FXUS63 KLOT 101154
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
554 AM CST WED MAR 10 2010

.DISCUSSION...
420 AM CST

POTENTIAL OF RAIN AND FOG THIS MORNING...AND THEN POSSIBILITY OF
HEAVIER RAIN THURSDAY WHICH MAY LEAD TO HYDROLOGIC ISSUES WILL BE
THE MAIN FORECAST CONCERNS.

WATER VAPOR IMAGERY EARLY THIS MORNING INDICATING UPPER VORT MAX
LIFTING NORTHWARD THROUGH NORTHEAST MISSOURI AND EASTERN IOWA. THE
STRONGEST MID/UPPER LEVEL FORCING WITH THIS SHORT WAVE WILL REMAIN
TO THE WEST OF THE AREA THROUGH THE EARLY MORNING HOURS...ALTHOUGH
RAIN SHOWERS EXTEND WELL TO THE SOUTHEAST INTO EAST CENTRAL ILLINOIS
IN WARM ADVECTION WING. SFC LOW ASSOCIATED WITH THIS WAVE AT 09Z
ACROSS SOUTH CENTRAL IOWA WILL BEGIN TO FILL THIS MORNING AS IT
LIFTS NORTHWARD. A SFC WARM FRONT EXTENDING EASTWARD FROM THIS LOW
INTO CENTRAL ILLINOIS WILL ALSO LIFT NORTHWARD THROUGH THE MORNING
BUT WILL LIKELY BEGIN TO STALL OUT ACROSS FAR NORTHERN
ILLINOIS...ESPECIALLY WHEN ENCOUNTERING COLD LAKE MICHIGAN WATERS AS
IS OFTEN THE CASE THIS TIME OF YEAR. THIS STALLING BOUNDARY SHOULD
SET UP A FAIRLY STRONG RANGE OF MAX TEMPS THIS AFTERNOON RANGING
FROM THE LOWER AND MID 60S SOUTH TO TEMPS STRUGGLING TO REACH 50
CLOSER TO LAKE MICHIGAN ACROSS THE NORTHERN SUBURBS. SOME CLEARING
NOTED IN SATELLITE IMAGERY THIS MORNING ACROSS NORTHEAST MISSOURI
AND SOUTHWEST ILLINOIS...AND TEMPS WILL BE DEPENDENT ON HOW FAR
NORTH THIS CLEARING WILL REACH. DO THINK SOUTHERN PORTIONS OF CWA
HAVE A CHANCE TO SEE SOME SUN TODAY...BUT FURTHER NORTH WHERE SFC
WARM FRONT WILL BE STALLING OUT...POTENTIAL ANY SUN IS LOW.

BY THIS AFTERNOON...FORCING WITH THE MID AND UPPER LEVEL SYSTEM WILL
HAVE LIFTED NORTH OF THE AREA. FORECAST SOUNDINGS SUGGEST SOME VERY
WEAK SURFACE BASED CAPE ACROSS FAR NORTHERN ILLINOIS OF 30-50 J/KG.
DESPITE FOCUS FOR STRONGER UPPER LEVEL FORCING SHIFTING NORTH OF THE
AREA...BROAD ISENTROPIC ASCENT WITH THE LOW LEVEL FRONT COMBINED
WITH STEEP LAPSE RATES COULD BE ENOUGH TO GENERATE A FEW WIDELY
SCATTERED LIGHT RAIN SHOWERS THIS AFTERNOON ACROSS THE FAR NORTH AND
WILL JUST MAINTAIN A MENTION OF 20 POPS ACROSS THE FAR NORTH THIS
AFTERNOON.

