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fxus63 kdtx 182006 
afddtx 


Area forecast discussion 
National Weather Service Detroit/Pontiac Michigan 
406 PM EDT Friday Jul 18 2008 


Short term...tonight 


A surface analysis shows an area of low pressure north of Lake Huron 
with a weak surface boundary extending westward across the lake into 
northern lower Michigan. A fairly zonal flow resides across the 
northern Continental U.S. With one shortwave over southern Ontario and another 
weak embedded shortwave seen moving into Minnesota. The upper level 
jet resides over the northern lakes region with a persistent area of 
middle and high clouds located to the south over central lower 
Michigan. This boundary has been the focus for a few light showers 
today and will continue to very slowly slide south towards the I-69 
corridor overnight. Dewpoints remain high along and north of the 
Tri-Cities where surface based convective available potential energy are approaching 1000 j/kg while 
dewpoints towards the Ohio border have mixed down to around 60 
degrees. Low level convergence will be better across the north and 
although coverage of storms will be limited...will carry 40 probability of precipitation 
across the north for the overnight period. Southern areas should be 
dry through the evening but will carry an isolated shower or 
thunderstorm overnight as the front approaches. Partly to mostly 
cloudy skies are expected overnight with another warm night in 
store. Lows will only drop to the upper 60s to near 70 for the metropolitan 
area. 


&& 


Long term...Saturday through Friday... 


Upper level pattern will change very little into the weekend... 
as a fast zonal flow remains across the northern third of the 
Continental U.S.... while broad ridging holds toward the south. This will 
create potentially unsettled conditions for the weekend... 
although the overall evolution/timing and placement of convection 
remains difficult to discern (especially saturday) with 
development largely dependent on mesoscale details that have yet 
to evolve. Weak low level frontal boundary dropping south will 
provide one focusing mechanism...although the convergence remains 
rather unimpressive. 12z NAM still suggesting upstream mesoscale convective system 
development late tonight along the low level jet...with a remnant 
mesoscale convective vortex/cold pool making a run eastward toward lower Michigan for 
Saturday. Otherwise...similar to today...considerable cloud cover 
and a general lack of good large scale forcing may act to limit 
overall convective coverage. Deep low level moisture...combined 
with just enough available instability and frontal convergence... 
does still warrant chance probability of precipitation Saturday for pulse-type showers/tstorms. 


Short range models in fair agreement in tracking a surface low along the 
frontal boundary Saturday night/early Sunday...as a stronger 
shortwave ejects east-southeast from southern Canada into the upper MS 
valley/Great Lakes. Still some differences in the placement of this 
low as it tracks through the Great Lakes. Favor GOES toward the 
more northerly NAM/European model (ecmwf) solution...with the GFS struggling to 
resolve the embedded shortwave energy and resultant convective 
feedback. This scenario takes an attendant cold front through 
southeast lower Michigan Sunday afternoon...and depending on exact 
timing...could place a more focused convergence zone over the area 
during peak heating. The latest NAM indicates convective initiation 
over areas east of I-75 by 21z. A strengthening wind field...with 
50 knots at 500 mb/bulk shear over 40 knots...will create more 
favorable conditions for convective organization. This assumes 
sufficient pre-frontal destabilization to sustain stronger 
updrafts. 


A more progressive evolution with this system now looks to bring 
drier air into the region as early as Monday...and will pull probability of precipitation 
accordingly. Aforementioned shortwave will deepen as it moves 
through the Great Lakes...eventually closing off over Quebec and 
leaving a broad upper level northwest flow into early next week. 
GFS appears to be an outlier in dropping another strong shortwave 
southeast within this flow Tuesday/Tuesday night...generating an area of 
precipitation under the increasing large scale forcing. Will continue to 
favor a drier solution depicted by the European model (ecmwf)...as the upper trough 
axis pivots through and high pressure builds in at the surface. This 
will bring temperatures down to around or just below seasonal norms 
through midweek...with highs in the upper 70s/lower 80s. 


&& 


Marine... 


A weak cold front remains stalled over Northern Lake Huron and will 
slowly slide south across the lake tonight. Winds will 
generally remain on the light side through Saturday with scattered 
showers and a few thunderstorms focused along this boundary. The 
next low pressure system will track northeast along this stalled 
frontal boundary on Sunday generating breezy winds and a continued 
chance of thunderstorms. 


&& 


Previous discussion...issued 145 PM EDT Friday Jul 18 2008 


Aviation... 


A weak frontal boundary will remain nearly stationary across central 
lower Michigan into tonight. An overcast deck of middle clouds is 
present along this boundary with a few scattered light showers 
possible at fnt/mbs this afternoon. An isolated thunderstorm is also 
possible but chances are low and will exclude from the taf until we 
see development upstream. Outside of a shower...VFR conditions are 
expected through tonight with chances of rain showers increasing at 
all taf sites by late Saturday morning. 


&& 


DTX watches/warnings/advisories... 
Michigan...none. 
Lake Huron...none. 
Lake St Clair...none. 
Michigan waters of Lake Erie...none. 


&& 


$$ 


Short term...kec 
long term....mr 
marine.......kec 
aviation.....Kec 




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