Weather

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Area forecast discussion 
National Weather Service Grand Rapids Michigan 
330 am EDT sun Sep 7 2008 


Latest update...all except aviation... 


Synopsis...(330 am EDT sun Sep 7 2008) 
an upper level disturbance will result in some showers this morning 
which will taper off this afternoon. A low pressure system will 
develop and track northeast and bring US our best chance of rain and 
some thunderstorms from late Monday through Monday night. Dry and 
cool weather is anticipated Tuesday behind the cold front after a 
few lingering morning showers. 


&& 


Short term...(330 am EDT sun Sep 7 2008) 
(today through Tuesday afternoon) 


Showers will continue to develop this morning (mainly near and south 
of I-96) due to positive vorticity advection and further aided by lake induced instability 
with 850 mb air temperatures of around 7 c crossing relatively mild Lake Michigan 
waters. The lake enhancement is apparent in occasional convective 
elements noted on radar reflectivity returns with some of the 
showers. 


Showers will taper off this afternoon and tonight. Our next big weather 
maker will move in late Monday and Monday night as low pressure 
tracks NE into our region. 


A time height x-section from WRF guidance shows strong omegas on the 
order of 10 to 20 microbars/second Monday night. Model guidance also 
shows strong fgen and vigorous middle level f-vector convergence 
oriented SW to NE across our County warning forecast area. We will also get into vigorous 
positive vorticity advection as the primary shortwave amplifies as it tracks east into our 
area with impressive potential vorticity as well. 


Therefore rain with some convection is likely Monday night. We 
raised probability of precipitation slightly (to 70 pct) and strongly considered going 
categorical. Future shifts may need to continue this trend of 
ramping up probability of precipitation a bit based on 12z guidance and model trends. 


Convective potential at first glance seems quite limited by several 
factors including (most notably) limited instability. However... a 
rather sharp baroclinic zone is noted with the front and a vigorous 
low level jet will develop Monday evening... with our County warning forecast area potentially right 
on the nose of it. Both speed and directional wind shear should 
become quite strong. 


Therefore... I would not be surprised if future shifts need to ramp 
up discussion of convective potential Monday evening. We feel that 
in spite of limited instability we may have to deal with some strong 
to potentially severe convection given strength of the front and 
wind fields with height... definitely a tricky forecast. 


Lingering showers across our eastern County warning forecast area Tuesday morning should 
taper off fairly quickly as brisk northwest winds behind the cold front 
advect much cooler and drier air in. Given the magnitude of cold air advection 
anticipated... we lowered the maximum temperature forecast for Tuesday just a bit. 


&& 


Long term...(330 am EDT sun Sep 7 2008) 
(tuesday night through saturday) 


The main challenge in the long term deals with the temperatures for 
the Tuesday to Wednesday timeframe...along with the potential for some 
rain Friday night into Saturday. Strong high pressure arrives for 
Tuesday night. This system has been trending stronger with each 
run. This high will dry our airmass out and also draw down cooler 
air from Canada. As a result I lowered temperatures several degrees 
for the Tuesday night into Wednesday night timeframe. The GFS shows a 
wave of low pressure bringing some rain to the area Friday night into 
Sat. However the European model (ecmwf) is slower with this system and still 
supports dry weather to start the weekend. As we currently have a 
dry forecast going for Friday night into Sat and the European model (ecmwf) supports 
this scenario...I will maintain the dry weather. 


&& 


Marine...(330 am EDT sun Sep 7 2008) 
winds/waves will stay below Small Craft Advisory criteria through 
Monday afternoon. Lake Michigan will likely turn quite choppy Monday night and 
Tuesday as strong northwest flow cold air advection develops behind the front. Rain and 
thunderstorms will also pose a hazard to mariners Monday afternoon 
into Monday night. 


&& 


Aviation...(1158 PM EDT Sat Sep 6 2008) 
the area of rain over eastern Iowa has moved east northeast into 
north central Illinois by 11 PM. This area will race northeast into 
Southern Lower Michigan after midnight tonight. That will bringing 
light to at times moderate rain to areas... near and south of 
Interstate 96... between 4 am and 9 am Sunday morning. This is 
thanks to coupled jet dynamics that will move across lower Michigan 
in the 8 am time frame of Sunday. That will bring the enhanced 
precipitation across most of the south half of lower Michigan. 


Now...add the lake effect to this. The 6c 850 mb moves into the Lake 
Shore areas by 12z Sunday. That in combination with the 18c surface 
lake temperature (coastwatch) will create some lake instability too. Thus 
as both the RUC and NAM show...there will be a lake enhanced 
precipitation area west of US-131 tonight thank to lake enhancement. 
As I write this... showers were already forming west of Saugatuck. I 
have to believe the combination of the dynamics and lake enhancement 
will result in MVFR ceilings/visible across the taf sites. I could even see 
IFR for a time by the way 12z and 15z at most of the taf sites (only mkg is 
to far northwest for this). 


Once the system moves through clearing should occur (2 pm). However 
the cumulus rule shows enough low level moisture for broken cumulus coverage 
in the afternoon. Model sounds show this too. The equilibrium level 
on most model soundings in the 18z to 21z time frame are near 15000 
feet. That by the way reaches the -10c isotherm. Isolated showers 
would not be out the question then either. The lake effect 
enhancement could best that. Bottom line here it the mostly likely 
time for solid VFR is from now till 09z at most taf sites. More 
showers and lake enhancement are possible early Monday morning as 
the next cold push moves in. 


&& 


Hydrology...(330 am EDT sun Sep 7 2008) 
the system on Monday night has the potential of producing rainfall 
amounts of a half an inch to an inch. A few urban and small stream 
type Hydro issues could develop in that time frame and linger into 
Tuesday morning. 


&& 


GRR watches/warnings/advisories... 
Michigan...none. 
Lm...none. 


&& 


$$ 


Synopsis: Laurens 
short term: Laurens 
long term: mjs 
marine: Laurens 
aviation: wdm 
hydrology: Laurens 

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