Forecast Discussions mentioning any of
"ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 04/30/23
Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
830 PM PDT Sat Apr 29 2023
.SYNOPSIS...29/810 PM.
An approaching upper level trough will bring a deepening marine
layer and cooling trend through Monday. Additional cooling and a
chance of rain and mountain snow is expected Tuesday through
Thursday as a low pressure system moves through the region.
&&
.SHORT TERM (SAT-TUE)...29/829 PM.
Another very warm day across the interior with highs in the
lower to mid 90s across the warmer valleys and deserts. The
upper level ridge of high pressure that brought the warming
the past couple of days will weaken and shift eastward as
Sunday as an upper level trough approaches the California coast.
This will lead to a deepening of the marine layer with low clouds
and fog spreading into many valley areas by Sunday morning.
Already this evening, satellie imagery showing low clouds and fog
widespread across the coastal plain. The latest ACARS data showing
the marine layer depth around 12000 feet across the LA Basin, but
is expected to increase to 1500 feet by Sunday morning. A
cooling trend is expected to begin across all areas on Sunday,
especially in the valleys with the greater marine layer influence.
By Sunday night into Monday, heights will continue to fall which
will allow further deepening of the marine layer, with low clouds
and fog likely reaching the lower coastal slopes. Clearing each
afternoon will be on the slow side with coastal areas likely
remaining cloudy. Additionally, there will be the potential for
some drizzle and a 10 percent chance of measurable light rain on
Monday for areas south of Point Conception (especially LA county).
Also with moderate to strong onshore gradients, there will be
gusty southwesterly winds during the afternoon/evening hours with
a chance of advisory-level winds across the Antelope Valley by
Monday afternoon.
*** From previous discussion ***
For Monday night and Tuesday, attention turns to the beginning of
the more direct impacts from the upper low. Deterministic and
ensembles all are in good agreement with the track of the upper
low, increasing confidence in the Tuesday and Tuesday evening
forecast. There will be a threat of rain developing across San
Luis Obispo and Santa Barbara counties Tuesday morning with the
rain threat working its way down to Ventura/LA counties Tuesday
afternoon and evening. Through Tuesday evening, do not anticipate
a lot of rain, just amounts under 0.10 inches. Additionally, it
looks like enough instability to warrant an inclusion of a slight
chance of thunderstorms Tuesday for San Luis Obispo county and the
adjacent coastal waters.
.LONG TERM (WED-SAT)...29/144 PM.
For the extended, main focus will remain on the upper low and its
impacts across the area Wednesday and Thursday. Surprisingly, the
12Z deterministic and ensembles of the GFS and ECMWF are in very
good agreement with the track of the upper low through Thursday.
So, confidence is a bit higher during this time period than
previous days.
For Wednesday and Wednesday night, the center of the upper low
will move across the Channel Islands. On Thursday, the center of
the low will move inland over Ventura/LA counties, eventually
moving off to the northeast Thursday night. The cyclonic flow
around the low should be able to pick up some decent moisture,
resulting in measurable rainfall for all areas Wednesday through
Thursday. At this time, the multi-run QPF trends on both the GFS
and ECMWF ensembles have been slowly decreasing from run-to-run.
So, the latest estimates of 0.25 to 0.75 inches with local amounts
up to 1.00 inch appear reasonable. Of course, given the fickle
nature of upper lows in early May, these numbers could change
over the coming days. As mentioned in the short term discussion,
there is some decent instability associated with the upper low.
So, have included a slight chance of thunderstorms for northern
areas on Wednesday then all areas on Thursday. Snow levels are
expected to drop into the 4500-5500 foot range. So, there will be
some measurable snowfall in the mountains with amounts around 6
inches possible.
For Friday, the upper low will be spinning over northern Nevada
with cyclonic flow continuing over the area. Models do not
indicate much moisture, so POPs are below 15 percent. However,
would not be surprised to see some mention of showers need to be
included in future forecasts.
For Saturday, dry conditions are expected with weak upper level
ridge over the area. This will allow for some nice warming,
compared to the Wednesday-Friday temperatures, with temperatures
in the mid 60s to mid 70s.
&&
.AVIATION...29/2349Z.
At 2340Z, the marine layer depth was around 1000 feet deep at
KLAX. The top of the marine inversion was around 2900 feet with a
temperature near 24 degrees Celsius.
Moderate confidence in 00Z TAFs, except high confidence in KPMD
and KWJF where VFR conds will prevail. Timing of flight category
changes may be off by two hours. IFR/LIFR conditions expected
through this evening, lifting to MVFR conditions by Sunday morning
as marine layer depth begins to rise. There is a 30-50 percent
chance that coastal TAF sites KSBA and south will have MVFR cigs
lingering into Sunday afternoon, with greatest chance at KOXR and
KLAX. There is a 10-20 percent chance that VFR conds prevail for
KVNY and KBUR.
