Area Forecast Discussion
Issued by NWS Phoenix, AZ

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000
FXUS65 KPSR 270356
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
855 PM MST Thu Apr 26 2018

.SYNOPSIS...
Very warm conditions will continue the remainder of the week with
widespread highs in the 90s at lower elevations. A slight chance for
thunderstorms will exist across southern Gila County Friday and
Saturday afternoon, however most locations will not see rainfall. A
distinct cooling trend will spread into the region for at least the
first half of next week with only occasionally breezy weather.

&&

.DISCUSSION...
Little has changed with respect to the weather pattern today; an
upper level high pressure ridge remained firmly in place across the
desert southwest ahead of a large upper low spinning off the CA
coast. IR imagery this evening showed quite a bit of rather thick
but mainly high level cloudiness spreading through the ridge and
across southern/central Arizona. Despite the cloud cover, high
temperatures over the lower desert still climbed into the mid or
even upper 90s today; Phoenix topped out at 97 degrees. For the rest
of tonight, the ridge will remain in place but the axis will shift
closer to the New Mexico border, allowing some clearing over the
western deserts and confining much of the thick mid/high cloud to
south central and southeast Arizona. Winds will stay generally on
the light side tonight but the cloud cover will result in rather
warm overnight lows over the central deserts including the Phoenix
area where the low may not fall out of the 70s in some spots.
Current forecasts for tonight look to be in good shape and no
updates are needed.

.PREVIOUS DISCUSSION...
A seasonally deep negative height anomaly over the east Pacific
basin has invigorated an active subtropical jet, pulling a stream of
upper level moisture into the SW Conus today. Despite the
detrimental insolation influence of this thicker band of high
clouds, 12Z TWC sounding data sampled both H5 heights and H7
temperatures near the 90th percentile yielding afternoon highs well
into the 90s. Further east, the highly amplified and partially
blocked downstream pattern, along with a deep PV anomaly over the
southern high plains has resulted in a stout westward moving cold
front which will pass into eastern AZ overnight. This front will
bring a short period of modestly strong southeasterly low level
winds across ridge top and foothill locations of southern
Gila/eastern Maricopa counties Friday morning potentially creating
large gradients in low temperatures.

A period of backed winds in the H8-H5 layer Friday morning will also
allow modest moisture advection through much of eastern Arizona with
BUFR sounding data suggesting well mixed 4-5 g/kg spilling into the
eastern CWA. With H5 heights still hovering around 580dm and good
diurnal heating, temperatures well above average juxtaposed with
this marginal moisture could result MUCape reaching aoa 50 J/kg and
a few very isolated towering Cu/thunderstorms through the eastern
sections of Gila County. The very dry subcloud layer will likely
preclude much, if any actual rainfall; and mean flow veering during
the afternoon should steer any activity and resultant gusty winds
away from the bulk of the forecast area. In all likelihood, the
greatest hazard will be potential fire starts from lightning strikes
given the lack of appreciable rainfall.

The larger east Pacific trough will slowly progress inland over the
weekend gradually unlocking the downstream blocking pattern with the
primary central circulation only reaching Idaho Sunday night. This
slow propagation will yield only modest height falls and pressure
packing along and behind a theta-e boundary crossing the region,
somewhat limiting the strength of winds than otherwise could occur
this time of year. Enough residual midlevel moisture will linger
over far eastern Gila County Saturday such that a few thunderstorms
could be possible, however strengthening SW flow should quickly take
any activity and impacts well into the White Mountains and western
New Mexico.

The overall longwave trough will open up and become more expansive
Monday into Tuesday while a potent shortwave digs southward down the
western periphery of this longwave along the west coast. Though
there`s still notable ensemble and operational model spread
regarding timing and depth of this shortwave, the general consensus
depicts another closed low (or family of closed shortwaves)
traversing the SW Conus during the middle of next week (most likely
on Wednesday). This scenario would bring at least a slight chance of
showers and thunderstorms to the high terrain, possibly even some
high based storms over the deserts in an ideal situation where
moisture levels are high enough and the low center passes through
southern Arizona. Temperatures next week will almost certainly take
a decent hit with highs easily falling into the middle 80s, possibly
lower 80s in some lower desert communities.

&&

.AVIATION...
South-Central Arizona including KPHX, KIWA, and KSDL: Although
strong high pressure aloft will persist, it will still allow
considerable mainly high clouds to spread across the terminals
tonight into Friday morning. Expect continued cigs with bases aoa
20k feet, and there will be FEW-SCT mid decks as well down to around
12k feet at times. By tomorrow afternoon we may see a few high based
cumulus in the greater Phoenix area but those decks - with bases
down around 10k feet - should mostly be on the FEW side. Any
significant cloud buildups Friday afternoon/evening will stay over
higher terrain well to the east of Phoenix. Winds will continue to
display typical diurnal tendencies with speeds less than 10kt;
expect east to southeast winds thru early Fri afternoon before
veering to the southwest. Overall, no significant aviation concerns
for the next 24 hours at the terminals.

Southeast California/Southwest Arizona including KIPL and KBLH: A
large upper low approaching from the west will result in generally
clear skies at the terminals through Friday evening but will also
result in somewhat stronger winds mainly during the afternoon and
evening on Friday. Winds to favor the west next 24 hours at KIPL but
should start to gust over 20kt by late Friday afternoon. Expect
winds to favor the south to southwest at KBLH with some afternoon
gustiness tomorrow mostly from the south.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Sunday through Thursday:
Much cooler weather will affect the districts during the first part
of next week with temperatures eventually falling to around 5F below
average. The weather disturbances causing this cooling will also
lead to locally breezy conditions with some afternoon gusts in the
20-30 mph range, particularly in the better upslope regions.
However, this cooler weather will also aid humidity levels with
afternoon minimums only falling into a 10-20% range. Overnight
recovery will also benefit into a fair to good category.

&&

.SPOTTER INFORMATION STATEMENT...
Spotters are encouraged to follow standard reporting procedures.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...None.
CA...None.

&&

$$

DISCUSSION...CB
PREVIOUS DISCUSSION...MO/Kuhlman
AVIATION...CB
FIRE WEATHER...MO



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