Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 200520 AAA

Area Forecast Discussion...Updated
National Weather Service Phoenix AZ
1020 PM MST Fri Oct 19 2018

.UPDATE...Updated aviation discussion.


Warmer temperatures with readings near to slightly above normal
will last into tomorrow, but more unsettled weather will move in
later tomorrow and last into early next week. Rain chances
tomorrow afternoon will favor higher terrain locations,
especially across central Arizona. There will be a slight cool
down Sunday into early next week under partly cloudy skies and
lingering rain chances mainly north and east of Phoenix. Drier
conditions will move into the region by the middle of next week
with temperatures remaining near seasonal normals.


The next weather system to affect our region is currently
centered off the northern coast of Baja California with this
system retrograding slightly tonight into early tomorrow morning.
The flow will become more southerly tomorrow, allowing moisture
to be advected into the region, however PWATs are expected to be
no more than a little over an inch. The greatest moisture will
exist near and east of the Colorado River and this is where the
greatest precipitation chances will also exist. With the trough
just to our west, there will be some weak dynamical support, but
overall it shouldn`t be a major factor. The threat for thunderstorms
will be there with enough shear to support some organized storms,
but storm movement will be around 20kts likely limiting the flood
threat outside of any cells that manage to train over the same
area. There is an outside chance of some small hail and gusty
winds, but it doesn`t seem all that likely given the brief period
of 500-1000 J/kg of MLCAPE. Hi-res models seem to keep showing a
similar trend, isolated showers during the day, mostly across the
higher terrain, with activity really ramping up after 00Z Sunday,
with an axis of showers and perhaps a thunderstorm or two
developing along western Pinal and Maricopa counties. Showers and
isolated convection then fade overnight with redevelopment of
showers possible once a gain early Sunday morning, but mainly
across Maricopa and Pinal counties.

The upper level low will begin to shift to the northeast on Sunday
shifting our winds more out of the southwest. South-southeasterly
moist low level flow will still be present across the eastern
half of Arizona on Sunday, so shower and thunderstorm chances will
shift a bit eastward on Sunday. Best rain chances Sunday look to
be north of Phoenix with more isolated activity across south-
central Arizona. Models are less clear on how much shower or
thunderstorm activity there will be on Sunday, but we are
thinking it will be a bit less than on Saturday. Temperatures on
Sunday will drop a few degrees off of Saturday`s highs, but
should remain close to normals.

There has been a slight shift in the models for Monday as a
stronger upper level Pacific jet is now shown entering the base of
the trough over the region. This should allow the trough to
deepen again with its center now over the Great Basin. Jet
positioning will become favorable for creating an area of moderate
vertical ascent late Monday over eastern Arizona. Plenty of
lingering low level moisture in combination with the jet dynamics
and weak southwesterly upslope low level flow should provide
another round of showers east of Phoenix. Rain chances will
greatly diminish by around noon Tuesday as the jet shifts eastward
and the upper level trough weakens again.

Drier air will gradually filter in from the west later Tuesday
into Wednesday while the trough finally exits well to the
northeast. Temperatures should hover around seasonal normals
through at least the middle of next week, but there are signs of a
potential warm up late next week into next weekend as a strong
Pacific ridge begins to shift eastward into the Desert Southwest.


South-Central Arizona including KPHX, KIWA, and KSDL:

Biggest aviation issue initially will be low level wind shear as a
strong 30kt low level SELY jet develops across the Phoenix area; as
surface east winds decouple and fall below 10kt shear will increase.
Thus a mention of LLWS in the TAFs from about 09z thru 16z. After
16z, mechanical mixing will increase surface winds and decrease
shear potential. Expect increasing clouds later tonight into the day
on Saturday with most bases 7-10kt feet. May see some CIGs at times.
By late afternoon may see a few showers developing; better showers
threats in the evening. Also thunder becomes possible in the evening
so added a VCTS to the TAFs starting about 02z Sunday. Wind shear
looks to redevelop again Sunday morning so added another wind shear
group into the KPHX TAF after 08z.

Southeast California/Southwest Arizona including KIPL and KBLH:

No significant aviation issues next 24 hours. Expect winds to favor
the north to northeast at KBLH with speeds mostly below 15kt. Winds
at KIPL to favor the west through late morning, then the
north/northeast and then back to west by Sunday evening. Expect some
mid level clouds to move into the area by Saturday morning with most
bases 8-12kt feet. CIGS are possible but most likely clouds stay in
the FEW-SCT ranges.

Aviation Discussion not updated for amended TAFs.


Monday through Friday:
Low pressure over the region will keep wetting rain chances
elevated during the Mon-Tue timeframe across south-central
Arizona. The best chances for rain continue to be over the high
terrain north and east of Phoenix. There is also a potential for
isolated thunderstorms as well on Monday. Dry conditions and
slightly lower humidities should return during the Wed-Fri period
as winds aloft become more westerly. Winds should remain at 15-
20mph or less area-wide. Minimum humidities in the 25-50 percent
range Mon (lowest over SE CA) will fall into the 20-40 percent
range (lowest over SE CA) by Thursday.


Spotters should follow standard reporting procedures.




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