Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 261132

Area Forecast Discussion
National Weather Service Grand Junction CO
532 AM MDT Fri Apr 26 2019

.SHORT TERM...(Today through Saturday)
Issued at 400 AM MDT Fri Apr 26 2019

An active weather day is in the works on Friday over portions of
eastern Utah and western Colorado. Thunderstorms, some possibly
strong to severe, will affect northeast Utah and northwest
Colorado through the afternoon. Additional showers and
thunderstorms are expected across much of the rest of the western
slope through the early evening hours. While this event has been
well advertised over the past two days, a few pitfalls preventing
a more robust thunderstorm event have emerged in tonight`s model

The primary culprit for the uptick in convective intensity is a
fast-moving shortwave trough, progged to track through the
northern Rockies on Friday afternoon. As it does so, a combination
of height falls, increasing mid and upper- level flow, and a
prefrontal vorticity maximum will provide upper- level support for
convective development over northeast Utah and northwest
Colorado. The timing of the prefrontal vort max emerges as the
first potential hindrance to stronger thunderstorm activity
today. Current high- res guidance matches well with satellite
trends showing this vort max over eastern Nevada. If this region
continues on its current pace, convective initiation may begin a
bit ahead of schedule in eastern Utah around 18z. An earlier start
would mean less instability available for convection, as well as
weaker flow aloft/shear as the 500mb speed max remains farther
north until later in the day. The speed of this vort max bears
watching throughout the morning hours.

At the surface, a slight increase in low-level moisture ahead of
this system will aid instability values throughout the western
slope. There are two sources of increasing Td values apparent on
high-resolution model data: moisture transport from the west
associated with current cloud cover depicted on GOES imagery, and
a weak but discernible plume of higher Td values modeled to move
north out of New Mexico through the late morning hours. Here we
arrive at our second potential pitfall - if Td values transported
from the west do not materialize and moisture return out of the
south is delayed, instability will run lower than forecast. A key
value to watch for on surface obs will be the 40 degree Td
contour: locations that eclipse this value bear watching through
the afternoon hours.

Finally, convective mode will ultimately determine the severe
threat. While DCAPE values in the 800-1000 range in forecast
soundings support a gusty wind threat, an earlier start time to
convection will place these thunderstorms in a lower-shear
environment. This may ultimately cause thunderstorms to cold pool
and gust-out, resulting in a more messy pop-and-drop type event.
Overnight runs of the NSSL WRF, HRRR, and NAM Nest all seem to
support the idea of two main rounds of activity - the first
beginning around noon in the Uinta Basin, and another later in the
afternoon as the front approaches from the Wyoming border.
Convective overturning may temper the later afternoon activity,
despite it coming with a better upper-level setup.

To summarize:
A marginal risk for strong to severe thunderstorms exists today
over northeast Utah and northwest Colorado, primarily north of
I-70. The main hazard will be strong wind gusts to 60mph, although
if more discrete, cellular convection can develop - a risk for
hail up to 1" can not be ruled out. An initial round will likely
develop around noon in the Uinta Basin and move east through
northwest Colorado. Other less intense thunderstorm activity is
expected throughout the western slope, and a cold front will
likely focus another stronger round across the US40 corridor
after 5pm.


Saturday will feature another round of showers and isolated
thunderstorms in the higher terrain of the central and southern
mountains of Colorado. Weaker forcing and instability values limit
overall activity through the evening hours.

.LONG TERM...(Saturday night through Thursday)
Issued at 310 AM MDT Fri Apr 26 2019

Instability showers continue across the area Saturday evening
through Monday morning as a trough and leading shortwave move across
the northern Rockies and a cut-off low move towards southern
California. Our CWA will be caught between the two systems with
moisture feeding into the region. The moisture ramps up Monday
afternoon with specific humidity values rising towards 5 to 6 g/kg
(a bit less than previous model runs). The southern California low
opens up and tracks across the Four Corners region Monday night,
bringing with it numerous thunderstorms and potential for heavy rain
through most the day Monday into Tuesday morning. This wave quickly
shifts east as another low drops down from the Pacific Northwest
from that large scale northern trough. Not much break looks to occur
between systems with this northern trough moving through Tuesday
evening into Wednesday afternoon. This latter system does bring in
some cooler air, but it does not look as cool as previous model
runs, so models are trending a bit warmer. Regardless, lots of
moisture and unsettled conditions will remain in place this weekend
through Wednesday. High pressure builds back in for the end of the
week with a drier northwest flow setting up Thursday as the ridge
moves in. Temperatures will remain warm and above normal through the
weekend, but do show some cooling to below normal levels in response
to increased clouds and showers, which is still welcome around the


.AVIATION...(For the 12Z TAFS through 12Z Saturday morning)
Issued at 511 AM MDT Fri Apr 26 2019

Aviation conditions will deteriorate this afternoon as showers and
thunderstorms develop after the noon hour. Brief VIS and CIG
lowerings to MVFR and perhaps IFR categories are possible. RIL,
EGE, ASE, GJT and MTJ will likely see the highest impacts between
2pm and 7pm today. Conditions will improve after 04z.




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