KLAX...Moderate confidence in 00Z TAF. IFR/LIFR cigs expected
through this evening, with MVFR cigs likely by Sunday morning
as marine layer depth increases. There is a 50 percent chance of
MVFR cigs lingering through Sunday afternoon. Any east wind
component will remain well below 8 kts.
KBUR...Moderate confidence in 00Z TAF. Return of IFR/LIFR
cigs/vsbys may be off by two hours. There is a 10 percent chance
that VFR conds prevail.
&&
.MARINE...29/807 PM.
Patchy dense fog is possible tonight across the coastal waters.
A Gale Warning is in effect for the Outer waters and inner waters
north of Point Sal for Sunday late morning or afternoon into
Sunday night. There is a 30-50 percent chance that Gales will
continue beyond this time frame, with the greatest chance across
the southern Outer waters.
Small Craft Advisory issued across western portions of the Santa
Barbara Channel late Sunday into Sunday night as probability has
increased to 70 percent. There is a 50-70 percent chance of SCA
conditions across the southern Inner Waters Monday into Monday
night with a 20 to 40 percent chance of Gale conditions, with the
highest probabilities north of Point Mugu.
Forecast confidence decreases Tuesday through Thursday, although
generally lighter winds are anticipated. Isolated thunderstorms
may impact the waters at times during this period as an unusually
strong May storm moves through the region.
&&
.LOX WATCHES/WARNINGS/ADVISORIES...
CA...NONE.
PZ...Gale Warning in effect from 11 AM Sunday to 3 AM PDT Monday
for zones 645-670. (See LAXMWWLOX).
Small Craft Advisory in effect from 3 PM Sunday to 3 AM PDT
Monday for zone 650. (See LAXMWWLOX).
Small Craft Advisory in effect until 11 AM PDT Sunday for
zone 670. (See LAXMWWLOX).
Small Craft Advisory in effect until 3 PM PDT Sunday for
zones 673-676. (See LAXMWWLOX).
Gale Warning in effect from 3 PM Sunday to 3 AM PDT Monday
for zones 673-676. (See LAXMWWLOX).
&&
$$
PUBLIC...Gomberg/Thompson
AVIATION...Gomberg
MARINE...Gomberg/Munroe
SYNOPSIS...Gomberg
weather.gov/losangeles
Experimental Graphical Hazardous Weather Outlook at:
https://www.weather.gov/erh/ghwo?wfo=lox
Area Forecast Discussion
National Weather Service Phoenix AZ
429 PM MST Sat Apr 29 2023
.UPDATE...
Updated Aviation section.
&&
.SYNOPSIS...
Temperatures well above seasonal normals will continue through
early next week with many lower elevation communities breaching
the 100 degree threshold. There will be a moderate risk for heat-
related health impacts with these temperatures. By the middle of
next week, a low pressure system will become established off the
California coast allowing temperatures to retreat back several
degrees below normal and yielding locally breezy conditions.
Despite this transition, a continuation of dry weather should be
expected.
&&
.DISCUSSION...
High pressure ridging continues to slide eastward over the western
CONUS with 500mb heights analyzed around 584-586dm. Aircraft
soundings out of Phoenix show a couple degrees Celsius of warming
in the 850-600mb layer. This slight bit of warming should
translate to highs in the Phoenix area of around 98-100F during
peak mixing this afternoon. So, there is a chance (~30%) for
Phoenix to experience its first 100F day today. Locals in
Southwest AZ and Southeast CA will see highs reach 100+ this
afternoon under mostly clear skies. Forecast soundings support
further lower tropospheric warming into Sunday across South-
Central AZ, with 850mb temps increasing to around 22-24C.
Confidence is still high Phoenix will reach or exceed 100F Sunday,
despite increasing high clouds in the afternoon. NBM
interquartile range is pretty small at 100-103F with the most
recent forecast. Phoenix will be flirting with with a daily record
Sunday, but perhaps the clouds will be what keeps the 102F record
from being reached. There are other low elevation spots that
could reach daily records as well (see Climate section). With the
abnormally warm temperatures this weekend, widespread moderate
HeatRisk will be common.
The larger scale synoptic blocking pattern across the CONUS, will
result in a gradual evolution of the sensible weather. The
ridging currently over the area will shift eastward into the
Plains during the first half of next week as a seasonally strong
negative PV anomaly intensifies along the California coast. NAEFS
output suggests H7-H5 heights closing in on 4 normalized standard
deviations below normal with model trends pointing towards a
deeper system. Given these trends, an enhanced pressure gradient
appears tighter and supportive of more widespread stronger winds
Monday afternoon. Recent ensemble output indicates H8-H7 winds
breaching the 90th percentile and speeds 30-40kt within the
boundary layer mixing depth. Wind advisory conditions may become
more prevalent in the typical areas of SW Imperial County while
the more concerning aspect appears to be fire weather issues given
the warm temperatures and low humidity levels.
