Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 281025
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
325 AM MST Thu Mar 28 2024

.SYNOPSIS...
Dry conditions and slightly above normal temperatures are expected
through at least Friday but with increasing winds each day through
Saturday. A strong low pressure system moving through the region
over the weekend is expected to bring windy conditions on Saturday
followed by a strong cold front moving through Saturday night and
Sunday morning. Much of the area can expect light to moderate
rainfall late Saturday and Sunday with some isolated thunderstorms
and brief heavy rainfall possible. Temperatures will drop to well
below normal for Sunday and Monday with rain chances lingering
through at least Monday across much of Arizona.

&&

.DISCUSSION...
The main forecast concerns over the next several days are the
strong gradient winds expected for Saturday across Arizona and the
potential for a squall line with embedded thunderstorms tracking
eastward across southern and central Arizona during the first half
of Sunday. In the meantime, the region will continue to fall
under weak ridging aloft through Friday resulting in overall quiet
conditions and temperatures at or slightly above seasonal
normals.

The weather system expected for our region over the weekend is
currently spinning off the coast of the Pacific Northwest and it`s
already quite strong even for that region. Little movement of the
upper level trough is expected today, but it is forecast to expand
in size with the base of the trough reaching southern California.
As a result, this will lead to an increasing pressure gradient
over the western deserts and gusty winds upwards of 35 mph in
some spots this afternoon and evening.

By tonight, a trailing shortwave trough is expected to very
quickly dive southward across the western periphery of the trough
reaching the base of the trough early on Friday as a new low
center develops. At the same time the trough is expected to link
up with a strengthening southern jet branch situated from west
west of Baja to Arizona. As this phasing occurs, the trough is
forecast to undergo a period of strengthening off the coast of
central California and then from Friday night through early Sunday
maintaining 500mb heights near climatological records for the
period in that area. For our area on Friday, the gradient will
continue to strengthen likely bringing near advisory level winds
across portions of southeast California into far southwest
Arizona.

For Saturday, as the unseasonably strong low pressure system
slowly tracks south southeastward just off the California coast,
very windy conditions are likely to develop across Arizona. Latest
guidance is now suggesting widespread advisory level winds across
the majority of Arizona Saturday afternoon/early evening,
including south-central Arizona. Forecast 850mb winds for
Saturday afternoon and evening shows a swath of 35-45 kts
centered across south-central Arizona, while skies are likely to
be just partly cloudy. The combination of the strong winds within
the boundary layer and some insolation should result in mixing
much of those winds down the the surface. Wind gusts of 35-45 mph
are likely to occur over much of southern and central Arizona on
Saturday with potential of reaching 50mph in some spots.

By Saturday evening, the advancing strong cold front with some
showers developing along and ahead of the front is expected to
move into southeast California. By the time the front reaches
southwest and then south-central Arizona during the overnight
hours Saturday night, we should see the rain band fill in with the
leading edge of the band potentially evolving into a broken
squall line with embedded thunderstorms due to strong frontal
forcing and favorable boundary-relative flow. Any available
instability is expected to be very minimal, but conceptually based
on the strong front, it should be possible for a few strong
thunderstorms to develop capable of producing heavy rainfall and
near to severe wind gusts.

For the rest of Sunday, the cold core portion of the upper low
will move east across the region with additional showers expected
later in the afternoon and through the evening. There also very
well could be a few isolated thunderstorms later on Sunday as
lapse rates improve and some weak instability develops mainly from
Phoenix and areas west of Phoenix. There is also model evidence
showing good shower potential lasting through the overnight hours
Sunday night across south-central Arizona. As the cold core low
remains in place over much of the region through Monday, shower
chances should persist. Depending on how much insolation can be
realized during the daytime hours Monday, its very possible some
areas could see some low-topped convective thunderstorms Monday
afternoon with small hail and brief heavy downpours the most
likely impacts.

As far as expected rainfall amounts, ample moisture will be
present this weekend, especially along and just ahead of the
front when there will be the most widespread rainfall. The
additional shower and potential thunderstorm activity later on
Sunday and Monday should also add a decent amount to the rainfall
totals from Maricopa County and areas to the east. The latest
storm total forecast rainfall amounts are calling for between
0.10-0.25" for areas west of the Colorado River to 0.50-1.00" in
the Phoenix area. Higher terrain areas northeast of Phoenix could
receive as much as 1.5" in a few spots. This rainfall is likely to
be spread out across a 2-day period, so at this point we are not
anticipating much if any of a localized flood threat.

Temperatures this weekend will also be something to take note of
as high temperatures Saturday will drop roughly 20 degrees to
Sunday`s daytime highs. Given the timing of the cold front during
the early morning hours Sunday, much of south-central Arizona is
likely to see highs reached just after midnight in the mid to
upper 60s with temperatures falling well into the 50s by sunrise
Sunday morning. Daytime temperatures will recover somewhat by the
afternoon, but depending on the cloud cover some areas Sunday
afternoon may struggle to get into the lower 60s.

For early next week as the weakening upper low is trying to linger
over eastern portions of the region, temperatures will be slow to
moderate. Forecast highs for Monday are 10-13 degrees below
normals with highs still in the 60s in Phoenix before likely
improving into the mid 70s on Tuesday. Ensemble guidance heavy
favors a ridge to begin building into the region by next Wednesday
and this should briefly push highs back into the 80s for 2-3 days
later next week.

&&

.AVIATION...Updated at 0535Z.

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

No aviation weather issues expected during the TAF period. Wind
pattern will follow a more typical diurnal tendency with light
speeds aob 7 kts. A few hours of southerly winds will be possible
during the diurnal transition. FEW-SCT high clouds are expected
through the period.

Southeast California/Southwest Arizona including KIPL and KBLH:

The main aviation weather issue during the TAF period will be
gusty winds late Thursday afternoon through Thursday evening. In
the meantime, winds during the overnight through the early
morning hours will be light and variable. Winds will acquire a
westerly component at KIPL and a south to southwesterly component
at KBLH Thursday morning. Wind speeds, especially during the late
afternoon through the early evening hours, will pick up with
gusts upwards to 25 kts, with the strongest gusts at KIPL.
FEW-SCT high clouds are expected through the period.

&&

.FIRE WEATHER...
Seasonably warm and dry conditions will persist through Friday,
but with increasing winds each day. Winds today will be light
across the eastern districts, but breezy to locally windy across
portions of southeast CA this afternoon and evening. Winds Friday
will increase further across the entire area with afternoon wind
gusts between 30-40 mph across the western districts to around 25
mph over the eastern districts. MinRHs both days are likely to
reach into the mid teens across the lower deserts to 20-25% over
higher terrain areas. Friday afternoon may bring a brief period
of elevated fire weather conditions within the Lower CO River
Valley. By the weekend, a strong low pressure system will be
moving in from the west bringing widespread windy conditions
during the daytime Saturday, fairly widespread wetting rains
Saturday night into Sunday, and cooler temperatures starting
Sunday.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...None.
CA...None.
&&

$$

DISCUSSION...Kuhlman
AVIATION...Lojero
FIRE WEATHER...Kuhlman


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