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089
FXUS65 KPSR 181746
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
1046 AM MST Tue Jun 18 2024

.UPDATE...
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Updated 18Z Aviation Discussion.
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&&

.SYNOPSIS...
Near normal temperatures are expected over the next couple of
days before stronger high pressure moves over the region leading
to another warm up later this week. Starting Thursday,
temperatures will likely top 110 degrees across the south-central
Arizona lower deserts with an Excessive Heat Watch in effect for
the area on Thursday and Friday. Moisture will also increase
across much of southern and central Arizona starting Thursday and
this will provide for some shower and thunderstorm chances over
the eastern Arizona high terrain into the coming weekend.

&&

.DISCUSSION...
A Pacific upper level trough has overtaken much of the Western
U.S. since yesterday allowing heights across the Desert Southwest
to fall. The near normal temperatures as the result of the lower
heights will persist through Wednesday with highs today mostly
between 101-104 degrees across the lower deserts. By tonight, the
main energy from the larger scale trough will already be lifting
to the northeast away from our region, while an unseasonably
strong ridge centered over the Mid-Atlantic States into New
England begins shift southwestward.

Going through the middle part of this week, the trough to our
north will continue to weaken and shift away from our region,
while the ridge over the Eastern U.S. continues to migrate west
southwestward closer to our region. At the same time, a tropical
disturbance will be moving westward off the Gulf of Mexico into
Mexico. This disturbance will quickly weaken as it progresses
inland, but it will leave an inverted trough behind which should
attempt to track west northwest across northern Mexico. Moist low
to mid level flow in between the tropical disturbance and the
upper level ridge across Texas will increase out of the east early
on Wednesday reaching New Mexico Wednesday evening. This moisture
is expected to continue to track westward Wednesday night
reaching eastern Arizona Thursday morning. Guidance shows this
moisture spreading westward through much of southern and central
Arizona on Thursday increasing surface dew points over the eastern
Arizona high terrain into the 50s and providing our first monsoon
thunderstorm chances east of Phoenix starting Thursday afternoon.

In addition to the moisture increasing later this week, the ridge
will also be spreading westward into our region increasing H5
heights to around 590dm as early as Thursday. Temperatures are
forecast to quickly heat up starting Thursday with highs between
110-114 degrees across the south-central Arizona lower deserts.
Temperatures should increase further on Friday areawide with all
of the lower deserts likely reaching or exceeding 110 degrees
with 112-116 degrees forecast for the Phoenix area. Given the
latest NBM forecast temperatures later this week, we have pushed
up the timing of the Excessive Heat Watch to start on Thursday and
going through Friday. The heat dome will continue to impact the
region through the weekend with any daily monsoon higher terrain
activity providing little relief from the heat over the lower
deserts. In fact, guidance suggests the ridge may strengthen
across the region and northern Mexico Sunday into early next week
and this could result in additional Excessive Heat headlines over
the lower deserts of southeast California.

Our first chances of monsoon showers and thunderstorms are likely
to mostly affect higher terrains as forecast moisture levels are
not quite high enough to provide the lower deserts with much of a
chance. Storm motion steering flow will also favor north
northeastward progressing convection which is not favorable for
activity making its way into the lower deserts. However, the
south-central Arizona lower deserts, including the Phoenix area,
may very well have to contend with gusty outflow winds and
possibly some blowing dust. This first round of monsoon activity
may only last through early next week as guidance favors the ridge
sinking farther to the southwest by the middle of next week. This
would shift the flow out of the southwest and gradually bring in
drier conditions across the region later next week.

&&

.AVIATION...
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Updated at 1750Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:

Winds will tend to follow their typical diurnal pattern. A period
of southerly winds, between now and approximately 21Z, is once
again expected to precede the westerly/southwesterly shift this
afternoon. Wind speeds this afternoon and early evening with
generally be around 8-12 kt, with occasional gusts up to 20 kt.
Winds will then go back easterly during the overnight hours
tonight. Mostly clear skies will continue.

Southeast California/Southwest Arizona including KIPL and KBLH:

Winds at KIPL will generally be aob 8 kt and have a westerly
component through early tomorrow morning before establishing out
of the SE. There may be a brief period of sundowner winds tonight
with gusts up to around 20 kt. At KBLH, northerly winds will
subside throughout the afternoon. Winds will then go westerly this
evening and shift south overnight. Mostly clear skies will
continue.
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&&

.FIRE WEATHER...
Overall light winds are expected over the next couple days with
near normal temperatures. The dry conditions will however persist
with daily MinRHs around 10% and overnight MaxRHs between 20-40%.
Starting Thursday, moisture will creep into much of southern and
central Arizona providing daily shower and thunderstorm chances
over the eastern Arizona high terrain. Humidities will also
noticeably improve later this week over higher terrain areas, but
CWR will stay low at mostly below 10%. Along with the increasing
moisture, high pressure is also expected to build back over the
region later this week resulting in very hot temperatures and
lower desert highs between 110-115 degrees.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Excessive Heat Watch from Thursday morning through Friday
     evening for AZZ534-537>555-559.

CA...None.
&&

$$

DISCUSSION...Kuhlman
AVIATION...Whittock/Berislavich
FIRE WEATHER...Kuhlman