Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 250740
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
240 AM CDT MON MAY 25 2015

.SHORT TERM...
209 PM CDT

THROUGH TONIGHT...

VERY SHARP WARM FRONT HAS BEEN GRADUALLY LIFTING NORTHWARD THROUGH
THE AFTERNOON HOURS. TEMPERATURES HAVE SOARED INTO THE 80S BEHIND
THE FRONT...WITH MUGGY MID 60S DEWPOINTS AS WELL. MEANWHILE FAR
NORTHERN ILLINOIS IS HOLDING ONTO THE 70S WITH TEMPS HOLDING AROUND
60 AT THE LAKE WITH ONSHORE WINDS.  ALOFT... A DEEP AND CLOSED UPPER
LOW IS SPINNING OVER NORTHERN COLORADO. A NEGATIVELY TILTED TROUGH
AXIS OUT AHEAD OF THE LOW INTO NORTHERN TEXAS WITH A MODESTLY STRONG
SHORTWAVE ALONG AND AHEAD OF IT. SHOWERS AND THUNDERSTORMS CONTINUE
AHEAD OF THIS FEATURE. CLOSER TO HOME...UPPER LEVEL RIDGING IS IN
PLACE ACROSS THE EASTERN HALF OF THE AREA...AND NO NOTABLE
SHORTWAVES OTHER THAN SOME WEAKER WAVES ARE FOUND IN SW FLOW ALOFT
ACROSS FAR WESTERN ILLINOIS. MORE ORGANIZED SHOWER ACTIVITY FROM
I-39 WESTWARD ASSOCIATED WITH THIS...BUT MUCH OF THE AREA REMAINS
IN BROAD AND MOIST SW FLOW. THEREFORE SCATTERED SHOWERS CONTINUE
TO FESTER FARTHER EASTWARD THOUGH THE ARE STRUGGLING A BIT MORE IN
THE LESS FAVORABLE ENVIRONMENT.

EXPECT AS THE AFTERNOON CONTINUES THAT SHOWERS REMAIN LIKELY IN OUR
WESTERN AREAS...WHILE MUCH OF ILLINOIS WILL BE SUSCEPTIBLE TO
SCATTERED SHOWERS THROUGH THE AFTERNOON PERIOD AS WELL. THE SHOWERS
PUT DOWN NEARLY TWO TENTHS OF AN INCH IN AN HOUR AT PEORIA...AND
WITH PW VALUES ON THE RISE...SOME BRIEF DOWNPOURS REMAIN POSSIBLE.
INSTABILITY IS MODEST AND FOCUSED ALONG AND WELL SOUTH OF I-80 WHERE
AN ISOLATED MENTION HAS BEEN HELD...BUT WE DO HAVE SOME STRIKES IN
LASALLE COUNTY AS OF THIS WRITING.

LOOKING FARTHER DOWNSTATE BEHIND THIS BATCH OF SHOWERS CUMULUS FIELD
IS FAIRLY TEMPERED WITH MORE AGITATED CLOUDS IN MISSOURI...AND MORE
SO FARTHER SOUTH IN THE ARKLATEX REGION. HAVE TRIED TO SHOW SOMEWHAT
OF A DOWNWARD TREND IN CHANCES FOR PRECIPITATION ACROSS NE ILLINOIS
TONIGHT IN BETWEEN THE CURRENT ACTIVITY BEFORE THE TROUGH AXIS
BEGINS PIVOTING NORTHWARD. MOIST ISENTROPIC LIFT REMAINS A BIT MORE
ROBUST ALONG AND WEST OF I-39 WHERE OCCASIONAL SHOWERS WILL CONTINUE
EVEN THIS EVENING AND HELD ONTO LIKELY POPS LONGER IN THESE AREAS.

