214 
fxus63 kbis 180812 
afdbis 


Area forecast discussion 
National Weather Service Bismarck ND 
315 am CDT Friday Jul 18 2008 


Short term...today through Sunday night... 
forecast concerns/highlights in the short term will be convective 
precipitation chances across the forecast area over the next couple 
of days. Forecast models remain in pretty good agreement with the 
overall large scale pattern through the weekend. Some differences on 
smaller scale features noted...which was expected due to convective 
feedback and model biases over the next 24-48 hours. 


Currently...a short wave upper level ridge is moving across the 
local region with high pressure at the surface. Mostly clear across 
west and central ND...with some upper level clouds advecting across 
the west from thunderstorm activity in eastern Montana. In regards to the 
thunderstorms...this nocturnal convection is being triggered by an 
embedded vorticity maximum moving across the northern High Plains of 
Wyoming and Montana. A few of these showers/thunderstorms may reach my far 
southwest counties later this morning as the embedded wave 
progresses further east. 


Will keep a mention of probability of precipitation over my far west...southwest...and south 
central counties through the morning hours as the above mention shortwave 
advects east across the Dakotas. Best chances will reside southwest 
and far south central. Thunderstorms look to redevelop across the 
west this afternoon as another embedded shortwave trough approaches the 
region...coinciding with diurnal destabilization and increasing cape 
values. Models indicate best 0-6km shear favorable for thunderstorm 
longevity will reside across the southwest...so have left higher 
probability of precipitation there along with severe wording. Difficult to ascertain where 
best chances for evening and overnight shower and thunderstorm 
activity will be due to the impact (if any) this afternoons storms 
will have (outflow boundaries and such). GFS...ECMWF...sref and 4km 
nmm WRF solutions are in fair agreement with suggesting an ongoing 
mesoscale convective system tracking across the ND/South Dakota border. Inherited grids and HPC quantitative precipitation forecast 
guidance agree well with this scenario so only a few edits were 
made. Will also continue to mention severe possibilities through 
tonight area wide. 


On Saturday daytime we may have ongoing morning convection from the 
night before...along with redevelopment of thunderstorms later in 
the day as a more dynamic upper level shortwave trough/closed low slides 
into southern Alberta during the day and across the ND/Canadian 
border Saturday night. Large scale ascent associated with this wave 
will combine with a weak surface trough moving across the County Warning Area and 
favorable shear profiles for increasing chances for strong to severe 
storms during the afternoon and into the evening hours. Best surface 
convergence and upper level dynamics are trending further to the 
north so have done the same with our grids. 


Upper level ridging Sunday will bring mainly dry and warmer weather 
with temperatures nearing the 90 degree mark in some locations. 


Long term...Monday through Thursday... 
forecast problem in the day 4-5 timeframe will be precipitation 
chances on Tuesday into Tuesday night. Overall medium range models 
are in pretty good agreement. Main differences in the day 4-5 
timeframe between the GFS and European model (ecmwf) center around a shortwave trough 
and its track into the northern plains. The GFS depicts a stronger 
upper ridge over the northern plains and the shortwave coming off 
the Pacific has a hard time making it into the local area. The European model (ecmwf) 
on the other hand brings the shortwave into the region...flattening 
the aforementioned upper ridge. 


Prior to this...models agree on a warm day on Monday with a surface 
trough over eastern Montana and a southerly surface flow over the 
region. Will keep our slight chance probability of precipitation over the southwest Monday 
afternoon and Monday night due to the possibility of an isolated 
thunderstorm developing in the vicinity of the trough. European model (ecmwf) does 
indicated some quantitative precipitation forecast in the vicinity of the trough Monday 
afternoon/evening...while the GFS remains dry. Will side with 
European model (ecmwf)/persistence here. Only some minor blending of temperatures as 
highs will range from the low-middle 80s east to around 90 west. 


For the Tuesday and Tuesday night time frame...will once again side 
with the European model (ecmwf) and persistence. We already have slight chance probability of precipitation 
in the forecast and the European model (ecmwf) certainly supports this. While the 
global Gem is dry and stronger with the upper ridge similar to the 
GFS...the European model (ecmwf) has some support from the gfsensemble which depicts 
a flattening of the upper ridge Tuesday night with some low 
probability probability of precipitation. 


Later in the long term period...we remain under the influence of the 
upper ridge...keeping temperatures above normal. We will still be 
along the periphery of the upper flow though...meaning an occasional 
risk of thunderstorms. 


&& 


Aviation... 
high pressure over southern prairie provinces of Canada...into North 
Dakota early this morning is keeping convection south of the state. 
Only some occasional middle-high clouds drifting by so except for some 
possible patchy ground fog from 12-14z...expect VFR conditions to 
start the period. Shortwave energy in a broad southwest upper flow 
will bring a chance of showers and thunderstorms into the west by 
this afternoon and spread east in the late afternoon through 
overnight. Will continue to monitor...but NAM guidance is suggesting 
areas of low clouds and fog in the wake of the convection tonight. 


&& 




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


$$ 


New Hampshire/twh