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Area Forecast Discussion
Issued by NWS Medford, OR
Issued by NWS Medford, OR
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814 FXUS66 KMFR 162349 AFDMFR Area Forecast Discussion National Weather Service Medford OR 449 PM PDT Fri Aug 16 2024 ...Updated AVIATION discussion... .SYNOPSIS... Saturday will be an active weather day with strong winds east of the Cascades and a slight risk of severe storms along and slightly west of the Cascades. Cooler than normal conditions persist through the short and long term forecasts. && .SHORT TERM...Now into Tuesday Plenty of cumulus clouds are on satellite this afternoon with some marine stratus holding on along the coast and portions of Douglas County. Smoke is still billowing out from active fires in the area and some of that smoke should be heading southwards this late afternoon. However, with a low off the Oregon coast, low level winds should become more southerly overnight through Saturday. This should result in more smoke from the Boise fire moving into sections of our forecast area. There is still an air quality alert out for Lake and Klamath Counties as smoke from the many complexes in the Central Cascades continue to flow into those two counties. By Saturday, an upper level low off the coast of Oregon will bring plenty of rain and thunderstorms to the region. There is a slight(15-25%) chance that these thunderstorms are severe. Convective available potential energy(CAPE) isn`t all that high with values around 500 to 1000. However, there is plenty of shear around 0-6 km, which is what we usually lack for severe storms in this section of the US. Lapse rates look good along the spine and east of the Cascades, which suggests some stronger and sustained updrafts with the right shear. Right now, hail seems to be the main concern, although strong gusty winds up to 60 mph are not out of the question. The high resolution ensemble forecast(HREF) system is picking up on some supercell structures in our CWA as well, which increases the risk of severe storms in the Oregon side of our warning area. As for flash flooding and debris flows, there is some risk of that happening given how wet the storms are. However, these storms will be moving quickly, which should decrease the amount of precipitation they can put down over a small area over a short period of time (15 minutes or so). These short intense bursts of rain is usually what triggers these events. Therefore, we decided to not issue a flash flood watch for Saturday`s thunderstorm event given the low probability of something occuring. However, it`s worth stating again that there is a low risk. It will also be very windy east of the Cascades on Saturday, which is more of a fire weather risk during this time of year. However, some model output statistical guidance suggested wind gusts around 45 to 50 mph over higher exposed terrain east of the Cascades. This would usually suggest a wind advisory, although we felt the fire weather products pretty much covered this threat. Heading into Sunday, the upper level low wanders a bit and moves a bit farther north. Southerly flow should still remain dominant, which will result in smoke from the Boise fire still moving into our area if it still burning with enough intensity. -Smith .LONG TERM...Tuesday into next Friday This rather progressive pattern will persist into the extended forecast with a deep wave swinging into Oregon with plenty of cooler moist onshore flow. This will keep temperatures below normal for Tuesday onwards. Our normal high in Medford is usually around 90 this time of year, and we`re forecasting highs the 80`s for all of next week with low temperatures in the mid to lower 50`s in many of the valleys. Models are currently showing another upper level low settle off the Oregon coastline sometime around Thursday afternoon and Thursday night. This will result in a low threat of thunderstorms across our forecast area. Right now, the probability is right around 10 percent, although that could trend higher as we move through the start of next week. -Smith && .AVIATION...17/00Z TAFS...A few scraps of marine stratus are clinging to the Oregon coast to start the TAF period but are quickly clearing out. The cloud layer is expected to rebuild later this evening, bringing MVFR and IFR ceilings that are expected to remain through the TAF period. Periods of MVFR visibilities are possible overnight into Saturday morning as well, but there is low confidence in the timing. Wildfire smoke is not expected to be a widespread problem as southerly winds will help to prevent accumulation. Visibility near the fires in eastern Lane and western Siskiyou counties may still be locally limited. Active weather is expected west of the Cascades late Saturday morning and into the next TAF period. 