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Area forecast discussion 
National Weather Service Indianapolis in 
540 am EST Monday Nov 23 2009 


Aviation...discussion for 231200z taf issuance. Area of low 
ceilings...003-005 above ground level...over the northern sections of the forecast area 
has been expanding west overnight...although not much expansion 
south has been noted so far. With the boundary layer winds veering 
this morning...not expecting these ceilings to make it to kind...but it 
will be close. Model guidance suggests it may be about 231700z 
before conditions improve at klaf. Otherwise...some areas of 
IFR/LIFR conditions in fog should lift by around 231500z. It may 
take a little longer for the lower visibilities to mix out this morning 
compared to the past few mornings given the more extensive cloud 
cover. 


Finally...area of MVFR ceilings based around 030 spreading north through 
Kentucky expected to continue to expand from south to north in the 
warm/moist advection pattern...reaching klaf by around 231900z. 


&& 


Discussion... 
remnant upper level low from weekend Gulf Coast system was 
traversing through the Ohio Valley early this morning. Extensive middle 
level cloud deck was expanding north into the forecast area. Swath 
of IFR stratocumulus/fog had dropped west/SW out of northern Indiana to 
encompass far northern portions of the forecast area. Temperatures 
at 08z ranged from the middle 30s to the middle 40s. 


Forecast challenges focus on clouds today and tonight and the 
impacts to temperatures...and the evolution of the colder airmass 
into the region for the second half of the week. 


Much of the deeper moisture associated with the upper level system 
was along and east of the central Appalachians closer to the center 
of surface low pressure. Model guidance in good agreement for today 
on keeping deeper moisture and stronger forcing from not getting 
much further west than Ohio and the eastern half of Kentucky. Both 
00z NAM and op GFS indicating that isentropic lift at 285-290k 
levels does progress into the southeast portion of the forecast area 
later today...but it appears that low level easterly flow and lack 
of significant forcing are doing their part to keep precipitation 
displaced well east of the forecast area. Have scaled probability of precipitation back to 
just low chances (20 or less) across far eastern sections of the 
forecast area today as not entirely out of the question that a few 
light showers sneak into eastern Indiana this afternoon as the upper 
level shortwave lifts into the Great Lakes. Main impact from this 
system however will be to allow for expansion of stratocumulus into 
the southeast half of the forecast area by late day. With broad 
stratocu deck across northern Indiana expected to retreat only 
slowly this morning as low level flow veers to more of a southeast 
direction...expect will end up being a fairly cloudy day for all but 
far western sections of the region where driest air will reside. 


Upper level ridging will briefly establish tonight as upper level 
shortwave becomes absorbed by broader upper trough expanding through 
the central/northern plains. With surface high pressure maintaining 
light wind flow and model soundings indicating plenty of moisture 
becoming trapped beneath a low level inversion...anticipate clouds 
will hold for much of the night. Some localized fog/mist will also 
be possible considering the saturated lower levels...and have 
inserted patchy fog for entire forecast area after midnight. 


Skies will remain mostly cloudy Tuesday as winds go southerly with 
the passage of a weak warm front. Model guidance favoring a slightly 
slower eastward progression to aforementioned upper low and 
associated cold front at the surface. Deeper isentropic lift ahead 
of the front does not arrive into western counties until late 
afternoon Tuesday...with main impacts predominantly Tuesday night as 
low level convergence develops at nose of 850mb low level jet. 
Several of the 00z runs have come in noticeably diminished with rain 
coverage along the front Tuesday night...and that appears to be a 
consequence of the Gulf being shut down as a major moisture source 
for the system. Low level forcing along the front justifies 
maintaining 50-60 probability of precipitation for entire forecast area however...with 
highest probability of precipitation over northern counties in closer proximity to track of 
surface low pressure. Front should be east of the forecast area by 
12z Wednesday...and have trended towards European model (ecmwf)/op GFS/Gem consensus 
placing front over Ohio by Wednesday morning. 


Initial upper level low is just The Table setter for more 
significant upper low racing out of the Canadian prairies Wednesday 
morning and diving into the region by Thanksgiving. Weather should 
remain unsettled Wednesday as surface low pressure lifts into the 
central Great Lakes Wednesday and favorable low level lift remains 
across much of the forecast area. Prefer more progressive track to 
secondary upper level low suggested by last few runs of European model (ecmwf)...with 
this system arriving into the region Wednesday night with another 
cold front passing through the forecast area. In wake of the 
front...low level cold advection commences immediately as a sharply 
colder airmass invades central Indiana. Low level thicknesses should 
support potential for snow showers to mix in with rain later 
Wednesday night and will continue to carry a mix of rain/snow for 
all but far southeast forecast area. 


Cold cyclonic flow around stacked upper level low will make for a 
cold raw Thanksgiving for central Indiana. Will maintain chance probability of precipitation 
all day with light rain/snow gradually transitioning to all snow 
showers Thursday night as colder air continues to feed into the low 
levels. Precipitation should gradually end by Friday as upper low 
shifts off to the east. 


Temperatures...remained close to mavmos guidance for highs today and Tuesday 
with temperatures in the 50s both days. Took split of warmer mav and 
cooler met for lows tonight and Tuesday night. Warmed highs 
Wednesday just a few degrees as appears secondary cold front will 
not arrive in the region until Wednesday night. Undercut guidance 
temperatures Thursday with an abundance of clouds/precipitation in 
the cold pool. 


&& 


Ind watches/warnings/advisories...none. 


&& 


$$ 


Public...Ryan 
aviation...jas 

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