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Area forecast discussion 
National Weather Service Phoenix Arizona
745 am MST Thursday Oct 20 2016

dry weather with mostly clear skies will continue across the Desert
Southwest the rest of the week and into the weekend. Temperatures
will continue to be unseasonably warm, and breezy weather at times
will last into Friday. By Sunday afternoon, a large Pacific low
pressure system will move into the western states. This will draw a
considerable amount of subtropical moisture into the region from the
southwest direction, with a slight chance of showers Sunday
afternoon persisting through Tuesday, mainly over south central



A high pressure system is still forecast to build into the region
today through Saturday. East to northeast breezes will continue to
develop today over portions over southwest and south central Arizona
today, diminishing late tonight. Otherwise clear skies and above
normal temperatures expected through Saturday. Current forecasts
look good. No updates planned.

Previous discussion...522 am MST...

As of 1 am this morning, high pressure aloft centered to our west
was providing dry subsident northwest flow aloft into the desert
southwest; the latest flg sounding showed just 0.17 inches of precipitable water
and tus was at just 0.42 inches. Infrared imagery showed clear skies area
wide and surface dewpoints over the central deserts at 1 am ranged
from the mid teens to near 40 at Phoenix. Over the next few days,
from today through Saturday, model guidance as well as gefs ensemble
guidance, calls for a broad upper ridge to gradually progress
inland, then eventually shift east of Arizona. With 500 mb heights
approaching 590dm in the center of the high, we can expect the
string of 90+ degree high temperatures to continue into the weekend,
with the warmer central and western deserts reaching into the mid to
upper 90s through Saturday with the warmest day in the Phoenix area
likely to be on Friday with a high near 97 forecast. Although highs
will be 5-10 degrees above normal, we are not looking at records at
this time. The record high in Phoenix through Saturday is at least
102 degrees each day. Of course, we can expect generally sunny days
and clear nights through Saturday.

It appears that the dry and unseasonably warm weather will come to
an end starting on Sunday, as a large area of low pressure
developing along the West Coast starts to tap subtropical moisture;
southerly low level flow begins to spread considerable mainly mid
and high level cloudiness across the area during the day on Sunday
as as upward vertical velocity fields start to increase - areas of mid level q-
convergence develop over the western deserts during the day -chances
for light precipitation start to increase. Pops have been raised
into the slight chance category over areas west of the greater
Phoenix area Sunday although most numbers are below about 15
percent. Expect a bit of cooling as well under mostly cloudy skies,
although warmer deserts will likely remain in the low to mid 90s.
Depending on how thick and persistent the cloud cover is, temps in
places may fall into the upper 80s.

Sunday night through Monday night we see a progressive series of
short waves move out from the main upper trough and pass through the
area, leading to increasing chances of precipitation, especially
over south central Arizona. By Monday afternoon, the most
significant qg forcing moves into south central Arizona and as the
atmosphere wets up from above, rain chances climb into the 15-25
percent ballpark over the deserts, reaching into the chance category
over higher terrain areas to the north and east of Phoenix. High
temps Monday will fall into the mid 80s to low 90s with a high right
at 90 now forecast for Phoenix Sky Harbor. Modest cooling aloft will
support weak instability Monday and we will look for a slight chance
of an afternoon thunderstorm mainly from central Phoenix eastward.
On Monday night the last in a series of short waves starts to exit
into New Mexico, and rain chances will primarily focus over the
higher terrain east of Phoenix with pops in the 30 percent ballpark
forecast, and 20 percent called for in the greater Phoenix area. By
Tuesday afternoon expect just a lingering slight chance of showers
over southern Gila County with skies becoming mostly sunny over the
central and western deserts. Overall with this precip event, pops
have followed the trends of the naefs guidance and have been raised
a bit from the previous forecast package.

High pressure aloft then builds into the area from the west on
Wednesday for mostly sunny skies and above seasonal normal high


south-central Arizona including kphx, kiwa, and ksdl:

Moderate to strong east-northeast winds can be expected today and
tonight in the lower levels. Before the inversion begins to break
after 15z, surface winds will be light and variable - even south or
southwest (except near foothill areas where northeast will be
favored). Expect east-northeast surface winds gusting 15-20 kts after
17z before weakening late afternoon/early evening. Otherwise, skies
will be clear.

Southeast California/southwest Arizona including kipl and kblh:

Moderate to strong northeast and east winds can be expected today
and tonight in the lower levels. Before the inversion begins to break
after 15z, surface winds will be light and variable except near
the lower Colorado River valley where north will be favored. Expect
north surface winds to prevail after 17z (locally gusting 15-20 kts
lcr valley) before weakening late afternoon/early evening.
Otherwise, skies will be clear.


Fire weather...
Saturday through Wednesday...

Temperatures start to decline over the weekend but will remain above
normal. Humidities will increase as well. A Pacific low pressure
system will lead to a slight chance of showers beginning Sunday over
portions of southwest Arizona and southeast California. Slight
chances will expand to the whole forecast area Sunday night and
Monday before trending down from west to east Monday night and
Tuesday. Wetting rains, if any, would be quite spotty and most likely
be limited to the higher terrain of south-central Arizona. Dry
conditions are expected Wednesday. Minimum humidities peak on Monday
with values of 20-30% at most locations before slowly trending down.
Overnight recovery will be good to excellent. No strong winds are
anticipated through the period.


Spotter information statement...
spotter activation will not be needed this week.


Psr watches/warnings/advisories...


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fire weather...aj

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