Preview of NWS' New Version of Forecast
This preview is not operational and should not be used for support decisions.

Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 241601

Area Forecast Discussion
National Weather Service Seattle WA
900 AM PDT Wed Apr 24 2019

.SYNOPSIS...A weak upper ridge will keep the weather dry today and 
Thursday. An upper trough will bring a chance of showers and cooler 
weather to the region by the weekend. 


.SHORT TERM /TODAY THROUGH FRIDAY/...A broad upper ridge will bring 
dry weather today and Thursday. Morning clouds are dissipating with 
the southwest interior mostly sunny already. The afternoon will be 
mostly sunny everywhere. Mid and high clouds will increase Thursday 
giving a mostly cloudy day. Highs will be slightly above normal 
today and Thursday--upper 50s to mid 60s today, and 60s Thursday.

A weak upper trough will cross the area Thursday night and Friday. 
While it looks mostly dry, a stray shower cannot be ruled out, 
especially in the mountains. Low level onshore flow will increase on 
Friday and highs will cool a few degrees. Schneider/Burke

.LONG TERM /SATURDAY THROUGH TUESDAY/...Previous discussion...In 
general, the region will be under the influence of an upper trough 
through the period for a chance of showers a times. The best chance 
for showers is probably Saturday when an upper short wave crosses 
the area. This will also probably be the coolest day with highs a 
few degrees below normal. The flow aloft will gradually turn more 
northerly the first half of next week with shower chances dwindling. 
Highs should also inch up to near or slightly above normal early next
week. Schneider


.AVIATION...A weak ridge of high pressure aloft over the offshore
waters will give moderate westerly flow aloft through Thursday.
High pressure extending from the central interior of British
Columbia into the offshore waters will give low level NW to N flow
at lower levels. The air mass is drying and stable. Some mid and
high level moisture will spread into the area from the west on

Visible satellite imagery shows stratus across most of
the interior with bases 4000-5000 ft. An area of lower ceilings at
around 2000 ft is seen across the central portions of the Puget 
Sound southward to around Olympia, but the lower clouds are
rapidly eroding. Expect the low level moisture to dissipate
shortly after 18Z leaving VFR conditions across the region this
afternoon through Thursday morning. Albrecht

KSEA...Expect sct-bkn025 bkn035 at about 18Z to quickly erode by
21Z with good VFR conditions this afternoon through Thursday
morning. Some high clouds will increase midday Thursday. Light
winds will become northerly 7-11 kt this afternoon through
tonight. Albrecht


.MARINE...High pressure extending from the interior of central
British Columbia southwestward into the offshore waters will give
northwest to north flow over the waters this afternoon and
tonight. West to northwest onshore flow will increase Thursday
afternoon as pressures fall east of the Cascades. Small craft
advisory conditions are expected Thursday afternoon and evening
over the coastal waters and through the eastern two thirds of the
Strait of Juan de Fuca. Mesoscale models appear to be
overforecasting the strength of northwesterly winds through the
Strait of Georgia getting into the northern inland waters north of
the San Juan Islands. Will maintain forecast winds at 20 kt or
less in this area through Thursday night and early Friday.

Expect onshore flow to significantly increase later Friday as a
trough of low pressure aloft moves by to the north of the waters.
Gale force winds are indicated for the eastern two thirds of the
strait Friday afternoon and night with small craft advisory
conditions across the remainder of the waters. 

Onshore flow will diminish Saturday then possibly become offshore
late in the weekend as a cold front moves into the area from the
north. Albrecht


.HYDROLOGY...The daily hydrology discussion has ended until the
start of the next rainy season; it will only be updated as needed
until then.