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fxus66 ksew 192356 
afdsew 


Area forecast discussion 
National Weather Service Seattle Washington 
330 PM PST Mon Feb 19 2018 


Synopsis...dry and cold northerly flow aloft will prevail over 
the region through tonight. A weak disturbance will bring some light 
snow to the southwest part of the state Tuesday and Tuesday evening. 
Another system will move southward off the coast late Wednesday into 
Thursday with a slight chance of snow showers. Another stronger 
front will bring rain or snow to the region Friday into the weekend. 


&& 


Short term...infrared satellite imagery shows a batch of high clouds 
racing southward along the b.C. Coastline toward Washington. These clouds 
should pass the area by later this evening giving a window of 
partial clear skies later tonight and early Tuesday. Low 
temperatures should fall into the low to mid 20s across The 
Lowlands, with mid/upper 20s right along the water. Colder valleys 
or low lying areas could reach the upper teens. If clouds linger 
over the area later than expected, then forecast lows could end up a 
few degrees too cold. 


An upper level trough embedded in northerly flow will split as it 
drives southward off the Washington coast Tuesday. Most models keep 
the bulk of precipitation offshore with the stronger trough and 
associated surface low just inside 130 west. Some models show a 
secondary trough moving south out of b.C. Over the area by late 
afternoon. There could be just enough moisture to generate some 
spotty light snow or flurries from around King County southward. 
Accumulating snow will probably stay confined to the southwest 
interior and possibly the central coast. The 18z NAM-12 is the most 
significant outlier of all the models. It holds back surface low 
pressure along the South Washington coast into Tuesday evening with a band 
significant precipitation over Puget Sound and southward. Quantitative precipitation forecast 
amounts over a half inch liquid seem way overdone with the cold dry 
air mass currently in place. The Canadian is the other model 
spreading moisture much further north with light snow accumulation 
as far north as Seattle. All other models including the ecwmf and 
WRF keep most snowfall south of around Tacoma. Ensemble means 
indicate around an inch or locally 2 inches over the southwest 
interior from Grays Harbor inland to Thurston and Lewis counties. 
Since most snow will fall during the afternoon and early evening 
will hold of on issuing an advisory. 00z model guidance might agree 
better on timing, location, and amounts of snowfall. 


Another weak system will dig southward late Wednesday into early 
Thursday on a similar trajectory as tuesday's system. Models 
indicate less moisture and precipitation making it into western 
Washington. Flurries or some spotty light snow is possible but 
little or no accumulation expected. Temperatures will remain cold, 
with highs struggling to reach 40 and lows still in the 20s. 


Long term...most models now advertise a strong front arriving from 
the north on Friday. The flow will go onshore with southerly flow 
increasing. Models also show cold air aloft driving southward which 
may keep snow levels rather low. At sea-level, a rain/snow mix is 
possible and hilltops could potentially see some snow accumulation. 
This is a ways out, so stay tuned. 


A cold and showery air mass holds over the area into the weekend 
with some models showing additional shots of cold air and enhanced 
precipitation bands. The mountains should get appreciable amounts of 
snow in this pattern. Snow levels will stay low, possibly not far 
above sea-level over the weekend. Mercer 


&& 


Aviation...northerly flow aloft will continue tonight and Tuesday. 
At the surface, northerly and offshore gradients will continue to 
ease this evening and turn southerly later tonight. A low center 
will move south well off the coast on Tuesday. The air mass is 
stable and dry except for some high clouds and a cumulus field over 
the southwest interior. The cumulus will evaporate after sunset but 
the high clouds will increase this evening, then probably diminish 
late tonight. 


VFR conditions will prevail through tonight. On Tuesday afternoon, 
as the low center moves south off the coast, some MVFR ceilings 2-3k 
ft are possible over the southwest interior, along with a chance of 
snow. Elsewhere there may be ceilings, but they will be VFR and 
conditions should remain dry. 


Ksea...VFR with no ceiling or a cirrus ceiling. Northerly wind 10-12 
kt will fall below 10 kt by 03z and become light southerly late 
tonight. Some mid and high level cloudiness is likely Tuesday 
afternoon. Chb 


&& 


Marine...a low center will move southeast well off the coast on 
Tuesday. Southeast winds of Small Craft Advisory strength are likely 
on the coast Tuesday and a Small Craft Advisory is now in effect. 
After the low moves south, some 20 kt northeast winds are likely 
over the inland waters, and some advisories may be necessary then. 


A weaker system is forecast to move southeast through western 
Washington on Wednesday night or so, but this is not forecast to 
bring any advisory level winds. A stronger system will arrive 
Friday. This system is likely to bring at least Small Craft Advisory 
strength winds to all waters, and could bring gale southerlies or 
westerlies to some waters. Chb 


&& 


Hydrology...river flooding is not expected the next 7 days. 


&& 


Sew watches/warnings/advisories... 
Washington...none. 
Pz...Small Craft Advisory from 4 am to 4 PM PST Tuesday 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. 






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