Area Forecast Discussion
Issued by NWS Pendleton, OR
Issued by NWS Pendleton, OR
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116 FXUS66 KPDT 020020 AFDPDT Area Forecast Discussion National Weather Service Pendleton OR 520 PM PDT Wed May 1 2024 .AVIATION...00Z TAFs...A weather system from the Pacific will bring increasing chances of MVFR or lower conditions (CIGS and/or VSBYs) overnight to KBDN/KRDM (80-95% chance of MVFR, 30-40% chance of IFR, and <20% chance of LIFR) as ample cloud cover and precipitation spreads over Oregon. Precipitation type will be a challenge with this system, leading to lower-than-average confidence in CIGs/VSBYs as precipitation may (20-40% chance) turn over to snow or a rain/snow mix by late night and early morning at KRDM/KBDN. Elsewhere, VFR conditions are very likely (85% chance or greater). Winds of less than 10 kts are forecast after this evening. Plunkett/86 && .PREV DISCUSSION... /issued 251 PM PDT Wed May 1 2024/ SHORT TERM...Tonight through Friday night...The forecast area is currently under a northwest flow aloft and observing a break in precipitation after last night`s shortwave trough. The air mass remains marginally unstable over most of the region. SPC Mesoscale Analysis shows SBCAPES between 100-500 J/kg and 7-8C lapse rates between H7-H5, but a mid level cap is keeping clouds primarily as towering cumulus. WSR-88D is picking up on a few weak returns over Wallowa County that could be isolated shower activity. Winds are a little breezy from the WSW sustained at 10-20 mph across the Lower Columbia Basin. Water vapor loops show a wave developing west of Vancouver Island, and this is the next system to bring precipitation to our southern forecast area late tonight through Thursday morning. The track of the system and the left exit region of the jet will bring the best potential over the southern half of Oregon, and the forecast will show 60-80% PoPs south and west of Madras and south of John Day (100% along OR Cascade crest). Looking at the HREF members, there are differences on the placement of the main band of precipitation which has made for some challenges in tonight`s forecast. This will be a quasi-stationary front resulting in steady precipitation for about 6-8 hours. Snow levels will be around 4000 feet and current wet bulb temperatures are in the 30s, therefore many areas will observe a snow or snow/rain mix that will affect some of the highways such as HWY 26 and 97. Using the 10:1 snow amounts of the HREF, the prob for 1" or more is around 70% and the prob for 2" or more ranges 50-70%. The high probabilities of snow accumulation at 3" or more are shown in the higher terrain. Confidence is high that Mt. Bachelor will have moderate to heavy snow with this front. There will be a sharp line of areas observing precipitation to areas with no precipitation, and eastern WA and most of northeast OR will have dry conditions tonight. It will be another chilly night as temperatures fall into the 30s to around 40. No Freeze Warnings are in effect, although overnight lows will fall into the mid-30s in the Kittitas and Yakima Valleys. The band of snow and rain will decrease in coverage Thursday as drier air in a NNW flow spreads across the region. The front will bring showers (30-50% chance) over the eastern mountains/valleys along with a slight chance (15%) of thunderstorms as it tracks eastward. CAMs show around 150 J/kg SBCAPEs and 15-20 kt of shear between 0-6 km so storms, if any, will likely be weak and diurnally driven (confidence 60%). High pressure aloft will bring mostly dry conditions Thursday night and Friday morning. This will be a weak transitory ridge quickly forced eastward due to a deepening low offshore. Models are in good agreement with the offshore low that will bring widespread precipitation to western WA/OR Friday and will gradually spread east of the Cascade Range Friday night. A southerly flow will increase snow levels to around 6500 feet Friday night. GFS is hinting of a weak AR over central OR in the IVT progs, although the ECMWF is less impressive. This still looks like a wet system to start the weekend, and Friday night looks particularly wet for central and north central OR and south central WA. Wister/85 LONG TERM...Saturday through Wednesday...An upper level low and associated trough will move inland over northern California/southern Oregon on Saturday and move generally eastward through Sunday. Some weak ridging will build in later Sunday into Monday, then the flow will become more zonal and another weaker system will move across the Pacific Northwest later Monday into Tuesday. Finally, the flow becomes more northwest and drier as we head into Wednesday. The first low pressure system will head mainly to the south, however wrap around moisture on the north side of the low is expected to be abundant. Model guidance has come into better agreement this model cycle after have quite a bit of discrepancies in previous runs. The ensemble clusters generally have an agreement between about 53% to about 57% through