Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 08/17/21

Area Forecast Discussion

National Weather Service Phoenix AZ

 515 PM MST Mon Aug 16 2021

.UPDATE...
Updated 00Z aviation discussion

&&

.SYNOPSIS...
Active monsoon weather will persist over much of Arizona into the
middle part of the week creating daily chances for showers and
thunderstorms. The best chance of thunderstorms through midweek
will be this afternoon and evening. The primary threats with any
storms will be localized heavy rainfall that may cause flash
flooding and strong winds. A steeper decrease in moisture starting
Wednesday should reduce rain chances across the area as early as
Wednesday afternoon, but especially by Thursday. Much drier
conditions are then anticipated by the weekend with temperatures
eventually warming back to normal readings.

&&

.DISCUSSION...
Convection is off to an earlier than normal start today with
midday radar and METSAT imagery showing areas of developing
thunderstorms from NE of Tucson to the White Mountains/W NM and
along the Rim NE of Phoenix. The storms SE-E of Phoenix and W-
Cent NM formed along a positively tilted H3 trough axis aloft
while the Rim storms were being triggered by a series of vort max
disturbances grouped near a weak H5 E-W trough and thermal/cold
trough embedded in the northerly flow over N AZ-NW NM. The
anomalously moist airmass remains in place across the region today
to complement the aforementioned triggers. The UL high has not
moved much and is centered over Cent-CA at H5 with moist NE flow
aloft and Nrly flow to the N. The latest ACARs mean soundings
maintained elevated PW of 1.91" owing to a moist column from the
BL through mid levels/H5. Also H7-H5 steering/flow remained
NEerly. Also, MU CAPE had jumped up to 2,381 j/kg with modest CIN.

Fairly decent chances for Monsoon storm activity will persist
through Wednesday for Phoenix and the deserts. However the very
best chance of thunderstorms, including an SPC Day 1 slight risk
of severe convection, through Wed will occur early evening when
NBM PPI/POPs spike up to 70%. Models agree on the area of
disturbances to N-NE of the area to shift SWrd into S-Cent AZ
including Phoenix and the lower deserts this afternoon and
evening. As a result An organized area of storms is expected to
fire along the rim with scattered to possibly/occasionally
numerous storms descending through Phoenix and the lower deserts
this evening and tonight.

These storms will be capable of producing strong winds, heavy
rainfall, and localized flash flooding, and hail as potential
threats. The latest HREF continues to call for a >=70% chance of
35 mph wind gusts this evening in and around Phoenix and a
>=10-20% chance of locally severe wind gusts of 60-75 mph.

The latest HREF probability of 3-hr QPF >1" for this evening has
increased to >=50% with WPC Day 1 ERO in the slight range for
Phoenix and the deserts. This generally puts the risk of rainfall
exceeding flash flood guidance at 10-20%. In preparation for this
system the Flash Flood Watch for S-Cent AZ continues through 2 AM
MST on Monday night. Although there remains 40-50% chance for
lingering scattered showers and storms to last until dawn for
Phoenix and the deserts. The HREF members are in good agreement on
the timing of scattered thunderstorms in the early evening for
Phoenix and the lower deserts. Although the FV3 is a slight
outlier with a bit larger coverage for storms even though the
other members already have pretty good coverage.

The shortwave trough should mostly exit east of the region on
Tuesday with the flow aloft weakening over the area. Guidance
mostly points toward only modest storm chances during the day
Tuesday, mostly focused over the high terrain. Can`t rule out some
of the high terrain storms making their way into the lower
deserts, but the steering flow will be quite weak and shifting out
of the west later in the day. Chances for showers and isolated
storms should then increase Tuesday night into Wednesday morning
from Phoenix and areas to the east of Phoenix as a unseasonably
strong trough digs southeastward into the Great Basin and a fairly
strong vort max tracks northward into southern Arizona later on
Wednesday. Increasing difluent flow aloft should provide some
broad scale ascent, possibly combining with potential vorticity
from the incoming vort max sometime on Wednesday. Overall, the
majority of the EPS and GEFS members support an area of rain
developing across southern into central Arizona sometime Tuesday
night into Wednesday morning, though instability will be fairly
limited and elevated. Forecast PWATs of 1.6-1.9" would support
some heavy rainfall, so an extension of the current Flash Flood
Watch may be needed.

