791 
fxus63 kmqt 212020 
afdmqt 


Area forecast discussion 
National Weather Service Marquette Michigan 
420 PM EDT Thursday Aug 21 2008 


Synopsis... 


Upper level ridge extends from the middle Atlantic coast into James 
Bay. Upper low over the southern states has quickly moved north with 
edge of rain now reaching Winona Minnesota to La Crosse WI. Area of 
steadier rain reaches as far south as southeast Iowa into northern 
Illinois. Farther south into MO/IL/AR are isolated/scattered thunderstorms and rain bubbling up in a 
humid airmass with dew point in the lower 70s. In addition to the well 
defined shortwave with a subtle baroclinic Leaf signature over upper 
Mississippi Valley...other shortwaves are showing up over southern MO 
and southern Arkansas. These features affect upper lakes through tomorrow... 
then the broad trough over Pacific northwest into western Canada plows into 
the region to start the weekend. Some rain possible on Friday night... 
but main impact with be gusty winds and cooler temperatures for later 
Saturday into Sunday. 


&& 


Short term (tonight and friday)... 


Seems like we are on track for some much needed rainfall. Main 
chance of rain is midnight tonight through middle morning Friday. 


Shortwave and area of 850 mb-500 mb deep moisture/broad lift should begin 
affecting southwest County Warning Area with showers around 00z and northern County Warning Area by 
06z. Added lift comes via rrq lifting in jet streak over Ontario. 
Earlier soundings showed fairly low dewpoint depressions from surface-800 mb 
at saw/rhi. Only dry air was in the middle levels...but given large 
scale lifting...this should be eroded quickly as has been case 
upstream all day. Upstream observation indicate a 10-12 hour duration of 
rain...fairly light to start...but intensifying for a time after 
midnight. Could see up to a quarter inch of rain...but lack of 
convection should prohibit much more than that. Steady rain ends 
tomorrow morning...but low clouds remain trapped below inversion. 
Could not even count out some fog in north central with upslope south 
winds. Eventually...skies should partially clear in afternoon. Strong 
winds just above inversion will eventually result in a gusty 
southerly surface wind in the afternoon. Also of note will be the higher 
dew point...in the middle-upper 60s...making it feel quite humid. Temperatures should 
rise toward lower 80s but it may be a situation with late day highs 
given the clouds in the morning. 


With similar T/dew point regime compared to that seen in lower Mississippi 
Valley today...there is some potential for convection to re-develop 
over County Warning Area in the afternoon. Main negative factor is 850 mb-h65 capping seen on 
NAM/GFS soundings which is beneath the dry slot seen on WV loop this 
afternoon from central Ontario into western Minnesota and south into neb/KS. Kept low 
chance probability of precipitation in the grids in case some convection develops...but best 
chances appear in the far south and east closer to lingering higher 
850 mb-700 mb relative humidity and near the most likely track of any shortwaves moving up 
in SW flow aloft. 


Longer term (friday night onward)... 


Trough across the northern plains affects the area Friday night and 
Sat. NAM takes the upper level trough and a closed 500 mb low to the 
north of area with the clsoed low going from Lake Winnipeg 00z Sat 
to northern Ontario 00z sun to east of Hudson Bay in western Quebec 
00z Monday. NAM showing 850-500 mb q-vector convergence Friday night along 
with a narrow ribbon of moisture which is along a cold front. Both 
parameters move across the County Warning Area Friday night and across the eastern County Warning Area Sat 
morning before the moisture moves out by 00z sun. GFS similar to the 
NAM as well with dynamics and moisture moving through Friday night and 
Sat morning. Not real impressed with pop chances with band of 
moisture narrow along the front. Will continue to go chance probability of precipitation 
until it becomes closer to the event to see how much convection 
breaks out with the cold front. Put sprinkles in the north and east 
for sun based on cold 850 mb temperatures...upper trough and a bit 
of wraparound moisture in the area and lake enhancement could happen 
as well. NAM and GFS 850 mb temperatures drop to 1c to 3c at 18z sun 
and then temperatures modify. Lake temperatures are around 13c-16c 
over the lake...so enough Delta-T to put sprinkles and definitely 
some lake effect clouds into the forecast for sun as well. Did not 
make too many changes other than above to going forecast. Sun looks 
cool with highs struggling to get out of the 60s in most places. 


In the extended...GFS...ECMWF and Canadian show a 500 mb trough axis 
east of the upper Great Lakes 00z Monday with surface high pressure in the 
area then. 500 mb ridging moving in by 00z Tuesday Onward. Looks dry for 
the extended with systems staying west and south of the area. 
Followed the 00z European model (ecmwf) for this forecast and kept remains of Fay out 
of the area Thursday with a 500 mb trough remaining still to the west 
then. Temperatures look to stay near normal to slightly above 
normal. Did not make too many changes to the going forecast. 


&& 


Aviation (for the 18z taf issuance)... 


Scattered-broken high clouds out ahead of upper level system moving into WI 
this afternoon. Based on upstream observations...plan on MVFR ceilings and 
IFR visibility with rain showers to begin affecting both terminals shortly 
after midnight. Visibility improves once rain ends Friday morning as low level 
winds are not very favorable for additional upslope. As for 
ceilings...expect overcast MVFR to persist through at least midday Friday. May 
even see IFR ceilings at ksaw late tonight into early Friday morning. 


&& 


Marine (for the 4 PM Lake Superior forecast issuance)... 


Low pressure organizing over southern Manitoba by Friday morning and high 
pressure just off the East Coast will result in srly winds prevailing over 
much of Lake Superior. Winds strengthen tonight through Friday night as 
the low lifts NE and a cold front approaches. Wind gusts should 
increase to around 25kt...with gusts to around 30kt possible Friday 
night just ahead of the approaching cold front. As is typical for 
srly flow...the strongest winds are expected to occur over the east 
half of Lake Superior. Cold front crosses Lake Superior on Saturday. 
In the wake of the front...expect west to northwest winds of 15-25kt with 
gusts to 30 knots. Winds will then diminish from west to east by sun as high 
pressure builds into the upper lakes. With high pressure over the area... 
light winds are expected Monday and Tuesday. 


&& 


$$ 


Synopsis...jla 
short term...jla 
aviation...jla 
marine...jla