FXUS66 KPDT 050549 AFDPDT Area Forecast Discussion National Weather Service Pendleton OR 949 PM PST Mon Mar 4 2024 .Updated Aviation Discussion. .UPDATE...Current radar and infrared satellite imagery showing light returns along the Oregon Cascades, Central Oregon, Blue Mountain foothills, and across portions of the Lower Columbia Basin under mostly cloudy skies. Persistent snow showers will continue along the Oregon Cascades and Central Oregon, pushing into the John-Day/Ochoco Highlands overnight and into the morning hours on Tuesday. Spotty Light snow showers are also anticipated to occur along the Blue Mountain foothills, Simcoe Highlands, and the east slopes of the Washington Cascades overnight before waning through Tuesday morning as precipitation chances push south. These zones further north and at lower elevations will see limited snowfall amounts of only a trace. However, an additional 2 to 6 inches of snow is expected along the east slopes of the Oregon Cascades, 3 to 5 inches of snow through the John- Day/Ochoco Highlands, and 1 to 4 inches of snow through Central Oregon as Winter Weather Advisories are still in effect through 10 AM Tuesday for these areas. Confidence in these snow amounts is moderate (40-60%) as the NBM highlights a 40-70% chance of 4 inches or more of snow along the east slopes of the Oregon Cascades, 30-50% chance of 4 inches or more snow over the John- Day/Ochoco Highlands, and a 15-35% chance of 2 inches or more over Central Oregon. These conditions are a result of a passing upper level trough and associated stationary front that is currently located west-to- east across Central Oregon. This front will migrate south tonight into Tuesday, taking with it precipitation chances as dry conditions will return and extend through much of the week. Northwest flow aloft will persist to allow cooler air to advect into the area, providing a slight decrease in high temperatures of 1-4 degrees from today. This colder air will be more obvious overnight and through the morning hours as low temperatures will stay in the low to mid-20s across the Basin, Eastern Gorge, Blue Mountain foothills, and the Yakima/Kittitas Valleys, with upper- teens likely through Central Oregon. 75 && .AVIATION...06Z TAFS...VFR conditions currently across all sites, with the exception of IFR conditions at KRDM and LIFR conditions at KBDN due to reduced visibilities of 1-2SM and ceilings of OVC001-028. LIFR conditions for KRDM/KBDN will extend through the overnight period before improving slowly through Tuesday afternoon. This is in response to light snow occurring at these terminals, which will persist through the morning hours before becoming more spotty through Tuesday afternoon and evening. Light snow showers are also anticipated to occur at KDLS/KPDT/KALW overnight, but a reduction in flight categories is not expected. Ceilings will hover around OVC050 for all sites, with KBDN/KRDM staying between OVC009-015 until late- morning. Winds will stay light and below 10 kts at all sites through the period. 75 && .PREV DISCUSSION... /issued 311 PM PST Mon Mar 4 2024/ SHORT TERM...Tonight through Wednesday...Satellite imagery this afternoon shows a weak upper low starting to push into the Olympic Peninsula, while organized showers are developing and pushing into western WA and northwest OR. Zooming out, a broad upper trough continues to sit over western Canada and the northern half of western CONUS, while keeping cool air advection into these areas today. The weak upper low just offshore the Olympic Peninsula is expected to swing east across WA this afternoon into tomorrow morning, bringing with it an increase of snow shower activity across the mountainous terrain in the forecast area. Meanwhile, a stationary front extending east to west is expected to develop between 43N and 44N latitudes overnight, with snow showers associated with this frontal boundary extending north into portions of Jefferson, Deschutes, Crooks, and Grant counties. Snow amounts of 3 to 6 inches in the OR Cascade east slopes and Ochoco-John Day Highlands is expected through late tomorrow morning, and 2 to 4 inches in the Redmond/Bend/Prineville areas with light snow flurries further north possible. Tuesday, the stationary front and the moderate snowfall associated with it will meander south of the forecast area as another shortwave rounds the broad upper trough. This shortwave passage tomorrow will help to keep light snow showers going across the eastern mountains and the southern portion of the forecast area. By Tuesday night, a cool and dry northwest flow will develop over the PacNW, resulting in shower activity decreasing area-wide. These drier and cooler conditions will continue