Area Forecast Discussion
Issued by NWS Sacramento, CA

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FXUS66 KSTO 222011
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
111 PM PDT Fri Sep 22 2023

.Synopsis...Cool and dry weather will continue through much of
the weekend. A pattern change will bring increasing rain chances
early next week along with gusty southerly winds. Dry weather
returns by Tuesday afternoon, with mild temperatures remaining
through the end of next week.

&&

.Discussion...Today through Monday.

As of noon today, we`re still under weak northwest flow aloft,
but by early this evening we will transition to short-wave
ridging, allowing for winds to weaken and shift to more of a
southerly direction. This wind shift will help aid the hazy skies
out of our area. This is supported with the latest HRRR smoke runs
with the bulk of the smoke/haze remaining east and north of the
Sacramento and San Joaquin Valleys by tonight.

By Saturday, a more potent upper-level trough in the Gulf of Alaska
will begin to influence our weather, by bringing increased southwest
flow. This will translate to increased onshore winds, bringing gusts
around 20-25 mph to much of the region, which will continue to push
any remaining haze out of the region. The upper-level low will be
very expansive and will aid in moisture across far northwestern CA
starting as early as Saturday evening. Before the low digs and
amplifies, some vorticity waves will move through the southwesterly
flow. As such, some hires models indicate this will allow for a
light, narrow band of precip to move across portions of interior
northern CA late Saturday night/early Sunday. The moisture
associated will be fairly meager/light, resulting in little to no
impacts for much of our area.

Before the arrival of the better moisture late Sunday-early next
week, increased pressure gradients and associated upper-level jet
will also result in gusty south to southwest winds across the
region. Expect sustained winds of 12-25 mph and gusts to 25-35+ mph,
with the strongest expected across the southern Cascades and
northern Sierra Monday afternoon. NBM probabilities of wind gusts >=
30 mph is generally 50-70% Sunday afternoon across the northern
Sierra and surrounding foothills with higher and more expansive
probabilities 60-90% north and east of the I-5 and I-80 corridors
Monday. When looking at probabilities >= 40 mph, only a few areas
have 15-30% in the northern Sierra/southern Cascades Monday
afternoon.

The better moisture will arrive later Sunday, with the bulk of the
`best` moisture impacting mainly far northwestern CA. There`s still
some uncertainty in the exact details regarding timing and
precipitation totals early next week, with the latest guidance
trending slightly drier. Even so, enough moisture will spill across
Western Shasta County and Coastal Range late Sunday night-Monday,
where storm totals of ~1"-2" will be possible. Since the bulk of the
dynamics will remain north and west of our region, the rain will
primarily fall as rain showers, but any embedded vorticity waves may
cause some embedded lightning and slightly heavier showers to move
across the southern Cascades- northern Sacramento Valley.
Probabilities of 0.50" or higher are mainly confined to Red Bluff
and areas north and west, with Red Bluff having about a 30-40%
chance and Redding having 65-75% chance. Areas south of Red Bluff
will see diminishing probabilities of rainfall, with around a 15-30%
of seeing between 0.25"-0.50" down to the I-80 corridor, with little
to no precipitation forecast for the far southern Sacramento
Valley and the northern San Joaquin Valley. Overall, rain impacts
are expected to minor.

Outside of winds and precipitation, temperatures will remain 8-12
degrees below seasonal normals through the weekend, with highs in
the low 80s to upper 70s across much of the Valley. Minimum
temperatures will be cool as well, allowing for frost development
through the weekend in the mountain valleys.
//Peters

.EXTENDED DISCUSSION (Tuesday THROUGH Friday)...

As the trough over the Pacific Northwest continues to move eastward
early next week, showers will likely continue over Northern
California on Tuesday, mostly in the Coastal Range and northern
Sacramento Valley. There is some disagreement between ensembles and
clusters as to how the system will progress late in the week, with
some showing the low dropping down over Northern California, and
others showing it moving further east before moving southward.
However, conditions should still generally be cool and dry starting
Wednesday, with temperatures in the high 70s to mid 80s in the
Valley throughout the rest of the week. The NBM is currently showing
slight chances of showers in the mountains, but probabilities are
still below 20% at the moment. //SP
&&

.AVIATION...Generally VFR conditions prevail at all TAF sites
through the forecast period. Some haze from wildfire smoke may
linger through the early afternoon across the northern San Joaquin
Valley. Otherwise, quiet weather and light winds of 10 kts or
less are anticipated at TAF sites. Breezy southwesterly winds
expected throughout the Delta.

&&

.STO WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$


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