936 
fxus63 kbis 081711 
afdbis 


Area forecast discussion 
National Weather Service Bismarck ND 
1111 am CST Thursday Jan 8 2009 


Update... 


What a mess...the previous forecast looks generally on track with 
amounts and timing...although the latest 12 UTC NAM does slow down 
the onset of precipitation. Have elected to slow precipitation 
down very slightly...though the NAM generally does not verify as 
good as the faster GFS/ECMWF. Have updated amounts with NCEP/HPC 
latest quantitative precipitation forecast and adjusted the band of heaviest snow to be just 
northeast of the 700 mb low...which passes just southwest of Bismarck 
in the 06 UTC GFS. Expect that amounts on the order of 5 to 8 
inches will pass through Williston/Minot/Bismarck/Jamestown. 


Mixed precipitation really complicates this system. Dickinson 
reports snow and ice pellets...while the Painted Canyon is 
reporting accumulating ice. The echoes in Williston have yet to be 
verified as precipitation reaching the ground...but over a quarter 
inch of ice has been reported in northeast Montana. Expect up to a 
quarter inch of ice accumulation is possible...with southern 
McKenzie County being the most likely to receive higher amounts of 
ice. However...uncertainty is too great to upgrade current 
advisory to Winter Storm Warning or Ice Storm Warning. Will leave 
headlines as they are and adjust for higher amounts and mention 
0.10 inch of ice accumulation possible. 


&& 


Previous discussion... /issued 405 am CST Thursday Jan 8 2009/ 


Short term...today through Saturday night... 
lots of issues to contend with this morning with a complex storm on our 
doorstep. The strong baroclinic zone draped across the region will 
serve as the focus for a banded precipitation event worthy of warning 
criteria +sn...but exactly where is the question. The stubborn 
Arctic air is making it difficult to assume that the band will be 
all that far NE...especially in light of the 00z GFS/NAM which shift 
the greatest quantitative precipitation forecast back SW toward a kisn-kbis-kjms line. The 00z European model (ecmwf) is 
not much help in this regard as it maintains a solution reminiscent 
of its predecessors with the +sn from kisn-kmot-kdvl. 


Having said that...the breadth of the cold air combined with the 
trend seen even in the 03z sref and 06z NAM toward the 00z GFS/NAM 
solutions has given US confidence to trend toward the latter. Much of 
the forecast was build around the 00z GFS since it has a much better 
handle on the surface low dropping southeast today...deepening it more 
realistically than the NAM through 06z Thursday. In addition...its low level 
temperatures are colder...a scenario that has had merit for quite some 
time given the deep snow pack. Given all of this...we upgraded all 
of the watch to a Winter Storm Warning...and after much discussion 
went ahead and did the same for the bis/man area. We will change 
the freezing rain advzy in SW ND to a winter weather advzy and extend its time 
through 00z Sat in order to simplify the headlines. The Arctic 
outbreak methodology/check list agreed with an advzy in the SW. 


Low/middle level frontogenesis ramps up quickly through 00z as the 
entrance region of a 110kt+ jet streak moves into the region. The 
result is a strong area of q-vector convergence in the vicinity of 
the 850-700mb frontogenetical forcing. Cross sections even suggest 
some instability above that layer with some folds noted in the Theta 
sfcs around 00z. Looking at all of this from the perspective of 
the isentropic sfcs at 290/295k provides much the same story with 
2-4 g/kg mixing ratios suggestive of 4-8 inches of snow per the 
Garcia method. There/S no question of forcing or moisture /with 
precipitable waters  of 0.50 inches/ but again just where all of this sets up 
shop is somewhat troublesome. There will likely be a tight snow 
gradient along the thermal ribbon. Again...in trending toward the 00z 
GFS we have arrived with liquid amounts near 0.50 inches centered 
near kn60. Using an average 12:1 liquid/snow ration nets storm totals 
of 4-8 inches from kisn-kbis-kabr and north. Admittedly...unless 
the 00z European model (ecmwf) is correct our far NE County Warning Area may not meet warning 
criteria snow...but we did not choose to split hairs over this at 
12+ hours from the event. The 700mb low tracks southeast through central ND 
tonight and to its right will lie the heaviest snow. 


