963 FXUS66 KPDT 201807 AFDPDT Area Forecast Discussion National Weather Service Pendleton OR 1007 AM PST Tue Feb 20 2024 Updated Aviation Discussion .MORNING UPDATE...Satellite reveals a closed low offshore with the forecast area under a deep, moist southwest flow. In combination with 7-8 C/km mid-level lapse rates and modest upper-level lift this is facilitating ongoing shower activity across the forecast area. Looking to the surface, patchy fog and dense fog persist in fog-prone areas, most evident from Madras north to north-central Oregon and along the foothills of the Blue Mountains. In Washington, most fog appears limited to ridgetop/summit locations of Kittitas and Yakima counties. Have opted to cancel the dense fog advisories early due to lack of coverage, but patchy fog, locally dense, will likely (70-100% chance) persist through the morning hours. This afternoon, the focus turns to convective potential. 12Z HREF members are in subjectively good agreement that modest (250-750 J/kg) MUCAPE will develop along preferentially heated slopes of the Ochocos north to the southern and northern Blue Mountains. While confidence is still low (10-20% chance) in any lightning activity with showers, satellite does show areas of clearing, and the aforementioned steepening mid-level lapse rates, developing surface-based instability in areas of clearing skies, modest synoptic lift, orographic lift, and weak shortwaves wrapping around the upper-level low has led to the inclusion of a "slight chance" mention of thunder for most of the eastern mountains of Oregon and southeast Washington down into the foothills. While not favored, 12Z HREF soundings do show weak low- level SRH (<50-100 m2/s2) so cannot rule out an isolated weakly rotating cell tracking off the Blues into the foothills. No severe threat is anticipated. Plunkett/86 && AVIATION...18Z TAFs...KDLS and KPDT are expected to be varying between MVFR and VFR due to mist and light precipitation, but KDLS is currently at LIFR with a low CIG at 17Z. However, CIGs for KDLS will improve through the day while the VSBYs fluctuate due to mist and showers within the vicinity until Wednesday morning when more light rain occurs. The CIGs and VSBYs for KPDT will be at VFR and with no precipitation. KALW is forecast to be at MVFR this morning with mist and showers but should improve tonight. Most sites are forecast to be at VFR by afternoon with winds below 10kts, with an exception of KDLS staying at MVFR. Feaster/97 && .PREV DISCUSSION... /issued 301 AM PST Tue Feb 20 2024/ SHORT TERM...Today through Thursday...A deep low continues to spin away offshore the southwest Oregon coastline tonight while pushing surges of moisture north across the PacNW. At the moment, radar shows scattered showers moving across the PacNW, though the heaviest showers are occurring outside of the forecast area. Overhead, however, mid level cloud decks continue to move forecast area while low stratus and areas of dense fog continue to shift around the Blue Mountain foothills and central/north central OR. Today, the deep low will weaken into an open trough as it makes it's trek towards Vancouver Island, all the while pushing weak plumes of moisture across the PacNW. This will result in light rain and mountain snow showers moving across the forecast area throughout today, with snow accumulations up to around 2 inches possible above 4.5kft to 5kft in the mountains. Otherwise, any rain accumulations in the mountains will be less than 0.2 inches, while the lower elevations will see less than 0.05 inches. As for fog concerns, areas of central and north central OR have cleared up tonight with the push of southerly winds, and with southerly winds persisting through the remainder of the morning, the dense fog advisories currently in effect will likely be ended early. As for the Blue Mountain foothills, dense fog continues to be noted from Pendleton to Walla Walla, with NBM guidance indicating that this trend will continue late into the morning. Tonight, areas that see precipitation throughout the day will see patchy fog develop, especially in portions of the Columbia Basin, Gorge, and adjacent valleys/foothills. Wednesday, the open trough will push east across the PacNW with scattered showers across the region. The Cascades will see the brunt of the precipitation accumulations through Wednesday, with liquid equivalent accumulations between 0.25 to 0.5 inches, and snow amounts between 5 to 7 inches along the high ridges and crest. It's a different story across the eastern mountains, Blues, and Wallowas as rain shadowing from the Cascades will only result in QPF amounts up to 0.15 inches, with snow amounts along the high ridges between 0.5 to 2 inches. While the lower elevations may see an isolated shower or two move across that will result in a few hundreths of an inch of rain, except along the Blue Mountain foothills where 0.05 to 0.1 inches are forecast. Wednesday night, the open trough will exit to the east and allow an upper ridge to build over the PacNW. This will result in dry and quiet conditions developing Thursday. Lawhorn/82 LONG TERM...Friday through Tuesday...The long term will start off on the quiet side as high pressure ridging will sit over the region and bring seasonably warm conditions, but the next system to bring winter back will arrive by the end of the weekend into next week, with lower snow levels and seasonably normal to below average temperatures in its wake. Friday starts under high pressure ridging leaving the region quiet overall. Some passing shortwaves and moisture through Sunday afternoon may bring a passing shower or snowfall in the Cascades or Northern Blues, but confidence is low (<30%) in any activity. On Sunday an upper level system will dive down from the Gulf of Alaska and follow the Canadian coastline before moving inland just to our north across the border. This deep low will bring cooler temperatures, a surge of moisture, and gusty winds to begin next week. The NBM shows a 30-80% chance of wind gusts in excess of 30 mph, and it's likely this will become more likely in the coming days as we get into higher resolution data. The GFS is already presenting pressure gradient differences of around 10-12 mb from Spokane to Portland, which would be favorable of at minimum wind advisory (45+ mph) gusts. On the side of precipitation, the forecast here is a bit more complex. The NBM is favoring fairly stout snowfall in the mountains with snow ratios back up into the 10+ inch range and 48-hour probabilities of 25-90% at the Blues, Wallowas, and Cascades seeing 10+ inches. But with the potential for onshore flow which would be a bit warmer than the forecast is allowing combined with high temperatures only a day or two ago in the mid 30's to low 40's for most of these mountain zones, the NBM may be overdoing the expected snowfall. For now, going to allow the forecast to play out as the NBM has produced, but expecting snowfall amounts to see a decrease in the coming days unless the cold temperatures truly hold as forecast confidence increases. Overall, there is moderate confidence (40-50%) in the expected conditions. The first few days of the forecast have generally good agreement in ensembles, but in the last few as the system is impacting us, we start to see guidance agreement fall apart in the strength of the troughing. If the troughing isn't quite as strong, we may get more zonal flow which would be warmer temperatures than forecast, and 40% of European members favor this solution; whereas, 53% members of the GFS expect a more northwesterly flow which would bring cooler temperatures, and overall comparing ensemble highs from both models shows the ECMWF running several degrees warmer both this weekend and next week. Still, we should see a fairly spring-like weekend, followed by a wintry return in the mountains with the lower elevations cooler but not quite an Arctic return. Goatley/87 && .PRELIMINARY POINT TEMPS/POPS... PDT 52 35 50 33 / 20 40 30 20 ALW 52 36 50 35 / 40 30 30 20 PSC 51 36 52 35 / 20 10 20 0 YKM 47 34 48 31 / 20 30 20 10 HRI 51 35 52 34 / 20 20 20 10 ELN 45 34 43 31 / 30 30 30 10 RDM 49 34 48 29 / 10 10 10 10 LGD 49 34 48 32 / 40 20 40 40 GCD 49 32 48 29 / 20 10 30 20 DLS 47 37 48 35 / 40 40 40 10 && .PDT WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. && $$ MORNING UPDATE...86 SHORT TERM...82 LONG TERM....87 AVIATION...97