Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 03/11/21

Area Forecast Discussion

National Weather Service Phoenix AZ

 515 PM MST Wed Mar 10 2021

.UPDATE...
Updated 00Z aviation discussion.

&&

.SYNOPSIS...
A slow moving weather disturbance will move through the region the
remainder of the week providing a period of unsettled and
occasionally breezy weather. Temperatures will cool to nearly 10
to 15 degrees below normal the next several days before
moderating over the weekend. A few light showers and even an
isolated thunderstorm will be possible through Friday though many
locations may not see rainfall. Precipitation amounts should not
be particularly heavy with the greatest chances and accumulations
over higher terrain areas of south-central Arizona.

&&

.DISCUSSION...
Drier air has moved in aloft and the atmosphere has cooled
considerably. The low in Phoenix this morning, 49F, was 15
degrees cooler than yesterday morning. Full tropospheric cooling
is noted in aircraft and neighboring area soundings. With the
cooler air in place, and trending cooler as a cold system slowly
approaches, highs this afternoon will also be notably cooler than
yesterday (~5F-10F cooler). Most lower desert locals are expected
to come up short of 70F today.

Objective analysis and satellite also shows a strong jet streak
has now positioned over the region ahead of the approaching
system. Tucson, AZ sampled a 130kt wind at 250mb with this
morning`s sounding. In the lower levels of the atmosphere the
height packing has shifted slightly to the southeast, thus
shifting the stronger geostrophic flow just to the southeast. So,
while it is not expected to be as windy as it was yesterday, it
will still be quite breezy through this afternoon, especially
across southeast CA and southwest AZ. Gusts could reach up to
30-35 mph in this area with up to 25-30 mph further east into
south-central AZ.

The expansive circulation which was centered off the coast of the
PacNW at this time yesterday has now slid down the CA coast to
just northwest of the Bay area. Multiple embedded vorticity
maxima are visible on satellite rotating around the larger
circulation. One shortwave axis moved into SoCal earlier today
with a weak IVT plume, spurring scattered showers and isolated
convection on the windward side of SoCal. For most of the day the
rain shadow effect has held on, but wave cloud coverage continues
to increase across and some weak echoes are beginning to pop up
across southeast CA. Through the rest of the afternoon and
evening, HREF members suggest a isolated showers will be possible
across the western warning area, with better chances just north of
the area. Some weak showers could also work their way into
central AZ tonight, but again remain mainly just north of the
warning area as NBM PoPs only reach 10-20% across the northern
Maricopa county foothills to Tonto Basin.

A strong jet streak currently seen on the western flank of the
expansive circulation will help reorient the broad low and begin
turning the track more toward the Desert Southwest heading into
Thursday. As this occurs, models indicate a deepening of the lower
level moisture field across the western warning area through most
of the day Thursday with increasing chances for showers. With
better dynamics and steepening mid level lapse rates, any surface
instability could also support an isolated storm Thursday
afternoon. Heading into Thursday night and Friday morning,
precipitation chances should increase across the eastern warning
area as the low continues to drift east. Based on some short-range
guidance this looks like the best timeframe for rain in south-
central AZ, including the Phoenix area. 6-hour NBM PoPs are now
up to 60-70% during this time.

By Friday afternoon, NAEFS members are in fairly good agreement
placing the cold core center somewhere over western Arizona. If
anything, the forecast trend points towards a slightly slower
forward progression albeit only enough to affect some subtle
details. Nevertheless given the magnitude of cold air aloft entering
the forecast area, some locations in Arizona will be primed
for shallow topped convection. Steepening lapse rates (H7-H5 layer
~8 C/km and sfc-H7 nearly 9 C/km) will likely become juxtaposed
with residual moisture and vorticity aided support. Boundary layer
moisture may be the key here where low 30 dewpoints result in SBCape
around 500 J/kg while more aggressive drying and dewpoints falling
into the low 20s could preclude deeper convective updrafts. Kept
POPs near 60% and low TS mention as conceptually this scenario
historically matches ones where convective activity and coverage
is much greater than models initially indicate.

There are still some minor timing differences heading through the
weekend which are resulting in limited forecast confidence. There
a minority of ensemble membership (mostly in the CMC and EPS camps)
showing the cold core and larger circulation still hovering over
Arizona Saturday while the vast majority of model output indicates
this center located somewhere near the New Mexico/Colorado border.
At this time, will stick with the preponderance of evidence even
incorporating a somewhat slower progression feeling only the
eastern part of the forecast area will still be influenced by the
western periphery of the cyclonic circulation and cold core.
Typically, these scenarios involve a vorticty spoke rotating
southeast and forcing moist ascent along the Rim. This familiar
pattern with NW-SE steering flow would bring a dusting of snow
across the higher terrain of Gila County, and potentially a few
decaying rain showers into the northeast portions of the Phoenix
metro.

By Sunday and Monday, a brief period of low amplitude shortwave
ridging between weather disturbances will control the SW Conus
weather. This setup should bring drying and moderating conditions
to the region with temperatures rebounding back close to the
seasonal normal. A quick moving shortwave may rapidly descend into
northern Arizona late Monday and Tuesday, however the trajectory,
depth, and impacts of this next system are highly uncertain this far
in advance.

&&

.AVIATION...Updated at 0015Z.

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

Gusty westerly winds through early evening and lowering ceilings
late Thursday morning and afternoon will be the main aviation
impacts through the TAF period. Westerly winds currently gusting
up to 20-25 knots will weaken by 03Z to around 10-15 knots before
becoming even lighter overnight. Easterly winds should briefly
return before sunrise at the East Valley sites, but remain
westerly or southerly at the West Valley sites Thursday morning,
with some uncertainty on whether they will switch at KPHX and
KDVT. Winds will remain southerly to westerly at 10-15 knots by
early afternoon while SCT-BKN ceilings decrease to 6-8 kft as
another Pacific disturbance slowly approaches from the west.

Southeast California/Southwest Arizona including KIPL and KBLH:

Strong westerly winds through late evening and isolated to
scattered showers over southeast California on Thursday will be
the main aviation impacts through the TAF period. Westerly winds
currently gusting up to 30-35 knots capable of causing patchy
blowing dust will continue at KBLH and KIPL through this evening
before gradually decreasing during the overnight hours. Low-level
wind shear impacts may briefly occur later this evening at KIPL
if winds weaken more quickly than forecast, but confidence is too
low to include that at this time because surface winds should not
decrease below 15 knots until late this evening. Confidence in
shower activity ahead of a strong Pacific disturbance is
increasing at enough at KIPL to include a PROB30 group there
between 15-21Z, but still too low to include at KBLH at this time.
Winds tomorrow afternoon should be a little weaker and a bit more
southwesterly than today, with gusts up to 20-25 knots.

&&

.FIRE WEATHER...
Friday through Tuesday:
Unusually cool temperatures at the end of the week will moderate
closer to normal next week as a weather disturbance exits the area.
Scattered showers will be common in eastern districts Friday with
some localized areas experiencing wetting rains. There is an outside
chance of an isolated thunderstorm Friday afternoon though heavy
rainfall is not anticipated. More mountainous areas east of the
Phoenix metro will see some light snow which could linger into
Saturday. Winds will exhibit typical seasonal gustiness with speeds
the strongest on Saturday and Monday. Minimum afternoon humidity
levels will mostly fall into a 20-30% range this week, then falling
back into the teens next week. Overnight recovery will be fair
to good in a 30-70% range.

&&

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

&&

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

$$

DISCUSSION...Benedict/MO
AVIATION...Hopper
FIRE WEATHER...MO