929 FXUS66 KMFR 270517 AFDMFR Area Forecast Discussion National Weather Service Medford OR 1017 PM PDT Sat Jul 26 2025 .UPDATE...Updated Aviation Discussion. && .AVIATION...27/06Z TAFs...Along the coast, areas of IFR will develop tonight into Sunday morning, mainly north of Cape Blanco and from Gold Beach southward. These low clouds will clear to VFR late Sunday morning. Inland, overall, skies are VFR and will remain that way through the TAF period. However, isolated to scattered thunderstorms Sunday afternoon and evening, mainly from the Cascades east and across NorCal. These could temporarily lower ceilings and/or visibility, but the main risks with thunderstorms will be gusty outflow wind gusts, cloud to ground lightning and even small hail. && .PREV DISCUSSION... /Issued 200 PM PDT Sat Jul 26 2025/ DISCUSSION...The main weather impact over the next several days will be thunderstorms. With strong upper ridging over the southern United States (east of the Four Corners) and an anomalous closed low centered over the Gulf of Alaska, SW Oregon and NorCal are in an area of SW flow aloft. We are mainly under the influence of an upstream trough, and this is keeping temperatures at levels that are fairly typical of late July levels -- highs in the upper 80s to mid 90s for the valleys west of the Cascades and generally 80-90F over the East Side. It will remain that way from now through the end of next week. A weakness within the upper trough is traveling into the Great Basin this afternoon and this will result in another round of isolated to scattered storms, mostly for SE portions of the forecast area. The isolated activity focused around the Mt Shasta region with more scattered (or slightly higher coverage of storms) storms from the Sierra up into south-central Oregon. Storms that form this afternoon will be much like those from Thu/Fri, with wetting rainfall, strong, gusty outflow wind gusts and even some hail. Models have maintained S-SW flow over the area for Sunday, so thunder chances have increased compared to yesterday`s thinking. With a bit more moisture expected in the mid-levels and the presence of a subtle shortwave disturbance moving through at peak heating, the resultant instability should bring another round of isolated to scattered thunderstorms from NorCal up across the East Side during the afternoon/evening. Storm motions should also increase a bit (15-20 kt) compared to recent days. We`ve issued a Red Flag Warning for this, please see Fire Weather section below. Newest CAMs (not all, but a few) are even showing an isolated cell tracking across SE Jackson County during the evening. The probability of this occurring is around 15%, so we`ve added that to the forecast, but convective inhibition is greater the farther NW you are west of the Cascades. It looks like best opportunity for this is from Phoenix/Talent southeastward. On Monday, the flow back subtly to the south. In the absence of a more distinct short wave, we think activity will be more isolated and shift to the north from around the Cascades north of Crater Lake across northern Klamath/Lake counties, then arcing down to around the Warner Mtns. Isolated thunderstorms could also pop up again over the Siskiyou Mtns of western Siskiyou County. However, with the flow being from the south, if strong enough, any isolated cells that form over western Siskiyou County could travel northward into Jackson County before dissipating. Right now, confidence is low in this scenario, so we`ll continue to monitor and adjust the forecast. As we`ve been talking about for a while now, Tue-Thu next week appear to be another active period of showers/thunderstorms across our CWA. A disturbance will round the base of the long wave trough and move onshore into the California Coast (south of SF Bay) on Tuesday. This system will head into the Great Basin Tuesday night, but the orientation of the trough will take on a negative tilt. This will allow for upper level flow from the south and perhaps even SE at times. This is when thunderstorms have their best chance of making in onto the west side of the Cascades. We have continued to increase PoPs during this period with even the potential for some nocturnal activity Wednesday night. It won`t be until Friday or Saturday that we get into a more typical WSW flow aloft and thunder chances decrease again. -Spilde MARINE...Updated 200 PM PDT Saturday, July 26, 2025...Sub-advisory north winds and low seas of 5 ft or less, will persist through the weekend into early next week with nightly stratus and fog that will last at least into the morning hours each day. Winds and seas may trend slightly higher around Tuesday of next week, especially south of Cape Blanco. FIRE WEATHER...Updated 100 PM PDT Saturday, July 26, 2025... Temperatures, humidities, and breezy afternoon winds will all remain near seasonable levels this week, so the main fire weather concern will continue to be daily thunderstorm chances. We`ll be in a relative lull in terms of thunderstorm coverage today through Monday, with a slight uptick in activity on Sunday. Then from Tuesday onward, we expect a heightened level of thunderstorm activity through Thursday/Friday, and this could include areas west of the Cascades. For today, another round of isolated thunderstorms is anticipated for northern California, mainly east of the Shasta Valley into the Modoc as well as southern and eastern Lake Counties. There is the potential for some scattered activity across far eastern Modoc/Lake Counties along the Warners, but given the recent rainfall and in coordination with partner agencies, a headline in the Fire Weather Planning Forecast (FWFMFR) is sufficient for today`s activity. Models have trended more active for Sunday`s activity, indicating scattered coverage from the Scott Valley in Siskiyou County eastward, extending across the Shasta Valley to the Medicine Lake area northward into Klamath, and Lake counties. We`ve decided to go with a Red Flag Warning for these areas for Sunday afternoon/evening, and details can be found at RFWMFR. Isolated storms are expected across the Modoc, and maybe even as far west as the Cascades and Rogue Valley (10-20% chance), but confidence wasn`t high enough in scattered coverage to include these areas in the Red Flag at this time. Guidance holds steady with isolated thunderstorms anticipated on Monday, focused over northern portions of Klamath and Lake counties into central and eastern Oregon. Current guidance shows a scenario where storms develop on the northern and eastern peripheries of FWZs 624/625, then continue moving north and eastward. Some isolated development is also possible across western Siskiyou County, but again, coverage does not warrant any watches/warnings at this time. For the upcoming week, attention remains focused on another trough that swings into central California and turns the flow southerly as it tracks northward into eastern OR/western ID. This pattern will maintain daily thunderstorm chances through at least Thursday, if not into Friday. Confidence is highest for areas east of the Cascades and across northern California to see at least scattered coverage during this pattern, but the more challenging aspect is how far west will storms make it, and will there be any overnight (nocturnal) thunderstorm potential this week. For now, the forecast supports increased thunderstorm activity for East Side/norCal beginning on Tuesday. Areal coverage of storms looks greatest on Wednesday with this day seeing the best potential for storms west of the Cascades. Current forecast maintains similar conditions on Thursday, and is even hinting at some nocturnal potential for the West Side Wednesday night into Thursday morning. By Friday, activity should trend less, though confidence in details is low at this point. Differences in timing and location of these shortwaves will be challenging to resolve until just a few days in advance, so expect changes to the forecast over the coming days. /BR-y && .MFR WATCHES/WARNINGS/ADVISORIES... OR...Red Flag Warning from 2 PM to 11 PM PDT Sunday for ORZ624-625. CA...Red Flag Warning from 2 PM to 11 PM PDT Sunday for CAZ280-281- 284. PACIFIC COASTAL WATERS...None. && $$