Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 08/18/21

Area Forecast Discussion

National Weather Service Phoenix AZ

 522 PM MST Tue Aug 17 2021

.UPDATE...
Updated Aviation Discussion.

&&

.SYNOPSIS...
Thunderstorm chances will increase by late evening and overnight
before falling off Wednesday evening and night, leaving very
little storm chances remaining by Thursday. Until that time, a
very moist environment will support scattered to isolated showers
and storms through south-central Arizona at times. Flooding
rainfall remains the greatest threat followed by gusty winds and
lightning. With much drier conditions arriving by the end of the
week, temperatures slightly below average will slowly warm back to
near normal levels early next week.

&&

.DISCUSSION...
Convection commenced by late morning again today as a quasi-
stationary upper low/inverted trough and vort max disturbances
remained positioned across S-SE-Cent to E to NE AZ. Midday radar
and METSAT imagery showed areas of developing thunderstorms mostly
over SE AZ, the White Mountains and along the Rim NE of Phoenix.
The anomalously moist airmass remains in place across the region
today and was assisting the aforementioned triggers. The UL ridge
axis was weakened and repositioned from S CA to the 4-Corners in
advance of a low pressure system moving through the PAC-NW with
drier, weak NE flow aloft. Late morning ACARs mean soundings
maintained elevated mean PW of 1.78" due to a moist column from
the BL through mid levels/H5. Winds from just above H7 to H5 were
still NEerly. With an overturned column the very skinny MU CAPE
was below 800 j/kg.

High to decent chances for Monsoon storm activity will persist
through Wednesday for Phoenix and the deserts. However the very
best chance of thunderstorms, including a WPC Day 1 slight risk
of excessive rainfall, through Wed 12Z, could occur on and off
from early Tue evening to early Wed morning when NBM PPI/POPs
spike up to 70% between 06-12Z. HREF members vary on the timing of
on and off storms through tonight as the upper level disturbances
essentially remain in place.

The NW-PAC low will slowly advance inland into the Intermountain
West through Wed night. On Wed morning SW flow develops aloft in
advance of the trough and begins to eject the local disturbances NE
into E AZ and beyond. During this time the HREF shows a second round
storm chances associated with an area of pre-trough instability.
Again the HREF members vary on their timing and coverage of
thunderstorms tracking NE through Phoenix and the deserts into the E
AZ high country. Some members favor storms developing in the mid-
late morning while others favor early-mid afternoon. The forecast
coverage also varies from very isolated to a bit scattered. The
uncertainty is likely due to the beginning of an accelerated
drying trend starting Wed although during a time when PW are still
a bit elevated near 1.4- 1.5".

The south-central AZ deserts are in a WPC Slight Risk of flash
flooding through tonight while S. Gila County remain in a Slight
Risk through Wed night. The latest HREF probability of 3-hr QPF
>1" for very early Wed morning for Phoenix and the deserts is
~10-20%, and 30% for S Gila Cty and far E Maricopa Cty and far NE
Pinal county by Wed afternoon. However given the wet soils and
limited amount of rainfall needed to create impacts over burned
areas, some flood threat will persist into Wed evening. The threat
for strong thunderstorm wind gusts remains in the forecast as
well. The HREF chances of strong thunderstorm winds >=35 mph is
10-20% late tonight and 10% again on Wed afternoon.

Virtually all ensemble output indicates a rapid drying trend
Thursday and beyond with forecast soundings showing 850-700mb
mixing ratios dropping below 8 g/kg Wednesday night, then eroding
the lower level moisture during diurnal mixing Thursday. Remaining
south of the trough base, anti-cyclonic midlevel warming will
also contribute to the lack of convection. With ensemble data
converging on a common solution, NBM POPs have fallen solidly
below 10% for the entire latter half of the week. Into the first
half of next week, some ensemble members indicate subtropical
ridging over the SE Conus retrograding back into the Southwest and
starting the cycle of increasing storm coverage again while other
members reload the Great Basin trough and maintain convectively
unfavorable westerly flow. So far, nothing suggests a return to
the current very active type pattern for at least the next 7-10
days.

&&

.AVIATION...Updated at 0022Z...
South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:

Thunderstorms over Gila and eastern Yavapai Counties are expected
to struggle to survive on the Valley floor this evening. Thus,
not anticipating anything like last night. However, they could get
close enough for VCTS (especially at KIWA and KSDL) and cause a
wind shift. Will need to continue monitoring this closely.
Otherwise, anticipate south-southwest winds early this evening
before transitioning back to southeast (light and variable) by
03Z. Later tonight, a disturbance currently centered over the
AZ/Mexico border is expected to track northward and generate
shower and thunderstorm activity over and near the Greater
Phoenix area. Once again, not anticipating a severe squall line
situation but instead scattered activity that has a good chance of
occurring within 5SM of each of the TAF sites roughly between
06Z-14Z with most active period 08Z-12Z. This would likely create
erratic winds in addition to the reduced ceilings and visibility
with the precipitation (remaining AOA FL050). As an aside, higher
terrain to the east of Greater Phoenix will have a better chance
of storms.

Anticipate thinning of cloudiness after 14Z as southwesterly flow
aloft develops. But, there will be one more window of opportunity
for storm development Wednesday afternoon. There is more
uncertainty about this period than tonight`s. Wouldn`t be
surprised to see development over the Estrella`s begin by midday
and then move east-northwestward. Activity Wednesday afternoon,
if any, would likely be finished by sunset if not sooner.


Southeast California/Southwest Arizona including KIPL and KBLH:

Winds will favor southerly directions at KBLH through Wednesday
with varying speeds (gusts 25-30kts Wednesday afternoon). Imperial
will have southeasterly winds early this evening before
transitioning to southerly then westerly late this evening.
Anticipate westerly winds to strengthen at KIPL during the
afternoon Wednesday with gusts to 25kts. Otherwise, no significant
cloudiness.

&&

.FIRE WEATHER...
Thursday through Monday:
Substantially drier conditions will overspread all districts during
the latter half of the week with almost no chance of thunderstorms
or wetting rains. These chances only increase marginally over
eastern districts early next week. With temperatures mostly hovering
slightly below normal, minimum afternoon humidity levels will
generally range between 20-40% through Friday, then potentially
falling closer to 15-25% over the weekend and early next week.
Overnight recovery will be good to excellent in a 40-80% range.
Winds should be relatively light with an afternoon upslope component
and overnight drainage common.

&&

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

&&

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

$$

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