Area Forecast Discussion
Issued by NWS Grand Junction, CO

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000
FXUS65 KGJT 181726
AFDGJT

Area Forecast Discussion
National Weather Service GRAND JUNCTION CO
1126 AM MDT Thu May 18 2017

.UPDATE...
Issued at 1126 AM MDT Thu May 18 2017

Upgraded the northwest Colorado valleys, including Craig, Meeker,
Hayden and Steamboat (COZ002-005) to a Winter Storm Warning around
1030 am this morning. Already received reports of 8 inches in
Steamboat, Yampa and Craig as well as 5 to 7 inches around and to
the north of Hayden. Models and time heights over the Steamboat
area indicate strong lift through this evening as well as a TROWAL
signature evident on the northern side of the low creating areas
of overrunning as the surface temps among these northern valleys
remain below freezing and will not warm up much throughout the day
due to persistent cloud cover. This should keep precipitation as
all snow for areas north of I-70 where the best overrunning and
dynamics are occurring with heavy snow bands setting up over some
areas. The dynamical support and TROWAL signature lessens and
shifts eastward this evening, so decided to extend the Winter
Storm Warning for COZ002 and COZ005 until midnight tonight.

Another update includes allowing the Freeze Warning to expire at
10 am this morning for UTZ024, COZ020 and COZ021. The Freeze Watch
remains in effect for tonight for COZ011, COZ020 and COZ021. Will
evaluate this later today. For now, the focus has been on the
current last season winter storm and near term updates.

UPDATE Issued at 944 AM MDT Thu May 18 2017

Upgraded the northern and central Colorado mountains to a Winter
Storm Warning as strong dynamics are occurring on the northern
side of the upper level low with an easterly flow across areas
north of Aspen. Easterly flow does not tend to favor snowfall
orographically, but it appears as though the dynamics and
overrunning associated with a TROWAL (Trough of Warm Air Aloft)
are overtaking any orographic influence and resulting in heavy
snow bands across this portion of the CWA. This is very evident
when looking at the latest GOES-16 IR satellite imagery.
Expecting this to continue through this evening and with reports
already coming in of a foot of new snow on the Grand Mesa and 4 to
8 inches in the northern and central mountains, 10 to 20 inches
of storm total snowfall is not out of the question. Vail Pass was
also closed at some point earlier this morning due to snowfall, so
the late season nature of this storm and impacts involved in
addition to expected snowfall are enough to push the advisory to a
warning for these areas (COZ004-013-009-010-012). The San Juans
received roughly 4 to 8 with upwards of 10 inches at Telluride.
The southern mountains will begin to see some improvement with
mainly a scattered convective nature to the precipitation for the
rest of today and some lulls from time to time, being on the
southern and less dynamical side of the upper low. This low will
track across the central mountain corridor through Friday morning.
The rest of the forecast remains on track. However, one other area
to watch for potential upgrade or extension is the Steamboat and
Upper Yampa River Basin area as reports of 8 inches of snow have
been reported. Easterly flow is unfavorable for Steamboat but as
said previously, the dynamics are overtaking orographics. Also,
suspect that overrunning is occurring here as well with surface
temperatures at 29 degrees. Will look at this area more closely as
the current advisory for Steamboat expires at noon. The other
valley zones in the advisory seem to be in good shape as reports
of 2 to 4 inches have come in already in Gunnison and Hayden
respectively.

&&

.SHORT TERM...(Today through Friday)
Issued at 430 AM MDT Thu May 18 2017

The closed low pressure system responsible for the return to more
wintry conditions was centered just south of the Great Salt Lake
early this morning. Moisture streaming northward across the
Colorado Rockies in the broad area of large-scale ascent fed
widespread precipitation, mostly falling in the form of snow.
Snotels, spotter reports and public reports from social media
indicated accumulations generally ranged from 4 to 6 inches with
some locally higher amounts.

A lobe of vorticity moving through the base of the trough was
just beginning to swing northeastward over the Four Corners area
and was forecast to move northward over the Colorado mountains
during the day. Models indicated that lift associated with this
feature will enhance mountain snowfall over the central and
northern Colorado mountains. Meanwhile, rising temperatures will
cause snow to transition to either a rain-snow mix or all rain in
the central valleys during the latter part of the morning and
early afternoon. Therefore, am comfortable with noon expiration
time for the Winter Weather Advisories in place for the central
and northwest Colorado valleys. In contrast, snow will continue to
pile up in the mountains as temperatures remain cold enough for
snow down to near 8000 feet with a mix of rain and snow at their
bases. The eastern Uinta mountains, which have seen a bit of lull
overnight, will see snow increase today as the low center wobbles
over central Colorado bringing a more favorable northeast flow.
Afternoon highs will be the cooler than yesterday and some of the
coolest experienced in some time.

Models suggest snowfall intensity will decrease as the low pushes
east of the Continental Divide late tonight, but coverage should
continue to be widespread. Another round of unseasonably cold
overnight lows in store for the region with some lower valleys
facing a potential freeze. Therefore, hoisted another Freeze Watch
for portions of southwest Colorado.

Drier air begins to fill in over eastern Utah and the westernmost
counties of Colorado on Friday. However, residual moisture and
instability is expected to continue to generate snow showers over
Colorado`s central mountains. However, additional accumulations
should be more localized as coverage becomes more scattered.
Partial sunshine and weak warm air advection will bring some
warming to the area. However, expect afternoon highs will peak
well short of seasonal norms.

.LONG TERM...(Friday night through Wednesday)
Issued at 430 AM MDT Thu May 18 2017

A ridge of high pressure building over the western states will
help push this trough eastward with a drier northwest flow taking
hold by Saturday morning. Some convection may develop over the
higher terrain on Saturday afternoon but coverage looks isolated
at best. This upper low will move northeast and park itself over
Ontario and the northern Great Lakes, with shortwave disturbances
rotating around the upper low and clipping our area in a
persistent northwest flow. This will result in afternoon and
evening convective showers developing each day from Sunday through
Wednesday as a series of disturbances move through each day. Kept
chance precip each afternoon over the higher terrain with an
overall improving trend in terms of temperatures, rising towards
more normal values for this time of year.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1108 AM MDT Thu May 18 2017

Aviation conditions will remain poor through the afternoon. Rain
in the lower elevations with mountain snows above 7000 feet will
result in ceiling heights in the MVFR to occasionally IFR range.
Visibility will be reduced in periods of heavier precipitation to
IFR levels. Mountains will be obscured especially in the higher
terrain to the east where heavier snow will continue. Expect a
general improvement in conditions later this evening and overnight
with precipitation decreasing in intensity overall across the
region.


&&

.GJT WATCHES/WARNINGS/ADVISORIES...
CO...Winter Weather Advisory until 6 PM MDT Friday for COZ003-017>019.

     Freeze Watch from late tonight through Friday morning for COZ011-
     020-021.

     Winter Storm Warning until 6 PM MDT Friday for COZ004-009-010-
     012-013.

     Winter Weather Advisory until noon MDT today for COZ008-014.

     Winter Storm Warning until midnight MDT tonight for COZ002-005.

UT...Winter Weather Advisory until 6 PM MDT Friday for UTZ023.

&&

$$

UPDATE...MDA
SHORT TERM...NL
LONG TERM...MDA
AVIATION...MAC



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