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Area Forecast Discussion
Issued by NWS Pendleton, OR
Issued by NWS Pendleton, OR
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090 FXUS66 KPDT 171005 AFDPDT Area Forecast Discussion National Weather Service Pendleton OR 305 AM PDT Sat Aug 17 2024 .SHORT TERM...Overnight satellite imagery reveals the calm before the storm: mostly clear skies over the forecast area, while a deep low pressure system churns just off the coast of the PacNW. This system will bring a myriad of impacts to us today, including strong to severe storms across the Cascade range, possibly extending into the east slopes, as well as a burst of hot and dry weather across the lower Columbia Basin eastward. The weather looks quieter after today, but Saturday does look to be awfully active. Looking at the timing of showers and storms today, CAMs more or less agree in cells materializing around 1 to 3 PM this afternoon, which is a bit later than previous guidance had suggested. Storms will then lift northward along the spine of the Cascades, reaching Washington by around 4 to 6 PM, lasting well into the evening and with some CAMs suggesting isolated activity across the central WA Cascades through the overnight hours heading into Sunday. Confidence is high in storms being strong to severe over the Cascades, as the upper-level dynamics are good with strong orographic lifting and plentiful moisture to work with (PWATs are as high as an inch to 1.5 inches, which is impressive across a high mountain range). Bulk shear is formidable as well at over 55 kts in some spots, which makes sense given the strong SW to S flow ahead of the low. SPC has painted the Cascades under a slight risk of severe storms as a result, with damaging winds and hail being the primary risks. QPF ranges from two tenths to even three quarters across the Cascades, so wetting rains should be widespread enough to preclude any fire weather headlines. Biggest challenge comes in predicting just how far east storms will extend beyond the Cascades. The 00z HREF reveals that CAM members are in a bit better agreement in storms spreading into the Washington Basin and not much south of that, however it is a bit concerning that models like the NAMNEST reveal scattered storms across much of the Columbia Basin in the evening, while the HRRR remains fairly dry east of the Cascades. Decent bust potential is in the forecast as a result, but more HREF members do appear to be siding with storms spreading eastward, so did opt to upgrade the Basin zones under a Fire Weather Watch into a Red Flag Warning. The synoptic setup is strong enough that current thinking is that it`s better to err on the side of caution. That being said, the stark drop-off in mid-level moisture according to models like the NAM12 reveal the more southerly orientation of the winds aloft as you trek eastward into the Interior Northwest. Southerly flow aloft will advect hot and dry Great Basin air into this area, with the NBM suggesting that even the Tri-Cities may see highs approaching the century mark. Down south, toward OR642, winds will be gusty, and instability elevated, so a Red Flag Warning remains in effect there. Thinking that the forecast area east of the Tri-Cities remains warm and dry with locally gusty winds, but critical fire weather conditions due to winds and low RH appear to be isolated to around southeastern Oregon for today. A blocking pattern keeps this low in place for the next several days, leaving us under amplified SW flow, however moisture aloft drops off, thus a dry forecast with seasonable temps prevails for Sunday and Monday. While we`re under this pattern, however, intermittent chances for high mountain showers will prevail so long as favorable moisture advection takes place under this pattern. Evans/74 .LONG TERM...Tuesday through Saturday...Models remain in good agreement about maintaining the current persistent weather pattern of an upper low and trough along the Pacific Northwest coast and a ridge over the Rockies and the Midwestern states through the long term period. Differences between models are fairly minor Tuesday through Thursday then begin to increase next Friday and Saturday. This will give us cooler than normal temperatures and showery conditions mainly in and near the Cascades most days. Thunderstorm activity will be fairly limited away from the Washington Cascade crest. The Kittitas Valley and eastern Columbia Gorge will have breezy 15-20 mph winds each afternoon though winds will be nothing unusual in the rest of the area. The Extreme Forecast Index is indicating little in the way of unusual weather aside from the below normal high temperatures in central Oregon Tuesday through Thursday. Models show the upper low in the offshore trough opening up into a wave Tuesday and coming ashore in the evening while another upper low moves south along the British Columbia coast. This will bring a chance of showers to the Washington Cascades and a slight chance of showers to most of the rest of the area in the late afternoon and Tuesday night. The Washington Cascades and Simcoe Highlands could get up to a tenth of an inch of rain but the rest of the area will have minor amounts of rain. NBM probabilities show less than a 15 percent chance of a tenth of an inch of rain over the entire area. Models aren`t showing any CAPE and LIs are +1.5 and higher, so do not expect any thunderstorms. Temperatures will be in the mid 70s to mid 80s. The Tri-Cities has a 41 percent chance of 90 degree temperatures and Hermiston and Hanford 21 percent