Ensemble membership is now exhibiting more agreement in edging
vorticity and lower heights inland during the middle and latter half
of the week. Guidance spread has narrowed and confidence is growing
that a brief period of below normal temperatures will be common
across the CWA. There is even potential for most lower deserts to
not eclipse 80F in the Thursday timeframe. Daily record cool
highs during the middle of next week are in the middle-70s. While
not the most optimal setup for rainfall, enough moisture could be
imported and coincident with cooling aloft and vorticity forced
ascent to support a few light showers Thursday. There is even the
potential for some frozen precipitation to fall at higher
elevations of Joshua Tree NP. Otherwise, the general blocked CONUS
pattern may hold through next weekend though a subtle realignment
could allow for a rapid return back to near normal temperature
readings.
&&
.AVIATION...Updated at 2329Z.
South-Central Arizona including KPHX, KIWA, KSDL, KDVT:
Northeasterly flow is prevalent in the lower half of the
troposphere. This will transition to SWly/Wly by midday Sunday. At
the surface, light and variable winds will become more consistently
SWly/Wly very early in the 00Z TAF period in response to thermally
driven upvalley flow. But expect sustained speeds to remain below
10kts and gusts, if any, below 15kts. Downvalley/drainage patterns
will begin to develop after 04Z with KPHX the last of the TAF
sites to respond. With morning surface heating, winds will favor
southeasterly directions before trending to SWly/Wly in the
afternoon. Skies will be clear except for some minor cirrus Sunday
afternoon.
Southeast California/Southwest Arizona including KIPL and KBLH:
A southerly component to the winds will be prevalent through much
of the troposphere through Sunday afternoon. At the surface,
upvalley/downvalley patterns will prevail through Sunday morning
before Sly/SEly directions become prevalent in the afternoon.
Anticipate gustiness (up to 25kts) at KBLH Sunday afternoon. Skies
will be mostly clear tonight before minor cirrus passes overhead
Sunday.
&&
.FIRE WEATHER...
Dry conditions with above normal temperature will continue through
early next week. Minimum afternoon humidity levels will mostly
fall below 10% with poor to fair overnight recovery of 15-35%.
Temperatures will steadily cool to below normal during the middle
of next week with modest improvements in humidity levels as
afternoon readings fall into a 10-20% range and overnight recovery
improves towards a 40-60% range. However, winds will increase
markedly during this transition, especially Monday where chances
of reaching critical thresholds are becoming more likely and a
Fire Weather Watch has been issued for most AZ districts. A
locally elevated fire danger may continue into the middle of the
week with periodically gusty winds and low afternoon humidity
values.
&&
.HYDROLOGY...Updated 429 PM MST 4/29/2023
Salt: Increased flow from Granite Reef Dam has prompted a renewed
Warning for the stretch from Granite Reef to Tempe Town Lake
through Tuesday evening. This is due to flow sufficient to
inundate the normally dry river bed including unbridged crossings,
gravel mining operations, and homeless encampments.
Gila: Flow in the Gila River continues to travel downstream through
Yuma County as water releases from Painted Rock Dam continue.
Releases from Painted Rock Dam remain above 3000 cfs. Continued
releases from the dam will lead to inundation of downstream
unbridged river crossings through Yuma County. Several unbridged
roads in Yuma County have already reportedly been closed due to
flooding. Flood warnings remain in effect downstream of Painted Rock
Dam to the Colorado River. The latest USGS gauge observation along
the Gila River near Dateland recorded a depth around 8.6 feet,
which is above action stage (8 feet). The latest forecast from
the CBRFC indicates that flow in the river will remain above
action stage through at least the next week. Thus, Flood Warnings
along the Gila River will remain in effect through at least May
2nd. Latest imagery from Sentinel-II satellite shows the main
floodwater is just west of Wellton, AZ.
&&
.CLIMATE...
Record high temperatures:
Date Phoenix Yuma El Centro
---- ------- ---- ---------
Apr 29 105 in 1992 106 in 1959 106 in 2013
Apr 30 102 in 1943 103 in 1943 104 in 2021
&&
.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Fire Weather Watch from Monday morning through Monday evening
for AZZ132-133.
CA...None.
&&
$$
DISCUSSION...Benedict/18
AVIATION...AJ
FIRE WEATHER...Benedict/18
HYDROLOGY...AJ/Benedict/Smith
CLIMATE...18