AVIATION
16/00Z TAFS...Conditions are mainly VFR early this evening, but with numerous showers and isolated thunderstorms. The thunderstorm activity will remain mainly over northern California this evening, before tapering off. Meantime, broader coverage of showers is mainly from the southern Oregon Cascades eastward, and across northern California. Showers will persist through Monday afternoon, while slowly shifting eastward. Mountain obscuration and MVFR ceilings are possible in the vicinity of showers, but conditions will remain mainly VFR through Monday afternoon.

Also, low stratus with MVFR ceilings are expected along the coast and into the lower Umpqua Basin around 08-18Z overnight into Monday morning. -DW

MARINE
Updated 800 PM Sunday, September 15, 2024...North winds will continue to strengthen tonight into Monday as low pressure gradually moves inland. North winds and steep to very steep seas will continue through Monday night, but an incoming front will turn winds to south and allow seas to diminish Tuesday.

The front will move into the region on Tuesday bringing widespread precipitation and lowering visibility. Moderate to occasionally strong north winds and steep seas return late Tuesday into Wednesday, likely bringing the return of conditions hazardous to small craft. -Wright



PREV DISCUSSION
/Issued 224 PM PDT Sun Sep 15 2024/

DISCUSSION...The first of two deep upper level troughs is affecting the region this afternoon, resulting in late October like weather. Temperatures are running about 5 to 10 degrees cooler than this time yesterday for areas west of the Cascades.
Meanwhile, east of the Cascades, temperatures are 10 to as much as 20 degrees cooler than this time yesterday. This cooler weather will continue through mid-week before temperatures rebound to values more typical for late September.

The best forcing for showers and thunderstorms today will extend from NorCal northeastward across SE Klamath and into Lake County, and this will be the focus area for wetting rainfall. There have been a handful of lightning strikes already across northern California and surface observations indicate generally a few hundreths to a tenth of an inch has fallen out of these showers.
Under the heavier returns, however, upwards of 0.25" has been recorded. We do expect shower/thunderstorm activity to increase within the next hour or two, but focus will remain east of the Cascades today. Meanwhile, as the low continues moving southward today, there will be an area of subsidence and mid-level drying on the NW periphery of the low and this will result in a relative minimum in shower activity, which will be for areas on the west side of the Cascades. That's not to say there isn't a chance of showers over here (including the Rogue Valley), but they'll be much more hit- or-miss than they are over Siskiyou/Modoc up into southern Klamath/Lake counties and put down much less precipitation compared to the East Side.

This first trough will progress eastward tonight into Monday, and wrap around moisture under easterly flow will maintain precipitation across the far eastern portion of the area in Lake and Modoc Counties. Widespread wetting rain is likely east of the Cascades where amounts of 0.10-0.25 of an inch will be common, with up to 0.50 of an inch in the mountains where showers tend to focus. Shower chances will diminish west of the Cascades, and temperatures will trend around 5 degrees warmer for the day Monday.

There will be a relative lull in activity Monday night, but the next trough will move in rather quickly behind the departing one. By early Tuesday morning, precipitation is expected at the coast, then will spread inland through the day Tuesday. This low will move into NorCal Tuesday night and showers will overspread the remainder of the region, then settle south and eastward Wednesday into Thursday.
We'll see showers gradually diminish across the area from NW to SE during this time frame. This system will be a more typical wet system for this area with the bulk of precipitation expected along and west of the Cascades. Overall 0.50-0.75 of an inch is likely, perhaps around an inch in the coast ranges. A tenth to 0.25 of an inch appear to be the most likely amounts across the remainder of the area, though with a showery regime, some areas could have more or less.

Shower activity comes to an end by Wednesday night. There will likely be some lingering cloud cover, and maybe even some patchy west side valley fog Thursday morning, but this will give way to sunny skies for Thursday. A period of ridging is expected for the latter half of the week and into next weekend, with temperatures trending warmer, but still remaining a few degrees below normal.
It's likely to remain dry through the weekend and into early the following week. However, guidance does show another system moving over the top to our north during the weekend. Right now, it looks like mainly a dry trough/frontal passage Saturday or Saturday night.
Rain chances at this time are less than 10%. While this could change, as of now, it appears this system is much more likely to cause an increase in afternoon/evening breezes over the east side than anything else. -Spilde/BR-y

FIRE WEATHER
Issued 200 PM Sunday September 15, 2024
The upper trough is digging south with the axis of the upper trough offshore.
Radar image is showing showers starting to develop and moving northeast in southeast Siskiyou county into southeast Klamath and central Lake County. So far no lightning has been detected however some of the returns on radar show higher reflectivity. Instability will be greatest along and east of the Cascades and eastern Siskiyou and Modoc county through this evening, thus the best chance for thunderstorms will be in these areas. However storms will likely produce rain, some of which could be moderate to heavy at times.

The upper low will dig south of the area overnight Sunday into Monday, and there's good agreement we'll have wraparound moisture east of the Cascades and northern Cal through the day with wetting rains likely in these areas. Some of the convective allowing models (CAMS), paintball, and SPC HRRR suggest storms could persist into at least the first half of the night, and possibly into the overnight hours. Not as confident we'll get any storms overnight tonight, but at the same tine cannot be ruled out. It could be one of those situations where if storms were to persist into the overnight hours, it would be the exception and not the norm, and not much more than a few strikes. Even then they will be rain producers.

Monday, showers will remain persistent in the southeast part of the area as we get caught up in some wraparound moisture. Meanwhile showers will tend to be far and few in between west of the Cascades in Oregon, and it could end up dry for the entire day. Instability Monday is marginal at best, but there's some evidence suggesting it could be enough for at least a few isolated storms Monday afternoon and early evening in southeast Lake and Modoc County.

Despite the variation of model solutions from run to run, one thing has been consistent. There will be a brief break in the weather Monday evening into Tuesday morning before precipitation moves back in during the day Tuesday as another upper trough drops in from the northwest, and pretty much remain over the area through Wednesday.
This system will also be wetter, and the chance for wetting rain will be high for most locations.

The upper trough will evolve into an upper low as it digs south of the area Wednesday night and we'll begin to see precipitation diminish from north to south. Wednesday night.

Dry weather returns Thursday and is likely to last through next weekend, and possibly into the first half of next week, with warmer temperatures for the interior. A thermal trough will set up along the southwest Oregon coast resulting in breezy east to northeast winds near and at the ridges, but if we get the expected rainfall in this area today through Wednesday, it won't pose a concern.
-Petrucelli

MFR WATCHES/WARNINGS/ADVISORIES
OR
CA
None.

PACIFIC COASTAL WATERS...Small Craft Advisory until 5 AM PDT Tuesday for PZZ350-356-370-376.

Hazardous Seas Warning until 5 AM PDT Tuesday for PZZ356-376.