Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 231544

Area Forecast Discussion
National Weather Service Phoenix AZ
844 AM MST Sun Apr 23 2017

High pressure will remain situated over the region today keeping
desert high temperatures well above seasonal normals. Some of the
warmer deserts may approach 100 degrees today. After today, a
gradual cooling trend will begin lasting through the end of the
week. Periods of mainly high clouds can be expected along with
breezy or even windy conditions during the afternoon hours each day.
There will be little if any chance for rain this week outside of
higher terrain areas mainly east of Globe.


Warmer top to the inversion layer via aircraft soundings this
morning when compared to 850mb-sfc values on a sampling of flights
thru KPHX Saturday AM. Also, surface conditions are 2 to 4 degrees
warmer than this time yesterday for many south-central AZ sites.
Atmospheric conditions may no longer support a solid/"slam-dunk"
run at 100F for the Phoenix-area, as outlined below in the
Previous Discussion, but conditions will still lead to one of our
warmer afternoons so far this calendar year. Some of the warmer
desert locales, like Tacna, have already hit the triple-digit mark
and Yuma came close with their high of 99F yesterday. While the
ingredients are not all there to tip the scale into the 100s,
there still is remote potential to mechanically reach 100F at KPHX
today with the warmer boundary layer, low-level jet development
and better mixing ahead of the approaching trough. Broken to
overcast, albeit thin, cirrus band currently cutting through the
far SoCal coast and across the AZ- CA-NV state lines will continue
to track eastward into AZ through the day. No updates planned for
the forecast package this morning.


For the past few days, the primary weather issue/impact has
centered around the expected high temperatures for today and
whether or not the lower deserts (especially Phoenix) would reach
100 for the first time this year. Earlier it seemed to be a "slam-
dunk" that Phoenix would reach 100, with forecasts running as
high as 101. Day by day it became more apparent that the ridge
would not be strong enough; it would be eroded by strong short-
wave energy moving inland over the western CONUS and boundary
layer temps would stay below the triple digit mark. This is still
the case today; the majority of available models keep desert high
temperatures in the upper 90s today and we will call for a high of
98 in Phoenix. Helping keep the temps below 100 will be patchy
high clouds spreading into the central deserts during the day; it
is possible that there will be enough breeziness to help mix the
last of the warm air to the surface and push Phoenix to 100, but
most likely this will not be the case. After today, we are looking
at a cooling trend for the majority of the week and we will
probably have to wait at least 7 days before the next decent
chance to have Phoenix reach 100 degrees.

Operational runs of the GFS and ECMWF as well as GEFS ensemble
output continue to agree that we will see the development of a very
large and broad upper trof over much of the western and central
CONUS this week. Initially the flow aloft becomes stronger and
rather zonal, and with time flow becomes more northwesterly, as a
series of disturbances move through the flow and across mainly
northern Arizona. All of this will result in a cooler airmass,
allowing high temperatures to fall off early this week, with even
more significant cooling to occur starting Friday and continuing
into the weekend. Most of the disturbances that pass by the area
will stay mostly to the north of the lower deserts and any chance
for precipitation will be confined to higher terrain areas north and
east of Phoenix - especially areas to the east of Globe. One such
disturbance will race southeast and across the 4 corners area
Tuesday, bringing the first shot of much cooler air to the area and
lowering high temps into the upper 80s over the central deserts.
Behind the exiting wave Wednesday into Thursday high temps will
rebound and return to the low to mid 90s.

The final push of cool air begins Thursday night as the upper trof
deepens even further and a closed low center dives south towards the
4 corners area, and a cold front swings into southern AZ. As this
low center moves through the four corners, a slight chance for
showers will develop over the higher terrain east of Phoenix with
best rain chances (such as it is) occurring on Friday. High temps
over the south central deserts will lower into the mid 80s for the
most part with the cooling persisting into the weekend. The far
western deserts (including far SE California) will stay a bit
warmer, mostly in the upper 80s and low 90s. There will be breezy
conditions over the area during the afternoon hours pretty much
every day this week as low/mid level gradients tighten, but some of
the strongest wind will likely occur Thursday and Friday associated
with the continued deepening of the main upper trof. It is possible
that wind advisories may be needed at some point and there may be
fire weather impacts as well, especially if humidities drop low
enough. We will be monitoring the potential for dangerous fire
weather during the days to come.


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

Briefly gusty winds and timing wind shifts will be the main aviation
concerns through this evening under scattered thin high cirrus.
An easterly LLJ should develop near sunrise over the Phoenix
vicinity with a wind speed of 25-30kt around 1500 ft AGL above a
light sfc wind creating some measure of LLWS. Feel the speeds
won`t be quite that strong, but could lead to some sfc wind gusts
immediately after mid/late morning mixing; and a few hours of
gusts near 20kt will be possible. After winds switch over to a
westerly direction early this afternoon, periodic gusts are likely
into this evening before diminishing.

Southeast California/Southwest Arizona Including KIPL and KBLH:

Increasing west to southwesterly winds today will be the primary
aviation concern for SE California terminals under scattered thin
high cirrus. Gusts near 25kt will be common from mid afternoon
through mid evening before diminishing somewhat into the overnight

Aviation discussion not updated for amended TAFs.


Tuesday through Saturday:
Temperatures will continue to hold above seasonal averages, but not
at readings as warm as the weekend. After Wednesday, a gradual
cooling trend will settle into the region as mostly dry storm
systems sweep through the Great Basin and down through the
Southwest. Outside of a remote chance at some light showers over
high terrain of Gila County Thursday, widespread dry weather will
cover the districts. Dry afternoons with 5-15% will be common with
overnight humidities varying between mostly fair to good. Active
storm track through the West will keep afternoons breezy for the
district, with upslope west winds and occasional gusts possible
through week`s end. While widespread areas of fire danger due to the
low humidities and winds look unlikely, some localized pockets of
elevated fire danger may be realized Thursday.


First Hundred Degree Day Facts for Phoenix and Yuma:

Average First 100F (30 yr Normal Period 1981-2010): May 2nd
Earliest (Period of Record 1895-2016): March 26, 1988
Latest first occurrence (1895-2016): June 18, 1913

Average First 100F (30 yr Normal Period 1981-2010): April 29th
Earliest (Period of Record 1876-2016): March 12, 1916
Latest first occurrence (1878-2016): June 13, 1971


Spotter activation will not be needed through the middle of next


CA...Wind Advisory from 5 PM this afternoon to midnight PDT tonight
     for CAZ562.



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