WEATHER SUMMARY AND FORECAST DETAILS
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FROM MEDFORD NWS 8/11
This is the Fire Weather Discussion. It is going to focus on the elements of greatest interest to us all.
The two main changes this morning are a continued model trend of reducing the probability of thunderstorms on Wednesday and Thursday, and a increasing confidence in a slight chance of afternoon thunderstorms on Friday and Saturday (especially Friday) that is sufficient to declare the highest probability (15-20%) will be for northern portions of Klamath and Lake counties, secondarily (15%) for western Siskiyou County, and lesser chances (10-15%) connecting those areas...across Jackson, far eastern Douglas, and far western Klamath counties.
Main message remains the same -- we expect dry conditions through Wednesday, then slight chances of thunderstorms/lightning in NorCal Thursday. These chances expand up the Cascades and over the East Side Fri/Sat. Latest guidance has diminished the overall risk/coverage area compared to yesterday, so confidence still remains low, especially with the specifics of where activity will be focused each day.
We'll stay in a pattern of very warm, very dry, and breezy conditions through Wednesday. This pattern includes overnight breezy east to northeast winds at ridge tops of SW Oregon and NW California (most notably the Coast Range and Kalmiopsis Wilderness) along with poor to moderate RH recovery tonight/Tuesday morning, trending to improved but still moderate recovery in subsequent mornings. A channeling of winds down the Chetco could result in some drier/warmer conditions along the SW Oregon coast as well (though the marine influence has held so far at the beaches).
The trend through midweek will be to lower daytime MaxTs down by a degree or two each day, and this should subsequently allow daytime MinRHs to be slightly higher. Even so, it'll be dry and unstable, so any fires on the landscape will still have the ability to grow significantly, especially during the peak burning periods during the afternoons/evenings. Overnight RH recoveries haven't been good the last couple of nights, generally only in the 20-35% range for some of the upper slopes and ridges. There should be slight nightly improvement in recoveries from NW to SE through the period. Things trend toward normal after Wednesday, but temperatures still probably remain a few degrees above normal for most locations.
Models and ensemble guidance continue to show a weakness or upper trough developing near the California coast Wed/Thu and also show an increase in monsoonal moisture coming up from the south late week. The best chance for isolated thunderstorms is Fri/Sat from NorCal up the Cascades and across portions of the East Side. Given plenty of uncertainty in strength of forcing mechanisms, overall instability and moisture advection, confidence is low in the locations where activity will be focused. Keep checking back for updates since any lightning brings with it the risk of new fire starts.
