Dust Storm in California, March 15, 2021

Area Forecast Discussion
National Weather Service Phoenix AZ
512 PM MST Mon Mar 15 2021

.UPDATE...Updated Aviation Discussion.

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.SYNOPSIS...
A fast moving weather system will affect the region this afternoon
into early Tuesday. This system will also bring a period of windy
conditions this afternoon and evening, particularly across
southeast California and southwest Arizona where wind gusts above
40 mph are expected. Chances for precipitation starting this
evening into tonight will be focused over the Arizona high
terrain, but a few light showers may affect the lower deserts.
High temperatures will cool into the 60s Tuesday, but increasing
high pressure and warming conditions through the latter half of
the week will push high temperatures into the 80s starting
Thursday.

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.DISCUSSION...
Mesoanalysis reveals a warm and very dry lower atmosphere. Above
about 500 hPa, the airmass is saturated, which corresponds to the
extensive high cloud cover that was present over Arizona this
morning. A closed low centered over central California is rapidly
moving to the southeast today and will pass through northern
Arizona this evening. This feature is associated with a surface
trough and modest height falls ahead of it, helping to increase
southerly winds this afternoon across the area. As the trough
moves through the area later today, high pressure will quickly
build in behind it with a corresponding rapid increase in
pressure. Accordingly, the primary forecast concern today will be
for strong westerly winds later this afternoon through the early
morning hours across southeast California and southwest Arizona.
Model guidance is in good agreement showing westerly winds of
20-35 mph in these regions, with gusts of 40-55 mph. As commonly
occurs in these type of set ups, the far southwest corner of
Imperial County may see even stronger winds, with a few gusts of
60-65 mph possible. Considering the dry soils, these winds will
likely kick up a significant amount of dust, leading to reduced
visibility and potential travel issues. These hazards are covered
by the existing and fairly widespread wind advisory and blowing
dust advisory through the late evening. A localized high wind
warning remains in effect for far southwest Imperial County.
Further east, breezy westerly winds will develop in the Phoenix
area late this evening. However, the pressure gradient will be
less pronounced than further west and will maximize after dark,
which will allow less opportunity to mix down stronger winds from
aloft. Therefore, wind gusts should peak around 30-35 mph.

In terms of precipitation, this system will be quite minimal. The
antecedent airmass looks quite dry and the inland track of the
system will not allow it to pick up much additional moisture.
Model forecast soundings show that there will be just enough
moisture increase in the 800-700 hPa layer to support a few
showers, but primarily to the north of the area where dynamics
supporting lift will be better and lower level dry air will be
less of a concern. NBM probabilities show about a 20-30% of
measurable rain around Phoenix, increasing to above 50% in the
foothills. As this system is fairly cold, precipitation may fall
as snow above about 4000 feet, but any accumulation would just be
an inch or two.

In the wake of this system cooler surface air will move in,
knocking high temperatures down to the low 60s in the Phoenix
area, and near 70F in southeast California. However, upper level
heights will rapidly rise as much warmer air moves in for the
midweek period under a robust ridge of high pressure. Modest high
cloud cover is expected Wednesday and Thursday but will clear out
by the weekend. The current forecast has high temperatures above
80F for the lower deserts Thursday through Saturday, with Friday
or Saturday being the warmest day. The probability of hitting 90F
remains near 10-20% in Phoenix on those days.

There are indications that another longwave trough may move into
the western United States next weekend or early next week. WPC
cluster analysis reveals uncertainty with respect to both the
timing and strength, but the greater uncertainty appears to be
with respect to timing. Sensible impacts will likely mirror many
of our recent storm systems, meaning a brief cool down with gusty
winds are relatively likely. Conversely, the odds of impactful
precipitation appear to be quite minimal.

&&

.AVIATION...Updated at 0000Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
A period of breezy southerly crosswinds at KPHX expected very
early in the period to transition to the southwest-west and become
very breezy to windy by 05-06Z at 12-17kt with gust near 24-30kt
with and ahead of an approaching cold front. Although hard to say
with any certainty, lofted dust from the west could work into the
metro area this evening and if any does it could impact slantwise
visibility. A few brief showers or sprinkles will also be possible
across the area late tonight. Localized erratic wind gusts as high
as 40kt in the vicinity of isolated showers can not be ruled out
overnight. Meanwhile, ceilings along the front may drop as low as
6-7kft for a few hours. Otherwise skies will be partly cloudy
with FEW-SCT low-mid cloud decks at 7-15kft. Wind speeds and gusts
will subside behind the front by around 12Z, though general
breeziness with a westerly flow will continue Tuesday with
occasional gusts into the teens expected again during the
afternoon.

Southeast California/Southwest Arizona including KIPL and KBLH:
Very strong W-SW winds and blowing dust reducing visibilities at
times are likely now through this evening. The strongest winds are
generally expected through 06Z tonight, with wind gusts at both
terminals likely reaching 40-45 kts during this time. A few gusts
could reach up to 50-55 kts. Patchy blowing dust will occur during
this period with more prolonged periods and areas of blowing dust
possibly causing IFR-MVFR visibilities as well. Widespread lofted
dust may also impact slantwise visibility. Winds may not fully
subside until after 12Z Tuesday. Skies will be partly cloudy with
FEW-SCT low clouds/dust from 2-10kft.

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.FIRE WEATHER...
Thursday through Monday:
Strong high pressure will settle into the region early in the
period bringing a strong warming trend through Friday. High
temperatures in the 80s are likely for Thursday through Saturday.
Minimum afternoon humidity levels will fall to around 10-15% each
afternoon. Overnight RH will maximize between 40-55% Thursday and
Monday, and 30-45% Friday to Sunday. Winds through Friday will be
fairly light, before increasing over the weekend with daytime wind
gusts of 20-30 mph possible.

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.SPOTTER INFORMATION STATEMENT...
Spotters should follow standard reporting procedures.

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.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Wind Advisory until 10 PM MST this evening for AZZ530>533-536.

     Blowing Dust Advisory until 10 PM MST this evening for
     AZZ530>533-536.

CA...Wind Advisory until 10 PM PDT this evening for CAZ560-561-
     563>570.

     Blowing Dust Advisory until 10 PM PDT this evening for
     CAZ564>567-569-570.

     High Wind Warning until 10 PM PDT this evening for CAZ562.

&&

DISCUSSION...Hodges
AVIATION...Sawtelle/Benedict
FIRE WEATHER...Hodges