000
FXUS63 KLOT 220104
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
804 PM CDT TUE MAY 21 2013

.DISCUSSION...
804 PM CDT

FOR EVENING UPDATE...

HAVE MADE ADJUSTMENTS TO LOWER NEAR TERM POPS BASED ON CURRENT OBS
TRENDS...IN ADDITION TO MINOR HOURLY TEMP TWEAKS THIS EVENING.

BROAD AREA OF SURFACE LOW PRESSURE WAS ANALYZED ACROSS THE UPPER
MIDWEST FROM FAR NORTHWEST IA INTO CENTRAL WI EARLY THIS EVENING.
WFO LOT CWA WAS WITHIN THE WARM SECTOR SOUTH OF THE ELONGATED LOW
WITH BROAD GENERALLY UNIDIRECTIONAL SOUTHWEST FLOW. 00Z ILX SOUNDING
DEPICTS SOME MODEST CAPE...ASSOCIATED WITH MOIST AXIS ACROSS
SOUTHEASTERN CWA...THOUGH LITTLE IN THE WAY OF ORGANIZED FORCING TO
INITIATE ANY SHRA/TSRA THROUGH THE AFTERNOON/EARLY EVENING. GUIDANCE
IS IN GENERAL AGREEMENT HOWEVER IN INCREASING COVERAGE OF SHRA/TSRA
FROM THE SOUTHWEST OVERNIGHT...WITH RADAR RETURNS INDEED RECENTLY
DEVELOPING ACROSS THE ILLINOIS RIVER VALLEY SOUTH OF PEORIA. THESE
SHOULD PROPAGATE NORTHEAST INTO SOUTHERN PORTIONS OF THE CWA LATER
THIS EVENING...LIKELY INCREASING IN COVERAGE LATE TONIGHT INTO
WEDNESDAY MORNING AS LARGE SCALE FORCING BECOMES MORE FAVORABLE WITH
THE APPROACH OF SHORT WAVE ENERGY LIFTING OUT OF MISSOURI.

SOUTHEAST/EAST PORTIONS OF THE FORECAST AREA APPEAR TO HAVE HIGHER
POTENTIAL FOR SHRA/TSRA IN ASSOCIATION WITH LOW LEVEL MOISTURE
PLUME...WITH RELATIVELY POOR MID-LEVEL LAPSE RATES NOTED FARTHER
NORTH/NORTHWEST LIKELY TO LIMIT THUNDER GREATEST THUNDER POTENTIAL
TO THE SAME AREAS. WITH ALL THIS IN MIND HAVE THEREFORE
SIGNIFICANTLY REDUCED OR ELIMINATED POPS ESPECIALLY NORTH/NORTHWEST
THROUGH MID-EVENING...BLENDING INTO HIGHER POPS OF PREVIOUS FORECAST
AFTER MIDNIGHT. HAVE ALSO BACKED OFF THUNDER MENTION TO SLGT CHANCE
ACROSS THE NORTHWEST PARTS OF THE FORECAST AREA.

RATZER

&&

.PREV DISCUSSION...
322 PM CDT

ANOTHER ROUND OF SHOWERS AND THUNDERSTORMS TONIGHT...MAINLY FOR THE
SOUTHEASTERN HALF OF THE FORECAST AREA...IS EXPECTED TO LINGER INTO
THE MORNING THEN GRADUALLY GIVE WAY TO LIGHT SHOWERS BY TOMORROW
EVENING BEFORE DRIER AIR STARTS TO MOVE IN TOMORROW NIGHT INTO
THURSDAY. AFTER TONIGHT`S ACTIVITY...COOLER TEMPERATURES ARRIVE
TOMORROW AS WINDS SLOWLY TURN FROM SOUTHWEST TODAY TO NORTHWEST
TOMORROW THEN ALL THE WAY TO NORTH BY THURSDAY AFTERNOON.  WITH
LIGHT NORTH WINDS AND CLEARING SKIES...TEMPERATURES FRIDAY MORNING
COULD BE A BIT CHILLY...PERHAPS EVEN A FEW DEGREES COOLER THAN THE
LOW TO MID 40S REPRESENTED IN OUR FORECAST GRIDS.  THIS COOLER AND
DRIER WEATHER APPEARS TO PERSIST THROUGH THE WEEKEND EXCEPT
PERHAPS FOR SOUTHWEST PORTIONS OF THE FORECAST AREA WHICH COULD BE
INFLUENCED BY RETURNING GULF MOISTURE.

