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FXUS65 KGJT 042307
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
507 PM MDT Sat May 4 2024

.KEY MESSAGES...

- Very strong winds remain likely for much of eastern Utah and
  western Colorado tomorrow, with gusts upwards of 65 mph for
  the valleys and 80 mph for the mountains. Numerous wind
  highlights are in effect.

- A strong cold front will move through Sunday night into
  Monday, bringing widespread precipitation and the potential
  for several inches of accumulating snow to the mountains.

- Much cooler and unsettled weather will remain for the coming
  week, with several disturbances keeping mountain showers in
  the forecast most afternoons.

&&

.SHORT TERM /THROUGH SUNDAY NIGHT/...
Issued at 323 PM MDT Sat May 4 2024

As expected, a shortwave tracking along the southern border of
eastern Utah and western Colorado has provided some extra lift and
moisture to kick off some light showers over the southern and
central Colorado mountains. No lightning as of yet, but the
potential for a thunderstorm or two remains through this evening.
Elsewhere across the region, conditions have been quiet, with mostly
sunny skies and relative light southwest winds. Change is on the
horizon, however, as the much discussed strong Pacific low has
already moved into the Pacific Northwest and the gradient ahead of
it has already begun to tighten up. The nose of this jet will be
into western Utah late tonight, and will be pushing into the Four
Corners by sunrise tomorrow morning. Between the increasing surface
winds in advance of this system and increasing cloud cover thanks to
midlevel moisture being advected in on the jet, tonight`s lows will
be milder than the last few nights, running around 10 degrees above
normal.

All eyes turn to the much discussed winds tomorrow. The approaching
southwesterly jet is stacked from 700mb all the way to 200mb, with
speeds starting out at 50-60 knots and increasing with altitude to
100 knots or more. Ensemble and climatological guidance all agree
that this is a rare event, with some of these wind speeds above the
99th percentile for this time of year, and with return intervals of
one day in every 5-10 years, depending on what source you look at.
Winds are expected to begin picking up after midnight tonight, with
gusts of 50-60 mph possible at the higher elevations of eastern Utah
and far western Colorado by 6AM. As the day progresses, strong
diabatic heating will promote deep mixing, tapping into those higher
wind speeds aloft, and bringing down gusts up to 60 mph in many of
the valleys, and up to 80 mph across the higher elevations. In
addition, virga showers will be possible in the afternoon as moist
midlevel air overrides very dry surface air, which will lead to
localized enhancement of already strong gusts. And on top of that,
downsloping off of terrain features such as the Uncompahgre Plateau
will locally enhance winds as well. With the consistency and
relative rarity of an event like this, have opted to issue High Wind
Warnings for eastern Utah and far western Colorado, with Wind
Advisories picking up as one moves east. The highlights for the
higher elevation zones kick in at 6AM, with the lower elevations
joining in at 8AM. All highlights run through midnight. The core of
the jet moves in late tomorrow afternoon and into the evening hours,
with peak winds expected between 3 PM and 9PM. Winds will gradually
drop after 9 PM from west to east as the cold front pushes through
and winds weaken aloft.

Aside from the winds tomorrow, deep mixing will promote warming
through the day, although clouds and moisture will limit how warm we
can get. Highs are expected to top out near normal, before rapidly
crashing as the above-mentioned cold front moves through Sunday
evening into Monday morning. The frontal band is progged to be on
the doorstep around 6PM tomorrow afternoon. Warm air ahead of the
front will keep precipitation as mostly rain, at least to start
with, with higher elevations seeing a rain/snow mix or all snow
possible with post-frontal showers. Little in the way of
accumulations are expected with the exception of mountains above
9000 feet. The southern mountains will see a quick couple of inches,
with the central and northern mountains seeing amounts approaching
advisory amounts. Will leave it to the midnight crew to make a call
on whether highlights are warranted for these areas, as amounts
continue to drop from model run to model run.