OVERNIGHT IS WHEN THINGS WILL RAMP UP ACROSS OUR LOCAL AREA. THE
NEGATIVELY TILTED TROUGH AXIS GETS SHUNTED NORTHEASTWARD ON A
POTENT MID/UPPER LEVEL JET. LOW LEVEL MOISTURE ADVECTION INCREASES
WHICH WILL BRING THE MOST ORGANIZED PERIOD OF SHOWERS AND
ISOLATED/SCATTERED THUNDERSTORMS ACROSS THE AREA...AFTER MIDNIGHT
AND INTO THE MORNING OF MEMORIAL DAY. WITH A VERY STRONG LOW LEVEL
WIND FIELD AND AND CLOSED SURFACE LOW PASSING ACROSS EASTERN
IOWA...SOUTHERLY WINDS WILL BE RAMPING UP CONSIDERABLY LATE
TONIGHT INTO MEMORIAL DAY MORNING.

MORE ON THE WINDS TOMORROW AND THE LONG TERM DISCUSSION SOON...

KMD

&&

.LONG TERM...
239 AM CDT

TUESDAY THROUGH SUNDAY...

TUESDAY THROUGH WEDNESDAY...
500MB TROUGH AXIS WILL SLOWLY PIVOT EAST ACROSS THE CENTRAL PLAINS
INTO THE MID-MISSOURI VALLEY TUE...WITH A SFC WAVE SLIDING NORTH
ACROSS IOWA INTO CENTRAL WISC LATE TUE. THE SFC FEATURE IS NOT
EXPECTED TO BE STRONG...HOWEVER THE GRAIDENT DOES TIGHTEN WITH
MODEST HEIGHT FALLS IN THE LOWEST LAYERS OF THE ATMOSPHERE. SOME
INSTABILITY DOES REMAIN ALONG WITH RELATIELY MOIST SFC CONDS AS DEW
PTS WILL REMAIN IN THE UPR 50S/LOW 60S. PWAT VALUES WILL ALSO HOVER
JUST OVER 1 INCH...WHICH IS SEASONAL. WILL HOLD ONTO MENTION OF RAIN
AND THUNDER...BUT COULD SEE THIS MAINLY FOCUSED IN THE TUE MORING
TIMEFRAME.

WITH CONSIDERABLE SOLAR SHIELDING PROGGED FOR TUE...TEMPS MAY
STRUGGLE TO WARM BEYOND THE MID/UPR 70S. BUT WITH GUIDANCE HINTING
AT A DRY SLOT LIFTING NORTHEAST ACROSS THE REGION TUE EVE...THIS
COULD INTRODUCE SOME EROSION TO THE CLOUD COVER IN THE AFTN HOURS
TUE AND ALLOW SOME SFC HEATING TO OCCUR AND PUSH TEMPS TO ARND 80.

GUIDANCE LIFTS THE DRY SLOT OVERHEAD LATE TUE NGT...WITH THE 500MB
TROUGH AXIS ARRIVING ARND DAYBREAK WED AND LIFTING EAST ACROSS LOWER
MICHIGAN. THIS WILL BRING DRY ONLY SLIGHT CHANCE POPS AS WEAK SFC
RIDGING BEGINS TO NUDGE EAST ACROSS THE MISSOURI VALLEY TOWARDS THE
FORECAST AREA MIDDAY WED. IT IS POSSIBLE THAT ENOUGH DRY AIR
INFLUENCE COULD ALLOW FOR REMOVAL OF THE SLIGHT CHANCE POPS WED
AFTN. THERMAL TROUGH LAGS BEHIND THE FRONTAL BOUNDARY WED...SO TEMPS
COULD STILL WARM BACK TO ARND 80 AND PERHAPS THE LOW 80S IN THE
SOUTHERN CWFA.

THURSDAY THROUGH SUNDAY...
ANTI-CYCLONIC FLOW ARRIVES THUR WITH AT LEAST ONE PERIOD OF DRY
WEATHER EXPECTED FOR NORTHERN IL/NORTHWEST IN. ONLY A COUPLE
ENSEMBLE MEMBERS CONTINUE TO PROG POPS ALONG THE SOUTHERN FORECAST
AREA THUR...BUT COULD SEE THIS TRENDING DRY IN THE NEXT FEW
SOLUTIONS. WITH WARM TEMPS ALOFT EXPECTED THUR AND THIN CLOUD
COVER...THUR HIGHS COULD PEAK IN THE LOW 80S.

