447
FXUS66 KSEW 142107
AFDSEW
Area Forecast Discussion
National Weather Service Seattle WA
207 PM PDT Mon Oct 14 2024
.SYNOPSIS...The first of a series of weather systems will impact
the region today, with light rain showers continuing through
Tuesday morning. A more substantial cold front Wednesday will
bring in cooler air, with the chance of rain with thunderstorms,
and snow in the higher elevations. Next weekend will warm up
slightly with the potential for heavy rain across the coverage
area.
&&
.SHORT TERM /TONIGHT THROUGH THURSDAY/...Morning sunshine this
morning was quickly replaced with clouds and rain showers via a
surface cold front moving across the region today. Most of the
rain so far today has been concentrated along the west Olympics
and the coastline, where radar shows a majority of the showers
continuing there, and up into Bellingham. A few light showers have
passed through Puget Sound, and high resolution guidance has most
of the showers wrapping up this evening (with a few continuing
overnight across the West Slopes of the Cascades. No major impacts
are expected with this front (in terms of precipitation totals
remaining light with most of the rain falling in the north central
Cascades, and west Olympics). There will be a break in the rain
activity for most of the day Tuesday. Temperatures will drop a
descent amount behind this front, with Tuesdays highs only topping
out around 60 for most places.
A stronger trough (from a low in northwest B.C.) will swing around
a stronger cold/occluded front and trailing trough Wednesday into
Thursday. This is aided by a 130 kt jet max off the coastline
helping to push the energy inland (along with limited moisture
offshore coming up in timing with the frontal boundaries). This
system has more cool air aloft, which will sink down helping to
bring snow levels down to around 4,000 feet by Thursday. This will
also cool daytime highs down into the mid 50s. Breezy winds are
possible in spots Wednesday (pre-front), but are not expected to
create any impacts.
Widespread precipitation is expected in the form of periods of
rain for most places. Snow will be possible at higher elevations,
which may be enough to stick to the ground at elevations greater
than 4,000 ft (including Stevens Pass). The deterministic forecast
trend for the precipitation with this system Wednesday/Thursday
decreased considerably (compared to yesterday, with a decrease in
moisture content). The amounts with this second system very quite
a bit, with lowlands seeing around a half an inch to an inch
Wednesday through Thursday. THis increases up to 1-2 inches of
QPF in the Cascades, and around 1-1.5 inches in the Olympics. The
biggest caveat with this system will be if any convective banding
takes place within the precipitation. With the risk of thunder
included in the forecast, it will be possible for these projected
amounts to be exceeded by as much as half an inch in the lowlands,
to an inch in the mountains/coast. This also applies to the
potential of snowfall in the upper Cascades and Olympics, where
there is around a 3 inch spread around the NBM mean in totals
(looking at the 25th and 75th percentiles). Most of the grounds as
well are well above freezing, so any snow that does fall may melt
on contact, or be slushy when sticking onto grounds/roads. Anyone
traveling on roads over 4,000 ft in elevation should check road
conditions ahead of time Wednesday/Thursday, and be prepared for
possible slick roads.
.LONG TERM /FRIDAY THROUGH MONDAY/...There will be a break in the
precipitation early Friday morning as the trough axis departs and
digs down into the Rockies. The ensembles/deterministic models
have more or less locked in on the potential for a more
traditional atmospheric river system from Friday night into next
weekend. IVT analysis shows a moist conveyor belt, or "pineapple
express" setting up a plume of moisture from the central Pacific,
up to the coast of B.C./northwest Washington. While the placement
of this low/atmospheric river may very well change between now and
next weekend, the amount of moisture with this system remains
impressive, and this has highlighted an area of concern for the
potential of excessive rainfall next weekend (highlighted by CPC`s
outlook). It is still too early to know exactly how much
precipitation everyone will receive with this system, but it may
be enough to cause hydro concerns following rainfall from systems
earlier in the week (see hydrology discussion below). The warmer
snow levels with milder temperatures returning this weekend may
also add to the impacts for higher elevations receiving snow.
