Area Forecast Discussion
Issued by NWS Pendleton, OR
Issued by NWS Pendleton, OR
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392 FXUS66 KPDT 311104 AFDPDT Area Forecast Discussion National Weather Service Pendleton OR 332 AM PDT Fri May 31 2024 .SHORT TERM...Friday through Sunday... Key Messages: 1. Breezy winds Saturday afternoon and evening. 2. Widespread rain showers return Sunday afternoon. 3. Warming temperatures through Saturday before cooling Sunday. Current radar and infrared satellite imagery showing dry condition as a few high level clouds pass over our northern zones. This is a result of a transient upper level ridge that will be passing through the Pacific Northwest today, keeping skies mostly clear to partly cloudy north-to-south and high temperatures about 10 degrees warmer than on Thursday. The ridge will exit to our east early Saturday as an upper level shortwave follows closely in its wake, tightening isobars between the two systems and allowing a weak pressure gradient to develop along the Cascades. As a result, winds will increase through Saturday morning before peaking in the afternoon between 25-35 mph across the Simcoe Highlands, Kittitas Valley, and along the Lower Columbia Basin of Oregon. Winds elsewhere are expected to stay between 15-25 mph primarily out of the west- northwest. Confidence in these wind values is moderate to high (70- 80%) as the HREF suggests a 60-80% chance over the Simcoe Highlands, a 35-45% chance along the Lower Columbia Basin of Oregon, and a 10- 20% chance across the Kittitas Valley of 30 mph gusts or greater. The NBM highlights more inflated probabilities of 35 mph or greater as 70-80% over the Simcoe Highlands, 40-60% along the Lower Columbia Basin of Oregon, and 20-40% over the Kittitas Valley. However, 850mb winds of between 24-35 mph are advertised by the NAM and HRRR, as the GFS, SREF, and NAM all keep the pressure gradient from Portland to Spokane between 5.5-7.5 mb - well shy of the normal advisory threshold (46 mph) of 12 mb. Thus, there is low probability (<10%) of advisory-level winds Saturday afternoon and evening. Saturday`s system is lacking in moisture as flow aloft is from the west-northwest. This will lead to only higher level terrain over the Cascades and Elkhorns receiving a chance (20-30%) of rain Saturday afternoon and evening. The shortwave originated from a strong upper level low pressure over the Gulf of Alaska, which will be responsible for a much stronger upper level trough to begin dropping along the British Columbia coast on Sunday. Flow aloft will shift more from the southwest Sunday afternoon and evening, allowing showers to begin to spill into the Basin and northern Blue Mountains Sunday afternoon before becoming widespread across the region Sunday evening and into the overnight as the system nears the coast. This system is tapping into an atmospheric river (AR), which will provide rain amounts Saturday afternoon and evening of 0.50-1.00" over the Cascades, 0.30-0.50" through the east slopes of the Cascades and the northern Blue Mountains, 0.15-0.30" along the northern Blue Mountain foothills, and 0.02-0.10" for lower elevations of the Basin, Central Oregon, Kittitas/Yakima Valleys, and the John Day-Ochoco Basin. Confidence is high (80%) in regards to these rain amounts as the NBM suggests a 90-100% chance over the Cascades, 60-80% chance over the east slopes of the Cascades and northern Blue Mountains, 35-55% chance along the northern Blue Mountain foothills, and a 10-30% chance through lower elevations of the Basin, Central Oregon, Kittitas/Yakima Valleys, and the John Day-Ochoco Basin of receiving rain amounts of 0.25" or more 11 AM Sunday through 5 AM Monday. The presence of the transient upper level ridge today will promote mostly clear skies as afternoon temperatures reach near to slightly above normal values, breaking into the upper 70s to low 80s across lower elevations of the Basin. Clouds will be increasing late this evening associated with the passing shortwave on Saturday, which will also lead to a brief period of southwest flow aloft. These two features will allow Saturday morning temperatures to be 7-11 degrees warmer than on Friday to allow high temperatures to bump up 2-4 degrees on Saturday. Cloud cover will break up overnight into Sunday ahead of the next, more robust system. Thus, overnight lows will be 1-4 degrees cooler than the previous night. These mostly clear skies will be short-lived as the next system nears, which will increase cloud cover rapidly through the morning and afternoon as high temperatures stay about 5 degrees below normal on Sunday. 