Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 08/10/19

Area Forecast Discussion...Updated

National Weather Service Phoenix AZ

 510 PM MST Fri Aug 9 2019

.UPDATE...Updated aviation discussion.

&&

.SYNOPSIS...
Increasing moisture and disturbances across eastern Arizona and
the southern Gulf of California will cause periods of scattered
showers and thunderstorms through Sunday morning over south
central Arizona. Temperatures will remain slightly below normal
for most areas except southeast California where temperatures will
be near normal with drier air preventing any rainfall chances to
the west. Dry air will begin to sweep eastward late this weekend
through most of next week, pushing thunderstorm chances east into
New Mexico and allowing above normal temperatures to return.

&&

.DISCUSSION...
Very isolated showers with broken to overcast midlevel clouds
continue to persist across south central Arizona this afternoon.
Although aircraft soundings currently suggest that there may be a
small amount of surface-based instability, there is a stable
layer evident at 500 mb just above the melting layer where drier
air is present, but is slowly moistening. In general, this should
keep any convective activity that forms mostly in the form of
shower activity with just a few thunderstorms. Thunderstorms this
afternoon and early evening will be most likely at the fringes of
the cloud shield out over the higher terrain of La Paz County
where daytime heating is considerably higher, likely causing
enough instability to overcome the midlevel stable layer. Showers
and very isolated thunderstorms will be possible over the higher
terrain north and east of Phoenix and the Phoenix metro as well,
but in general the cloud cover should prevent instability from
increasing enough to allow more than a rogue thunderstorm or two
through early evening. This could change over the Phoenix metro
if we warm up a few more degrees more towards our convective
temperatures in the 103-104 degree range, but this is not
currently expected. Storm coverage over the higher terrain should
remain very isolated, with weak steering flow this afternoon
keeping outflow boundaries out of Phoenix.

Confidence is increasing in strengthening southerly steering flow
later tonight with an increased coverage of showers and
thunderstorms over southern Arizona. A weak inverted trough
currently in far southeast Arizona and southwest New Mexico into
Mexico near the Sonora-Chihuahua border is currently associated
with convection over Cochise County and northern Sonora. Models
generally do not appear to be handling the position of this
feature very well, with perhaps up to a 100 mile bias south and a
little east of where convection is developing now. In addition, it
may be combining it with the weak inverted trough that is located
further south over the Gulf of California and Sinaloa that is
expected to move through the region later tomorrow. Given that
confidence in any particular model solution is low, we are
favoring ongoing observational trends which models are slowly
latching on to that suggest the inverted trough over southeast
Arizona combined with one further south over the southern Gulf of
California may help trigger convection over southern Arizona
tonight. If this occurs, some model solutions show scattered to
widespread showers and a few thunderstorms moving into Pinal,
Gila, and perhaps southern Maricopa Counties overnight which
would have relatively efficient rain rates capable of producing
flash flooding. Thus, we have issued a Flash Flood Watch for
these areas beginning at 1 AM MST tonight to account for this
potential, though the best chances for heavy rainfall associated
with flash flooding are likely not until later on Saturday.

At some point tomorrow, the much advertised pronounced inverted
trough that is currently triggering convection over the Gulf of
California, Baja California, and southern Sonora will begin to
provide larger-scale lift to help get convection going across
south central Arizona. Although what happens tomorrow afternoon
and evening will be heavily modulated by whatever evolves tonight,
the broader model ensemble continues to show a swath of heavier
rainfall from Pinal County north-northeast into Gila and far
eastern Maricopa Counties. Specifically, the operational GFS
shows a large area of 1-1.5" rainfall totals over three hours in
southern Gila County late tomorrow afternoon and early evening.
Although the general depiction of this seems reasonable given PWAT
values in the 1.6-1.8 range with favorable upslope development,
the axis of deeper moisture could end up further west or east and
the timing could be earlier or later. The Flash Flood Watch is
primarily for this feature and is lowest in confidence for areas
at the far western and northern fringes of the watch including the
East Valley of the Phoenix metro. There is higher confidence for
the watch the further south and east you go, particularly for Gila
and Pinal Counties. Models continue to trend faster, so it seems
reasonable that Sunday may end up being drier and warmer than
previously thought with PoPs generally in the 10 percent range for
Phoenix by Sunday afternoon, with isolated to scattered storms
confined to the higher terrain of Gila County.