Tuesday. By Wednesday, there is much less agreement, mainly due to the position of the upper low over the northern plains and how it handles upstream energy (or lack thereof). Agreement is no more than about 40% at this time, and this is day 7, and normal model variability has to be taken into account at this time scale. Latest QPF values are over an inch in most of the mountain locations and even over 0.75 inches in many of the lower elevation locations. The only place that seems to miss out on the rain is central Oregon, though they still get over 0.25 inches. This is a significant increase, especially across many of the lower elevation areas. Snow levels will start out over 5000 feet everywhere, and in most areas over 6000 feet on Saturday, but by Sunday morning, will decrease to between 3000 feet in central Oregon to about 4500 feet elsewhere. So a mix of rain and snow or even all snow is possible Sunday morning, though accumulations are expected to be light, except at the highest elevations where there could be a few inches. During the day, snow levels will rise again to between 4500 and 5500 feet. Precipitation should begin to wind down, especially across the lower elevation areas, Sunday into early Monday. However, chances will increase again later Monday as the next system moves through. Finally, by later Tuesday into Wednesday, precipitation chances will decrease most everywhere. The ECMWF EFI shows QPF values of 0.8 to 0.9 over central Oregon on Saturday, with the highest values west of the Cascades. For snow, there is a small area 0f 0.7 to 0.8 over the Oregon Cascades, high shift of tails values of 1 to 2 extend northeastward into the higher terrain of the southern Blue Mountains. These values indicate higher uncertainty due to the low and trough and at least some low potential for snow. Any snow would be anomalous in May. On Sunday, the EFI QPF anomalies shift north and eastward to northeastern Oregon and southeastern Washington, where values are 0.7 to 0.8, with a shift of tails of 2, centered over the Columbia Basin. The snow is generally 0.6 to 0.7 over the northern Blue mountains and a shift of tails of 2. So the ECMWF while indicating some target areas is also indicating some continued uncertainty in the amounts. Winds will also be gusty, especially later Saturday into Sunday, mainly across the Columbia basin, Gorge and Foothills of the Blue Mountains, Yakima Valley and Kittitas Valley. Breezy winds are expected to continue into Monday, and could become more widespread across the region. Winds will generally gust 25 to 35 mph. The ECMWF EFI is not keying in on anything regarding winds on Saturday, however it does focus in on winds on Sunday with a large area of 0.7 to 0.8 and a small 0.8 to 0.9 across the Oregon Columbia Basin and Blue Mountain Foothills. This area shifts east and north on Monday into Washington, over the Blue Mountains and nearby foothills and Columbia Basin with a large area of 0.6 to 0.7 and the focus of 0.7 to 0.8. The NBM probabilities of winds >= 39 mph on Sunday across the Blue Mountain Foothills, Columbia Basin, Gorge, Simcoe Highlands Yakima Valley and Kittitas Valley are as high as 70 to 90%. However, the same NBM probabilities for winds >=47 mph in the same locations are only as high as about 40 to 60% across a much smaller area. Finally, temperatures are expected to be below normal through the period due to the presence of the trough initially. They will warm a bit by midweek and will end up closer to normal, but still below, especially in central Oregon. On Saturday, highs will range from the 50s in central Oregon to the 60s across the Columbia Basin. In central Oregon, these high temperatures would be about 15 degrees below normal, where elsewhere, they may only be about 3 to 5 degrees below normal. Sunday will be the coolest day. Highs will be near 50 in central Oregon, to the upper 50s in the Basin (and could be cooler). Again, this is about 15 degrees below normal in central Oregon and about 10 or so degrees cooler elsewhere. By Monday, temperatures will begin to warm, but will still be below normal, especially in central Oregon. The ECMWF EFI also keys in on the anomalous high temperatures beginning Sunday, mainly from central Oregon west, with values of -0.7 to -0.8. The area spreads eastward on Monday, but the values are less. Most of the state of Oregon is covered on Monday in the - 0.6 to -0.7 range. On Tuesday, virtually all of Oregon and most of Washington is covered in -0.6 to -0.7 anomalies for highs, with a large portion of Oregon in -0.7 to -0.8, with similar results Wednesday, though a bit further east. && .PRELIMINARY POINT TEMPS/POPS... PDT 35 62 36 70 / 0 20 0 0 ALW 37 66 40 73 / 0 10 0 0 PSC 39 69 42 75 / 0 0 0 0 YKM 34 67 37 70 / 0 0 0 0 HRI 40 68 39 75 / 10 10 0 0 ELN 33 66 36 67 / 0 0 0 0 RDM 34 55 33 64 / 90 70 0 10 LGD 32 58 32 65 / 10 40 10 0 GCD 34 56 33 65 / 50 60 20 0 DLS 42 67 42 70 / 40 30 0 10 && .PDT WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. && $$ SHORT TERM...85 LONG TERM....77 AVIATION...86