25-50% thunderstorm chances will be present over west side areas on Sunday, with the highest chances over Jackson, Josephine, and eastern Douglas counties. Abundant lighting, erratic gusty winds, and localized heavy showers are possible. Hail development will also be a concern. Thunderstorms are expected to move quickly to the north. Approximate timing for activity to start has been included in the Medford and Roseburg TAFs, with thunderstorm chances lingering into early Sunday morning. -TAD && .MARINE...Updated 200 PM Friday, August 16, 2024...Relatively calm conditions are expected for the next several days with light to moderate winds and seas remaining 5 feet or less. A rather strong upper low for this time of the year will move into the waters Saturday and will likely bring showers with a slight chance of thunderstorms into early Sunday morning. -DW/Smith && .FIRE WEATHER...The main fire weather concerns over the next 24-36 hours will be two-fold. One, for some strong, gusty winds in combination with low relative humidity over portions of the East Side and in NE California Saturday afternoon and evening. And two, for thunderstorms expected near and west of the Cascades during the same time period. First, the winds. A strong mid-level jet will move across NE Cal and south-central Oregon in advance of low pressure spinning offshore Saturday afternoon and evening. This will bring increasing S-SW winds east of the Cascades with models indicating a high probability (60-80% chance) of wind gusts exceeding 40 mph. In-house fire weather RAWS guidance is showing peak wind gusts near 50 mph for some of our more exposed RAWS locations. In addition, afternoon relative humidity will bottom out in the 10-15% range. We have upgraded the Fire Weather Watch for FWZ 285 (Modoc) in California and FWZs 624/625 in Oregon, to a Red Flag Warning (2-8 pm PDT). mainly south and east of Summer Lake. The main area of concern includes the Warner Peak fire in eastern Lake County. Winds will also be gusty in the Shasta Valley up into portions of the Klamath Basin, but humidities there should be slightly higher. Next, the thunderstorms. The low offshore will send a very potent shortwave northward across NW California and SW Oregon Saturday, providing plenty of forcing/lift. With good mid-level moisture available and enough (modest) instability, this will induce showers and thunderstorms across a large portion of the west side (from the Cascades west) during the afternoon/evening. Models are showing some showers moving onshore as early as Saturday morning, but it should take until midday for the thunderstorms to really get going. This is where the forecast gets tricky. This isn`t your typical mid-August thunderstorm event since we`ve had a cool-down in the last week or so and subsequent moistening of the low-levels and fuels to some degree. Usually, these events are preceded by hot, dry weather. In that case, this would be a slam dunk fire weather regime. But, these storms are also likely to be decent rain producers with PWs in the 0.75-1.00 inch range (even up to 1.25 farther north). Recent model runs of the SPC HREF convection allowing models (CAMs) are showing a high probability (60-80% chance) of lightning from the Cascades westward to central Douglas/portions of Jackson, Josephine and western Siskiyou counties. It is in these areas that SPC SREF also shows a fairly high probability of >100 cloud to ground lightning strikes. These storms will be moving north at a pretty good clip (40- 50 mph) and they may be more cellular (as opposed to clusters) at least initially. This could allow for strikes outside rain cores. Local fuels agencies are still indicating high fire danger despite the recent cool down and there is potential for numerous new fire starts, due to the expected amount of lightning. For these reasons, we have decided to issue a Red Flag Warning for FWZs 616, 617, 620, 621, 622, and 623 in Oregon and FWZs 280 and 281 in California. It should also be noted that the Storm Prediction Center has much of the aforementioned area in a marginal to slight risk for severe storms, which means there`s a 5-15% chance of wind gusts exceeding 58 mph and large hail in excess of 1 inch in diameter. It is important for aircraft and ground resources/personnel to be aware of this risk for safety concerns. Individual storm cells will have a tendency to become clustered/more organized as they move north (toward Lane County), where sustained heavy rainfall rates (>0.50"/hr) are more likely. The shortwave disturbance should lift north of the area by around 9 pm Saturday evening and the lightning risk will end. By Sunday, the low will wobble a bit more offshore and lift northward, so while there is still a slight chance of showers near and west of the Cascades, most areas will be dry. It will be breezy again over portions of the East Side and humidity will continue to be down around 15%, but wind gusts should be at least 10-15 mph lower than on Saturday. We`ll headline this in the Fire Weather Planning forecast. Beyond that, things should calm down a bit with the low moving northward toward Vancouver Island Monday, then opening up into a trough Tuesday and eventually onshore Tuesday night. This should put the area in a mostly dry SW flow aloft. However, models are showing a shortwave moving through NorCal Tuesday night, so we`ll have to watch for some shenanigans with that. If you can believe it, yet another upper low is shown to drop south along the British Columbia Coast mid next week, then offshore the PacNW Thu/Fri. Will we do this all over again? We`ll see. -Spilde && .MFR WATCHES/WARNINGS/ADVISORIES... OR...Red Flag Warning from 1 PM to 9 PM PDT Saturday for ORZ616-617- 620>623. Red Flag Warning from 2 PM to 8 PM PDT Saturday for ORZ624-625. CA...Red Flag Warning from 1 PM to 9 PM PDT Saturday for CAZ280-281. Red Flag Warning from 2 PM to 8 PM PDT Saturday for CAZ285. PACIFIC COASTAL WATERS...None. && $$