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fxus63 kbis 211618 
afdbis 


Area forecast discussion 
National Weather Service Bismarck ND 
1118 am CDT Thursday Aug 21 2008 


Update... 


Expanded likely precipitation chances for tonight and added 
mention of severe to grids this morning based on higher confidence 
in storm evolution this evening/tonight. NCEP/Storm Prediction 
Center convective outlook calls for a large hail and damaging wind 
threat...so have indicated such. 


Right now...expect some afternoon/early evening convection across 
north central North Dakota as peak daytime heating coincides with 
cold air advection in the middle levels to destabilize 
thermodynamic profiles. This is consistent with the 4 km WRF nmm run. 
Shear profiles support multicell clusters with embedded 
supercells. After sunset...a second fast-moving squall line 
should progress out of eastern Montana and sweep across the area 
overnight. As cold pools merge...expect a widespread damaging 
wind event...though there is some uncertainty concerning low-level 
lapse rates allowing for higher winds to mix down. If earlier 
convection occurs in such a way to stabilize the low- 
levels...wind threat would decrease. Likewise...diurnal 
stabilization of low-levels could inhibit mixing down if the 
timing of the nocturnal squall line is too fast because it would 
allow for more radiational cooling prior to arrival. 


&& 


Previous discussion... /issued 339 am CDT Thursday Aug 21 2008/ 


Short term...today through Saturday night... 
forecast concerns/highlights in the short term period will be 
another day of strong winds over south central and east central 
North Dakota today...convective chances today through Friday 
daytime...and yet another day of advisory level winds on Friday. 
Models are once again in fair agreement so went with a blend. 


Currently...an upper level trough continues to eject northeast into 
south central Canada ahead of a potent closed upper low now moving 
onshore over the Pacific northwest. Slow eastward moving surface 
trough is stretched from far southeastern Montana...through western 
ND...northeast into south central Canada. Elevated thunderstorms are 
currently ongoing along the trough axis and have prompted several 
warnings across northwest ND this early morning. Some weak echos 
depicted over northwest South Dakota as well...most likely induced by the 
nocturnal low level jet. Will continue to monitor any further development of 
these echos as they progress into central ND through the overnight 
hours. Winds also continue to be on the breezy side across central 
portions of the state as the pressure gradient remains tight ahead 
of the surface trough. 


Another day of warm and windy conditions today as the surface trough 
slowly inches eastward across the western Dakotas. Strongest winds 
(sustained 20-30 with gusts up to around 40 mph) will once again be 
over the south central and eastern portions of the forecast area and 
will issue another Wind Advisory for these areas. Will continue to 
monitor convection this morning...but currently have isolated probability of precipitation 
primarily northwest and north central along the trough axis. If 
low level jet induced storms develop early this morning may need to modify 
12-18z probability of precipitation further to the south before finalizing the forecast. 
Kept mainly slight chance probability of precipitation for this afternoon along and behind 
the trough axis/surface cold front (west and north central) as we look 
to be strongly capped in the warm sector (south central and 
southeast). Noticed however that latest model runs now bring an 
embedded shortwave northeast across the state 18-00z...with an area of 
enhanced surface convergence over the northeastern County Warning Area around 
Pierce...Bottineau...and Rolette counties. Cap is also weaker here 
so upped probability of precipitation a bit and will mention severe. 


The strong closed upper low moves into southern Saskatchewan and 
northeastern Montana tonight...finally ejecting the surface trough 
eastward and pushing a strong cold front west to east across North 
Dakota through the overnight hours. Kept highest probability of precipitation northwest and 
north central tonight where the greatest middle/upper level dynamics of 
the closed upper low will track. Ample 0-6km shear in place over 
much of west and central ND this evening and overnight so mentioned 
severe within the zone forecast product for all but the southeastern counties. 


Continued chances of precipitation Friday daytime across the north 
as the upper low tracks across south central Canada along with some 
weak instability in place. The main story for Friday however will be 
the affects of the strong cold frontal passage from tonight as much cooler air 
moves into the local region along with gusty westerly winds 
developing west to east overnight Thursday night into Friday 
morning. Another Wind Advisory looks likely for Friday with peak 
gusts expected to be up to around 35 - 45 miles per hour...this time 
encompassing all of west and central ND. Not expecting elevated fire 
weather concerns on Friday despite the strong westerly winds and dry 
air advecting into North Dakota as temperatures will mainly be in the 70s. 
However...if surface dewpoint temperatures mix out more than expected fire 
concerns may still become problematic so will closely monitor over 
the next 24 hours. 


Saturday should be mostly sunny and pleasant as surface high 
pressure dominates the weather. Temperatures recover a bit from 
Friday but will still be cool with highs ranging from around 80 
southwest to the low/middle 70s north central. 


Long term...Sunday through Thursday... 
ridging aloft Sunday and Monday will keep a high pressure system 
over the area with warming temperatures and little chance of 
precipitation. By Tuesday morning the ridge will move east as the 
next trough moves into The Rockies pushing a surface trough into the 
Central Plains. Increasing conditional instability combined with 
surface convergence along the trough will lead to a chance of 
thunderstorms Tuesday. The GFS and European model (ecmwf) differ on the movement and 
intensity of the trough...with the GFS digging the trough into the 
central rockies and closing it off as an upper level low...while the 
European model (ecmwf) keeps the trough moving east as the next ridge moves onto the 
West Coast. Either way...there would be a continued chance of 
precipitation through most of the rest of the extended period. 
Temperates would trend toward normal or a little lower. 




Aviation...strong winds and the potential for thunderstorms will be 
the main aviation problems today. Any convection that occurs 
initially should be high based...but should lower through the day as 
continued southeast winds advect moisture into the area. The surface 
trough was in the western Dakotas this morning but should move 
slowly east through the day. Surface winds east of the trough will 
be strong from the south 15 to 30 kts early this morning...but 
increasing to 25 to 35 kts with stronger gusts by late morning. A 
strong southerly jet at 3 to 10 thousand feet above ground level with winds up to 
45 kts can be expected in central North Dakota this morning... 
transitioning east through the day. 


&& 


Bis watches/warnings/advisories... 
Wind Advisory until 9 PM CDT this evening for ndz005-013-022-023- 
025-034>037-045>048-050-051. 


&& 


$$ 


New Hampshire/tsw/scheck 

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