289 
FXUS65 KBOU 092256
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
356 PM MST Sat Feb 9 2019

.SHORT TERM...(This evening through Sunday)
Issued at 339 PM MST Sat Feb 9 2019

Shortwave trough moves across the area overnight, producing 
scattered snow showers mainly in the mountains with slight chance
on the adjacent foothills out into the northern plains. Left exit
region of 300 mb jet crosses the northern mountains and northeast
CO 00-06z time frame, preceding the trough as it crosses 06-12z. 
QG ascent linked more to jet dynamics, while model soundings 
showing mid level cooling and increasing lapse rates with the 
shortwave trough itself. Moisture is limited early in the evening 
and then increases with the arrival of the trough. With that in 
mind POPs increase to likely category over the higher terrain and 
more favored west to southwest facing slopes 03-06z, and slight 
chance out on the plains toward 06z. Amounts will be light, 1-3 
inches of fresh powder for the mountains, and any snow showers 
that venture onto the plains will produce only a dusting. 

Lows tonight near normal single digits and teens in the mountains, 
and on the plains the Platte River Valley will continue to hold its 
cold pool of air with single digit lows and teens south of I-70. 
Northerly surface winds early-mid morning Sunday as the upper low 
passes will help reinforce the cool air. By mid day Sunday surface 
flow turns southerly with warm air advection, with highs similar to 
today. Warmer 40s south of I-70 over the Palmer ridge, and in the 
30s north of I-76. Clearing towards Sunday morning in drying and 
subsidence behind the departing trough, then an increase in 
mid/upper level moisture later in the day in advance of the next 
system.

.LONG TERM...(Sunday night through Saturday)
Issued at 339 PM MST Sat Feb 9 2019

A string of shortwaves will move across Colorado over the next
week in westerly flow aloft. There is reasonable agreement on
these features, though some uncertainty about strength and exact
location of the late Wednesday/early Thursday wave. 

The next trough comes Monday morning with a quick shot of snow
for the mountains. It looks like enough westerly low level
component to kill the chance of showers east of the mountains.
There will be very strong flow at mountaintop level with this one,
so lots of wind (probably lots of 40-60 mph speeds Sunday night) 
over the higher ridges. The plains will get windy behind the 
trough, but it looks like only a slight high wind threat.

Tuesday will be drier as flow goes more westerly under shortwave
ridging. There will still be a fair amount of wind in the
mountains and a little warming southwest wind on the plains. 

Another, stronger wave is slated for late Wednesday or early
Thursday. Model agreement is not bad, but the operational ECMWF
does have a sharper southern end to the trough which would bring a
better chance of precipitation to the plains. With everything else
over or north of us, compromising on the PoPs seems reasonable.
This leads to another quick but possibly significant shot of snow
in the mountains and a chance of mainly snow on the plains--it
could be warm enough Wednesday afternoon for some rain showers.
Models also agree with a surge of cool air on Thursday which may 
actually give a better chance of showers on the plains. The
temperature gradient with this is a point of contention though.
There's a lot of spread in the ensembles for temperatures Thursday
and Friday with plains highs ranging from the upper 30s to the mid
50s. Again, a compromise seems reasonable at this point. A sharp
trough in fast flow means another round of wind as well,
especially Thursday but to a lesser extent before and after.

By Saturday the next trough should be approaching and we'll have a
chance of snow again coming into the mountains. There may be some
chance of precipitation on the plains again with this system, but 
chances during the day Saturday still look pretty low.

&&

.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 1042 AM MST Sat Feb 9 2019

VFR the rest of this afternoon with IFR ceilings overnight KBJC and
KBJC. Scattered snow showers may move out to KBJC this evening and
a little later at KDEN and briefly lower vis to MVFR. Any snow
showers will be short lived and produce only trace amounts of
accumulations. Denver cyclone to set up this afternoon and turn
winds around to north KDEN/KBJC. This will also bring low level
moisture up the terrain, and slight upslope along with night time
temperature inversion will trap low stratus deck for IFR ceilings
much of the night. IFR to improve 12z with more of a westerly 
downslope flow at KBJC and some drying, lingering at KDEN to 14z
for daytime heating and mixing out of low level inversion.

Wind...as mentioned cyclone will shift wind around to northerly
this afternoon, then after sunset move through west to southwest
drainage flow. South gradient wind will increase 18-24z Sunday at
KDEN to around 15 kts.


&&

.BOU WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$

SHORT TERM...Hanson
LONG TERM...Gimmestad
AVIATION...Hanson