Severe Thunderstorm Warning 4, NWS Chicago, IL

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
809 PM CDT MON APR 5 2010

.DISCUSSION...
809 PM CDT

MESO UPDATE...

CONVECTIVE TRENDS CONTINUING TO BE MONITORED THIS EVENING WITH
RATHER CONDITIONAL THREAT OF SEVERE WEATHER EXISTING ACROSS THE
CWA.

WARM FRONT THAT HAS BEEN SITUATED SOUTH OF THE AREA ALL DAY
CONTINUES TO LIFT NORTH THIS EVENING...STRETCHING FROM NEAR
MACOMB TO CHAMPAIGN TO JUST NORTH OF INDIANAPOLIS AT 00Z.
ALOFT...VWP/PROLIER DATA INDICATE COUPLE OF LOW AMPLITUDE
SHORTWAVES EMBEDDED IN SW FLOW. FIRST IS MAKING ITS WAY NORTHEAST
NEAR DVN...WITH STRONGER WAVE LOCATED FURTHER SOUTH NEAR KANSAS
CITY. 00Z RAOBS AND SPC/RUC MESOANALYSIS INDICATE ROUGHLY 2000 J/K
MUCAPE SOUTH OF WARM FRONT...WITH A RATHER SHARP GRADIENT NORTH OF
THE WARM FRONT TOWARD IL/WI STATE LINE. ELEVATED CONVECTION ACROSS
IA HAS BEEN FORMING IN AIR MASS WITH RATHER WEAK MUCAPE VALUES
/LESS THAN 500 J/K/...BUT ALONG NORTHERN EDGE OF 850 MB THETA-E
ADVECTION.

BIG QUESTION THIS EVENING SURROUNDS WHETHER SEVERE CONVECTION WILL
FIRE ACROSS OR SOUTH OF THE AREA THIS EVENING AS 850 MB WIND
SPEEDS/LLJ START TO INCREASE THIS EVENING...AS WELL AS IN RESPONSE
TO PREVIOUSLY MENTIONED SHORTWAVE NEAR KANSAS CITY. ASSUMING THAT
INITIATION DOES INDEED TAKE PLACE...NEXT QUESTION IS JUST HOW LONG
THE LOT CWA REMAINS IN ZONE OF ELEVATED CONVECTION BEFORE WARM
FRONT SURGES NORTH OF THE AREA AND FOCUS FOR THUNDERSTORMS MOVES
INTO WISCONSIN.

AT THIS TIME...APPEARS THAT MOST FAVORED TIME FOR
CONVECTION MAY BE THROUGH THE 07-08Z TIME FRAME AS NORTHERN MO
SHORTWAVE BEGINS TO INFLUENCE AREA NORTH OF SURFACE WARM FRONT AND
850 MB THETA-E ADVECTION INCREASES SOMEWHAT. ENVIRONMENT BULK
SHEAR VALUES ARE QUITE STRONG...IN THE NEIGHBORHOOD OF 80 KTS FROM
0-6KM. FOR THOSE UPDRAFTS THAT ARE ABLE TO BECOME
ESTABLISHED...SHEAR VALUES WOULD SUPPORT RAPID STORM ORGANIZATION.
STABLE BOUNDARY LAYER SUGGESTS THAT CONVECTION WOULD REMAIN
ELEVATED IN NATURE. THIS...COMBINED WITH RATHER LOW FREEZING
LEVELS...WOULD SUPPORT LARGE HAIL AS THE PRIMARY THREAT WITH ANY
STRONG STORMS.

BOXELL

&&

.PREV DISCUSSION...
311 PM CDT

FORECAST CONCERNS FOCUS ON CONVECTIVE TRENDS IN NEAR TERM AND
THROUGH EARLY PART OF THE WEEK. CONDITIONAL SEVERE AND HEAVY
RAINFALL POTENTIALS ARE MAIN CONCERNS.