but the rest of the area is under 10 percent. Wednesday will have the trough move overhead while the next upper low continues south along the British Columbia coast. This will keep a a chance of showers along the Washington Cascade crest with a slight chance extending into the Kittitas and Yakima Valleys. Models hint at enough instability along the immediate Cascade crest in the late afternoon, so have a slight chance of thunderstorms there. Due to the colder air with the trough, temperatures drop about 4-5 degrees to the upper 60s and 70s with a few places reaching the lower 80s in the Columbia Basin. Chances for 90 degree temperatures are virtually zero and chances for 80 degrees are 80-100 percent in the Columbia Basin and 45-70 in the rest of the lower elevations. Thursday will see the deepening low arrive in the base of the trough off the Pacific Northwest coast. A slight chance of showers with very light rain amounts will continue mainly along the Cascades though a few showers will develop in central Oregon in the late afternoon. Models indicate some instability in the late afternoon in central Oregon but the window appears to be short and the NBM doesn`t add any thunderstorms so have kept them out of the forecast for central Oregon. The chances of a tenth of an inch of rain are no more than 15 percent along the Cascade crest. Temperatures will warm 1-2 degrees to the 70s and lower 80s. The Tri-Cities and Hermiston will have a 15-25 percent chance of 90 degrees but chances will be under 10 percent elsewhere. By Friday, models have a deep trough but differ significantly as to the location. About 60 percent of the model ensemble members have the trough a few hundred miles offshore, 20 percent right at the coast and the remaining 20 percent have a weaker trough centered near the Cascades. The NBM favors the majority and has the southwest flow developing a slight chance of showers along the Cascades and central Oregon in the afternoon then into the eastern mountains in the evening. Central Oregon has a slight chance of afternoon thunderstorms as well. Rainfall amounts look to be very light with chances of a tenth of an inch no more than 15 percent. Due to cloudiness and showers, temperatures remain stationary in the mid to upper 70s in central Oregon but the rest of the area warms another 3 degrees or so to the lower to mid 80s with mainly 70s in the mountains. The Columbia Basin has a 30-55 percent chance of reaching 90 degrees while the rest of the area is under 25 percent. On Saturday, model clusters show four significantly varying solutions, each with support from 23-28 percent of the members, so no one solution appears more likely than the others. The choices are a trough with a strong closed low off the Oregon/California border, a trough with a weaker low a few hundred miles further south off the northern California coast, a strong trough with the low opening into a wave at the northern California coast and a shallow trough with its axis near the Idaho border. Most of these favor continued showery conditions and the NBM has a chance of showers over the mountains and a slight chance of showers elsewhere. It also have a slight chance of afternoon thunderstorms over the eastern mountains. It drops temperatures 3-5 degrees due to the clouds and showers to the 70s and lower 80s. Through the long term, the NBM brings breezy winds of 15 to 20 mph through the eastern Columbia Gorge and Kittitas Valley, but the chances of 25 mph winds in those locations are only 30-50 percent Tuesday, Wednesday and Saturday and under 30 percent Thursday and Friday. The rest of the area will have normal afternoon wind speeds under 15 mph. Perry/83 && .AVIATION...06Z TAFs (Previous Discussion)...Quiet weather and VFR conditions, consisting of winds of 10 kts or less coupled with sky cover ranging from SKC to FEW-SCT mid-high cloud will persist overnight through Saturday morning. Saturday afternoon, confidence is very high (>90% chance) that thunderstorms will initiate over the Cascades and propagate northward through evening. Along the east slopes, confidence is lower in shower and thunderstorm coverage...generally 30% or less for central Oregon. By evening, showers and thunderstorms will likely expand northward to north-central Oregon and south-central Washington, impacting DLS and YKM (>50% chance of precipitation). Afternoon thunderstorms along the Cascades are expected (>75% chance) to produce outflow winds of 40-50 mph, with lower chances (<20%) of 60 mph. These outflow winds will manifest as a stark wind shift at DLS/PDT, and outflow may also reach the remainder of TAF sites, though confidence in magnitude/timing is lower. Showers and thunderstorms will be capable of producing temporary MVFR or lower conditions. Plunkett/86 && .PRELIMINARY POINT TEMPS/POPS... PDT 93 56 84 52 / 10 20 0 0 ALW 99 62 88 57 / 0 20 0 0 PSC 99 65 89 60 / 0 30 0 0 YKM 93 58 84 54 / 10 80 10 0 HRI 98 62 88 56 / 0 30 0 0 ELN 93 59 84 56 / 0 80 10 0 RDM 87 47 82 46 / 40 60 0 0 LGD 94 53 89 51 / 0 10 0 0 GCD 93 52 88 50 / 0 10 0 0 DLS 95 61 83 60 / 20 80 10 0 && .PDT WATCHES/WARNINGS/ADVISORIES... OR...Red Flag Warning from 2 PM this afternoon to 11 PM PDT this evening for ORZ641. Red Flag Warning from 1 PM this afternoon to 8 PM PDT this evening for ORZ642. WA...Red Flag Warning from 5 PM this afternoon to 2 AM PDT Sunday for WAZ691. Red Flag Warning from 8 PM this evening to 5 AM PDT Sunday for WAZ690. Flash Flood Watch from 2 PM PDT this afternoon through late tonight for WAZ522-523. && $$ SHORT TERM...74 LONG TERM....83 AVIATION...86