IN THE NEAR TERM...A FEW THUNDERSTORMS HAVE STARTED TO DEVELOP THIS
AFTERNOON IN WISCONSIN AND LOWER MICHIGAN WELL NORTH OF THE FORECAST
AREA...WHILE MORE WIDESPREAD ACTIVITY HAS BEEN FIRING FAR TO THE
SOUTH...FROM THE SOUTHERN PLAINS TO THE SOUTHEAST.  THE ACTIVITY TO
OUR NORTH APPEARS TO BE SUPPORTED BY A WEAK WEST TO EAST ORIENTED
SURFACE TROUGH UNDER SHORTWAVE ENERGY SPINNING AROUND AN UPPER LOW
POSITIONED OVER THE EASTERN DAKOTAS.  SUPPORT FOR THIS ACTIVITY HAS
BEEN TRENDING FARTHER NORTH...AND THIS WOULD NOT BE EXPECTED TO
EXPAND INTO THE IL/IN AREAS.  VIS SATELLITE IMAGERY CONTINUES TO
INDICATE VERY LITTLE WIDESPREAD CUMULUS DEVELOPMENT THIS
AFTERNOON...LIKELY DUE TO WEAK CAPPING AT LOW LEVELS AND POOR LAPSE
RATES ALOFT AS EVIDENCED BY LATEST ACARS SOUNDINGS FROM ORD AND
MODEL FORECAST SOUNDINGS.

AS MENTIONED IN THE AFD UPDATE EARLIER THIS AFTERNOON...BY TONIGHT
THE DIURNAL HEATING WILL OF COURSE NOT BE FAVORABLE BUT BETTER
SHORTWAVE ENERGY...SFC AND MIDLEVEL MOISTURE...AND COOLER
TEMPERATURES ALOFT WILL SUPPORT AN INCREASED CHANCE OF TSRA AHEAD OF
AN APPROACHING COLD FRONT.  THE BEST CHANCES ARE ACROSS THE
SOUTHEASTERN HALF OF THE FORECAST AREA WHERE MOISTURE FLUX MAXIMIZES
IN CONJUNCTION WITH A PRONOUNCED LOW LEVEL JET NOSING INTO INDIANA.

THE APPROACH OF THE UPPER TROUGH ITSELF...THE CONTINUOUS PRESENCE OF
CONSIDERABLE MOISTURE AT LOW LEVELS...AND THE PROXIMITY OF THE
SURFACE TROUGH WILL SUPPORT AN ONGOING CHANCE OF TSRA
TOMORROW...GRADUALLY GIVING WAY TO SHOWERS BY EVENING AND OVERNIGHT
INTO THURSDAY.

THE AXIS OF THE UPPER TROUGH IS STILL OVER THE AREA ON THURSDAY BUT
A REINFORCING COLD FRONT WILL DRY OUT LOW LEVELS AND LEAD TO
DECREASING PRECIP CHANCES.  BY FRIDAY THE AXIS IS WELL TO OUR EAST
AND NORTHERLY FLOW WILL MAINTAIN COOL DRY AIR AT ALL LEVELS.

FOR THE EXTENDED...MAIN FORECAST CONCERNS/CHALLENGES WILL BE WITH
SHOWER/THUNDERSTORM POTENTIAL...WITH LOW CONFIDENCE AT THIS TIME.

