NWS Seattle, WA wrote its last area forecast discussion before the first event at Nov 17, 8:20 PM PST, 40 minutes earlier.
Area forecast discussion, Nov 17, 8:20 PM PST, 40 minutes before
Area Forecast Discussion
National Weather Service Seattle WA
820 PM PST Tue Nov 17 2020
.UPDATE...The 03 - 04Z mesoanalysis trends shows that instability
is waning across Western Washington - a trend that is supported
by the downtick in thunderstorm activity over the past half hour
or so. Effective bulk shear has decreased dramatically as well -
falling from around 40 to 45 knots to below 20 knots from Puget
Sound northward in the past hour. The current environment in place
across the region no longer supports thunderstorm activity for
this evening. A pocket of 20-30 knots of effective bulk shear
along the central and southern Olympic coast will support showers
capable of briefly gusty winds, particularly as these showers move
inland off of the water. Otherwise, expect shower activity to
continue to gradually decrease in coverage through the remainder
of the night. The remainder of the forecast remains on track and
details outlining Wednesday onward are in the sections below.
Pullin
&&
.PREV DISCUSSION... /issued 228 PM PST Tue Nov 17 2020/
SYNOPSIS...Active weather will continue through Wednesday as
several frontal systems pass across the area. Unsettled weather
will carry into Thursday and Friday, but precipitation will not be
as widespread. A brief break in the wet weather is expected on
Saturday before another stretch of active and wet weather begins
Sunday and carries into next week.
&&
SHORT TERM /TONIGHT THROUGH FRIDAY/...The synoptic pattern this
afternoon is characterized by a longwave trough extending from the
Gulf of Alaska into the NE Pacific and into the PNW. Just
downstream from this feature, a fairly high amplitude ridge
extends from the Rockies into the Plains, with a broad, yet
weaker trough also noted across the Great Lakes and NE. At the
surface, a very impressive low center (~964 mb per 18z WPC
analysis) was noted near the northern tip of Vancouver Island,
associated with a strong mid level shortwave perturbation within
the longwave trough in the same general vicinity. A cold front,
associated with this strong low extends across Vancouver Island
and into W WA and W OR. Meanwhile, expansive high pressure
dominates much of the CONUS east of the Rockies.
Quite an active weather day across W Washington! The main story
has been the wind, which is associated with the deep sfc low near
northern Vancouver Island and its associated pressure gradient.
As it moved northward towards its current location, winds
substantially increased across the offshore waters and near the
Coast. Gusts in this vicinity generally peaked between 55-65 mph,
with Destruction Island reporting the highest gust of 80mph
earlier this morning. With these areas now in the wake of the cold
front and experiencing increasing displacement from the sfc low,
winds have peaked but will remain breezy through the afternoon. As
the front continues to make inland progress, winds have
noticeably picked up from east of the Olympic Peninsula into the
Cascades. Areas from around Port Townsend and through much of
Island County and northward toward the US/Canada border have been
windiest, with gusts generally 30-50 mph with a few gusts near 65
mph over the water. Locations from about Olympia to Seattle and
Everett have reported gusts generally 30-40 mph. Winds across
these areas will peak this afternoon before gradually winding down
through the evening.
In addition to the winds, have had widespread rain (heavy at
times) in the vicinity of the cold front, with some small enhanced
linear segments noted within. Thus far these segments have been
benign, aside from carrying locally very heavy rainfall, and are
largely the result of frontal forcing coupled with strong deep
layer shear. Rain should transition to more showery precipitation
in the wake of the front, with some additional widespread rain
developing along a secondary trough near the Coast. This area of
precipitation is expected to affect the area from Puget Sound
westward through the offshore waters through the evening. With the
aid of some lower level instability offshore/near the Coast
combined with steepening mid level lapse rates from Puget Sound
westward amidst sufficient shear, could see the development of a
few thunderstorms through midnight. Small hail, brief gusty
winds, and heavy rain will be the main threats with any
thunderstorm. Enough helicity may also remain in place over the
next few hours for rotation and the potential for a
waterspout/brief weak tornadic spinup (offshore and near the Coast).
River flooding is not expected to be an issue into Wednesday,
however, will watch the Skokomish River closely (see hydro
section). Could see ponding on water on roadways within heavier
convective segments and/or any thunderstorm. Snow levels will drop
in the wake of the cold front, to between 3-4k ft by daybreak
Wednesday. With precipitation expected to have largely tapered
off, not expecting much impactful snow, although Stevens Pass
could receive a dusting.
Another shortwave trough (with associated sfc front) within the
broader scale cyclonic flow will approach the area early
Wednesday. After our brief break in widespread precipitation
overnight, the next round of rain will push from south to north
through the area, mainly during the morning hours. Showery
conditions are then anticipated behind the front/initial rain
shield by the afternoon. With much colder conditions aloft
expected, the threat for thunderstorms will return, the main
threats being small hail and heavy rain. Snow levels will also
remain lower, still remaining between 3-4kft for most of the day.
Stevens Pass will likely pick up a few inches of new snow, and as
snow levels then dip down below 3000 ft overnight into early
Thursday, Snoqualmie Pass may pick up an inch or so.
