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Area Forecast Discussion
Issued by NWS Medford, OR
Issued by NWS Medford, OR
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391 FXUS66 KMFR 141152 AFDMFR Area Forecast Discussion National Weather Service Medford OR 452 AM PDT Wed Aug 14 2024 .DISCUSSION...Satellite imagery this morning shows mostly clear skies across the region. There is some marine stratus out over the waters and along the coast south of Cape Blanco. There may be some additional stratus development along the coast north of Cape Blanco and into the Coquille Basin over the next few hours, but this should be the extent of the marine layer for this morning. We`ll be under the influence of an upper level trough through the weekend and well into next week. This will continue the trend of below normal temperatures with afternoon highs in the mid to upper 80s for West Side Valleys and upper 70s to low 80s for areas east of the Cascades. Despite the reprieve from the summer heat, smoke will be a continued nuisance through the forecast period, and probably longer. An Air Quality Alert (issued by partnering agencies) remains in effect for eastern Douglas, northern Jackson/Klamath and Lake counties. Oregon DEQ has some air quality advisories out and we have relayed those via AQAMFR. A shortwave trough will move into the region late this evening and overnight, and this will bring thunderstorm chances to the region today and again Thursday. Models are showing modest instability and best moisture from the Cascades over to the east side, but also extending down into portions of NorCal. High resolution guidance shows isolated thunderstorms developing over northern California late this afternoon, then moving north-northeastward into Oregon this evening and continuing into the overnight hours. While we expect mostly isolated thunderstorm coverage (15% chance) today, guidance does show the potential for more scattered coverage (25-40% chance) as the trough moves overhead around sunset. There is some uncertainty regarding how far west the shower/thunderstorm threat is as some guidance shows some slight chances (15% chance) as far west as the Rogue Valley/Medford and far eastern portions of Douglas County. Have added a slight chance mention for these areas, but the better chances are expected to be farther east. This system will slow down as it moves through Oregon tonight into Thursday, so this will keep the risk of showers and thunderstorms going. After a brief lull in shower/thunderstorm activity early Thursday morning, we expect another round of isolated to scattered thunderstorms in the afternoon. The thunderstorm potential looks higher for Thursday with the trough overhead during peak heating hours. The better dynamics, however, will have pushed farther east and north, so thunderstorm activity will shift slightly farther east and north as well. Scattered coverage (25-40% chances) looks best over northern Klamath/Lake counties centered on the Christmas Valley. The atmosphere is more stable on Friday. Lingering moisture and weak instability will be present, but thereisn`t a defined trigger coming through the region, so thunderstorm chances drop out of the forecast for Friday. A deep, large scale trough will develop off the PacNW Coast late Friday into Saturday and linger there well into next week. This will continue the cooler than normal pattern, and also potentially bring some precipitation chances to the region. This low will wobble eastward and northward, and these "wobbles" could result in big changes to the forecast depending the extent of the "wobble". This is leading to low confidence in precipitation potential and location. That said, there are a good amount of ensemble members showing measureable precipitation for locations west of the Cascades on Saturday afternoon. The other weather element to note with this trough will be an increase in south to southwesterly winds. Guidance is currently showing the potential for gusts of 30-40 mph east of the Cascades and in the Shasta Valley on Saturday and Sunday afternoons, so we`ll have to monitor this for fire weather concerns. Despite the low confidence in the precipitation potential, confidence is higher for these stronger winds as the location of the low pressure will result in tightening pressure gradients, and subtle "wobbles" likely won`t have big changes on this element. Subtle changes in position/strength of this upper low can and will play a role in if, when and where additional showers/thunderstorms set up, so be on the lookout for daily forecast changes. /BR-y && .AVIATION...14/12Z TAFS...Marine stratus has brought LIFR ceilings over coastal areas south of Cape Blanco and is slowly approaching areas north of Cape Blanco. This layer is expected to move over those northern areas early this morning. There could be a few hours of clearing this afternoon, with stratus likely to return this evening. Other areas should generally be at VFR through the TAF period. An upper trough remains in place over the Pacific northwest, bringing more westerly winds that help to limit smoke accumulation west of the Cascades. However, that does also mean that smoke may impact visibilities east of active fires. For this area, that includes possible periods of moderate to heavy smoke over Klamath, Lake, eastern Douglas, and western Siskiyou counties. Upper level instability could bring some thunderstorm chances later today. 15-30% thunderstorm chances are present over Lake, Klamath, and eastern Siskiyou and Jackson counties this afternoon and evening. These chances will decrease early Thursday morning, with 15- 20% thunderstorm chances lingering over Klamath and Lake counties through the end of the TAF period. Lightning, locally heavy rain showers, and gusty erratic winds are possible hazards around any storms that do develop. -TAD && .MARINE...Updated 200 AM Wednesday, August 14, 2024...Seas will remain below advisory levels through the forecast period, with light winds and low north-northwest swell. Showers over marine waters are possible this weekend and going into next week. -TAD && .FIRE WEATHER...Issued 400 AM Wednesday August 14, 2024...Upper level troughing will remain the dominant weather pattern through the forecast period. This will continue the trend of cooler than normal temperatures, moderated humidities and breezy afternoon winds. The two main fire weather concerns on the horizon are thunderstorm potential today through Thursday and the development of a deep upper low offshore over the weekend which could bring some strong winds to the area. First the thunderstorm potential...a shortwave trough will move into the region later today and linger over the area through Thursday. With ample moisture and modest instability, isolated to scattered thunderstorms are expected beginning late this afternoon. Initially, isolated thunderstorms are expected over portions of northern California, then spreading north-northeastward into Oregon after sunset and into the overnight hours. Isolated thunderstorm coverage is the most likely scenario today mainly along and east of the Cascades (possibly as far west as the Rogue Valley/Medford), but scattered coverage is possible as storms move into Oregon across Fire Weather Zones (FWZ) 624/625. We do expect a break in thunderstorm activity early Thursday morning. The trough will linger over the region Thursday afternoon, however, and this will provide a better opportunity for scattered thunderstorms over FWZ 624/625 due to daytime heating effects. Due to these factors, the Fire Weather Watch has been upgraded to a Red Flag Warning for abundant lightning on dry fuels...details can be found at RFWMFR. Storms are expected to have some precipitation with them, but lightning strikes outside of precipitation cores are possible and cause for concern. Additionally, expect gusty and erratic winds near thunderstorms with strong outflows that can travel a good distance from the originating storm. The atmosphere will stabilize some on Friday. There will be lingering moisture and instability, but without a sufficient trigger/forcing mechanism, it`s likely there will be some cumulus buildups east of the Cascades and over the higher terrain. The next concern turns to a deep, large scale trough that develops off the PacNW Coast late Friday into Saturday and lingers there well into next week. This will bring some precipitation chances to the region, though confidence is low on the details regarding where and amounts. Ensembles suggest the best chances are along and west of the Cascades, but there could thunderstorms to accompany these precipitation chances. The other element of concern with this low pressure (and where the higher confidence lies) is the potential for strong south to southwest winds. Guidance suggests gusts of 30-40 mph for areas east of the Cascades and in the Shasta Valley on Saturday and Sunday afternoons. These strong winds could combine with low humidities (which ultimately depends on where precipitation chances occur) and result in critical fire weather conditions. Stay tuned for updates as subtle shifts in the position of this low could have big changes on the forecast. /BR-y && .MFR WATCHES/WARNINGS/ADVISORIES... OR...Red Flag Warning from 8 PM this evening to 11 PM PDT Thursday for ORZ624-625. CA...None. PACIFIC COASTAL WATERS...None. && $$ MNF/TAD