Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 221646

Area Forecast Discussion
National Weather Service GRAND JUNCTION CO
1046 AM MDT Thu Mar 22 2018

.SHORT TERM...(Today through Friday)
Issued at 457 AM MDT Thu Mar 22 2018

Mid and high clouds cast off from the trough over the West Coast
was moving through the ridge of high pressure over eastern Utah
and western Colorado early this morning. Advection of warming air
in the prevailing southwesterly flow ahead of this system combined
with passing clouds resulted in early morning temperatures 5 to 10
degrees warmer than this time last night.

Warm air advection will continue today as a shortwave ejected from
the West Coast trough tightens the height gradient aloft resulting
in a stronger southwest flow. This will drive 7h temperatures to
+6 C in the south to +2 C in the north by late afternoon. Daytime
mixing will bring the warmer temperatures to the surface resulting
in a very mild day (around 10 degrees above normal) despite
increasing cloud cover ahead of the approaching trough. In
addition, breezy southwest winds will develop across the area
this afternoon as mixing couples the lower elevations to the
stronger flow aloft. However, increasing cloud cover is expected
to limit wind intensity to sub-advisory levels. Meanwhile,
vestiges of the moisture from the atmospheric river (AR) event
over southern California begin to push over the area during the
afternoon causing precipitable water levels to rise to near 0.7 of
an inch over southeast Utah to 0.6 of an inch over northwest
Colorado. Dynamic forcing associated with difluent flow aloft and
conditionally unstable air working on this moisture is expected to
generate scattered showers over the mountains with the the
eastern Utah ranges favored. Given the warm temperatures, snow
levels will range from near 10,000 feet in the north to close to
11,000 feet in the south.

The heaviest precipitation associated with this event is expected
to occur tonight into Friday morning as the shortwave and a 115 kt
jet sweep over the area. The best AR moisture shifts south and
east of the area this evening, but PW levels remain near the 0.5
of inch mark which is still pretty juicy for this time of year. In
addition, thundershowers are possible as colder air aloft moves
overhead keeping the airmass unstable. Meanwhile, diurnal cooling
combines with cold air advection overnight to cause snow levels
to lower to between 9000 and 9500 feet by sunrise Friday. Levels
dip a bit further Friday morning before stabilizing toward noon.

Per previous discussion, impacts from snow a real challenge with a
system as warm and moist as this one as melting will be
prevalent, especially early and even at mountain pass level. This
is not easily quantified. However, believe the latest model
guidance indicates a Winter Weather Advisory is needed for
Colorado`s mountain passes tonight through Friday with
accumulations of 5 to 10 inches likely above 9500 feet. As a side
note, some peaks above the timber line in the southern and central
mountains, where temperatures will be colder and snow to water
ratios higher, could potentially receive up to two feet of snow.
The Uncompahgre and Roan plateaus are too low to be included in
the advisory. In addition, Vail Pass was also left out for now as
southwest flow less favorable for snow production there. However,
strongly encourage the day shift to reevaluate with the 12Z model

Snow showers will linger over the higher terrain Friday afternoon,
though fall rates will decrease as a shortwave ridge forms in the
wake of the trough.

As alluded to previously, temperatures will be well above normal
today before dipping by around 5 degrees on Friday, which is still
above normal. Overnight lows tonight will be quite mild with
valleys mainly in the 40s with 30s in the mountains.

.LONG TERM...(Friday night through Wednesday)
Issued at 457 AM MDT Thu Mar 22 2018

An interesting but difficult forecast is shaping up for the long
term period, especially going into early next week. Precipitation
will be ending during the evening hours on Friday as a transitory
ridge brings in drier air aloft to scour out the dendritic moist
layer. Cold air will not have time to work in behind this system
with strong southerly flow moving back overhead for Saturday.
Guidance is suggesting Saturday could be on the breezy side as
instability helps mix these winds to the surface while also aiding
in some isolated to scattered afternoon convection. It`s really
after this period that the forecast becomes less certain as a
strong blocking pattern sets up along the mid latitudes. Much of
this is due to an anomalously strong system developing off the New
England coast by Monday which creates an upstream blocking ridge
along the Lower MS, TN and OH Valleys. This in turn slows the
progression of the next upstream trough over the Great Basin which
is initially phased with the northern stream near the Canadian
border. Late this evening the medium range models showed the
southern energy forming a closed low over Utah as it separates.
This low is basically stopped in its tracks by the downstream
pattern then is forced to dig southward by a strong jet dropping
into the upstream side of the low on Tuesday. This system will be
slow to move out of the Southwest until later in the week when a
northern stream kicker drops down a developing ridge along the
Left Coast. Initially there is some colder air moving in from the
west as the low stalls in Utah but things become barotropic as
the system drifts to the SW. Models really don`t jive on the jet
rounding the base of the low Monday and there is some suggestion
of drier air wrapping into the system. There`s likely to be some
convectively driven precipitation in the forecast just due to the
proximity of the low but a lot of details to be chiseled out.
Overall temperatures appear to hover around seasonal for this


.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1041 AM MDT Thu Mar 22 2018

An approaching storm will bring widespread rain and snow to the
region beginning this evening and continuing through the
remainder of the night and into Friday. Strong southwest winds
will develop after 21Z this afternoon with speeds ranging from 10
to 15 kts and gusts to 25 kts for most TAF sites. This storm
system is very warm, so all TAF sites will experience rain
exclusively, even late tonight. Consequently, VFR conditions will
prevail, even during showers, though brief MVFR visibility is
possible. For KHDN, KRIL, KEGE, KASE, KMTJ and KGUC expect
prolonged periods with CIGS below ILS breakpoints after about 03Z
this evening. Strong southwest flow will continue aloft and as
surface winds decrease, LLWS is expected at KHDN, KASE and KTEX.


CO...Winter Weather Advisory from 6 PM this evening to 6 PM MDT
     Friday for COZ009-012-018-019.

     Winter Weather Advisory from 9 PM this evening to 6 PM MDT
     Friday for COZ004-013.



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