COLD FRONT WHICH PUSHED THROUGH FORECAST AREA LAST NIGHT HAS SETTLED
ACROSS SOUTHERN IL/IND TODAY...WITH WEAK HIGH PRESSURE RIDGE SLIDING
EAST ACROSS CWA. WESTERN PORTION OF FRONT ACROSS KS/MO ALREADY
SURGING NORTHWARD AS WARM FRONT HOWEVER...IN RESPONSE TO PRESSURE
FALLS ACROSS THE PLAINS INDUCED BY SERIES OF SHORT WAVE IMPULSES
PROPAGATING THROUGH FAST SOUTHWEST FLOW IN MID/UPPER LEVELS. LATEST
PROFILER AND SPC/RUC MESOANALYSIS DATA INDICATES PERSISTENT 35 KT
LOW LEVEL JET ACROSS FRONTAL ZONE FROM EASTERN KS INTO WESTERN MO/SW
IA...AND THIS HAS MAINTAINED AREA OF ELEVATED CONVECTION THROUGH THE
DAY WHICH IS NOW EXPANDING NORTHEASTWARD IN REGION OF BROAD
WARM/MOIST ADVECTION ACROSS IA/WESTERN IL. NUMERICAL GUIDANCE
INDICATES THIS ELEVATED CONVECTION WILL CONTINUE TO INCREASE IN
COVERAGE AND INTENSITY THROUGH LATE AFTERNOON/EVENING AHEAD OF
DEEPENING LOW PRESSURE ACROSS THE PLAINS AS LARGE SCALE UPPER TROUGH
CONTINUES TO AMPLIFY AND MOVE EAST. WRF-NAM FORECAST SOUNDINGS
ACROSS NORTHERN IL INDICATE ELEVATED INSTABILITY TO INCREASE THIS
EVENING IN MOIST ADVECTION ON NOSE OF LOW LEVEL JET...EVENTUALLY
INCREASING TO 500-1000 J/KG FOR ELEVATED PARCELS OVERNIGHT. COUPLED
WITH STRONG 0-6 KM BULK SHEAR ON THE ORDER OF 60 KTS SUGGESTS
POTENTIAL FOR LARGE HAIL AND POSSIBLY DAMAGING WIND GUSTS WITH
STRONGER ORGANIZED STORMS. TORNADO THREAT HIGHER WEST/SOUTHWEST OF
CWA WHERE SFC WARM FRONT JUXTAPOSED WITH BEST NEAR-SFC BASED
INSTABILITY INTO THIS EVENING...AND WHILE PERHAPS NOT NIL THE
TORNADO THREAT SHOULD DECREASE OVERNIGHT AS FRONT LIFTS INTO
NORTHERN IL/IN AND BOUNDARY LAYER BECOMES MORE STABLE. SFC WARM
FRONT FORECAST TO LIFT ACROSS THE CWA LATE THIS EVENING AND
OVERNIGHT...WITH GUIDANCE IN PRETTY GOOD AGREEMENT IN LOCATING
BOUNDARY OVER NORTHERN COUNTIES OF CWA ROUGHLY NORTH OF I-88 BY
MORNING.

GIVEN WEST-EAST ORIENTATION OF THE WARM FRONTAL ZONE AND
SOUTHWESTERLY LOW LEVEL JET...SOME THREAT OVERNIGHT FOR TRAINING
CONVECTION PARTICULARLY NORTH OF BOUNDARY FROM NORTHERN IA INTO FAR
NW IL/SOUTHWEST WI. FORECAST PRECIPITABLE WATER VALUES BETWEEN
1.25-1.50 INCHES ESPECIALLY THROUGH THIS EVENING...THOUGH SOME MID
LEVEL DRYING NOTED IN FORECAST SOUNDINGS LATE TONIGHT. FLASH FLOOD
GUIDANCE ACROSS NORTHWESTERN PARTS OF CWA IN 1.5-3.0 INCH RANGE
WHICH IS ABOVE CURRENT QPF FOR TONIGHT. CONSIDERED HOISTING FLASH
FLOOD WATCH...THOUGH HAVE CHOSEN TO HOLD OFF AT THIS TIME. FRONT
LIFTS INTO WISCONSIN TUESDAY...WITH FORECAST SOUNDINGS INDICATING
FAIRLY STOUT CAP ACROSS WARM SECTOR. THUS EXPECT A SHARP CUTOFF TO
ONGOING CONVECTIVE THREAT FAR NORTH...AND WARM...HUMID BUT PRECIP
FREE CONDITIONS ACROSS THE SOUTHERN PARTS OF THE CWA.

