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fxus63 kbis 182032 
afdbis 


Area forecast discussion 
National Weather Service Bismarck ND 
332 PM CDT Friday Jul 18 2008 


Short term...Friday through Monday night... 


The main difficulty with todays forecast is nailing down 
timing...areal coverage...and convective Mode of precipitation 
Friday night through Sunday evening. 


While the 12 UTC NAM/GFS...00 UTC European model (ecmwf)...09 UTC sref mean all key 
in on southwestern North Dakota for Friday afternoon/evening...the 
remainder of western and central North Dakota has little agreement 
on precipitation. 


The last couple runs of the NAM have indicated a second cluster of 
thunderstorms that develops over northeastern Montana and clips 
northwestern North Dakota. The 00 UTC 4 km nmm WRF...which is only 
20 or 30 miles north of dead on with the elevated convection along 
the North Dakota/South Dakota border...also develops an mesoscale convective system that 
clips northwest North Dakota. 


Meanwhile...the 06 UTC GFS is plagued with grid scale feedback over 
southeastern Montana/southwestern North Dakota. Given the 
situation...there is a good chance of developing convective feedback 
and a resulting mesoscale convective vortex...which would make this a 
reasonable forecast...just on too large a scale. 
However...subsequent forecast hours indicate questionable 
propagation of said mesoscale convective vortex/feedback vorticity center. That said...would 
tend to stray from the GFS solution by Friday evening. 


The 00 UTC European model (ecmwf) seems to be a nice compromise between the two 
solutions. Again...forcing is maximized in the southwestern part of 
the state...but the model does allow for some convection to move 
across northwestern North Dakota. 


It is Worth noting that NCEP/Storm Prediction Center has placed 
southwestern North Dakota in a slight risk for severe storms...with 
wind and large hail the primary threats. Isolated tornadoes are also 
possible through the evening. 


For Friday night...it appears that the Friday evening complex will 
most likely propagate eastward across southern North Dakota 
overnight. Additional convection is possible across most of the 
state as favorable middle level dynamics spread eastward over 
convectively unstable thermal profiles. To compose the forecasts...a 
blend of the 12 UTC NAM...00 UTC 4 km nmm WRF...00 UTC European model (ecmwf) seems 
most appropriate. The threat for severe weather should continue 
overnight with hail being the main threat and wind a secondary 
problem. 


On Saturday/Saturday night...the primary middle/upper level 
shortwave is forecast to swing through western North Dakota with a 
strong negative tilt. Thermal profiles in the 12 UTC NAM/GFS 
indicate 1500 to 2000 j/kg surface based cape...and shear appears 
sufficient for all convective modes. Storm relative flow vectors 
would suggest a line of storms with embedded supercells. NCEP/Storm 
Prediction Center convective outlook for Saturday is also a slight 
risk...but strong dynamics in place could be an indication that the 
risk will increase as timing uncertainty is reduced. 


A few residual showers associated with cyclonic flow aloft are 
possible after the cold front passage on Saturday night...but 
chances for precipitation should drop off considerably by Sunday 
afternoon. 


Temperatures should remain near climatology until middle/upper level 
heights rise in the wake of our weekends trough. Then...temperatures 
should begin to rise steadily into the extended range. 


Long term...Tuesday through Friday night... 
the GFS and European model (ecmwf) are in agreement Tuesday swinging a short wave 
across The Rockies and into the forecast area Tuesday night. This 
period has the best chance of precipitation with the usual Lee side low 
developing as the short wave moves across The Rockies. Southeast 
low level winds will advect moisture into the area...increasing 
the cape with time. As the low moves east...it will be the focus 
for convergence and potential for thunderstorms. The models begin to 
diverge in their solutions by midweek and will trend toward the 
European model (ecmwf) but for now will not mention precipitation. Temperatures should be 
seasonal through the period. 




&& 


Aviation...looks like an active weather night as thunderstorms 
in western North Dakota move eastward across the state. K index 
is above 35 which is a good indication that the thunderstorms 
could produce heavy rain. In addition....there is some potential 
for MVFR or IFR conditions in the southwest later tonight due to 
abundant low level moisture developing from the thunderstorms 
combined with light surface winds and diurnal cooling. 
Otherwise...except for the thunderstorms...conditions should be 
VFR through the night. 




&& 


Bis watches/warnings/advisories... 
none. 
&& 


$$ 


Jws/thunderstorm with snow showers