Area Forecast Discussion
Issued by NWS Pendleton, OR
Issued by NWS Pendleton, OR
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560 FXUS66 KPDT 142222 AFDPDT Area Forecast Discussion National Weather Service Pendleton OR 322 PM PDT Sat Sep 14 2024 .SHORT TERM...This afternoon through Monday night...Isolated showers along the east slopes of the Washington Cascades are evident in radar and satellite imagery. 12Z HREF soundings suggest surface-based CAPE of 150-300 J/kg capped around 15 kft AGL, likely too shallow for lightning. Radar returns paint a more optimistic picture for lightning chances with returns as high as 20-25 kft, where temperatures are roughly -25 to -30C, perhaps just sufficient for an isolated strike. Looking ahead, a couple vorticity lobes over the Pacific will carve out a deepening upper-level trough just offshore through the remainder of the day and tonight. Ensemble guidance is in excellent agreement (>95% chance) that the trough will develop into a closed low through Sunday with the main circulation center over southwest OR and northwest CA. This will place our CWA under southwesterly to southerly flow in the mid- to upper levels, while low-level flow will become northerly. With sufficient moisture in place, rain showers are anticipated to develop overnight through Sunday across the eastern mountains aided by synoptic-scale lift from an upper-level jet. Sunday afternoon, low (10-15%) chances of embedded thunderstorms are forecast for southeastern portions of our CWA, primarily southeastern Deschutes, Crook, and Grant counties. Monday through Monday night, the upper-level low will eject northeastward across the northwestern Great Basin. While the upper jet will weaken overhead, NWP guidance is suggesting diffluent flow aloft coupled with a modestly unstable atmosphere over the Blue Mountains. Afternoon convection is more likely over the Idaho Rockies, but activity may track into our CWA. Probabilities of additional upslope showers range from chance (25%) to likely (60%) across the northern and eastern Blue Mountains overnight. While confidence is very high in the overall synoptic pattern, ensemble clusters suggest some uncertainty remains regarding the precise track of the low as it comes onshore and tracks across the region. The most impactful forecast variable that the track will modulate is QPF. While NBM chances of wetting rain (0.10" or greater) are medium-high (40-70%) for the 48-hr period tonight through Monday night across most of the Blue Mountains, individual global ensemble QPF clusters place a sharp southwest-northeast gradient across eastern Oregon; the most likely solution (~50% of members) places the heaviest QPF (cluster mean of 0.10-0.50") over south- central and southeast Oregon into western Idaho, the remaining clusters are split (25%/25%) in placing a substantial (0.5-1.5") or lighter (up to 0.50") amount of QPF over the Blue Mountains, heaviest over northeast Oregon. Winds will switch to westerly and become breezy to windy Monday evening through Monday night. Plunkett/86 && .LONG TERM...Tuesday through Saturday... Key Messages: 1. Breezy winds midweek. 2. Scattered showers Tuesday and Wednesday. 3. Below normal temperatures extend through the period. The extended period is characterized by an incoming upper level trough Tuesday and Wednesday, followed by an upper level ridge building across the area through the remainder of the week. The digging upper level system and associated cold front will bring scattered showers to the area through Wednesday before dryer and warmer conditions return. These synoptic features will also allow pressure gradients to tighten across the Cascades, leading to breezy conditions through the Gorge and Kittitas Valley. The most impactful concerns will primarily be focused on these enhanced winds through the Gorge, foothills, and the Kittitas Valley Tuesday. Sustained winds of 25-35 mph will be possible with gusts of 30-40 mph out of the west-northwest over these areas. Elsewhere, 10-15 mph sustained winds and gusts of up to 20 mph are expected. Confidence is moderate to high (60-80%) in these wind values as the NBM advertises a 70-90% chance of sustained winds of 30 mph and gusts of 35 mph across the Gorge and Kittitas Valley over Tuesday afternoon. Peak timing for these increased winds will be late morning through the early afternoon on Tuesday before winds look to slacken through the evening into Wednesday morning. Showers associated with the incoming system on Tuesday will predominately occur over mountainous terrain across the Cascades and Blues until the afternoon, before extending into Central Oregon, Blue Mountain foothills, and portions of the Basin by late afternoon. This is in response to the trough axis dropping to our south as the associated cold front passes through the region overnight into Wednesday morning. Wetting rain (>.10") chances will be low to moderate (10-35%) for the area, with the exception of Wallowa County and the east slopes of the Cascades (60-80%). Rain chances (20-45%) will linger into Wednesday afternoon across the Cascades and the eastern mountains before upper level ridging begins to push into the area from the west. The chance for a wetting rain over mountainous terrain in the Cascades and eastern mountains on Wednesday is rather lacking, as only 10-30% is showcased by the NBM. Ensembles and deterministic models are in good agreement with the overall pattern, but slight differences in system speed is observed via the 500mb EOF patterns. This would correlate to a slightly earlier onset of precipitation and elevated winds Tuesday morning/afternoon. However, this is only depicted by 49% of ensemble members, so if it does shift earlier it is expected to be slight. 44% of members also hint at a stronger incoming system, versus 34% suggesting a weaker system, and 22% staying similar to the current forecast. A stronger system would lead to a better potential for showers, cooler temperatures, and higher winds, but confidence in a stronger system is currently low to moderate (20-40%) due to the variable spread of ensemble members. However, ensembles are in good agreement with the incoming upper level ridge moving into the region late Wednesday into Thursday and strengthening into the weekend, leading to a 80-100% chance of high temperatures staying 5 to 10 below normal through this period. 75 && .AVIATION...18Z TAFS...VFR conditions currently at all sites, which will stay the course through the period. Winds are increasing for KDLS and KPDT as gusts of 18 to 25 kts will persist through the afternoon before subsiding this evening. Breezy winds are also anticipated to occur at KRDM and KBDN later this afternoon and evening between 20 and 25 kts, but will subside late tonight and overnight into Sunday morning. These winds are associated with an approaching system that will also drop ceilings during the early morning hours on Sunday for most sites. 75 && .PRELIMINARY POINT TEMPS/POPS... PDT 50 65 49 72 / 10 30 30 20 ALW 53 67 50 75 / 10 40 40 20 PSC 56 72 55 80 / 10 10 20 10 YKM 46 74 47 82 / 10 10 10 0 HRI 54 71 53 78 / 10 20 20 10 ELN 47 74 48 81 / 10 10 10 0 RDM 44 64 42 69 / 10 20 30 20 LGD 49 64 48 69 / 30 60 60 40 GCD 47 63 48 67 / 30 60 60 40 DLS 52 75 53 80 / 0 10 10 0 && .PDT WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. && $$ SHORT TERM...86 LONG TERM....75 AVIATION...75