ATTENTION FOR TONIGHT WILL THEN SHIFT BACK TO THE SOUTHWEST AND THE
EVOLUTION OF A STRONG UPPER LEVEL WAVE CURRENTLY TRACKING THROUGH
NEW MEXICO. THIS WAVE WILL EJECT INTO THE SOUTHERN PLAINS TONIGHT
RESULTING IN SFC CYCLOGENESIS COMMENCING. STILL DOES APPEAR THE LULL
IN PRECIP CHANCES FOR FORECAST AREA WILL CONTINUE INTO THIS EVENING.
BY LATE TONIGHT...NAM/GFS IN GENERAL AGREEMENT IN TAKING 90-100 KNOT
UPPER JET STREAK INTO THE REGION DOWNSTREAM OF THIS UPPER
LOW...ALONG WITH 40 KNOT SOUTHERLY LOW LEVEL JET AXIS SHIFTING INTO
THE AREA. DO THINK BY LATE TONIGHT...RAIN WILL BE MOVING INTO FAR
SOUTHERN SECTIONS OF FORECAST AREA AND DID TREND TO LIKELY POPS
AFTER 09Z TONIGHT. THIS SECOND MOISTURE ADVECTION SURGE TONIGHT AND
OLD STALLED OUT BOUNDARY ACROSS THE FAR NORTH DO SUGGEST THE
POSSIBILITY OF MORE FOG TONIGHT AND PREVIOUS FORECAST ALREADY HAD
THIS WELL IN HAND. FOG MAY BE LOCALLY DENSE ESPECIALLY ACROSS THE
FAR NORTH WHERE SOME SNOWPACK STILL RESIDES BUT WILL HOLD OFF ON ANY
ADVISORIES AT THIS TIME.

SYSTEM TO THE WEST WILL BEGIN TO OCCLUDE EARLY THURSDAY AND
EXPECTING LARGER RAIN SHIELD WITH PERHAPS SOME EMBEDDED THUNDER TO
OVERSPREAD FORECAST AREA FROM SOUTHWEST TO NORTHEAST DURING THE DAY
THURSDAY. WILL ALSO HAVE TO WATCH FOR ADDITIONAL CONVECTIVE ACTIVITY
ALONG EASTWARD ADVANCING OCCLUDED FRONT THURSDAY AFTERNOON. SOME
BACKING OF NEAR SFC FLOW EXPECTED TO THE EAST OF THE SFC OCCLUDED
BOUNDARY THURSDAY AFTERNOON WITH NAM STILL GENERATING 500-600 J/KG
OF SURFACE BASED OR NEAR SURFACE BASED CAPE ACROSS FAR SOUTHERN
PORTIONS OF THE FORECAST AREA. NEW SPC SWODY2 DOES INCLUDE MOST OF
FORECAST AREA IN FIVE PERCENT SEVERE THREAT FOR THURSDAY...MAINLY
FOR MARGINAL HAIL WITH RELATIVELY LOW FREEZING LEVELS AND LOW END
THREAT FOR A BRIEF TORNADO DUE TO BACKING OF LOW LEVEL FLOW AND
SUFFICIENT SHEAR PROFILES...ESPECIALLY ACROSS SOUTHERN PORTIONS OF
FORECAST AREA. THIS MARGINAL THREAT IS QUITE DEPENDENT ON LOW LEVEL
THERMODYNAMICS WHICH ARE STILL QUITE UNCERTAIN ESPECIALLY GIVEN
EARLIER RAINS EXPECTED ON THURSDAY WITH STRONGEST SYNOPTIC FORCING.
GIVEN THIS UNCERTAINTY WILL JUST MENTION POSSIBILITY OF SOME SMALL
HAIL IN HWO AND HOLD OFF ON TORNADO THREAT. THREE DAY RAINFALL
TOTALS BY FRIDAY MAY VARY QUITE A BIT ACROSS THE AREA ESPECIALLY
GIVEN POSSIBILITY OF PRECIP MORE CONVECTIVE IN NATURE THURSDAY WHEN
HEAVIER RAIN IS POSSIBLE...BUT GENERAL THREE DAY TOTALS OF THREE
QUARTERS TO AN INCH TO IN EXCESS OF AN INCH WILL BE POSSIBLE.
CONTINUED SNOWMELT AND RAINFALL WILL POSE CONCERN ESPECIALLY FOR
AREA RIVERS IN TERMS OF FLOODING AND WILL CONTINUE TO HIGHLIGHT THIS
POTENTIAL IN ESF THIS MORNING.

