000
FXUS66 KSEW 301016
AFDSEW
Area Forecast Discussion
National Weather Service Seattle WA
316 AM PDT Mon Mar 30 2020
.SYNOPSIS...Unsettled weather will continue through much of the
week and into the upcoming weekend. The most organized system will
move across the area today, bringing a quick round of heavy
lowland rain, mountain snow, and gusty winds to area. Rain will
transition to widespread showers through the day, with a chance
for a few thunderstorms. Unstable conditions with scattered
showers will then continue through much of the week.
&&
.SHORT TERM /TODAY THROUGH WEDNESDAY/...Water vapor imagery this
morning suggests broad cyclonic flow in the upper levels of the
atmosphere extending roughly from the Aleutian Islands/Western AK
south/eastward into the northeastern Pacific and into the western
US. From there, the flow becomes more split, with the northern
stream extending into the northern Rockies and Canada, and the
southern stream into the desert SW and southern Great Plains. In
the lower levels, the most significant feature is an organized
frontal system located just offshore the Washington and Oregon
coasts.
Focusing closer to home, an active period of weather is already
underway. Within the broader scale cyclonic flow that was
mentioned in the previous paragraph exists a robust area of closed
low pressure, centered roughly just west of Haida Gwaii. Several
vort maxes are also detected rotating around this area of
circulation, one of which is currently /as of 10z/ taking on a
negative tilt as it approaches the Washington Coast. This
shortwave/vort max is aiding the larger scale lift, via PVA and
attendant left exit region of jet streak, coincident with the sfc
frontal boundary. The result is a rather organized swath of rain
beginning to push into western Washington, per 10z radar imagery.
Convective features have been noted, per data from KLGX, within
this swath of precip. SPC mesoanalysis suggests sufficient deep
layer and low level shear and helicity, which is suggestive of the
possibility of a few rotating cells near the frontal boundary.
Also given sufficient mid level lapse rates, a few lightning
strikes are expected as well (In fact, a few strikes were noted
earlier in the night across Grays Harbor County). The frontal
boundary itself will be rather quick-moving this morning, with hi-
res guidance suggesting its location near the Cascades by
daybreak. Generally can expect increasing wind speeds/gusty winds
and heavy rain with its passage in the lowlands, with isolated
lightning also possible. For the higher terrain, decent snowfall
is expected, especially above 2500ft.
In the wake of the cold front, snow levels will tank to around
1500ft within an unstable environment by this afternoon. As mid
level temperatures continue to fall, mid level lapse rates will
steepen. This suggests the development of widespread post-frontal
shower development. These showers will carry the threat for brief
heavy downpours, small hail, and lightning in the lowlands, and
brief bursts of heavy snowfall and lightning for the mountains.
The overall winter weather headlines for the mountains will remain
in effect through the day today with generally the highest
snowfall amounts in the Northern Cascades. There will still be
enough snowfall across the Central Cascades to create some travel
headaches across the Passes.
Unfortunately, not seeing a pronounced decrease in shower
coverage into the overnight hours tonight. Unstable conditions
will continue, along with disturbed mid and upper levels to
support this trend. Given cool temps aloft, will opt to keep
mention of thunderstorms in the forecast overnight tonight.
Tuesday looks to similar to the latter half of Monday with
unstable conditions continuing. The closed upper level low will
begin to sink southward towards the local area. Ongoing scattered
shower activity is likely to become enhanced as forcing from the
approaching low comes into closer proximity. Thunderstorms will be
possible yet again.
The closed low will pass to the south into Wednesday, however,
another shortwave will drop across the area from north. This
feature will have the ability to spark a few additional showers
through the day, though coverage currently does not look as high
as the previous few days.
