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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 211107

Area Forecast Discussion
National Weather Service Seattle WA
306 AM PST Thu Feb 21 2019

.SYNOPSIS...Upper level ridging with dry northerly flow aloft will 
continue today. A frontal system will arrive from the northwest on 
Friday. A cool upper trough will follow the front for the weekend 
into the early part of next week. Low snow levels will continue 
through the period. 


.SHORT TERM /TODAY THROUGH SUNDAY/...Current obs over the area 
showing the majority of temps at the time of this writing generally 
right around or just below freezing...with some exceptions such as 
Tacoma...Seattle...and many island locations...all sitting in the 
upper 30s. SW interior sites such as Olympia and Chehalis are 
reporting low level stratus...but no fog development just yet nor 
any reductions in visibility. Will leave freezing fog in the 
forecast for this area as the potential still exists for it to 
develop within the next few hours.

One more dry day looks to be on tap for today as the upper level 
ridge will pass through W WA today. The leading edge of the next 
frontal system looks to hold off until early Friday morning with 
best threat of widespread precip waiting until the late morning to 
afternoon hours. Interior snow levels will be pretty low during the 
morning any precip that kicks off early will likely be 
snow or a rain/snow mix. As already mentioned however...lowland 
amounts during this time period are expected to be light. By the 
time POPs climb into likely category...snow levels will have jumped 
to or near 1000 ft...almost 2000 ft along the coast...and as such 
remainder of precip expected to fall as rain. The front moves on 
Friday night but a broad area of upper level low pressure will 
remain...keeping an active...showery period over the CWA for the 
bulk of the weekend. As the core upper level low finally approaches 
the area Sunday...the system looks to stall. This will keep the 
prospect for showers in the forecast for the bulk of the 
CWA...however model disagreement on the actual position of the low 
may allow for the northernmost quarter of the CWA to meet with dry 
conditions. Given model disagreement...confidence in this is fairly 
low...but no harm in positive thinking at this point in the winter.

Snow levels during the short term will oscillate between 1000-2000 ft 
during the day and 500 ft to near sea level during the overnight 
hours. As such...the threat for snow or mixed precip never really 
goes away. The plus side of this is that any lowland snow that does 
occur will be minimal with not much in the way of accumulations 
expected...and any localized accumulations that do occur will be 
gone by mid to late afternoon. Adding more proof to minimizing any 
snow threat will be both high and low temps throughout the 
period...with highs in the low 40s and overnight/morning lows either 
right around freezing or 2-3 degrees above. At the very 
can surmise that the gradual march into...well...March may finally 
be taking a bit of the sting out of our lingering winter weather. SMR

.LONG TERM /MONDAY THROUGH THURSDAY/...Models fall seriously out of 
sync for the ECMWF takes the upper low out into the 
Pacific and allows for a minor upper level ridge to take root Monday 
into Tuesday. The GFS shows this happening too...just almost 24 
hours later than its Euro counterpart. And the current runs remain 
out of phase like this for the remainder of the forecast 
period...although...again...their general patterns remain fairly 
similar...generally dry conditions early next week followed by a 
return to active weather offset by a gradual climb in snow levels 
and daytime highs during the middle of the week. Needless to 
say...even though specifics are hard to come by and confidence 
remains low...the general trend seems to show that any future 
lowland appearances of that other four letter s word...snow...may 
have to wait until next winter.  SMR


.AVIATION...Northerly flow aloft over the region today as weak upper 
ridging centered well offshore pushes into the area. The air mass is 
generally dry except for some patchy low level moisture. The flow 
aloft will back to west-northwest tonight as another upper trough 
slides down the British Columbia coast toward Western Washington. 
Except for some patchy areas of MVFR low clouds this morning, VFR is 
expected across the region today. High and mid level moisture will 
begin to increase tonight ahead of the next system. N-NE surface 
winds will weaken this evening and back to southerly early Friday AM.

KSEA...Some patches of stratocumulus in the vicinity of the terminal 
for some possible BKN020-030 until around sunrise, then expecting 
VFR the remainder of the day. High and mid-level clouds increase 
tonight and lower to MVFR toward 15-18Z Friday as rain spreads into 
the area with the next frontal system. Surface winds N-NE 4 to 8 
knots today shifting to southerly near or after 06Z tonight.  27


.MARINE...Northeasterly offshore flow will ease this afternoon 
and turn southerly tonight as high pressure over British Columbia 
shifts to the southeast and a front approaches from the northwest.
West swell 10-12 feet will subside this afternoon and evening.

The aforementioned frontal system will move through the area on 
Friday. A trailing weak low over Haida Gwaii will move to Oregon 
Saturday and Saturday night. 

Northeasterly offshore flow will develop on Sunday and continue into 
Monday with high pressure over British Columbia and low pressure 
over Oregon. Schneider


.HYDROLOGY...River flooding is not expected for the next 7 days.


PZ...PZ...Small Craft Advisory for hazardous seas until 6 PM PST this 
     evening 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 for rough bar until 8 PM PST this evening 
     for Grays Harbor Bar.

     Small Craft Advisory until noon PST today for 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.