textproduct: Seattle/Tacoma

SYNOPSIS

Upper level trough moving through the area today. Upper level low moving south over British Columbia today and tonight will veer southwest off Vancouver Island Wednesday. The low will drift south Wednesday night and Thursday ending up off the Washington coast Thursday night. The low will remain off the coast Friday then weaken and move inland Saturday. Upper level ridge building offshore Sunday moving over Western Washington Monday.

SHORT TERM /TODAY THROUGH THURSDAY/

If we are going to be dealing with slow moving upper level lows into the first part of next year it is going to be a long forecasting season.

Satellite imagery shows small circulation just off the North Oregon coast early this morning. Doppler radar has showers spinning around the low over the Central and Southern Coast spreading into the Southwest Interior. With the cloud cover temperatures in a narrow range at 3 AM/10Z, mid 40s to lower 50s.

Small upper level low opening up into a trough and moving mostly into Northwest Oregon this morning with the highest pops over the southern half of the area. Upper level low over Northern British Columbia this morning moving south during the day. Vort lobe spinning out of this feature over mainly the northern portion of the area this afternoon producing more showers. Snow levels in the 4500 to 5000 foot range confining snow showers to the higher elevations. Highs in the lower to mid 50s.

Upper level low to the north moving over Vancouver Island by late tonight. Showers rotating around the low moving through the area overnight. Air mass cooling with snow levels lowering to 3500 to 4000 feet. Up to 4 inches of new snow in the Cascades overnight. Higher amounts possible from around Mount Rainier southward. Lower snow totals in the Olympics, less than 2 inches. Lows in the upper 30s to mid 40s.

Upper level low not moving much Wednesday with showers rotating around the low continuing to move through Western Washington. Snow levels lowering slightly to 3000 to 3500 feet. Up to another 4 inches of new snow in the Cascades with slightly higher amounts possible again Mount Rainier southward. Highs in the lower to mid 50s. Winter weather advisory for the Cascades tonight and Wednesday looks good and will be continued with this forecast package.

Little change in the forecast Wednesday night into Thursday as the upper level low hangs out off the Washington coast. Flow aloft becoming southwesterly ( versus westerly tonight and Wednesday ) as the low drifts slowly south. This will reduce the orographics over the mountains lowering the precipitation amounts. The air mass will still cool with snow levels 2500 to 3500 feet so most of the passes will continue to see light snow with another couple of inches of new snow. Lows mid 30s to lower 40s. Highs in the lower to mid 50s.

LONG TERM /THURSDAY NIGHT THROUGH MONDAY/

Upper level low hanging out off the Washington coast through Friday night will finally weaken and shift inland Saturday. Pops slowly decreasing Friday and Friday night with just chance to slight chance pops by Saturday. Snow levels remaining in the 2500 to 3500 foot range but precipitation amounts will only produce another couple inches of new snow.

Upper level ridge building offshore Sunday moving inland Monday. Atmospheric river on the backside of the ridge aimed at the Central British Columbia coast. As the ridge moves inland warm front may nick the coast Monday night.

Highs in the lower to mid 50s warming into the mid and upper 50s Monday. Lows in the mid 30s to lower 40s dropping to the upper 20s and 30s Monday. Felton

AVIATION

Northwesterly flow aloft as upper level troughiness remains in place over the Pac NW. Surface winds mostly southwesterly with speeds less than 5 kts with some areas of light and variable. More pronounced southwesterlies expected by late morning/early afternoon for all terminals with speeds generally increasing to 5-10 kts with some terminals possibly seeing wind gusts up to 20 kts.

Wide selection of cigs in place this morning. Majority of obs show VFR conditions, however some terminals in the south Sound area, including Sea-Tac, are reporting MVFR to IFR conditions. Cigs will continue to lower throughout the early morning with widespread MVFR to IFR conditions expected by 12-14Z. Cigs begin to recover by late morning/early afternoon with generally VFR conditions returning, save for those terminals that favor lower cigs, where clouds may only lift to high-end MVFR. Showers will remain a contributing factor to lower cigs throughout the day, with scattered activity expected in the morning becoming more widespread this evening. Will begin to reflect increased chances of precip during this time frame with 12Z issuance. As rainfall becomes more prominent, widespread MVFR conditions expected to return by tonight.

KSEA...Conditions continue to deteriorate this morning with latest special ob reporting high-end IFR conditions as opposed to model- advertised low-end MVFR. Will likely favor the latter as the dominant condition for 12Z update with TEMPO group reflecting the former. As southwesterly winds increase to 4-8 kts by 12-14Z, this should help cigs to lift a little bit more, nudging upward into low to mid MVFR. As speeds reach their peak this afternoon and early evening, generally ranging 8-12 kts with gusts up to 20 possible, should see cigs return to low-end VFR. As rain becomes more widespread tonight, cigs will droop again back to MVFR as wind speeds lower slightly to 5-10 kts.

18

MARINE

Onshore flow returns to the area waters today. Northwesterly winds along the outer coastal waters will become breezy this morning and linger into Wednesday, with wind gusts approaching 25 kt at times. That said, inherited SCA looks good and will remain in place. Another frontal system will move into the waters tonight into Wednesday, increasing winds and seas across the area. Increasing onshore flow will bring a brief push of small craft westerly winds to the central and eastern Strait late this afternoon into this evening. New SCA will cover this. Southerly winds will also pick up along the Puget Sound waters tonight, with latest guidance highlighting a 60-80% chance of gusts exceeding small craft criteria. Even though the time frame for these gusts seems to be pretty limited, will opt to cover with an additional headline here as well.

Winds will ease again across the area waters on Wednesday as the accompanying surface low drops into the area and tracks southward along the coastal waters. This low will allow for flow to turn back offshore, with offshore flow persisting the second part of the week.

Seas 4-6 ft this morning will increase to 6-8 ft by this afternoon and will persist at this height into Wednesday. Conditions slowly subsiding back to 4-6 ft on Thursday and remain that way into the weekend.

18/14

SEW WATCHES/WARNINGS/ADVISORIES

WA...Winter Weather Advisory from 5 PM this afternoon to 5 PM PDT Wednesday for Cascades of Pierce and Lewis Counties- Cascades of Snohomish and Northern King Counties-Cascades of Southern King County-Cascades of Whatcom and Skagit Counties.

PZ...Small Craft Advisory until 5 AM PDT Wednesday 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.