Area Forecast Discussion
Issued by NWS Grand Junction, CO

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000
FXUS65 KGJT 180632
AFDGJT

Area Forecast Discussion
National Weather Service GRAND JUNCTION CO
1232 AM MDT Thu May 18 2017

.UPDATE...
Issued at 1232 AM MDT Thu May 18 2017

Snow was falling along a continuous band stretching from north to
south along the spine of the Colorado Rockies reaching all the way
to valley floors. Models indicate the low system to the west
centered over the Great Salt Lake will remain quasi-stationary
during the remainder of the night and into the daylight hours.
Therefore, hoisted an advisory for the Gunnison Basin and Upper
Colorado River basin through noon today. Accumulations should be
fairly light and confined mainly to grassy areas. However, have
concerns for road conditions around the time of the morning
commute with lows already at or below the freezing mark at KEGE
and KGUC. In contrast, cancelled the WW.Y for CO zone 1 where
precipitation is unlikely until sometime after sunrise. Remote
higher terrain on the eastern end of the Uinta range may pick up
a few inches during the day, but Rangely and surrounding areas are
unlikely to see additional accumulating snow.

&&

.SHORT TERM...(Tonight through Thursday night)
Issued at 412 PM MDT Wed May 17 2017

A mild morning to start quickly turned showery and cooler as a
cold front made its way through northeast Utah and into northwest
Colorado, changing any precipitation over to snow. As of the late
morning and early afternoon hours, webcams indicated wet roads
with some light snow accumulation on grassy surfaces among the
northern valleys near Craig with snow-packed roads already over
Douglas Pass. Expect snow to continue overnight and begin
accumulating on the road surfaces over the northern valleys this
evening and continue through Thursday morning before temps warm up
Thursday afternoon and showers become more convective in nature.
Therefore, decided to include the northern Colorado valleys
(COZ001-002-005) north of I-70 in a Winter Weather Advisory for 1
to 3 inches through noon Thursday, since its also late season and
impacts will be felt overnight. Also, included the Tavaputs
Plateau (COZ003) which includes Douglas Pass in the Winter Weather
Advisory through Friday for 3 to 7 inches. Rest of the highlites
were left as is.

This cold front is currently working its way through the rest of
western Colorado this evening and will be east of the divide by
Thursday morning. This front is associated with a strong upper
level low pressure system currently spinning over southern Idaho.
Colder air will be spilling in behind this front as the upper low
moves into central Utah by Thursday morning. The biggest challenge
was the temperatures to try and determine how cold we would get
and whether any valleys would see widespread freezing conditions
to warrant an upgrade to a Freeze Warning. H7 temps reach their
coldest point tonight into Thursday morning with values in the -7C
to -9C range, before trends show moderating temperatures Thursday
night and once again Friday night as the system slowly moves
eastward. Due to the slower moving nature of this upper low, it
appears that enough cloud cover will be hanging around overnight
in the central valleys to prevent a widespread freeze and help
keep temps mostly above freezing in the low to mid 30s overnight.
So, decided to drop the Freeze Watch for the Grand Valley, Delta-
Montrose area and I-70 corridor up towards Rifle as latest
guidance appears to indicate overnight lows staying above freezing
for the majority of those zones. The areas that are at greatest
risk of seeing more widespread below freezing temperatures are on
the western and southern fringes of our forecast area, including
the Uinta Basin (Vernal area), Paradox Valley and southwest
Colorado zone that includes Cortez area. May be less cloud cover
over these areas and guidance supports values closer to freezing
or below freezing temps, so upgraded UTZ024, COZ020 and COZ021 to
a Freeze Warning.

Heavier precipitation rates will occur along this front through
this evening, so expecting the best snowfall accumulation through
Thursday morning over the mountains that are in the advisory. The
upper low will track across the central mountain corridor during
the day on Thursday with west to northwest flow over the south and
east-northeasterly flow across the north. This northeast component
favors the eastern Uintas in northeast Utah to pick up some decent
snowfall. However, the strong easterly component also really
favors the Front Range and higher foothills over eastern Colorado
where they can pick up significant amounts of snow. Our northern
mountains tend to be shadowed in this flow and not do as well.
Regardless, convection should pick up Thursday afternoon and
evening and lend itself to some periods of moderate to heavy
snowfall continuing in the mountains. There may be some lulls in
the action on Thursday as the nature of the precip looks scattered
and convective. Decided to keep the same timing of the Winter
Weather Advisory as snowfall will last through Friday. Can re-
evaluate this later as the system evolves to see how significant
the lulls may be. Impacts will be felt regardless over the
mountain passes as well as Highway 40 in northwest Colorado and
the I-70 corridor from Vail and areas eastward towards Vail Pass
and beyond.

.LONG TERM...(Friday through Wednesday)
Issued at 412 PM MDT Wed May 17 2017

By Friday morning, drier air is already moving into western
portions of the forecast area with northwest flow keeping snowfall
going over mainly the northern and central divide mountains of
western Colorado favoring the northwest facing slopes. 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 06Z TAFS through 06Z Thursday night)
Issued at 1145 PM MDT Wed May 17 2017

IFR common for most TAF sites this morning due to low ceilings and
visibility thanks to snowfall. Little change for the overnight
hours with some lifting tomorrow during the day but still expect
MVFR to occur as rain and snow showers move over TAF sites. Some
fog is also expected for TAF sites this morning also causing
aviation problems. Anticipate most ceilings to be around the 2K to
3K foot mark across the forecast area. Visbilities will vary with
the worst visibility expected for our mountain TAF sites with
some light snow and rain continuing off and on through the day.
Visibilities will range from 1SM to 3SM. Conditions will slowly
start improving after 00Z though more fog is certainly possible
tomorrow night. Mountains will be obscured through much of the
day. ILS breakpoints will be met for mountain TAF sites and hover
near those breakpoints for KGJT, KRIL, KMTJ, and KDRO.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...

CO...Freeze Warning until 10 AM MDT this morning for COZ020-021.

     Winter Weather Advisory until 6 PM MDT Friday for COZ003-004-009-
     010-012-013-017>019.

     Winter Weather Advisory until noon MDT today for COZ002-005-008-
     014.

UT...Freeze Warning until 10 AM MDT this morning for UTZ024.

     Winter Weather Advisory until 6 PM MDT Friday for UTZ023.

&&

$$

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



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