75 .LONG TERM...Monday through Friday... Bottom Line Up Front (BLUF) 1. Widespread rain and mountain thunderstorms overnight Sunday into Monday. 2. Breezy to windy conditions Monday. 3. Drier conditions return midweek with warmer temperatures. Models are in firm agreement with the upper level shortwave that is being pushed into the region ahead of an upper level trough. Guidance is showing a moderately strong atmospheric river (AR) is being pulled into the synoptic flow which will lead to a increase in precipitation Monday. Probabilities of the AR is hovering around 80- 90% and has been categorized as an AR 3 which is considered moderate. That being said, widespread precipitation will grace the majority of the region on Monday with over 60% of the ensembles agreeing the lower elevations, such as the Basin and the Blue Mountain foothills, will see greater than 0.25 inches of rain. Over 60% of the ensembles are also in agreement the mountains will see over 0.5 inches or more over a 24 hour period. Models remain in firm agreement with the upper level shortwave weakening Monday and flattening through Wednesday. Precipitation will remain mostly over the Cascades with 60% of the ensembles showing only 0.10 inches of rain for Tuesday. With the instability associated with the shortwave, model derived soundings are showing some slight surfaced based CAPE values over 50 J/kg and lapse rates over 6.5 C/km with warm surface temperatures. This is enough to cause some orographic uplifted thunderstorms over the higher terrain of the Cascades and the Northern Blues. However, confidence is these occurring is low (<20%) with guidance showing 15% probabilities of thunder. Not only will there be an increase with the precipitation, but also winds will pick up again with the incoming upper level wave. Guidance is showing a tightening of the gradients along the Cascades with models showing a roughly 7-9 mb squeeze. This will bring back the breezy to windy conditions across the majority of the region with the key areas being the Cascade Gaps. 60-70% of the ensembles are showing the Simcoe, Gorge, Horse Heaven Hills, Yakima and Kittitas Valleys all seeing sustained winds over 30 mph with gusts nearing or above 40 mph with isolated ridgelines seeing over 45mph. Models remain in firm agreement with the upper level shortwave weakening ahead of another upper level ridge Wednesday. Clusters are in pretty good agreement with the solutions regarding the ridge, however the clusters so show a slight variance with the amplitude. This could cause the temperatures to vary slightly depending the amplitude that follows through. Regardless, EFI is picking up on some above average temperatures gracing the region. Raw ensembles are showing 70% agreement that Pendleton and the surrounding area will see temperatures in the upper 70s to low 80s, over 50% shows the Columbia and John Day Basin, central and northern OR and Kittitas Valley in the mid to upper 80s, while elsewhere will be in the mid 70s. Temperatures will steadily increase through the remainder of the period. && .AVIATION...12Z TAFS...VFR conditions prevail this TAF period with current clear skies and variable winds. CIGs will return towards the end of the period but remain 25kft. Bennese/90 && .PRELIMINARY POINT TEMPS/POPS... PDT 75 50 76 49 / 0 0 0 10 ALW 77 53 78 54 / 0 0 0 10 PSC 79 55 82 55 / 0 0 0 10 YKM 78 53 78 47 / 0 0 10 10 HRI 79 52 81 54 / 0 0 0 0 ELN 75 52 70 48 / 0 10 10 10 RDM 76 46 71 44 / 0 0 0 10 LGD 72 46 72 46 / 0 0 10 10 GCD 76 47 74 46 / 0 0 0 10 DLS 82 56 76 55 / 0 0 10 10 && .PDT WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. && $$ SHORT TERM...75 LONG TERM....90 AVIATION...90