Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 262332

Area Forecast Discussion
National Weather Service Grand Junction CO
532 PM MDT Thu Apr 26 2018

.SHORT TERM...(Tonight through Friday night)
Issued at 400 PM MDT Thu Apr 26 2018

Sunny and mild conditions prevailed across the region again today
as an amplified ridge of high pressure remained over the the
Great Basin. A deep closed low pressure system over the eastern
Pacific will shift eastward overnight causing the ridge to
reposition itself over the area tonight. Some high clouds will
begin to filter over southeast Utah late tonight, otherwise clear
skies and cool overnight lows are expected for the remainder of
the region.

The Pacific low will continue to move east Friday approaching the
bordering coasts of Oregon and California by days end. As the
system shifts mid-level moisture will begin to move into the
region from the southwest in the broad difluent area between the
ridge and the low to west. Models suggest a slight chance for
showers and thunderstorms over the San Juan Mountains and eastern
Uintas late in the afternoon and into the evening. This activity
will be largely driven by diurnal heating and instability so in
the absence of any significant forcing. Though GFS suggests
nocturnal showers are possible late Friday night, ECMWF and NAM
say otherwise and personal experience has me leaning toward the
drier solutions. Overnight lows should be a bit milder as clouds
increase overhead.

.LONG TERM...(Saturday through Thursday)
Issued at 400 PM MDT Thu Apr 26 2018

The extended forecast period continues to be an adventure
according to the 12Z models. At least the big four (GFS, ECMWF,
NAM and Canadian) all agree that, as the closed low pressure
system over the eastern Pacific comes ashore over the Pacific
Northwest on Saturday, showers will increase significantly over
the area. The southwest and central Colorado mountains and the
eastern Uinta Mountains are favored. The mild temperatures will
result in elevated snow levels and impacts to snow accumulations
aren`t likely to have much impact. Then, as the Pacific low moves
over the northern Intermountain region, a dry slot moves over the
area limiting or shutting down shower activity altogether. On
Monday, a vort-lobe moving through the base of the Intermountain
low is forecast to sweep over the area bringing another round of
shower/thunderstorm activity.

For the rest of the week to come, the outlook is a bit murky.
Both models generate a split system over the western U.S. from
Tuesday to Thursday. However, positioning of the southern branch
differs and this has a profound impact on expected weather. In
general, the ECMWF depicts a less coherent split between the
northern and southern branches and as a result is wetter. In
contrast, the GFS generates a separate closed over the
Southwest leaving the forecast in a relatively dry col Tuesday
through Wednesday morning. Given the difference in model solutions
forecast confidence is low and will avoid attempting to add much
detail in these later periods.

As for temperatures, as the moisture moves into the region at the
end of the week highs will fall from levels of 5 to 10 degrees
above normal to near normal by Monday where temperatures are most
likely to hover during the remainder of week. Overnight lows will
show a similar trend.


.AVIATION...(For the 00Z TAFS through 00Z Friday evening)
Issued at 527 PM MDT Thu Apr 26 2018

A ridge of high pressure aloft will keep winds light from the
west northwest shifting to terrain driven overnight. CIGS will
come down late in the period tomorrow as high level cirrus build
in. VFR conditions are expected at all regional terminals over
the next 24 hours.




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