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fxus63 kbis 110913 
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Area forecast discussion 
National Weather Service Bismarck ND 
413 am CDT Sat Oct 11 2008 


Short term...today through Monday night... 
forecast concerns/highlights in the short term period will focus on 
the large Fall storm system impacting the local region over the next 
2-3 days. Specific challenges remain precipitation type and amounts 
today through Sunday. Model guidance is in decent overall agreement 
over the first 24 hours or so...with solutions then unfortunately 
diverging rather drastically Sunday into Monday. The good news is 
there is good agreement with the brunt of the impending storm system 
tonight into Sunday morning. Have used an overall blend here. Later 
Sunday and into Monday the 12z ec is an outlier and much faster than 
other guidance. For this period (sunday night through Monday night) 
will side closer to a blend of the slower 00z GFS and latest sref 
guidance. 


Currently...the large closed upper low continues to deepen across 
the southwestern United States. Closer to home an area of moderate 
to heavy rain is pushing into eastern ND/western Minnesota at this time...where 
strong isentropic upglide is migrating ahead of a stationary 
boundary stretched from central New Brunswick northeast through central Minnesota. 
Enhancement in precipitation is also underway just to our south- 
southwest across eastern Wyoming/western South Dakota as another area of low 
condensation pressure deficits and 15-25kts of isentropic lift at 
the 300k surface advects north. This shows up best with the latest 
GFS...which if holds true will bring light precipitation across all of 
western ND this morning most likely as a rain and snow mix given 
surface temperatures are in the middle/upper 30s and 850mb temperatures are -2c. Will 
take one last look at short term guidance and latest radar before 
finalizing. 


The upper low starts to slowly lift northeast today into the 
intermountain west region of southeastern Idaho/northern Utah. As this 
occurs weak warm air advection gradually shifts northeast this afternoon across the 
forecast area...with decent Theta-E advection pushing north into the 
northern plains 18-00z. Isentropic upglide looks to thus increase 
middle to late afternoon and should see showers increasing in aerial 
coverage from Dickinson east through Bismarck to around the Devils 
Lake basin area and points south. Precipitation type will likely be 
in liquid form with a possible mix west underneath subfreezing temperatures 
aloft. 


The main event is tonight into Sunday morning. Upper level 
divergence in the left exit region of an upper level jet streak 
rotating around the closed low develops surface cyclogenesis over 
eastern Colorado this afternoon and evening. As the jet streak continues 
to rotate counter-clockwise around the upper low and lifts 
northeast...divergence aloft increases dramatically across the 
northern plains...which in turn further develops surface low 
pressure and results in an increase in the strong moisture 
transport/Theta-E advection already underway over western and 
central North Dakota. Nearly 50kts of isentropic flow on the 300k 
Theta surface in the 850-700mb layer within the warm conveyor belt 
of the Colorado low will be present throughout tonight. This is 
similar to what models had advertised the past few days. Southerly 
flow at 850mb will transition to the east at 700mb within the 
trowal feature of this system over western ND. Thus an enhanced 
precipitation area will exist over my western counties with high 
quantitative precipitation forecast amounts expected (1-2 inches of water). In addition...strong 
support both at the surface and aloft across western ND as coupled 
circulation of the lower jet and upper jet will enhance the upward 
vertical motion. Biggest challenge though is precipitation type 
with both thermal and dynamic variables in play. Looks as though 
precipitation will change over to all snow along the ND/Montana border 
early tonight...with a sharp cutoff another County or two to the 
east of this line where precipitation remains all rain. Thus it still 
looks as though a heavy wet snow event (6 to possibly 14 inches) 
is in the offing for far western ND so have gone ahead and 
upgraded the Winter Storm Watch to a warning. Have also placed a 
Winter Weather Advisory one County to the east of the warned area 
to account for mixed precipitation and chances for light snow 
accumulations if dynamic cooling produces all snow. Speaking of 
which...this will need to be monitored in case an upgrade to a 
warning is needed a bit further to the east. The upper low moves 
into the region Sunday and is slow to move out as per the GFS and 
sref solutions. Thus have winter headlines valid from 00z this 
evening through 06z Sunday evening to account for timing 
uncertainty. 


The upper low finally pushes east across the local region Monday 
and Monday night...where we may still see isolated-scattered rain or snow 
depending on surface temperatures as temperatures aloft support all light snow. 




&& 


Long term...Tuesday through Saturday... 
system influencing the northern plains through the short term period 
finally moves out of the area on Tuesday. A northern stream 
shortwave trough moving across the prairie provinces of Canada will 
drag a cold front across the County Warning Area Tuesday afternoon into Tuesday 
evening. Little moisture associated with this system over our local 
area so will keep forecast dry. Some colder air will drop south 
behind the cold front Tuesday night into Wednesday. Adjusted 
temperatures a bit to make Wednesday a bit cooler than Tuesday. 
Surface high moves across the area Tuesday night through Wednesday. 
GFS and European model (ecmwf) bring another system into the region in the latter 
periods of the extended period. European model (ecmwf) is more vigorous with the 
shortwave trough and the resultant surface pressure pattern...but 
both bring some light quantitative precipitation forecast into the west Wednesday night. This is 
handled well by the previous forecast so little changes expected to 
probability of precipitation Wednesday night. 


This system looks wetter than previous runs of the GFS and 
European model (ecmwf)...so day shift may need to raise probability of precipitation on Thursday if models 
continue this trend. Current global Gem is weaker with this system 
and dry across our County Warning Area on Thursday so still some uncertainty. Models 
indicate a warming trend toward the weekend. 


&& 


Aviation... 
widespread MVFR-IFR ceilings and visibilities will persist through the forecast 
period. Could see a few periods of VFR ceilings today...but will lean 
toward the pessimistic side. Tonight...expect IFR-LIFR ceilings and 
visibilities across the County Warning Area. Main problem will be precipitation type. Will confine 
snow to dik and isn taf sites...with rain at bis Mot and jms. 






&& 


Bis watches/warnings/advisories... 
Winter Storm Warning from 7 PM CDT /6 PM MDT/ this evening to 1 
am CDT /midnight MDT/ Monday for ndz001-009-017-031-032-040-043. 


Winter Weather Advisory from 7 PM CDT /6 PM MDT/ this evening to 
1 am CDT /midnight MDT/ Monday for ndz002-010-018-033-041-044. 


&& 


$$ 


New Hampshire/twh 

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