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000
FXUS63 KLOT 132036
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
336 PM CDT WED MAY 13 2009
.DISCUSSION...
330 PM CDT
SEVERE WEATHER POTENTIAL THIS EVENING AND RAINFALL AMOUNTS WILL BE
THE MAIN FORECAST CONCERNS.
SOME RESIDUAL SHOWERS PERSIST ACROSS MAINLY SOUTHEAST PORTIONS OF
THE FORECAST AREA. THUNDERSTORMS FROM EARLIER TODAY HAVE ACTED TO
SLOW THE NORTHWARD PROGRESS OF THE MAIN SURFACE WARM FRONT. WARM
FRONT MAINLY IDENTIFIED BY DEW POINT FIELD APPROACHING FAR
SOUTHERN PORTIONS OF THE CWA THIS AFTERNOON...WHILE IT APPEARS
ANOTHER OUTFLOW ENHANCED BOUNDARY LINGERS WELL SOUTH OF THE AREA
JUST NORTH OF STL. CLOUDS AND PRECIPITATION ASSOCIATED WITH
MORNING SHOWERS/STORMS HAVE LIMITED SURFACE BASED INSTABILITY
WHICH IS EVIDENT IN REGIONAL 18Z SOUNDINGS. A RATHER IMPRESSIVE
UPPER LEVEL SHORT WAVE ACROSS THE DAKOTAS WILL APPROACH THE
MID/UPPER MS RVR VALLEY THIS EVENING INCREASING LARGE SCALE ASCENT
AND ACTING TO COOL MID LEVELS. EXPECTING THE MAIN COLD FRONT TO
BECOME MORE ACTIVE TOWARD 22Z-00Z AS THIS TROUGH APPROACHES. MAX
INSTABILITY MAY TEND TO ALIGN IN FAIRLY NARROW CORRIDOR ALONG AND
JUST AHEAD OF THE FRONT IN RESPONSE TO STEEPENING LAPSE RATES WITH
COOLING MID LEVEL TEMPS AND SECONDARY LOW LEVEL MOISTURE ADVECTION
SURGE AHEAD OF THE FRONT. ONCE THUNDERSTORMS INITIATE LATER THIS
AFTERNOON EXPECTING SEVERE WEATHER THREAT TO INCREASE AND POSSIBLY
EVOLVE INTO A SQUALL LINE FEATURE WITH POSSIBILITY OF
SUPERCELLULAR STRUCTURES EMBEDDED IN THE LINE GIVEN FAVORABLE
SHEAR. CANNOT COMPLETELY DISCOUNT POSSIBILITY OF MORE DISCRETE
DEVELOPMENT WITH THE NORTHWARD MOVING WARM FRONT ACROSS THE AREA
LATE THIS AFTERNOON...BUT POTENTIAL OF THIS ACTIVITY BEING SURFACE
BASED APPEARS TO BE MINIMAL FROM THE SAMPLING OF THE 18Z RAOBS.
EXPECTING THAT CONVECTIVE LINE SHOULD BE MOVING OUT OF FAR
SOUTHEAST PORTIONS OF THE CWA SHORTLY AFTER MIDNIGHT WITH SKIES
CLEARING OVERNIGHT. SEVERE WEATHER THREATS STILL APPEAR TO RUN
THE FULL RANGE FROM HIGH WINDS...HAIL...AND POSSIBILITY OF
ISOLATED TORNADOES. FOR MORE DETAILS REGARDING SEVERE
POTENTIAL...PLEASE SEE MESOSCALE DISCUSSION BELOW.