DRY SLOT EXPECTED TO BUILD INTO FORECAST AREA THURSDAY PROVIDING
ANOTHER LULL IN BETTER PRECIP CHANCES FOR LATER THURSDAY NIGHT INTO
EARLY FRIDAY...BUT ANOTHER SOUTHERN STREAM VORT IS EXPECTED TO WRAP
AROUND MAIN UPPER LOW FRIDAY KEEPING NORTHERN ILLINOIS/NORTHWEST
INDIANA IN SCATTERED SHOWER ACTIVITY FOR FRIDAY/SATURDAY. VERY SLOW
MOVING NATURE OF UPPER LOW COULD EVEN ALLOW SOME SCATTERED LIGHT
SHOWERS TO LINGER INTO SUNDAY BUT WILL HOLD OFF ON POPS AT THIS
TIME. HAVE KEPT REST OF FORECAST DRY AT FORECAST REMAINS DOWNSTREAM
OF BROAD WESTERN CONUS/SOUTHWEST CANADIAN UPPER RIDGE INTO EARLY
NEXT WEEK WITH SEASONAL OR SLIGHTLY ABOVE SEASONAL TEMPS EXPECTED.

MARSILI

&&

.AVIATION...

1200 UTC TAFS...

AVIATION ISSUES CONTINUE TO REVOLVE AROUND CEILINGS AND VISIBILITY
TRENDS...AND TIMING PERIODS MOST LIKELY FOR PRECIP NEXT 24-30 HOURS.
HIGHEST PROBABILITY FOR IFR/LIFR CONDITIONS APPEAR TO BE LATER THIS
MORNING AND AGAIN LATE TONIGHT.

EARLY MORNING ANALYSIS PLACES SFC LOW PRESSURE OVER CENTRAL
IA...WITH WARM FRONT ARCING SOUTHEAST THROUGH CENTRAL IL. MID LEVEL
SHORT WAVE WRAPPING NORTH ACROSS THE AREA WITH BAND OF RAIN SHOWERS
NORTH OF SFC FRONT...AND THESE EXPECTED TO AFFECT TERMINALS FOR THE
NEXT FEW HOURS BEFORE LIFTING OFF TO THE NORTH BY 14-15Z. WHILE
CONDITIONS HAVE BEEN VFR OR HIGH END MVFR ACROSS CHICAGO TERMINAL
AIRSPACE MUCH OF THE NIGHT...RAIN AND APPROACH OF WARM FRONT IS
RESULTING IN LOWERING OF CEILINGS INTO SOLID MVFR RANGE...AND BAND
OF IFR/LIFR CIGS/VIS EXISTS CLOSER TO SFC BOUNDARY OVER SOUTHEAST IA
AND CENTRAL IL. 00/06Z GUIDANCE INDICATE WARM FRONT WILL CONTINUE TO
LIFT INTO NORTHERN IL BY MIDDAY...BUT IS EXPECTED TO STALL AS LOW
OVER IA BEGINS TO FILL BENEATH STACKED MID LEVEL SYSTEM. THIS WOULD
SUGGEST TERMINALS MAY BE IN PRECARIOUS POSITION OF BEING VERY CLOSE
OR JUST TO THE NORTH OF SFC WARM FRONT...WITH FRONTAL INVERSION
TRAPPING RELATIVELY COOL MOIST BOUNDARY LAYER AND MAINTAINING IFR
CIG/VIS. CONFIDENCE NOT EXTREMELY HIGH WITH EXACT EVENTUAL FRONTAL
POSITION...OR IN IFR CONDITIONS PERSISTING...THUS HAVE INDICATED
SOME IMPROVEMENT INTO MVFR THIS AFTERNOON.

