fxus66 ksew 201730 

Area forecast discussion 
National Weather Service Seattle Washington 
1030 am PDT Sat Oct 20 2018 

Synopsis...high pressure aloft will remain over the area through 
Monday. Expect a fairly strong temperature inversion to persist 
through at least Monday morning. This will keep cooler air trapped 
at the lower elevations while the mountains experience well above 
normal temperatures. A dissipating occluded front will bring some 
light rain to the area on Tuesday. A stronger system is expected 
on Thursday. 


Short term... 
a fairly strong temperature inversion was over the area this 
morning. Stratus and fog blanketed much of The Lowlands at this 
time, and there were areas of light drizzle. At this time, 
temperatures across The Lowlands were mainly in the 40s while the 
mountains were sunny and mild. For example, it was sunny and 67 
degrees at Paradise (on mount rainier) at 9 am PDT, this morning. 
Based on current trends, it looks like the clouds/fog across The 
Lowlands will be slow to clear out. It may not clear out at all in 
some areas. The forecast was updated earlier to add drizzle and 
to knock down temperatures over The Lowlands but will update 
again to keep clouds in longer. Will also bump up temps in the 

Meanwhile, an upper level ridge will remain over the Pacific 
northwest this weekend, resulting in a stable and slightly stagnant 
air mass over western Washington. Most air quality monitoring sites were 
in the good to moderate range this morning. The exception was 
Maple Falls, where it was reading unhealthy for sensitive groups. 
The degradation in air quality was being caused by the temperature 
inversion, which acts like a lid and keeps pollutants trapped 
near the surface. This also explained the hazy skies yesterday 
afternoon. It doesn't look like there will be much change on 
Sunday. Will knock down high temps for Sunday or go with the NAM 
guidance since it has been performing better than the other 

The ridge axis will shift east on Monday. There is a possibility 
that it may remain cloudy all day across much of The Lowlands on 
Monday, resulting in highs being similar to or slightly cooler 
than on Sunday. 

Long term from the previous discussion... 
00z extended model runs continuing the trend of a slower and 
weaker front with what is left of the system moving through 
western Washington Tuesday night. With the trend of slower 
eastward movement of the front have dry weather in the forecast 
Tuesday from about the Seattle area eastward and just chance pops 
over the area for Tuesday night. Some differences in the solutions 
start showing up on Wednesday with the GFS a few hours faster 
with the onset of precipitation ahead of the next approaching 
front. The GFS has precipitation into western Washington by 
Wednesday afternoon while the European model (ecmwf) gives the area a little longer 
break with the precipitation not starting until Wednesday night. 
The flow aloft is consolidated so can't rule out the GFS solution 
at this point. Wednesday forecast has a chance of rain. Stronger 
and wetter system arriving by Thursday with rain across the entire 
area. Still the possibility of some windy conditions along the 
coast Wednesday night into Thursday. Another system trying to get 
organized quickly behind the front for Friday. Main message of the 
extended is we are going back to our regular fall pattern next 
week. Felton 


Aviation...a flat upper ridge over the Pacific northwest will 
amplify over the region tonight and Sunday. Light flow aloft with 
low level north-northeast flow becoming increasingly offshore 
tonight and Sunday. The air mass will remain very stable, with 
dry air above a moist, shallow boundary layer. This setup will be 
conducive for development of late night and morning dense fog both 
this morning and Sunday morning. On both days given the time of 
year, IFR fog will probably be slow to mix out or burn off. Some fog 
prone locations may not scatter until late in the day. 

Ksea...very slow lifting of cigs from low IFR to IFR then expected 
in the 19z-21z time frame. North-northeast wind to 10 kt becoming 
north-northwesterly to 10 kt this afternoon. NE wind 5 to 10 kt after 
03z tonight with low IFR expected again late, after 10z Sunday 
morning. Dtm 


Marine...light to moderate northerly flow will becoming more 
easterly or offshore tonight and Sunday. 

An approaching front on Monday will turn the low level flow 
southeasterly. Advisory level winds are expected over the coastal 
waters for a time Tuesday, but the system is forecast to dissipate 
rapidly. A stronger system will arrive around Wednesday, with 
advisory level southerly wind expected to focus once again over the 
coastal waters. Dtm/smr 



Sew watches/warnings/advisories... 


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