Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 172047

Area Forecast Discussion
National Weather Service Denver/Boulder CO
247 PM MDT Sat Mar 17 2018

.SHORT TERM...(This evening through Sunday)
Issued at 240 PM MDT Sat Mar 17 2018

There is some weak convective cloudiness in the high mountains
right night, everywhere else is nearly cloud free. There is a
pretty uniform wind field of south-southeasterlies over most of
the plains. Models have moderately strong southwesterly flow aloft
tonight into Sunday morning. The flow aloft weakens and become
more due southerly in the afternoon as an upper trough gets into
western Colorado. The QG vertical velocity fields have weak upward
motion over the CWA tonight, then strong upward energy Sunday,
especially in the afternoon. The boundary layer winds should be
mostly drainage tonight, although a surface cyclone will set up
overnight, more so on Sunday as the upper trough nears. By Sunday
afternoon, the surface low will be somewhere over the CWA, from
our central plains to the southeastern corner of the CWA. For
moisture, I had to back off on the increase of cloudiness for the
GFE grids this evening/overnight based on real data. Even the
Models are slower. Moisture gets pretty good by late Sunday
morning through 00Z in the afternoon. Will start a Snow Advisory
highlight for the high mountains and the northern foothills at 18Z
Sunday. The orographic enhancement is poor up there on Sunday but
the dynamics with the approaching trough are strong. Mountain top
flow gets better in the third period. Will get pops into the
foothills and plains from late Sunday morning through late
afternoon out on the eastern border. It is still to warm for any
snow out on the plains through Sunday late afternoon. It will be
getting close on the Palmer Ridge however by then. Pops look to
be mostly above 60%s for much of the CWA by 00Z late Sunday
afternoon. For temperatures, Sunday`s highs are about 2-6 C cooler
than this afternoon`s.

.LONG TERM...(Sunday night through Saturday)
Issued at 235 PM MDT Sat Mar 17 2018

Satellite water vapor shows an old, wrapped up low with embedded
vort maxes pushing through Idaho bringing snow to the region. Thing
to note is that for the areas in southern ID that are currently
reporting snow have temperatures hovering just around the freezing
mark. This could be the cause of some warmer theta e values moving
into the mid levels helping to increase the instability.

For our region in the long term, looking at the main low pressure
system associated with the elongated trough organizing over the four
corners region by Sunday afternoon. According to current model
trends it will then push into the western KS border and OK/TX
panhandle by early on Monday morning. At this time it appears that
the GFS, NAM and Canadian regional models are pushing for a more
northerly low pressure center while the EC is pulling it further
south. After analyzing some model PV fields overlaid on WV it
appears the NAM is handling the current placement pretty well so
will stick with that solution for this forecast.

In the mountains, upper level flow starts out as southwesterly ahead
of the main trough with increasing moisture. Snow will be in full
swing in the mountains by Sunday evening with periods of moderate to
heavy snow...especially over the Park and Gore range with mid and
low level flow turning more northwesterly by late Sunday. Upslope
along the foothills looks short-lived with the surface low
developing and moving southeast along and east of Lincoln county by
00z Monday helping to pull the surface winds more northerly along
the plains and base of the foothills by late Sunday. This is one of
the factors keeping me from having high confidence in warning
criteria numbers for the higher terrain at this time, however model
trends are showing advisory numbers for the Cameron Pass area in
zone 33 and possibly the higher foothills of zone 35. Will most
likely issue a Winter Weather Advisory for these areas with the
afternoon packages reaching into monday morning/afternoon. Current
amounts are looking at 5 to 10 inches across the northern mountains
and foothills with locally higher amounts east of the divide near
Cameron Pass and into Red Feather Lakes region.

The plains are a bit tricky with the lining up of the best energy
with the right temperatures to deliver snowfall. Starting out, flow
will be northwesterly, which will help to downslope off the
foothills and Cheyenne ridge keeping amounts low in those areas.
Looking at 700 mb theta fields show some potential for a TROWAL set-
up over portions of the eastern plains by late Sunday that could
help to bring increased snowfall amounts to areas east of I-25. The
placement of this warm air aloft can be hard to pin down even a few
days out so expect amounts to change. However, looking at model
guidance there is an area that could reach advisory criteria or
higher east of I-25 and a line stretching from southern Morgan south
into Elbert county. Another factor that will affect snow amounts for
the plains will be the changeover from rain to snow. Model 700 mb
temperatures show warmer values on Sunday changing over to colder
temperatures with CAA from the north into Monday morning. The best
lift and moisture for the plains is expected to be between 00 and
06z Monday so anticipate most of the precipitation to fall as snow
or a mix to start before quickly changing over to all snow around
03z. Current model trends are showing 1 to 4 inches along I-25 and
into Denver with higher amounts favoring South Denver. For areas
east of I-25 in the Douglas, Elbert and Palmer Divide areas where
conditions seem more favorable looking at amounts in the 5 to 10"
range or possibly higher if current trends continue. Will hold off
on any highlights for now as model consensus on placement of the
higher amounts are still not great. Will give the models another few
runs before pulling the trigger. Winds will increase the plains with
the inflow into the developing low pressure system pushing SE. Gusts
in the 30-35 mph range will be possible on the plains by early
Monday. This could cause some blowing and drifting snow across the
plains Monday.

Snow will taper off from west to east Monday morning into the
afternoon across the region. Scattered, light snow showers will
continue in the higher mountains through the day finally ending late
Monday into early Tuesday. Upper ridging will return by midweek
bringing drier conditions and increasing temperatures.


.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 235 PM MDT Sat Mar 17 2018

Models are all over the place concerning wind directions at DIA
tonight. There is a surface low to develop somewhere later this
evening. Will lean towards drainage patterns at normals or a tad
weaker than normal speeds. There will be no ceiling issues
through mid day Sunday.


Winter Weather Advisory from noon Sunday to noon MDT Monday for



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