Weather

975 
fxus65 kbou 071020 
afdbou 


Area forecast discussion 
National Weather Service Denver Colorado 
415 am MDT Tuesday Oct 7 2008 


Short term...dry and stable air will be in place over the entire 
state today as a broad upper ridge builds over the southwestern 
U.S. Moderate westerly flow aloft will continue over the mountains. 
At the surface...weak pressure gradients will lead to a light wind 
field through today and tonight on the plains. Temperatures today 
and tonight should return to typical seasonal values under sunny 
skies. Temperatures this morning have fallen to the lower 30s in 
normally cooler locations. Will let this mornings advisory run 
through 9 am. With the airmass moderating over the next 24-48 
hours...nighttime temperatures should not fall as far as the did 
this morning. Will not need any additional highlights for frost or 
freezing. 


Long term...westerly flow aloft will be over the area on Wednesday and 
then become more southwesterly on Thursday in advance of an intense upper level 
trough that will eventually develop over the western US. With dry air and 
subsidence over the region both days will be precipitation free. As far as 
temperatures both NAM and GFS a weak fnt is forecast to affect northestern Colorado on Wednesday 
which will have the greatest affect over the far northestern plains. Thus 
will have the warmest temperatures closer to the foothills with slightly 
cooler readings along the Colorado-WY-NE border area. 850-700 mb temperatures 
would support highs near or slightly above seasonal norms. On Thursday 
with the low level flow becoming more southwesterly should see warmer temperatures 
as 850-700 mb readings rise around 4 degrees c which should allow 
for highs over northestern to rise well into the 70s with possibly 80 
closer to the foothills. 


By Friday an upper level trough will intensify over the western US with 
increasing southwesterly flow aloft over northern Colorado along with an increase in middle 
level moisture. Meanwhile the GFS is showing a cold front moving into 
northestern Colorado by Friday afternoon or evening. High temperatures over northestern Colorado on Friday will 
depend on the timing of the fnt. If the fnt comes in quicker then 
highs will be considerably cooler over northestern Colorado with only 50s and 60s 
for afternoon highs while if frontal passage holds off until late afternoon or 
evening then highs would be warmer. 


For Friday night into Sat a strong closed low will develop over the Great 
Basin with south-southwest flow aloft across northern Colorado with an increase in 
moisture. There are considerable differences between the European model (ecmwf) and 
the GFS with the handling of surface features by Friday night into Sat. 
The European model (ecmwf) has strong surface low pressure over eastern Colorado while keeping the 
main cold front further north over Wyoming while the GFS shows surface high 
pressure building south across northestern Colorado with upslope flow developing 
as cold front moves into southeastern Colorado. As a result there are huge differences 
in temperature and precipitation forecasts between the two models as the GFS is 
colder and wetter over the foothills and northestern Colorado while the European model (ecmwf) is 
drier and warmer. At this time its not clear which solution is the 
best so see no reason to drastically change the previous forecast. In 
the mountains although moisture will be increasing and lapse rates will 
become more unstable 700 mb winds will be south-southwest from Friday night through 
Sat which is not a favorable direction for orographics. 


By Sat night into sun both the European model (ecmwf) and GFS agree somewhat that the 
strong upper level low over the Great Basin region will eventually lift 
northeast across the northern rockies into the northern hgh plains as a 
secondary weaker trough moves slowly across northern Colorado. The orographic 
component in the mountains will still remain unfavorable Sat night through 
sun although the GFS is showing increasing qg ascent as secondary 
weaker trough moves across. Thus will likely see some snow however 
at this time amounts look to be on the light side. Over lower 
elevations once again there is quite a difference between the European model (ecmwf) 
and the GFS with regards to low level features. The European model (ecmwf) shows 
more of a northwesterly low level component Sat night into sun with limited 
precipitation while the GFS has more of an upslope component and much more 
precipitation. This far out its really hard to say which model will be 
correct. If precipitation does occur it appears temperatures would be cold enough 
for some snow over lower elevations Sat night into sun. 


&& 


Aviation...VFR conditions will continue through today and tonight. 
Weak pressure gradients over northeast Colorado will allow winds to 
follow typical diurnal patterns. 


&& 


Bou watches/warnings/advisories...none. 


$$ 


Kdrby/rpk