NWS Phoenix, AZ wrote its last area forecast discussion before the first event at Mar 14, 4:38 AM PST, 4 hours 22 minutes earlier.
Area forecast discussion, Mar 14, 4:38 AM PST, 4 hours 22 minutes before
Area Forecast Discussion
National Weather Service Phoenix AZ
538 AM MST Thu Mar 14 2024
.UPDATE...Updated 12z Aviation Discussion.
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.SYNOPSIS...
Low pressure will settle near the AZ/CA border later today and
stall through the weekend. As it does so, it will bring cooler
than normal temperatures as well as increased rainfall potential,
with the best rain potential expected late today through Friday.
The chances for showers will linger through the weekend, but
mostly concentrated across the higher terrain areas of eastern AZ.
In addition, breezy to windy conditions are expected through
today, mainly along and to the west of the Lower Colorado River
Valley.
&&
.DISCUSSION...
Latest water vapor satellite imagery as well as streamline analysis
show a closed upper-level low diving southward across southern
Nevada as of early this morning. This upper-level low later today is
expected to settle across the AZ/CA border and stall through the
upcoming weekend as part of a Rex Block pattern that will set up
across the western CONUS.
The main concern in the near term will be the winds. The
combination of the upstream ridge just off the west coast and the
the upper-low that is diving southward is setting up a strong
pressure gradient. As a result, strong northerly winds are
expected to materialize along and to the west of the Lower
Colorado River Valley through the morning. Wind gusts are
expected to peak at 50 mph across the higher terrain areas of
southeast California, in the vicinity of Joshua Tree National Park
and the Chocolate Mountains, while wind gusts will peak to
between 30-40 mph across the lower elevations. As a result, wind
advisories are in effect for all of these areas through midday
today. With these strong winds, areas of blowing dust temporary
causing poor visibilities as well as strong crosswinds along east
to west oriented highways, especially along I-10 and I-8, will be
a concern. Thus motorists traveling across these areas will need
to take extra precautions. Winds will begin to subside by this
afternoon as the closed low settles along the CA/AZ border and the
pressure as a result weakens substantially.
The other aspect of this low pressure system will be the rainfall
potential, especially across south-central AZ later today through
Friday. Once the low settles along the AZ/CA border later today,
southerly flow out ahead will advect moisture from the Gulf of
California through Friday, with PWATs expected to rise as high as
0.6-0.7" according to the EPS and GEFS mean. This moisture
increase along with favorable strong upper-level jet dynamics of
around 100 kts at 250 mb, enhancing upper-level divergence, will
result in the development of showers beginning later this evening,
but especially during the day on Friday when the latest Hi-res
model guidance is showing waves of showers affecting most of
south-central AZ, including the Phoenix area. As a result, the
latest NBM PoPs have been adjusted upwards significantly with now
the higher terrain areas over 80% and the Phoenix area between
60-70%. Some showers may also develop across southwest Arizona and
southeast California but should be more isolated in coverage with
PoPs much lower at between 10-30%. Accompanying these waves of
showers will also be isolated thunderstorms as instability from
the cold core of the low will be enhanced with the latest HREF
showing CAPE values of between 100-200 J/KG Friday afternoon and
evening. Thus with any thunderstorm activity will be the threat of
locally heavy rainfall, small hail, and gusty winds potentially
up to 30 mph. In terms of the QPF, average rainfall amounts range
between 0.15-0.25" across the Phoenix metro area to between
0.25-0.75" across the higher terrain to the north and east of
Phoenix. Locally higher amounts are possible, especially from any
thunderstorm activity that enhances rainfall rates.
Heading into the weekend, as previously mentioned, the upper-level
low will still meander near the Lower Colorado River Valley but will
be on a weakening phase. Thus, even though precipitation chances
linger, chances will be on a gradual decline with the best chances
expected mainly across the higher terrain areas of eastern and
northern Arizona. By early next week, the upper-level low will
still meander over the area but where exactly it will be
positioned is still uncertain as the guidance is exhibiting some
differences. Finally, there is agreement by the ensembles that the
low should finally exit the region by middle of next week as the
Rex Block breaks down.
Temperatures through Friday will be significantly cooler compared to
the last couple of days as readings will be a good 8-10 degrees
below normal with highs struggling to reach 70 degrees across the
lower deserts. A very gradual warming trend commences by the
weekend with highs in the low to mid 70s, which is still below
normal for this time. Thereafter, through the early to middle
portion of next week, temperatures will then rebound to near to
slightly above normal levels with highs in the low to mid 80s.
&&
.AVIATION...Updated at 1230Z.
South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Minor aviation impacts are expected throughout the forecast
period, with the main concerns being considerable amount of
uncertainty in prevailing wind directions and potential for shower
activity this evening and into the overnight hours. Currently,
winds continue to have variable directions across the metro, but a
light easterly component remains the expectation for early this
morning. However, due to increasing clouds and weak gradient
winds, periods of variability are anticipated to redevelop this
morning and persist into the afternoon hours. Weak SW winds may
prevail during the late afternoon hours, but confidence remains
moderate (40-60%) of that occurring. The more certain timeframe
for winds will be this evening when scattered shower activity is
expected to enhance winds out of the NW across the metro and
persist into the overnight period. Ceilings are expected to remain
aoa 7 kft, with only brief drops in ceilings during periods of
shower activity. Due to limited coverage and probabilities of
showers prevailing at any of the terminals, elected to insert VCSH
conditions generally from 23z-06z, with continuing the PROB30
group for more specific timing (generally 02-06z) for best chances
for shower activity to occur at each terminal going into the
overnight hours.
Southeast California/Southwest Arizona including KIPL and KBLH:
Gusty winds are the main aviation concerns through the forecast
period, with winds already gusting 30-35 kts at IPL. Northerly
wind gusts in excess of 20 kts are anticipated through most of
today until finally subsiding by this evening. The stronger wind
gusts during this period will be capable of generating blowing
dust, which may briefly lower visibility below VFR. Due to the
higher certainty of stronger winds, the blowing dust potential is
only reflected in the TAF at BLH via a TEMPO group. VFR skies
will persist through the TAF period with SCT to BKN clouds aoa 10K
ft at times.
&&
.FIRE WEATHER...
An approaching low pressure system will lead to strong gusty north
winds through early this afternoon along and to the west of the
Lower Colorado River Valley with gusts as high as 50 mph across the
higher terrain areas of southeast California and up to 40 mph
across the lower elevation areas. Winds begin to subside by later
this afternoon with much lighter speeds expected for Friday.
Winds will generally remain light through the weekend and into
early next week. As the low pressure system settles along the
CA/AZ border, the chances for wetting rains begin to increase
later today across the eastern districts but more so heading into
Friday as the coverage becomes more widespread. The chances for
showers linger through the weekend, with the best chances confined
mainly to the higher terrain areas of eastern AZ. MinRH values
today will remain in the teens across the western districts and
20-30% across the eastern districts. The MinRH ramps up
significantly on Friday with values ranging between 30-60% area
wide. The MinRH will remain elevated through the weekend before
declining heading into early next week. Well-below normal
temperatures expected through Friday before a gradual warming
trend commences over the weekend.
&&
.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Wind Advisory until noon MST today for AZZ530.
CA...Wind Advisory until noon PDT today for CAZ560-561-564-565-
568>570.
Wind Advisory until noon PDT today for CAZ563-567.
&&
DISCUSSION...Lojero
AVIATION...Young/Benedict
FIRE WEATHER...Lojero