Persons in western Washington should stay tuned for future updates
of this system (as more will be known later this week).
HPR
&&
.AVIATION...The frontal system has been slow moving this afternoon
as its made its way into western Washington. Precipitation has been
light, with generally MVFR to IFR cigs observed as it has passed
over. Conditions are somewhat gusty throughout Puget Sound this
afternoon, notably near BLI and PAE. Surface flow has been
southwesterly and is likely to continue until between 06Z and 08Z
when post frontal winds will turn to the north or northwest.
Precipitation will taper off for much of the region at this point as
well, with only isolated showers left over. Additionally, cigs are
expected to remain lower in the wake of the front, largely
oscillating between MVFR and IFR. Some of the usual spots such as
OLM, PWT, and HQM may experience sporadic LIFR conditions through
Tuesday morning.
KSEA...VFR at the terminal presently, as rain has been much slower
to move overhead than originally anticipated. A band of showers
moving east from the Kitsap peninsula at this time will likely be
the next chance for rain to begin at the terminal, granted that it
holds together on its way. Shortly after precipitation begins, cigs
are expected to drop to MVFR with isolated occurrences of IFR cigs
throughout the night. Light, post frontal northerly flow is expected
to begin between 06Z and 08Z tonight. While not an ideal combination
for terminal operation, there is little to no support in the model
guidance to suggest that surface wind speeds from the north will be
in excess of 5 to 6 kts.
Kristell
.MARINE...Southerly winds are continuing across the Coastal Waters
as a frontal system is still moving across the area today. Small
Craft Advisory winds are expected to continue for the Coastal Waters
through before winds transition northwest and taper off later this
afternoon. Winds for the interior waters have generally become
lighter this afternoon as the front has slowed down and weakened as
it`s moved over the interior waters. Due to the weakened state of
the front, the post frontal northwesterlies are not expected to be
strong. Present headlines for the northern inland waters will be
allowed to expire this evening.
Winds are expected to continue to ease tonight through Tuesday.
Another front will move through Tuesday night into Wednesday with
increasing southerly winds during this period. Another, stronger
westerly push is then expected on Wednesday for the Strait of Juan
de Fuca where headlines will likely be necessary. Winds will
transition more northerly for the Coastal Waters on Thursday, before
another potential front into the weekend.
Small Craft Advisories continue for the Coastal Waters for
heightened seas through tonight. There may also be a brief period of
seas near 10 feet for the far westernmost portion of the western
Strait of Juan de Fuca, but not expected to be widespread across the
zone to warrant a SCA. Seas will then subside below 10 feet on
Tuesday. Seas may then build again to 10 feet Wednesday through
Thursday.
Kristell
.HYDROLOGY...A series of frontal systems will track into western
Washington throughout this week, bringing periods of rain into the
weekend. There is a signal of an atmospheric river later on in
the week and into the weekend that could be bringing more
plentiful moisture throughout the area, although timing and
placement of this atmospheric river is not certain at this time.
QPF will be need to monitored closely over the next couple of days
to determine if there will be any elevated river flows. At this
time, there are no area rivers expected to rise into action stage
or minor flood stage.
Mazurkiewicz
&&
.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 5 PM PDT this afternoon for Grays
Harbor Bar-Northern Inland Waters Including The San Juan
Islands.
Small Craft Advisory until 11 PM PDT this evening for Coastal
Waters From Cape Flattery To James Island Out 10 Nm-Coastal
Waters From James Island To Point Grenville Out 10 Nm-
Coastal Waters From Point Grenville To Cape Shoalwater Out
10 Nm.
Small Craft Advisory until 8 AM PDT Tuesday for Coastal Waters
From Cape Flattery To James Island 10 To 60 Nm-Coastal
Waters From James Island To Point Grenville 10 To 60 Nm-
Coastal Waters From Point Grenville To Cape Shoalwater 10
To 60 Nm.
&&
$$
NWS SEW Office Area Forecast Discussion
Discussion Script by Ken True at Saratoga Weather - WFO Script by SE Lincoln Weather
