Weather

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afddtx 


Area forecast discussion 
National Weather Service Detroit/Pontiac Michigan 
355 am EDT Friday Aug 29 2008 


Short term...today 


A pattern of showers will move through Southeast Michigan this morning 
ahead of a slow moving cold front. The bulk of the showers will exit 
to our east by afternoon but the front will take most of the day to 
move through the area. This will allow an isolated shower to be 
possible later in the day as humid air lingers over the region and 
high temperatures make a run at 80f. 


Early morning satellite imagery showed a respectable upper level 
trough driving the shower pattern and surface frontal system into 
lower Michigan. Model analysis fields indicated the showers 
concentrated in the low level Theta-E ridge ahead of the surface 
front with lift provided by moisture advection and some weak 
differential positive vorticity advection. The lack of lightning strikes verifies the weak 
elevated instability indicated in the model data. While this will be 
a limiting factor for convective strength, there has been a modest 
increase in areal coverage since midnight, giving the nod to NAM 
among the 00z models handling the pattern best. That being the case, 
expect numerous showers in the Saginaw Valley and thumb this morning 
with scattered showers south toward the Ohio border. 


The deeper moisture will exit to our east by afternoon but the front 
will still be in the area as it is left behind by the upper wave. 
The weak surface pressure gradient will become altered by 
differential heating due to the lakes and varying cloud cover. This 
will enhance surface convergence and support at least an isolated 
shower or thunderstorm during peak heating. High resolution WRF 
model output run locally and from NSSL depict this scenario nicely 
and look reasonable with the front finally exiting to our east and 
south during the evening. 


&& 


Long term...tonight through next Thursday 


An extension of surface high pressure will nose into Southern Lower 
Michigan at the start the period. Support for rising isobars and 
midlevel heights is provided by the right exit region of an 
anticyclonically curved upper level jet. With the surface front 
safely eastward and quick dry air advection will maintain the 
precipitation free forecast during the overnight. Given a lack of 
boundary layer moisture and the predominate dry soil conditions not 
anticipating any fog issues. A good radiational cooling environment 
will allow lows to dip into the lower 60s heat island/lower 50s 
remaining. 


A shortwave will zip along the US/Canada border Saturday within the 
developing zonal west to northwesterly flow. The shortwave has a 
fairly impressive satellite depiction this morning...but the main 
absolute vorticity energy is expected to remain well north of any 
land areas of the County Warning Area. A weak surface reflection/trough will work 
southward through the northern part of lower Michigan before 
stalling/washing out due to lost upper level support and diabatic 
effects Saturday. With no column moisture and lower levels that 
are strongly divergent...expect mostly sunny skies and dry 
conditions. Highs will solidly reach the lower 80s. 


The models indicate an emergence of the wind shift line again 
Saturday night as the center of surface high pressure passes through 
Northern Lake Michigan and into Northern Lake Huron. This action 
would force the weak front to clear the rest of County Warning Area and give the 
area an easterly fetch on Sunday and potentially into 
Monday...albeit weak. 


There is strong model agreement on the development of an Omega block 
wide ranging from the Rocky Mountains to the Atlantic Seaboard early 
next week. This should allow temperatures to climb markedly above 
normal...with the readings reaching the middle to upper 80s Monday and 
potentially near 90 Tuesday. Will not tweak the forecast much for 
this issuance but will need to watch the future model solutions in 
regards to a lingering easterly wind fetch immediately south of the 
surface high pressure cell. 


Will raise high temperatures for Wednesday as any cold front/deeper 
moisture remains well west of the area. 


&& 


Marine... 


A weak cold front will move through Lake Huron and lower Michigan 
today. Other than some showers and an isolated thunderstorm, mainly 
this morning, this front will move through the region will little 
fanfare and will not disrupt marine conditions to any significant 
degree. Winds shifting toward the northwest will be 15 knots or less 
and will then settle back to the west tonight. High pressure will 
then move in for the rest of the Holiday weekend. 


&& 


Previous discussion...issued 110 am EDT Friday Aug 29 2008 


Aviation... 


An increasingly moist low level airmass will maintain favorable 
conditions for MVFR fog early this morning. Model soundings 
suggest the potential for conditions to briefly drop into IFR 
prior to sunrise...mainly at dtw/fnt. An approaching cold front 
will be accompanied by showers through middle morning...although 
chances appear too slim to include except at mbs where deeper lift 
will exist. MVFR visibility restrictions may prove difficult to 
scour out through the morning hours...especially at 
dtw/det...where sufficient low level moisture will remain in place 
prior to the frontal passage by midday. A scattered/broken 4-5 kft cumulus field 
will persist into at least the early evening behind the front. 
Winds will generally remain below 7 knots through the taf 
period...shifting from south to north behind the front for the 
afternoon hours. 


&& 


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


&& 


$$ 


Short term...bt 
long term....cb 
marine.......bt 
aviation.....Mr 




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