Kovacik
.LONG TERM /THURSDAY THROUGH SUNDAY/...The beginning of the long
term forecast period will have the state of Washington embedded
within broad scale upper level cyclonic flow extending from just
offshore the coast of BC into the Northern Great Plains. A few
pesky showers may be lingering early Thursday morning but overall
coverage does not appear to be very impressive. A weak shortwave
will pass through during the afternoon, which is likely to be
strong enough to generate a few showers. Conditions will improve
into Friday, with model guidance picking up on a closed area of
low pressure offshore approaching the Pacific Northwest. This
system is currently expected to pass south of the area into Oregon
and northern California. A few showers still cannot be ruled out
on Friday, given the proximity of this passing system as well as
the overall larger scale pattern. Showers are likely to be more
diurnally driven at this point in time.
Unfortunately, this general pattern will take us through the
weekend. Most ensemble guidance is suggestive of the broader scale
trough lingering around the Pacific Northwest and perhaps even
retrograding a tad through the weekend. This will likely continue
the threat for showers off and on, but the good news is neither
day appears to be a washout at this point.
Kovacik
&&
.AVIATION...Southwesterly flow aloft as the stronger upper level
trough continues to move into the area overnight. Cigs will be on
their way down as the rain showers spread inland. MVFR to IFR, down
to LIFR in the stronger shower activity. The upper level pattern has
destabilized some, lending itself to some isolated lightning strikes
in convection moving onshore this evening, particularly in Grays
Harbor County. Additional instability across all terminals will be
limited overnight, but will increase into Monday and Tuesday.
Southwesterly winds are expected to pick up across all terminals,
with gusts from 10-20 knots becoming 25-30 knots associated with the
frontal passage by early Monday morning.
KSEA...Current conditions are low end VFR. Cigs are expected to
deteriorate down to MVFR to IFR as rain shower activity spreads
inland. A stronger frontal system is approaching the area overnight
and will move through Monday morning, bringing stronger winds to the
terminal. Winds remain predominantly southwesterly overnight, at 10-
16 knots, before becoming increasingly gusty ahead of the frontal
boundary passing through. Gusts will increase to 20-25 knots at
times.
Kristell
&&
.MARINE...A stronger frontal system is moving onshore overnight into
Monday morning. Southerly gales for the coast, northern inland
waters, and the Strait of Juan de Fuca tonight into Monday morning.
Small Craft Advisory winds elsewhere as the front moves through.
Winds will remain elevated until Monday evening into Tuesday,
however the seas will remain elevated over the coastal waters in
advance of another system possibly moving through late week. Kristell
&&
.HYDROLOGY...River flooding is not expected over the next 7 days,
despite an unsettled period of weather. A fast-moving frontal
boundary will carry the threat for a short period of heavy rain
and some mountain snow this morning. Given the progressive
nature, total QPF is not expected to be impressive. Snow levels
crash to below 2000 feet through the day today with rain turning
to scattered showers. Some of these showers will produce short
bursts of heavy rain and mountain snow, but with coverage
remaining scattered, this will not cause river concerns. Showers
then look like a good bet for much of the week, but again, given
the overall scattered nature, not expecting river concerns.
Kovacik
&&
.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Weather Advisory until 11 AM PDT Tuesday for Cascades of
Pierce and Lewis Counties-Cascades of Snohomish and King
Counties.
Winter Storm Warning until 11 AM PDT Tuesday for Cascades of
Whatcom and Skagit Counties.
Winter Weather Advisory until 5 AM PDT Tuesday for Olympics.
PZ...Small Craft Advisory from 8 AM this morning to 5 PM PDT this
afternoon 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-
Northern Inland Waters Including The San Juan Islands.
Gale Warning until 8 AM PDT this morning 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-East Entrance U.S. Waters Strait Of
Juan De Fuca-Northern Inland Waters Including The San Juan
Islands.
Small Craft Advisory until 2 PM PDT this afternoon for Central
U.S. Waters Strait Of Juan De Fuca-West Entrance U.S.
Waters Strait Of Juan De Fuca.
Small Craft Advisory until 8 PM PDT this evening for Admiralty
Inlet-Puget Sound and Hood Canal.
&&
$$
www.weather.gov/seattle
NWS SEW Office Area Forecast Discussion