National Weather Service Raw Text Product

378 
FXUS66 KPQR 212237
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR 
337 PM PDT Thu Oct 21 2021

.SYNOPSIS...Expect a very active weather pattern to bring valley rain
and high mountain snow every 12-24 hours through early to mid next
week. Strong winds will be possible along the coast Sunday. 


&&


.SHORT TERM...THIS EVENING THROUGH SATURDAY NIGHT...Rain is 
gradually overspreading the area from the west, with a heavier wave 
of semi-convective activity expected to impact the Willamette Valley 
during the 4 to 9 PM timeframe. Briefly breezy winds may be possible 
with this activity in inland locations, but nothing substantial is 
expected there at this time. Along the coast and in the Coast Range, 
however, wind gusts to 40-45 mph appear likely for a period over the 
next couple of hours. Nearly continuous rain, potentially heavy at 
times, will continue over the area through late tomorrow morning, 
after which time much of the high resolution guidance is suggesting 
a brief break in activity. However, potential exists for a couple 
thunderstorms to develop in coastal and inland locations alike when 
surface heating destabilizes the atmosphere in the afternoon. Weak, 
large-scale sinking motions should be favored in cold air advection 
and anticyclonic vorticity advection behind the trough axis during 
the afternoon tomorrow, and vertical wind shear will weaken 
dramatically under the core of the upper trough. Yet, with the moist 
marine layer in place and any surface heating through cloud breaks 
(HREF is suggesting 40-60% probabilities for clouds over the 
Willamette Valley areas west), the area could see around 100-200 
J/kg of surface based CAPE which could support a few rumbles with 
isolated to scattered showers.

Late Friday into Saturday, cyclogenesis will be favored on the back 
side of a ridge which will amplify in the wake of the bomb cyclone 
now moving northeast across the eastern Pacific. This system, 
however, will be significantly weaker than its predecessor, and the 
only effects from it expected in our area are additional rain. The 
deterministic models are now in reasonable agreement showing another 
cold front arriving onshore during the early morning on Saturday. 
Ahead of it we'll get some gentle rain and perhaps additional breezy 
winds - particularly along the coast. Behind it, we should see more 
showers. Precipitation totals with this system are not looking like 
much in comparison to tonight's system, with only a general two to 
four tenths of an inch expected (vs. 1/2" to 1" today through 
tomorrow evening) in the lowlands. -Bumgardner


.LONG TERM...SUNDAY THROUGH WEDNESDAY...A ridge tries to build in to 
bring a break in precipitation for about 12 hours or so Saturday 
night before another storm system approaches the region right on its 
heels. Whether or not showers taper early enough to provide the area 
a break in rain before more arrives ahead of the next surface low is 
not clear at this time, but it's at least a reasonable possibility. 
Even so, Saturday looks cool, with the NBM only giving PDX a 15% of 
making it out of the 50s.

Sunday's forecast high is trickier, as timing, strength, and 
position of the low are still uncertain. A stronger low would favor 
deeper cloud cover in isentropic lift ahead of the front, but it 
would also feature stronger low-level warm air advection which could 
offset the inhibition of surface warming resulting from those 
clouds. A proper arrival time of the system could lead to 
significantly warmer temperatures than what is forecast as well, 
since cold frontal passage during the morning hours would clear out 
Saturday night's radiational cooling-inhibiting clouds and allow for 
efficient surface heating. If this timing pans out, convection 
concerns would be on the increase as well, since 200+ J/kg of 
surface based instability would result area-wide in an environment 
characterized by upwards of 40-50 knots of 0-6km shear (assuming 
model forecasts from the NAM and GFS are not completely out to 
lunch).

Models are struggling to forecast this system in general, but are in 
much better agreement compared to yesterday. The EPS system of 
ensemble members looks much less disorganized, with most forecasting 
the low deepening to 945mb to 960mb near 130W - a track slightly 
closer to shore than the today/tonight system. Despite the intensity 
of the system, its distance from the coast should prevent an extreme 
wind event there, but still several ensemble members are suggesting 
6 hour peak wind gusts of greater than 50 mph will be possible along 
the coast Sunday/Monday. Meanwhile, the GEFS ensemble is slightly 
weaker than (albeit in the same general area as) the EPS, with lows 
bottoming out in the 950-965mb range. The GFS solutions are somewhat 
more concerning though, since they are trying to develop additional, 
compact (meso)lows closer to the coast. If a small-scale low with a 
tight enough gradient can develop in sufficiently close proximity to 
the coast, then stronger winds would ensue, though this seems like a 
low-probability scenario on this time based on previous experience. 
Oddly enough, a smaller proportion of GFS members (in fact, only 4 
of 30) are forecasting high winds along the coast. One additional 
thought: The track of the ECMWF ensemble's lows are generally west-
southwest to east-northeast, which is not as favorable for high winds 
- especially in the Willamette Valley - as a more meridional (i.e., 
south to north) low track. Ultimately, this system will bear 
watching in the coming days to determine whether or not impacts can 
be expected, though high winds along the coast look possible (not 
likely, but possible) at this time. Greater impacts would likely be 
from convection if it can develop in the strongly sheared 
environment east of the upper trough on Sunday.

