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fxus63 kmqt 240800 
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Area forecast discussion 
National Weather Service Marquette Michigan 
400 am EDT Thursday Jul 24 2008 


Discussion... 


00z radiosonde observations/latest WV imagery and RUC analysis show hi amplitude trough 
centered over southeast Ontario...with sharp upper ridge from Minnesota into northwest Ontario 
between the trough and falling heights over the northern plains/scntrl can 
associated with a pair of strong shrtws riding east-northeastward out of a mean 
trough in the Pacific northwest/SW can. 00z grb radiosonde observation depicts a very dry...stable 
airmass (kinx 8...precipitable water 0.74 inch) dominating the upper Great Lakes over 
surface hi pressure centered across lake supply at 03z. There are some middle clouds 
moving into the eastern County Warning Area associated with shortwave rotating around deep 
trough to the east. The central County Warning Area is clear...but there are some hi clouds to 
the west pushing eastward through the upper ridge axis and over very dry...stable 
air also shown on the 00z inl radiosonde observation (kinx -13...precipitable water 0.55 inch 
there). Leading edge of this cirrus is tending to fade as it pushes eastward 
into the larger scale subsidence over Upper Michigan to the west of trough to the 
east. A very shapr gradient in 850 mb dewpoint is apparent from Minnesota into the northern 
plains. While 00z 850 mb dewpoint at inl is only 5c...the value is 17c at 
bis in return SW follow between ridge over the Great Lakes and troffing in the 
Pacific northwest. The interaction between the resulting instability shown on the 
00z bis sdng and lead shortwave aprchg Lake Winnipeg has resulted in some 
cnvctn as far east as the Minnesota/ND border. However...rather dry nature of 
the middle levels (00z 500 mb dewpoint depression 28c at bis) appears to be 
limiting covg of rain showers/thunderstorms and rain in that area. 


Main forecast concern tday through tmrw will be timing breakdown of ridge/dry 
air now in place and then covg/strength of cnvctn that can spread 
into the upper Great Lakes associated with shortwaves to the west. 


For tday...the day will start on the quiet side with ridge still in 
control. The low level flow will gradually shift to the SW once the surface-850 mb ridge 
axis shifts to the east. 00z NAM/GFS show isentropic lift on the 305k 
surface (arnd h75) resulting in incrsg relative humidity at that level over the west in the 
afternoon. 00z NAM/ruc13 also appear to generate some isolated cnvctn over 
the interior west half of the forecast area in the afternoon. But since the higher 850 mb 
dewpoints are to the west and not the SW (23z tamdar sdng from msp showed 850 mb 
dewpoint only 6.5c there in absence of 00z mpx raob)...prefer the drier 
00z GFS/00z local WRF-arw run and sister model run with kf 
convective scheme that generates scattered cnvctn only over Western Lake supply 
closer to the deeper large scale 850 mb-3 qvector cnvgc/DPVA associated 
with shortwave now near Lake Winnipeg that is riding into the upper ridge 
over Ontario. Have thus removed probability of precipitation over the land forecast area except for 
some schc values over the far west after 18z. Elevated moisture (mid cld) 
return/ weak destabilization shown on forecast sdngs warrants exclusion 
of ts except over far Western Lake supply. Going maximum temperatures forecast needed little 
adjustment. 


Second shortwave upstream that was moving through the northern rockies is 
prognosticated to reach near Lake Winnipeg by 12z Friday...further weakening the 
upper ridge in place over the Great Lakes as its accompanying surface cold fnt 
aprchs the Western Lake supply shore by 12z. With the low level flow forecast to veer 
more west overngt...looks like a better opportunity for the higher dewpoints 
to the west to spread into the forecast area. In fact...00z GFS/NAM show some 
sharper isentropic lift/moistening on the 300k surface spreading west-east during 
the night and supporting chance probability of precipitation. Will go with the higher probability of precipitation (40) across 
the northern tier closer to sharper 850 mb-7 fgen forecast by NAM/GFS. Adjusted 
probability of precipitation close to sharper isentropic lift...resulting in reduced probability of precipitation 
over the west with guidance showing drier air influx in the middle levels/ 
capping. Did increases probability of precipitation again late over Western Lake supply with aprch of 
cold fnt and where 12z European model (ecmwf) persists in showing sharper DPVA ahead of 
incoming shortwave. Moisture influx/clouds guarantee a much warmer overngt 
than early this morning. 


On Friday...00z GFS has trended a bit toward the 12z European model (ecmwf) showing 
sharper height falls with a shortwave that digs a bit more and exhibits a 
bit more diffluent upper flow in the left exit region of a more potent 
looking upper jet. But while the 12z European model (ecmwf) showed low level dry advection 
overspreading the forecast area even ahead of the cold frontal passage...the 00z GFS slows 
down the boundary and allows for additional moisture advection/much higher quantitative precipitation forecast 
as the dynamics interact with the greater moisture flux. The 00z UKMET 
appears to trend in the direction of the GFS...but is not as extreme 
even though its forecast 90kt 300 mb jet maximum in southern Minnesota by 00z Sat looks like 
the stronger 12z European model (ecmwf). Since the 00z GFS is such a drastic change 
from previous runs...will tend more toward the NAM/cndn/UKMET/local 
WRF-arw with kf convective scheme scenario...which supports the 
going forecast higher afternoon probability of precipitation over the scntrl and east where later arrival 
of cold fnt will be in better sync with daytime heating/possible lake breeze 
interaction. Despite the potential destabilization with drying 
aloft... weak low level cnvgc and the dry advection will limit probability of precipitation. New Storm Prediction Center 
day2 convective outlook has County Warning Area just north of slight thunderstorms and rain risk area... but 
incrsg unidirectional middle level winds/0-6km bulk shear aprchg 40 knots/ 
modified NAM forecast sdng for maximum temperatures in the low 80s and dewpoints around 
63 (vs explicit model forecast to 68) SBCAPE in excess of 1100 j/kg/mid 
level drying...suggest a potential for severe storms. 


Lingering evening cnvctn on Friday over the scntrl and east will diminish with 
cold frontal passage...influx of low level dry air...loss of diurnal heating and 
departure of left exit dynamics. But with trend toward more digging 
of shortwave/lower heights...brought backwash moisture probability of precipitation over Northern Lake supply a 
bit farther S to tickle the northern County Warning Area Friday night. But with bulk of deep 
moisture prognosticated to remain in Ontario closer to deep upper trough for middle 
Summer...nothing more than schc probability of precipitation warranted. GFS seems to be the 
outlier in maintaining higher backwash moisture/relative humidity over lake supply on Sat... 
but with trend toward deeper trough in southeast can...hesitant to discount 
its forecast scenario. With that trend toward lower heights in mind... 
tended toward the somewhat cooler GFS MOS temperatures for Friday ngt/Sat. 


Coordinated with apx/grb/dlh. 


&& 


Aviation (for the 06z taf issuance)... 


VFR conditions will continue through the period with high pressure 
in control and convection remaining to the west until Thursday night. 
Only thing convection will do is continue to give off some middle and 
high level clouds in this forecast through 00z and then will get 
close enough to have a probability group in kcmx Thursday night. 


&& 


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


Winds should be at or less than 20 knots through the period. Winds will 
remain out of the south today as the high pressure slowly shifts 
southeastward. The winds will veer to the west as the cold front 
slides across the lake on Friday and Friday night. Then when the 
high pressure system returns...winds will diminish and shift south 
again on Monday. 


&& 


MQT watches/warnings/advisories...none. 


&& 


$$ 


Discussion...kc 
aviation...GM 
marine...dlg 

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