Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 09/06/21

Area Forecast Discussion

National Weather Service Phoenix AZ

 431 PM MST Sun Sep 5 2021

.UPDATE...Updated Aviation Discussion.

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.SYNOPSIS...
Only isolated thunderstorms will form over the mountainous areas of
eastern Arizona the next several days with little to no chance of
storms descending into lower deserts. Afternoon high temperatures
will remain a few degrees above normal much of the week with the
warmest readings across SE California. During the latter half of the
week, there may be a modest increase in thunderstorm chances,
however trends are showing many locations likely staying dry.

&&

.DISCUSSION...
Pronounced high pressure with H5 heights around 594dm has become
stationed over central California early this afternoon with an
extension of the ridge axis and large scale subsidence displaced
eastward into southern UT/northern AZ. Concurrently, a sharp trough
axis and cold pool aloft were descending into southern NM/SE Arizona
providing lift and tapping one area in the region with uncapped
instability. HREF members remain consistent in developing clusters
of storms along this trough axis from the White Mountains through
the Tucson area this afternoon; and with DCape greater than 1500
J/kg, organized outflows should spill into lower elevations of Pinal
County. Per ACARS soundings, quality moisture at lower elevations
around the Phoenix area is extremely shallow essentially precluding
any chance storms could persist of redevelop off higher terrain. At
most, wind gusts 30-40 mph along the outflow with decaying showers
and blowing dust could impact the I-10 corridor around the Casa
Grande vicinity.

Through the middle of the week, the aforementioned high pressure
system will intensify, expand in breadth, and migrate into central
UT. Trends among ensemble members has been decidedly towards a
stronger and more influential ridge dominating the weather pattern
over the forecast area. The current forecast configuration will tend
promote drier air and hinder to ability to maintain deep moisture
profiles limiting more organized thunderstorm activity to northern
Mexico. Even NBM POPs for preferred higher terrain areas within the
local CWA are solidly below 15%; and not while completely precluding
any chance for a storm, this output suggests minimal convective
potential and impacts. Temperatures will remain slightly above
normal and follow near persistence type trend as heights and thermal
profiles through the atmospheric column hover in a near steady
state.

During the latter half of the week, the ridge will transition from
having a fairly high amplitude structure over Utah Wednesday to
becoming flattened over New Mexico by Saturday. During this time,
shower and thunderstorm chances may expand through southeast
California and southwest Arizona. This could be associated with
moisture shed from a yet to develop tropical system. NHC is
forecasting a disturbance currently in the vicinity of 17N 103W
moving slowly to the northwest and developing into a depression by
midweek. Global ensembles depict this system tracking relatively
close to southern Baja, but eventually dissipating and drifting
west. There are still a lot of uncertainties in how the disturbance
will evolve and what the impacts to moist advection will be as well
as subtleties in the positioning of synoptic flow features over the
Desert Southwest, but the prospects for a more robust moisture surge
and widespread rainfall are not particularly great.

&&

.AVIATION...Updated at 2330z

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
The main aviation weather concerns will focus on distant
thunderstorms to the east and south for the rest of this afternoon
and evening. Chances for terminal area storms remains low, less
than 10%, but distant storms will be capable of sending outflows
into the terminal area. Trends and recent guidance indicate a
south to southeasterly boundary with speeds in the 15-25 kt
range. Stronger gusts capable of blowing dust cannot entirely be
ruled out with chances no greater than 30%, particularly for KIWA.
If that outflow boundary does realize, expect mostly easterly
winds to continue through the overnight hours. For tomorrow, the
diurnal westerly windshift is anticipated by the late
morning/early afternoon.

Southeast California/Southwest Arizona including KIPL and KBLH:
Aside from the potential for gusty winds late Sunday night into
Monday morning, minimal aviation impacts are expected. At KIPL,
winds will mostly favor the east to southeast with a few hours of
an afternoon westerly. Speeds will generally fall in the 7-12 kt
range with some late evening and overnight gusts of 15-20 kts. At
KBLH, winds will generally favor the SSE to SSW with speeds
reaching 8-12 kts and occasional gusts up to 20 kts.

&&

.FIRE WEATHER...
Tuesday through Saturday:
Storm chances Tuesday will be meager - only about 5-10% and mainly
over the higher terrain of south-central AZ. During the latter half
of the week, storm chances trend upward - most noticeably over
southeast CA and southwest AZ. A key factor driving the forecasted
trend is anticipated moisture from a developing tropical system far
to the south (well south of Cabo). A lot of uncertainties remain
though. High temperatures will be close to normal Tuesday through
Thursday before trending down late in the week. Min RH levels will
range from 20-30% most places through the forecast period (15-20%
Tue-Thu over the south-central AZ deserts). Max RH will generally
range from 40-70% (locally higher). Apart from thunderstorms, strong
wind events are not anticipated.

&&

.SPOTTER INFORMATION STATEMENT...
Spotters should follow standard reporting procedures.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...None.
CA...None.
&&

$$

DISCUSSION...18/AJ
AVIATION...12
FIRE WEATHER...AJ