Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 200439
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
939 PM MST Sun Feb 19 2017

.UPDATE...
Issued at 935 PM MST Sun Feb 19 2017

Scattered convection banded along the Front Range and out across
Weld...Adams and Arapahoe counties is aligned with the 700-500 mb
trough axis translating northeastward across the area. Back edge
of large scale ascent region associated with the trough now over
the Front Range mountains. Have observed a steady reduction in
snowfall coverage and intensity in the high country. Since around
3 pm this afternoon snow sticks at area ski resorts indicate
anywhere from 3 to 7 inches of new snow accumulation...with the
greater amounts up along the Continental Divide in Grand and
Summit Counties. Out across the plains, rainfall has been spotty
and amounts no more than a hundredth of an inch.

Over the next couple hours...light rainfall wrapped around a
weak mid-level vort max now over southeast Weld County will
migrate northeast across the northeast corner of the state.
Measurable rainfall chances in the 20-30 per range along its
path. A second area of light rain showers now forming along the
Palmer Divide in Douglas and Elbert Counties may not linger much
beyond 10 or 11 pm this evening once the last of the upper level
energy/shear exits the area to the northeast.

Rest of the tonight...west-northwest mid-level flow on the back
side of the exiting upper trough will strengthen and so will the
mtn top static stability. Model cross sections show the cross mtn
wind component steadily increasing after 06z and peaking between
09z and 12z. Some indicate a max wind approaching 65 kts over the
east slope of the Front Range around the 11000 ft level, others
closer to 40 kts. Nevertheless, should see gusty westerly-
downslope winds developing on the lee slope of the Front Range
and nearby plains after midnight.

Changes to tonight`s forecast include adjusting pops to better
account for band of showers moving across the northeast plains
and reduce pops sooner and from west to east across the high
country. Also made minor adjustments with hourly and overnight
minimum temperatures and increased winds winds by 10-15 mph on the
Front Range and 5-10 mph on the plains...mainly along the base of
the foothills and up along the Wyoming and Nebraska borders.


UPDATE Issued at 551 PM MST Sun Feb 19 2017

Main area of lift ahead of the trough is currently producing
moderate snow over the mountains and light rain and gusty winds
over the urban corridor at the moment. The moderate snow should
occur for the next 2-3 hours before moisture rapidly decreases and
subsidence occurs over the back of the trough. A few rain showers
will still be possible over the eastern plains through midnight.

&&

.SHORT TERM...(This evening through Monday)
Issued at 349 PM MST Sun Feb 19 2017

Area radars show some showers over the mountains, but they are
light. There is nothing over the plains right now. Models push the
upper trough across the CWA overnight and then increasing
northwesterly flow aloft is progged on Monday. There is upward
motion on the QG Omega fields this evening for the CWA, then all
downward motion is progged overnight and Monday. Models all have
northwesterly low level winds after the upper trough passes this
evening. Downsloping winds are progged on Monday, weaker and
westerly over the foothills and western plains; stronger and
northwesterly over the eastern half of the plains. Moisture is
wide spread this evening and relatively deep in the mountains.
After the upper trough axis pushes across, things dry out
significantly. The QPF fields have some measurable precipitation
over the western half of the CWA this evening, then a tiny bit
more over the high mountains overnight, then nothing on Monday. So
for pops, 60-90%s look good in the mountains tonight, less in the
high valleys and foothills, down to 10-20%s over the plains
early. Although the winds could help with orographic enhancement
tonight in the mountains, by the time the directions get
favorable, the moisture is gone. There is no CAPE progged this
evening over the plains like there was yesterday on the 12Z models
for this evening. There is a decent looking mountain wave set up
on the cross sections tonight, but speeds don`t look high enough
for highlights. For temperatures, Monday`s highs will be 0-2 C
cooler than today`s highs.

.LONG TERM...(Monday night through Sunday)
Issued at 139 PM MST Sun Feb 19 2017

Monday night, the ridge axis will shift to the east with an
increasing westerly flow aloft over the region. The models show
some increase in moisture in the mountains west of the Continental
Divide but dry from The Front Range Foothills eastward. The
pattern will be the same on a slight chance of pcpn will be
possible in the mountains mainly north of Interstate 70. The
pattern will continue through the day on Tuesday. The NAM12
spatial cross-section shows a weak mountain wave developing around
12z Tuesday morning so gusty winds in the mountains and foothills
will be a good possibility. Could see some gusts to 50 mph over
exposed eastern slopes. Gusty winds will persist in the mountains
and foothills, with dry conditions with temperatures climbing into
the lower to mid 70s across the urban corridor and northeast
plains. The dry conditions will elevate the fire danger across the
northeast plains, refer to the fire weather discussion below.
Tuesday night/Wednesday, Pacific moisture increases in the
mountains below 600 mb so chc of pcpn will increase in the
mountains west of the divide, but persistent dry conditions with
gusty winds at time over the northeast plains. A weak front is
progged to back into the Urban Corridor around 18z Wednesday so
there will be a slight drop in temperatures on Wednesday, with
weak cold air advection aloft but temperatures will remain well
above normal. Wednesday night, a stronger southwesterly flow aloft
will be over Colorado with moisture increasing in the mountains
ahead of the next trough. Stronger cold air advection with the
next trough axis moves across northern CO on Thursday. Best chc of
accumulating snow in the mountains will be Thu/Thu night. Chc of
pcpn will carryover on the northeast plains, with the highest pops
across the northern tier counties. Cooler with a strong sfc
pressure gradient as the system exits the region Thursday night.
Short wave ridging and subsidence on Friday with continued cool
temperatures. Increasing moisture across northern and western CO
Friday night through Saturday night, as another system moves into
the Desert Southwest and is progged to lift northeast into CO late
Sunday/Sunday night.

&&

.AVIATION...(For the 06Z TAFS through 06Z Monday night)
Issued at 935 PM MST Sun Feb 19 2017

Now until 0530z...we could still see a stray light rain shower or
two pass over the Denver metro area. However, the chance of
measurable rainfall form these weak showers no more than 10
percent. Temperatures will remain too warm for any snow to form.
As for winds, north to northwest winds of 7-15 mph next hour or
two are predicted to shift to a northeast direction at less than
10 kts. Then later tonight as skies begin to clear, winds will
quickly transition to their normal drainage pattern, which for
most locations in the Denver metro area is south-southwest at
4-10 kts. An exception to this will be for areas along the base
of the foothills, such as at KBJC, where enhanced westerly
drainage winds could gust 20-30kts at times beginning around 09z
and lasting through 16z tomorrow morning.

&&

.FIRE WEATHER...
Issued at 349 PM MST Sun Feb 19 2017

Winds come up fairly strong for the eastern plains Monday
afternoon, and humidity progs drop the RHs into the mid and upper
teens. The conditions will get close to RED Flag Warning criteria
over the eastern plains Monday afternoon, but no highlights for
now.

Unseasonably warm temperatures with relative humidities dropping
into the teens will be possible across the northeast plains of
Colorado Tuesday afternoon, elevating the Fire Danger. Stronger
winds will be in the foothills, but gusty winds over the Palmer
Divide could warrant a Fire Weather Watch at some point. The
threat will remain elevated on Wednesday but not as high as
Tuesday. Temperatures will remain above normal but will be
slightly cooler with a weak frontal passage Wednesday aftn.

&&

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

$$

UPDATE...Baker
SHORT TERM...RJK
LONG TERM...Cooper
AVIATION...Baker
FIRE WEATHER...Cooper


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