919 
fxus63 kmqt 062040 
afdmqt 


Area forecast discussion 
National Weather Service Marquette Michigan 
439 PM EDT Sat Sep 6 2008 


Synopsis... 
12z radiosonde observations/latest WV imagery and RUC analysis show large scale upper 
trough centered over central namerica between rdgs off the west and east coasts. 
Shortwave responsible for rain showers last night/this morning is heading northeastward into 
Ontario this afternoon...and subsidence in the wake of this system has 
contributed to the development of a middle level inversion around h75 as shown on 
12z mpx radiosonde observation/15z tamdar sdng from rhi that is likely spreading northeastward 
into the forecast area because the 12z radiosonde observation from grb showed inversion base at h65. 
Despite plenty of lingering low level moisture as shown on recent tamdar 
sdngs...the strengthening/lowering inversion as hinted as well on RUC 
forecast sdngs has caused most of the rain showers to dspt by 16z. The only 
exceptions are over the eastern zones...where backedge of lingering deeper 
moisture has not quite passed...and over the west closer to a weaker middle 
level inversion as shown on the 12z inl radiosonde observation. The west is also closer to a 
weak surface low present just west of cmx at 15z. Although the lowering 
inversion has tended to diminish the rain showers...the trapped low level moisture below 
the inversion base has resulted in quite a bit of low cloud. Where skies 
had clrd earlier over the scntrl...daytime heating resulted in the 
development of more low cloud. The back edge of this low cloud extends west into 
central Minnesota and S into scntrl WI. The 12z bis/abr radiosonde observations depict a drier 
surface-800 mb layer where there is an absence of low cloud near surface hi pressure ridge 
axis. However...there was some fog this morning in that area. Another 
fairly strong shortwave rotating through the mean trough in the plains is 
digging through Saskatchewan early this afternoon. Another shortwave rounding 
the base of the upper trough is noted over Iowa. 


&& 


Short term (tngt/sun)... 


Main forecast concerns during this time are cloud and rain showers trends/fog chances 
tonight... then timing/covg of precipitation on sun associated with shortwave now 
dropping through Saskatchewan. 


For tonight...shortwave now over Iowa is prognosticated to move east-northeast just S of the 
forecast area...so no concerns about precipitation associated with it even though some 
hi cloud on its northern fringes may impact mainly the scntrl and east through 
much of the night. Otherwise...larger scale 850 mb-5 qvector dvgc is forecast to 
dominate through the night...with surface hi pressure ridge now to the west expected 
to nose into the forecast area overngt. Despite these expectations...NAM 
persists in showing light quantitative precipitation forecast over the nctrl and east. Local hi res 
models also depict some isolated rain showers impacting mainly the north central and 
eastern zones despite the suspected larger scale trends. The GFS shows little 
in the way of quantitative precipitation forecast. Given this disparity in forecasts...opted to maintain 
some low chance probability of precipitation over the northern tier before loss of diurnal heating causes 
going rain showers to weaken in absence of sgnft dynamic forcing. The rain showers 
will persist longest over the east closer to departing low center moving 
into Eastern Lake supply/more resilient cyclonic flow. Since fog was obsvd 
upstream last night close to this ridge axis...will add mention of some 
fog late tonight under the upper drying over the interior west/scntrl closer 
to hi center passing to the S. Sharper pressure gradient/west-northwest flow over 
the north will minimize fog potential...but sustain low cloud. Lowest min 
temperatures over the interior near the WI border where low level winds are 
most likely to decouple. 


On sun...shortwave over Saskatchewan this afternoon is prognosticated to reach northern 
Minnesota by 00z Monday. Surface cold fnt associated with this feature will reach 
the central zones late in the day. Expect the day to start on the dry 
side with forecast sdngs showing a dry atmosphere...but aprch of shortwave/ 
fnt in the afternoon will increase precipitation chances. Negatives for precipitation include 
abundant dry air...surface dewpoints this afternoon ahead of the fnt in the 
Dakotas are in the low 40s...and relatively weak 850 mb cnvgc along the 
boundary as well as absence of any larger scale jet forcing/upper dvgc. 
Despite the negatives...introduced chance ts given obsvd lightning strikes 
upstream ahead of the fnt...GFS/NAM forecast 700 mb-5 lapse rates aprchg 
7c/km...and favorable peak heating time of day for arrival of boundary. 
Modified GFS forecast sdng for surface T/dew Point of 70/40 yields a modest SBCAPE 
up to 500 j/kg...but steep low level lapse rates/unidirectional westerly flow 
may result in gusty winds under the heavier rain showers. 


Long term (sun night through sat)... 