MAIN WESTERN TROUGH SLOWLY APPROACHES WEDNESDAY WITH SFC SYSTEM AND
COLD FRONT MOVING THROUGH DURING THE DAY. THIS WILL HERALD A SHIFT
BACK TO COOLER WX. SHOWER CHANCE LINGERS INTO THURSDAY ESPECIALLY
EASTERN COUNTIES AS A SECONDARY SHORT WAVE AND ASSOCIATED SFC TROUGH
ROTATES SOUTHEAST ACROSS REGION...THEN DRY AND WARMER WEATHER LOOKS
TO RETURN FOR THE END OF THE WEEK AS BROAD UPPER RIDGE DEVELOPS
ACROSS THE AREA ONCE AGAIN. FAIRLY DECENT MODEL AGREEMENT THROUGH
EXTENDED...WITH INDICATION OF COLD FRONT SAGGING ACROSS NORTHERN
LAKES AND WI/MICH OVER THE WEEKEND. FOR NOW APPEARS FRONT WILL
REMAIN JUST OFF TO OUR NORTH.

RATZER

&&

.AVIATION...
720 PM CDT

0000 UTC TAFS...CONVECTIVE POTENTIAL AND WINDS WILL REMAIN THE
MAIN AVIATION FORECAST ISSUES.

SURFACE WARM FRONT EXTENDING FROM NORTHERN MISSOURI INTO CENTRAL
ILLINOIS SHOULD CONTINUE TO LIFT NORTHWARD OVERNIGHT. STRONG LOW
LEVEL WARM ADVECTION NORTH OF THIS SURFACE FRONT ASSOCIATED WITH
A STRENGTHENING LOW LEVEL JET SHOULD CONTINUE TO PROMOTE SCATTERED
THUNDERSTORM DEVELOPMENT ACROSS THE TERMINALS. COVERAGE IS ALSO
EXPECTED TO INCREASE LATE THIS EVENING AS A SHORT WAVE ACROSS
WESTERN IOWA MOVES INTO SOUTHERN WISCONSIN. IT STILL APPEARS AS
THOUGH GREATEST THREAT OF THUNDER WILL BE ACROSS FAR NORTHERN
ILLINOIS INTO SOUTHERN WISCONSIN WITH GYY LIKELY SEEING THE LOWEST
PROBABILITIES OF THUNDER. HAVE INCLUDED THUNDER MENTION IN TAFS
AFTER THE 03Z-04Z TIME FRAME WITH EARLIER TIMING AT RFD IN CLOSER
PROXIMITY TO STRONGER LOW LEVEL JET FORCING. GIVEN STORMS ARE
LIKELY TO BE ELEVATED IN NATURE...WIND THREAT IS FAIRLY LOW AND
MAIN CONCERN GIVEN ELEVATED INSTABILITY WILL BE ON HAIL POTENTIAL.
TOWARD 12Z...SHOULD START TO SEE MAIN FOCUS FOR ADDITIONAL SHOWERS
AND STORMS SHIFT NORTH AND NORTHEAST OF THE FORECAST AREA AS LOW
LEVEL WARM FRONT LIFTS NORTH AND AS IOWA SHORT WAVE SHIFTS INTO
GREAT LAKES REGION.

