810 
fxus63 kiwx 151802 
afdiwx 


Area forecast discussion 
National Weather Service northern Indiana 
202 PM EDT Monday Mar 15 2010 


Short term... 
tonight through Tuesday night. 


Two challenges for the short term...the first being cloud cover and 
the other temperatures. 


Large stratocu field observed on visible Sat from southeast Wisconsin into 
central lower Michigan. North of this generally clear skies prevail. This area 
has been progressing slowly southward through the day with hints at 
thinning in spots. Both GFS and NAM shows similar timing of clearing 
arriving with leading edge moving into far NE areas around 23 to 00z 
and progressing south. This looks good based on extrapolation of 
current Sat data. Will have skies becoming mostly clear to clear by 
late evening. Temperatures will fall off more than past couple of nights 
with good radiational cooling. Lows should bottom out in the lower 
30s...close to dewpt values. 


A good deal of sunshine should prevail on Tuesday allowing temperatures to 
be at least 7 to 10 degrees warmer than today's highs. Have only 
made minor adjustments to afternoon highs to nudge up just a bit. 
Overnight lows Tuesday night will be similar to tonight with 
readings into the lower 30s. 


&& 


Long term... 


Wednesday through Monday 


Large scale flow progression to a more dominant northern stream 
remains the main forecast concern in the long term period. All 
forecast guidance remains very agreeable with respect to the large scale pattern 
evolution. However...the fine details of the flow transition remains 
very murky at this point. Forecast models and ensembles continue to 
forecast a plethora of wave solutions with respect to to the late week progressing 
trough...with much of the variability stemming from the handling of 
the southern periphery vorticity lobe. The GFS may be too aggressive with 
its development of strong rotating impulse on the southern periphery 
of the progressing long wave trough...but its persistence may have 
some weight given uncertainty per the resultant upper flow over The 
Rockies in the day 3-4 time period. In addition...strength of the 
northern stream upper wave counterpart is adding to forecast 
uncertainty. One can see this clearly in the GFS model alone when 
comparing the 06z run to the 12z run. Details per long wave trough 
fracture/digging processes per upper jet influence/and strength of 
ll cold air advection under the northern stream counterpart still needs to be 
pinpointed...and given poor sampling of the eastern Pacific wave that 
will initiate all these processes...confidence per surface reflection 
speed and resulting cold front passage remains low. Given the large 
model/ensemble spreads per 500 mb trough speed/depth with resultant 
thermal fields and pop influence...feel a trend to a mean solution 
which encompasses a wider window of solutions is prudent at this time. 
Hence...have remained close to the previous forecast...in days 5-7 
opting for a trend to a mean gefs solutions...which handles 
uncertainty/concerns well. 


Wed-Fri...mean surface ridging is expected with general light flow. Warm air advection 
in the presence of strong insolation will support temperatures much above 
seasonal norms with highs in the middle to upper 50s...with lower 60s 
expected by thur-Fri. Much cooler conditions can be expected near Lake 
Michigan given expected light wind fields and ample bl cooling per lake moderate. 
Better mixing and flow will support warmer conditions near the lake by 
Friday...possibly as early as Thursday...with 50s likely. Expect mostly 
clear skies through the period. 


Friday night-Monday...long wave trough and a possibly strong tail end 
impulse will eject toward the region...supporting an advancing surface 
cold front. Depending on the strength of the northern stream 
wave...a front may drop into the forecast area as early as Friday 
night....supporting NE flow per a possible northern stream 
impulse...similar to 12z European model (ecmwf) and 06z GFS solutions. This would 
certainly be the case if the northern stream splits and out runs the 
southern stream counterpart...given the cold sink of the Great 
Lakes. However...if a solution Akin to the 12z GFS verifies....much 
warmer conditions would be expected on Sat...but feel the 12z GFS 
solution is very questionable and certainly is not typical given the 
large scale setup...and this agrees well with Gem/ECMWF/UKMET forecasts. 
Hence...have opted for a solution that trends heavier toward a 
strong northern stream influence...supporting cooler thermal 
fields...and an earlier bias to precipitation chances. This remains the 
preference until better sampling/handling of the eastern Pacific trough 
is realized. Sat pop grids look good with precipitation expected this 
weekend...but timing remains a question. Given northwest flow with an 
active northern stream...added a low chance probability of precipitation on Sunday...with a 
rain/snow mix mentioned. A trend to eastern Continental U.S. Troughing would 
offer increased clouds and cooler fields into day 7. 


&& 


Aviation... 
18z tafs. 


Extensive stratocu deck with some thin spots continues to progress 
south with the back edge moving across central lower Michigan. 
Extrapolation brings clearing into both taf sites around 23z to 
01z time frame with at most some high clouds in its wake. Have 
adjusted timing in tafs to reflect that and opted for sky clear after 
3z...which should persist through the period as cumulus rules suggests 
little or no development. Winds will decrease this evening and 
remain on the lighter side on Tuesday. 


&& 


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


$$ 


Short term...Fisher 
long term...jc 
aviation...Fisher