EXPECT THE START OF THE EXTENDED PERIOD TO BE ON THE QUIET SIDE WITH
THE HIGHLY AMPLIFIED UPPER LEVEL TROUGH EXITING TOWARDS THE EAST
COAST...AND WITH A LARGE EXPANSIVE SURFACE HIGH BUILDING SOUTH
THROUGH MUCH OF THE REGION. ALTHOUGH...DO EXPECT CHANCES FOR PRECIP
TO INCREASE BY THE START OF THE WEEKEND WITH ACTIVITY INCREASING
OVER THE CENTRAL PLAINS POSSIBLY SHIFTING EAST TOWARDS THE CWA. AN
UPPER LEVEL RIDGE SHIFTING EAST ACROSS THE CENTRAL CONUS WILL
DEFINITELY BE KEY WITH SHOWER/THUNDERSTORM POTENTIAL AS GUIDANCE
VARIES WITH THE EXTENT AND STRENGTH OF THIS RIDGE OVER THE CENTRAL
CONUS. THE ECMWF IS LESS EXCITED WITH THE STRENGTH OF THIS RIDGE
BREAKING IT DOWN AS SEVERAL WAVES OF ENERGY TRAVERSE THE FLOW. WITH
THIS OCCURRING...STRENGTHENING AND VEERING LLJ WILL ALLOW ANY
DEVELOPMENT ACROSS THE PLAINS ON FRIDAY TO SHIFT EAST TOWARDS THE
CWA INTO FRIDAY NIGHT BRINGING THE EDGE OF THE PRECIP SHIELD ACROSS
THE SOUTHWEST PORTIONS OF THE CWA. MEANWHILE...THE GFS HOLDS THE
UPPER LEVEL RIDGE IN PLACE AND KEEPS THE LLJ ORIENTED TOWARDS THE
PLAINS WITH ANY PRECIP STAYING WELL WEST OF THE CWA FRIDAY NIGHT.

WITH SOME UNCERTAINTY REMAINING WITH REGARDS TO SYSTEM
EVOLUTION...HAVE MADE VERY LITTLE CHANGE FRIDAY NIGHT KEEPING LOW
CHANCE/SLIGHT CHANCE POPS FOR PRIMARILY THE WESTERN HALF OF THE CWA.
WHILE THE ECMWF DRIES OUT THE CWA SATURDAY INTO THE REMAINDER OF THE
WEEKEND...THE GFS FINALLY SWINGS THE PRECIP ACROSS THE CWA. WITH THE
GFS ADVERTISING PERSISTENT FLOW OVER A LINGERING BOUNDARY FOR THE
REMAINDER OF THE WEEKEND INTO EARLY NEXT WEEK IT BRINGS PERIODS OF
PRECIP ACROSS THE CWA...WHILE THE ECMWF STILL REMAINING PRIMARILY
DRY. WHILE THE ECMWF IS INDICATING HIGHER DEWPOINT AIR RESIDING JUST
TO THE SOUTH OF THE CWA WITH THIS BOUNDARY ALSO NOT TOO FAR
AWAY...IT WOULD NOT BE OUT OF THE QUESTION FOR A FURTHER NORTHWARD
SHIFT WITH THIS BOUNDARY AND THUS ANY PRECIP. SO HAVE MAINTAINED LOW
CHANCE/SLIGHT CHANCE POPS FOR MAINLY THE SOUTHERN CWA THROUGH EARLY
NEXT WEEK. THUNDER APPEARS LIKELY...BUT WITH INSTABILITY/MOISTURE
APPEARING MARGINAL AT THIS POINT.

LENNING/RODRIGUEZ

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 00Z...

* CHANCE TSRA LATE TONIGHT.

* CHANCE MVFR CIGS/VIS IN SHRA TOMORROW AFTERNOON.

KREIN

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

THE AREA IS IN A RELATIVE CALM AND STABILITY BETWEEN THE LINES OF
THUNDERSTORMS TO THE SOUTH AND A STRONG DEFORMATION ZONE OF
SHOWERS TO THE NORTH AND WEST. THE SHORT RANGE MODEL GUIDANCE
INDICATES A CHANCE FOR ONE OF THE THUNDERSTORM CLUSTERS TO EXPAND
ON THE NORTHERN EXTENT...POSSIBLY REACHING INTO EAST-CENTRAL
ILLINOIS AND INDIANA. THESE IS A CHANCE THAT SOME OF THE ACTIVITY
COULD REACH INTO THE CHICAGO AREA TERMINALS BY AROUND
MIDNIGHT...BUT CONFIDENCE IS LOW ON ANY PCPN OR TS TO DIRECTLY
IMPACT THE TERMINALS. SO...WILL CARRY THE VCTS GROUP IN THE TAF
FOR NOW AND MONITOR CLOSELY TO REFINE THE FORECAST IN LATER
UPDATES. FOR THE REMAINDER OF THE PERIOD...WILL FOCUS ON THE
CHANCES FOR LOWER CIGS/VIS AS THE PERSISTENT UPPER LOW BEGINS TO
LIFT NEWD TOMORROW. ANOTHER SHORTWAVE ROTATING AROUND THE SOUTHERN
PERIPHERY OF THE UPPER LOW WILL BRING ANOTHER CHANCE FOR SOME
SHOWERS TOMORROW AFTERNOON...WITH CIGS/VIS LOCALLY LOWERING TO
MVFR INVOF THE SHOWERS. BY TOMORROW EVENING...THE REGION SHOULD BE
IN THE COLD ADVECTION AS THE UPPER LIFTS OFF TO THE
NORTHEAST...WITH WINDS GRADUALLY VEERING TO NORTHWESTERLY AND A
MORE WIDESPREAD ST/STRATOCU DECK OVERSPREADING THE REGION WITH
MVFR CIGS.

