Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 072218
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
218 PM PST Sun Mar 7 2021

.SHORT TERM...Through Tuesday Night...Lightning strikes continue
beyond 60 NM, but so far the SW flow has been pushing these
electrified showers north into Portland`s outer water area. Will
continue to monitor an area of showers that moving into our outer
waters in about 6hrs time. The cloud top temperatures of these
showers are on par with those that have been producing lightning,
so there may need to be an update to add this later this evening
as the current forecast does not include them.

Though temperatures along the immediate coast dip to around 36
degrees they do not last long enough for frost to form. winds will
also be increasing during the early morning. This should prevent
frost formation and therefore have not issued a frost advisory.

Added a winter Weather Advisory for southern Siskiyou County south
of weed, including portions of I-5, route 89, Mount Shasta City,
Dunsmuir, and McCloud. It also includes areas NE of Mt. Shasta to
Medicine Lake. Flow will be from the SW and favors the upslope
flow effects in this area. Snowfall rates could approach 1"/hr
Monday afternoon/evening (roughly 4-8 PM PST). Roads should be
warm enough to cause snow to melt, but the one inch rates could
cause a buildup of slush on roads which will offer a cooler
surface for further snow to settle on top. See WSWMFR for further
details on this advisory.

Heavy snow will also occur in the Coast range and western
Joesphine County, but should be isolated to higher terrain when it
precipitates the most. As snow levels rise from 2300-2500 feet to
2700-3000 feet, the environment becomes more showery, allowing
for breaks in snowfall and better melting along warm roads, snow
should not pose a large impact. Therefore have held off on issuing
a winter Weather Advisory for this area. Local passes, like Hayes
Hill and areas along route 199 south of Obrien, could observe a
few tenths of snow accumulate along the roads. Siskiyou summit and
surrounding portions of I-5 could also accumulate a few tenths of
snow. As a whole though, not much precipitation is expected to
advance beyond the coast range except with the leading edge of the
front. The precipitation along this leading edge is expected to
have enough momentum to bring at least a trace of rain to the
eastern portions of Lake County. The limited nature of this
precipitation means that other passes along I-5, like sexton
summit, and other Cascades passes are not expected to accumulate
an impactful amount of snow.

A south wind will develop in the Shasta valley Monday and Tuesday
afternoons. The winds are expected to be stronger Tuesday than
Monday. Wind gusts should stay between 25-30 mph, but a gust to 40
mph Tuesday afternoon can`t be ruled out. Snow in the Weed, CA
area should have minimal impact wrt to snow because of how light
the amounts should be. These winds will approach speeds that could
impact high profile vehicles, so attention to this portion of the
forecast will be critical as the forecast seems likely to change.

Expanded the slight chance of thunderstorms to the entire marine
waters and brought the slight chance onshore up to the coastal
range. In addition to the lightning threat there is also a chance
that small hail forms in these storms. CAPE approaching 500 J/kg
and lapse rates of 8 C/km are closer to summertime values, the
only limiting factor is available moisture, which at times dips
below 50% RH over the 700-500 mb layer. Combined, small hail with
gusty winds, near 30 mph in the headlands, could pose a potential
hazardous travel situation.

This pattern of activity continues into Tuesday afternoon before
beginning to taper off into Tuesday night. The cutoff low will
move south along the coast, so northern areas will be the first to
have improving conditions. Tuesday night could also be the last
chance that areas of Modoc and southern Lake Counties have to
accumulate some light amounts of rain before the system totally
leaves the region. -Miles


.LONG TERM...(Wednesday through Sunday)...Wednesday, the closed low
will gradually move SSE and then inland Wednesday night over south-
central California. Models show continued scattered to numerous
showers over the area on Wednesday as this low gradually shifts
southward. Snow levels are expected to be around 2000 to 2500 ft
increasing to 2500-3500 feet late in the day Wednesday. Shower
activity will decrease Wednesday when the low is projected to move
inland over south-central California. Thursday, an upper ridge
will build into the area. Models and ensembles indicate the
potential for some continued, light wrap-around showers Thursday as
the low and upper trough move eastward. Then drier weather is likely
as an upper level ridge strengthens over the region late Thursday
into Friday. Models and ensembles continue to show variability on
how long this ridge will remain in place Friday night into the
weekend. The ECMWF and EC ensembles favor keeping the ridge in place
through Saturday night then show a weakening front moving into the
area Sunday. The GFS, GEFS and Canadian models favor a more
progressive pattern with a front moving into the PacNW Saturday
followed by the upper trough moving overhead on Saturday night.
Ensemble variability remains high. So there is low confidence in the
details of the weekend forecast and when this front will bring
precipitation into the area. -CC

&&

.AVIATION...For the 07/18Z TAF Cycle...Overall conditions are
generally VFR  through the TAF period. However areas of mountain
obscurations are expected through the TAF period, especially tonight
into Monday morning when CIGS lower some and stratus is expected to
develop along the Southern Oregon Cascades west. Patchy fog is
possible early Monday morning west of the Southern Oregon Cascades,
with best chances in the Grants Pass area and portions of the Umpqua
Basin. -CC

&&

.MARINE...Updated 200 PM PST Sunday 07 Mar 2021...A very high and
very steep long period west-northwest swell will in the waters late
this afternoon, peaking late this evening at 16 to 18 feet, then
gradually diminishing late tonight. An upper low will move south
towards the waters and remain to the west Monday night through
Tuesday. Ahead of the upper low will be a series of systems moving
through the waters late tonight through Tuesday with high and steep
seas expected to continue. A cold air mass aloft will also move into
the area which could bring enough instability for isolated
thunderstorms during this time.

On Monday, a cold front will bring gusty south winds, followed by a
another front Monday night and Tuesday with continued south winds.
Winds will gradually diminish Tuesday night as a surface low moves
over the waters. This low will move south on Wednesday with winds
becoming north and increasing, with the strongest south of Cape
Blanco. The pattern may turn active again at the end of the week. -
-Petrucelli

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...High Surf Advisory until 7 AM PST Monday for ORZ021-022.

CA...Winter Weather Advisory from 4 PM Monday to 4 AM PST Wednesday
     for CAZ082.
     Winter Weather Advisory from 4 PM Monday to 10 AM PST Tuesday
     for CAZ083.

Pacific Coastal Waters...Small Craft Advisory until 4 PM PST this afternoon for
     PZZ350-356-370-376.
     Small Craft Advisory from 4 AM Monday to 4 PM PST Tuesday for
     PZZ350-356-370-376.
     Hazardous Seas Warning until 4 AM PST Monday for
     PZZ350-356-370-376.

$$


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