FXUS63 KIWX 270509
AFDIWX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE NORTHERN INDIANA
1209 AM EST FRI NOV 27 2009
.AVIATION /06Z TAFS/...
MVFR CIGS ENTRENCHED EARLY THIS MORNING AS LAKE EFFECT RAIN AND
SNOW SHOWERS CONTINUE. EXPECT VERY GRADUAL IMPROVEMENT THROUGH THE
PERIOD...WITH PRECIPITATION ENDING BY DAYBREAK AND MVFR CIGS
IMPROVING TO 2.5KFT DURING THE NEXT FEW HOURS...AND THEN
SCATTERING OUT DURING THE EARLY AFTERNOON ON FRIDAY. VFR
CONDITIONS WILL THEN PERSIST THROUGH THE END OF THE TAF PERIOD
WITH SLOWLY CLEARING SKIES.
WEST NORTHWEST WINDS 10-15KTS WILL GRADUALLY SUBSIDE TO AROUND
10KTS FOR THE DAY TODAY...AND THEN BECOME LIGHT /LESS THAN 5KTS/
TONIGHT...SHIFTING SOUTHERLY AT SBN BEFORE THE END OF THE TAF
PERIOD.
&&
.PREV DISCUSSION... /ISSUED 1142 PM EST THU NOV 26 2009/
UPDATE...
HAVE UPDATED OVERNIGHT FORECAST AS PLUME OF LAKE ENHANCED
PRECIPITATION CONTINUES WITH A GRADUAL TRANSITION TO A LAKE EFFECT
SCENARIO WITH UPPER SHORTWAVE DEPARTING TO THE EAST. UPPED POPS
FOR THE NEXT FEW HOURS...BUT STILL EXPECT A DIMINISHING TREND AS
UPSTREAM DEWPOINTS...AND REGIONAL RADARS OVER LAKE SUPERIOR
SUGGEST DRYING UPSTREAM AIRMASS COMBINED WITH DEVELOPING MID LEVEL
SUBSIDENCE SHOULD WEAKEN THE LAKE PRECIP LIKELY ENDING IT TOWARDS
DAYBREAK.
NO OTHER CHANGES ATTM.
SHORT TERM...
TONIGHT THROUGH FRIDAY
SHARP UPPER TROUGH IS BEGINNING TO SLOWLY EJECT NE FROM THE
FA...WITH DEFORMATION PRECIP CONTINUING TO SHIFT EAST. ONE FINAL
BAND OF PRECIP CONTINUES TO ADVECT ACROSS THE WESTERN FA...LIKELY
BECOMING MODIFIED BY LONG FETCH FLOW OVER LK MI. ANOTHER MID LEVEL
IMPULSE IS CLEARLY EVIDENT ON WATER VAPOR...PUSHING SE ACROSS
WESTERN ONTARIO. THIS FEATURE IS EXPECTED TO PHASE WITH THE GREAT
LAKES UPPER LOW AND HELP SUPPORT VIGOROUS CYCLOGENESIS OVER THE NE
CONUS. TEMPS WERE GENERALLY HOLDING IN THE UPPER 30S WITH FEW LOW
40S PRESENT OVER THE FAR NW ZONES WITH FLOW VEERING NW BEHIND THE
ADVECTING SFC TROUGH. MAIN CONCERN IN THE PERIOD IS PRECIP CHANCES
THIS EVENING AND EVENTUAL CLEARING ON FRI. HAVE TAILORED FORECAST TO
NAM AND LOCAL WRF PROGS...GIVEN LAKE MI RESPONSE WITH CAA...AND
GENERAL GOOD CONTINUITY WITH THERMAL AND MOMENTUM PROGS.
TONIGHT...UPPER TROUGH AND SFC REFLECTION WILL CONTINUE TO PROPAGATE
EAST THROUGH THIS EVENING...SUPPORTING SHOWER CHANCES ACROSS THE
CENTRAL AND EASTERN FA AS THE SFC TROUGH AND LL DEFORMATION ZONE
PUSHES EAST ACROSS THE REGION. CAA WILL SUPPORT H85 T FALLS DOWN TO
AROUND -7C...SUPPORTING MARGINAL DELTA T/S AROUND 13 C BY LATE
TONIGHT. FLOW IS EXPECTED TO CONTINUE TO VEER TONIGHT WITH NEGATIVE
THETA-E ADV SUPPORTING LL RH FALLS. NAM H85 RH PROGS UPSTREAM LK MI
FALL TO AROUND 30 PERCENT COINCIDENT WITH THE COLDEST THERMAL FIELDS
ALOFT...SUPPORTING AN UNFAVORABLE ENVIRONMENT FOR THE PRODUCTION OF
PRECIP. IN ADDITION...INVERSION HEIGHTS WILL CONTINUE TO FALL AND
WITH LK INDUCED CAPES ONLY PROGGED AT AROUND 200 J/KG IN NAM AND WRF
BUFR PROGS...HAVE DECIDED TO LIMIT POPS TONIGHT IN THE NW...WITH A
LOWERING TREND THROUGH THE NIGHT. HAVE OPTED ALSO TO KEEP THE TREND
TO SCT SKY COVER IN THE SW TOWARD DAWN AS DRY AIR CONTINUES TO ADV
INTO THE AREA. COOLEST READINGS ARE EXPECTED IN THE SW WHERE THE
HIGHEST POTENTIAL FOR SCT-BKN CLOUD COVER IS.