Bufr soundings from the 00z GFS/NAM both suggest less risk of freezing rain 
from kisn-kbis than earlier thought. Warm layer temperatures only get to 
about 1 c...not sufficient for complete melting. There/S a 
different story from kdik-khei and SW though...where the layer 
near 850mb still gets to +2-4 c. Thus we moved the freezing rain threat a 
bit further SW...but still expect it to be a problem in areas. 


Forcing wanes Friday...but by then strong cold air advection begins on Stout nearly 
flow. 850mb temperatures fall to near -15 c on 30kt winds...and expect to 
mix a good part of this to the surface as nothing less than impressive 
pressure rises dump into wstrn ND. Note the 6 hour pressure rises of 
15-20mb. Forecast soundings east of a kisn-kbis line only have about 
25kts of wind to mix...so while we may not have to worry about 
true blizzard conditions...the deep snow and accumulation from 
tonight will make for sig blsn. This helped tip the scales toward 
issuing the warning in Burleigh/Morton counties. 


Long term...Sunday through Wednesday... 
GFS and European model (ecmwf) are in agreement in transitioning to a more meridional 
flow over the U.S. In the day 4-5 timeframe with an upper ridge in 
the eastern Pacific and a mean trough over the eastern U.S. This 
will bring another surge of Arctic air into the region on Monday. 
Models differ on the track of the clipper system moving through the 
area on Sunday that Ushers in the Arctic air. The GFS track is 
farther to the east than the European model (ecmwf). The GFS would bring more snow to 
the east...while the European model (ecmwf) solution would bring a better chance of 
snow to the west. Will compromise and broad brush chance probability of precipitation across 
the County Warning Area on Sunday into Sunday night. Made only minor adjustments to 
probability of precipitation Monday and Monday night. With Arctic air settling over the 
area...Don/T think there would be much in the way of any 
accumulating snow. The European model (ecmwf) is farther east with the Arctic high 
and does bring some warm advection snow into the far west Monday 
night...while GFS keeps warm air advection snows west of the County Warning Area. Current package 
does have slight chance probability of precipitation in the west so see no reason to mess 
with it now. Would probably have to add it back later if we did take 
it out. 


As far as temperatures are concerned...no changes were made to maximum 
temperatures on Sunday. Beyond this...we lowered temperatures 
significantly...especially north and east. 


Another problem will be winds on Sunday night through Monday 
evening. Strong gradient will be in place between the exiting 
clipper and the approaching Arctic high. Will likely need highlights 
for wind chills. Depending on the amount of snow with the clipper 
and all the snow already on the ground...could definitely see 
problems with blowing and drifting snow as well. 


And by the way...in the latter portions of the extended...do not see 
a big change. Cold air will remain over the region. Any warmup would 
likely be associated with some warm advection snows followed by a 
reinforcing shot of cold air. 


Aviation... 
low pressure will move southeast across the region through the 
forecast period...bringing a swath of snow from northwest to 
southeast. Will see MVFR-IFR ceilings spread southeast from Williston 
through Minot and Bismarck to Jamestown. Models have shunted the 
warm air intrusion farther south. Therefore will likely only bring 
freezing rain into the dik area...keeping snow over the remainder of 
the taf sites. 


&& 


Bis watches/warnings/advisories... 
Winter Storm Warning until 6 PM CST /5 PM MST/ Friday for 
ndz018>020-034-035-047. 


Winter Weather Advisory until 5 PM MST Friday for ndz031>033-040- 
041-043-044. 


Winter Weather Advisory until 6 PM CST /5 PM MST/ Friday for 
ndz042-045-046-050. 


Winter Storm Warning until 6 PM CST Friday for ndz001-009-017. 


Winter Storm Warning from 6 PM this evening to 6 PM CST Friday 
for ndz002>005-010>013-021>023-025-036-037-048-051. 


&& 


$$ 


Cs/twh/scheck 

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