
WINDS INTO FRIDAY EVENING. COULD BE SOME WET MICROBURSTS WITH PWAT
VALUES IN 1.5 TO 1.75 INCH RANGE.

A STRONG COLD FRONT WILL SWEEP ACROSS THE AREA FRIDAY
NIGHT...RESULTING IN MUCH COOLER WEATHER FOR THE LAST WEEKEND OF
MAY. HIGHS ON THE LAKEFRONT ARE LIKELY TO STAY IN THE 50S ON BOTH
SATURDAY AND SUNDAY...WITH ONLY 60S AND 70S INLAND ON SATURDAY AND
60S ON SUNDAY. WILL NEED TO WATCH FOR A WAVE OF SURFACE LOW
PRESSURE FORMING SOUTH OF THE AREA ON SATURDAY...SO KEPT HIGHER
CONSENSUS POPS FOR NOW SATURDAY MORNING...BUT IF TRENDS ON MUCH OF
GUIDANCE CONTINUE MAY BE ABLE TO LOWER POPS SATURDAY THROUGH
MONDAY WITH EXPANSIVE COOL HIGH PRESSURE OVER THE GREAT LAKES.
TEMPERATURES WILL THEN WARM BACK TO NEAR NORMAL FOR EARLY JUNE BY
TUESDAY WITH SOUTHERLY FLOW ON BACK SIDE OF HIGH TO EAST...THOUGH
ONSHORE FLOW LOOKS TO CONTINUE FOR AT LEAST A PORTION OF THE
LAKEFRONT.

RC

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 03Z...

* SCATTERED SHRA THRU OVERNIGHT.
* MVFR CIGS POSSIBLE OVERNIGHT INTO WEDNESDAY MORNING.
* LAKE BREEZE WIND SHIFT TO NORTHEAST POSSIBLE LATE AFTERNOON INTO
  EARLY EVENING WEDNESDAY. CMS

//DISCUSSION...UPDATED 00Z...

LOW PRESSURE LOCATED OVER CENTRAL WISCONSIN RESULTING IN GUSTY
SOUTHWEST WINDS AND THESE GUSTY WINDS WILL CONTINUE THROUGH MID
EVENING. AFTER NARROW AREA OF SHRA CLEARS THE EASTERN
TERMINALS...ANOTHER UPPER DISTURBANCE ROTATING NORTHEAST FROM IA
SHOULD BRING SCATTERED SHRA INTO NORTHERN ILLINOIS AGAIN BY MID
EVENING...BUT TRAJECTORY WOULD FAVOR RFD AND HAVE LEFT OTHER
TERMINALS DRY FOR NOW. NEXT CONCERN IS FOR POTENTIAL PERIOD OF
LOWER CIGS OVERNIGHT INTO EARLY WEDNESDAY. GUIDANCE VARIES ON HOW
LOW/WIDESPREAD CIGS WILL GET AND TIMING...BUT GIVEN LOWER
CONFIDENCE HAVE MAINTAINED PREVIOUS TIMING. DID PULL MVFR VSBY AS
STEADY SOUTHWEST WINDS SHOULD PREVENT VSBY RESTRICTIONS.

A COLD FRONT AND UPPER TROUGH WILL MOVE ACROSS THE AREA ON
WEDNESDAY...WITH WINDS VEERING TO WEST AND THEN WEST-NORTHWEST.
GUSTS TO 15 TO 20 KT ARE POSSIBLE IN THE MORNING THROUGH EARLY
AFTERNOON. GRADIENT IS EXPECTED TO WEAKEN ENOUGH TO ALLOW LAKE
BREEZE TO SLIP INLAND LATE...BUT SPEEDS SHOULD BE LIGHT BEHIND
SHIFT. AS UPPER TROUGH MOVES OVERHEAD...ONLY VERY MINIMAL
INSTABILITY SHOWN ON FORECAST SOUNDINGS AND SHOWER COVERAGE IS
LIKELY TO BE ISOLATED...SO HAVE BACKED OFF TO A PROB30 DURING THE
AFTERNOON AT THE EASTERN TERMINALS.

RC

//ORD AND MDW CONFIDENCE...UPDATED 03Z...

* MEDIUM FOR SHRA THRU OVERNIGHT AND AGAIN WEDNESDAY AFTERNOON.
* LOW FOR MVFR CIGS WEDNESDAY MORNING.
* MEDIUM FOR LAKE BREEZE LATE WEDNESDAY AFTERNOON. CMS

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

THURSDAY...MAINLY VFR WITH CHANCE SHRA/TSRA LATE THURSDAY NIGHT.
LAKE BREEZE SHIFT TO EAST WINDS.

FRIDAY...PERIODIC SHRA/TSRA...ESPECIALLY IN MORNING AND THEN AT
NIGHT.

SATURDAY...STRONG NORTH-NORTHEAST WINDS. SHRA AND MVFR
POSSIBLE...MAINLY IN THE MORNING.

SUNDAY AND MONDAY...VFR. NORTHEAST WINDS.

RC

&&

.MARINE...
417 PM CDT

WINDS REMAIN GUSTY THIS AFTERNOON AND DECIDED TO ISSUE A SMALL
CRAFT ADVISORY FOR STRONG WINDS INTO THIS EVENING. THE SOUTH WINDS
BECOME SOUTHWEST THIS EVENING. LOW PRESSURE MOVES OVER QUEBEC
TOMORROW AFTERNOON AND COLD FRONT TRAILING FROM THE LOW SWINGS
THROUGH. WINDS WEAKEN WEDNESDAY AFTERNOON AND A LAKE BREEZE IS
EXPECTED LATE WEDNESDAY AFTERNOON.

MODEST SOUTHERLY FLOW IS EXPECTED ON THURSDAY...WITH LAKE BREEZE
INFLUENCES IN THE SOUTHERN PORTION OF THE LAKE...THEN A PERIOD OF
SLIGHTLY STRONGER SOUTHERLY FLOW ON FRIDAY AHEAD OF A STRONG COLD
FRONT...THOUGH WARM AIRMASS WILL LIMIT MIXING. WILL LIKELY NEED TO
ADD AREAS OF FOG OVER THE LAKE. THE STRONG COLD FRONT WILL SWEEP
ACROSS THE LAKE FRIDAY NIGHT INTO SATURDAY...BRINGING NORTHERLY
WINDS TO 30 KT. WILL NEED TO WATCH FOR A PERIOD OF NORTHERLY GALES
SHOWN ON SOME MODEL GUIDANCE. WILL NEED A SMALL CRAFT ADVISORY
SATURDAY MORNING THROUGH SUNDAY MORNING UNTIL WAVES SUBSIDE.

RC

&&

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

&&

$$

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