Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 05/19/23

Area Forecast Discussion

National Weather Service Phoenix AZ

 509 PM MST Thu May 18 2023

.UPDATE...00Z Aviation Discussion.

&&

.SYNOPSIS...
Storms over the higher terrain of Arizona are expected to affect
the lower deserts today and Friday with large areas of strong
outflow winds...especially on Friday with impacts extending into
southeast California. New storm development over the valley
floors will tend to be isolated but those storms will be capable
of producing brief heavy rain. Anticipate somewhat cooler high
temperatures on Friday as well. For the weekend and next week,
atmospheric humidity decreases. In the process, storm chances
retreat to the higher terrain over the weekend before dwindling
altogether after that. With lower humidities and less cloudiness,
temperatures trend upward through Monday reaching 5 to 7 degrees
above normal. Anticipate little change thereafter.

&&

.DISCUSSION...
Isolated thunderstorms have started to develop over the northern
half of AZ with very isolated development over SE AZ. An upper low
that had been centered off the Baja Coast is now centered over the
northern Baja peninsula. That is helping to produce more
cloudiness over southern AZ than in previous days. At the surface,
dew points have increased by around 10 degrees over portions of
SW AZ and SE CA compared to 24 hours ago with little change or
slight declines elsewhere over the deserts. SPC mesoanalysis and
latest ACARS data show MLCAPE values of around 500-1000 J/kg over
south-central AZ and with little CIN. The CAPE drops off west of
there and the CIN increases. This is fairly consistent with HREF
data for early this afternoon. However, it goes on to show a
temporary decline of CAPE for south-central AZ followed by some
recovery by late afternoon and early evening as some Gulf of
California air moves in. But, as CAPE tries to improve early this
evening, CIN increases. This is likely why the NBM PoPs remain
relatively modest over the lower elevations. However, there is
more to the story. While there may be very localized brief heavy
rain, areas of strong winds and possibly blowing dust are expected
to affect a much larger area. HREF probabilities of
reaching/exceeding 30kts is 30-50% over south-central AZ (10-30%
over a sliver of adjacent SW AZ). Winds reaching/exceeding 50kts
looks to be less than 10%. For the Phoenix area, outflow from the
higher terrain to the north/northeast will be the most likely
wind scenario. But, storms over Pima County may also produce some
outflow that reaches NW Pinal and SW Maricopa Counties.

On Friday, a lobe of the upper low moves through portions of AZ.
There is good agreement amongst the CAMs and associated ensembles
that convection will initially be focused over northern AZ
(especially Yavapai County) and generate strong southwestward
moving outflows that affect Maricopa, La Paz, Yuma, and eastern
Riverside Counties (less so Imperial County). This is supported by
experimental NCAR ensemble data as well. Probabilities of
reaching/exceeding 30kts on Friday increases to 50-70% across AZ
portions of our CWA and 10-50% over southeast CA. At this time,
HREF probabilities of 50kts+ is less than 10%. Wouldn`t be
surprised if the increases with subsequent runs. Likelihood of
blowing dust increases as well. With strong gust front activity,
new storm development is likely but it is unclear how widespread
the storms will be. Lingering cloud cover as well as residual
effects from any convection tonight could lead to higher CIN.
PoPs are higher Friday than for today but not dramatically. Expect
highs to be a little lower as well.

For the weekend and next week, global ensemble data continues to
show a slow decline in PWAT values as the Baja low dissipates, the
coastal ridge gets flattened and there is more influence from the
Westerlies. In the process, storm chances retreat to the higher
terrain over the weekend before dwindling altogether after that.
With lower humidities and less cloudiness, temperatures trend
upward through Monday (reaching 98-103) with little change
thereafter.


&&

.AVIATION...Updated at 2355Z.

South-Central Arizona including KPHX, KIWA, KSDL, KDVT:
Strong, erratic outflow winds and abrupt wind shifts will once
again be the primary aviation weather concern going through this
evening. Currently, breezy westerly to northwesterly winds will be
common across the TAF sites. Thunderstorms will continue to
develop across northern Arizona and the Rim as well as places
across southeast Arizona going through the early evening, which
are anticipated to generate outflows into the Valley.

Confidence remains low in exact timing, magnitude, directional
shifts with outflows this evening. As of now, multiple outflow
boundaries are expected to push into the Phoenix area this evening
with the northerly outflow anticipated to be the stronger
boundary. Convection across southeast Arizona may also send an
outflow from the south into Phoenix this evening as suggested by
several CAMs. Thus, several wind shifts will be possible this
evening. HREF guidance shows probabilities for outflow winds in
excess of 30 kts climb to around 40-50% late afternoon and
evening. Additionally, CAMs suggest that VCSH/SHRA and isolated
VCTS could develop along these boundaries with chances around
15-25%. The primary impacts will be the gusty, erratic wind
shifts, lightning, and the potential for some blowing dust. SCT-
BKN cloud bases aoa 10 kft will continue throughout the period.

Southeast California/Southwest Arizona including KIPL and KBLH:
No major aviation weather concerns are expected through the next
24 hours. Winds will favor the southeast at KIPL and south at KBLH
with occasional gusts upwards of 20-25 kts through the evening
hours. Overnight, winds will diminish and shift to the
east/southeast at KIPL and south at KBLH. Breezy conditions are
anticipated to return late tomorrow morning. SCT-BKN clouds bases
will remain aoa 12-15 kft.


&&

.FIRE WEATHER...
Storms over the higher terrain of Arizona are expected to affect
the lower deserts today and Friday with large areas of strong
outflow winds...especially on Friday with impacts extending into
southeast California. New storm development over the valley floors
will tend to be isolated but those storms will be capable of
producing brief heavy rain. Anticipate somewhat cooler high
temperatures on Friday as well. Humidities will trend up through
Saturday morning. For the weekend and next week, atmospheric
humidity decreases. In the process, storm chances retreat to the
higher terrain over the weekend before dwindling altogether after
that. With lower humidities and less cloudiness, temperatures
trend upward through Monday reaching 5 to 7 degrees above normal.
Anticipate little change thereafter. MinRH drops to 10-20% by
Monday with little change thereafter. Max RH drops to 20-40% lower
deserts by Tuesday morning (40-70% higher terrain) with little
change thereafter. Anticipate relatively light winds on Saturday
followed by an uptrend in afternoon breeziness (favoring south and
southwest directions) Sunday through Tuesday with little change
thereafter.

&&

.HYDROLOGY...Updated 315 AM MST 5/15/2023

GILA: Ongoing releases above 3000 cfs from Painted Rock Dam continue
to travel down the Gila River through Yuma County. These releases
will continue to impact unbridged crossings with multiple roads
reportedly closed due to flooding. The latest USGS gauge observation
along the Gila River near Dateland reported a depth of 8.9 feet,
which is just under minor flood stage (9 feet). Further downstream,
another USGS gauge near Dome, north of Yuma, shows a depth of 22.46
feet, which is above action stage (20 feet). Flow at Dome has for
the most part flat lined, with a CBRFC forecast that remains level.
Flood Warnings in effect along the Gila River between Painted Rock
Dam and the Colorado River have been extended out another 7 days
through at least May 23rd.

&&

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

$$

DISCUSSION...AJ
AVIATION...Smith/Leffel
FIRE WEATHER...AJ
HYDROLOGY...Benedict/Smith