506
FXUS64 KSJT 050424
AFDSJT

Area Forecast Discussion
National Weather Service San Angelo TX
1024 PM CST Wed Dec 4 2024

...New AVIATION...

.KEY MESSAGES...

- Patchy fog is expected to form across the southern and eastern
parts of the forecast area tomorrow morning.

- Shower and thunderstorm chances remain intact from Friday
morning and through the weekend.


&&

.SHORT TERM...
(This evening through Thursday)
Issued at 227 PM CST Wed Dec 4 2024

19Z upper air analysis reveals a split-flow pattern encompassing the
Lower 48, where an amplified, shortwave trough continues to pivot
across the Great Lakes region. Farther west, a Rex Block remains
intact over the Great Basin, with the broad, cyclonically-curved,
southern-stream jet streak rounding the base of the open trough
wobbling over the Desert Southwest. The superposition of this jet
streak relative to the Rex Block and downstream shortwave trough has
resulted in a persistent channel of Pacific moisture to advect over
the Edwards Plateau and Hill Country, where thick cirrus banding
has, and will continue, to advect over the southern half of the CWA
throughout the remainder of the short-term period. The mid-level
circulation embedded within the southern tranche of the Rex Block
was ill-defined on the 12Z objectively analyzed upper air data this
past morning; however, trends in water-vapor imagery have shown an
organization of the primary vorticity lobe rotating about the base
of the Rex Block. This vorticity lobe has taken on a more-southward
progression the last few hours as it rotates over the Mojave Desert
via the influence of a well-defined, mid-level deformation zone that
continues to propagate southward along the inflection point of the
Rex Block and shortwave trough over the Upper Midwest. This will
have implications for shower and thunderstorm chances heading into
the weekend, which will be discussed further in the long-term
section below.

At the surface, a synoptic-scale cold front extends along a line
from near HOB-LBB-CDS-HBR, and continues to progress southward
towards the Big Country/northern portions of the CWA. A secondary
cold front is also located north of the I-40 corridor, extending
along a line from DHT-GAG-PNC, and will also continue to move
southward and into the CWA by tomorrow morning. To the south of this
front, a pre-frontal, west-to-east-oriented surface trough is draped
across the Edwards Plateau and into the Lower Pecos River Valley,
with a weak, lee cyclone slightly west of the CWA. The presence of
this surface trough, albeit diffuse, has caused surface winds to
veer southwestward over the last few hours, where the weak component
of adiabatic compression and related improvements to boundary-layer
mixing have steadily eroded the low-level stratus deck spread across
the CWA this early-afternoon. Further erosion of the stratus deck is
expected heading into the evening, especially as the weakening cold
front moves in from the north and advects additional dry air into
the region. Temperatures for this afternoon were lowered a few
degrees to account for the significant delay in diabatic heating.

Later tonight, winds will veer northward following the passage of
the weakening cold front with speeds remaining light/less than 10
mph. Reinforcement of the weak CAA in the wake of the secondary cold
front should occur shortly before sunrise, with an expectation for
some patchy fog to develop across the southern and eastern portions
of the CWA tomorrow morning as the boundary-layer saturates. The
extent of fog is a bit less certain owing the redevelopment of
stratus, and whether or not dense fog forms. Surface winds will also
veer northeastward following the passage of the secondary front,
followed by a slight uptick in speed as pressure tendencies rise
from a 1032 mb surface high settling in from the north. Temperatures
tomorrow morning will also range from the upper 30s near the I-20
corridor to the middle-upper 40s across the I-10 corridor.