Drier air aloft will begin working into the area from the west
southwest later in the day Wednesday, but chances for showers and
storms are likely to continue through Wednesday evening across
eastern Arizona. The drier air should really be noticeable
starting Thursday with PWATs lowering to around 1.25", which
should completely end rain chances across the lower deserts. May
see a few isolated showers and storms across the high terrain on
Thursday, but NBM PoPs are only 15-25%. Ensembles then support
continued dry westerly flow aloft through the upcoming weekend,
likely even ending rain chances over most of the high terrain.
Temperatures will also begin to warm later this week, but we may
not see highs back to normal readings until maybe the weekend.

&&

.AVIATION...Updated at 0015Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:

Thunderstorm impacts to the Phoenix terminals this evening will
the main aviation impact through the TAF period, with potential to
cause severe winds, heavy rainfall, and frequent lightning. Strong
winds may also pick up some dust, with KIWA having the best chance
of visibility reductions due to dust. There is still a 70% chance
of northerly to easterly outflows above 30 knots moving through
the terminals, with a 15% chance for outflows to exceed 50 knots
and even very isolated gusts up to 70 knots within thunderstorms.
However, the timing is more uncertain as storms over the higher
terrain are showing signs of weakening. If those push strong
outflows into Phoenix between 01-02Z, thunderstorms would likely
occur between 02-04Z when we have the TEMPO in place. If not, a
disturbance over northeast Arizona may help trigger storms between
05-07Z. For now, we are shading towards the earlier period with
VCSH from 04-06Z, but will amend as needed to reflect trends. A
Severe Thunderstorm Watch is currently in effect until 07Z.

Light and variable winds are expected overnight once the shower
activity ends, with perhaps a brief 1-2 hour period of southerly
winds behind any organized lines of storms. Winds will eventually
settle upon easterly headings at 5-10 knots before becoming
westerly between 20-22Z with a brief period of southerly winds for
a couple hours ahead of that switch. Another round of showers and
thunderstorms will be possible again tomorrow evening or during
the overnight hours into Wednesday morning, but the evolution of
tonight`s convection or lack thereof will strongly affect our
storm chances tomorrow. Thus, for now we have not included any
convection in the extended TAF for KPHX and will reevaluate for
the next package. Ceilings will generally remain above 6-8 kft
aside from brief decreases to 4-5 kft within thunderstorms.

Southeast California/Southwest Arizona including KIPL and KBLH:

Gusty winds this evening and again on Tuesday afternoon will be
the main aviation impacts through the TAF period. In addition,
isolated thunderstorms capable of producing blowing dust and
strong to severe winds will be possible late tonight over La Paz
County and western Yuma County, but the more northerly steering
flow should keep impacts generally east of the region. At KBLH,
winds will generally favor a southerly component with speeds
around 10-15 knots aside from gusts up to 25 knots tonight and 30
knots on Tuesday afternoon. Winds at KIPL will have more southeast
headings tonight before southerlies persist through Tuesday as an
upper level disturbance approaches from the west, with gusts up to
25 knots expected. FEW-SCT cloud decks will stay above 10-15 kft
aside from any thunderstorms in southwest Arizona.

&&

.FIRE WEATHER...
Wednesday through Sunday:
The threat for showers and thunderstorms will continue through
Wednesday before a drying trend sets in late in the week with
dwindling chances for wetting rains on Thursday and likely little
if any rain chances by Friday. Minimum humidities mostly between
30-45% on Wednesday will gradually lower to 15-30% late in the
period. Temperatures will remain below normal across much of
Arizona through the period, while the western deserts see near
normal readings.

&&

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

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Flash Flood Watch until 2 AM MST Tuesday for AZZ534-537>563.

CA...None.
&&

$$

DISCUSSION...Sawtelle/Kuhlman
AVIATION...Hopper
FIRE WEATHER...Kuhlman