Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 011639

Area Forecast Discussion
National Weather Service Phoenix AZ
939 AM MST SAT OCT 1 2016

This weekend will feature seasonably warm temperatures and mostly
clear skies though some breeziness will enter the picture by Sunday
afternoon. Increasing moisture across south central Arizona later on
Sunday will lead to a slight chance of showers or thunderstorms over
the higher terrain of southern Gila County Sunday night. Dry weather
will dominate the weather pattern next week though a sharp cooling
trend will arrive early in the week as a strong area of low pressure
passes to our north.


A few isolated showers managed to develop early this morning across
portions of northern Maricopa County, but have since dissipated.
Models show somewhat drier air mixing down below 700 mb, resulting
in mostly clear skies across the lower deserts this afternoon. Near
to slightly above normal temperatures are also expected with a high
temperature near 96 degrees in Phoenix. Current forecast is in good
shape and only minor adjustments to the short-term temperatures were
needed this morning.



The deep Pacific trough to our northwest will eventually lead to
breezy and much cooler conditions as a cold front pushes across the
deserts Monday and a much drier westerly flow aloft will follow into
the early to mid part of next week behind the exiting trof. Latest
GFS and ECMWF runs as well as GEFS ensembles support this basic
idea. However, the past few operational runs of the GFS and ECMWF
are starting to agree that unsettled conditions will again develop
across south-central Arizona later Sunday and Sunday night with
another threat of showers or thunderstorms developing mainly east of
Phoenix. On Sunday, the deep Pacific upper low is forecast to drop
southeastward and inland over Northern California, and low level
southerly flow ahead of this low will start to tap an area of
disturbed weather now located  off the southern tip of Baja CA.
Variable amounts of tropical moisture will spread north/northeast
and into south-central AZ later Sunday with PWAT values starting to
increase and exceed one inch while 850mb dewpoints rise above 10C.
As such we will keep a slight chance of afternoon thunderstorms in
place over southern Gila County Sunday afternoon with POPs climbing
to around 20 percent overnight on Sunday night.

Rain chances in the greater Phoenix area should stay slightly below
10 percent but cloud cover will certainly increase on Sunday.
Despite the increasing clouds, high temps in south central AZ should
stay in the mid 90s, due to the effects of the low level southerly
flow across the area leading to modest WAA effects. One additional
note; on Sunday we can expect windy conditions across southeast
California as low level gradients tighten ahead of the advancing
low. Winds should remain remain below wind advisory criteria for the
most part but a few favored locations such as areas to the southwest
of Imperial may see gusts reaching advisory levels for a short time.
Winds will also lead to potentially dangerous fire weather
conditions for a few hours as well given RH values will drop to
around 15 percent during the afternoon.

Later Sunday night through Monday the main upper low center is
forecast to progressively move east and pass by well to our north,
moving through central NV and Utah. Following the passage of the
cold front, high temperatures over the deserts will fall into the
80s; Phoenix will drop to around 85 degrees, almost 10 degrees below
the normal of 94. Except for a lingering 10 percent chance of a
shower or storm to the east of Globe, much drier westerly flow will
rapidly overspread the deserts giving sunny skies on Monday. On
Tuesday, as the low quickly ejects off to our northeast, dry
subsident northwest flow will move across the area for continued
sunny skies along with much cooler than normal high temperatures.
High temperatures Tuesday should remain in the middle 80s across the
greater Phoenix area.

For Wednesday through Friday, following the exit of the Pacific
upper low, dry and relatively zonal flow will spread across the
desert southwest leading to mostly sunny or sunny days, clear nights
and warming temperatures. Highs over all of the deserts will rise
back into the 90s by Thursday with mostly mid 90s expected over the
warmer deserts on Friday.


South-Central Arizona Including KPHX, KIWA, and KSDL:
A decrease in moisture along with more stable conditions will mean
less cloudiness and only isolated weak showers anywhere near metro
Phoenix today. Still enough moisture leftover for some few-sct mid
clouds (locally broken at times) with bases AOA FL100. Surface winds
will be light and follow familiar diurnal trends.

Southeast California/Southwest Arizona including KIPL and KBLH:
A decrease in moisture along with more stable conditions will mean
less cloudiness and only isolated weak showers over La Paz County.
Still enough moisture leftover for some few-sct mid clouds (locally
broken at times east of the Lower Colorado River Valley) with bases
AOA FL100. Surface winds will be light through the morning before
strengthening this afternoon with some gusts up to 20 kts after 21Z
(after 23Z Imperial Valley). Of note, there will be a layer of relatively
strong winds in the lower levels above the inversion this evening but
not enough to meet LLWS criteria.


Monday through Friday:
A cold front will usher in very dry air Monday and create breezy to
windy conditions leading to an increase in fire danger. Winds will be
significantly lighter thereafter. Minimum humidities drop below 15%
on the lower deserts Monday (at or below 20% higher terrain) and will
remain low the rest of the week. Overnight recovery will only be
fair. Temperatures will be much cooler on Monday (highs in the 80s
on the lower deserts) followed by a slow warming trend beginning


Spotter activation will not be needed.




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