Monday through Wednesday, the pattern is as uncertain as (in fact, 
possibly more than) it is Sunday, with EPS and GEFS low pressure 
system tracks scattered all over the place from beyond 130W to areas 
east of the Cascades. A few solutions portray reasonably strong lows 
moving north/northeastward just off the Oregon and Washington coasts 
which could ultimately lead to another shot at strong winds along 
beaches and headlands (and possibly the Willamette Valley). Yet, 
solutions are, in general, weaker with wind speeds area-wide. At 
this time, the system with the greatest potential to have impacts is 
forecast to move through on Sunday, but more rain looks to be a good 
bet for early next week. By mid-week, 3 out of 4 solutions in WPC's 
cluster analysis suggest an amplification and westward expansion of 
the ridge currently centered in the Great Basin. Uncertainty is 
high, but this is a promising sign for warming and drying 
conditions. -Bumgardner   


&&


.AVIATION...00Z TAFS: Cold front is coming onshore right now.
Winds have eased at the coast with cigs briefly falling to IFR
along with brief IFR vsbys under the moderate rain band. Most 
gusty wind inland right now is from the easterly Gorge outflow 
affecting KTTD and KPDX. The outflow should end when the front 
arrives in the next couple hours, otherwise don't expect much 
significant wind inland with this specific frontal passage. 
Conditions behind the front appear to bring MVFR cigs with 
trailing showers continuing through most of the remaining TAF 
period.

For detailed regional Pac NW aviation weather information, go online 
to: https://weather.gov/zse

KPDX AND APPROACHES...Main impact will with the cold front crossing 
during the afternoon arrivals centered around 00Z. Expect a fairly 
rapid drop in flight conditions with IFR cigs under the primary rain 
band from 00-02Z. Cascades will likely remain obscured for much of 
the 12 hours following. /JBonk

&&

.MARINE...Current surface low is well off the Strait of Juan de
Fuca and nearly made a direct track over buoy 46036. The lowest
pressure at the buoy was 957 mb recorded at noon PST. This would
give the 12Z GFS as the better performer of the NAM, GFS, and
ECMWF regarding the surface low strength. Unfortunately, that
particular forecast did not translate well to the observed 
strength of the winds. Peak sustained winds at buoy 89 were
around 30 kt with the highest gust of 45 kt. That was compared to
all model forecasted speeds of 34-41kt and much higher gust
speeds. Winds have dropped off pretty quickly immediately behind
the front but then the wrapped up occlusion will cross the waters
overnight bringing another round of marginal gales. Given the
overdone performance of first round today, feel no need to extend
the current gale warning. A Small Craft Advisory should suffice.
Widespread wind gusts then drop below 21 kts near daybreak
Friday. Buoy 89 seas briefly peaked at 20 feet earlier today and
appear to have reached the mid-teens as the front crosses the
buoys closer to shore. Forecasted seas for this evening may be a
bit overdone given the slightly weaker winds. 

Somewhat of a break tomorrow as surface high pressure crosses the
waters through trailing westerly swells will keep seas 12-14 ft.
Another front, though decaying, will bring increased winds across
the waters late Friday night with the front crossing inland by 
daybreak. Seas will probably not see much change and remain in
the lower to maybe mid teens.

Models continue to widely vary with the position of the next
strong low pressure area for Sunday heading into Monday. Each run
seems to bring a new spread of solutions. As such, feel it is not
time to start jumping on any single solution or timing. In
general, however, there will be a surface low center maturing
well off the Columbia mouth between 130 and 135W. Lowest
pressures seem to be in the 945-950 mb range and the trajectory
seems to be generally toward the NE and coming onshore somewhere
over western British Columbia. Many models are indicating storm
force gust potential over the coastal waters of Oregon and
Washington from any time between daybreak Sunday through 
daybreak Monday. Combined seas 25-30 feet are plausible from
Sunday night through Monday night but again, continue to see a
wide variance of wave output. At this point, feel it is best to
significantly push the forecasted winds and seas higher toward
those values given the very broad model agreement, but hold off
on fully jumping in an any single or blended solution. /JBonk

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Gale Warning until 8 PM PDT this evening for coastal waters 
     from Cape Shoalwater WA to Florence OR from 10 to 60 nm.

     Gale Warning until 5 PM PDT this afternoon for coastal waters 
     from Cape Shoalwater WA to Florence OR out 10 nm.

     Small Craft Advisory until 10 PM PDT Friday for Columbia River 
     Bar.


$$