Sun night...shortwave bringing precipitation to the forecast area sun will move east 
of the forecast area by 12z Monday. Will thus diminish probability of precipitation from west to east 
overnight Sun night. Thunder potential will also end with boundary 
layer cooling. On Monday...if there are any lingering -shra over the 
east...they will end in the morning. Attention then turns to next 
shortwave dropping into the plains Monday and then lifting NE into the 
Great Lakes Monday night. NAM/GFS (especially gfs) are stronger and 
more nearly with associated surface low lifting into the great laeks. 12z 
UKMET/ecwmf offer a more suppressed weaker surface low while the global 
Canadian shows very little surface reflection at all. At this point... 
will lean toward the more consistent European model (ecmwf) solution which is 
strongly supported by the UKMET. Weak global Canadian solution also 
suggests leaning away from the GFS/NAM at this time. Impressive 
upper divergence in right entrance of 300mb jet will yield a 
significant stripe of precipitation lifting into the Great Lakes north of surface low 
track. However...in favoring the flatter European model (ecmwf)/UKMET solution...bulk 
of that rain will pass by to the southeast of the forecast area Monday night. 
Should a more nearly solution pan out...a much needed widespread 
rainfall would develop over the forecast area. Grids will be structured 
to show highest probability of precipitation southeast (still just chance category) with dry weather 
northwest Monday aftn/night. 


Wednesday through Sat...medium range models (ecmwf/UKMET/Canadian/gfs) are in 
good agreement on the overall pattern over namerica during this time 
frame. All feature a progressive regime across the northern Continental U.S. To the 
S of the polar vortex centering itself over north central Canada. There is 
one main shortwave trough of interest that will swing across the northern 
Continental U.S....and medium range models have been and continue to be in 
quite good agreement on timing the trough. Given agreement on the 
overall pattern and individual features...confidence during the 
Wednesday-Sat period is above normal. Surface high pressure over the western Great 
Lakes Tuesday night will slip quickly to the east. With high pressure over the 
forecast area for at least the early part of Tuesday night...will go at or 
below the low end of MOS guidance for mins...especially over the east. 
May see some patchy frost interior central and east. On Wednesday...surface low 
pressure trough will emerge over the Western Plains in response to the 
approaching shortwave trough. Warm air advection pattern will develop Wednesday...though it 
appears the best warm air advection will still remain west and SW of the forecast area 
through the day. In addition...low-level flow remains somewhat 
anticyclonic...especially as depicted in the ecwmf. For these 
reasons...considered removing probability of precipitation from all but Western Lake Superior. 
For now...will trim probability of precipitation back from the inherited forecast to include 
just the far western forecast area Wednesday afternoon. Rain showers/thunderstorms and rain will then progress east 
across the forecast area Wednesday night/Thu. With 12hr height falls nearing 
100m and a nice ribbon of deep moisture...this system offers the 
potential for widespread much needed precipitation though quick movement will 
work to limit amounts. For now...did not go any higher than the high 
chance range as model timing changes of 12hrs or more are certainly 
possible 4-6 days out into model runs. Eventually...high probability of precipitation should 
be required once timing is certain. Although some -shra/sprinkles 
may linger into Friday morning in backside cyclonic flow...will dry 
forecast out for Friday/Sat in the wake of system. As for temperatures...readings 
through the period should not stray too far from normal for early/middle 
sept. 


&& 


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


Expect MVFR ceilings to persist at least occasionally at both sites the 
rest of this afternoon with lingering low level moisture shown on recent tamdar 
sdngs/small surface dewpoint depressions in the presence of diurnal heating. But 
the trend should be for a gradual improvement with this surface heating lifting 
the ceiling base. MVFR ceilings will likely redvlp at cmx tonight with expected 
upslope westerly flow and diurnal cooling before the arrival of drier air 
now in western Minnesota tends to break up any low cloud. Saw will be more sheltered 
from this flow...so radiation fog is a possibility there. Then VFR 
conditions will be the rule after any morning fog Burns off at saw on sun 
through 18z. 


&& 


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


Expect low pressure over western Upper Michigan to drift into 
Eastern Lake Superior overnight. In its wake...a weak 30.0 inch 
ridge of high pressure will Cross Lake Superior late tonight into 
Sunday morning. Winds over the lake will be light during this 
period. A 29.9 inch low pressure over northwest Ontario early Sunday 
morning will move over James Bay late Sunday night...dragging a cold 
front across Lake Superior. Northwest winds behind this front will 
increase to 30 knots over the east half of Lake Superior. A low will 
form over northern Illinois Monday evening and deepen to as it moves 
into western Quebec on Tuesday. Look for high pressure to build over 
the Central Plains and Minnesota Tuesday afternoon and move to the 
eastern Great Lakes Wednesday evening. A low pressure trough... 
extending from western Ontario to the Central Plains Wednesday 
evening...will Cross Lake Superior Thursday. 


&& 


MQT watches/warnings/advisories... 
Upper Michigan...none. 
Lake Superior...none. 
Lake Michigan...none. 


&& 


$$ 


Synopsis/short term...kc 
long term...rolfson 
aviation...kc 
marine...kc 

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