995
FXUS65 KPSR 021018
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
318 AM MST Tue Jul 2 2024

.SYNOPSIS...
The weather pattern over the next few days will eject the
lingering monsoon moisture southward out of the region leaving
only minimal chances for showers and a few thunderstorms
generally over higher terrain areas through Wednesday. Starting
Thursday, very strong high pressure and dry conditions will
overtake the region with excessive heat conditions likely
spreading eastward through much of the lower deserts of Arizona.
Very hot temperatures with highs around 115 degrees are expected
starting July 4th, lasting through at least the coming weekend.

&&

.DISCUSSION...
A major pattern swing has begun with monsoon moisture getting
kicked eastward as a trough passes by to our north before strong
ridging and northerly flow scours out the rest of the moisture on
Thursday. Water vapor imagery continues to show drier air
spreading eastward through all but far southeast Arizona. A couple
days ago, guidance was keeping enough moisture around for at
least some modest rain chances through Wednesday, but as we now
see this was too optimistic. For today, the drier air will largely
end rain chances across all of the lower deserts in Arizona and
then even over higher terrain areas starting Wednesday. By
Thursday, PWATs are forecast to drop to below 1" with surface dew
points dropping to as low as 35-40 degrees on Friday. This kind of
dry air is more like what we typically see in June before the
monsoon gets going.

The main forecast concern through the rest of the week is the
building heat and the excessive heat conditions expanding into
much of southern and central Arizona. A very strong high pressure
ridge with H5 heights near 600dm has developed off the coast of
California over the last couple of days and this heat dome is
forecast to gradually shift eastward over our region later this
week. Temperatures will continue to rise over the next several
days with highs nearing 115 degrees across the southeast
California lower deserts by Wednesday and topping 110 degrees in
the Phoenix area.

Starting Thursday, the center of the ridge is likely to get closer
to the California coast while the ridge elongates and spreads over
the Desert Southwest. This should raise our H5 heights from the
current 590-592dm to 594-596dm starting around Thursday. We will
also fall under an increasing dry northerly flow, scouring out the
rest of the boundary layer moisture on Thursday. NBM forecast
temperatures continue to trend hotter for later this week and
through the holiday weekend, partially due to the drier conditions
and also because the ridge is now likely to eventually move
directly over the region late in the weekend and early next week.
Temperatures are now expected to reach between 110-114 degrees in
the Phoenix area on July 4th to 112-116 degrees across far
southwest Arizona into southeast California.

Thursday may be the start of a more expansive and extended heat
wave across much of the region. Excessive heat is a near certainty
across all of the western deserts Thursday and through at least
the weekend (likely longer), while the eventual positioning of
the ridge will determine how far east the excessive heat will
spread. Forecast confidence has increased enough to issue an
Excessive Heat Warning for all of southeast California starting
Thursday and an Excessive Heat Watch for southwest and south-
central Arizona. The end time is currently next Tuesday, but given
the good agreement among guidance through at least the middle of
next week, the excessive heat conditions are likely to persist
through much if not all of next week. Any good moisture return
could end this upcoming heatwave, but as of now that probably
won`t happen through at least the middle of next week.

Forecast temperatures Friday through early next week are now
showing highs ranging from 112-116 degrees across south-central
Arizona to 113-119 degrees across far southwest Arizona and
southeast California. Given it seems models are playing a little
catch up with the ridge and the dry air coming in over the next
few days, it would not be surprising to see these forecast ranges
increase another couple of degrees.

&&

.AVIATION...Updated at 0505Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
No major weather issues are the most likely outcome through Tuesday
evening under partly cloudy skies with decks mainly focused over
mountain areas and bases above 10K ft AGL. Easterly winds should
quickly sweep over the metro terminals by midnight with a reversal
to a westerly component around noon Tuesday. Isolated SHRA/TSRA will
again be common mainly outside the terminal footprint Tuesday
afternoon with a 10% chance of any TSRA directly affecting any
aerodrome. Chances of outflow winds and abrupt wind shifts are
around 25% with a shift to the southeast most likely during the
early/mid evening, however with very little enhanced gustiness.

Southeast California/Southwest Arizona including KIPL and KBLH:
No significant weather issues will exist through Tuesday night under
clear skies. Wind directions will favor the typical variation
between daytime SE and evening SW with occasional afternoon/evening
gusts to around 20kt.

&&

.FIRE WEATHER...
Drier conditions will continue to filter into the region over the
next few days. Isolated afternoon showers and a few thunderstorms
are expected today over the Arizona higher terrain with chances
on Wednesday even lower. As conditions continue to dry out, even
hotter temperatures are expected, especially later this week.
Humidities will drop considerably today over the majority of the
area. The lower deserts will see MinRHs mostly fall to around
15%, while higher terrain areas east of Phoenix will see 25-35%.
Winds will follow diurnal trends, occasionally gusting to 20 mph
during the afternoon hours. Starting Thursday, essentially all of
the monsoon moisture will get ejected from the area leaving very
dry and hot conditions, likely well into next week. MinRHs
starting Thursday will mostly be between 10-15% over the lower
deserts to 15-20% over higher terrain areas. The dry air will also
end any rain chances during this time. Temperatures are expected
to climb to well above normal by Thursday with lower desert highs
between 112-118 degrees.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Excessive Heat Warning until 8 PM MST Tuesday for AZZ530.

     Excessive Heat Watch from Thursday morning through Tuesday
     evening for AZZ531>544-546-548>551-553>555-559.

CA...Excessive Heat Warning until 8 PM PDT Tuesday for CAZ562-566-567-
     569-570.

     Excessive Heat Warning from 10 AM Thursday to 8 PM PDT Tuesday
     for CAZ560-561-563>565-568.

&&

$$

DISCUSSION...Kuhlman
AVIATION...18
FIRE WEATHER...Kuhlman