Area Forecast Discussion
Issued by NWS Medford, OR
Issued by NWS Medford, OR
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802 FXUS66 KMFR 302138 AFDMFR Area Forecast Discussion National Weather Service Medford OR 238 PM PDT Tue Apr 30 2024 .DISCUSSION... .Short Term...Tonight through Thursday night...Showers continue across the Cascades and points west, but so far they have been light with very little in the way of vertical extent. With onshore flow and the upper level trough axis passing just to our north, this activity will likely continue into this evening, with activity and intensity diminishing to the north. By tonight, the area should be dry, with upper level ridging building in aloft. Tonight, with skies clearing and winds on the weaker side, cool overnight lows are expected. This is especially true of the Illinois and Applegate valleys, where lows tonight are expected to once again drop below freezing. While this is not unheard of for this time of year, area agricultural interests may need to be concerned given the level of development seen in area orchards and wineries. A freeze warning has been issued for those areas, and details can be found at PDXNPWMFR. With the ridge overhead, Wednesday will be dry and about 10 degrees warmer than today. Mostly clear skies for most of the day will start to give way to increasing clouds late in the afternoon and evening as the next trough approaches from the northwest. This next system, smaller in scale and weaker than most, will arrive Wednesday night, then pass overhead through Thursday. Due to the angle of the system, there will be considerable westerly onshore flow on the front side, along with relatively little in the way of strong winds out of the south. This type of pattern typically produces a good shot of wetting rain across the region, especially along and west of the Cascades, with little in the way of downslope drying. As a result, most areas in Oregon should see measurable rain, although northern California may not be as lucky, as those areas will remain well south of the main moisture flow and dynamics. Temperatures will trend towards normal for this time of year, which means snow levels with this event will remain at or above 5000 feet, so winter impacts will remain confined to the backcountry and the highest passes, and even then, those impacts will be minimal. Most precipitation will dissipate or move out of the area by Thursday night as the next ridge briefly builds in overhead. -BPN && .LONG TERM...Friday through Tuesday, May 3-7, 2024... The axis of a short wave upper ridge will be centered over the area Friday morning with a surface warm front just offshore. The upper ridge will be transient as an active jet over the north Pacific allows another deep upper trough to carve out a closed low over the Gulf of Alaska. Any warming that occurs Friday (compared to Thursday) will be modest at best and where it stays sunniest the longest (in NE California/East of the Cascades). These areas probably have high temps 5-10F higher than on Thursday. But, west of the Cascades, warming will be muted due to the increased cloud cover, with temps topping out mostly in the 60s. As the warm front pushes northward and an associated surface cold front well offshore approaches the PacNW, rain chances increase along the coast with rain becoming likely there (50-70% chance) by Friday evening. This frontal system will be fairly potent in terms of precipitation Friday night into Saturday with good moisture transport into the region and the lack of a significant downslope component that would otherwise lead to rain shadowing. PoPs peak at 80-100% area wide. Don`t get me wrong, there won`t be as much rainfall in the typically drier areas as say, the coast, but it probably won`t be as large a difference as usual. So, the front should be an efficient rain producer for a larger part of the area. Widespread wetting and beneficial rainfall is expected with preliminary amounts for the coast in the 1.25-2.50 inch range and many areas inland receiving 0.50-1.25 of an inch. A little less rain is expected for the Klamath Basin and over the deserts east of Winter Rim where 0.25-0.50 of an inch appears to be the most likely amount. Snow levels will be up above 7000 feet as precipitation arrives Friday night, so no significant winter impacts are expected initially as precipitation begins. Snow levels will lower to around 5000 feet late Friday night/Saturday morning as the cold front moves through, so some snow will accumulate over the mountains (mostly above 5500 feet). Several inches (4-8") are possible up at Crater Lake, but down on the more traveled roadways around Diamond Lake and also over Highway 140 near Lake of the Woods, it`ll probably just be wet or slushy (especially due to the time of year and warmer ground temps). Model wind fields aren`t particularly impressive with this system, with most suggesting peak wind gusts in the 40-50 mph range over usual spots east of the Cascades (eastern Siskiyou/Modoc and up into Klamath/Lake counties). The cooler, wetter weather pattern with post-frontal showers will continue Saturday afternoon and also Saturday night into Sunday as the deep upper trough/closed low swings through the area, but coverage of showers will lessen with time. While showers are possible just about anywhere and at just about any time on Sunday, much of the time for most areas will be rain-free. Highs on the weekend in the west side valleys will be mostly in the 50s to around 60F. East side temps will top out in the upper 40s to mid 50s. Models are showing moist onshore flow continuing into early next week with renewed (and increased) chances for rain showers (50-70%) along the coast and into the Umpqua and over to the Cascades Monday/Tuesday. But, precip chances are lower (20-40%) across SE sections. We may get a reprieve from the active weather as the Climate Prediction Center 8-14 day outlook favors a return toward more "normal" conditions (temps/precip) for the second week of May. -Spilde && .AVIATION...30/18Z TAFs...VFR will prevail over a large portion of the forecast area today through Wednesday morning. A weak front pushing onshore now is leading to onshore flow and scattered showers for areas near and west of the Cascades. Temporary MVFR ceilings or briefly reduced visibility can occur in any of the heavier showers as well as partial terrain obscurations, but even then lower conditions should only last for a half hour or so. Gusty NW winds are expected in Medford (~20kt) and Klamath Falls (~30kt) this afternoon/evening til around 3-4Z. Tonight, breezy conditions ease and showers end. Low-level moisture could lead to MVFR ceilings in portions of the Umpqua Basin, including Roseburg, later tonight (after 06Z) until early Wednesday morning (around 16-17Z). But again, most areas remain VFR. -Spilde && .MARINE...Updated 230 PM Monday, April 30, 2024...Seas will be hazardous to small craft through this evening with a combination of wind-driven seas and dominant northwest swell. A weak surface high will move in over the waters Wednesday with moderate north winds, highest south of Cape Blanco. Then, another front will move through the waters Wednesday night into Thursday with winds and wind wave dominated seas just shy of small craft advisory levels. A break with calmer conditions is expected Thursday night into early Friday. A series of fronts is likely to follow Friday into early next week with stronger winds and building seas with multiple chances for rainfall. -DW && .MFR WATCHES/WARNINGS/ADVISORIES... OR...Freeze Warning from 2 AM to 8 AM PDT Wednesday for ORZ024. CA...None. PACIFIC COASTAL WATERS...Small Craft Advisory until 11 PM PDT this evening for PZZ356-376. Small Craft Advisory until 11 PM PDT this evening for PZZ350- 370. && $$