Enjoy the sunshine today, and make sure to prepare for a big change
in the coming days. Take time to secure loose outdoor objects if
possible, and stay up to date with the latest changes to this
complex forecast.

&&

.LONG TERM /MONDAY THROUGH SATURDAY/...
Issued at 323 PM MDT Sat May 4 2024

The main trough will be swinging through northeast Colorado and the
Front Range into the northern High Plains by Monday afternoon with
some lingering northwest flow behind it over our northwest Colorado
and central Colorado mountains. This will allow high elevation
snowfall to continue although rates are expected to drop off quite a
bit as the strongest forcing exits the region. Showers may lessen
mid day Monday before picking up again Monday evening into Tuesday
morning and potentially continuing through much of the day Tuesday
as an additional shortwave moves across the north following the main
trough. Snowfall amounts with this second wave favor the Park Range
and to a lesser extent the Flattops with the better accumulations.
Through much of the week, we will stay in this unsettled flow
pattern as the broad low pressure trough remains anchored over the
Intermountain West and Northern Rockies as additional weak
shortwaves drop in from the north across our Continental Divide
mountains, resulting in a chance of showers over the northern and
central high terrain, but snow accumulations among the higher peaks
are expected to be limited with minimal impact. Temperatures during
this period will continue to remain cooler than normal through at
least mid week, by about 10 to 15 degrees below climatology for
early May. In fact, Tuesday night into Wednesday morning may see the
potential for more widespread freezing temperatures, with some lower
elevations like the Grand Valley and Uncompahgre Basin being most
susceptible due to budding vegetation.

As this large trough exits to the east by Wednesday finally, we will
begin to warm up for the latter half of the week with temperatures
warming to 5 degrees below normal Thursday and Friday and near
normal by next weekend. There is some model discrepancy though in
the synoptic pattern as the GFS shows a low dropping southward
through the Great Basin by late in the coming week and potentially
closing off from the main flow, leaving our CWA underneath a broad
col or deformation region. The ECMWF is much further west with this
cut off low feature, bringing it all the way out to northern
California, with less influence over the western slope.  This cut
off low shows signs of becoming Rex Blocked, but it`s just a matter
of how far west and the positioning of these features is key to our
sensible weather and forecast. Still a ways out but much of the
coming week looks to be cooler than normal and unsettled for the
most part with a slow warm up towards the latter half of the work
week.

&&

.AVIATION /00Z TAFS THROUGH 00Z MONDAY/...
-- Changed Discussion --
Issued at 507 PM MDT Sat May 4 2024

Clouds and isolated high-based showers continue across the
southern and central Colorado mountains late this afternoon as a
system passes through. Quiet weather is expected tonight area-
wide but clouds will once again increase from west to east as
the next cold front approaches. This will bring a tightening
gradient with widespread sustained southwest winds of 20 to 30
kts and gusts of 40 to 50 kts as early as 12Z Sunday but more-so
from 15Z onwards. Scattered showers are also expected over the
higher terrain throughout the day. Snow levels will remain above
9kft before dropping Sunday evening.
-- End Changed Discussion --

&&

.GJT WATCHES/WARNINGS/ADVISORIES...

CO...High Wind Warning from 8 AM Sunday to midnight MDT Sunday
     night for COZ001-006-011-020.
     Wind Advisory from 8 AM Sunday to midnight MDT Sunday night
     for COZ002-007-008-021-022.
     High Wind Warning from 6 AM Sunday to midnight MDT Sunday
     night for COZ003-017.
UT...High Wind Warning from 8 AM Sunday to midnight MDT Sunday
     night for UTZ022-024-027-029.
     High Wind Warning from 6 AM Sunday to midnight MDT Sunday
     night for UTZ025-028.
     Wind Advisory from 6 AM Sunday to midnight MDT Sunday night
     for UTZ023.

&&

$$

SHORT TERM...BGB
LONG TERM...MDA
AVIATION...TGJT