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fxus63 kiwx 182050 
afdiwx 


Area forecast discussion 
National Weather Service northern Indiana 
450 PM EDT Thursday Mar 18 2010 


Short term /tonight through Friday night/... 


Our stretch of amazing weather will gradually come to an end through 
the short term forecast period but not before we squeak out one more 
great day on Friday. 


Surface ridge axis has settled south of the region this afternoon 
with the middle level ridge axis overhead as of 12z...but also settling 
south as the western Continental U.S. Ridge continues to flatten in response to 
shortwave energy over Manitoba...and additional upstream energy 
diving south along the West Coast. A band of enhanced 700 mb-5 lapse 
rates is promoting some middle/high level cloudiness just to our 
west...and this will filter the late afternoon/evening sun some. 


Broad southwest flow continues tonight ahead of the Manitoba 
shortwave mentioned above and it/S associated cold front currently 
located over Minnesota and South Dakota this afternoon. Through 
tonight...this front will only make slow southeastward progress as 
it slowly becomes more aligned with the steering level flow. Ahead 
of it...positive thetae advection already underway this afternoon 
will continue. With the westerly flow aloft...Don/T see much other 
than a few high clouds. The moisture advection and clear skies 
signals some fog potential given a profile likely to show moisture 
increasing with height in the llevels. Main question will be 
whether overnight mixing due to the stronger surface pressure 
gradient will keep the fog in check. At this point...thinking is 
that southeastern areas may be able to decouple...or at least see 
the winds die off enough for some fog formation. In the 
northwest...with a stronger llevel pressure gradient...expect 5-10kt 
winds to prevent fog formation. In all areas...expect a warmer 
night given the surface winds...increasing moisture...and warmer 
previous afternoon highs. Will go with highs in the lower 40s over 
northwest areas down to the upper 30s over the southeast given the 
expected lower winds in this area. 


One more fine day Friday with aforementioned cold front nearing the 
region towards evening. Certainly expect some increase in 
cloudiness with the approaching boundary and with the increased 
llevel moisture...could even pop a few cumulus...but still expect a 
mostly sunny day. Even with continued deep southwesterly flow ahead of 
the surface front models want to slowly erode the warmer temperatures 
aloft...with t85 cooler than today. Am not sure if I want to fully 
buy this given the source region of the flow...and the fact that 
t92s still would support middle 60s. Will keep highs close to today/S 
values...which is a little above guidance. 


Things get interesting Friday night as surface cold front pulls into 
the County Warning Area with dramatic temperature drop behind it. With the 12z 
operational guidance package now in better agreement in keeping the 
northern and southern stream separate /an increasingly likely 
scenario given that all of the important shortwaves were sampled 
with the 12z radiosonde observation set/...the eventual position where the front 
stalls appears to be right overhead. Needless to say...this makes 
for a very difficult temperature forecast. Would tend to favor the 
NAM in this type of scenario...but luckily there is pretty good 
agreement in frontal position by 12z Saturday. So...will use a 
consensus approach to temperatures/winds with lows falling into the 
middle/upper 30s over the northwest with flow off the lake...and temperatures 
not falling below the middle 40s over the southeast given cloud cover 
and continued residence in the warmer airmass. With flow pretty much 
parallel to the front except in the very llevels...and with a very 
shallow cold airmass...the front will take on an Ana-front structure 
with most of the lift/precipitation upstream of the surface boundary. 
Here...the models do feature some differences with the GFS/European model (ecmwf) 
much more aggressive with bringing precipitation in after midnight 
Friday night. Given the current expectation that the front will 
reach about halfway through the County Warning Area by daybreak...feel it/S prudent 
to add a pop northwest of this...but certainly keep in the chance 
category. Ptype looks to be rain through daybreak...but it will be 
a chilly rain near the lake and wouldn/T be surprised if a few wet 
snowflakes mixed in by daybreak. 


&& 


Long term... 


..Saturday through Thursday... 


Yet another stellar flop of the models for the storm system this 
weekend is leading to even more doubt and uncertainty in what will 
eventually transpire. Medium range models have continued a trend 
toward less phasing of northern and southern stream short waves 
beginning with last nights 00z suite and continuing with 12z output. 
A dramatic separation now leaves a closed low drifting east across 
the Mississippi Delta Region into the Tennessee Valley region from 
Sunday into Monday. Meanwhile...northern stream wave is being 
prognosticated to move quickly by to our north and now shown to remain 
separated completely from the closed low to our south. The result is 
a weak cold front which will be sagging into our north on Saturday 
with a shallow cold airmass immediately behind it. Models begin to 
differ drastically with this front...the cold air and precipitation 
type with it and how it will move through the area. GFS and European model (ecmwf) 
coming into some agreement but NAM and sref are in a different Camp. 
Nearly all models have significantly moderated temperatures for 
Monday with the 850mb temperatures of -10 to -13c now confined to the 
Canadian/US border region rather than over our region. For now plan 
on following GFS/European model (ecmwf) idea with HPC guidance which indicates that 
operational GFS/European model (ecmwf) are slow compared to their ensemble 
counterparts. 


Some drastic changes were made to grids with this package in light 
of recent model solutions though confidence remains low. Shallow 
cold air looks to sag into far northern areas with difference 
amongst models in how far south the cold air will move before 
moderating. European model (ecmwf)/GFS Camp brings the colder air and associated 
rain/snow line into central forecast area on Saturday while NAM/sref 
keep it just into far north. NAM/sref quickly bring warm wedge back 
into the area Sat night and Sunday while European model (ecmwf)/GFS continue to sag 
cold air south with little warm air return but a moderating trend to 
airmass on Sunday. Given the potential thermal fields immediately 
behind this front supporting snow or at least a rain and snow mix 
have opted to introduce mixed precipitation in the north Saturday 
into Sunday. Still could be some light accumulations on grassy areas 
in extreme northern areas but should be light. 


Given the warming trend and lack of significant cold air intrusion 
with less phasing have trended Monday highs up a little but still 
remained well short of latest operational mex numbers. Remained 
closer to men ensemble mean numbers as a hedge given uncertainty. 
Remainder of forecast periods generally followed HPC numbers which 
indicates a warming trend for middle week followed by another weak 
wave..chance for rain showers Wednesday and cooler temperatures by day 7. 


&& 


Aviation /18z tafs/... 


VFR conditions should dominate the period with only caveat being 
some potential fog tonight as upper shortwave passes north of the 
region and trailing cold front nears the area on Friday. Dewpoints 
much higher this afternoon than previous days given developing 
positive thetae advection ahead of the approaching boundary. With 
moisture increasing through the column we have a pretty good setup 
for fog with the only question remaining with surface winds and 
whether enough llevel mixing will keep fog development at Bay. 
Right now...expect best fog potential at FWA given further 
distance from the approaching upper cloudiness...and a slightly 
weaker llevel pressure gradient...which...when combined with 
mclear skies should allow winds to drop sufficiently for fog. 
Confidence not high in dense fog given the winds...but this isn/T 
out of the question if winds fully decouple...with increasing 
moisture continuing to stream in above the llevel inversion. Given 
this...will go ahead and add at tempo MVFR br mention at FWA and 
let shifts tonight monitor the winds. If winds go calm...which I 
think is pretty unlikely...watch out for some thicker fog than 
currently forecast. 


West winds 10g16kts this afternoon will veer more southerly and 
decrease to 5-10kts overnight before strengthening back to 
14g20kts for the daytime on Friday. 


&& 


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


$$ 


Short term...Arnott 
long term...Lashley 
aviation...Arnott 

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