Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 05/12/19

Area Forecast Discussion

National Weather Service Phoenix AZ

 551 PM MST Sat May 11 2019

.UPDATE...Updated Aviation Discussion.

&&

.SYNOPSIS...
A low pressure system will impact the region this weekend,
bringing an increased threat of showers and isolated
thunderstorms to the Desert Southwest. The best rain chances
across south central Arizona and the Phoenix area will be during
the overnight hours tonight and Sunday afternoon. Rain chances
will linger across the eastern Arizona high terrain into Monday,
before high pressure causes warmer and drier conditions through
Thursday. A return to below normal temperatures is likely by
Friday as another Pacific low pressure system affects the
intermountain West.

&&

.DISCUSSION...
Scattered showers and isolated thunderstorms are currently
developing along the Mogollon Rim with deeper cells suggesting the
weak midlevel steering flow has become east-northeasterly. Limited
instability should prevent much if any development into the lower
deserts this afternoon and early evening, but weak east-northeast
outflows will probably make it into the Phoenix metro after 5-6
pm. PWAT values are gradually increasing across south central
Arizona and will continue to increase from 0.6-0.7 inches now to
0.8-1.0 inches overnight as the closed low currently just moving
onto northern Baja California ejects east across the Gulf of
California tonight. At the current moment, instability is fairly
limited to primarily the higher terrain with weak midlevel
stability present on aircraft soundings into KPHX. Therefore,
thunderstorms are generally not expected in the Phoenix metro
until after sunset, with none expected over the lower deserts of
southeast California and southwest Arizona.

However, the aforementioned stable layers over the lower deserts
of south central Arizona are forecast to erode by early evening
as moisture advecting into the region continues to increase CAPE
into the 250-1000 J/kg range overnight. In addition, hi-res models
show convection developing over southeast Arizona late this
afternoon that should push to the east-northeast into northern
Pinal and southern Maricopa Counties later tonight and possibly
hold together into Yuma County. Therefore, we expect to have at
least isolated to scattered showers and thunderstorms tonight into
at least northern Pinal County and southern portions of the
Maricopa County and the Phoenix metro, with some potential for
more widespread light to moderate showers on the north side of the
low tracking over the northern Gulf of California. Rainfall
amounts should generally stay below a tenth of an inch with
locally heavy amounts of a quarter to half inch in slow-moving
convective cells. Gusty winds will be possible, but DCAPE values
will be below 500 J/kg in the absence of daytime heating tonight.

Model agreement remains high in the track of the closed upper
level low as it slowly pushes east over northern Sonora, becoming
centered south of Nogales by late tomorrow afternoon. Therefore,
we should have stronger northerly or northeasterly steering flow
off of the Mogollon Rim tomorrow afternoon to support movement of
thunderstorms developing off of higher terrain into the Phoenix
metro. However, overnight showers and thunderstorms may leave
behind significant midlevel and high cloud cover that may reduce
our instability if they persist into the afternoon hours. We
currently expect that afternoon insolation will give way to
steepening lapse rates and increasing instability enhanced by the
close proximity of the closed low to allow for isolated to
scattered showers and thunderstorms in the Phoenix metro. SREF
paints high probabilities of widespread MUCAPEs exceeding 500
J/kg across Arizona, with 25-30 knot bulk shear values supporting
organized convection with the potential for gusty winds and small
hail in the strongest cells. Rainfall amounts will once again
generally be below a tenth of an inch of rain, but very localized
minor flooding will be possible in stronger, more organized cells
that produce heavier downpours up to a half inch.

The low pressure system is expected to weaken overnight Sunday
into Monday, with storms dissipating in the early evening as a
more subsident pattern develops. Although a ridge of high pressure
will build across the region early in the week, lingering
moisture will support isolated showers and thunderstorms mainly
across the Mogollon Rim on Monday and Tuesday afternoon.
Otherwise, the rest of the lower deserts will remain dry as
increasing heights translate into a warming trend that will
continue through mid-week. High temperatures will again flirt
with the 100 degree mark in the Phoenix area Wednesday, with
temperatures on Thursday a bit uncertain due to the timing of our
next westerly upper-level disturbance. A quicker progression of
this trough supported by the ECM ensemble would yield high
temperatures in the upper 80s across the lower deserts, whereas a
slower progression would result in highs in the upper 90s which
is more in line with the NBM and our forecast. Regardless, a
cooling trend is anticipated by Friday with high temperatures
likely falling back below normal into the upper 80s. This next
system currently looks to be relatively dry and windy under
southwest flow aloft with NBM PoPs generally below 5-10 percent.

&&

.AVIATION...Updated at 0050 UTC.

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

A low pressure system currently centered just off the western
coast of Baja California will continue to sponsor unsettled
weather through the TAF period. Storms over the higher terrain
northeast of Phoenix have produced an outflow boundary, which is
currently moving to the east/southeast through the Phoenix metro.
Easterly/northeasterly winds with gusts up to 20 kts will be
possible as the boundary moves through. Winds may become erratic
this evening as outflows from nearby showers and thunderstorms
move through. Model guidance shows showers moving through the
Phoenix metro early Sunday morning. Have VCSH in the TAFs starting
around 14-15Z but this may change as there is still uncertainty in
timing. Variable amounts of mid and high level clouds are expected
with bases aoa 6 kft and cigs aoa 10 kft. More showers and
thunderstorms are expected to develop tomorrow afternoon across
south-central Arizona.

Southeast California/Southwest Arizona including KIPL and KBLH:

Winds at KIPL and KBLH will favor the S/SE this evening before
KIPL takes on a westerly component tonight. Winds are expected to
remain fairly light through the period with wind speeds below 10
kts. Variable amounts of mid and high level clouds are expected
with bases remaining aoa 7 kft. A slight chance of a vicinity
shower cannot be ruled out during the TAF period but confidence
remains too low to include VCSH in the TAFs.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Tuesday through Saturday:
Aside from very isolated showers and thunderstorms on Tuesday
afternoon across the higher terrain north and east of Phoenix, dry
conditions are expected for the rest of next week. A ridge of high
pressure will quickly build across the region to usher in much
drier and warmer conditions for the middle of next week as
temperatures warm back up into the mid to upper 90s. Another
upper-level disturbance moving in from the west will drop
temperatures back below normal late in the week with wind gusts in
excess of 20-25 mph across the lower deserts and stronger gusts
at higher elevations. Minimum humidity levels will drop into the
teens across the lower deserts and the 20-25 percent range for
higher elevation locations each day with good overnight recovery.

&&

.SPOTTER INFORMATION STATEMENT...
Amateur radio Skywarn net activation is not expected at this time.

&&

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

$$

DISCUSSION...Hopper/Hirsch
AVIATION...Smith
FIRE WEATHER...Hopper