385
FXUS65 KGJT 080604 CCA
AFDGJT

Area Forecast Discussion...CORRECTED
National Weather Service Grand Junction CO
1204 AM MDT Wed May 8 2024

.KEY MESSAGES...

- Windy conditions with gusts of 45 to 55 mph decrease through
  the evening.

- Sub-freezing temperatures for the lower valleys where
  vegetation is susceptible to damage is forecast overnight.
  Other areas may see frosty conditions and anyone with concerns
  for irrigation or sensitive vegetation should take
  precautions.

- Cool and unsettled weather will linger through the remainder
  of the week. Persistent snow over the northern mountains will
  bring some accumulations to the higher elevations through
  Wednesday...before this threat shifts Southward to end out the
  week.

&&

.UPDATE...
Issued at 1202 PM MDT Tue May 7 2024

Snow has decreased over the northern and central Colorado
mountains late this evening. Additional accumulations are expected
to be light and more localized. Therefore, the Winter Weather
Advisory has been allowed to expire.

&&

.SHORT TERM /THROUGH WEDNESDAY NIGHT/...
Issued at 251 PM MDT Tue May 7 2024

Nothing like being behind the curve ball all day to inspire
confidence in the forecast going forward. A persistent
frontogenetically driven band of heavy precipitation along the
I-70 corridor has been the main culprit of my struggles with
snow rates likely well in excess of an inch per hour creating
some January-like conditions on Vail Pass. This fronto band was
being forced by a strong jet aloft rounding the base of the
very anomalous Rex pattern residing over the central NOAM. This
pattern will haunt of for several more days...and likely bring
another strikeout or two. About the only thing to go as planned
today is the very gusty winds cranking up with gusts in the 45
to 55 mph becoming more widespread this afternoon. These will
slowly taper off near sunset and then if all goes as planned
allow cold air to settle into many valley locations as we
decouple. There are still a few low confidence areas where the
winds may not totally decouple and clouds may linger...but went
ahead an upgraded the Freeze Watch area to Warnings with mainly
the higher elevations impacted over SE Utah. There will be some
higher clouds invading from the north during the early morning
hours from the North but it looks like their arrival will be too
late to impact radiational cooling. These clouds will be on the
leading edge of the shearing axis of the Rex low over Plains.
This shearing axis will eventually lead to another low forming
upstream of us over the Intermountain West by late tomorrow
night...while also providing weak ascent and orographics over
our northern CWA the next 24 to 36 hours. Again we are looking
at a prolonged snow event over our far northern mountains with
the greatest impacts over the highest elevations. Allegedly.
Will have to keep a track on trends of this for more winter
highlights during this prolonged cool and active period. For now
it looks like impacts at lower elevations including Rabbit Ears
will be limited to a several inches over this period with
higher amounts along the spine of the divide mountains. Luckily
for the most part winds looks to be the missing link as the main
jet stream is forced well south in this pattern. Temperatures
start out cold tomorrow and end up well below normal tomorrow
afternoon. Lows also look to moderate a tad tomorrow night and
freezing impacts will be a much lower probability (<20%)
outcome.

&&

.LONG TERM /THURSDAY THROUGH TUESDAY/...
Issued at 251 PM MDT Tue May 7 2024

A piece of energy rotating around the large cutoff low over the
Northern Plains will, by Thursday morning, have retrograded into the
Great Basin and begun forming a secondary cut-off low. In the
meantime, anticyclonic wave breaking will be pushing a ridge of high
pressure into the Pacific Northwest, even extending into the
Canadian Prairie Provinces. This situation sets up a Rex Block over
the Western CONUS, which fairly effectively cuts that region off
from the steering flow. Unsurprisingly, when a feature such as a cut-
off low becomes disconnected from the steering flow aloft, it tends
to sit and spin in the same location for long periods of time, days
and sometimes even weeks. Under this Rex Block pattern, which models
are in quite good agreement over, the cut-off low will set up shop
just to our west by Friday, leading to southerly flow that will tap
into some Gulf of Mexico moisture Friday into Saturday. Weak but
persistent large-scale ascent associated with the low will remain in
place through the period, adding some dynamic lift to diurnally
driven showers and thunderstorms acting on pooled moisture from that
Gulf moisture tap. A gradual warming trend through the weekend and
into next week will increase instability and make convection more
efficient day by day, which will increase the amount of QPF these
showers and storms can put down, which will then increase the pool
of usable moisture. Under this set up, look for increasingly
widespread showers and thunderstorms as we move into early next
week, with activity expected to fire initially off the terrain and
adjacent high valleys. Storms will initially be capable of producing
gusty outflow winds, as dry surface air will take a day or two to
moisten. After that, look for the threat to switch to heavy rain.

Models aren`t in the best agreement over how this Rex Block scenario
develops, with some solutions continuing the low`s retrograde right
off the West Coast, and others keeping it firmly over the Great
Basin. What matters most is that all model solutions do keep an area
of low pressure over the Southwest and high pressure over the
Northwest, maintaining the Block through early next week and keeping
conditions unsettled over eastern Utah and western Colorado. The
above-mentioned gradual warming trend will take temperatures around
5 degrees below normal on Thursday all the way up to near 10 degrees
above normal by Monday. Thanks to increasing moisture and lingering
evening convection, overnight lows will also follow a similar
trajectory. Over the course of a few days, eastern Utah and western
Colorado will jump from early spring conditions right into early
summer-like weather.

&&

.AVIATION /06Z TAFS THROUGH 06Z THURSDAY/...
Issued at 1152 PM MDT Tue May 7 2024

Showers will continue overnight over the northern and central
high elevations with KHDN, KASE and KEGE standing the best
chance at seeing MVFR so left VCSH through 12Z. Otherwise, BKN
skies and occasionally breezy winds will continue with VFR
elsewhere. Conditions should improve after 12Z Wednesday with FEW
to SCT cloud cover Wednesday afternoon and much lighter winds
than previous days but still occasionally breezy. Showers should
come to an end with exception of some higher elevations and
possibly HDN which could linger through the afternoon.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...

CO...Freeze Warning until 10 AM MDT Wednesday for COZ006-007-011.
UT...Freeze Warning until 10 AM MDT Wednesday for UTZ022-027-029.

&&

$$

UPDATE...NL
SHORT TERM...15
LONG TERM...BGB
AVIATION...TGJT