Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 10/22/23

Area Forecast Discussion

National Weather Service Salt Lake City UT

 357 PM MDT Sat Oct 21 2023

.SYNOPSIS...Dry and mild conditions will persist through the
remainder of the weekend. A cooler and more unsettled pattern
then begins late Sunday into early next week, with considerable
uncertainty noted on the pattern evolution thereafter.

&&

.SHORT TERM (Through 12Z/6AM Tuesday)...Early afternoon analysis,
satellite and radar observations indicate the presence of a mid
to upper-level shortwave trough axis located across northern Utah.
Total moisture through the column is scant, but just enough mid-
level moisture is combining with the mid-level lift associated
with the shortwave to produce some high-based cumulus across extreme
northern and northeast Utah as well as across SW Wyoming early
this afternoon. Some sprinkles have been observed with this
activity. Given strong mid-level lapse rates on the order of
8-8.5C/km and DCAPE values ranging from 700-1000J/kg from SW
Wyoming southwest to north-central Utah, we can`t rule out an
isolated microburst with this activity. Mitigating factor is the
presence of a nocturnal inversion, which is still apparent on 19Z
aircraft soundings. This inversion will act as a barrier
preventing most microburst activity from reaching the surface
except for in very isolated instances. Model forecast soundings
suggest nocturnal inversions will mix out across SW Wyoming,
suggesting best potential for gusty outflow winds/microburst
activity here. Indeed, CAMs do seem to reflect this potential
here, with ensemble max gusts advertised around 35 mph late this
afternoon across SW Wyoming. CAMs suggest this activity will
quickly diminish around sunset tonight thanks to waning forcing
with the shortwave forecast to depart coupled with the lull in
daytime heating.

Model consensus shows shortwave ridging building across the area
Sunday morning behind the departing shortwave. This will signal a
return of dry, clear conditions. These conditions appear short-
lived as a closed area of low pressure is forecast to drop into
the Great Basin late Sunday. As this low approaches, the pressure
gradient will strengthen across Utah and SW Wyoming, and as such
SW winds in the 25-35 mph range are forecast to develop across
western valleys during the afternoon. As the low drops south into
the Desert Southwest and evolves into a wakening trough across
Utah on Monday, a cold front will drop across the state in the
process. The front is forecast to drop into northwest Utah Sunday
evening before reaching southern Utah by Monday morning. This
will drop temperatures by ~10F between Sunday and Monday, bringing
temperatures back down to near seasonal normals for this time of
year. A period of gusty NW winds will accompany the frontal
boundary across wind-prone areas of NW Utah mainly after midnight
Sunday night into early Monday morning, with pockets of NW wind
gusts in the 25-35 mph range, although widespread, gusty winds
are not expected. Behind the front, a smattering of mainly light
showers are expected to develop across northern Utah late Sunday
night and continue through Monday morning.

.LONG TERM (After 12Z/6AM Tuesday)...The extended period continues
to have a lot of uncertainty as to what will transpire. In general
two main scenarios exist and they are: some version of a longwave
trough impacting the area, or a ridge/zonal flow scenario. Within
these two main scenarios exist four possible outcomes: a weak trough
that clips the northern portions of Utah and southwest Wyoming, a
deep trough that would impact much of the Great Basin, a zonal flow
that would keep temperatures near normal, or a ridge that would have
above normal temperatures and quiet and dry weather conditions
through the week.

When looking at ensemble guidance, each of these scenarios has a
decent chance of occurring. The weak trough outcome has the lowest
overall chance of occurring based on the ensemble members with
around a 20% chance. The majority of this outcome stems mainly from
the EPS and CMCE guidance. Out of the three deterministic models,
the EC has this solution. This scenario would result in mostly dry
conditions for southern Utah and some increased chances of
precipitation across northern Utah and southwest Wyoming.
Temperatures with this system would dip slightly below normal
depending on how far south this system would get.

The other impactful scenario and the most favorable amongst ensemble
guidance is the strong trough. The GFS and GEFS are all in on this
solution occurring. Greater than 90% of all GEFS members support
this scenario. This would bring cold temperatures and the greatest
chance at seeing precipitation across Utah and southwest Wyoming.
Depending on how cold we get from this system we could be looking at
our first freeze of the year along portions of the Wasatch Front.
This would also mean the potential at seeing the first snowfall of
the season in many low elevation valleys in northern and central
Utah. Current ensemble guidance gives this about a 35% chance of
occurring.

Looking at the drier solutions, the first outcome is one that
supports a more zonal flow for the area. This scenario is similar to
the weak trough as the majority of ensemble members supporting this
are from the CMCE and EPS. This would be a mostly dry and quiet
solution with temperatures being slightly above normal. Current
ensemble guidance gives this between 20% and 25% of occurring.

The final solution is a ridge. This outcome is dominated by the CMC
and CMCE. Ensemble guidance has around 25% of all ensembles in
support of this. Weather conditions would be dry and this would give
us the warmest temperatures out of the four options.

In summary, there is a lot of uncertainty surrounding the outcome.
Our forecast area could continue on with below normal temperatures
after a trough moves through early in the week and increased
precipitation chances or see warm and dry conditions return.


&&

.AVIATION...KSLC...VFR conditions will persist at KSLC through the
TAF package. Mostly clear and sunny skies will continue with light
northwest winds through the evening. Normal southerly wind shift is
expected ~03Z with an increase in cloud cover with it as well.

.REST OF UTAH AND SOUTHWEST WYOMING...VFR conditions will persist at
all terminals in Utah and SW Wyoming through the TAF package. Light
and diurnally driven winds will remain for the day with increasing
high based clouds overnight. Showers and an isolated storm linger
over northern UT and SW Wyoming through sunset this evening but
expected to be benign.


&&

.FIRE WEATHER...Southwesterly flow will increase Sunday as an
upstream storm system approaches, bringing locally breezy
conditions to mainly the western Utah valleys while maintaining
warm temperatures. Across these western Utah valleys, southwest
wind gusts in the 25-35 mph range are forecast to develop during
the afternoon with minimum RH values dipping into the mid to upper
teens once again. The upstream storm system will spread a cold
front across Utah on Monday, dropping temperatures around 10
degrees across the state while providing an opportunity for
isolated showers across northern Utah as well as high terrain
areas of central and southern Utah. Behind the front, RH values
will increase for Monday, with a few pockets of teens across SE
Utah as well as good overnight recovery most valleys into Tuesday
morning. Dry but seasonable temperatures are then forecast to be
in store for Tuesday with min RH values staying out of the teens
all areas.

There remains significant model spread regarding the pattern
after Tuesday. Solutions continue to range from a dry and mild
near-westerly flow pattern through late week, to high pressure
building back into the area, to a strong storm system dropping
from the north Wednesday or Thursday bringing a strong cold front
and potential for significant precipitation including snow to the
valley floors. At this point, all three scenarios remain in play
and there isn`t enough confidence to lean the forecast in the
direction of the three scenarios.


&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...None.
WY...None.
&&

$$

ADeSmet/Cecava/Selbig

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