Area Forecast Discussion
Issued by NWS Grand Junction, CO

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000
FXUS65 KGJT 161752
AFDGJT

Area Forecast Discussion
National Weather Service GRAND JUNCTION CO
1152 AM MDT Tue May 16 2017

.UPDATE...
Issued at 934 AM MDT Tue May 16 2017

Had a quick look at recorded low temperatures over the last few
weeks for several of our zones and discovered that many of them
have been hitting freezing or below for many nights. Another night
of freezing temperatures will be nothing new for those zones so
went ahead and cancelled the Freeze Watches. Also removed a few
zones that are higher elevation where there will be minimal
impacts. Remaining watches will remain in place for now but will
keep an eye on latest guidance to see how temperatures trend.

&&

.SHORT TERM...(Today through Wednesday)
Issued at 454 AM MDT Tue May 16 2017

Mild conditions and generally light winds are in place across the
forecast area this morning as the first in a series of storm
systems approaches the forecast area. This storm system will
result in cooler temperatures today and will also support
scattered showers and isolated thunderstorms, mainly in the
afternoon and evening hours.

This storm system which is currently centered over southern Nevada
will exit the area late tonight with cool air and instability
filling in behind it. The cool air and instability will support
showers and isolated thunderstorms into Wednesday evening along
with mountain snow.

A cold front will also impact the area on Wednesday as a second
deeper and colder storm system approaches from the northwest. This
cold front will provide additional lift and instability which will
increase snow rates over the mountains Wednesday afternoon and
evening. These high snow rates will result in periods of moderate
to heavy snowfall over the mountains with accumulations ranging
from 4 to 8 inches over the Eastern Uintas to 8 to 11 inches over
the mountains of the western slope of Colorado. Therefore a Winter
Weather Advisory has been issued to alert the public of this late
season snow event.

.LONG TERM...(Wednesday night through Monday)
Issued at 455 AM MDT Tue May 16 2017

Over the past few days model solutions have remained consistent
with this second storm system which is currently pushing into the
Pacific Northwest. The 0000Z ECMWF and 0600Z GFS20 have
initialized well with this storm system and remain in good
agreement sliding it south into the Intermountain West Wednesday.
This storm system is expected to linger over Utah and Colorado
through at least Friday evening.

One of the big stories associated with this storm system is how
much cold air is accompanying it. According to the 0600Z GFS20
700mb temperatures over Grand Junction will fall to around -5 C
Wednesday night into Thursday morning. This cold airmass will
drive overnight surface temperatures at many low elevation
locations to near or slightly below freezing Wednesday night and
Thursday night. Therefore, we have issued a Freeze Watch for these
low elevation locations. These record low temperatures will be
capable of damaging sensitive plants and irrigation systems as
well as swamp coolers. So please take action to protect these
sensitive plants and equipment.

Temperatures will rebound this weekend as a warming trend begins
to develop with high pressure building over the Eastern Pacific
and western CONUS. In the extended portion of the forecast the
forecast models are suggesting that a trough of low pressure will
be in place over the Rockies and Intermountain West next Monday,
supporting the potential for another round of showers and
thunderstorms.

&&

.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 1146 AM MDT Tue May 16 2017

Clouds will be increasing this afternoon across the forecast area
with isolated to scattered showers developing over mainly the
higher terrain as a disturbance approaches and passes to the south
of the Four Corners late this afternoon into early evening. These
showers may impact TAF sites so put VCSH in during the afternoon
at most TAF sites as some showers drift into nearby adjacent
valleys. Expecting more wind than rain with showers today as the
low levels remain dry with some isolated lightning strikes with a
few cells. Winds will be breezy this afternoon as mixing occurs
and in vicinity of showers. Showers will die off after sunset with
some partial clearing of clouds before the next stronger storm
system enters Utah Wednesday morning. This system will produce BKN
to OVC skies with widespread precipitation developing after 15Z
Wednesday, leading to lowered cigs and vsby in showers and mainly
higher elevation TAF sites reaching ILS breakpoints. Until then,
VFR conditions will prevail.

&&

.HYDROLOGY...
Issued at 0930 AM MDT Tue May 16 2017

Above normal temperatures, especially temperatures staying
above freezing at night, have allowed the remaining snowpack
at high elevations to melt at an accelerated rate.

With cooler temperatures arriving with the next storm system,
melting of the snowpack will slow. The response will be seen over
the next 24 hours as rivers levels and flows slowly decline.

A few of the rivers still being closely monitored include: the
the North Fork of the Gunnison at Hotchkiss the Green River at
Jensen and the Mancos River near Mancos. It should be noted that,
at this time, rivers across eastern Utah and western Colorado are
not forecast to reach flood stage but rather just approach or
reach bankfull conditions.

Increases in river stages lead to much higher river flows making
for swift and unpredictable currents. Debris in the river
could create numerous snags. Water temperatures will be quite
cold with the introduction of the snowmelt. If recreating or
working in or near high flow waterways, use appropriate safety
gear. Unstable banks along fast moving waterways may quickly
give way and should be avoided.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...
CO...Freeze Watch from late Wednesday night through Thursday morning
     for COZ006-007-011-020>023.

     Winter Weather Advisory from 6 AM Wednesday to 6 PM MDT Friday
     for COZ004-009-010-012-013-017>019.

UT...Freeze Watch from late Wednesday night through Thursday morning
     for UTZ024-027.

     Winter Weather Advisory from midnight tonight to 6 PM MDT Friday
     for UTZ023.

&&

$$

UPDATE...TGR
SHORT TERM...Larry
LONG TERM...Larry
AVIATION...MDA
HYDROLOGY...TGJT



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