FXUS66 KPDT 182315 AFDPDT Area Forecast Discussion National Weather Service Pendleton OR 315 PM PST Mon Nov 18 2024 .SHORT TERM...Tonight through Wednesday...Lingering showers and snow showers are noted across the mountains this evening as we wrap up on the back side of a trough exiting the region, but zonal flow through Tuesday morning will continue to promote moisture moving into the Cascades with periods of light snowfall possible along and just past the crests. The real story now for the short term comes in the form of a very strong offshore low that will descend from the Gulf of Alaska and deepen considerably near the offshore borders of Canada and the US. Major models are all indicating this system will near or drop just below 940mb, bringing hurricane force offshore winds and a steady stream of southerly flow to our inland region. Although frictional effects will help to alleviate some of the strength of these winds, CAMs and ensemble solutions all agree we're looking at a noteworthy wind event for our area, with widespread gusts of 45-60+ mph possible during the overnight Tuesday into Wednesday morning timeframe. The mountains in particular will be impacted by these winds, and combined with falling or already laid ground snow will present a significant reduction of visibilities. The ECMWF ensembles all show a widespread agreement on wind gusts of 55-60 mph during that timeframe for Snoqualmie Pass for example, and this alongside short term model agreement has led to a blizzard warning being issued for the Washington Cascades. Meanwhile, the east slopes of the Washington Cascades and the Oregon Cascades will see noteworthy snowfall alongside very gusty winds, but blizzard level conditions of limited visibilities should be much more limited for these locations, but still expect significant hindrances to travel for these areas. On the note of precipitation, the initial heaviest wave of rain and mountain snow will arrive with that stronger push of winds overnight Tuesday into Wednesday. By the time we reach the end of Wednesday, almost every location across the CWA is expected to have received 0.1+ inch of new rainfall outside of the Columbia Basin and portions of Central Oregon, with the NBM giving a high probability to this occurring at 50-100% chance. Otherwise, southwesterly flow aloft will begin to overspread warmer air and increase our temperatures, allowing snow levels to rise and limit future snowfall to the crests and peaks as we move into the end of the week. Goatley/87 .LONG TERM...Thursday through Monday... Key Messages: 1. Widespread rain chances through period. 2. Above normal temperatures extend into weekend. 3. Localized flooding possible. The extended period is characterized by a strong, upper level low pressure system that stalls off the Washington/Oregon coasts before moving onshore and through the Pacific Northwest late in the weekend. Southwest flow aloft will be dominant under this synoptic pattern, leading to widespread rain chances each day as a weak atmospheric river sets up through the workweek. The ECMWF EFI indicates Friday's rainfall as being above the climatological normal and the wettest day in the forecast, but rain amounts on Thursday will be rather similar. Rain amounts of 0.25-0.45" are expected across lower elevations of the Basin, Central Oregon, and the Blue Mountain foothills with 0.35-0.85" of rainfall expected at higher elevations of the Cascade east slopes, John Day Highlands, and the Blue Mountains both Thursday and Friday. Confidence in these rain amounts is moderate to high (60-80%) as the NBM suggests a 25-55% chance of 0.25" of rain or more over lower elevations of the Basin and a 50-75% chance along the Blue Mountain foothills Thursday and Friday. The upper level low opens and broadens out on Saturday ahead of moving onshore mid-to late Sunday. This weakened and less organized system will still bring widespread rain showers across the area. Rain amounts of 0.50-0.75" at elevation over the Cascade and Blue Mountains, 0.20-0.40" along the Blue Mountain foothills, and 0.01-0.10" across lower elevations of the Basin and Central Oregon are expected on Saturday. Conditions will continue to dry out into Sunday, with lower elevations of the Basin staying dry as rain amounts stay confined to our foothills and mountain zones. Snow levels will hover between 5000-6000 feet Thursday and Friday, dropping to 3500-4500 feet Saturday and 2500-3500 feet Sunday. These values will keep snowfall over our mountainous terrain of the Cascades, Blue Mountains, John Day/Ochoco Highlands, and the Simcoe Highlands through the period. 1-5 inches of snowfall is anticipated to occur across the Cascade passes Thursday and Saturday. Southwest flow aloft will help to bring in a warmer and wetter airmass through the week. This will keep high temperatures in the upper 40s to low 50s through the period, which will be 2-5 degrees above normal for this time of year. Overnight lows will also be rather mild through the workweek, with morning temperatures dropping into the upper 30s to low 40s Thursday and Friday. A cooling trend develops over the weekend as the system offshore opens, weakens, and begins to push onshore. Monday morning temperatures will drop to near or below freezing over the Basin, with values dropping into the mid to upper 20s across Central Oregon. Confidence in experiencing freezing morning temperatures on Monday is moderate (50-70%) as the NBM suggests a 40-70% across lower elevations of the Basin and a 75- 95% chance over Central Oregon. There are significant rain amounts in the long-term forecast, which may lead to isolated or localized areas of flooding - primarily in and around urban and low-lying areas. Snowpack is not yet established, so the probability of rain-on-snow events is low as snow levels begin to decrease Saturday. However, there may be times when cells train and persistent moderate to heavy rainfall may lead to ponding on roadways and localized flooding near bodies of water. 75 && .AVIATION...18Z TAFS...VFR conditions currently at all sites, but are expected to degrade to MVFR for KDLS/KRDM/KBDN due to light snowfall leading to reduced visibilities during the early morning hours on Tuesday. Ceilings will also drop late in the period as a result of the incoming system, with KDLS dropping the lowest to OVC025. Winds will be breezy this afternoon for KDLS/KPDT/KRDM/KBDN/KALW/KPSC with sustained winds of 10-15 kts and gusts of 20-25 kts. Winds will decrease this evening and continue to dissipate through the end of the period. 75 && .PRELIMINARY POINT TEMPS/POPS... PDT 27 47 36 52 / 20 20 90 70 ALW 30 47 37 52 / 30 30 90 80 PSC 33 49 41 55 / 30 20 90 50 YKM 23 43 29 46 / 30 60 100 60 HRI 31 51 40 55 / 40 20 90 50 ELN 23 40 29 44 / 30 60 100 70 RDM 24 43 35 49 / 50 20 90 60 LGD 21 37 31 42 / 20 40 90 90 GCD 20 40 32 44 / 50 50 100 90 DLS 32 46 39 52 / 60 80 100 80 && .PDT WATCHES/WARNINGS/ADVISORIES... OR...Wind Advisory from 4 PM Tuesday to 10 AM PST Wednesday for ORZ044-049-505. High Wind Warning from 4 PM Tuesday to 10 AM PST Wednesday for ORZ507-508-510-511. Winter Weather Advisory from 4 PM Tuesday to 10 AM PST Wednesday for ORZ509. WA...Winter Weather Advisory from 4 PM Tuesday to 10 AM PST Wednesday for WAZ026. Blizzard Warning from 4 PM Tuesday to 10 AM PST Wednesday for WAZ522. Winter Storm Warning from 4 PM Tuesday to 10 AM PST Wednesday for WAZ523. && $$ SHORT TERM...87 LONG TERM....75 AVIATION...75