KREIN

//ORD AND MDW CONFIDENCE...UPDATED 00Z...

* LOW CONFIDENCE IN TSRA DIRECTLY IMPACTING THE TERMINALS THIS EVENING.

* MEDIUM CONFIDENCE IN CIG/VIS TRENDS.

* MEDIUM TO HIGH CONFIDENCE IN WIND TRENDS.

KREIN

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

THURSDAY...CHC SHRA IN THE DAY. VFR OVERNIGHT.

FRIDAY...VFR. WX NIL.

SATURDAY - MONDAY...PERIODIC CHANCES FOR SHRA/TSRA.

KREIN

&&

.MARINE...
314 PM CDT

LOW PRESSURE CURRENTLY OVER SOUTHERN MINNESOTA AND CENTRAL WISCONSIN
WILL WEAKEN AND DRIFT OVER THE WESTERN GREAT LAKES TONIGHT.  THE
LOWS COLD FRONT WILL PASS SOUTH OF THE LAKE TONIGHT INTO TOMORROW.
WINDS WILL DIMINISH THIS EVENING THEREFORE THINKING THE SMALL CRAFT
WILL BE ALLOWED TO EXPIRE THIS EVENING AS WELL.

AS THE LOW SHIFTS EAST OVER THE NEW ENGLAND COAST...NORTHWEST TO
NORTH GALES WILL SPREAD SOUTH ACROSS THE LAKE.  THINKING GALES WILL
DEVELOP OVER THE NORTHERN END OF THE LAKE LATE WEDNESDAY
NIGHT...SPREAD ACROSS DOWN THE LAKE ON THURSDAY...THEN DIMINISH
THURSDAY EVENING.  ISSUED A GALE WATCH AS THE EXACT TIMING AND
STRENGTH OF THE GALES IS UNCERTAIN AT THIS TIME.  MODEL SOUNDINGS
INDICATE THERE WILL BE A STRONG CAP OVER THE LAKE WHICH MAY LIMIT
MIXING.  THAT BEING SAID GUIDANCE INDICATES 40+ KT WINDS WILL BE
ONLY 1200 FT ABOVE THE SURFACE SO THINKING GALES ARE CERTAINLY
POSSIBLE.

A RIDGE OF HIGH PRESSURE SPREADS OVER THE LAKE FRIDAY MORNING WITH
NORTH WINDS DIMINISHING AND BECOMING EAST ON SATURDAY.

JEE

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...NEARSHORE WATERS
     UNTIL 10 PM TUESDAY.

     GALE WATCH...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-LMZ745...10 AM
     THURSDAY TO 9 PM THURSDAY.

     GALE WATCH...LMZ080-LMZ669-LMZ671-LMZ673-LMZ675-LMZ777-LMZ779-
     LMZ870-LMZ872-LMZ874-LMZ876-LMZ878...3 AM THURSDAY TO 9 PM
     THURSDAY.

     GALE WATCH...LMZ261-LMZ362-LMZ364-LMZ366-LMZ563-LMZ565-LMZ567-
     LMZ868...3 AM THURSDAY TO 7 PM THURSDAY.

&&

$$

VISIT US AT HTTP://WEATHER.GOV/CHICAGO (ALL LOWERCASE)

FOLLOW US ON FACEBOOK...TWITTER...AND YOUTUBE AT:
WWW.FACEBOOK.COM/US.NATIONALWEATHERSERVICE.CHICAGO.GOV
WWW.TWITTER.COM/NWSCHICAGO
WWW.YOUTUBE.COM/NWSCHICAGO