519 
fxus65 kbou 111026 
afdbou 


Area forecast discussion 
National Weather Service Denver Colorado 
410 am MDT Sat Oct 11 2008 


Short term...cold air has pushed southward overnight with stratus 
into southeastern Colorado. Strong south winds will be pushing over 
the strong inversion and there does not appear to be a westerly 
component to try to push it off the mountains. So...our expectation 
is that the stratus deck will hang pretty tough today with continued 
northeasterly low level flow. Not much additional cooling expected 
but we are already about 5 degrees colder than expected for this 
time. Will trim highs into the 40s in most areas and add the drizzle 
which is likely already occurring in many places. Meanwhile...we are 
under the dry air aloft ahead of the approaching system. Mountain 
probability of precipitation for the morning were likely overdone...though the showers may 
be spreading in from the west by noon. Model soundings show a warm 
moist neutral layer with fierce south winds this afternoon. 
Combination of moisture arriving and orographic lift should trigger 
showers and a few thunderstorms in the afternoon...and with the 
cooling aloft arriving there will likely be a hefty shower band with 
strong convergence along it. At first glance the setup looks pretty 
good for the mountains along the park/Summit County border with the 
southeast flow...but the winds are so strong the resulting 
convection will likely dump the snow over Summit County. I will up 
snow amounts in zone 34 as there could be a good round of this in 
the late afternoon and early evening before the flow GOES more 
southwest. Another wrinkle is the temperatures as model soundings 
suggest the snow level remaining around 10 thousand feet until the 
trough passes. There should be some cooling where there is heavy 
precipitation. 


For tonight...mountains will transition to a more typical southwest 
flow unstable regime which is normally not too productive. Still 
some instability but also still rather warm...will drop snow levels 
but hold back on snow amounts. I will issue a Winter Weather 
Advisory though for the combination of snow and strong 
winds...expecting the main impact to be above 10 thousand feet again 
like the last storm. On the plains the forecast temperature profiles 
are isothermal in the lowest 6 thousand feet just above freezing. 
This often eventually does result in snow through the layer...but 
with the southerly winds continuing at 700 mb that level just does 
not want to cool so it could stay rain at colder temperatures than 
usual. If you adjust the model sounding two degrees for its usual 
errors you could get all rain even high in the foothills...all 
snow...sleet or freezing rain. Freezing rain probably least likely 
given the lack of low level cold air. Will stick with the idea of 
mainly rain in the evening with the snow level lowering...but unsure 
of how much. In any event plains and foothill elevations have warm 
ground so impact should be low. 


Long term...the strong southwesterly flow aloft is prognosticated to 
continue over the forecast area Sunday into Monday afternoon. Then 
the upper trough axis pushes across. By 06z-09z Tuesday morning 
..weaker northwesterly flow aloft is prognosticated over the forecast 
area. The qg vertical velocity fields have pretty strong downward 
motion prognosticated over the northern half of the forecast area Sunday. 
By Sunday night into Monday morning...upward motion is over the 
forecast area in advance of the trough axis. The low level pressure 
and wind fields show a more downsloping component to the boundary 
layer flow over the plains early Sunday...but more northeasterly 
flow is prognosticated Sunday afternoon and evening. By Monday 
afternoon...northwesterly surface winds are prognosticated as the upper 
trough axis moves east. For moisture...it dries out somewhat Sunday 
morning...but by afternoon there is a return of some low level 
moisture. This continue into Monday. It dries again Monday night 
..slowly. The moisture is gone by early Tuesday morning. With 
the moisture and synoptic scale energy in mind...will adjust probability of precipitation 
here and there on Sunday...nothing significant however. Will linger 
minimal probability of precipitation Sunday night into Monday morning in appropriate areas. 
For temperatures...sunday's highs should not differ a whole lot from 
today's. There should be more clouds than expected so will lower the 
gfe grids a bit. Monday's highs are close to sunday's. For the 
later days...Tuesday through Friday...models show zonal flow to weak 
upper troughing...than upper ridging by the end of the weak. It 
looks dry. Temperatures are near normal from Wednesday through 
Friday. Will change little. 


&& 


Aviation...IFR ceilings will persist most of the day...with areas 
of LIFR ceilings this morning. Some improvement is expected in the 
afternoon but ceilings will likely remain below 2000 feet. Areas of 
drizzle will give way to a widespread rain during the late afternoon 
and evening which will lower ceilings and visibilities again. The 
rain will end late tonight but low ceilings will persist. 


&& 


Bou watches/warnings/advisories...dense fog advisory until 3 PM for 
the Front Range foothills...zones 35 and 36. 
Winter Weather Advisory from noon today through 6 am Sunday for 
the high mountains...zones 31..33 and 34. 


$$ 
Gimmestad/rjk