FXUS63 KIWX 011730
AFDIWX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE NORTHERN INDIANA
1230 PM EST TUE DEC 1 2009
.AVIATION /18Z TAFS/...
CLEAR SKIES TO PREVAIL THROUGH 00Z WITH CIRRUS STREAMING NORTH
INTO THE AREA AFTER 00Z. WHILE IT WILL THICKEN OVER TIME...ANY
LOWER CIGS WILL HOLD OFF TILL AFTER THE VALID TAF PERIOD.
&&
.PREV DISCUSSION... /ISSUED 355 AM EST TUE DEC 1 2009/
SHORT TERM /TODAY THROUGH WEDNESDAY NIGHT/...
SHORTWAVE RIDGING WILL BUILD INTO THE REGION TODAY BETWEEN CLIPPER
SYSTEMS UNDER RIBBON OF STRONG NORTHWESTERLY FLOW ALOFT. DON/T SEE
MUCH MORE THAN JUST SOME HIGH CLOUDS TODAY GIVEN LITTLE UPSTREAM
LLEVEL MOISTURE TO SUPPORT ANY CU DEVELOPMENT...PARTICULARLY WITH
THE MID LEVEL SUBSIDENCE. WITH RELATIVELY ROBUST SURFACE PRESSURE
GRADIENT /ALBEIT RELAXED COMPARED TO WHAT IT IS ATTM/ AND NEARLY
FULL SUN...EXPECT THAT MIXING TO H92 SHOULDN/T BE A PROBLEM. FOR
REASONS DESCRIBED BELOW...FAVOR THE ECMWF/GGEM/SREF HANDLING OF
THE NEXT CLIPPER SYSTEM APPROACHING FROM THE UPPER MIDWEST AND
IT/S STRONGER DEPICTION OF THIS SYSTEM. THIS SHOULD ALLOW FOR
PLENTIFUL WARM ADVECTION AND GIVEN THE ABOVE-MENTIONED
MIXING...EXPECT A MUCH WARMER DAY ACROSS THE AREA. PRETTY BIG
MODEL DISPARITY IN T92 VALUES WITH THE NAM/RUC MUCH MORE
AGGRESSIVE IN BRINGING THE WARM AIR EAST. UPSTREAM OBS TODAY
SUGGESTED THAT THE AIRMASS ARRIVING IN MY SOUTHWESTERN ZONES
SUPPORTED HIGHS IN THE MID 50S...SO WILL BUMP UP GOING HIGHS BY
2-3F...MOST IN THE SOUTHWEST...WITH HIGHS GENERALLY IN THE LOWER
50S EXCEPT UPPER 40S ACROSS THE NORTHEAST...AND IN BERRIEN COUNTY
WHERE THE SOUTHWESTERLY FLOW WILL LIKE HAVE A SLIGHT COOLING
EFFECT OFF THE LAKE.
NORTHERN STREAM LOW PUSHES EAST INTO NORTHERN MICHIGAN TONIGHT.
HIGH CLOUD COVER WILL GRADUALLY INCREASE AS UPPER LEVEL MOISTURE
ASSOCIATED WITH GULF LOW FLOWS NORTH AND EAST AT UPPER LEVELS INTO
DOWNSTREAM 160KT JET STREAK. EXPECT THIS TO LIMIT TEMP FALLS
SOMEWHAT...BUT ENOUGH CLEAR EVENING SKIES AND DIMINISHING WINDS
SHOULD ALLOW TEMPS TO FALL INTO THE MID 30S...JUST A TAD ABOVE
GUIDANCE.
RUN TO RUN AND MODEL TO MODEL INCONSISTENCIES CONTINUE TO PLAGUE THE
WEDNESDAY-WEDNESDAY NIGHT PERIOD. THE NAM AND GFS CONTINUE TO
PROVIDE THE GOALPOSTS FOR THE UPCOMING GULF LOW AS IT TRAVELS NORTH
AND EAST...WITH THE SREF/GGEM/ECMWF/GEFS ALL BASICALLY IN BETWEEN
THESE SOLUTIONS. EARLY MORNING WV IMAGERY CONTINUES TO SHOW SLOW
MOVING CUTOFF LOW OVER THE DATA SPARSE REGION OF NORTHERN MEXICO.
MODELS APPEAR TO ALL HAVE A GOOD/SIMILAR HANDLE ON THIS FEATURE AS
SEEN IN AVAILABLE 00Z OBS. PRIMARY DIFFERENCES ARISE OUT OF EACH
MODEL/S HANDLING OF ENERGY NOW OVER BRITISH COLUMBIA/ALBERTA...AS
WELL AS THE IMPACT OF GULF COAST CONVECTION ON LOWER TROP PV
REDISTRIBUTION AND IT/S IMPACT ON THE INITIAL SURFACE LOW FORMATION.
THE OVERALL EVOLUTION OF THE NORTHERN STREAM IS QUITE COMPLICATED AS
A NUMBER OF MID LEVEL SHORTWAVES WILL PLAY A ROLE AS EASTERN PACIFIC
RIDGING BUILDS...AND SHARPENS THE DOWNSTREAM TROUGH. BELIEVE THE
12/00Z GGEM HAVE THE BEST HANDLE ON THE PRIMARY VORT CENTER AT
00Z...LOCATED NEAR 59N/108W. AT THE SAME TIME...BELIEVE THE
GGEM/SREFS/ECMWF ARE ON THE RIGHT TRACK OF STRONGER EPAC RIDGING
AHEAD OF INTENSE NEGATIVE HEIGHT ANOMALY NOW LOCATED OVER THE
ALEUTIANS. FINALLY...THE SREF/GGEM ARE MUCH PREFERRED OUT OF THE
GATE WITH THEIR DEPICTION OF THE SURFACE PATTERN OVER THE
NORTHWESTERN CONUS. FOR THESE REASONS...AND THE FACT THAT THE
EJECTING SOUTHWESTERN CUTOFF WILL TAKE A NEGATIVE TILT AS IT LIFTS
OUT OF THE GULF FAVORS A FURTHER WEST SURFACE LOW TRACK...TUCKED
UP NEAR THE H5 HEIGHT ANOMALY...WILL LEAN WITH THE WESTERN 00Z
GGEM/21Z-03Z SREF/00Z ECMWF EVOLUTION FOR THE UPCOMING EVENT.