LOW LEVEL FRONT WILL STALL SOUTH OF THE FORECAST AREA TODAY
SETTING UP A QUIET DAY TOMORROW AS A SURFACE HIGH BUILDS IN FROM
THE WEST. LOW LEVEL WESTERLIES WILL BE DIMINISHING TOMORROW WHICH
MAY ALLOW A LAKE BREEZE TO FORM CLOSE TO THE SHORELINE. LOW LEVEL
MODEL 900 HPA THERMAL PROGS MIX DOWN TO THE SURFACE GENERALLY
SUPPORT HIGHS FROM THE LOW 70S FAR SOUTH TO MID TO UPPER 60S NORTH
FOR THURSDAY. THE NEXT SIGNIFICANT UPPER LEVEL TROUGH IS ALREADY
EVIDENT ON WATER VAPOR IMAGERY ENTERING PACIFIC NORTHWEST COAST AT
THIS TIME AND IS EXPECTED TO TAKE A SIMILAR TRACK TO THE SHORT
WAVE MOVING THROUGH THE REGION THIS EVENING. ENHANCED SOUTHERLIES
AHEAD OF THIS SHORT WAVE WILL ALLOW THE STALLED BOUNDARY TO THE
SOUTH TO LIFT BACK NORTHWARD AS A WARM FRONT FRIDAY WITH STRONG
ISENTROPIC ASCENT AND INCREASING LOW LEVEL MOISTURE CONVERGENCE
ESPECIALLY ACROSS SOUTHERN PORTIONS OF THE FORECAST AREA BY FRIDAY
MORNING. HAVE INCREASED POPS A BIT FROM PREVIOUS FORECAST FOR
FRIDAY. THIS SYSTEM FOR FRIDAY PERHAPS NOT QUITE AS DYNAMIC AS THE
SYSTEM THIS EVENING AND CURRENTLY APPROXIMATELY SOUTHERN HALF IS
OUTLOOKED IN SLIGHT RISK FOR DAY 3.
FORECAST AREA WILL GET GRAZED BY UNSEASONABLY COLD AIR MASS BEHIND
MOVING ACROSS ONTARIO SATURDAY BEHIND THIS SYSTEM...AND THE
COOLEST DAY OF THE PERIOD IS EXPECTED FOR SATURDAY. BROAD SURFACE
HIGH WILL BUILD INTO THE GREAT LAKES REGION FOR SUNDAY WITH DRY
CONDITIONS. MEDIUM RANGE MODELS ARE BEGINNING TO COME INTO A BIT
BETTER AGREEMENT FOR EXTENDED PERIOD WITH THE 12Z ECMWF TRENDING
MORE TOWARD NCEP ENSEMBLE MEANS/DETERMINISTIC GFS IN SLOWER
PROGRESSION WITH NEXT WESTERN CONUS TROUGH AND MORE IN THE WAY OF
BROAD DOWNSTREAM RIDGING FROM THE NORTHERN PLAINS INTO THE GREAT
LAKES REGION. THIS WOULD SUPPORT DRY CONDITIONS AND MODERATING
TEMPERATURES THROUGH THE END OF THE PERIOD.
MARSILI
&&
.DISCUSSION...
MESOSCALE UPDATE
312 PM CDT
WARM FRONT/OUTFLOW FROM THIS MORNING`S CONVECTION IS BECOMING
INCREASINGLY DIFFUSE AND DIFFICULT TO IDENTIFY IN SURFACE OBS.
SOUTHERLY WINDS ARE INCREASING ACROSS NORTHERN ILLINOIS WITH
WARMER AND MORE HUMID AIRMASS BEGINNING TO STREAM RAPIDLY
NORTHWARD. WHILE HIGHER THETA-E AIRMASS STREAMS NORTHWARD INTO THE
AREA SHOULD CONTINUE TO SEE ATMOSPHERE BECOME INCREASINGLY
UNSTABLE. DVN/ILX/SGF 1800 UTC SOUNDINGS ALL SAMPLED THE
IMPRESSIVE ELEVATED MIXED LAYER ADVECTING EASTWARD...THOUGH ALL
ALSO SHOWED A STOUT CAP AT THE BASE OF THIS EML WHICH SHOULD
PREVENT ANY SURFACE BASED CONVECTION FROM FORMING IN THE WARM
SECTOR UNTIL STRONGER FORCING CAN LIFT THROUGH THE INVERSION.
SHALLOW CONVECTION IS INCREASING IN COVERAGE OVER IOWA APPARENTLY
IN RESPONSE TO THE STRENGTHENING LARGE SCALE ASCENT ASSOCIATED
WITH THE APPROACHING TROUGH. CONTINUED LOW LEVEL CONVERGENCE ALONG
THE FRONT COMBINED WITH STRONGER SYNOPTIC SCALE ASCENT SHOULD
SUPPORT THE DEVELOPMENT OF SUSTAINED DEEP CONVECTION OVER IOWA
WITHIN AN HOUR OR SO...WITH CONVECTION LIKELY FILLING INTO A
FAIRLY SOLID LINE AND ACCELERATING EASTWARD ACROSS IOWA INTO NW
IL.
