Forecast Discussions mentioning any of
"ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 09/20/21
Area Forecast Discussion
National Weather Service Phoenix AZ
501 PM MST Sun Sep 19 2021
.UPDATE...
Updated Aviation Discussion.
&&
.SYNOPSIS...
Drier air will spread east through the region today with only
isolated storm chances of less than 5 percent through late
afternoon. High temperatures will stay near normal through the
early part of the week. By midweek increasing high pressure across
the the region will bring a slight warming trend along with
continued dry conditions. An increase in moisture late in the week
could bring highs back down closer to normal along with a slight
chance of storms.
&&
.DISCUSSION...
The UL pattern depicts dry W to SW flow aloft with a
weak, positively tilted open wave trough axis and weak elongated
vort max from SW to Central AZ. A very moist BL still occupied
Central to E AZ with SPC mesoscale analysis depicting elevated
dewpoints up through at least 850 mb. The Phx ACARs sounding trend
from late morning to early afternoon confirmed significant moisture
below H7 and very dry air above that, MLCAPE in the 1200s j/kg and a
dramatic downward drying trend with PW falling from near 1.57" to just
above 1". Midday METSAT showed large fields of CU cloud streets
across Central-E AZ with WV showing the mostly dry airmass intrusion
into the region south of a longwave trough to the north.
Models are in good agreement on a dramatic plunge in PW through
this evening to about 0.5-0.6" as the weak UL trough fills and
ejects very slowly across AZ today. Model soundings show the BL
slowly drying throughout the afternoon and into the evening as
instability and CAPE gradually diminish. But because the drying
takes all afternoon the HREF/family members indicate very isolated
and fairly modest showers and cells popping up in SE-E AZ
including portions of Pinal and Gila counties this afternoon.
However the preferred NBM PPIs remain at less than 5% in and
around Phoenix, and only 5% in Gila Cty east of Globe.
Nevertheless any isolated cells that do form will still have about
a 20% chance of producing winds up to 35 mph.
Early this week models agree on dry NW flow aloft dominating the
region as a low pressure trough pushes through the Rockies and N.
Plains. This will help confine a ridge of high pressure well to the
west and southwest of the area. As a result highs through Tuesday
are likely to remain fairly near normal, or around 98-102 degrees.
Beginning Tuesday a ridge of high pressure builds into the W Coast
and then strengthens over the Desert Southwest and W CONUS by
midweek. The relatively strong high pressure anomaly could push H5
heights to around 596 dam. This will result a midweek warming
trend with highs favored to reach roughly 5 degrees above normal,
or around 103-105 degrees around Phoenix and a couple of degrees
warmer out west.
Some areas of moderate heat risk are also likely, especially around
Phoenix, near Yuma and some parts of SE CA including Imperial and
Blythe. The latest NBM now favors a 49% chance of a high of 103
degrees for Phoenix on Wednesday (the Autumnal Equinox), the warmest
day this week. At the same time it also indicates a 35% chance of a
high of 107 degrees (the 75th percentile) for Wednesday.
For the second half of the week and into the weekend the clusters
and ensemble means show at least a moderate recovery of regional
moisture from the south, although with plenty of model spread. This
is the result of low pressure system developing near coastal CA
under high pressure to the N-NW in a quasi rex block. For now low-
end POPs/storm chances are favored across the region ranging from 8-
15% though with considerable uncertainty.
There is also moderate uncertainty on a slight cooling trend with
highs dropping back down to the upper 90s to low 100s late this
week and into the weekend.
&&
.AVIATION...Updated at 0001z.
South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
An upper level disturbance (on the southern periphery of a larger
upper trough over the northwest CONUS) is moving through
southeastern AZ. Shower/thunderstorm activity the rest of the day
is expected to be well removed from the Phoenix area and limited
to southeastern Arizona. Otherwise, just some scattered cumulus
with bases at FL080-100 that will slowly decrease this evening
with a few lingering well into the night. There may some temporary
redevelopment of FEW100 during the late morning and early
afternoon before clearing. Surface winds will favor westerly
components into the evening before slowly trending toward light
downvalley/drainage patterns. That transition is expected to be
pretty slow at KPHX and not happen until after 10Z. Above the
surface through about FL120, winds will be southwest and west
through the daytime Monday (northwest at higher altitudes).
Southeast California/Southwest Arizona including KIPL and KBLH:
A weather disturbance over southeast AZ this afternoon has left
behind dry and stable conditions and northerly flow above roughly
FL120 which will continue through the day Monday. Below that,
southwesterly directions will prevail through about 12Z before
transitioning to southeasterly. At the surface, KIPL will have a
somewhat abrupt switch to westerly directions between 01Z-02Z
along with some gusts up to 20 kts before speeds weaken. Southerly
surface winds redevelop by midday Monday. At KBLH, southwesterly
winds are expected this evening (some gusts to 20kts through about
02Z) before transitioning to a northerly downvalley/drainage
pattern by 12Z. Anticipate a lot of variability in the surface
winds during the day Monday. Otherwise, clear skies.
&&
.FIRE WEATHER...
Tuesday through Saturday:
Dry conditions will dominate through most if not the entirety of
the week as high pressure strengthens across the region through
the first half of the week. A weather system may move toward the
region late in the week, but there is still considerable
uncertainty in the evolution of the system at this time. High
temperatures through the period will be near to slightly above
normal. Afternoon RH values will mostly be in the 10-20% range
with overnight recoveries to 30-45% through the first half of the
week and 35-55% during the later half. Winds will be fairly light
each day, with typical afternoon breezes, while favoring local
diurnal/terrain influences.
&&
.SPOTTER INFORMATION STATEMENT...
Spotters should follow standard reporting procedures.
&&
.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...None.
CA...None.
&&
$$
DISCUSSION...Sawtelle
AVIATION...AJ
FIRE WEATHER...Benedict