000
FXUS63 KLOT 212029
AFDLOT

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

.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 20Z...

* STRONG GUSTY SOUTH TO SOUTHWEST WINDS THROUGH 00Z.

* SLIGHT CHANCE TSRA THIS AFTERNOON AND EVENING.

* TSRA LIKELY AFTER MIDNIGHT.

PAW

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

IN THE WAKE OF LAST NIGHTS SHOWERS AND STORMS...THINGS HAVE
STABILIZED FOR NOW. EXPECT MOSTLY CLEAR SKIES THROUGH THE MORNING
HOURS. BY MID DAY EXPECT ATMOSPHERE TO BEGIN TO DESTABILIZE AGAIN
BUT DONT SEE ANY REAL FOCUS FOR THUNDERSTORMS TO DEVELOP.
SCATTERED THUNDERSTORMS ARE POSSIBLE IN THE AFTERNOON AND EVENING
BUT DONT HAVE ENOUGH CONFIDENCE IN TIMING OR LOCATION TO PUT
THUNDER IN THE TAFS AT THIS TIME. WAVE ROUNDING THE BASE OF THE
PLAINS TROF OVER EASTERN NM THIS MORNING WILL LIFT NORTHEAST
TOWARD ILLINOIS LATE TONIGHT. THIS SHOULD BE THE FOCUS FOR MORE
WIDESPREAD SHOWERS AND THUNDERSTORMS LATE TONIGHT INTO WEDNESDAY
MORNING.

SOUTH WINDS WILL BE INCREASING THIS MORNING AND TURNING SW THIS
AFTERNOON. WINDS WILL DIMINISH AND GUSTINESS WILL END THIS
EVENING. WINDS WILL BECOME LIGHT AND VARIABLE LATE TONIGHT AS WEAK
LOW PRESSURE MOVES OVERHEAD.

ALLSOPP

//ORD AND MDW CONFIDENCE...UPDATED 20Z...

* HIGH CONFIDENCE IN WIND TREND.

* LOW CONFIDENCE IN THUNDERSTORMS THIS AFTERNOON AND EARLY
  EVENING.

* MEDIUM-HIGH CONFIDENCE IN THUNDERSTORMS AFTER MIDNIGHT.

PAW

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

WEDNESDAY NIGHT...CHC SHRA. POSSIBLE MVFR.

THURSDAY...CHC SHRA IN THE MORNING. VFR AFTN.

FRIDAY...VFR.

SATURDAY - MONDAY...SLIGHT CHANCE TSRA.

ALLSOPP

&&

.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.

&&

$$

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