646 
fxus63 kiwx 192026 
afdiwx 


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


Short term...tonight through Saturday night... 


Very busy short term period in store as our fantastic Spring weather 
is about to take a temporary hiatus as a cold front currently 
approaching the area will return US to reality beginning tonight. 
Strong split flow pattern will exist through the short term as a 
potent upper wave closes off over Texas and remains independent from 
northern stream energy prognosticated to graze the US/Canada border without any 
substantial penetration into the Continental U.S.. however...this upper energy 
will be potent enough to allow a cold front to drag into County Warning Area tonight 
and Saturday with exact location/timing of front imperative to 
sensible weather forecast. Most significant concern for this period the 
possibility of some light accumulating snows in the northwest on 
Saturday. 


Tonight... 
cold front will move into the County Warning Area with strong model agreement of 
frontal position from Henry co, Ohio to Grant co, in by 12z Sat which 
necessitates a slight decrease in low temperatures tonight in the northwestern 
portions of County Warning Area. Will leave going rain chance in the northwest after 06z to 
coincide with faster frontal timing. 


Saturday... 
temperatures expected to rise very little in northwest half of County Warning Area tomorrow 
and used a non diurnal curve to keep things steady in the low to middle 
30s as clouds...precip...and cold air advection all contribute to a cold day. In 
the southeast County Warning Area...temperatures a little trickier as cold front slowly clears southeast 
County Warning Area in the afternoon and felt a decrease in highs necessary here as 
well with strong model agreement showing faster frontal passage that 
has been evident in the past several runs. All ptype tests from a 
top down perspective showing that precipitation falling tmrw morning in the 
northwestern County Warning Area will be in the form of snow and have trended the grids 
further that way leaving in only a slight chance of some rain mixing 
in. Not expecting more than an inch of accumulation on grassy 
surfaces and/or vehicles tmrw given how recent stretch of weather 
has warmed soil surface temperatures immensely and timing of precipitation during the 
daylight hours will further hamper accumulations. Across the cen 
part of the County Warning Area... expect to see a mix of precipitation as wet bulb temperatures 
sit right around the 0c isotherm with no snow accumulations. In the 
southeast County Warning Area...expect only low chances for a little rain as most impressive 
isentropic ascent stays further northwest and forcing remains meager southeast of 
a Monticello to Defiance line and have lowered probability of precipitation in the southeast to 
account. 


Saturday night... 
not overly impressed with synoptic forcing for ascent tmrw night but 
did not want to jump on a single batch of model runs so did leave 
likely probability of precipitation over most of County Warning Area once again tailoring probability of precipitation lower in southeast 
County Warning Area. Lows pretty well on track with a cool night near freezing 
expected for most. 


&& 


Long term... 


..Sunday through Friday... 


Better overall agreement among medium range models continues this run 
with very good agreement through middle week. By end of week we again 
see large differences in models with respect to separate northern 
and southern stream waves and degree of phasing. 


Weak and diffuse surface front looks to be through forecast area at 
beginning of this period with shallow cold air in place Sunday 
morning. Depth of this cold air will be greatest across extreme 
northern counties where chance of lingering snow will continue. 
Models in agreement on moderating low level temperatures Sunday as 
southern stream closed low slowly moves east through the lower 
Mississippi River valley region and into the Tennessee Valley region on 
Monday. Concerned that with a persistent northeast wind models are 
warming low levels too fast and with clouds and any lingering precipitation 
we will see temperatures below latest MOS values. Several waves will rotate 
around the upper low and southern areas most likely to see 
precipitation on Sunday...though should be rather light north and 
more widespread south. Kept likely probability of precipitation all areas for now until we 
see a more confident drying in the north. 


Lingering precipitation Sunday night in the southeast. Temperature 
profiles suggest all rain given the moderation and warmer air 
wrapping around the upper low. Again concerned models may be a bit 
too warm so remaining on cool side of guidance but will go with all 
rain given model agreement on thermal profiles. As with most closed 
lows feel a slower trend is best and expect models to continue 
slowing things down with departure of this system. We saw this last 
weekend and no reason to expect anything different this time around. 
Will linger clouds and cooler temperatures into Monday and begin clearing 
skies Monday night into Tuesday. 


Warm air advection prognosticated to begin in earnest Tuesday. 00z European model (ecmwf) 
most aggressive with 850mb temperatures warming back to around +6 to +9c 
while GFS from +3 to +5c. With ridge axis being depicted overhead 
through early part of day and better mixing not occurring until 
later in the day will probably not fully mix to this level so again 
remained conservative with temperatures. Better mixing expected 
Wednesday but likely offset some by increasing clouds ahead of next 
system. Should still be able to get back into the 60s on Wednesday and 
possibly Tuesday with faster mixing. 


Chances for rain continue later Wednesday into Thursday. Models have 
slowed somewhat with best chances now appearing to be Wednesday night into 
Thursday. Confidence low as this is a split wave with northern 
stream short wave to our north and another southern stream wave 
lagging to our southwest. Given recent trends would expect a slower 
and split regime with possibility of southern wave closing off yet 
again. Have opted to keep day 7 dry rather than introduce a low 
chance pop with little confidence. Temperatures near normal for end 
of week. 


&& 


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. 


&& 


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


$$ 


Short term...Simpson 
long term...Lashley 
aviation...Simpson 

National Weather Service Glossary of Abbreviations

Non-Expanded Version (with abbreviations)