Area Forecast Discussion
Issued by NWS Pueblo, CO

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366
FXUS65 KPUB 281135
AFDPUB

Area Forecast Discussion
National Weather Service Pueblo CO
535 AM MDT Fri Apr 28 2017

.SHORT TERM...(Today and tonight)
Issued at 535 AM MDT Fri Apr 28 2017

...Late Spring storm targeting southern CO with a prolonged period
of heavy wet snow...

150+ kt upper jet digging down the back side of the broad upper trof
across the Rockies is still progged by models to carve out a closed
low across the Four Corners region by this evening which then tracks
slowly southeast then eastward across New Mexico tonight through
Saturday, before lifting back northeastward through the Texas/OK
Panhandles by Sunday morning.  Although models are still latching on
to the more southern route through New Mexico, they have trended
slower and farther north as it lifts out into the southern/central
plains in the latter parts of the period.  The degree of
strengthening and position of the H7 low has also changed with this
set of runs.  GFS and ECMWF were the preferred 00Z models (see
PMDHMD) and have leaned forecasts in that direction.  The one thing
that has been consistent all along is that this system will bring
with it sufficient cold air for a switch over to snow across all of
the southeast plains. In fact this system looks colder than any of
the systems we`ve seen in the past month.  This will lead to an
early switch over to snow which should start around 00z for the
crest of the Palmer divide...spreading southward and lowering into
the COS area shortly before midnight...then into most of the
southeast plains by 06z to 12z. Other factor that has been
consistent from run to run is that areas south of highway
50...including the Wet Mountains, Sangre De Cristo Mountains and the
southern I-25 corridor will get hit hardest.  The challenges that
still exist is how rapidly will the upper low intensify which will
determine how quickly H7 winds will pull around to the east and
spread moisture back westward into the southeast mountains.  00Z NAM
was slower to do this which cut back on snow amounts across northern
portions of the forecast area including the Pikes Peak region and
Chaffee and Lake counties.  This run was the outlier and
interestingly the 06z run has trended back stronger which has goosed
up snowfall totals across all of the area.  (So has 06z GFS!)

Given that this event begins in the tonight period have started to
make some decisions with highlights with this package.  Snowfall
will begin across the northern and western mountains around
00z...spreading south and eastward through the night with rain
changing to snow over most or all of the plains by 09z-12z.  Have
hoisted advisories for the central and southwest mountains and
warnings for the southeast mountains.  As upper difluence moves
overhead and storm wraps up...should see best window for heaviest
snowfall across the southeast mountains during the late evening
through Saturday morning period as intense isentropic upglide
develops. Southeast mountains, southern I-25 corridor and Raton
Ridge should see solid warning amounts with snow accumulations of
around a foot for many of these areas...and up to 2 feet for the
northeast and east facing slopes of the southeast mountains.  Teller
county and northern El Paso should also fair well...but amounts up
that way will be more in the 6 to 12 inch range given it is farther
away from the best upper level lift.  Northern extent of the heavy
precipitation is always a challenge with strengthening storm
systems...so have kept southern El Paso and the Upper Arkansas River
Valleys in the Advisory snow amount range for now. Its important to
note however that if the storm does intensify sooner then some of
these areas may need to be upgraded to warnings.

For the southeast plains there are still some timing and storm track
differences to work out...particularly in the latter portions of the
storm where snow may end up lingering much longer into Sunday.  Have
decided to go with Watches for the remainder of the southeast plains
for now. Models have upwards of a foot of snow south of highway 50
and more in the range of 4 to 6 to the north. However a storm track
that is more westward as the latest GFS and NAM suggest, could
result in heavier amounts.  Will keep these areas watches for now.
Other complication will be how much snow will accumulate given the
warm ground and temperatures near or slightly above freezing on
Saturday.  Snowfall rates look impressive so do think there will be
impacts to travel as well as a concerning potential for power
outages and downed tree limbs to to the heavy wet nature to the snow
on already leafed out trees.  There is even the possibility of white
out conditions over the Raton Ridge on Saturday.

