Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 262101

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

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.


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.


South-Central Arizona including KPHX, KIWA, and KSDL:
No major aviation weather concerns today with light and diurnal
winds across the Phoenix area. Thick high clouds will likely
persist throughout the day and overnight, but bases should remain
above 20K feet. Some clouds around 11-12K feet may also develop
overnight into Friday, but coverage should remain FEW or SCT.

Southeast California/Southwest Arizona including KIPL and KBLH:
Stronger winds will be of concern this afternoon, especially for
KIPL, with gusts up to 20-25 kts. The winds at KBLH will not be as
strong but some afternoon breeziness can be expected. Otherwise,
the wind speeds should settle a bit after sunset.

Aviation Discussion not updated for amended TAFs.


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.


Spotters are encouraged to follow standard reporting procedures.





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