FXUS66 KMFR 240550 AAA AFDMFR Area Forecast Discussion 1050 PM PDT Tue Jul 23 2024 ...Updated AVIATION discussion... .DISCUSSION...The minor heat risk we have today will lower further through the remainder of the week. This will be caused by low pressure moving north of the region. The main impacts will be to fire weather, so please read that section below if your interest lies there. From a smoke perspective, the winds generated from this low pressure will blow new smoke west to east and should limit the smoke entering southwest Oregon from the fires in Lane and Douglas County. However, this will increase the amount of smoke in northern Klamath County around Crater Lake and across most of Lake County. Medium confidence exists that the winds will be able to scour all the smoke from the valleys west of the Cascades, but even if this isn't achieved, a noticeable difference is expected. If only for a short while. The cautious optimism is driven by uncertainty in the thunderstorm forecast. While across NorCal and east of the Cascades confidence is good that there will be at least isolated thunderstorm activity on Wednesday (more on this later), there is very low confidence about the chance for nocturnal isolated storms tonight. An energy rich environment tonight lends our thoughts to there being a low chance for an isolated thunderstorm tonight or early tomorrow morning west of the Cascades. The low confidence is being driven by single models showing this occurring and the environment generally being without a source of initiation and the only moisture being too high in the atmosphere. Therefore, we have not included this ~10% chance in the forecast, but want to highlight here in the event a no- notice warning is needed. The trough moving through will also increase ridgetop winds. This will mean an increasing chance for mountain wave formation in the lee of north-south oriented ranges, but also for the thunderstorms on Wednesday to transport these strong wind speeds down to the surface. The Storm Prediction Center is highlighting Lake County in a 5% risk for severe thunderstorms due to strong winds. We expect any thunderstorms that develop to have at least 40 mph outflow winds (58+ mph are severe strength winds). Then, Thursday will be a windy day, with critical fire weather expected east of the Cascades. But after that, weather conditions into the weekend are expected to be relatively normal and calm, although smoke from existing or any new fires will remain a large unknown. -Miles && .AVIATION...24/06Z TAFS...VFR levels are generally expected through the TAF period across northern California and southern Oregon. Wildfires in eastern Douglas County may locally lower visibilities. Light daytime winds will limit smoke movement in the morning, then an approaching upper trough will bring westerly afternoon winds that will push smoke over northern Lake and Klamath counties. In other areas, afternoon winds will be stronger than usual with the trough's influence. The strongest winds will be over Lake and Modoc counties. These winds are not expected to approach advisory strength. Unstable conditions will also support slight to moderate (15-30%) precipitation chances over areas east of the Cascades on Wednesday afternoon and evening, with the highest chances over Lake and Modoc counties. Storms that do develop can locally lower visibilities as well as produce unpredictable gusty winds. -TAD && .MARINE...Updated 145 PM Tuesday, July 23, 2024...A thermal trough bringing gusty northerly winds will result in continued steep to very steep seas. Broad areas of gale gusts and very steep seas will be present south of Cape Blanco, with steep seas in waters north of Cape Blanco. Gale Warnings and Small Craft Advisories are in place through tonight. Early Wednesday morning, winds will lower slightly but with gusts remaining near gale strength and very steep seas persisting, mainly from Gold Beach south. Steep seas linger for other areas from Reedsport southward. Winds and seas will trend slightly lower Wednesday afternoon and night with small craft advisory conditions likely to continue for areas from Cape Blanco southwards. The thermal trough will remain in place into Saturday, and long term guidance supports area of continued small craft advisory conditions south of Cape Blanco. -CC && .FIRE WEATHER...Issued 200 PM Tuesday July 23, 2024...Water vapor satellite imagery is showing an upper low well off the coast of Baja California. Moisture is beginning to stream northward ahead of this upper low and some convection continues to break out along the Sierra. Isolated thunderstorms are possible in portions SE Modoc and SE Lake counties in this evening. A few buildups are also possible near the Mt. Shasta region where there is a small risk of a thunderstorm (~15% chance). Mid and high level moisture and elevated instability will continue to surge northward tonight into Wednesday. Most guidance is showing no activity with this due to a lack of upper level forcing. In these scenarios, we usually get mostly mid- high level clouds and nothing else. But, there is a low probability (10% or less) of an isolated high-based shower/virga or perhaps even a dry thunderstorm that moves up into NorCal/SW Oregon west of the Cascades during this time. It's low confidence, but potentially high impact should there be lightning. On Wednesday, we are maintaining an isolated thunderstorm risk across most of NorCal, but scattered thunderstorms with abundant lightning on dry fuels are expected in eastern Modoc and eastern Lake counties where a Red Flag Warning is in effect. Inverted-v soundings indicate these storms will have the ability to produce very strong outflow winds (50-60 mph). The Storm Prediction Center has easternmost portions of our forecast area in a Marginal Risk for severe storms. Meanwhile, an upper level trough will move into western Canada and this will increase mid-level flow area wide Wed-Thu. So, even without thunderstorms, we are expecting some very gusty SW winds (30- 40 mph) to develop Wednesday afternoon/evening over the east side. We'll continue to headline for near critical fire weather conditions for this expected wind Wednesday, but a limiting factor will be the surge in moisture resulting in afternoon humidity values in the 15- 25% range. This is still enough winds to move fire, but not technically meeting the criteria for a Red Flag Warning. Things change on Thursday. The trough passage will sweep the moisture eastward, ending the risk of thunderstorms. But, we still expect gusty winds with lower afternoon minimum RH (10-15%). For this reason, we have issued a Fire Weather Watch for strong, gusty winds and low RH for fire weather zones 624, 625, and 285. Late this week into the weekend, models continue to show a closed low off the California coast, which gets absorbed into the WSW flow aloft and eventually cruises across NorCal Friday and into northern Nevada/southern Idaho Saturday. Normally, with the potential for forcing, we'd be concerned about more thunderstorms, but by this time, the best mid-level moisture will be off to the east and thunder risk remains low. We're still seeing lower temperatures to levels that are more typical for this time of year (highs in the upper 80s to mid 90s for west side valleys and in mid to upper 80s east of the Cascades). -Spilde && .MFR WATCHES/WARNINGS/ADVISORIES... OR...Fire Weather Watch from Thursday afternoon through Thursday evening for ORZ624-625. Red Flag Warning from 11 AM to 11 PM PDT Wednesday for ORZ625. CA...Fire Weather Watch from Thursday afternoon through Thursday evening for CAZ285. Red Flag Warning from 11 AM to 11 PM PDT Wednesday for CAZ285. PACIFIC COASTAL WATERS...Gale Warning until 2 AM PDT Wednesday for PZZ356-370-376. Small Craft Advisory until 11 PM PDT Thursday for PZZ350-356-370- 376. Hazardous Seas Warning from 2 AM to 11 AM PDT Wednesday for PZZ356-376. && $$