Weather

179 
fxus65 kbou 052230 
afdbou 


Area forecast discussion 
National Weather Service Denver Colorado 
312 PM MST Monday Jan 5 2009 


Short term...real data indicating middle and high cloudiness moving 
across the forecast area at this time. Radar indicating little or 
no precipitation over the forecast area right now...but a decent 
wind signature came up just east of The Divide in Boulder County 
since around 1 PM MDT. Wind gusts are mainly in the 30 to 50 miles per hour 
range right now over the foothills and immediate plains. There were 
a couple 60 miles per hour plus gusts at Rocky Flats and the ncar Mesa lab. 
Area profilers are currently showing a perfect direction on the 
cross-barrier flow...as well as little direction or speed shear with 
height. Overall a decent wind set up for the foothills and 
immediate plains. Models do not have the upper trough axis through 
the forecast area until 12z Tuesday morning...somewhat slower then 
yesterday's runs indicated. There is weak upward motion prognosticated for 
the forecast area from now through about 12z Tuesday morning...then 
downward synoptic scale energy on Tuesday. The low level pressure 
gradient is prognosticated to decrease a bit from its current condition. 
Although it will stay fairly decent overnight and Tuesday...enough 
so to insure a downsloping component to the boundary layer flow for 
much of the forecast area. For moisture...both models have deep 
moisture over the mountains all night into Tuesday morning...then a 
decrease somewhat. The east will just have mainly upper level 
moisture moving over through the two periods. The quantitative precipitation forecast fields do 
show some measurable snowfall for the mountains tonight and 
Tuesday. For probability of precipitation...none for the plains. In the mountains...will 
keep the high probability of precipitation going into Tuesday morning...with the moisture 
and orographic enhancement expected. There is no appreciable cold 
air advection ... lapse rates are poor and the synoptic scale 
energy is pretty weak. Amounts will be under advisory number for 
sure. On Tuesday will reduce them to 60%s with the moisture 
decrease. Amounts will be light. For winds...it looks like the 
best high wind set-up GOES away by early/middle evening. No 
highlights. It should stay fairly windy overnight and 
Tuesday...will make sure blowing snow is mentioned in the high 
mountains. For temperatures...Tuesday highs look to be 1-3 c cooler 
than this afternoon's readings. 


Long term...jet core shifts over the area Tuesday night with winds 
also increasing at mountain top level. Warm advection intensifies in 
this period...keeping fairly stable air above the mountains. There 
looks to be a bit less flow aloft than in the December 29-30 event 
but there will be a little stable layer and possibly a critical 
layer or close to it up high with the northerly jet. Ran the Brown- 
Weaver local wind guidance and got fairly high numbers for 
Wednesday...a 40 percent chance of 60 miles per hour in Boulder and gusts to 68 
knots in Fort Collins...about as high as this guidance GOES. Despite 
what is at first a marginal appearance...there may be the right 
combination of factors for a stronger high wind event. Threat should 
begin with the passage of the jet core...which looks to be after 06z 
so I will begin a high wind watch at midnight Tuesday night. If the 
timing of the jet speeds up this may need to be adjusted. Expect the 
threat will initially be confined to the immediate vicinity of the 
foothills...but we are lacking cold air at the surface on the plains 
so it could spread eastward behind the shortwave...especially during 
the day Wednesday. 


As for snow...stability will limit accumulations. West-northwest direction with 
speeds of 40 knots is favorable for the Rabbit Ears Pass area...so 
we will have a Winter Storm Watch for the slopover into zone 31. Put 
the other mountains into the high wind watch as the wind should be 
the main factor there. 


Shortwave ridge moves over Thursday with warm temperatures...even in 
the mountains where there could be some 40s in places that do not 
usually see them in midwinter. Still pretty good agreement on a 
shearing shortwave about Friday night...without a lot of wind or 
moisture. More discrepancies after that with some model runs showing 
a return of the northwest jet with decent cold advection and 
moisture on Sunday or Monday...others with the baggy trough shifting 
eastward but a less dynamic northwest flow behind it. For now will 
show some mountain probability of precipitation and a little cooling...but it could be more. 


&& 


Aviation...VFR conditions expected for the next 12 hours. 


&& 


Bou watches/warnings/advisories...high wind watch midnight Tuesday 
night to midnight Wednesday night Front Range mountains/ 
foothills/adjacent plains...zones 33..34>36..38..39. 
Winter Storm Watch Tuesday night and Wednesday zone 31. 


$$ 
Rjk/cmg