Area Forecast Discussion (AFD)
Issued by Seattle/Tacoma, WA (SEW)
000 FXUS66 KSEW 161204 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 404 AM PST Sat Feb 16 2019 .SYNOPSIS...An upper level trough over the area today will move south tonight and Sunday. Dry northerly flow aloft Sunday night and Monday with an upper level ridge nosing into British Columbia. The ridge will flatten Monday night with a system arriving from the northwest on Tuesday. Unsettled weather will continue through the end of the week with a break between systems on Thursday. && .SHORT TERM /TODAY THROUGH MONDAY/...Satellite imagery shows upper level trough over Western Washington early this morning. Doppler radar has a few showers over the area with the heaviest concentration over the central Puget Sound. Temperatures at 3 am/11z were in the 30s. Upper level trough lingering over the area today keeping showers in the forecast. The more organized bands of showers will remain offshore and to the south with the trough slowly drifting south during the day. Snow levels will be low this morning, 500 to 1000 feet for the most part from the Puget sound eastward, so some of the higher hills could see some flakes in the air. No snow accumulations in the lowlands expected. As the trough digs south this afternoon the snow levels will rise up into the 1000-2000 foot range. Highs today will be in the upper 30s to mid 40s. Chance of showers lingering into the evening hours with the trough still in the vicinity. Shower chances decreasing rapidly after 06z as the trough southerly movement increases. Fraser river outflow creeping back into the picture with the Bellingham to Williams Lake gradient down to -13 to -16 mb by 12z Sunday. Lows will be in the upper 20s and 30s. Sunday looking like a dry day at this point. Fraser river outflow pretty much confined to Whatcom Skagit and San Juan county. For the remainder of the area surface gradients fairly light. Although the upper levels of the air mass will be drying out with the northerly flow aloft, light gradients and plenty of low level moisture stratus a good possibility at least through Sunday morning. Will word the forecast partly sunny. Highs will mostly be a couple of degrees either side of 40. Dry weather continuing Sunday night into Monday with an upper level ridge nosing into British Columbia keeping the flow aloft northerly over Western Washington. Northerly surface gradients increasing over the entire area which will help dry out the lower layers of the air mass resulting in sunny skies on Monday. Windy conditions in the north but the models have the Bellingham to Williams Lake gradient peaking around -16 mb. Will keep the winds just below advisory, in the 20 to 30 mph range with gusts to 40 mph. Highs once again will be a couple of degrees either side of 40. .LONG TERM /MONDAY NIGHT THROUGH FRIDAY/...Models very inconsistent early on in the extended with the GFS bringing a cool upper level low down over the area Monday night while the ECMWF does not have this feature at all. Snow levels will be low with model 1000-850 mb thickness values on the GFS below 1300 meters, cold enough for snow. With the ECMWf showing nothing Monday night will go with just a chance pops forecast with low, 500 feet or less snow levels. Better consistency with a shortwave moving down from the northwest later Tuesday into Tuesday night. This system does not have a lot of moisture with it but once again snow levels will be low, 500 feet or less, so there is a chance of some light accumulations Tuesday night. Upper level trough behind the shortwave moving through Wednesday into Wednesday night keeping low snow levels in the forecast. Drying trend on Thursday with the next shortwave embedded in the northwesterly flow aloft a little slower on the 00z models, arriving Friday night into Saturday. Will lower the pops out of the likely category and go for just a chance of rain or snow on Friday. Snow levels will still be low on Friday and Friday night so there is another chance for some light accumulations especially above 500 feet. Highs through the period well below normal, upper 30s and lower 40s with lows below freezing most of the mornings. Felton && .AVIATION...South to southwesterly flow aloft will continue through today as an upper level trough lingers over the area. Residual moisture will keep spotty showers through today, though not everywhere will see precipitation, best chance of light rain will be during the afternoon and evening hours. Ceilings will continue to fluctuate between VFR and MVFR through the morning with mainly MVFR cigs by afternoon. Surface winds will mainly be light to 10 knots through today. KSEA...VFR cigs currently are expected to drop to MVFR by late morning. A few spotty showers around this morning with light rain possible during the afternoon. Fluctuation between VFR and MVFR cigs expected Saturday night into Sunday morning. Winds generally under 10 kts. JD && .MARINE...A 1010 mb surface low will consolidate off the north Washington coast later today, then drift southward. High pressure will build to nearly 1040 mb over southern British Columbia late tonight and Sunday. Small craft winds over the coastal waters in addition to westerly swell of 12 to 14 feet at 14 seconds will support keeping the small craft in effect there through tonight. Elsewhere, winds have eased between systems and should remain 20 kt or less. Fraser outflow will increase late tonight and Sunday across the northern inland waters with BLI-YWL gradient peaking near -15 to -17 mb Sunday morning. This will support low-end gales over the northern inland waters, with a Gale Watch now in effect there late tonight through Sunday evening. && .HYDROLOGY...Precipitation that falls through Saturday night will mainly be in the form of rain, or a rain snow mix. The precipitation, combined with all the snow on the ground that will be melting over the same period, will result in some ponding water on roadways and yards, especially with snow and ice plugging storm drains. In addition, melting snow is efficient at saturating the ground and is increasing the threat of landslides more than the rainfall-based indices would suggest. The SPS for the landslide threat will be continued through Saturday. As far as river flooding, none is expected for the next 7 days. && .SEW WATCHES/WARNINGS/ADVISORIES... WA...None. PZ...Small Craft Advisory until 4 AM PST Sunday for Coastal Waters From Cape Flattery To James Island 10 To 60 Nm-Coastal Waters From Cape Flattery To James Island Out 10 Nm-Coastal Waters From James Island To Point Grenville 10 To 60 Nm- Coastal Waters From James Island To Point Grenville Out 10 Nm-Coastal Waters From Point Grenville To Cape Shoalwater 10 To 60 Nm-Coastal Waters From Point Grenville To Cape Shoalwater Out 10 Nm. Small Craft Advisory for rough bar until 4 AM PST Sunday for Grays Harbor Bar. Gale Watch from late tonight through Sunday evening for Northern Inland Waters Including The San Juan Islands. && $$ www.weather.gov/seattle