176 
fxus63 kmpx 081936 
afdmpx 


Area forecast discussion 
National Weather Service Twin Cities/Chanhassen Minnesota 
236 PM CDT Friday Aug 8 2008 


Discussion... 
latest water vapor imagery shows Sharp Ridge over the western 
Continental U.S....poking up through Alberta and Saskatchewan... perched 
between upper lows parked off the northwest coast and over eastern 
Canada. This ridge... and is slow erosion over the next week... 
will continue to be the main player in our weather with chances 
for precipitation tied to shortwaves rotating over it and descending into 
our area while the main baroclinic zone and truly soupy air 
remains off to the southwest. One such shortwave can currently be 
seen diving southeast through far southern Manitoba. The enhanced 
upper level jet along the southern periphery of this feature is 
helping to kick of some rain showers/thunderstorms and rain over the Dakotas and northwest 
Minnesota... and that activity should dive southeast across a 
decent portion of the forecast area tonight. This morning/S 
sounding from kmpx gave a good indication of the dryness of the 
antecedent airmass... so precipitation will continue to have a tough time 
being too widespread or consistently heavy... particularly with 
only marginal elevated instability present over the area. 
However... the shortwave and some middle level warm advection... 
along with the marginal instability should allow for the continue 
of some rain showers with embedded thunder... to move through the area 
tonight. Will continue to going probability of precipitation and not raise them too much. 
Thought they could possibly be raised... but with the paucity of 
coverage in currently observed precipitation am reluctant to do so at this 
time. The 12z GFS and NAM are in decent agreement through the 
short term... and a blend of their guidance and the going forecast 
will be used. 


Chances for precipitation with tonight/S shortwave will depart most of the 
area by late tonight... but will maintain some chance probability of precipitation over 
the south into Saturday morning with some weak qg forcing still 
indicated over that area along with showalter indices of 0 to -1c. 
Will sag the chances of precipitation farther southwest to and nearly out 
of the area Saturday afternoon... just leaving some low chance 
probability of precipitation in that area on the edge of the moisture gradient. The bulk 
of the area... including the Twin Cities should see increasing 
sunshine through the day along with dry weather. Saturday night 
looks to be mostly clear and dry and low level riding works over 
the area... although there is a slight chance for a rain showers/thunderstorms and rain over 
the far southwest with a shortwave possibly diving toward that 
area toward morning on Sunday. Sunday looks to be a dry and mostly 
sunny day over the entire area... with ridging present surface and 
aloft. Chances for precipitation will slowly encroach once again from west 
to east Sunday night and Monday as a more potent shortwave is 
prognosticated to move through the upper ridge and take the first crack 
and eroding it. This feature looks to bring a decent slug of DPVA 
and 925-700mb warm advection across the area from late Sunday 
night through Monday. Instability does not look overly 
impressive... with marginally negative 850-500mb differential 
Theta-E values coincident with the forcing and better instability 
lagging it to the west. However... there is still certainly 
sufficient instability to include thunder along with the chance 
probability of precipitation for rain showers late Sunday night over the west and across the 
remainder of the area on Monday. 


The medium range period is still somewhat fraught with uncertainty 
and overall low confidence in the forecast details. The medium range 
guidance is not in good overall agreement on the forecast details... 
mainly due to differences in how they break down the current 
western-central Continental U.S. Ridge during the start of the extended 
period... which leads to differences in the degree of troughing over 
our region by midweek as well as obvious differences in the passage 
of individual shortwaves and fronts. While the 00z European model (ecmwf) and GFS are 
in decent agreement through Sunday... differences increase by Monday 
and by Tuesday the GFS is much more aggressive with the development 
of an upper low near the US/Canada border north of Montana. The 
European model (ecmwf) and Canadian are in better agreement... and exhibit a bit more 
continuity with yesterday/S Canadian and HPC forecasts. The general 
notion here is that some ridging will remain tucked between an upper 
low over the northeast Continental U.S. And southeast Canada and an upper low 
over central Canada with the most substantial height falls in our 
area holding off until the end of the period. Forecast for the 
Tuesday through Friday period will be based on a blend of the European model (ecmwf) 
and Canadian solutions... which actually matches the HPC manual 
forecasts quite well. 


With a weak shortwave and frontal boundary swinging through on 
Tuesday... will continue to advertise chance probability of precipitation. The timing of 
things looks like the best chance could be Monday night into early 
Tuesday... so will have somewhat higher probability of precipitation over the east vs the 
west. Will shift chance probability of precipitation into the far east Tuesday night as the 
frontal moves east and washes out. Plan to keep some low probability of precipitation over 
the far east through Wednesday with some chance for rain showers/thunderstorms and rain on the 
backside of the eastern upper low. Shortwave ridging over the 
remainder of the area could keep things dry for Wednesday. Will 
allow for some chance of precipitation to creep back into the west Wednesday 
night as the surface trough axis to our west creeps eastward and 
perhaps a weak shortwave arrives on northwest flow. Will spread 
chances eastward Thursday through Friday as it appears this is when 
heights may finally fall more substantially and the main upper low 
will drop toward northern Minnesota... helping to propel a more 
coherent surface trough/front into the region. It still appears as 
through this will be a slow process... will will unfortunately need 
to blanket a fairly long period with some chance probability of precipitation... basically 
from Thursday through Friday... with some hint of west to east 
motion indicated in the details. Stuck close to HPC guidance for 
temperatures... but given the uncertainty in the timing of 
features... cloud cover... et cetera there is certainly some margin of 
error expected. 


A quick note... addendum... to the extended forecast discussion. 
The 12z GFS has trended toward the 00z European model (ecmwf) and Canadian 
solutions... which adds support to the discussion and forecast 
reasoning used in the HPC and local forecasts. 


&& 


Aviation.../18z taf issuance/ 


Main concern is trend of precipitation as it moves southeast across the area 
during the period. Appears best 09z sref rain probs moving from the 
kaxn-kstc-krwf-kmsp corridor mainly in the 00z-10z period. This 
matches 12z NAM and GFS fairly well. Will leave thunder out of taf 
sites as believe it will be fairly scattered in nature with mainly middle 
cloud deck associated with rain showers. Will bring ceiling down to 4k feet in cumulonimbus 
at most of these sites. Believe krnh/Keau will remain dry with some 
middle/high clouds. Forcing along warm front/short wave expected to exit to 
the southeast of the area during early Saturday morning...with 
mainly VFR ceilings. Southerly surface wind flow to become light/variable over eastern 
areas...becoming light east or northeast as boundary sags into 
southern Minnesota. 


&& 


Mpx watches/warnings/advisories... 
Minnesota...none. 
WI...none. 
&& 


$$ 


Trh/dwe 

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