Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 070006

Area Forecast Discussion
National Weather Service Phoenix AZ
506 PM MST Mon Apr 6 2020

.UPDATE...Updated Aviation discussion.


Warm, sunny, and dry weather will gradually give way as a storm
system moves through California and Arizona during the middle part
of this week. Increasing clouds and a few showers will be
possible for southeast California starting Tuesday, continuing
into Thursday. Rain chances spread eastward into Arizona into
Phoenix Wednesday, before exiting the region Friday. Temperatures
will be coolest on Thursday, some 10 to 20 degrees below normal.
Warmer and dry weather returns by the weekend.



A strong upper low remains along the CA coastline this afternoon
with two very distinct smaller circulations. A steady band of
moderate to heavy rain has been pushing into southwest CA most of
today with a noticeable rain shadow into the southeast CA deserts
where it is still sunny. The core low is still on track to drop
down to about San Diego`s latitude before pushing inland on
Wednesday. Moisture content linked with this storm and QPF amounts
have trended upward, primarily across southeast CA and southwest
AZ. Rainfall amounts still trail off towards central and southeast
AZ, including the Phoenix area. Models are starting to come into
better agreement on convective banding developing Wednesday in
southeast CA as the vorticity maximum and with low level
frontogenesis shifts eastward into the area. Rain rates still do
not look too impressive, but training echoes of moderate rain and
possible isolated storms may still lead to localized higher
rainfall amounts and potential for flash flooding. Elected to
issue a Flash Flood Watch for southeast CA and the Colorado river
for Tuesday afternoon through Wednesday afternoon, in coordination
with neighbor WFOs. Expansion of the watch to include La Paz and
Yuma counties is possible for Wednesday as global models have been
depicting a strengthening IVT plume developing with the band of
rain as it pushes east of the Colorado River.

Thursday still looks to be the coldest day of the week with most
lower desert locations staying below 70 degrees. Warmer and drier
weather is still anticipated by the weekend. However, there is
still noticeable uncertainty with the upper low progression
beyond Wednesday. There are some indications the low may hang on
into Saturday morning, but after Wednesday most precip chances
remain north and west of the CWA.


A strong upper level low continues to slowly drift southward
along the central California coast this morning with the southern
periphery of the system beginning to move into the Desert
Southwest. For today, the dry airmass over our region will remain
in place, but moisture levels will be on the increase across much
of southern California. A fairly steady band of rain currently
entering the Los Angeles area will continue to slowly progress
southward today, but only JTNP may see any rainfall by the end of
today. For southern Arizona, today will be sunny with highs in the
upper 70s to lower 80s across the deserts.

Models are in agreement the low center will be just off the coast
of San Diego by late Tuesday afternoon with better moisture
advection into southeast California throughout the day on Tuesday.
Rain chances will also improve on Tuesday west of the Colorado
River with a broken band of showers likely affecting Imperial and
eastern Riverside Counties during the latter half of Tuesday.
Farther east into Arizona, Tuesday should almost be a carbon copy
of today.

Wednesday still looks like the most active day of the week as the
low center finally pushes ashore into southeast California
sometime during the morning or early afternoon. There remains some
model differences with the track of the low center and the timing
as it tracks through our region and this will definitely play a
role in the timing of the rain on Wednesday. Currently, the most
likely scenario points toward a band of showers and possibly a few
thunderstorms moving through the Colorado River Valley Wednesday
morning. This area, especially eastern Riverside County into La
Paz County, still looks to be the most likely area for higher
rainfall amounts with localized amounts up to an inch. The band
should then move through southwest Arizona and south-central
Arizona into Wednesday afternoon, but the timing and the extent of
the shower and thunderstorm activity is still somewhat uncertain.
Additional scattered showers and thunderstorms will remain likely
through the rest of Wednesday west of Phoenix as the low center
eventually tracks northeast through the area.

Models diverge even more going into Thursday with the Euro
ensemble mean keeping the upper level low centered near Las Vegas
while the GEFS slowly tracks it northeastward through northern
Arizona. Even though drier air is forecast to overtake much of
southern Arizona into Thursday, there will still be quite a bit of
residual moisture across the northern half of Arizona keeping
chances for showers mostly across the high terrain during the
daytime hours Thursday. Due to the uncertainty in the progression
of the track of the low, forecast confidence for Thursday into
Friday is fairly low. Both the Euro and the GEFS show a potential
weak upper level low center and/or vort max hanging around the
region through Friday. However, rain chances for Friday do not
look very good as the system will be considerably weaker and
moisture levels will be even lower. Finally by Saturday, the
region should be under weak ridging and rain chances will have
completely come to an end.

Temperatures this week will start off near normals for much of
Arizona with below normal temperatures initially across southern
California. As the weather system progresses through the region
during the middle part of the week, highs will drop into the upper
60s as early as Wednesday for areas west of Phoenix with
widespread readings only in the upper 60s for Thursday. High
temperatures will quickly rebound Friday into Saturday with
readings back to near normal starting Saturday.


.AVIATION...Updated at 0005Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Breezy conditions will again subside this evening before shifting
to the east around 09z. TAFs more or less will follow a
persistence forecast for Tuesday, with a period of southerly
crosswinds expected followed by a switch to the west during the
afternoon. However, speeds are generally anticipated to be weaker
than those observed Monday afternoon.

Southeast California/Southwest Arizona including KIPL and KBLH:
Breezy conditions will again subside later this evening. Main
aviation impact will be the increase in stratocu as early as
Tuesday morning. There also is the potential for showers, as well
as ceilings below 5k ft Tuesday afternoon at KIPL. However, most
likely scenario keeps this activity confined to western Imperial
County through 00z Wednesday.


Wednesday through Sunday:
A low pressure system will move from west to east across the
region through at least Thursday bringing widespread shower
chances on Wednesday with an isolated chance of thunderstorms.
Lingering chances for mostly isolated showers are likely through
Thursday, possibly even into Friday. Cooler conditions will occur
early in the period with desert highs mostly in the upper 60s to
lower 70s through Thursday. Min RH values mostly around 40% are
expected through Thursday, gradually lowering closer to 25-30%
into the weekend. After widespread breezy conditions on Wednesday,
winds will be relatively light through the rest of the period.
Dryer air and warmer temperatures are expected to return to the
region late by the coming weekend with highs possibly recovering
back the low to mid 80s by Saturday.


Spotters should follow standard reporting procedures.


AZ...Flash Flood Watch from Tuesday afternoon through Wednesday
     afternoon for AZZ530-532.

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

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



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