AT THIS TIME...CONFIDENCE IS GROWING SOMEWHAT IN A STRONGLY FORCED
LINE OF CONVECTION RACING ACROSS THE CWA THIS EVENING. ROUGH
TIMING WOULD BE FOR LINE TO ARRIVE IN OUR WESTERN CWA EARLY THIS
EVENING...THEN MOVE QUICKLY ACROSS THE AREA. THE EXPECTATION WOULD
BE FOR INSTABILITY TO BE NOT OVERLY IMPRESSIVE...THOUGH RUC WOULD
SUGGEST OTHERWISE WITH MLCAPES INCREASING TO BETWEEN 1000-1500
J/KG. EVEN IF MLCAPES ONLY CLIMB TO AROUND 500 J/KG...THE VERY
STRONG SHEAR PROFILES ARE QUITE CONCERNING FOR THE POTENTIAL FOR
DAMAGING WINDS. VERY STRONG 0-1KM SRH VALUES PROGGED TO BE UP TO
400 M2/S2...SO THERE WOULD ALSO BE A POSSIBILITY OF SHORT-LIVED
TORNADOES TO FORM WITHIN THE LINE AS WELL.
AS MENTIONED YESTERDAY...THE SEVERE THREAT WILL EXIST ACROSS OUR
ENTIRE CWA...HOWEVER IT DOES APPEAR AS THOUGH THE GREATEST SEVERE
POTENTIAL WILL BE OVER OUR WESTERN CWA WHERE MLCINH IS LIKELY TO
BE WEAKER BEFORE SLOWLY INCREASING DURING THE EVENING AS BOUNDARY
LAYER COOLS.
IZZI
&&
.AVIATION...
1800 UTC TAFS...PERSISTENT AREAS OF RAIN HAVE SATURATED SFC LAYERS
AND RESULTED IN CIGS AND VSBY LOWERING TO IFR. SEVERAL BREAKS IN
PCPN MOVING THRU NRN IL NOW MAY RESULT IN BRIEF LIFTING TO MVFR OR
EVEN VFR AT TIMES. RETURN OF RAIN WILL LOWER US BACK TO IFR. TSTMS
IS THE MAIN CONCERN HOWEVER. ADDED SUNSHINE WILL AID IN SPARKING
AFTN TSTMS. THIS IS NOT CLEAR CUT NOW IN IL BUT CAN SEE DRY TONGUE
PUSHING UP THRU CENTRAL IOWA. NATIONAL RADAR MOSAIC INDICATING
SOME CONVECTION BUILDING IN THIS CLEARING SLOT. THIS LIKELY TO
MOVE EAST THIS EVENING AND BRING ACTIVE WX SCENARIO HERE BY
EVENING. WARM FRONT TO THE SOUTH HAS SLOWED ITS NWD PROGRESS AND
MAY NOT BE MOVG THRU TERMINALS UNTIL 20-22Z. THIS WILL SWITCH
WINDS MORE TO SW AND RAISE TEMPS FURTHER THRU THE CONVECTIVE
TRIGGER VALUES IN THE LOW 60S. COLD FROPA LIKELY AFTER MIDNGT...
PUTTING AN END TO TSTMS AND DRYING AND CLEARING BY DAYBREAK.
RLB
&&
.MARINE...
320 AM CDT
A STRONG LOW WITH A PRESSURE OVER SOUTHERN MANITOBA WILL MOVE
NORTHEAST TO NORTHERN ONTARIO BY THURSDAY MORNING. THIS LOW WILL
DRAG A FRONT INTO MICHIGAN AND INDIANA BY MIDNIGHT THURSDAY
MORNING. A LARGE HIGH WILL MOVE INTO MISSOURI BY THURSDAY
AFTERNOON. THE COOL WATER HAS SET UP AN INVERSION WHICH PREVENTS
STRONG WIND SPEED ALOFT TO REACH THE LAKE SURFACE. BECAUSE OF THE
INVERSION WE DO NOT EXPECT GALES OR HIGHER WIND SPEEDS AT THIS
TIME. WILL KEEP THE SMALL CRAFT ADVISORY IN EFFECT THROUGH
THURSDAY MORNING.
&&
.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...NEARSHORE WATERS
UNTIL 7 AM THURSDAY.
&&
$$