LAKE BREEZE WHICH PUSHED TROUGH ORD/MDW HAS NOW MOVED THROUGH GYY
AND RADAR IMAGERY SFC OBS INDICATING LAKE BREEZE BOUNDARY BECOMING
LESS DISTINCT WITH GENERAL FAVORING TO SOUTHEAST GRADIENT ACROSS
ALL TERMINALS NORTH OF SHARPENING LOW LEVEL WARM FRONT. WINDS WILL
SHIFT TO THE SOUTH OVERNIGHT ACROSS GYY/MDW AND THEN BY EARLY TO MID
MORNING TUESDAY FOR ORD/MDW/DPA. FRONT MAY GET HUNG UP ACROSS
NORTHERN SUBURBS BUT AT THIS TIME EXPECTING TERMINALS TO BE SOUTH
OF THE WARM FRONT BY AFTERNOON. EXTENT OF CONVECTIVE ACTIVITY
OVERNIGHT ALSO WILL PLAY SOME ROLE IN DETERMINING WHERE THIS
BOUNDARY SETS UP. GUSTS ALSO TO BECOME STRONGER AGAIN SOUTH OF THE
WARM FRONT WITH FORECAST SOUNDINGS SUGGESTING 25 TO 30 KNOT GUSTS
POSSIBLE TUESDAY AFTERNOON. KEPT TAFS DRY AFTER EARLY MORNING
TUESDAY WITH CONFIDENCE LOW ON ADDITIONAL PRECIP UNTIL LATE
TUESDAY NIGHT INTO WEDNESDAY.

MARSILI

&&

.MARINE...

300 PM...A LOW PRESSURE SYSTEM MOVING OUT OF THE CENTRAL ROCKIES WILL
LIFT NORTHEAST OVER THE NEXT FEW DAYS. A STRONG WARM FRONT ASSOCIATED
WITH THIS LOW WILL MOVE SOUTHERN ILLINOIS NORTH TO NEAR THE IL/WI
BORDER BY TUESDAY MORNING.  THIS WARM FRONT WILL REMAIN NEARLY
STATIONARY OVER SOUTHERN LAKE MICHIGAN THROUGH WEDNESDAY MORNING.
WINDS OVER THE SOUTHERN TIP OF THE LAKE WILL INCREASE TO NEAR 30 KT
SOUTH OF THE WARM FRONT.  AS THE FRONT SURGES NORTH TUESDAY NIGHT
AND WEDNESDAY...STRONGER SOUTH WINDS WILL SPREAD NORTHWARD.  AS THE
LOW TRACKS OFF TO THE NORTHEAST...WINDS WILL TURN AROUND NORTHERLY
AND INCREASE TO AROUND 30 KNOTS AGAIN.  WHILE GALES ARE NOT EXPECTED
AT THIS TIME...A SMALL CRAFT ADVISORY WILL LIKELY BE NEEDED FROM
TUESDAY NIGHT THROUGH THURSDAY.

KREIN

&&

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

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SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
849 PM CDT MON APR 5 2010

ILZ003-008-060215-
OGLE IL-WINNEBAGO IL-
849 PM CDT MON APR 5 2010

...A STRONG THUNDERSTORM WILL AFFECT OGLE AND SOUTHERN WINNEBAGO
COUNTIES...

AT 843 PM CDT...A STRONG THUNDERSTORM WAS LOCATED 16 MILES SOUTHWEST
OF FREEPORT...MOVING EAST AT 55 MPH.

THIS STORM WILL PRODUCE HAIL UP TO PENNY SIZE. EXPECT INTENSE
LIGHTNING FROM THIS STORM. DO NOT STAY IN THE OPEN OR SEEK SHELTER
UNDER TREES WHEN LIGHTNING THREATENS...MOVE INDOORS WHEN A
THUNDERSTORM APPROACHES.

THIS STORM WILL BE NEAR FORRESTON AROUND 855 PM...LEAF RIVER...
ADELINE...AND MOUNT MORRIS AROUND 900 PM...OREGON AROUND 905 PM...
BYRON AROUND 910 PM...ROCKFORD AIRPORT...STILLMAN VALLEY...NEW
MILLFORD AND DAVIS JUNCTION AROUND 915 PM.

LAT...LON 4220 8969 4220 8941 4227 8941 4236 8895
      4196 8896 4196 8969 4219 8970

BOXELL

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
850 PM CDT MON APR 05 2010

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0845 PM     HAIL             MORTON GROVE            42.04N 87.79W
04/05/2010  E0.25 INCH       COOK               IL   TRAINED SPOTTER

&&

IZZI

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
851 PM CDT MON APR 05 2010

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0845 PM     HAIL             BATAVIA                 41.85N 88.30W
04/05/2010  E0.50 INCH       KANE               IL   PUBLIC

&&

NMARSILI

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