Sincavage

&&

.LONG TERM...
(Thursday night through next Wednesday)
Issued at 227 PM CST Wed Dec 4 2024

The Rex Block over the western U.S. should begin to collapse on
Thursday as a high-frequency wave breaking pattern persists over the
northern waters of the Pacific Ocean. The aforementioned vorticity
lobe rotating over the Mojave Desert is forecast to evolve into a
closed, mid-level low by tomorrow evening as a neutrally-tilted
shortwave trough over the far northeastern Pacific Ocean becomes
negatively-tilted heading into Friday morning. This will cause the
amplified shortwave trough over the northeastern U.S. to progress
farther eastward, while the surface high lagging behind rotates
eastward into the Mississippi River Valley. Temperatures will be
nearly 10 degrees cooler tomorrow as overcast remains amidst
easterly flow at the surface and low-levels, capping mixing heights
near or below 1-km AGL beneath the broadly cyclonic flow aloft.

The split-flow pattern across the U.S. will persist into Friday,
with an amplifying, positively-tilted trough evolving over the
Desert Southwest where a closed low will be embedded within.
Increasingly difluent, high-level flow is expected to gradually
overspread the Edwards Plateau heading into Friday morning, with
area-averaged and individual forecast soundings indicating a classic
profile for elevated thunderstorms. However, most-unstable parcels
will yield <250 J/kg, and updrafts will struggle to reach heights
high enough to produce lightning on Friday. Therefore, only a slight
chance mention of thunderstorms have been reflected in the official
forecast with a preference towards mainly rain showers area-wide as
this precipitation shield is driven by moist, isentropic ascent
beneath a gradual increase in divergent, mid/upper-level outflow.
Temperatures will also trend even cooler on Friday, with highs
struggling to breach 50 degrees as dense overcast restricts diabatic
heating amidst the upslope flow regime.

Global NWP guidance is in excellent agreement with the progression
of the positively-tilted trough heading into the weekend, with
indications of the closed low eventually dissipating by Sunday as a
northern-stream shortwave trough digs into the northern Rocky
Mountains. Prospects for storms embedded within the broader shield
of rain showers will improve by Saturday as geopotential height
falls steepen upon the arrival of the closed low, though organized
convection is not expected as the CWA will remain bereft of any
mesoscale boundaries and/or synoptic-scale fronts. Therefore,
coverage of showers and storms should remain scattered as it
continues to be driven by WAA/moist, isentropic ascent. The elevated
mixed layer (EML) should be inhibited from improving further as high
relative humidity continues to advect through the low- and mid-level
theta surfaces, with the EML garnering <250 J/kg throughout the
weekend. Locally heavy rainfall will be possible, particularly as
warm-cloud depths near 10 kft AGL beneath a belt of intense 300 mb
divergence on Saturday, and the severe weather threat remains low.

Chances for showers and storms will begin to wane on Sunday
afternoon as the positively-tilted trough becomes open while also
dampening in amplitude, and it is forecast to become absorbed into a
longer-wave trough enveloping the Lower 48 by early next week. A
warming trend will follow into Monday as NVA/subsidence on the
backside of the trough advects into the region, with a polar cold
front forecast to move through the entire CWA by Tuesday morning.
Cooler and drier weather will then prevail through middle of next
week as full-latitude troughing develops over North America.

Sincavage

&&

.AVIATION...
(06Z TAFS)
Issued at 1023 PM CST Wed Dec 4 2024

IFR ceilings will develop and move in from the south between
06-09Z, with visibilities also lowering in fog from 09-12Z for
some locations. Ceilings should hold in place through at least
15Z, followed by gradual improvement through 21Z.

&&

.PRELIMINARY POINT TEMPS/POPS...
Abilene     67  42  57  35 /   0   0   0   0
San Angelo  69  43  60  38 /   0   0   0  10
Junction    70  45  63  38 /  10   0   0  10
Brownwood   64  43  58  33 /  40   0   0   0
Sweetwater  67  41  56  36 /   0   0   0   0
Ozona       69  46  61  41 /   0   0   0  10
Brady       67  45  60  37 /  20   0   0  10

&&

.SJT WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$

SHORT TERM...99
LONG TERM....99
AVIATION...SK