THEN FOCUS TURNS TOWARDS THE UPCOMING WEEKEND AND A PATTERN SHIFT
AWAY FROM THE WARM/HUMID CONDITIONS. ENSEMBLES SHOW MINIMAL SPREAD
IN THE LATE WEEK PERIODS...WITH THE UPSTREAM RIDGE OVER
ALASKA/WESTERN CANADA AND A DOWNSTREAM MID-LVL RIDGE ACROSS THE
SOUTHEAST CONUS. BOTH OF THESE FEATURES IN TIME BEGIN TO
WEAKEN...WITH THE LONGWAVE PATTERN TRANSITIONING INTO A SEMI-ZONAL
ORIENTATION BY THE SECOND HALF OF THE WEEKEND.

OPER GUIDANCE HAS BEEN CONSISTENT WITH PROGGING A ROBUST 500MB
TROUGH TO SLIDE SOUTH FROM CENTRAL CANADA FRI...ALONG WITH A STRONG
CANADIAN SFC RIDGE ON IT/S HEELS. WITH A CHANNEL OF MOISTURE
REMAINING ACROSS THE SOUTHERN PLAINS STRETCHING NORTHEAST THROUGH
THE UPPER MIDWEST...THIS BOUNDARY WILL BECOME ACTIVE WITH A SFC WAVE
DEVELOPING ACROSS THE SOUTHERN PLAINS. THIS LOW WILL THEN LIFT
NORTHEAST TOWARDS THE CWFA FRI WITH A COLD FRONT ARRIVING ACROSS THE
FORECAST AREA SAT. WINDS LOOK TO SHIFT TO THE NORTH...WITH MUCH
COOLER AIR SLIDING SOUTH AND LIKELY KNOCKING TEMPS BACK TO SUB-
SEASONAL CONDS IN THE UPR 60S/NEAR 70 DEGREE RANGE SAT/SUN. IF
NORTHEAST WINDS MATERIALIZE...NEXT WEEKEND AREAS ADJACENT TO LAKE
MICHIGAN ALONG THE IL/IN SHORELINE COULD BE LOCKED IN THE UPR 50S TO
LOW 60S FOR HIGHS WITH A COLD FLOW OFF OF LAKE MICHIGAN.

BEACHLER

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 06Z...

* SCATTERED SHOWERS AND ISOLATED TSRA OVERNIGHT.

* WINDS...TURNING SOUTH AND INCREASING OVERNIGHT.

* STRONG AND GUSTY SOUTH WINDS MONDAY MORNING THROUGH EARLY
  AFTERNOON.

KREIN

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

MAIN CONCERN FOR THE OVERNIGHT HOURS WILL BE STRENGTHENING WINDS
BECOMING PREVAILING SOUTHERLY. AT 06S...WINS HAVE INCREASE T0
10-15KT AND SHOULD GRADUALLY INCREASE THROUGH THE NIGHT...BECOMING
GUSTY TO 20-25KT THROUGH THE EARLY MORNING HOURS AS THE SFC
PRESSURE GRADIENT TIGHTENS WITH THE SFC LOW MOVING INTO NWRN WI.
LATEST RADAR TRENDS INDICATE THAT PCPN HAS ENDED OVER NERN
IL...BUT AN AREA OF SCT SHRA/ISOLD TSRA PERSISTS FROM NCNTRL IL
WEST INTO ERN IOWA...INCLUDING THE RFD AREA. THERE IS A BAND OF
PCPN OVER CNTRL IL TRACKING ENELY WHICH COULD REACH THE CHICAGO
AREA TERMINALS DURG THE EARLY MORNING HOURS...SO WILL MAINTAIN THE
GOING FORECAST OF THE TEMPO TSRA FOR 07-10Z.

