000
FXUS65 KPSR 072320
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
420 PM MST Tue Apr 7 2020

.UPDATE...Updated aviation discussion.

&&

.SYNOPSIS...
A slow moving storm system will mainly affect southern California
today with warm and sunny weather continuing across Arizona and
most of southeast California. This system will move inland on
Wednesday to bring widespread showers and isolated thunderstorms
across southeast California and southwest Arizona with a slight
chance of showers and thunderstorms over south central Arizona.
Lingering chances for showers will continue on Thursday and Friday
before the storm system finally exits the region on Saturday.
Temperatures will be 10 to 15 degrees below normal on Thursday
and Friday before warmer and dry weather returns for the weekend.

&&

.DISCUSSION...
Another warm day is ongoing across most of the lower deserts of
southeast California and Arizona as shortwave riding remains in
place at mid-to-upper levels ahead of a deep quasi-stationary
closed Pacific low centered off the coast of southern California.
Temperatures have warmed up into the upper 70s to lower 80s this
afternoon with a couple more degrees of daytime heating possible.
One exception to this is over western Imperial and Riverside
Counties where low clouds and isolated shower activity is
gradually pushing into the region. However, the heavier showers
are staying well west of the region into San Diego and far western
Riverside Counties with only light rainfall accumulations up to a
tenth of an inch expected over southeast California. One exception
to this will be over Joshua Tree National Park where locally
heavier amounts and a Flash Flood Watch is currently in effect
through tomorrow afternoon.

The broader global and hi-res model ensemble continue to delay
the onset of heavier rainfall deeper into the overnight and
morning hours on Wednesday as the main lobe of vorticity swings
counter-clockwise around the low tonight in the Pacific and into
southeast California tomorrow morning. Nearly all hi-res model
runs keep rain totals below a quarter inch through about 2-5 AM
PDT before rain rates increase dramatically in an area of upper-
level diffluence and increasing differential vorticity advection
tomorrow morning. Therefore, we have delayed the onset of the
Flash Flood Watch for southeast California (excluding JTNP) and
the Colorado River Valley in far western Yuma and La Paz Counties
until 5 AM MST/PDT. There is some indication in both the ECMWF and
GEFS that a focused IVT plume may move over eastern portions of
Yuma and La Paz Counties late tomorrow morning and afternoon just
east of the current watch area. If models continue to trend in
this direction, the watch may need be expanded a bit further east.

Further east into south central Arizona, showers and thunderstorm
activity should decrease in coverage as this initial shortwave
and associated vorticity lobe moves north-northeast, with only
isolated activity expected in the Phoenix metro. High temperatures
may approach 80 degrees once again to help increase surface
instability a bit, so a rogue thunderstorm or two capable of
producing gusty winds, small hail, and a brief heavy downpour are
not out of the question. The more sensible weather impact for all
locations in south central Arizona will be gusty winds, which are
currently predicted to be sustained at around 15-20 mph with
gusts approaching 30-35 mph. Although this is currently below our
Wind Advisory threshold which makes the most sense since 850 winds
should remain below 30 knots, a small portion of ECM members
suggest gusts approaching 35-40 mph are not out of the question.

By Wednesday evening, the shortwave will be north of the region
with the main low center moving into southern Nevada where it
most likely will briefly stall out as another lobe of vorticty
rotates around it into southern and southeast California on
Thursday. Rain chances should mostly remain north and west of our
area Wednesday night into Thursday, but we can`t rule out some
shower activity for areas west of Phoenix as it all depends on the
eventual track of the main vort max. Models are now in better
agreement showing the low center finally swinging back to the
southeast late Thursday into Friday, most likely tracking through
southern California into northern Mexico on Friday. There will
likely be some isolated to scattered showers and maybe even a few
cold-core based thunderstorms near the low center during the
daytime hours Friday. The best chances for these storms currently
appear to be over Imperial and Yuma Counties, but it is too early
to tell where the best chances will reside. Thursday and Friday
still look to be the coolest days of the week with highs mostly in
the upper 60s to lower 70s over the deserts. Warmer and drier
conditions will settle into the region beginning Saturday with
highs nearing seasonal normals most likely by Sunday.

&&

.AVIATION...Updated at 2320Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Gusty southerly to southeasterly crosswinds are expected to shift
to predominantly southwesterly to westerly direction between
00-01Z. Winds will taper off later this evening and switch to the
southeast after 9Z. Stronger gusts to 20-30 kt are likely by late
Wednesday morning. Southerly crosswinds may create some concerns
between roughly 17-20Z before switching to a westerly direction.
Clouds will gradually increase overnight, but the most notable
increase will occur mid-day with a BKN layer generally ranging
between 7k-9k ft. SCT clouds below 5k ft become more likely by
late afternoon. Some showers will be possible near/after sunset
across the Phoenix area, but latest trends suggest this activity
will remain scattered and may not impact all terminals.

Southeast California/Southwest Arizona including KIPL and KBLH:
Cloud cover is overspreading much of southeast California this
evening, with isolated showers noted across far western Imperial
County. Bumped up the timing of VCSH for KIPL to 03Z in the latest
TAF package to account for a long period of intermittent shower
activity. Still expecting more widespread showers to affect KIPL
by 12-13Z, which will is forecast to bring CIGS below 5k ft and
potentially reduced visibilities at times. Timing for KBLH will be
slightly later, roughly between 15-20Z. Confidence is high in
widespread shower coverage. Should see a decrease in activity into
the early afternoon across most of southeast California as rain
chances become focused across western Arizona. Wind gusts to 15-25
kt will also be possible, generally out of a westerly direction.

&&

.FIRE WEATHER...
Thursday through Monday:
A low pressure system will linger over the region through Friday,
possibly even into early Saturday with slight chances for showers
mainly during the daytime hours Thursday and Friday. Cooler
conditions will last through Friday with desert highs mostly in
the upper 60s to lower 70s. Min RH values between 30-40% are
expected through Friday, gradually lowering closer to 25-30% into
the weekend. Winds will be relatively light through the bulk of
the period with some afternoon breeziness likely, especially this
weekend. Dryer air and warmer temperatures are expected to return
to the region this weekend into early next week with highs
possibly recovering back to near normal readings by Sunday.

&&

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

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Flash Flood Watch from late tonight through Wednesday afternoon
     for AZZ530-532.

CA...Flash Flood Watch through Wednesday afternoon for CAZ560-561.

     Flash Flood Watch from late tonight through Wednesday afternoon
     for CAZ562>570.

&&

$$

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

NWS PSR Office Area Forecast Discussion