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fxus63 kiwx 191812 
afdiwx 


Area forecast discussion 
National Weather Service northern Indiana 
212 PM EDT Friday Mar 19 2010 


Aviation... 


VFR conds expected through first 12-18hr of taf period before lowering 
ceilings in cold air advection behind cold frontal passage provide MVFR flying conds. Strong 
west-southwesterly winds early this afternoon will begin to wane as surface trough 
approaches and gradient relaxes. Winds will then shift nearly and 
eventually nerly after cold frontal passage tonight and timed frontal passage through 
ksbn and kfwa in tafs. Could see some snow possibly mixing with a 
little rain at ksbn after 12z as better isentropic lift moves over a 
cool boundary layer. 


&& 


Previous discussion... /issued 346 am EDT Friday Mar 19 2010/ 


Short term... 
one more superb Spring like day before the bottom falls out this 
weekend. 


Focus for this period turns toward amplifying SW trough and developing deep 
cutoff across the Southern Plains over the weekend. Lead SW over western on will 
continue eastward near term and aide southward progression of surface cold front stretched out 
from northern WI back southwestward into eastern NE this morning. No doubt the cold 
waters of the Great Lakes will augment this process and have sided 
west/details of WRF surface pattn as a result yet not too dissimilar to 00z 
spectral solutions which have come into much btr agreement across the 
board. 


Little concern today as weakening thermal ridge advances S across the area. 
Upstream radiosonde observations yesterday out of mpx/grb still point to a deeply mixed 
boundary layer once again this afternoon even west/encroaching cirrus shield and 
expect temperatures near persistence /mid 60s/ once again. 


Much bigger changes on tap tonight west/quicker frontal passage assumed and dramatic 
drop in surface temperatures given strong ll cold air advection. Continued ll Theta-E advection today in advection 
of frontal boundary will lead to rapid Post frontal cumulus/stratocu by lt 
afternoon/evening as winds veer quickly to NE by morning. Some penchant for 
rain far northwest and previous pop alignment not out of the qn...however deeper 
saturation looks to hold off later into Sat morning west/far greater 
isentropic lift developing (esp northern half) and will bump probability of precipitation further. 
Depth of cold layer and greater diabatic/adiabatic latent heat 
reduction across northwest zones still heralds a legitimate snow chance although 
would tend to doubt any accumulation Sat afternoon or Sat night for that 
matter west/strongest ascent now prognosticated further S of prior 12z 
guidance and fitting west/a deeper surface frontal intrusion into central in. 
Regardless...it will certainly feel raw west/a chilly rain and 
increasing NE winds for all. 


Temperature guidance Sat is troubling per implied small diurnal in face of 
expected rain/cold air advection which casts considerable doubt and seen in GFS 
ensemble MOS deviations. Wet bulb temperatures would suggest readings a Cat 
lower Sat afternoon and something the day shift may need to jump on 
assuming 12z MOS guidance trends lower. 


Long term.../Sunday through Thursday/... 


With a general consensus finally emerging across the model suite 
with regards to the early extended...a reasonable level of confidence 
in the overall scenario allows the addition of some forecast details 
in the Sunday-Monday time frame. GFS/ensemble mean/NAM/12z European model (ecmwf) 
all more or less agree that upper low initially centered near the 
arklatex Saturday night will progress easterly to the Atlantic coast 
near Virginia/NC border by 12z Tuesday. Surface frontal boundary still prognosticated to be 
stalled somewhere along the southern County Warning Area which will allow cooler 
conditions to permeate for the beginning of the week...but the lack 
of phasing with longwave trough over southern Canada leaves severely 
colder air hung up along and north of the US Canadian border and 
initially cool temperatures aloft are shown to warm for sun/Mon...and 
after 00z Monday guidance/2m temperatures/bufr soundings unsupportive of snow 
with temperatures rising to above freezing surface through 850 mb. Have therefore 
introduced probability of precipitation to the southeast half Monday afternoon given slower 
progression of upper system and surface wave still angling up the Ohio 
Valley...while keeping ptype all rain after 12z Monday. This is a 
conservative pop addition and kept rain or snow mention Sunday night 
especially given a quick look at the 00z European model (ecmwf) shows a southerly 
shift after crossing the Mississippi River which would trend Monday 
afternoon probability of precipitation even further south and allows cooler temperature profiles 
across the north Sunday night. 


Behind this system warmer temperatures return for midweek. GFS brings weak 
frontal boundary through on Wednesday while European model (ecmwf) holds a stronger system 
for a bit later in the week. With no reason to favor a particular 
solution no changes to days 5-7 pop/wx.&& 


&& 


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


$$ 


Short term...T 
long term...lud 
aviation...Simpson