WINDS WILL THEN BECOME THE PRIMARY CONCERN BY THE PREDAWN HOURS AS
SFC RIDGING BUILDS THROUGH THE OHIO VALLEY WHILE THE SFC LOW MOVES
INTO NRN WI WITH TROUGHING EXTENDING TO THE EAST. THIS WILL SET
OF A STRONG SWLY GRADIENT AT THE SFC. ALSO...LATEST MODEL GUIDANCE
SUGGESTS THAT A STRONG LOW LEVEL JET WILL MOVE OVER THE
AREA...WITH 925MB WINDS OF 45-50KT. SINCE THE LOWER LEVELS SHOULD
REMAIN WELL MIXED OVERNIGHT...STRONGER GUSTS SHOULD HAVE LITTLE
TROUBLE MIXING DOWN TO THE SFC...WITH GUSTS OF 30-35KT LIKELY
THROUGH ARND NOON. INTO THE EARLY AFTERNOON...THE LOW LEVEL JET
WILL WEAKEN TO ABOUT 30KT WHILE VEERING MORE WLY...SO SFC GUSTS
SHOULD DROP OFF INTO THE AFTERNOON AS WELL. A SECONDARY LOW IS
EXPECTED TO DEVELOP OVER THE UPPER MISSOURI VALLEY TOMORROW
EVENING...KEEPING A MODEST PRESSURE GRADIENT OVER THE REGION...SO
MODERATE...GUSTY WINDS SHOULD PERSIST INTO TOMORROW NIGHT.

KREIN

//ORD AND MDW CONFIDENCE...UPDATED 06Z...

* MEDIUM CONFIDENCE IN WIND TRENDS OVERNIGHT BUT HIGH TOMORROW.

* MEDIUM CONFIDENCE IN CONVECTIVE TRENDS OVERNIGHT.

* MEDIUM CONFIDENCE IN CIG TRENDS OVERNIGHT...ESPECIALLY INVOF
  PCPN/TSRA.

* HIGH CONFIDENCE IN ALL OTHER FORECAST ELEMENTS.

KREIN

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

TUESDAY...SHRA LIKELY/CHANCE TSRA. GUSTY SOUTHWEST WINDS.

WEDNESDAY...CHANCE OF SHRA/TSRA EARLY. WEST WINDS TURNING EAST.

THURSDAY...CHANCE SHRA/SLIGHT CHANCE TSRA. SOUTHEAST WINDS.

FRIDAY...CHANCE SHRA/SLIGHT CHANCE TSRA. SOUTHWEST WINDS.

SATURDAY...CHANCE SHRA. EAST WINDS.

LENNING

&&

.MARINE...
330 PM CDT

A WARM FRONT WILL LIFT NORTHWARD INTO THE SOUTHERN PART OF THE
LAKE THIS EVENING...OR POSSIBLY NOT UNTIL OVERNIGHT...BUT EITHER
WAY WINDS WILL TURN SOUTHERLY DURING THE NIGHT. HIGHER DEW POINT
AIR WILL ADVECT NORTHWARD ON THESE SOUTHERLY WINDS ALLOWING FOR
POTENTIAL FOG DEVELOPMENT. AS LOW PRESSURE LIFTS NORTH THROUGH
WISCONSIN ON MEMORIAL DAY MORNING....SOUTH TO SOUTHWEST WINDS
WILL ENCOMPASS THE ENTIRE LAKE. ALSO...WIND SPEEDS WILL RAPIDLY
INCREASE IN THE ILLINOIS AND INDIANA NEARSHORE WATERS...WITH
OFFSHORE WINDS REACHING GALE CRITERIA DURING THE MORNING AND
LASTING INTO AT LEAST EARLY AFTERNOON. OVER MUCH OF THE OPEN
WATERS...STABLE AIR WILL LIKELY KEEP THE STRONGEST WINDS FROM THE
SURFACE...ALTHOUGH STILL 20 TO 30 KT WINDS ARE EXPECTED AND
LASTING INTO MONDAY NIGHT.

MTF

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...NEARSHORE WATERS
     UNTIL 9 AM MONDAY.

     GALE WARNING...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-LMZ745...9 AM
     MONDAY TO 3 PM MONDAY.

     SMALL CRAFT ADVISORY...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-
     LMZ745...3 PM MONDAY TO 4 AM TUESDAY.

&&

$$

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