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-- Discussion containing changed information from previous version are highlighted. --
744
FXUS65 KPSR 182359
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
459 PM MST Sun Apr 18 2021

.UPDATE...
Updated 00Z aviation discussion.

&&

.SYNOPSIS
Isolated sprinkles and showers are expected this afternoon and
evening across central Arizona as a low pressure system drifts
southward through the state. An isolated thunderstorm capable of
producing strong wind gusts is also possible. Above normal
temperatures will resume Monday with highs climbing into the low
90s across the lower deserts. Another weather system during the
middle of next week will bring a return of somewhat cooler
temperatures and increased breezy conditions.

&&

.DISCUSSION...
Latest water vapor imagery reveals cyclonic flow across Arizona
with drier air positioned across the Mojave Desert. RAP analysis
confirms the presence of a closed low at 700 mb, which is evident
on water vapor imagery across southern Maricopa and Pinal
Counties. Surface observations also suggest the presence of a
surface low or trough a bit further south. Given the positive tilt
of the upper trough, anomalously strong northeast flow is
resulting in breezy conditions across much of the forecast area.

Main impact from the aforementioned low pressure system will be
the potential for isolated convection across central Arizona this
afternoon. Latest mesoanalysis does not indicate any organized
areas of instability across the Desert Southwest, however 12z run
of the HREF and GEFS suggest max SBCAPE/MUCAPE up to 250 J/kg will
be sufficient for isolated showers and storms to develop this
afternoon.

Latest hi-res guidance including the HRRR indicates showers and
isolated thunderstorms will blossom rapidly across central Arizona
early this afternoon. Meanwhile, neighborhood probabilities
indicate the threat could extend as far west as La Paz and western
Maricopa counties as the low drifts due southward. Although
dewpoints have increased considerably since this time yesterday,
conditions remain relatively dry near the surface with dewpoints
in the 20s and 30s. This will yield strong evaporational cooling
along with areas of virga/sprinkles, and consequently an
environment favorable for downburst winds. The strongest cells
will be capable of producing gusts in excess of 40+ mph as well as
localized blowing dust. The abundant dry air will also limit rain
totals, with only a few hundredths of an inch likely in the
strongest cells. Conditions are expected to improve rapidly early
this evening as subsidence builds in behind the trough axis.

&&

.PREVIOUS DISCUSSION...
High temperatures will begin to climb on Monday and Tuesday as the
storm system moves out of the area. Tuesday will likely be the
warmest day this week with mid 90s across southeast California and
low 90s for south-central Arizona. The Imperial area shows a
10-15% chance of reaching as high as 100 F according to NBM
temperature probabilities. By Wednesday, yet another shortwave
will begin to move in from the west. WPC cluster analysis shows
that the uncertainties with this system have shrunk since
yesterday, but still revolve around how far southwest the system
will dig and how tight of a pressure gradient will develop over
the forecast area. The ECMWF ensemble firmly supports the idea of
a deeper and more elongated trough and has little overlap with
the GFS ensemble, which has a weaker trough that does not dig to
the southwest. The main implications this will have on the
forecast will be with respect to the amount of cooling we see and
how breezy conditions will become. The mostly overland trajectory
of this system along with the lack of favorable dynamic support
given its positive tilt will make precipitation unlikely, which
ensemble guidance all agree upon. ECMWF EFI shows the potential
for anomalously strong winds on Wednesday as this system
approaches. The potential for strong wind combined with dry
conditions may lead to heightened fire weather concerns. The
trajectory of this system will likely favor the strongest winds in
the Imperial area, but breezy conditions would be expected
everywhere. By Thursday, temperatures look to cool into the mid
to upper 80s for highs across the lower deserts as heights fall
with the passage of this system. Temperatures quickly rebound
going into the weekend as ensemble members show good agreement in
upper level ridging returning.

&&

.AVIATION...Updated at 2359Z.

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

Isolated convective activity will continue to dissipate over the
next hour with no showers expected by 01Z. Gusty N-NW winds that
have frequently been sustained at 10-20 knots with gusts up to
30-35 knots will continue weakening over the next hour as well,
with some variability with the winds as showers dissipate over the
East Valley. Blowing dust impacts and dust devils observed near
KIWA should begin to subside as well over the next hour with
ceilings continuing to lift above 10-15 kft and eventually
clearing after sunset. Winds will gradually become E-NE overnight
at 5-10 knots with light and variable or easterly winds tomorrow
morning before becoming westerly between 20-22Z. Winds should
generally remain below 10 knots with only very brief gusts.

Southeast California/Southwest Arizona including KIPL and KBLH:

Strong northerly 15-25 knot winds with brief gusts up to 30 knots
should subside below 10-15 knots by 02Z. Winds this evening and
overnight will generally favor N-NW headings at KBLH and more W-NW
headings at KIPL before becoming light and variable tomorrow
morning across the region. Clear skies will persist through
Monday afternoon with winds generally remaining below 10 knots.

&&

.FIRE WEATHER...
Wednesday through Sunday:
Hot, dry and windy conditions are likely Wednesday ahead of a low
pressure system. The greatest fire danger will be across southern
Arizona, where MinRHs will fall into the single digits. Conditions
will improve somewhat Thursday and Friday as temperatures and RHs
return to seasonable values. A warming trend is then anticipated
next weekend.

&&

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

&&

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

$$

DISCUSSION...Hirsch
PREVIOUS DISCUSSION...Hodges
AVIATION...Hopper
FIRE WEATHER...Hirsch