Through much of next week, a series of progressive northern
stream waves will create unusually strong troughing over the
western Conus and further dampen and force subtropical ridging
southward. By Sunday evening, most of southwest and south central
Arizona will be out of its monsoonal pattern with the entire
atmospheric column comprised of westerly flow. This pattern will
result in PWATs plummeting down towards 1 inch or below with
boundary layer mixing ratios falling to 5-7 g/kg which generally
does not support convection. Even storms over favored higher
terrain locations will be tough to come by, with most deep
convection relegated to areas along the New Mexico border for
multiple convective days. As this drier pattern evolves, H5
heights will still hover in a 592-595dm range yielding another
round of potentially excessive heat as afternoon highs approach or
exceed 110 degrees over the lower deserts by Wednesday.

&&

.AVIATION...Updated at 1755 UTC.

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

Very interesting weather pattern with potential aviation concerns
through the TAF period. Initially, there is the possibility of a
first weak wave moving north out of southeast AZ, bringing a few
showers or a stray storm into the greater Phoenix area after
midnight and into early morning tomorrow. Currently, we are rather
stable over the central deserts due to cloud cover/cool high temps,
and storms not that widespread from Tucson south and east. So,
confidence is low in this initial wave so decided to remove the VCSH
from the TAFs later tonight. Decided to just go with variable
mid/high level debris decks with FEW-SCT down to around 7k feet. If
showers develop will amend the TAFs later tonight or in the 06z
package.

Much stronger and more vigorous wave will move north and through
eastern AZ later Saturday and through Saturday night, bringing
scattered to numerous showers and embedded storms...some of which
may affect the greater Phoenix area. Before this big wave arrives,
may see some clearing during late morning and early afternoon before
convection really spins up. This will be an issue for the subsequent
06z and 12z TAF packages to add this increasing/lowering cloud and
weather. Actually, blowing dust Saturday night may be less of an
issue if widespread rains or heavy rain develop and really wet the
ground.

One of the problems in the TAFs lately have been winds. Although
winds have not been very strong (mostly less than 12kt), the
directions have been tricky and poorly forecast by much of our
guidance. Currently it appears that the preponderance of our wind
guidance suggests winds turning to the southwest or west early this
evening and continuing much of tonight. Decided to make the shift to
light southwest after 02z...but turn winds back to the south or
southeast after about 06-07z. Most of the showers/storms that
develop tonight will be over southeast AZ and we may eventually see
a broad, weaker SELY outflow wind move up into the Phoenix area and
winds will probably stay out of the south or southeast rest of
tonight and into the day Saturday. After 19z opted to turn winds
back to the southwest less than 10kt due to model consensus and the
fact that that conforms to typical diurnal tendencies. Again, with
the weather/disturbances moving through over the next 24+ hours,
winds will be very trick and often will not conform to guidance.
Thus there will be numerous updates likely needed to the TAFs as
winds make sudden and unexpected changes. Stay tuned with regards to
the winds tonight through Saturday.

Southeast California/Southwest Arizona including KIPL and KBLH:

Dry southwest flow aloft will keep skies mostly clear next 24 hours.
Winds to favor the south at KBLH through the period, with gusts OVER
20KT likely during the afternoon. Winds to favor the south to
southwest at KIPL with elevated wind speeds in the afternoon between
10-15 kts. Some tendency for winds to turn to the southeast by mid
morning Saturday with speeds below 10kt.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Monday through Friday:
A much drier pattern will take hold beginning Monday with
thunderstorms very limited even over higher terrain area.
Temperatures will warm above normal amidst the drier air mass,
with winds following typical diurnal trends. Humidity values will
generally drop into the upper single digits to mid-teens across
the lower elevations while remaining slightly higher above 20
percent over the higher terrain. Overnight recovery will be poor
to fair except at higher elevations where it will be better.

&&

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

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Flash Flood Watch from 1 AM MST Saturday through late Saturday
     night for AZZ547>563.

CA...None.
&&

$$

DISCUSSION...Hopper/MO
AVIATION...CB
FIRE WEATHER...Hopper/Rogers/MO