AS OLD LOW FILLS ACROSS IA THROUGH TONIGHT...NEW LOW PRESSURE
DEEPENS AND MOVES INTO MISSOURI BY EARLY THURSDAY. RESULTING BACKING
OF LOW LEVEL WIND FIELD EXPECTED TO PROVIDE A LITTLE MORE EASTERLY
COMPONENT TO SFC WINDS LATER TONIGHT...WHICH AGAIN IS EXPECTED TO
LEAD TO DETERIORATION OF CEILINGS AND VIS BACK DOWN INTO IFR
TERRITORY IN MOISTURE RICH WARM FRONTAL VICINITY. APPROACH OF NEXT
UPPER WAVE THURSDAY MORNING MAY SUPPORT SOME DRIZZLE INITIALLY
BEFORE SUNRISE...THEN STEADIER SHOWERS EXPECTED IN ORD 30 HOUR
PERIOD BEYOND 12Z.

WINDS...FAIRLY UNIFORM FROM SOUTHEAST TODAY ROUGHLY 120-150 DEG...
BACKING MORE EAST/SOUTHEAST 090-120 DEG TONIGHT. SPEEDS AROUND 10-12
KTS THIS MORNING THEN SLACKING THIS AFTERNOON AND THIS EVENING AS
GRADIENT WEAKENS IN VICINITY OF WARM FRONTAL TROUGH.

RATZER

&&

.MARINE...
317 AM CST

MAIN MARINE CONCERNS DEAL WITH POTENTIAL FOR A COUPLE OF BRIEF
PERIODS OF GUSTY EAST/NORTHEAST WINDS ON THE LAKE OVER THE NEXT
FEW DAYS. ONE IS LATE THURSDAY THROUGH EARLY FRIDAY MORNING AS
SECOND LOW PRESSURE SYSTEM LIFTS INTO THE MIDWEST AND BRIEFLY
TIGHTENS GRADIENT ACROSS THE LAKE BEFORE WEAKENING. THE OTHER
PERIOD IS LATE FRIDAY NIGHT THROUGH SUNDAY AS NORTHERLY WINDS
DEVELOP IN WAKE OF DEPARTING LOW. FORECAST SOUNDINGS DEPICT VERY
SHALLOW MARINE BOUNDARY LAYER DURING THE NEXT SEVERAL DAYS WITH
RELATIVELY MILD EARLY SPRING AIR SPREADING ACROSS VERY COOL LAKE
WATERS. THIS SHOULD HELP LIMIT HIGHER END OF GUSTS. GUIDANCE
DEPICTS 20-25 KT WINDS FROM THE EAST/NORTHEAST ESPECIALLY OVER
NORTHERN AND CENTRAL LAKE MICHIGAN THURSDAY NIGHT...THEN AGAIN BY
EARLY SATURDAY. COULD SEE A FEW GUSTS NEAR 30 KNOTS...ESPECIALLY
SATURDAY MORNING OVER NORTHERN PART OF THE LAKE...BUT IT APPEARS
WITH LOW IN WEAKENING PHASE AND NOT MUCH IN THE WAY OF COLD AIR
SPREADING IN BEHIND IT THAT 25 KNOTS SHOULD SUFFICE FOR SUSTAINED
WINDS FOR NOW.

ONE OTHER CONCERN IS POTENTIAL FOR DEVELOPMENT OF FOG ACROSS THE
LAKE AS WARM...MOIST LOW LEVEL AIR MASS LIFTS NORTH ACROSS
ESPECIALLY SOUTHERN PART OF THE LAKE LATE THURSDAY/EARLY FRIDAY.
WEAKER WINDS ON SOUTH PORTION IN VICINITY OF WARM FRONTAL BOUNDARY
COMBINED WITH DEW POINT TEMPS ABOVE LAKE TEMPERATURE MAY PRODUCE A
PERIOD OF LOW VISIBILITY IN DENSE FOG.

RATZER

&&

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

$$



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