Overall a high impact event for most of southern CO...particularly
for the southeast mountains and plains south of highway 50.
Forecasts are still vulnerable to change so please monitor the latest
forecasts.  Expansions and upgrades to warnings will be possible for
the southeast mountains and plains over the next 24 hours. -KT

.LONG TERM...(Saturday through Thursday)
Issued at 535 AM MDT Fri Apr 28 2017

Active meteorological pattern anticipated into Sunday as vigorous
storm system impacts the forecast district during this time-frame.

Primary longer term meteorological concerns continue to include
but are not limited to locally heavy precipitation(including
heavy snow), winds and temperatures.

Recent PV analysis, forecast model soundings, and longer term
computer simulations indicate that dynamic(and cold for this time
of year) closed upper low centered over east-central New Mexico
at 00Z Sunday shifts into the Texas panhandle by 12Z Sunday before
moving into south-central Kansas by 00Z Monday.

With favorable track and strength of incoming system, various
winter weather highlights have been issued over the forecast
district into Sunday with precipitation shield expected to move
east of the forecast district by later Sunday.

This incoming potentially high impact storm system has the
potential to produce heavy snow and strong winds, especially over
eastern portions of the forecast district into Saturday night and
Sunday, therefore a Winter Storm Watch has been issued over these
locations into Sunday. Some recent model solutions have impressive
wind and precipitation amounts during the later Saturday into
Sunday time-frame over easter 1/3 or so of the forecast district.
As always, WFO Pueblo will issue additional winter weather
highlights as needed based on the progression/development of this
incoming storm system.

In addition to the expected snow and wind, still project that
below seasonal temperatures will be experienced into the weekend
with moderating temperatures then anticipated next week.

Switching gears to the forecast for the upcoming work week, still
project that a return to lower grade to nil pops in combination
with moderating temperatures should be noted from Monday into
Thursday as the northwesterly upper flow of Monday into Wednesday
is replaced by upper ridging by next Thursday. At this time, it
appears that the highest potential for precipitation during the
Monday into Thursday time-frame should be experienced from later
Monday into Tuesday night as a trailing upper disturbance impacts
the region with basically dry conditions then anticipated
Wednesday and Thursday.

Finally, the highest potential for stronger winds(favoring eastern
sections) during the longer term should be experienced from
Saturday night into Monday morning.

&&

.AVIATION...(For the 12Z TAFS through 12Z Saturday morning)
Issued at 535 AM MDT Fri Apr 28 2017

VFR conditions will quickly deteriorate into IFR to LIFR this
evening for all three terminals.  KCOS and KPUB will have the
potential for several inches of snow (most likely in the 3 to 6 inch
range) for both sites by Saturday afternoon, however this will be
subject to change depending on storm track and evolution.  Also warm
pavement surfaces will help melt snow at times...though some slushy
runway surfaces can be expected at times.  Winds will shift around
from the east to northeast with gusts up to 25 kts this afternoon
before becoming northerly with gusts to 30-35 kts tonight after 02
to 04z.  Expect the switch over to snow at KCOS around 03-04z and
for KPUB between 04-06z.

KALS will see west winds increase to around 15-25 kts in the
afternoon before shifting around from the north tonight.  Best
window for snow and IFR to LIFR conditions will come in after 06z
with the potential for an inch or two by mid day Saturday. -KT

&&

.PUB WATCHES/WARNINGS/ADVISORIES...

Winter Storm Watch from late tonight through Sunday morning for
COZ089-093-095>099.

Winter Storm Warning from 10 PM this evening to 6 AM MDT Sunday
for COZ074-075-087-088.

Winter Storm Warning from 6 PM this evening to 6 AM MDT Sunday
for COZ072-073-079-080.

Winter Storm Warning from 6 PM this evening to 6 PM MDT Saturday
for COZ078-081-082-084.

Winter Weather Advisory from 6 PM this evening to 6 PM MDT
Saturday for COZ058-060-061-063-065>068-076-077.

Winter Storm Warning from midnight tonight to 9 AM MDT Sunday
for COZ094.

Winter Storm Watch from late tonight through Saturday afternoon
for COZ086.

Winter Weather Advisory from 10 PM this evening to 6 PM MDT
Saturday for COZ083-085.

&&

$$



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