Weather

148 
fxus65 kbou 100948 
afdbou 


Area forecast discussion 
National Weather Service Denver Colorado 
345 am MDT Friday Oct 10 2008 


Short term...main upper level trough will intensify over the western US 
as strong surface low pressure resides over Colorado. First challenge to 
forecast today is location of main surface low and associated surface boundary. 
Latest NAM continues to show main low setting up to the south and 
west of northestern Colorado by afternoon as a cold front backdoors its way across the 
plains during the morning hours. Widespread stratus is developing 
behind this fnt over southeastern Wyoming early this morning and as this boundary 
moves south into northestern Colorado will likely see stratus fill in across much 
of the northestern plains and northern fnt range urban corridor by 12z. Main 
question is how far south will this low cloud deck get and when will 
it begin to burn off. At this point appears frontal boundary may end up 
stalling from just south of Denver to around Akron. North of this 
boundary just like we saw on Thursday temperatures may not get out of the 50s while 
south of the boundary with southwesterly low level flow highs may rise well into 
the 70s. On Thursday the low cloud deck stuck around much of the day so 
I expect a similar situation may occur today as well. I have 
dropped afternoon highs quite a bit north of expected location of boundary 
for this afternoon. Meanwhile in the mountains rather strong southwesterly flow will be 
in place through tonight. Some increase in moisture will occur through 
the day and as some qg ascent moves across later this afternoon into this 
evening shower/thunderstorm activity may pick up especially over zone 31. 
In addition an upper level jet maximum is forecast to move across the 
mountains which could enhance precipitation this evening. Soundings show wet bulb 
zeros lowering to around 10500 feet by midnight so will likely see 
some snow occur over the higher peaks. As far as precipitation chances over 
lower elevations appears best chance for showers from late afternoon through 
tonight will be in the northern foothills and over the plains near the 
Wyoming-Colorado border where some qg vertical velocities are forecast to occur. 
Meanwhile overnight a secondary surge of colder air will move across 
northestern Colorado which should allow for widespread lows clouds along with 
some drizzle and fog after midnight. 


Long term...models have a decent upper closed low over the southern 
Great Basin...with strong southwesterly flow aloft for the forecast 
area. This strong southwesterly flow aloft continues through late 
Sunday afternoon with the upper feature tracking northeastward into 
eastern Montana. The jet level wind speeds are 90 to 130 knots over 
the forecast area through all the periods. The qg vertical velocity 
fields show upward motion for the forecast area Saturday and 
Saturday night. The best is still well to the west and north of the 
forecast area with the system. There is upward motion on Sunday 
over the southern forecast area... downward over the north. The low 
level pressure and wind fields have upslope boundary layer flow for 
the plains Saturday and Saturday night...then again Sunday night. 
The upslope is better and somewhat deeper on the NAM compared to the 
GFS. Concerning moisture...it increases on Saturday...with both 
models showing the deepest moisture late Saturday afternoon through 
about much of Saturday night. The GFS dries it out quite a bit 
after that...the NAM does it slower. On Sunday...the NAM has more 
moisture than the GFS...which is pretty dry in most areas. The best 
moisture is over the western forecast area. The same GOES for 
Sunday night...the GFS continues drier than the NAM. On 
Saturday...there is a Theta-E axis over the southern and eastern 
forecast area. There is some cape as well. Quantitative precipitation forecast fields have the 
best precipitation from 00z Saturday afternoon through 12z Sunday 
morning. The NAM has more snow indicated...but the GFS does have 
some. It's mainly all in the mountains. Forecast soundings point 
to a 5000-6000 mean sea level snow level later Saturday night/Sunday morning. 
Will leave a mix going over the higher plains...all snow for the 
mountains and foothills Saturday night. So as the previous shift 
saw...the best precipitation window is from 21z Saturday afternoon 
through 06z-12z Sunday morning. Will go with "likely's then in all 
areas. No highlights for mountain snows at this time...orographic 
enhancement is poor with the nearly due southerly mountain top 
flow...but we'll see. For temperatures... both models have a 5 to 
12 c cool off for highs Saturday. Their agreement remains poor 
however. The GFS remains warmer. Sunday's highs cool a bit more 
from Saturday on the NAM. The GFS only has the northeast corner 
cooling down some more. For the later days...Monday through 
Thursday...many of the models have the upper trough axis pushing 
eastward across the forecast area Monday night into Tuesday. Behind 
it there is upper ridging. By Thursday a weak trough with strong 
northwesterly flow aloft behind it is prognosticated. Will make a few 
small changes to Monday...and leave the rest alone. 


&& 


Aviation...stratus will likely redevelop by 12z as a boundary moves 
south into the area with IFR ceilings at dia and bjc. Ceilings may 
improve to MVFR by by 18z however the stratus may not break up until 
around 21z. Over apa the boundary may stay north of the Airport so the 
threat of low clouds may be much lower. By tonigth a secondary 
surge of cooler air will move into the area with additional 
development of low clouds along with some fog and drizzle possible 
after midnight. Winds will be variable early this morning but then 
become east-northeast by 15z with speeds around 10 miles per hour. 




&& 


Bou watches/warnings/advisories...none. 


$$ 
Rpk/rjk