EXPECT A SURFACE LOW TRACK FROM NEAR VICKSBURG MS AT 12Z WEDNESDAY
TO NEAR LOUISVILLE KY BY 00Z THURSDAY INTO SOUTHWEST ONTARIO EARLY
THURSDAY. THETAE SURGE AT H85 ARRIVES BY ABOUT NOON...AND WILL HAVE
AN INITIALLY DRY AIRMASS TO OVERCOME. BY AFTERNOON...THIS STRONG
ISENTROPIC LIFT COMBINED WITH DEVELOPING H85-H7 FRONTOGENESIS...
POSITIVE DIFFERENTIAL PVA...AND FAVORABLE JET DYNAMICS SHOULD HAVE
NO PROBLEM ALLOWING RAIN TO DEVELOP THROUGHOUT THE CWA...WITH BEST
CHANCES OVER THE SOUTHEASTERN HALF IN THE AFTERNOON. HIGHS WILL BE
DEPRESSED GIVEN THE OVERSPREADING MORNING CI/CS AND AFTERNOON
PRECIP. WILL CONTINUE HIGHS IN THE LOWER 40S...A CATEGORY BELOW
GUIDANCE.
MOST INTERESTING PERIOD COMES WEDNESDAY NIGHT WHEN FGEN/DEFORMATION
BAND OVERHEAD COMBINES WITH LOWER TROP COOLING...BOTH THROUGH
ADVECTION AND DIABATIC EFFECTS TO BRING THE BEST CHANCES FOR A
CHANGEOVER TO SNOW. TEMPS ABOVE THE BL WILL BE RAPIDLY DROPPING AS
THE H85 LOW CENTER BEGINS PULLING EAST OF OUR LONGITUDE. BL TEMPS
LOOK MARGINAL AND IT WILL LIKELY BE PRECIP INTENSITY THAT GOVERNS
ANY CHANGEOVER. ALSO...FAVORABLE FGEN/DEFORMATION WILL BE
WEAKENING AFTER ABOUT 6Z...SO PRECIP RATES WILL BEGIN WINDING DOWN
AS WELL AS THE COMMA HEAD WEST OF THE LOW PULLS AWAY. HAVE ADDED A
CHANGEOVER BETWEEN 3 AND 6Z FOR LOCATIONS NORTH AND WEST OF A
MARION TO FORT WAYNE TO BRYAN LINE EXCEPT FOR WARMER LOCALES NEAR
THE LAKE SHORE. CAN/T SEE MUCH MORE THAN AN INCH OF SLUSHY
ACCUMULATION...LIKELY ON GRASSY SURFACES AND WHILE A FEW SLICK
SPOTS ARE POSSIBLE...CURRENT EXPECTATIONS ARE FOR A PRETTY MINIMAL
IMPACT. WILL CONTINUE TO HIGHLIGHT IN THE HWO GIVEN THAT MOST
SPOTS HAVEN/T SEEN MORE THAN A FEW FLAKES SO FAR THIS FALL.
LONG TERM...
BY ONSET OF EXTENDED PD SYSTEM WILL BE EXITING NE OF THE CWA WITH
COLD AIRMASS ESTABLISHING BENEATH UPPER TROUGH/CLOSED LOW CENTERED
OVER THE GREAT LAKES. AS LINGERING -SN OR FLURRIES RELATED TO
SYNOPTIC SYSTEM DIMINISH BEHIND SPEEDILY EXITING WAVE LAKE RESPONSE
WILL BEGIN TO KICK IN PERPETUATING SNOW SHOWERS ACROSS THE NW HALF
OF THE CWA. GENERAL AGREEMENT THEREAFTER BETWEEN NAM/GFS/ECMWF
THAT AS H85 TEMPS FALL TO AROUND -14C THURSDAY NIGHT LLEVEL WINDS
INITIALLY NW QUICKLY BACK TO WEST BY 00Z FRI. THIS SHOULD SERVE TO
RESTRICT MAJOR FOCUS TO NORTHERN PORTIONS OF CWA INTO FRIDAY NIGHT
BEFORE WINDS BACK FURTHER PUSHING ACTIVITY INTO CENTRAL MICHIGAN.
POORLY RESOLVED SHRTWV ROTATES AROUND UPPER VORTEX AND FOR NOW
HAVE CARRIED FLURRIES AWAY FROM LAKE EFFECT AREA WITH MAIN
ADJUSTMENT TO GRIDS BEING TO LOWER POPS IN THE SOUTH THURSDAY
NIGHT/FRIDAY. AS UPPER LOW RETREATS ON SUNDAY FLOW BECOMES MORE
ZONAL WITH SLOW MODERATION OF TEMPS BUT NOT EXPECTING ANY
SIGNIFICANT WARMUP THROUGH MONDAY.
&&
.IWX WATCHES/WARNINGS/ADVISORIES...
IN...NONE.
MI...NONE.
OH...NONE.
LM...SMALL CRAFT ADVISORY UNTIL 1 PM EST THIS AFTERNOON FOR LMZ043.
&&
$$
SHORT TERM...ARNOTT
LONG TERM...LUDINGTON
AVIATION...FISHER