through Wednesday as northwest flow prevails aloft. Temperatures will remain below normal through the middle of the week, with afternoon temperatures ranging from mid 30s (mid-elevation valleys) to upper 40s (Columbia Basin)...except 20s to lower 30s mountains. Lawhorn/82 LONG TERM...Thursday through Monday...The long term period will see a few days of quieter weather as a ridge build overhead before the weather pattern becomes more active this weekend into early next week as a trough and pair of fronts move through the area. The Extreme Forecast Index shows little in the way of unusual weather aside from breezy winds Saturday along the Blue Mountain Foothills and in central Oregon. This fits with the forecast of south to southwest winds of 15 to 25 mph with gusts to 35 mph in those locations as well as the Grande Ronde Valley. There is a small Shift of Tails area in the Pendleton area suggesting the potential for a southerly downsloping wind event along the base of the Blue Mountains. This is something that will have to be watched in the upcoming days. Thursday will see the trough that had been over the area moving off to the Rockies while a drier northwest flow from an offshore ridge will move overhead. This will give us dry weather and slightly warmer temperatures with highs in the 40s and in the mid 30s to lower 40s in the mountains. This will still be several degrees below normal for early March. Thursday night through Friday night, models continue to be in excellent agreement in having the ridge come ashore and move through the area. This will keep us dry and give us temperatures rising to near normal with highs in the upper 40s to mid 50s in the lower elevations and in the 40s in the mountains. The weather pattern will change Saturday as the ridge departs to the east and a weakening frontal system will move ashore and into our area. There will be a chance of rain and snow in the mountains on Saturday afternoon and evening while the lower elevations will just have a slight chance of rain with a few hundredths of an inch at most. Snow amounts in the mountains will be on the order of an inch or two along the Cascade crest. There is a 65-70% chance of an inch of snow along the crest and a 40% chance of 2 inches but values drop of significantly at higher snow amounts. In the eastern mountains, the probability of an inch of snow is 35-45%. Temperatures Saturday will be similar to Friday. Taking a closer look at the winds mentioned above, The Grande Ronde Valley has an 85-95% chance of wind gusts to 39 mph and the southern Grande Ronde Valley has an 80% chance of gusts to 47 mph but the chances drop off sharply at higher wind speeds. In the Blue Mountain Foothills, there is a 70-80% chance of 39 mph wind gusts from Pendleton to Milton-Freewater, but drops to about 40% for 47 mph gusts. Chances are much lower elsewhere in the Blue Mountain Foothills. It's far too early to make a decision, but a wind advisory for the southern Grande Ronde Valley and the Blue Mountain Foothills looks possible if the models stay consistent with these wind speeds. After a break late Saturday night and Sunday morning, models bring in a stronger frontal system, though there are differences about the timing and strength of the system. The ECMWF is weakest and fastest with the system and the GFS is strongest and slowest. The NBM has a chance of mountain snow developing Sunday afternoon with a slight chance of rain in the lower elevations. This becomes likely mountain snow and a chance of lower elevation rain Sunday night and Monday. This looks like a good compromise between the models and the NBM solution was used. The NBM has a 75-85% chance of 4 inches of snow along the Cascade crest though it drops off to about 25% for 6 inches. The eastern mountains have a 40-60% chance of 2 inches of snow but only about a 30-40% chance of 4 inches in the highest portions of the mountains. Rainfall in the lower elevations are a tenth of an inch or less on Sunday and Monday. Winds will remain breezy on Sunday but significantly lower than on Saturday. Temperatures will remain in the mid 40s to mid 50s in the lower elevations and in the upper 30s to mid 40s in the mountains. Perry/83 && .PRELIMINARY POINT TEMPS/POPS... PDT 29 43 25 44 / 40 10 0 0 ALW 30 44 26 46 / 30 20 0 0 PSC 31 49 28 48 / 10 0 0 0 YKM 26 47 23 47 / 10 0 0 0 HRI 31 48 27 48 / 20 0 0 0 ELN 23 43 20 43 / 10 0 0 0 RDM 25 37 18 41 / 60 40 10 0 LGD 26 39 18 42 / 50 20 10 0 GCD 25 37 19 42 / 80 50 20 10 DLS 32 49 27 47 / 40 0 0 0 && .PDT WATCHES/WARNINGS/ADVISORIES... OR...Winter Weather Advisory until 10 AM PST Tuesday for ORZ506-509- 511. WA...None. && $$ UPDATE...75 SHORT TERM...82 LONG TERM....83 AVIATION...75