Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

Current Version | Previous Version | Text Only | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

804
FXUS66 KSEW 182139
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
239 PM PDT MON JUL 18 2016

.SYNOPSIS...An upper low will produce scattered showers through
Tuesday night, with isolated thunderstorms over the Cascades in
the afternoon and evening hours. The low will weaken as it moves
slowly inland Wednesday, and the threat of showers will decrease
as a weaker trough sits well offshore. A weak upper ridge should
bring dry and warmer weather this weekend.

&&

.SHORT TERM...A 5680 meter upper low remains over the Oregon
offshore waters this afternoon, centered near 48N 128W. Weakly
diffluent southerly flow aloft continues over Western Washington,
and the air mass is unstable and moist below 10000 ft. Scattered
showers continue to move northward across the area, especially
over the Cascades and the adjacent lowlands. A few of the cells
are still developing into isolated thunderstorms with the radar
showing reflectivity up to around 55 dBZ. These conditions should
change little through this evening, and then there will be diurnal
stabilization of the air mass overnight.

The upper low will move slowly northeast Tuesday, reaching the
Washington coastal waters late Tuesday night. It will keep a
chance of showers across the forecast area, and we have decided to
add a slight chance of thunderstorms to the Cascades for the
afternoon and evening hours. On the other hand, the combination of
weaker onshore flow and the instability should bring partly sunny
skies to much of the interior lowlands in the afternoon.

The remains of the low will weaken and move inland Wednesday,
leaving a broader upper trough offshore out near 130W. It will
maintain somewhat unstable southwest flow aloft over the forecast
area, although the air mass should be less moist during the
afternoon. The threat of showers on Wednesday should be limited to
the coast, northern lowlands, and mountains. And again, the
morning clouds should give way to partly sunny skies over parts of
the interior lowlands during the afternoon. Highs will again range
from the mid 60s at the coast to the mid 70s inland.

The offshore upper trough will progress slowly eastward Wednesday
night and Thursday, probably moving into Western Washington and
Oregon Thursday evening. The chance of showers looks limited to
parts of the Olympics and Cascades, with partly to mostly sunny
skies elsewhere. McDonnal

.LONG TERM...A rather flat upper ridge will begin building
offshore on Friday. It looks like a final weak shortwave will move
through Western Washington, weakening though as 500 mb heights
rise to around 5820. It should be a dry day, with morning marine
stratus mainly at the coast. Upper heights will rise over the
area on Saturday, then fall a bit Sunday as an upper trough digs
into British Columbia and Monday as it moves into the Canadian and
Northern Rocky Mountains. The weather is likely to remain dry as
the trough moves by. For now we`re forecasting minimal effects
associated with it, but it could trigger a marine push Sunday
night or Monday night. McDonnal

&&

.AVIATION...A broad upper low over the coastal waters will remain
stationary through Tuesday giving light southerly flow aloft. A
ridge of high pressure extending northward from the Oregon coastal
waters along the Washington coastline will give low level south to
southwest flow. The air mass will be moist through Tuesday. The
lower levels will be mainly stable with a marine layer while
conditions in the mid and upper levels will be mostly unstable
through Tuesday.

Showers and isolated thunderstorms are moving northward through
the Cascades and the eastern Puget Sound lowlands. There appears
to be a decreasing chance that the shower activity will shift
westward as far as the waters of Puget sound this evening.
Low level moisture has been slow to scatter out over the Puget
Sound lowlands this afternoon, but the lower stratus should
scatter out by 00Z this evening. Stratus will likely redevelop in
the marine layer overnight with ceilings most locations falling to
around OVC020. Lower stratus will likely scatter out around 20Z
Tuesday. Albrecht

KSEA...Expect the cigs to rise to 035-050 about 00Z this afternoon
then to fall to OVC015 about 10Z tonight. It appears that most
shower activity will remain east of the terminal. Isolated TSTMS
appear that they will remain more than 15 SM east of the terminal.
Winds will be SW to S 5-7 KT. Albrecht

&&

.MARINE...A weak ridge of high pressure extends from the Oregon
coastal waters northward into the Washington coastal waters near
the immediate coastline. This will result in southerly flow over
the coastal waters and south to southwest flow inland. Westerly
inflow through the Strait of Juan de Fuca will remain at or above
small craft advisory criteria through late tonight. Small craft
advisory conditions are also likely for a small portion of the
Northern Inland waters south of the San Juans and at the southern
approaches to Haro Strait and Rosario Strait this evening.
Onshore flow will weaken Tuesday and Wednesday.

The surface ridge will move into the offshore waters later Wednesday
through Friday resulting in a more west to northwest flow over the
coastal waters. Expect daily inflow through the Strait of Juan de
Fuca resulting in typical late afternoon and overnight small craft
advisory conditions Thursday through Saturday. Albrecht

&&

.SEW Watches/Warnings/Advisories...
WA...None.
PZ...Small Craft Advisory 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 11 PM PDT this evening for Northern
     Inland Waters Including The San Juan Islands.

&&

$$

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.weather.gov/seattle/gafd/latest_webafd.html



USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.