945 
fxus63 kiwx 062006 
afdiwx 


Area forecast discussion 
National Weather Service northern Indiana 
406 PM EDT Sat Sep 6 2008 


Short term... 
tonight through Sunday night. Upstream shortwaves becoming more 
impressive on WV imagery. Two waves embedded in longwave 
trough...first/fastest near NE/Iowa border beginning to phase into southern 
MO wave. Strong 1000-850 mflux convergence overnight as this 
feature continues to lift NE into western Great Lakes. 1000-850 mb mixing 
ratios increases to 9-9.5 g/kg late tonight coincident with 40-45kt 7h 
jet and surface-925mb convergence axis. Isent upglide on 300-305k surface 
with parcels quickly saturating and condensation pressure deficits 
below 10mb between 06-09 UTC. Scattered rafl developing mainly after midnight...then 
lifting northeast into southeastern Michigan by midday. Quantitative precipitation forecast amounts should 
remain rather light with initial dry subcloud layer and speed of 
system...but will afford wording as scattered for anticipated hir 
coverage/probability of occurence...this also to stress forecast chngs 
in light of earlier dry forecast. Given cloud cover/precipitation chances late 
tonight into first half of sun have shrunk diurnal 
fall/rise...with a slight bump to overnight lows including a 
slight non-diurnal temperature curve/faster initial fall and low level 
cold air advection/cloud cover to squelch insolation sun am. Removed pop mention 
northern half on Sunday night with timing of next upstream wave holding 
off until Monday. 


&& 


Long term... 


Monday through Saturday 


Mean troughing over the eastern Pacific will remain steadfast in 
reponse to nearly stationary deep upper level low pressure over the 
Gulf of Alaska. This will favor mean troughing over the central 
northern Continental U.S.. as a series of timed disturbances are still on track 
to eject from this mean trough...affecting the northern tier 
including the Great Lakes. Forecast models and ensembles continue to 
display relatively good agreement with forecast details in the long 
term period...with a few small differences still noted. Things still 
a little complicated by late week regarding forecast details with a 
prognosticated system. There are still concerns on the track of Ike and its 
possible local dynamical effects. Have followed very close to the 
NAM 12 as it seems to be lead the pack with respect to forecast 
trends regarding Monday-Tuesday system. Have also trended later portion of 
the forecast to the 12z GFS along with the mean gefs given 
uncertantity present. 


Monday into Tuesday...have opted for a slower system evolution with respect to 
to advancing deepening upper trough and associated surface reflection. 00z 
Sat NAM 12 was the first to trend slower with respect to this system and 
now...the 12z GFS and Gem continue to opt for this solution due to a 
slightly deeper upper trough with robust 850 mb cold air advection /which becomes 
slightly negatively tilted/ and slightly deeper surface low...which all 
points to a stronger system and slower eastward propagation. Have 
shifted probability of precipitation to Monday afternoon and increased to 50 in the north 
Monday night where better upper level large scale ascent and forcing 
will be present. Low dewpoint bias Monday morning with minimal cloud 
Clover/increasing ll flow/and warm air advection will allow southeast half of the forecast area to 
quickly warm Monday and so have warmed daytime highs across the 
board...especially southeast half. Northwest portion of the forecast area may also need 
warming...but current indications suggest increasing clouds and 
northerly flow north of a surface warm front may preclude anything 
warmer than the lower 70s. Increasing ll moisture and cooling aloft 
with robust deep shear values may support a severe threat Monday 
night with frontal passage given strong dynamic system...even passed daytime 
heating as low level jet revs up...with the main threat being a damaging 
winds...possibly small hail. However...given 
period/questionable/system evolution/and questions on ll moisture 
return...will not mention in severe weather potential statement at this time. Cold air advection on the backside 
of the front will lock stratus into the area similar to last 
Friday...supporting a much cooler day Tuesday...especially with gusty 
northwest winds. Deep dynamics with potent cold air advection at 850 mb will support 
vigorous momentum Transfer to surface...allowing breezy conditions. This 
will especially be the case over Lake Michigan where Warm Lake surface temperatures/long 
fetch flow/and a rather strong surface pressure gradient will be 
supportive of excellent momentum Transfer. Have opted for the cool 
end of the MOS spectrum Tuesday night per surface ridging/crashing 
dewpoints and clearing skies. 


Strong surface high pressure /approx 1025 mb/ will build into the region 
middle week and provide dry conditions through Wednesday...the next 
upper level disturbance will take aim at the region Thursday-Friday and 
provide the focus for precipitation chances. Still much uncertainty to 
track/strengthen/and moisture return to be able to warrant any 
changes here. Have introduced a dry Post frontal day 7 as current 
indications support another round of surface and upper level ridging for 
the region. Have opted to dismiss the 12z European model (ecmwf) at this point given 
its rather extreme changes in the day 6-8 period...demonstrating 
very poor run to run continuity. Went with highs slightly above 
climatology given expected low cloud cover and dry conditions. 


&& 


Aviation... 
primarily VFR through the forecast period. Growing concern with respect to plains 
shortwave. 12 UTC NAM/GFS Idaho system well and with decent upstream 
low/middle level moisture tap and isent upglide for at least spotty rain/rain showers 
late tonight/early sun as subcloud layer saturates some MVFR conds 
possible...espcly in vicinity of ksbn where best moisture plume to reside. Remainder 
of sun should dry out into a VFR broken cumulus field. 


&& 


Iwx watches/warnings/advisories... 
in...none. 
Michigan...none. 
Ohio...none. 
Lm...none. 
&& 


$$ 


Short term...Murphy 
long term...Chamberlain 
aviation...Murphy 

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