Weather

800 
fxus63 kbis 112107 
afdbis 


Area forecast discussion 
National Weather Service Bismarck ND 
405 PM CDT Sat Oct 11 2008 


Synopsis... 


A very strong autumn storm is taking shape over the northern High 
Plains/northern plains as a middle/upper level low continues to 
deepen over northwestern Utah. Intense upper level diffluence 
over Montana is associated with upper level jet coupling...which is 
forecast to gradually push into North Dakota. Several low/middle 
level shortwaves are expected to eject out of the vortex before it 
finally pushes northeast across the plains. Thereafter...a more 
zonal general flow pattern is forecast to direct several weak 
shortwaves through the northern plains. 


&& 


Short term...Saturday through Tuesday night... 


The aforementioned autumn storm looks to be on track with previous 
forecast thinking. The forecast problem of the day is precipitation 
type. Finding the rain/snow demarcation line is always difficult 
when isentropic lift is paired with strong dynamics aloft...since 
this tends to result in more vertical isentropic surfaces and 
resultant frontogenetical circulations. This results in an 
environment more conducive to dynamic cooling on a mesoscale Beta 
scale...which means narrow moderate/heavy snow bands can occur while 
surrounding areas experience light rain. Along this line of 
thinking...is the idea that dynamic cooling on a larger scale can 
offset warm air advection effects. Forecasting this becomes a 
balancing act. 


All that said...it appears that more pronounced snowfall 
accumulations will remain confined to the western part of the state. 
Meanwhile...a sharp drop in accumulations from west to east should 
keep all of central North Dakota in rain...though after sunset 
Saturday evening a few snowflakes may mix in with the stronger 
bands. NCEP/HPC winter weather desk is calling for a moderate risk 
of 8 inches in western North Dakota. 


The big model discrepancy begins at 12 UTC Monday when the 12 UTC 
GFS becomes a strong outlier with an eastern Pacific trough that 
kicks the High Plains low out faster than the 12 UTC NAM/UKMET...09 
UTC sref mean..00 UTC European model (ecmwf). Complicating the problem is the fact 
that the NAM/European model (ecmwf) had some problems initializing winds in the base 
of the southwestern U.S. Trough and there is missing upper air data 
at desert rock Nevada. Will defer to NCEP/HPC knowledge and follow a 
two-thirds 00 UTC European model (ecmwf) one-third 12 UTC GFS for grids Monday 
through Tuesday night. 


Long term...Wednesday through Saturday evening 
near zonal flow on Wednesday will let a weak disturbance move 
through the area Wednesday night and Thursday and...consequently... 
have added a slight chance of precipitation to the forecast. Ridging will 
develop over The Rockies by Saturday keeping the remainder of the 
forecast dry. Temperatures will be below normal Wednesday...but normal for 
the remainder of the forecast period. 


&& 


Aviation...the surface low will move slowly into the plains tonight 
and Saturday...then move east of the forecast area Sunday and Sunday 
night. This will cause an extended period of adverse weather...with 
snow along the Montana border and west...and rain in central North 
Dakota with potential for thunderstorms in the east tonight. IFR conditions 
can be expected in the west through Sunday...with MVFR to IFR 
conditions in eastern North Dakota. 




&& 


Bis watches/warnings/advisories... 
Winter Weather Advisory from 7 PM CDT /6 PM MDT/ this evening to 
1 am CDT /midnight MDT/ Monday for ndz002-003-010-011-018-021- 
033-041-044. 


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. 


&& 


$$ 


Scheck/thunderstorm with snow showers