790 
fxus63 kmqt 181735 aab 
afdmqt 


Area forecast discussion...updated 
National Weather Service Marquette Michigan 
135 PM EDT Friday Jul 18 2008 


Updated aviation discussion for 18z tafs 


Discussion (405 am edt)... 
00z radiosonde observations/latest WV imagery and RUC analysis show zonal flow across 
the northern tier Continental U.S. On northern flank of Summer-time ridge over the southern 
states. Disturbance responsible for the rain showers/thunderstorms and rain yesterday is now 
shifting east through southeast can and has taken the bulk of the deep moisture/ precipitation 
with it. Although drier air is spreading in aloft from the west...low level moisture 
lingers ahead of another shortwave now just east of Lake Winnipeg and its 
attendant cool fnt pressing eastward through Minnesota. So some fog and St has 
developed over the forecast area under the middle level drying. Isolated rain showers/thunderstorms and rain noted 
ahead of the cool fnt moving in from the west...but this activity appears 
to be waning with the loss of diurnal heating and some middle level 
drying/capping noted on the 00z mpx/inl radiosonde observations. Skies are mostly clear over 
the northern plains under hi pressure ridge to the west of Lake Winnipeg shortwave and 
where 700 mb/5 dewpt depressions are 10 to 20c. Next shortwaves of concern 
embedded within zonal flow are over the Pacific northwest/British Columbia. 


Main forecast concerns tday are fog/St trends this morning along with any 
chance of rain showers associated with Lake Winnipeg shortwave. Focus shifts to 
mainly temperatures tonight/Sat and then to timing next chance of precipitation 
accompanying shortwaves now in the Pacific northwest. 


For tday...expect fog this morning to burn off with return of diurnal 
heating. Otherwise...shortwave now over Lake Winnipeg prognosticated to reach Northern Lake supply 
at 18z tday and then the Quebec/Ontario border by 00z Sat. Concern 
is that there may be enough insolation to pop some cnvctn over the 
scntrl and east where passing cool fnt may interact with lake breezes in 
spite of dnva/drying aloft in the wake of departing shortwave overwhelms 
any sb instability. Local WRF-arw model run with kf convection 
scheme and NAM hint isolated rain showers/thunderstorms and rain will develop this afternoon over the 
scntrl and/or east. Modified sdng near esc for 80/65 yields sb cape 
near 1100 j/kg. But will go no more than a 20 pop given modest lapse 
rates/hint at some middle level capping/kinx forecast around 25. 


Hi pressure ridge/dry air will dominate under shortwave rdging...so expect a 
tranquil night with nothing more than some hi cloud spilling into the 
upper Great Lakes in zonal flow aloft. Various model MOS forecasts are similar 
showing the Mercury bottoming out near 50 over the interior...and 
this seems rsnbl. Will throw in patchy fog across the interior areas. 


Significant model differences become apparent on Sat as 00z NAM follows lead of 
12z European model (ecmwf) and brings a shortwave now in the Pacific northwest quickly east with 
fairly sharp DPVA/deep layer qvector cnvgc/deep moisture moving into the west 
late in the day. The GFS on the other hand is much weaker with this 
shortwave and farther S so that surface hi pressure/dry air is more resilient. 
Since the 00z Canadian/UKMET models support the NAM solution...will tend 
toward that scenario...which is closer to going forecast albeit a bit 
slower with arrival of dynamics/deep mstr/qpf. Since all guidance is 
dry through 18z...cut all probability of precipitation in that time period and lowered the numbers 
in the 18z-24z time period as well...maintaining schc probability of precipitation only over Western 
Lake supply/WI border. 


Favored 00z NAM/UKMET/cndn and 12z European model (ecmwf) models show upper dvgc/DPVA/ 
deep layer qvector cnvgc/deep moisture in left exit of supporting jet maximum 
of shortwave shifting eastward across the forecast area Sat night. Bumped up probability of precipitation about 10 
percent for now with the higher probability of precipitation across the southern counties closer to 
where NAM/UKMET show 850 mb warm fnt. Went no higher than chance probability of precipitation for 
now given dry GFS forecast...but later shifts will likely have to raise 
probability of precipitation if the NAM/UKMET/cndn models maintain continuity. 


Although responsible shortwave for Sat night precipitation chances drifts to the east on 
sun...shortwave now over British Columbia forecast to follow into the upper 
Great Lakes and maintain upper troffing and semblance of cyclonic NE flow over 
the upper lakes. Maintained chance probability of precipitation into Sun night to reflect persistence 
of the troffing/cyclonic flow. Then ended probability of precipitation faster west-east on Monday per Canadian 
model showing departing upper trough to transition to going dry forecast on 
Monday night. 


Coordinated with grb/dlh. 


&& 


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


Conditions will continue to improve at kcmx this afternoon and will 
go VFR soon. Kcmx will then drop back to LIFR tonight with light 
winds and plenty of low level moisture around to form fog and ground 
fog before heating in the morning mixes this back out. Ksaw...VFR 
conditions will give way to LIFR conditions again tonight with low 
level moisture and light winds around causing fog/ground fog. 
Heating will mix this back out Sat morning and will go back to VFR 
conditions. 


&& 


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


Expect fog to persist through much of the day on the lake due to 
light winds and abundant low level moisture. Drier air pushing 
across the lake tonight should scour out the fog...or at least make 
it less widespread. East to northeast winds expected to start the 
weekend as low pressure develops in the plains and a high pressure 
ridge is present over the upper Great Lakes. Northeast winds may 
reach 25 knots on Sunday as the low pushes into the central Great 
Lakes...then again Monday into Tuesday...this time from the 
northwest...as the low slowly exits the Great Lakes region. 


&& 


MQT watches/warnings/advisories...none. 


&& 


$$ 


Discussion...kc 
aviation...GM 
marine...jla 

National Weather Service Glossary of Abbreviations

Non-Expanded Version (with abbreviations)