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Area Forecast Discussion
Issued by NWS Medford, OR
Issued by NWS Medford, OR
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448 FXUS66 KMFR 171101 AFDMFR Area Forecast Discussion National Weather Service Medford OR 401 AM PDT Sat Aug 17 2024 .DISCUSSION...An impactful afternoon and evening of wet, windy weather is imminent, with showers and scattered thunderstorms from the coast, across the west side to Klamath and Siskiyou counties, and also southwest wind gusts of 25 to 40 mph east of the Cascades. Both conditions prompted the issuance of Red Flag Warnings /PDXRFWMFR/. The forecast for today and tonight exhibits very little change from yesterday`s expectations. As such, the main points from the previous discussion are still fully valid. This includes a low, but mentionable risk of flash flooding, and a higher, 5-15% probability that thunderstorms could become severe, reaching thresholds of 1 inch diameter hail or 58 mph wind gusts. Convective available potential energy(CAPE) isn`t all that high with values around 500 to 1000 J/Kg. However, there is plenty of 0-6 Km shear forecast, along with steep lapse rates. A broad, strong, slow-moving upper trough is offshore from Oregon. It is expected to shift south-southeastward today, then northeastward toward the Washington coast tonight through Sunday. Rainfall amounts from the coast to the Cascades will be highly variable, representing the nature of showers, but amounts of a tenth of an inch to an inch should be common. Meantime, amounts in far eastern Siskiyou and Klamath counties should largely be less than a tenth of an inch. Beyond this evening, late tonight into Sunday morning, lower level moisture, closer to the center of the trough, will move onshore. The main result is expected to be additional light rain from the coast to the Coast Range, mainly for Coos, Curry, and western and northern portions of Douglas counties. Forecast confidence is high for Today through Wednesday, and more generally is high for a continued relatively cool, trough dominated pattern through next week. The upper low is expected to lift sufficiently far to the north during Sunday to allow the light, near coastal precipitation to taper off before noon on Sunday. Sunday afternoon through Wednesday morning looks to be a period of dry weather across our area with continued below normal temperatures, variable cloud cover, and typical late day breezes. A minority of ensemble members, as well as the operational ECMWF do indicate a few light showers on Monday and/or Tuesday, and we have continued with a slight chance mention for near coastal showers. But, this scenario doesn`t favor anything more than some virga. Another trough and associated shortwave are expected to track inland Wednesday afternoon with some slight potential for showers and thunderstorms, highest from the Cascades east-northeastward into south central and central Oregon. Meantime, on Wednesday, the next strong, closed upper low is likely to be situated over western Canada. From Thursday into Saturday, there are important, signficant differences in the strength, speed, and track of this next low. The extent of similarity with today`s weather is highly uncertain. But, compared with the past few days, the probability is trending higher for another episode of wet, windy weather during the latter portion of the week. && .AVIATION...17/12Z TAFS...Marine stratus is rebuilding overnight, bringing MVFR and IFR ceilings that are expected to remain through the TAF period. Wildfire smoke is not expected to be a widespread problem as southerly winds will help to prevent accumulation. Visibility near the fires in eastern Douglas, eastern Lane, and western Siskiyou counties may still be locally limited. Active weather is expected west of the Cascades later this morning and into the next TAF period. 25-50% thunderstorm chances will be present over west side areas today, 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 as early as late morning, mainly in the afternoon and evening, then tapering off tonight. -TAD/Hermansen && .MARINE...Updated 245 AM Saturday, August 17, 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 today and will likely bring showers with a slight chance of thunderstorms into early Sunday morning. -Hermansen && .FIRE WEATHER...(From Friday afternoon). 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 this afternoon to 9 PM PDT this evening for ORZ616-617-620>623. Red Flag Warning from 2 PM this afternoon to 8 PM PDT this evening for ORZ624-625. CA...Red Flag Warning from 1 PM this afternoon to 9 PM PDT this evening for CAZ280-281. Red Flag Warning from 2 PM this afternoon to 8 PM PDT this evening for CAZ285. PACIFIC COASTAL WATERS...None. && $$