Area Forecast Discussion
Issued by NWS Phoenix, AZ

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000
FXUS65 KPSR 242330
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
430 PM MST Sat Oct 24 2020

.UPDATE...
Updated Aviation

&&

.SYNOPSIS...
Temperatures will start to cool as a strong fall weather system
moves into the region with readings falling at least 10 degrees
below normal Monday and Tuesday. The potential for showers and
possibly a few thunderstorms will increase Sunday evening, mainly
across Gila and Pinal counties before potentially spreading into
eastern portions of Maricopa County Monday. Across southeast
California and southwest Arizona, a strong northerly wind is
expected on Monday, which may lead to areas of blowing dust. Drier
and warmer conditions are then expected during the latter half of
next week.

&&

.DISCUSSION...
An expansive region of high clouds is spread across the region,
streaming in ahead of the positively tilted trough just off the
California coast. The clouds should stick around through Sunday
morning and model soundings show the clouds thicken/lower as
moisture continues to stream into the region. The clouds should
moderate temperatures this afternoon, keeping highs across the lower
deserts generally in the mid to upper 80s.

The moisture streaming in ahead of the trough just to the west
will basically be the best moisture we have to work with over the
next few days. ENS and GEFS shows a tongue of 0.8" to 1.0" by midday
Sunday, which is near the 90th percentile when comparing to TUS
sounding climatology. So there will be decent mid and upper level
moisture to work with Sunday into Sunday night, but forcing will be
limited Sunday. Models show the trough currently to the west kicking
east-southeast and passing just south of the U.S. border Sunday
evening through Monday morning. This occurs just before the more
prominent trough digs down from the north. Model soundings and HREF
do show the potential for up to 500 J/kg MUCAPE Sunday afternoon
through the evening, but mainly in a narrow zone currently depicted
from Pinal county through southern Gila county. Add in a little bit
if isentropic ascent and isolated storms may develop in this
corridor beginning as early as late Sunday afternoon within. Bulk
shear (0-6 km) will be quite high, around 40-50 kts. So with a
strong enough updraft an isolated severe storm could develop.

Rain chances will likely increase over time Sunday night through
Monday morning as the better dynamics with the robust trough from
the north reaches the central and southern AZ deserts by 12 Monday,
but chances will likely continue to favor upslope and higher terrain
regions east and northeast of the Phoenix area. There very well
could be a few showers as far west as the Phoenix area on Monday,
but with the low center now most likely tracking more across the
eastern half of Arizona, rain chances for Phoenix have diminished
considerably. The past few iterations of the ENS and GEFS continue
to trend down the QPF for the Phoenix area. Most GEFS members are
now completely dry for Phoenix while only one member of ENS (of 50!)
has QPF over 0.1". QPF has also trended down east of Phoenix where
the best rain chances are. Likely due partially to the track and
more progressive nature of the trough, but also due to the immense
amount of dry air that will get wrapped into this low. PWATs plunge
through the day Monday across much of the region, falling from near
the 0.8-1.0" it`s expected to be Sunday night down to around 0.2" in
central AZ by the end of the day.

To the west of the low track on Monday, especially across southeast
California and southwest Arizona, a southward surging cold front
will lead the charge of much drier air and windy conditions. Post-
frontal wind gusts up to 40-50 mph are likely at times. These strong
winds should mostly persist through the daytime hours, likely
creating some patching blowing dust and localized reduced
visibilities. A wind advisory has been issued for this area for all
of Monday. Very chilly air behind the cold front will keep daytime
highs Monday mostly in the middle 60s across our western deserts to
around 70 degrees for the Phoenix area. The low is likely to shift
quickly southward into southeast Arizona by late Monday night while
snow levels quickly lower to just below 4000 ft by Tuesday morning.
There could be a quick change over to snow across Gila Co. late
Monday evening, but any lingering shower activity in the area should
be quickly diminishing as dry air surges southward starting late
Monday afternoon. At this time, we are not expecting anything other
than a light dusting of snow late Monday across eastern Gila Co.

By late Tuesday morning, the low center may already be into northern
Mexico leaving a dry northerly flow across all of Arizona. Winds
Tuesday may still be fairly breezy, but mainly in the morning hours
as the gradient will relax quickly by Tuesday afternoon. Highs
Tuesday are now looking a bit warmer due to the more progressive
track of the low with readings from the lower 70s in the Phoenix
area to closer to 75 degrees across our western deserts. The latter
half of next week still looks be in warming trend with temperatures
back to or even above normal likely by Thursday. There could also be
a day or two from next Friday into the following weekend that would
see highs around 90 degrees.

&&

.AVIATION...Updated at 2330Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Trends in winds will be the primary weather challenge Sunday
afternoon as thick high clouds gradually begin to clear. Light west
winds will switch to the traditional easterly overnight, with a
period of variability (140v220) likely later Sunday morning as a
result of an approaching weather system. Confidence is good that
gusty W/SW winds 20-25kt will develop by mid afternoon Sunday and
persisting into the evening hours.

Southeast California/Southwest Arizona including KIPL and KBLH:
Strengthening winds will be the main weather hazard Sunday afternoon
as thicker high clouds gradually clear. Southerly to southwesterly
winds will be preferred through the entire period with increasing
gusts Sunday afternoon. Gusts around 20kt will be common at KBLH
while stronger 25-30kt gusts will impact KIPL.

&&

.FIRE WEATHER...
Tuesday through Saturday:
Much cooler temperatures expected region-wide Tuesday through
Wednesday, following a strong weather system. Temperatures then
warm several degrees back above normal by Friday. Relative
humidity will remain quite low through the period with min RHs
generally around 10-15% (a bit drier across western districts
Tuesday and Wednesday) across the region with recoveries to around
25-40% during the overnight hours. Winds will still be breezy out
of the north across western districts Tuesday. Otherwise, winds
will be relatively light each day with typical afternoon breezes
and favoring typical diurnal tendencies.

&&

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

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Fire Weather Watch from late Sunday night through Monday evening
     for AZZ131.

     Wind Advisory from 11 PM Sunday to 11 PM MST Monday for
     AZZ530>533.

CA...Fire Weather Watch from late Sunday night through Monday evening
     for CAZ230-231.

     Wind Advisory from 11 PM Sunday to 11 PM PDT Monday for CAZ560-
     561-564-565-568>570.

&&

$$

DISCUSSION...Benedict/Kuhlman
AVIATION...MO
FIRE WEATHER...Benedict



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