343 
fxus65 kbou 201100 
afdbou 


Area forecast discussion 
National Weather Service Denver Colorado 
352 am MST Thursday Nov 20 2008 


Short term...cold and moist upslope flow has pushed all the way up 
against the Front Range early this morning. Local web cams show 
areas of fog and freezing drizzle...while WSR-88D and tdwr also 
showing freezing drizzle becoming more widespread especially over 
the Palmer Divide and southwestern two-thirds of the Denver metropolitan 
area. Meanwhile...there was a sliver of drier low level air from 
around Fort Collins to Greeley to near dia preventing drizzle 
formation there for the time being. However...there was yet another 
band of low level moisture now working across the Wyoming/NE border area 
and this should get to the Front Range by or shortly after 
daybreak. As a result...dia and other locations along the Front 
Range should have a better shot of freezing drizzle. Finally...the 
q-g vertical motion fields show weak but sustained lift this morning 
associated with a nice area of middle level frontogenesis sitting off 
just to our east. Deeper moisture associated with this may help 
produce some light snow as temperatures in the deeper moist layer are 
closer to -10 to -12c...borderline for ice/supercooled liquid water. 
Have added some flurries in the forecast to account for this. 


Given the icing from freezing drizzle and dense fog in the higher 
terrain...have already issued a Winter Weather Advisory for the 
Front Range foothills and Palmer Divide. Will be adding Denver and 
Boulder metropolitan areas as well considering temperatures should drop enough to 
allow freezing of drizzle to bridges and overpasses. 


Precipitation is expected to gradually decrease late this afternoon/ 
evening as drier air works into the region. However...areas of low 
level moisture will linger especially in/near the Front Range 
foothills. The upslope component gradually weakens as flow turns 
more southerly by or shortly after midnight...so any light freezing 
drizzle/flurries should be ending then. 


Long term...after today...the upper air pattern across the state 
will be mostly ridge-like through late next week. A weak trough 
will move through the ridge and over Wyoming on Friday. But it 
appears that all we will see in northern Colorado will be some 
breezy conditions from a brief mountain wave that develops under the 
nose of the approaching upper level jet. Given that the upper jet 
will only be about 120 knots...over Wyoming...and the forecast wind 
speeds of 35 knots at 700 mb over northern Colorado...the threat of 
damaging winds is minimal. Have held on to the slight chance of 
snow forecast in the mountains from this passing impulse...even 
though moisture with the system looks pretty limited and the quantitative precipitation forecast 
fields from the GFS are dry. 


Beyond Friday we will see continued warming with high temperatures 
around to slightly above normal through the period. The forecast 
package needed little adjustment from the prior package other than 
toning down the wind forecast for Friday...especially over the 
mountains. Dry...dry...dry ends up being the main message this time 
around. 


&& 


Aviation...slightly drier air has allowed dia to stay out of the 
freezing drizzle during the very early morning hours...but this will 
change before daybreak as low level moisture increases once again 
and freezing drizzle expected to commence. The freezing drizzle... 
light fog with visibilities mainly 2-4 miles...and IFR ceilings will then 
continue into early afternoon...gradually decreasing by 20z-23z. 
Cant rule out brief visibility (about 30% chance) around 1 mile with 
period of heavier drizzle in the 13z-16z period. Easterly surface 
flow will turn southerly this evening which will bring lifting 
ceilings and a return to VFR conditions around 06z. 


&& 


Bou watches/warnings/advisories...Winter Weather Advisory for the 
Denver/Boulder metropolitan area until 2 PM MST...zones 39 and 40. 
Winter Weather Advisory for the Front Range foothills and Palmer 
Divide area until 5 PM MST...zones 35..36..and 41. 


$$ 
Barjenbruch/kdrby