Unsettled weather will close out the short term for Thursday and
Friday as additional perturbations move across the area. These
systems will be weaker and more progressive, being ushered in part
by the development of larger scale cyclonic flow over the
Bering.Sea. Expect scattered showers both days.
Kovacik
LONG TERM /SATURDAY THROUGH TUESDAY/...As we begin the long term
forecast period, longwave troughing will become better established
over the Gulf of Alaska, resulting in the development and
progression of short wave ridge downstream of it, of which will
push across W WA on Saturday. This will provide for a brief (and
let's emphasize brief) break in the active weather, so if wanting
to get outdoors this weekend, Saturday currently looks to be the
day. By Saturday evening/night, the next frontal system will be
approaching the area, aiding by a mid level shortwave. This system
appears to be progressive and should push across the area by the
evening hours, however, this may just be the beginning. The
majority of the ensemble solutions by early next suggest a weak
split flow pattern developing across the far western US/West Coast.
with deep troughing becoming better established over the Gulf of
Alaska and strong high pressure over the subtropical Pacific.
Given this pattern, it appears PNW could reside within the storm
track/polar jet trajectory between the two larger scale features,
resulting in another active and wet weak. It seems possible these
systems would have access to subtropical or tropical Pacific
moisture based on some of the latest IVT guidance. Will have to
monitor the development of this pattern in the coming days.
Kovacik
&&
AVIATION...Scattered SHRA will continue to taper off from south to
north over the next 3 to 6 hours. Minor reductions in vsby will
continue under heavier pockets of rain, with vsbys as low as 4sm
possible. Otherwise, low end VFR conditions will persist through
much of tonight as BKN to OVC CIGs will generally range from
around 3000 to 5000 feet. The break from SHRA will be temporary,
as another round of SHRA will return to the region, affecting all
TAF sites. Reductions in CIGs and vsbys will be possible once
again, particularly under any heavy showers. Prevailing wind out
of the south at most TAF sites around 10 kts, with the exception
of KHQM and KCLM, where easterly and northwesterly winds will
prevail respectively. Occasional gusts to 20 kts possible
Wednesday afternoon.
KSEA...Scattered SHRA will continue at the terminal for the next
several hours, yielding light to moderate RA. CIGs and vsbys will
largely remain VFR through this timeframe, with brief reductions
in vsby possible under heavier rain. Otherwise, RA is expected to
taper off between 05 and 06Z tonight, with a few stray showers
lingering across the Puget Sound region. Another round of SHRA is
expected to reach the terminal around daybreak Wednesday, with
occasional reductions in vsbys possible under any moderate to
heavy SHRA. Prevailing wind out of the south through the period
around 10 kts. Occasional gusts to 20 kts will be possible
Wednesday afternoon.
Pullin
&&
MARINE...Winds have decreased across coastal and inland waters this
evening, with advisory level conditions being observed. Expect
hazardous conditions for small crafts to persist through Wednesday
before calmer conditions prevail across all waters. A stronger
storm will approach the coast once again by this weekend, with
hazardous winds potentially making a return by late Saturday into
Sunday.
Large, hazardous southwesterly to westerly swells continue to
gradually decrease this evening. Swells will become more westerly by
late Wednesday morning, dropping below 10 feet by the afternoon and
remaining between 5 and 8 feet into the weekend. Hazardous swells
above 10 feet will be possible once again by the Sunday/Monday
timeframe with the approach of a stronger system.
Pullin
&&
HYDROLOGY...The Skokomish River near Potlatch is on the rise again
this afternoon following this morning's cold front. Additional rain
is expected on Wednesday, though amounts look to be fairly light.
Current forecast holds the rise today into tomorrow below flood
stage but will continue to monitor. Have continued the flood watch
given uncertainty and proximity to minor flood stage.
Elsewhere, no river flooding is expected into the weekend. Model
guidance suggests the appearance of another atmospheric river system
early next week with the potential for heavy rainfall. It's too
early to speculate on the details but it's something we're keeping
an eye on.
Carpenter
&&
.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 10 PM PST Wednesday for Coastal
Waters From Cape Flattery To James Island 10 To 60 Nm-
Coastal Waters From Cape Flattery To James Island Out 10 Nm-
Coastal Waters From James Island To Point Grenville 10 To
60 Nm-Coastal Waters From James Island To Point Grenville
Out 10 Nm-Coastal Waters From Point Grenville To Cape
Shoalwater 10 To 60 Nm-Coastal Waters From Point Grenville
To Cape Shoalwater Out 10 Nm.
Small Craft Advisory until 10 AM PST Wednesday for Grays Harbor
Bar.
Small Craft Advisory until 1 AM PST Thursday for Admiralty Inlet-
Central U.S. Waters Strait Of Juan De Fuca-East Entrance
U.S. Waters Strait Of Juan De Fuca-Northern Inland Waters
Including The San Juan Islands-Puget Sound and Hood Canal-
West Entrance U.S. Waters Strait Of Juan De Fuca.
&&
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