Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 212056

Area Forecast Discussion
National Weather Service Phoenix AZ
156 PM MST Wed Mar 21 2018

High pressure aloft will continue to bring warmer temperatures
with readings peaking Thursday over south-central AZ in the mid
80s to around 90 degrees. Thereafter, a weather system will pass
through the region bringing a chance of showers late Thursday into
Friday. Cooler conditions can be anticipated in the wake of this
system Friday afternoon through early next week.



The upper level low that has been nearly stationary in the
Eastern Pacific this week is beginning to open up and move inland
today. The southwesterly flow out ahead of this system is already
bringing rain and clouds to portions of CA this afternoon and can
be clearly seen on satellite imagery. For the Desert SW, few to
broken high clouds are being observed this afternoon as our
region sits underneath a ridge of high pressure downstream of this

Compared to 24 hours ago, 500 mb heights are up 3 dm while 850 mb
temperatures are up 4 degrees C. The building ridge has
undoubtedly contributed to warming surface temperatures today with
most locations running 4-8 degrees warmer than at this time
yesterday. The ridge will continue to build into Thursday over
south-central AZ as the flow downstream from the aforementioned
low pressure system gets amplified. There is a slight chance a
few locations in south-central AZ could even hit their first 90
degree reading of the year on Thursday. One huge caveat is that
even more high level clouds will begin spreading into our area
Wednesday night into Thursday that could substantially impact our
daytime warming. Model statistics show temperatures in Phoenix
could be anywhere from 83 to 91 F tomorrow, quite a large spread
for just 24 hours out! If these clouds are thicker/thinner than
expected, temperatures will certainly be warmer/cooler than

Out in SE California, temperatures will actually be cooler on
Thursday due to its closer proximity to the incoming low pressure
system. Here, the low pressure system will bring a chance of
precipitation starting Thursday morning. While rainfall amounts
are expected to be very light in most areas (<=0.05 inches),
Joshua Tree National Park could see a bit more. Over the park,
there is large model spread (0 to 0.5 inches) as there are
difficulties handling the rain shadow. For now, we have gone with
the median value among all models which is roughly 0.1 inches.
Along with the showers, all areas of our CWA will see breezy
conditions on Thursday with the strongest winds confined to SE

By Thursday evening into Friday morning, the trough will swing
through south-central AZ bringing the best chance of precipitation
to the Phoenix area. Light sprinkles could begin over Phoenix as
early as Thursday evening along with gusty westerly winds.
Unfortunately, the best dynamics will remain to our north so
precipitation amounts will be meager (<=0.05). By late Friday
morning, the system will quickly move off to our east and much of
Arizona will be basking under mostly sunny skies as cooler and
drier air filters into our region.

This weekend looks to be dry but another weakening low pressure
trough will approach from the northwest keeping temperatures near
normal. Winds should also be on the breezy side as the system
slowly approaches but skies should still remain mostly clear.
While model spread increases into early next week, model
consensus shows the approaching trough digging southward into the
Desert Southwest and potentially becoming closed off through the
middle of next week. Even if this trough sits over our region for
a couple days, rain chances will still be fairly low due to the
likely low amount of system moisture. Forecast temperatures do
lower starting Sunday and stay slightly below normal through early
next week. However, this is highly dependent on the closed low
affecting the region.


South-Central Arizona including KPHX, KIWA, and KSDL:
Mid and high clouds will continue to increase ahead of an
approaching Pacific Atmospheric River. Although measurable
precipitation is generally not expected across the Phoenix area
Thursday afternoon, abundant upper-level moisture will likely
produce areas of virga and localized gusty winds. Otherwise, a
general trend towards increasing breeziness is expected Thursday
afternoon with gusts to 20 to 25 kt possible.

Southeast California/Southwest Arizona including KIPL and KBLH:
Gusty winds will be the primary aviation weather concern. Windy
conditions will develop Thursday afternoon at KIPL/KBLH, with
gusts to 30 kt possible in the afternoon. Abundant upper-level
moisture will also generate areas of virga/sprinkles, which will
locally enhance the wind gusts.

Aviation Discussion not updated for amended TAFs.


Friday through Tuesday...A fast moving weather disturbance will
bring cooler high temperatures Friday along with isolated to
scattered showers east of the lower Colorado River Valley. Best rain
chances will be over higher terrain areas north and east of Phoenix.
Expect high temperatures to drop to near seasonal normals. Humidity
will be a bit elevated with minimum RH values over the deserts
mostly 20 percent and higher. Dry southwest flow aloft on Saturday
will keep skies partly sunny and temperatures will remain near
seasonal normals. A large area of low pressure will gradually
develop over the southwestern states Sunday into early next week;
little if any rain is expected but temperatures will cool with highs
falling into the low to mid 70s each day over the deserts. Humidity
levels stay mostly in the teens each day across the deserts.
Relatively light west wind are expected Friday afternoon; after that
winds will increase and become rather breezy during afternoon and
early evening hours each day, favoring the west to northwest.


Spotters should follow standard reporting procedures.




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