Area Forecast Discussion
Issued by NWS Medford, OR
Issued by NWS Medford, OR
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916 FXUS66 KMFR 092210 AFDMFR Area Forecast Discussion National Weather Service Medford OR 210 PM PST Sat Nov 9 2024 .SHORT TERM...Through early Tuesday morning, Nov 12, 2024... Satellite imagery shows mostly mid and high level cloudiness over the area this afternoon in advance of an incoming trough offshore. The trough is supporting a weak frontal boundary, which is attached to a swirling low about 275 miles due west of Cape Mendocino. Radar is picking up on some precipitation over the coastal waters, and even some light echoes west of the Cascades at times, but a dry layer below 12,000 feet has prevented precip from reaching the ground. The trough offshore will move onshore tonight into Sunday morning with the surface front getting strung out and weakening over the area. This will moisten the air mass and lead to some rain chances across the area, highest along the coast and over to the Cascades with diminishing probabilities farther south and east. But, rain amounts remain meager, generally a hundredth or two and largely less than a 0.10 of an inch. One area though, the SW coast and coast ranges, might get a little help with some orographic lift, which could enhance precip for a time Sunday. There might be 0.25 of an inch or so in some of those locations. We might get a few drips here in Medford, but right now it looks like we`ll be lucky to get anything measurable. Overall, high temperatures Sunday will be at least a few degrees above normal (in the upper 50s to low 60s for the west side valleys; mid 50s over the east side), but with the cloud cover, it may not feel like it. Meanwhile, a Pacific jet stream disturbance coming through the southern reaches of a deep closed low over the Gulf of Alaska will energize another front which will move onshore Sunday night into Monday. Model depictions of Integrated Water Vapor Transport (IVT) show a healthy, but narrow atmospheric river (~500 kg/m/sec) associated with this front, with PW values around or just over 1.00 inch. Model wind fields in advance of the front show speeds of 45-55 kt at 700 mb traversing the area during this time. Gradients support peak wind gusts along the coast of 40-50 mph, but well shy of the criteria (>58 mph) we`d issue a warning on. Expect S-SE winds to pick up in the Shasta Valley Sunday afternoon, then especially Sunday evening/night. Similarly, we expect gusty S-SW winds to develop over the higher terrain of the east side and also near Summer Lake. These locations have been included in a Wind Advisory for wind gusts up to 50 mph. The band of moderate to heavy precipitation associated with the front will move onshore Sunday evening, then push to the Cascades by Monday morning. Rain amounts at the coast look to be in the 1-2 inch range for most locations with up to or perhaps even a little more than 3 inches in the Curry coast ranges. Here in the Rogue Valley, the most likely amount is 0.25-0.50 of an inch. Most other areas east of the Cascades and in NE Cal should also receive a wetting rainfall with this front, though the Christmas Valley and northern Lake County appear to get the least with 0.10 of an inch or less. In terms of snow, snow levels up around 7000-7500 feet will drop quickly to around 5000 feet Monday morning, so we do expect some winter hazards over the Cascades. We have issued a Winter Weather Advisory from 4 am PST Monday through 10 am PST Tuesday. Snowfall rates could reach 1"/hour at times (most likely between 6-10 am PST Monday) up at Crater Lake as the front moves through. While snowfall will also occur farther south, including at Mt. Ashland, impacts should be less since the main thrust of post-frontal showers will be in the Cascades from around Highway 140 northward Monday night into Tuesday morning as snow levels drop a bit more to around 4500 feet. This could bring a couple of inches of snow along Highway 97 north of Chiloquin. -Spilde .LONG TERM...Tuesday through Saturday, Nov 12-16... A warm front will pass through the area during Tuesday, producing spotty precip, mainly along and west of the Cascades with snow levels rising again. Generally speaking, winds will be a bit breezy out of the south due to an increasing north/south pressure gradient, and this will result in some increased winds in the Shasta Valley and across the East Side, but nothing too out of the ordinary. A brief period of dry weather and near normal temperatures is expected late Tuesday afternoon into the evening as the warm front lifts to the north, then conditions deteriorate Tuesday night as the next storm system approaches the area. The first concern with this system is rain. The leading edge of precipitation should pass over the coast overnight Tuesday night, with rain spreading west to the Cascades by midday, then to the East Side through the afternoon. With IVT values of between 500 and 750, moisture inflow will be strong, however, much of that is due to the stronger winds associated with the system (more on that below). What is important is the direction of the prevailing flow, which in this case, will be out of the southwest. This will provide good upslope mechanics along the coastal ranges, the Cascades, and the Siskiyous, and especially in the Brookings area. These are where the heaviest rainfall amounts will be, although totals will not be altogether unusual for systems like these and during this time of year. Forecast rain amounts were bumped up above NBM/WPC in these areas, especially along the south coast near Brookings. That wind direction will, however, also produce downslope flow in the Rogue Valley and east of the Cascades. Rainfall totals here were reduced below NBM/WPC amounts, but we still expect a good precipitation event for all of the