FRI...SFC HIGH PRESSURE WILL BEGIN TO BUILD INTO THE AREA WITH UPPER
LEVEL RIDGING AND HEIGHT RISES EXPECTED AHEAD OF THE NEXT SYSTEM.
WAA AND BACKING FLOW WILL GRADUALLY SUPPORT THE DEPLETION OF PRIMARY
AND SECONDARY LK CLOUDS...BUT AT A SLOW PACE. MOST NORTHERN AREAS
WILL LIKELY SEE AMPLE CLOUD COVER THROUGH THE DAY PERIOD. STRONG
SUBSIDENCE INVERSION WITH WEAK MIXING WILL ONLY OFFER HIGHS INTO THE
LOWER 40S AT BEST.
EXPECT MOSTLY CLEAR SKIES AND LIGHT WINDS OVER MUCH OF THE FA FRI
NIGHT. DRY AIR IN PLACE COMBINED WITH GOOD RATIONALLY COOLING SETUP
WILL SUPPORT LOWS IN THE 20S AT MANY PLACES...TO AROUND 30 IN THE
FAR SW WHERE SFC WINDS MAY BEGIN TO RESPOND TO PRESSURE FALLS WEST
OF THE FA TOWARD DAWN.
LONG TERM...
SHRT TERM PREFERENCE STILL LIES W/LESS PROGRESSIVE SPLITTING MID LVL
TROUGH DEPICTIONS TO START. THIS FITS WELL W/PRIOR GRIDS AND
SUGGESTS FEW IN ANY CHGS YET FOR SUN AFTN/SUN NIGHT. HWVR WILL AT
LEAST SHADE WRN AREAS DOWN POP WISE MON IN RESPECT OF WRN NEIGHBOR
XPCNS AND COLLABORATION CONSTRAINTS.
OTHERWISE FAR GREATER PROBS MANIFEST TWD END OF THE PD W/CONTD HUGE
MODEL SPREAD RELATED TO HOW POTENT SRN STREAM ENERGY EJECTS OUT AHD
OF IMPRESSIVE NRN STREAM TROUGH DVLPMNT OVR THE NRN PLAINS. HPC
PREFERENCE OF TAKING NRN STREAM HGT FALLS FURTHER W THROUGH THE WRN
US FITS W/CURRENT XPCN OF LEAD SW COMING ONSHORE OVR THE PAC NW THIS
AFTN SPLITTING W/SRN STREAM SW CUTTING OFF OVR NRN MEXICO AND
ULTIMATELY SLWR TO EJECT NE AHD OF POTENT NRN STREAM TROUGH DVLPG
ACRS THE NRN PLAINS MID WEEK. THUS WILL CONT TO RIDE DRY FCST
THROUGH END OF DY7 YET LIKELY ADD A MID RANGE CHC MENTION TOMORROW
IF PLACEMENT OF SRN STREAM WV LIFTING UP THROUGH THE OH VALLEY HOLDS
AS HAS BEEN THE CASE IN SVRL DIFF MODEL CYCLES THE LAST FEW DAYS.
WRN PERIPHERY OF THIS SYS HOLDS THE POTENTIAL FOR SIG SNOW
ACCUMULATION PENDING HOW ENERGY ALOFT PHASES AND HOW QUICKLY
MODIFIED ARCTIC AIR GETS ENTRAINED UNDERNEATH WRN EXTENT OF PCPN
SHIELD. CERTAINLY SOMETHING TO KEEP WATCHING ALTHOUGH THE HIGH LEVEL
OF UNCERTAINTY SHOULD BE STRESSED.
&&
.IWX WATCHES/WARNINGS/ADVISORIES...
IN...NONE.
MI...NONE.
OH...NONE.
LM...SMALL CRAFT ADVISORY UNTIL 4 PM EST THIS AFTERNOON FOR LMZ043-
046.
&&
$$
SHORT TERM...JC
LONG TERM...HOLSTEN
AVIATION...ARNOTT
