Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 09/30/22

Area Forecast Discussion

National Weather Service Phoenix AZ

 523 PM MST Thu Sep 29 2022

.UPDATE...00Z Aviation Discussion...

.SYNOPSIS...
Sufficient moisture has lingered across the region warranting
continued slight chances for afternoon showers and thunderstorms the
next several days. The main activity should be focused over terrain
features with localized gusty winds and localized blowing dust the
primary impacts at lower elevations. Temperatures will remain near
to above average with desert communities afternoon highs reaching
the mid 90s to near 100 degrees.

&&

.DISCUSSION...
An extremely complex Rex block has developed over western North
America under a split flow regime where northern jet energy has been
retreating poleward and subtropical energy too far south to exert a
strong progressive influence on the Conus flow pattern. In effect,
this could be termed a double-stacked Rex block with 1) high
amplitude ridging extending into northern British Columbia while a
cutoff negative height anomaly was spinning over the northern
Rockies, and 2) a remnant easterly wave over northern Sonora with a
narrow ridge sandwiched between the two cyclonic circulations. This
configuration places much of the forecast area under periodic
favorable divergent flow aloft while lobes of vorticity rotate
through southern Arizona extending from the aforementioned Sonoran
low pressure. This pattern will begin to partially open and progress
during the first half of next week awaiting the downstream
absorption of tropical system Ian into the northerly jet and an
upstream Pacific system capable of better defining and strengthening
West Coast ridging.

In the interim, early afternoon ACARS soundings and objective
analysis indicate persistent easterly flow in the H8-H7 layer has
substantially eroded moisture in the eastern parts of the CWA such
that mixing ratios just off the surface have fallen near 5-6 g/kg.
In fact at midday with initial mechanical mixing, sfc dewpoints in
the east part of the Phoenix metro had fallen into the mid 30s while
the western metro was still hovering in the mid 50s. With further
erosion this afternoon, more preferred 7-8 g/kg moisture juxtaposed
with H5 temperatures of -10C will be relegated to southern Maricopa
County and SW Arizona. While instability may be limited, synoptic
forcing ahead of rotating vorticity arms and lowering CinH should
allow for a few showers and storms (initiation off the Kofa range
most likely). Given the higher T/Td spread and DCape, the main
thunderstorm threats would be gusty winds and localized blowing dust
(very small chance storms become anchored on terrain features and
pose a flood threat).

As alluded to above, very little will change in the overall pattern
through early next week as the Sonoran closed low only slowly begins
to open and lift northeast. However, this subtle change through the
weekend will allow easterly midlevel flow to veer more to a
southeast direction allowing better moisture to return and be
sustained at the top of the boundary layer. It`s still likely
synoptic vorticity forced ascent will still be in play along with
divergent flow aloft resulting in isolated afternoon/evening storms.
Similar to the past couple days, instability should be fairly low
(MLCape < 750 J/kg) albeit combined with limited inhibition (CinH <
25 J/kg) such that NBM POPs of 10-20% over the eastern half of the
CWA seem appropriate.

Into the beginning and the middle of next week, somewhat more
progressive flow will overtake the Conus though the preponderance of
ensemble evidence points towards high amplitude ridging being
maintained along the coast while small shortwaves and vorticity
centers rotate into eastern Arizona in building NW flow. Without
stronger flow, moisture may still be difficult to completely scour
away; and historically, these NW flow, small negative height anomaly
patterns result in showers readily developing along the Rim and
Whites and descending into the northeast parts of Phoenix as they
decay. Monday and Tuesday appear most primed for this outcome, and
it would not be surprising if automated NBM POPs start increasing
slightly as the event nears. Forecast uncertainty becomes more
abundant during the latter half of next week as some model output
suggests much stronger ridging and subsidence descending into the
forecast area, while other ensemble members maintain weakness in
the height field and persistent unsettled weather (albeit still
isolated and tied more closely to terrain features).

&&

.AVIATION...Updated 0023Z.

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

A weak weather disturbance situated across northern Mexico will keep
east southeasterly winds favored across the area through Friday
morning. However, there will be a period thru the early evening
where winds will try to become more southerly at times. Winds are
expected to become westerly by Friday afternoon as the afomentioned
weather disturbances drops southward away from the region, with
winds reverting back to a more typical easterly drainage flow Friday
night. Drier air filtering into the region is expected to keep
chances for showers/t-storms near-zero thru Friday. SCT-BKN mid/high
level cloud decks will persist through this evening with some
gradual reduction in cloud cover tonight and Friday.


Southeast California/Southwest Arizona including KIPL and KBLH:
Winds at KBLH will favor a southerly direction, while southeasterly
winds are favored at KIPL thru the TAF period. Shower activity that
is currently affecting the region around KBLH to dissipate over the
next hour or two. Current SCT-BKN mid/high level CIGS at KBLH to
become FEW-SCT late tonight thru Friday.

&&

.FIRE WEATHER...
Slightly cooler temperatures relative to the last few days will be
the trend over the next several days, with afternoon highs hovering
around 100F across the Phoenix metro and lower deserts. Modest
moisture across the western districts will continue to provide
chances for isolated showers and thunderstorms through the rest of
the week over higher terrain areas across southeastern CA and
southwestern AZ. Drying conditions by early next week will mostly
end rain chances across the majority of the area as high pressure
rebuilds across the region. Afternoon MinRHs through the end of the
week will increase slightly going into the weekend, with today`s
MinRH`s between 17-26%. Winds are expected to shift back to a more
typical diurnal pattern with lighter winds speeds beginning today.

&&

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

&&

$$

DISCUSSION...18
AVIATION...Percha/Feldkircher
FIRE WEATHER...Young