623
FXUS65 KPSR 180927
AFDPSR

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

.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...Updated at 0543Z.

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

W to SW winds will shift E by 8-9Z tonight with speeds aob 10 kts
through Tuesday morning. A period of southerly winds, between
15-22Z, is expected before W to SW winds take hold. Afternoon wind
speeds will peak upwards of 8-12 kts with gusts topping out
around to 20 kts. Skies will remain mostly clear, with some
passing cirrus around midday.

Southeast California/Southwest Arizona including KIPL and KBLH:

Breezy W winds will continue for another hour or two at KIPL
before subsiding. Winds at KBLH will increase and shift N to NW
as a dry front pushes through around 12Z. Sustained speeds up to
around 12-15 kts are anticipated with potential for gusts as high
as 25 kts. The N to NW winds at KBLH will prevail most of the day
Tuesday, with speeds subsiding through the afternoon, while KIPL
sees a period of NW winds followed by westerly sundowner winds in
the evening. Skies will remain mostly clear aside from passing
cirrus early Tuesday morning.

&&

.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...Benedict
FIRE WEATHER...Kuhlman