Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 04/18/21

Area Forecast Discussion

National Weather Service Phoenix AZ

 508 PM MST Sat Apr 17 2021

.UPDATE...Updated Aviation Discussion.

&&

.SYNOPSIS...
Temperatures will remain near normal through the remainder of the
weekend with most locations staying in the 80s. Conditions will
generally be dry, but a few showers and potentially a few isolated
thunderstorms will be possible on Sunday, primarily across the
high terrain north and east of Phoenix. Additionally, breezy
conditions are expected each afternoon, with Sunday being
particularly breezy. Above normal temperatures will resume early
next week with highs climbing into the low 90s across the lower
deserts. Another weather system during the middle of next week may
bring a return of somewhat cooler temperatures and increased
breezy conditions.

&&

.DISCUSSION...
Objective analysis showed broad and amplified upper level ridging
over the eastern Pacific extending up towards Alaska. Across the
Desert Southwest, a trough, with the axis extending from the
Utah/Arizona border through south-central California, continues to
push toward the south and into Arizona. Afternoon radar showed light
showers ongoing across the northern Arizona high terrain in
response to this approaching trough. Across southeast California
and southwest Arizona, visible satellite imagery showed mostly
clear skies while cumulus development and a few virga showers were
seen over parts of south-central Arizona. Temperature for the
weekend will remain around seasonal normals as lower elevation
highs remain in the mid to upper 80s.

The previously mentioned trough is expected to close off as it
drops south through the state tomorrow and bring increasing rain
chances to south-central Arizona. This system will be quite
moisture starved given the overland trajectory with model
soundings keeping any moisture above 700 mb. As the system moves
overhead, HREF guidance shows PWAT values increasing to near 0.50"
Sunday afternoon compared to the ~0.30" values seen on ACARS
soundings today. This system will be characterized by unusually
cool upper level temperatures for this time of year, with 500 mb
temperatures dropping to -22 to -23C on Sunday; these temperatures
fall into the 10th percentile of NAEFS climatology. Additionally,
mid level lapse rates will steepen to rates of 7-8 C/km as the
system moves over central Arizona and will allow for some elevated
CAPE to develop. Models continue to advertise CAPE values of less
than 200 J/kg or so. Although these parameters are fairly meager,
it does appear to be just enough to get a few showers and
isolated thunderstorms going over the higher terrain Sunday
afternoon. The main concern with any isolated thunderstorms that
develop will be lightning along with gusty outflow winds given
the very dry sub cloud layer. As the circulation in the upper
level low moves south to southeast of the area, north to
northeasterly flow will develop, which may allow for an isolated
shower or thunderstorm to drift into the Phoenix area. As of now
the probability of this is fairly low, but it remains a
possibility. Somewhat higher PoPs can be found over Gila county,
with a 30-40% chance of measurable rain.

The other forecast concern for Sunday will be the wind potential.
Breezy northerly winds will develop over the Colorado River Valley
as the upper level low moves southeast of the area. Wind speeds
should remain below advisory levels, but some gusts to 30-35 mph
will be possible in favored areas, leading to elevated fire weather
concerns.

Going into Monday, the upper level low will eject to east and allow
for heights to temporarily rebound for the first part of the week as
shortwave ridging builds into the region. This will allow for a
warming trend to commence on Monday with highs likely climbing
into the low 90s on Tuesday. NBM probabilities continue to show
about an 80-90% chance for reaching 90 degrees on Tuesday. The 90
degree temperatures look to continue again Wednesday for many
lower desert locales, though temperatures may only get into the
upper 80s across southeast California.

The early week tranquil weather conditions will change going into
the middle part of the week as another shortwave trough progresses
southwest from the Pacific Northwest into our region. Ensemble
cluster analysis continues to show good agreement in this trough
moving through the western CONUS with the main uncertainty focused
around how far southwest it will dig. EPS and CMC members are more
on board with a deeper solution while GEFS members are mostly
leaning toward a less amplified solution. The main implications
this will have on the forecast will be with respect to the amount
of cooling we see and how breezy conditions will become. The
mostly overland trajectory of this system along with the lack of
favorable dynamic support given its positive tilt will make
precipitation unlikely, which ensemble guidance all agree upon.
ECMWF EFI shows the potential for anomalously strong winds on
Wednesday as this system approaches. The potential for strong wind
combined with dry conditions may lead to heightened fire weather
concerns. By Thursday, temperatures look to cool into the mid to
upper 80s for highs across the lower deserts as heights fall with
the passage of this system. Temperatures quickly rebound going
into the weekend as ensemble members show good agreement in upper
level ridging returning.

&&

.AVIATION...Updated at 0000Z.

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

Breezy westerly winds will persist into the early evening with
gusts near 17-24 kts. with the stronger gusts at KIWA and KDVT.
Windspeeds will gradually taper off and favor the west or light
and variable conditions through the night for KPHX. However breezy
northerly component winds with gusts into the teens and low 20s
will be possible for KSDL and KDVT overnight. A very late E-NE
wind switch is expected in the early-mid morning around 14-15Z
with breezy E-NE winds prevailing through most of the day Sunday
with winds increasing by late morning and early afternoon. There
will be a slight chance for isolated showers and possibly a few
isolated storms Sunday afternoon between 20-00Z, likely
developing over and just north of the metro and drifting
southward. The activity Sunday afternoon is expected to produce a
gusty northerly component winds with outflow gusts upwards of
25-30 kts. Skies will be partly cloudy with SCT-BKN cumulus mid
cloud decks around 14 kft early this evening before scattering out
and becoming clear. Then more SCT-BKN decks around 10-14 kft are
expected by Sunday afternoon.

Southeast California/Southwest Arizona including KIPL and KBLH:

Northerly flow over the area will persist through the period and
strengthen into Sunday. KIPL will favor NW winds with speeds
around 6-8 kts.tonight with some gusts developing by Sunday
morning near 20 kts. Northerly gusts up to 20-27 kts will be
possible through much of the period at KBLH. Skies will be clear
to mostly clear aside from a few occasional mid level clouds.

&&

.FIRE WEATHER...
Tuesday through Saturday: A continuation of breezy afternoons and
warm temperatures are expected. Warm temperatures in the low 90s
are expected Tuesday and Wednesday across the lower deserts before
briefly dropping into the 80s Thursday. RH maximums will be
15-30% through Wednesday with minimums of 5-10%. For Thursday and
Friday maximums will rise to 30-50% and minimums to 10-20%. The
highest values will be in California. Wednesday is expected to
have the windiest weather of the period.

&&

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

&&

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

$$

DISCUSSION...Smith/Hodges
AVIATION...Sawtelle/Benedict
FIRE WEATHER...Smith/Hodges