NWS Phoenix, AZ wrote its last area forecast discussion before the first event at Mar 21, 10:38 PM MST, 1 hour 22 minutes earlier.
Area forecast discussion, Mar 21, 10:38 PM MST, 1 hour 22 minutes before
Area Forecast Discussion
National Weather Service Phoenix AZ
1038 PM MST Tue Mar 21 2023
.UPDATE...
Updated Aviation discussion.
&&
.SYNOPSIS...
Cloudy, wet, and breezy conditions will continue through Wednesday
morning as a system pushes through the region. This system will
bring elevated chances of rainfall, long-term flooding impacts,
breezy to windy conditions, and continued below normal
temperatures. In the wake of this system, drier conditions are
expected, but temperatures will continue to run well-below normal
through at least this weekend.
&&
.DISCUSSION...
Rain has been falling in the upslope regions north of Phoenix
since early this morning. Rain rates have not been anything too
impressive (0.1-0.3"/hr), but the rain has been falling
consistently for several hours and it is adding up. Rain gages in
the New River Mesa area have already measured in excess of 1.25"
today and many other gages in the foothills have already measure
0.50" or more. Responses in some of the washes and streams are
already being observed as well, with New River above the town of
New River having gone above the designated alarm stage shortly
after 1 PM MST. Rainfall amounts drop off quickly away from the
upslope areas to mostly less than 0.25", and even less than 0.10"
in Phoenix. Mesoanalysis and satellite does show the most
anomalous moisture is working its way into Central and Northern AZ
early this afternoon. A PWAT value of 1.1" was measured with a 19Z
sounding out of the Yuma area. This appears to be a record value
based on sounding climatology for Yuma, with data dating back to
1955. However, the sounding in Yuma also still showed some drier
air below 750mb. This likely explains the minimal rainfall
amounts in parts of Southeast CA and Southwest AZ, despite what
radar may be suggesting. Another major factor is the limited
forcing with the upper low still well removed from the region,
back near the Central CA coastline. The main forcing mechanism is
the orographic lift.
Radar does appear to be showing a more distinct break in the
showers and water vapor satellite is showing some drier mid-level
air slowly pushing in from the west. A notable dry slot, if you
will. This mid-level drying has been noted in forecast soundings
leading up to this event. It does not mean the rain will
completely come to an end, as the deep southwesterly flow persists,
but the moisture will become more limited to the lower
troposphere, below 600mb. With the limited forcing, the rain will
likely become even more isolated to upslope regions through this
evening. Hi-res models continue to show re-flourishing of showers
tonight through early Wednesday, becoming more convectively
induced with minimal CAPE (100-200 J/kg). A cold front will also
be moving west to east across the region, likely passing through
the Phoenix area between 4-7 AM MST. This additional forcing will
bring the greatest chances (PoPs up to 70-90%) for more
appreciable rainfall in the lower deserts and higher rain rates
could lead to more rapid runoff. Hi-res models support rain rates
more in the 0.30-0.60"/hr tonight and with the linear wind
structure, training of cells over the same area could lead to more
significant localized flooding impacts. The overall thunder
threat is minimal, given the limited CAPE, but still cannot rule
out some storms (10-20% chance). A thunderstorm could also briefly
push rain rates up to 0.75-1.00"/hr.
The latest WPC storm total QPF for this event still lines up with
the general thinking, with a sharp contrast in QPF between the
foothills/high terrain and the lower deserts. However, so far
rainfall amounts seem to be slightly. There will also remain
greater uncertainty in the transition areas from the foothills to
the lower elevation deserts. Rainfall amounts in excess of 2.00"
will remain possible in the foothills/higher terrain. The primary
concern with this event will be elevated runoff in the washes,
streams, creeks, and rivers, as well as new rises and dam releases
in some of the major waterways like the Salt and Verde Rivers,
and Tonto Creek. Despite lower rainfall amounts in the lower
deserts runoff from up north will be capable of reaching further
downstream and causing impacts. Rain should decrease quickly
through late morning Wednesday following the cold front, but
flooding impacts will likely continue beyond Wednesday.
Strong winds so far have not fully been realized. There have been
some strong gusts in some elevated spots, but the cloud cover is
likely limiting the mixing potential. Winds are still expected to
pick up later this evening into the overnight hours as jet max
winds pass over the area. Hi-res models continue to support the
development of a 45-50 kt 850mb LLJ later this evening in South-
Central AZ. Winds will only increase with height while the jet
passes over the area. A new Wind Advisory has been added for
higher terrain areas of Southern Gila County for tonight through
early Wednesday, as winds could gust up to 40-50 mph. The other
Wind Advisories and High Wind Warning remain mostly unchanged,
with the exception of an extension in time for Joshua Tree NP and
far Southwest Imperial County.
