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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 180359

Area Forecast Discussion
National Weather Service Seattle WA
859 PM PDT Mon Jun 17 2019

.SYNOPSIS...Tuesday will see more moisture and perhaps a little 
morning drizzle or rain, especially around the central sound. An 
upper trough will move through the region Wednesday and Thursday. 
Weak high pressure might build toward the end of the week before an 
upper level low arrives Sunday.


.UPDATE...Current satellite imagery indicates cloud cover gradually
increasing over the region, with stratus already starting to move
inland along the coast. A thicker marine layer will move in tonight
and models continue to hint at the possibility of a Puget Sound 
Convergence Zone setting up across the central Sound early Tuesday.
Thus, have maintained current POP trends. Clearing will not be as
widespread tomorrow-however, could see some late afternoon sun 
across portions of the northern and southwest interior. Expect 
skies around the central Sound to remain mostly cloudy, especially
if a PSCZ does develop. Overnight lows tonight will be in the mid
50s across the interior and low 50s along the coast. 14


discussion...Vort lobes wrapping around an upper level trough 
pushing through BC will lead to continued onshore flow and chances
for precipitation Tuesday night through Wednesday before the 
trough shifts south Thursday.

.LONG TERM /THURSDAY THROUGH SUNDAY/...The chance of showers will 
continue through Thursday--and the sharpest shortwave upper trough 
arrives on Thursday, so that day could see heavier scattered showers 
develop. The GFS now shows a little brief ridging toward the end of 
the week, and then an upper low arrives on Sunday. So, we might see 
a little more sunshine Friday and Saturday and then go back into 
clouds and showers on Sunday. The ECMWF is similar, but with 500mb 
heights only rising to about 573dm, perhaps the most we can expect 
is for marine layer clouds to break up a little faster in the 
afternoons--and just for a day or two. It is interesting to note 
that 500mb heights are 100m higher today then they are forecast to 
be under that weak ridging late in the week, so today might very 
well be the last warmish day for awhile.


.AVIATION...Onshore flow will continue to increase tonight as an 
upper level trough approaches the region from the northwest. As of
8 PM, satellite imagery indicates that stratus has started to 
move inland along the coast and down the Strait. A thicker marine
layer will move inland this evening, resulting in mostly MVFR to 
IFR ceilings across most western Washington terminals by Tuesday 
morning. Current conditions for Puget Sound terminals remain VFR 
at this hour, however HQM and CLM ceilings have already dropped to
MVFR and IFR. The thicker marine layer should produce some spots 
of drizzle and light rain across the area during the morning 
hours. Ceilings will gradually improve Tuesday, however at a later
time than today. Flow aloft is west- northwesterly.

KSEA...VFR conditions are expected to continue through 07-08Z, 
then drop as stratus returns to the terminal. Current guidance
suggests that stratus will arrive by 09Z and will drop ceilings 
to low-end MVFR. Surface winds are currently southwesterly at 5 
to 10 knots. Winds could veer to northwest for a brief period 
this evening before backing to southwesterly again overnight.


.MARINE...High pressure over the coastal waters with lower
pressure inland will result in varying degrees of onshore flow 
throughout the week. Gale force westerlies will continue across
the central and eastern Strait of Juan de Fuca during the late 
afternoon and evening hours today and again on Tuesday, with 
small craft westerlies likely in the morning and early afternoon 
hours. Expect small craft strength winds to bleed into Admiralty 
Inlet and the Northern Inland waters during the evening hours for
the next couple of days as well. A small craft advisory has also
been issued for northwesterlies over the outer coastal waters for later
tonight and Tuesday as the onshore flow pattern increases ahead 
of an approaching trough. 27/14


.HYDROLOGY...While big precipitation producing storms 
occurred last fall and early winter, it has been much drier than 
normal since the start of 2019. The warmer, dryer spring has left 
the area with growing water supply issues. The snow pack was lower 
than normal in May and has since mostly melted out already. Most 
western Washington rivers are running below normal with a handful 
having daily record low flows. And for much of western Washington, 
the water supply forecasts are for below normal volumes, many in the 
5 percentile range of lower. Please see our Drought Information 
Statement, Water Supply Outlook, and Drought Information webpage at:


PZ...Small Craft Advisory from midnight tonight to 5 PM 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.

     Gale Watch from Tuesday afternoon through late Tuesday night for 
     Central U.S. Waters Strait Of Juan De Fuca-East Entrance 
     U.S. Waters Strait Of Juan De Fuca.

     Gale Warning until 5 AM PDT Tuesday for Central U.S. Waters 
     Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of 
     Juan De Fuca.

     Small Craft Advisory until 5 AM PDT Tuesday for Admiralty Inlet-
     Northern Inland Waters Including The San Juan Islands.