Winter Storm in California, February 18 to 20, 2024

Area Forecast Discussion
National Weather Service Medford OR
354 PM PST Sun Feb 18 2024

.DISCUSSION...UPDATED AVIATION SECTION...

.AVIATION...19/00Z TAFs...VFR ceilings will last into early this
evening.

Freezing levels of 5000 to 6000 feet MSL will drop to 4500 feet MSL
overnight, but are likely to return to over 5000 feet MSL during the
day Sunday.

After a very short break in precipitation, precipitation will return
to the area early this evening. VFR will likely still prevail,
except for brief periods of MVFR in heavier precipitation.

Low level wind shear and mountain turbulence could be a concern at
the Medford and North Bend terminals, and this has been added to the
TAFs, and this will begin this evening. There was a concern about
the possibility of fog tonight. Although winds will die down
tonight, the extensive cloud cover will limit fog development, so
fog has not been added to TAFs this period.
-Petrucelli/Hermansen

&&

.PREV DISCUSSION... /Issued 221 PM PST Sun Feb 18 2024/

SHORT TERM (Today through Tuesday night)...

Overview:

Moderate rainfall across southern and western parts of Siskiyou
County as well as Curry County and Josephine County still looks
likely through Tuesday night. Snowfall also appears likely for
higher elevations through this same period, and a Winter Storm
Warning is in effect for parts of Siskiyou County. An additional 2
to 5 inches of rainfall will be possible across the aforementioned
areas with southern Siskiyou County having the highest forecast
totals for this event (both rain and snow). Elsewhere across the
area we have rainfall accumulations but amounts will not be as high
as the areas mentioned above. For example, the Rogue Valley may only
see and additional tenth to two tenths of an inch of rainfall
tonight through Tuesday night. Umpqua Valley may only see a quarter
of an inch of additional rainfall. Lastly, wind speeds will remain
an issue through this stretch  as strong south to southeasterly flow
continues. A Wind Advisory is in place for Shasta Valley and
southern parts of the Rogue Valley goes through tonight, and we may
very well need another advisory on Monday but some uncertainty's
exist.

Further Details:

Mid-level water vapor imagery shows a distinctive low pressure
system circling between 140W and 130W as of writing this (~2pm).
Model consensus is this closed low approaches the coast between
Oregon and California but never comes inland. The center appears to
meander off the coast and slowly push north through Tuesday night
before dissolving. This pattern will allow for a constant fetch of
moisture advection across the region through the short term. The
vorticity field indicates sporadic pieces of energy kicking out ahead
of the parent low, and this will likely lead to on/off periods of
rainfall. In other words, we are not expecting a constant rain event
for several days, but rather periods of moderate--to even heavy at
times--rainfall across southwestern and southern parts of the
forecast area. Due to better forcing, much of the heavier rainfall
will be south of our area across central/northern California.
However, 2-5 inches of rainfall is still a decent amount for roughly
48 hours. Western Siskiyou, Curry, and Josephine Counties are more
in the realm of 1-3 inches, while Southern Siskiyou County may see
around 2-5 inches.

While rainfall is accumulating at the lower elevations levels, our
colder/higher elevation areas in Siskiyou County will see snowfall
accumulations as snow levels hover around 4500-5000 feet through
tomorrow. However, snow levels start to rise late Monday (above 5000
ft) before lowering again Tuesday, but QPF starts to wane by Tuesday
night. Therefore, the Winter Storm Warning ends Tuesday night as
snowfall begins to taper off.

Another day of strong southerly to southeasterly winds have
developed, but here in the Rogue Valley those winds haven't been as
breezy. That said, the pressure gradient and wind speeds aloft could
produce some occasionally gusty winds around 45 mph, so a Wind
Advisory will remain in effect for parts of the southern Rogue
Valley and Shasta Valley through tonight. We will need to consider
another advisory for tomorrow, and for more of an area that might
include the eastside in addition to similar areas as today.

-Guerrero

LONG TERM (Wednesday through Sunday)...

Low pressure responsible for multiple periods of gusty winds and
precipitation in SW Oregon and especially NorCal earlier in the
week will lift northward to near Vancouver Island on Wednesday.
The upper trough associated with this low will swing onshore and
then push east of the Cascades Wednesday night. Showers will
linger over most of the area on Wednesday, but will be focused
along the coast, just inland and over the higher terrain. Overall
though, things will calm down compared to the early week
storminess.

As the upper trough exits to the east, an upper ridge axis will
develop across the PacNW and this should result in a drying trend
on Thursday. It will stay on the mild side with high temperatures
remaining near or just above normal levels (highs in the mid 50s
to near 60F west side valleys and 45-55F across norCal and over
the east side).

The dry pattern is likely to persist Thursday night through
Saturday. We are watching an offshore upper level low (out near
40N and 140W) that may send some clouds and light rain (20%
chance) toward the coast Thursday night into Friday, but odds
favor this low wobbling southeastward toward California and
precipitation from it remaining well offshore. Ultimately, the
upper ridge axis likely wins out resulting in a couple of more
mild days Friday into Saturday (highs of 60 to 65F in the west
side valleys).

After that, upstream, a strong upper level jet streak will move
across the Gulf of Alaska and eventually allow a potent fast-
moving disturbance to dig southeastward across the NE Pacific on
Sunday. We may eke out one more mild day Sunday, but there is a
fairly wide spread in ensemble members for high temps here in
Medford (52-63F), largely the result of timing differences of the
incoming cold front. Either way, this system seems to be on track
to bring a pattern change beyond day 8, Sunday night and into
early the following week, Mon/Tue, Feb 26-27. Models/clusters show
a fairly strong cold front sweeping through Sunday night into
Monday accompanied by some wind/rain and mountain snow along with
a rapid drop in snow levels behind it. It's still way too early to
say just how low snow levels will get and how much precipitation
will occur, but it's definitely something we'll be watching
intently over the next several days with the potential for more
widespread wintry conditions toward the end of the month.

-Spilde

MARINE...Updated 215 PM Sunday February 18, 2024...Winds have
ease, but seas will remain steep resulting in a continuation of
Small Craft conditions for all of the waters. The high res guidance
supports the increase of southeast winds over the southern outer
waters through this evening which could result in steep wind driven
seas.

Steep swell dominated seas are expected to persist until a low
pressure system approaches the California Monday into Tuesday,
bringing another round of elevated and gusty southerly winds.
Periods of gale winds are possible, along with large very steep
seas. Thus a Gale Warning has been issued for the southern waters,
first Monday afternoon, followed by the northern waters Monday
evening. Please see MWWMFR for more details.

Conditions are expected to improve through the latter half of the
week and through next weekend.

-Petrucelli/Hermansen

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Wind Advisory until 10 PM PST this evening for ORZ026.

CA...Winter Storm Warning until 10 PM PST Tuesday above 5000 feet for
     CAZ080-082-083.

     Wind Advisory until 10 PM PST this evening for CAZ081.

PACIFIC COASTAL WATERS...Small Craft Advisory until 10 AM PST Monday
     for PZZ350-356-370-376.

     Gale Warning from 10 AM Monday to 10 PM PST Tuesday for PZZ350-
     356-370-376.

&&