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Forecast Discussion for Phoenix, AZ

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FXUS65 KPSR 181134

Area Forecast Discussion
National Weather Service Phoenix AZ
434 AM MST Thu Oct 18 2018

.UPDATE...Updated aviation discussion.


Drier air and warmer temperatures will gradually spread into the
region for the remainder of the week as a low pressure system lifts
to the north. Temperatures will finally warm back near normal by the
end of the week after this prolonged period of cool conditions.
Rain chances will return to much of the area this weekend while
also resulting in partly cloudy skies. Temperatures this weekend
into next week look to stay near seasonal normals.


The warming trend will continue over the next few days as the
upper level low shifts out of the region, but broad scale
troughing still reigns across the Southwestern U.S. Highs today
should top out around 80 degrees in the Phoenix area with the
middle 80s commonplace for the deserts of southeast California.
The broad troughing over the region will shift a bit westward
Friday and Saturday as the low closes off just off the California
coast. This low is forecast to deepen slightly into Saturday, but
the lack of a strong upper level jet will keep the 500mb low
center only at around 575dm.

The positioning of the low to our west Friday night through
Saturday will bring modest southerly upper level flow into
Arizona while the low and mid levels turn out of the southeast.
500mb heights over Arizona rise to around 582dm on Saturday while
the southerly flow advects in warmer air mainly below 700mb. Highs
Saturday should approach 90 degrees across the lower deserts even
as moisture will be on the increase due to a low level jet
setting up across southeast and south-central Arizona. 850mb
southeasterly winds increase to 20-30kts on Saturday while decent
mixing should bring breezy conditions and some downslope warming.

The increased southeasterly low level flow Friday night into
Saturday will advect moisture out of northern Mexico into Arizona
allowing for 1000-700 mixing ratios to increase to around 8-9
g/kg. Forecast soundings indicate the moisture will be fairly
shallow and focused mainly in a 800-700mb layer. Given the
expected warm day on Saturday and increased low level moisture,
airmass instability will be on the increase. The upper level cold
core to our west will extend far enough into the Desert Southwest
to allow for good to excellent lapse rates. CAPE will increase
during the day Saturday, but forecast soundings show a pronounced
stable layer centered at 600mb. The combination of weak vertical
ascent associated with the upper level low to the west, upslope
southerly flow, and modest low level instability should be enough
for scattered showers and a few thunderstorms on Saturday. Best
rain chances seem to be focused across the high terrain in central
Arizona, but there should be at least some isolated activity
across southern Arizona. Shear will be fairly strong to support
some organized storms, so there could be a stray strong
thunderstorm or two with small hail and gusty winds. Rainfall
amounts Saturday will be on the low side with most areas seeing
less than a tenth of an inch.

On Sunday the closed low to our west will begin shifting to the
northeast turning flow more out of the southwest. Rain chances
will remain across south-central and eastern Arizona, but drier
air will start shifting eastward into southwest Arizona.
Instability will lower somewhat throughout the day Sunday as the
drier air slowly works eastward, but there should still be enough
to give a slight chance of thunderstorms along with the shower
activity. This activity will slowly shift east of Phoenix Sunday
night with lingering rain chances across eastern Arizona on
Monday. Temperatures Sunday into early next week will drop a bit
from the warm Saturday readings, but will stay at or just below
seasonal normals. We should see another warming trend at some
point during the latter part of the next week as a strong ridge to
our west shifts into the Desert Southwest.


South-Central Arizona including KPHX, KIWA, and KSDL:
Moderate LLWS concerns will continue through mid-morning with
easterly winds above 20 kt just above the surface, but should
weaken after sunrise. The surface pressure gradient will favor
easterly flow persisting longer than normal today, although still
expecting a wind of westerly winds developing at KPHX and KIWA
after 23Z. May see another round of low-end LLWS into early Friday
morning, but winds above the surface will be slightly weaker
compared to this morning, with lower confidence in significant
concerns. Skies will remain mostly clear throughout the period.

Southeast California/Southwest Arizona including KIPL and KBLH:
No aviation concerns under clear skies through Thursday evening. Sfc
winds will favor a northerly direction through the entire period
with speeds remaining below 12kt.

Aviation Discussion not updated for amended TAFs.


Saturday through Wednesday:
Rain chances and a noticeable increase in humidities will return to
South-Central AZ during the Sat-Mon period as low pressure along the
southern-central CA coast pulls moisture back into the region from
the south. The best chances for rain will be over the high terrain
north and east of Phoenix. Dry conditions and slightly lower
humidities should return during the Tue-Wed period as winds aloft
become more westerly. There will be a noticeable increase in winds
on Saturday as the moisture moves into the area, especially in the
mountains east of Phoenix, but otherwise winds should remain at 15-
20mph or less area-wide. Minimum humidities in the 25-50 percent
range during the Sat-Mon period (lowest over SE CA) will fall into
the 20-45 percent range (lowest over SE CA) by Tuesday.


Spotters should follow standard reporting procedures.




FIRE WEATHER...Rogers/Percha

Previous Forecast Discussions may be found at
NWS Phoenix, AZ (PSR) Office Forecast Discussions.
(Click 'Previous Version' there to view past versions successively.
Some may differ only in time posted.)

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