forecast area. Overall, rainfall amounts will be healthy, but given the low river levels, no major flooding is expected. Nuisance flooding, such as small creeks and streams, blocked ditches, or urban areas are possible. Winds will also be a concern. Forecast surface pressure gradients along the coast between Arcata and North Bend are approaching -10mb, and gradients between Redding and Medford of around -7mb, and model solutions are suggesting 60 to 70 kt winds at 700 mb. With this in mind, it is likely that Wind Advisories and Warnings will be necessary, especially along the coast and across the East Side. Wind headlines may also be necessary in the Shasta Valley and the Rogue Valley near Ashland, but the prevailing southwest flow will not align well with the valleys, so these areas will be mainly gradient driven and therefore a bit weaker than otherwise. Winds at the coast will peak Wednesday afternoon, with inland areas peaking into the evening and overnight. The final concern is snow. With the southwest flow tapping into warmer air to the south of us, snow levels will be a bit on the high side, beginning the event at around 6000 feet, then only dropping to around 4500 to 5000 feet after the front passes, when precipitation has likely transitioned to a more showery regime. This will keep snow from being a major impact, but heavier showers, or the very last gasp of the front itself, could produce just enough snow at the higher passes to cause some travel difficulties. With all of these factors, there is still time for the forecast to change, so we will be keeping a sharp eye on the latest model runs over the next several days. That being said, it is never to early to begin preparations for impactful wind and rain. Once the front passes through the area (exiting by Thursday morning), the system slows, keeping upper level troughing overhead through Friday afternoon. Temperatures will cool to about 5 to 10 degrees below normal as the cold air settles into the region. Along with ongoing onshore flow, precipitation chances will continue through this time, with snow levels hovering between 3000 and 4000 feet. There will not be continuous precipitation, as the NBM suggests with near continual high PoPs, but instead we expect hit-or- miss and off-again-on-again showers will be the rule Thursday and Friday. Precipitation amounts will vary greatly from place to place, and some showers may be strong enough to produce graupel or very small hail. Friday night into Saturday, the trough shifts just far enough to the southeast to allow ridging to nudge in from the west. The NBM keeps showers across the whole region during this time, but it is much more likely that the area dries out Saturday as high pressure settles in. -BPN && .AVIATION...09/18Z TAFS...Look for some light winds and plenty of high clouds through the day ahead of a weak front. VFR conditions will continue through the day. The exception is when rain chances increases along the coast this evening as this rain could locally drop ceilings to MVFR. Shear from the surface to 2,000 ft is nearing 15 kt tonight into tomorrow morning, but for now this has not been added to the TAFs so more guidance can be reviewed with the next TAF series. -Smith/Hermansen && .MARINE...Updated 200 PM PST Saturday, November 9, 2024... Two fronts will move through this weekend, one later this afternoon/evening and another Sunday night into Monday. The first front, the weaker of the two, will bring hazardous to small craft conditions for much of the outer waters through 10 PM tonight north of Cape Blanco. Farther south, where winds will be lighter, seas will be swell dominated and remain mostly below 10 feet. That front will stall and dissipate over the waters tonight. The second front, the stronger one, moves through late Sunday into Monday. This will likely bring a period of moderate to heavy rain, gales, and very steep and chaotic seas. Latest forecast has wind gusts up to 40 knots over the waters and perhaps 45 knots off the coast of Port Orford. Models also continue to show this system being followed by another large, long period swell, peaking around next Tuesday. Early indications are showing peak swells 17-21 feet with a period of 15-16 seconds. This would maintain very high and very steep seas through midweek. -Smith/Hermansen .BEACH HAZARDS...Updated 200 PM PST Friday, November 8, 2024... Another large, long period swell is expected to arrive early next week. Model guidance has been consistently showing NW swells arriving following the front Sunday night into Monday. Seas briefly subside immediately behind the front Monday (6 hours or so during the morning), but then build significantly Monday afternoon and Monday night, reaching a peak of 17-21 feet, 16 seconds by early Tuesday morning. This will bring large breaking waves (23-27 feet) to the beaches and coastline through Tuesday afternoon. Seas will gradually subside Tuesday night. A high surf advisory has been issued for 10 PM Monday through 10 PM Tuesday because of these dangerous surf conditions. Large breaking waves create hazardous conditions along and within the surf zone, and could inundate beaches and low lying shorelines. If you have plans to venture out to the beaches early-mid next week, pay close attention to the ocean, stay away from the surf zone and off of jetties. Also, keep checking back for forecast updates. -BPN/Hermansen && .MFR WATCHES/WARNINGS/ADVISORIES... OR...Wind Advisory from 7 PM Sunday to 7 AM PST Monday for ORZ030- 031. Winter Weather Advisory from 4 AM Monday to 10 AM PST Tuesday for ORZ027-028. CA...Wind Advisory from 7 PM Sunday to 7 AM PST Monday for CAZ081- 085. PACIFIC COASTAL WATERS...Gale Warning from 1 PM Sunday to 10 AM PST Monday for PZZ350-356-370-376. Small Craft Advisory until 10 PM PST this evening for PZZ370- 376. && $$ MAS/BPN/ANH