Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 01/31/19

Area Forecast Discussion

National Weather Service Phoenix AZ

 855 PM MST Wed Jan 30 2019

.UPDATE...Updated aviation discussion.

&&

.SYNOPSIS...
Ample high cloudiness with above normal temperatures and humidity
will continue through Saturday as a pair of weather systems move
through the region. The first disturbance will bring a quick
chance of showers and possibly a few thunderstorms overnight
Thursday into Friday. The second and more significant system will
impact southern California and Arizona this weekend with the best
rain chances beginning Saturday morning in southeast California
and Saturday afternoon or evening in Arizona. Rain chances should
decrease on Sunday with below normal temperatures and slight rain
chances expected to continue through the first half of next week.

&&

.DISCUSSION...
Above normal temperatures with most lower desert locations in the
low to mid 70s have continued today under midlevel broken cloud
decks with ceilings as low as 10-12 kft. Aircraft soundings
display a fairly thick 3-4 kft layer of saturation at midlevels,
but they also show fairly dry conditions at lower and upper
levels. This midlevel cloudiness with relatively dry near-surface
conditions is expected to continue overnight and through most of
the day tomorrow, allowing low temperatures tonight to stay up in
the upper 40s to mid 50s and high temperatures tomorrow to stay
in the lower to mid 70s.

Moisture will begin to increase at lower and upper levels during
the day tomorrow ahead of a compact closed upper level low
approaching from the W-NW. Precipitable water values increasing
to 0.6-0.8 inches with S-SW to N-NE oriented isentropic ascent
and PVA at midlevels ahead of this system will cause scattered to
widespread showers over southeast California late tomorrow
afternoon and evening. Rainfall amounts out there will be around a
tenth to quarter of an inch for many locations, with a few spots
seeing up to a half inch particularly near Joshua Tree NP. There
is more uncertainty as to how much this will hold together into
Arizona, but the increase in moisture with increasing isentropic
ascent should support at least isolated to scattered showers
overnight Thursday into Friday. Rainfall amounts should generally
remain below a tenth of an inch, but the presence of modest cold
air and weak elevated instability aloft may allow for a few
thunderstorms to occur that would cause local amounts of a quarter
to half an inch. However, it is more likely that locations will
receive only a few hundredths of an inch to nothing at all than
see these higher totals, especially over southwest Arizona. This
fast-moving shortwave disturbance will exit the region by late
Friday morning to perhaps allow some clearing on Friday afternoon
as drier air briefly fills in aloft. Thus, Friday generally looks
to be a pleasant day with highs in the lower 70s.

Moisture begins to increase again on Saturday as a larger and
more significant shortwave disturbance approaches from the west.
Models continue to waver a bit on the timing and amount of rain
expected from this system, but the latest model cycle suggests the
timing of rainfall may be pushed back a bit to Saturday morning
for southeast California and Saturday afternoon or evening for
Arizona. This system will have better upper level support with the
northern jet stream phasing with a southern branch as it
approaches the California coast and the midlevel shortwave itself
maintains reasonable strength as it ejects to the northeast as it
traverses the Desert Southwest. The heaviest rain showers will be
expected just ahead of an along a surface front that should push
through southeast California Saturday afternoon and south central
Arizona during the overnight hours from Saturday into Sunday.
Deeper moisture and better dynamics with this system should result
in widespread rainfall amounts of around a quarter inch over
southeast California and southwest Arizona with around a half inch
over south-central Arizona. However, locally heavy rainfall up to
an inch will be possible for most areas with higher terrain
locations north and east of Phoenix possibly receiving up to an
inch and a half. Localized minor flooding issues in flood prone
areas will be possible with this rainfall, especially if any
convective shower or thunderstorm activity is enhanced by terrain.

There should be a brief break later on Sunday through the first
part of Monday, but another weaker system is shown approaching
from the west sometime early next week. This system should have
much less moisture to work with as models show the system moving
through Monday night into early Tuesday. A slight chance of
rainfall has been maintained on Monday and Tuesday, but it seems
unlikely that this system will provide widespread rains. Snow
levels do look to 5000-6000 feet by Monday and Tuesday, so snow
showers at higher elevations will not be out of the question. Below
normal temperatures are also expected for the first half of next
week as broad trough remains situated across the western U.S., but
there is significant uncertainty as to how the pattern will
evolve behind the third system expected early next week.

&&

.AVIATION...
South-Central Arizona including KPHX, KIWA, and KSDL:

Expect moist northwest mid-level flow to continue across the
terminals tonight and through the day Thursday ahead of a wet
Pacific low pressure system. SCT-BKN mid level decks 10-14k feet
will persist until Thursday evening when cloud bases will lower and
scattered showers will develop over the Phoenix area. Most of the
lower CIGs will develop after 11 pm Thursday but bases will commonly
be 5-6k feet into the morning hours on Friday. Do not expect much in
terms of wind Thursday; initially winds should be east to southeast
and generally less than 10kt; by mid to late afternoon they will
swing towards the south/southwest but cloud cover should limit
mixing and as such afternoon speeds should stay below 12kt for the
most part.

Southeast California/Southwest Arizona including KIPL and KBLH:

Do not expect any significant aviation impacts rest of tonight and
through the morning on Thursday as moist northwest mid level flow
continues across the western deserts. Look for SCT-BKN decks 10-14k
feet into Thursday afternoon before bases lower as a wet Pacific low
starts to move east and into far SE California. Scattered showers
should start after 21z Friday with CIGs lowering to 8k feet or so;
by evening some CIGs could drop to around 6k feet depending on how
much rain develops. No visibility restrictions are expected. Winds
will be relatively light, favoring the southeast at KIPL and KBLH
mainly during afternoon hours.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Friday through Tuesday: There will be no fire weather concerns. A
series of weather systems will move over the region through the
early part of next week bringing an increase in moisture. Minimum
humidities can be expected to rise from 40-50 percent today to
45-60 percent Saturday. Moisture will decrease slightly Sunday
through Tuesday as the main system pushes off to the east. The
wettest system will move through the region on Saturday with the
breeziest conditions expected during the day with gusts to 15 to
25 mph.

&&

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

&&

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

$$

DISCUSSION...Hopper/Kuhlman
AVIATION...CB
FIRE WEATHER...Hernandez