textproduct: Seattle/Tacoma

SYNOPSIS

Upper level ridge over the area today will slide off to the east tonight and Thursday. Weakening front arriving late Thursday into Friday morning. Developing upper level trough offshore Friday afternoon. System spinning out of the trough moving through Saturday into Sunday. Upper level trough moving across area Monday. Upper level ridge trying to build Tuesday with warm front moving into British Columbia.

SHORT TERM /TODAY THROUGH FRIDAY/

Fog continues this morning around southern and central Puget Sound, with stratus into the Strait of Juan de Fuca. Fog will likely linger into the afternoon around Olympia towards Seattle due to light surface flow. Otherwise, a mix of clouds and sunshine elsewhere. No major forecast updates this morning, previous discussion below.

Satellite imagery shows high clouds embedded in southwesterly flow aloft over the northwest portion of the area. Dense fog formed late last night/early this morning along the I-5 corridor from the King/Snohomish county line down to south of Chehalis and westward through the Lower Chehalis Valley to Grays Harbor. Visibilities a quarter of a mile or less common in these areas. In addition to the fog a stratus layer has set up along the Strait of Juan de Fuca. Temperatures at 3 am/11z range from the upper 20s to mid 30s.

Have issued a dense fog advisory for the I-5 corridor from Lewis county through Seattle with widespread visibilities a quarter of a mile of less until 10 am. Some temperatures are below freezing in this area bringing up the possibility of a thin sheet of black ice on the roads. Lets be careful out there. Models have pretty sharp but shallow inversion over the area this morning. Tops around 500 feet ( Top of Columbia Center probably sticking out of the cloud tops for you early morning photographers ). Cross Cascade gradient around -6 mb is keeping the fog out of the Cascade foothills and the gradient will stay near -6 mb through the morning hours. With the fog layer so shallow some light slightly drying easterlies will help dissipate the layer. On the flip side the high clouds could slow down the breakout but at this point the high clouds look to be moving mostly north of the foggy areas. End result midday breakout with just some high clouds this afternoon. The exception to this will be along the Strait where the stratus will hang on a little longer. Highs in the mid 40s to lower 50s.

Upper level ridge over the area today shifting east tonight. System embedded in the southwesterly flow aloft still well south of Western Washington by 12z Thursday. High clouds spreading over the southern portion of the area overnight but likely not in time to prevent at least some areas of fog to form again along the I-5 corridor south of Seattle. It will be another cool night with lows in the upper 20s to mid 30s.

Increasing middle and high clouds Thursday as front continues to move north through Western Oregon. Could be a good morning for lenticulars off of Rainier. Rain out ahead of the front still mainly south of Lewis county by late Thursday afternoon with just a chance of rain from about Seattle south in the afternoon. Increasing clouds will keep highs in the mid 40s.

What is left of the front moving into Western Washington Friday with rain spreading over the area Thursday night. System weakening with rainfall amounts for Thursday night into Friday mostly in the tenth to a quarter of an inch range. Rain decreasing Friday afternoon. Cloud cover keeping lows near 40 Friday morning with highs Friday near 50.

LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/

Some timing differences in the models with regard to when the rain out ahead of the next system spinning out of the upper level trough offshore will reach the area. The ECMWF is faster then the GFS on the operational runs. Ensemble solutions leaning toward the faster solution so will have rain likely in the forecast for Friday night. With the system moving up from the southwest away from the parent low it will be a slow mover keeping rain in the forecast for Saturday and Saturday night.

Next forecast problem is where will the surface low associated with the front end up. The GFS tracks the low well offshore while the ECMWF brings the low up the coastline Saturday night into Sunday morning ( where have we seen this before? ). The ECMWF does have the low weakening as it moves inland which helps to limit the wind impact. Still could be a little windy Sunday morning as the rain comes to an end with the frontal passage. Snow levels with the south southwesterly flow aloft starting out above pass levels Saturday lowering down to around 3500 feet Sunday. South southwesterly flow aloft not conducive to decent snow amounts in the mountains.

Confidence low for the last two days of the extended with a lot of variability in the solutions. The ECMWF has the upper level trough moving through with an upper level ridge trying to build over the area. The GFS has the trough retrograding back to the southwest. For now will go with a broadbrush mostly cloudy chance of rain forecast until there is more clarity in the models. Felton

AVIATION

An upper level ridge will continue to weaken over the region today with light southwest flow aloft. Winds remain mostly calm through the interior with light offshore flow being the overarching flow across western Washington and will continue to be through the TAF period. A strong surface inversion has allowed for fog and freezing fog to persist across much of the Puget Sound lowlands into the Southwest Interior. Conditions are already starting to slowly improve in areas along the edges where the fog is most shallow, but in the central Sound where the fog layer is the thickest, this area will have the toughest time clearing out with tops around 700 ft or so. Guidance is hinting at a period of improvement from around 21-03Z this afternoon in visibility but ceilings may not fully scatter out. The pattern looks to continue into tonight and early Thursday morning with more widespread fog and LIFR cigs. An earlier scatter- out time is possible on Thursday with increasing surface winds and the inversion beginning to weak as a weak disturbance begins to approach the area by Friday.

KSEA...LIFR conditions continue this morning with dense fog and limited vertical visibility as well. Visibility should gradually improve by around 21Z this morning with ceilings lifting slightly, but a full break-out of LIFR cigs seems less likely. LIFR conditions redevelop with fog tonight through Thursday morning. Winds mostly calm or light and variable, or SE less than 5 kt. 62

MARINE

High pressure will weaken over the waters today. A system moving up from the south will reach the area later Thursday into Thursday night. A stronger frontal system will move through the waters Saturday into Sunday.

Dense fog continues for portions of the interior waters this morning. A Dense Fog Advisory has been issued for Puget Sound through early afternoon. Winds increasing with system Thursday look to stay under small craft advisory criteria. Small craft advisory winds over the outer coastal waters with the next system later Friday into at least Saturday. Small craft advisory winds in the outer coastal waters could last into Sunday before easing.

Seas in the 5 to 8 foot range building Friday night with seas in the 10 to 14 foot range over the weekend. Higher seas over the outer coastal waters. JD/Felton

HYDROLOGY

No river flooding expected in the next 7 days. The Skokomish River will rise back into action stage over the weekend.

SEW WATCHES/WARNINGS/ADVISORIES

WA...Dense Fog Advisory until 10 AM PST this morning for City of Seattle-Eastside-Lowlands of Lewis and Southern Thurston Counties-Lowlands of Pierce and Southern King Counties- Middle Chehalis River Valley-Olympia and Southern Puget Sound-Shoreline / Lynnwood / South Everett Area.

PZ...Dense Fog Advisory until 1 PM PST this afternoon for Puget Sound and Hood Canal.