.PREVIOUS DISCUSSION...
Showers will linger across the higher terrain areas on Wednesday,
with residual moisture and clearing skies allowing shower
potential across this region during the daytime hours across a
more limited area of higher terrain on Thursday. Ensembles are in
excellent agreement of the troughing feature exiting the region
by Friday under westerly flow due to another trough digging across
the Intermountain West. Clusters begin to diverge on the depth of
this trough as early as Saturday, with about an equal chance the
trough digs across the Great Basin, or ejects further to the
northeast. The weakest of solutions would cause temperatures to
trend warmer more quickly, while the stronger solutions would
dampen the warming trend that is likely to occur going into next
week. Going into early next week, ensembles and clusters depict
another trough digging across the Pacific Northwest. There are
currently a large variance within the cluster solutions by Monday,
but the range of solutions does not decrease confidence that the
forecast area should remain dry primarily from Thursday into early
next week under mostly clear skies and temperatures trending
warmer, but remaining below normal.
&&
.AVIATION...Updated at 0538Z.
South-central Arizona including KPHX, KIWA, KSDL, and KDVT:
An upper low centered near KSFO is driving strong southwesterly
flow aloft and advecting considerable low to mid level moisture
over the region. Ceilings at FL060-080 the next few hours along
with isolated light showers which will cause temporary lowering
of those ceilings by another 1000ft or so. A cold front is
expected to track through the Greater Phoenix area roughly between
11Z-15Z. The passage of the front will lead to an distinct uptrend
in shower activity. That will lead to lower ceilings as well
(FL050-070 common; occasionally FL030-050). Ceilings slowly
improve from west to east behind the front the rest of the day
with very little cloudiness by late afternoon.
Surface winds late this evening are highly variable from one TAF
site to another. But, S/SWly directions will become favored by 08Z
along with some gustiness of 20-25kts. At locations where winds
are around 5kts or less, conditions may flirt with LLWS criteria.
The breeziness will become more consistent just ahead of, and with
the frontal passage (gusts of 25-30kts possible). With the
frontal passage, directions will trend to Wly. There may be a
period of highly variable winds for a short time in the wake of
the front. W/NWly directions may not even hold for the rest of the
day before SEly directions become favored by late afternoon. But,
speeds are anticipated to be below 10kts for the most part after
17Z or so.
Southeast California/Southwest Arizona including KIPL and KBLH:
An upper low centered near KSFO is driving strong southwesterly
flow aloft. The approach and passage of a cold front will cause
redevelopment of ceilings in the FL060-080 range along with
isolated light showers for a few hours (roughly between 08Z-11Z).
Cloudiness will thin out considerably after that. As for surface
winds, KIPL will go from light and variable to W/SW gusts of
20-30kts with the frontal passage and have prolonged breeziness
the rest of the day. At KBLH, southwest winds of around 10kts are
expected to strengthen considerably with the frontal passage
(gusts to 25 kts) with lingering breeziness the rest of the day.
&&
.FIRE WEATHER...
Our next storm system will begin to move through the region
starting today and last through Wednesday, brining elevated
chances of rainfall and breezy to windy conditions. MinRH values
during that span will generally run between 30-50%. Good overnight
recovery is expected with MaxRH values between 50-90% at least
through the end of the week. Probabilities for wetting rains range
from 10-40% in the southern and western districts of our CWA to
50-90% in the northern and eastern areas. Wind gusts of 40 mph or
greater are likely in the western districts, while gusts of 30-35
mph are possible in the eastern districts. Our region should begin
to see a warming and drying trend beginning Thursday, but
temperatures should remain well below average through at least
this weekend.
&&
.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Wind Advisory until 3 AM MST Wednesday for AZZ530>533-535-536.
Flood Watch through Wednesday evening for AZZ541-545-547-549-552-
555>558-560>563.
Wind Advisory until 10 AM MST Wednesday for AZZ556>558-562-563.
Wind Advisory until 7 AM MST Wednesday for AZZ539-554-559.
CA...Wind Advisory until 3 AM PDT Wednesday for CAZ563>570.
Wind Advisory until 7 AM PDT Wednesday for CAZ560-561.
High Wind Warning until 1 PM PDT Wednesday for CAZ562.
&&
DISCUSSION...Benedict
PREVIOUS DISCUSSION...Young
AVIATION...AJ
FIRE WEATHER...RW