Area Forecast Discussion
Issued by NWS Pendleton, OR
Issued by NWS Pendleton, OR
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190 FXUS66 KPDT 191751 AFDPDT Area Forecast Discussion National Weather Service Pendleton OR 1051 AM PDT Sun May 19 2024 Updated aviation discussion .AVIATION...18Z TAFs...VFR conditions are expected through the next 24 hours. Expect mainly FEW-SCT clouds at 5K-10K feet AGL though some clouds at around 4K feet AGL at KALW will linger for a few more hours. KDLS will have west winds of 10-20 kts with gusts to 30 kts through 04Z then start decreasing and drop below 12 kts after 11Z. KPDT and KALW will continue at 10-15 kts with gusts to 25 kts through 22Z then drop off to below 12 kts. KRDM, KBDN and KYKM will have northwest winds at 10-15 kts with gusts to 25 kts through 05Z-08Z before dropping off to below 10 kts. KPSC will remain below 12 kts for the next 24 hours. Perry/83 && .PREV DISCUSSION... /issued 236 AM PDT Sun May 19 2024/ SHORT TERM...Today through Tuesday...We`re staring down a wet and windy forecast as a train of upper-level lows threatens the forecast area through the next week. With the northwesterly origin of these systems, expect temps to remain at or below seasonal averages as well, with some chilly overnight lows possible over the next couple of nights. Over the last couple of days, we`ve been under the influence of a broad western Canadian low, whose southern flank has placed a jet over the forecast area. Mountain showers and gusty winds have resulted, as well as some weak bands of showers across the Basin, but Sunday will mark the low`s departure, and much of the forecast today looks drier and calmer wind-wise as a result. That being said, we remain under a broad NW flow regime, meaning winds through the Cascade Gaps will remain locally gusty, and showers may still develop across our mountain zones, mainly through the Washington Cascades and eastern mountains of Oregon. The airmass remains dry, so PoPs are low (20-30%), but the environment does support weak orographic shower development over the aforementioned areas. As the flow behind the departing low turns more northerly, colder air will advect in, making for a chilly Sunday night/Monday morning. Overall looking like winds and cloud cover will be enough to preclude subfreezing overnight lows across areas eligible for Freeze Warnings, but temps may approach freezing overnight for the Grande Ronde Valley and foothills of the Southern Blues in particular, as far as these eligible zones are concerned. Monday also looks mostly dry, but the trough axis of the departing low tilts toward our forecast area just enough to invite instability across mainly the eastern mountains. MUCAPE is supportive for storms, running around 400-600 J/Kg Monday afternoon, but moisture once again is a limiting factor. PoPs remain on the lower end at around 20-40%, mainly for Wallowa County and the eastern mountains of Oregon. Finally, Tuesday marks the initial arrival of an upper-level low almost due north from Canada. Precip chances look to envelop the forecast area by early Tuesday evening as a result. Expecting mostly rain showers, as the atmosphere will be cool given the northerly approach of this system. Confidence is high (60-70%), however, that the Tuesday into Wednesday period will be wet as a result of this system. The arrival of this low will also welcome a return to windy conditions, although guidance thus far does not suggest anything headline worthy for now. Evans/74 LONG TERM...An upper level low will move across the region Wednesday bringing showers, mainly over the mountains and a chance of thunderstorms. However, due to the proximity of the low, there will even be rain chances over the lower elevations. By later Wednesday into Wednesday night the low will have moved eastward and out of the area and rain chances should be decreasing. Due to the proximity and path of the low, there will be instability aloft for thunderstorms, but being 4 days away, the exact details will come into better focus as the day gets closer. Based on what is shown in both the deterministic GFS and ECMWF, the NBM precip fields look overdone on Thursday. Thursday night should be dry as well in advance of the next low moving southeast across the Pacific Northwest. The GFS is stronger and has a closed low, and also further south, while the ECMWF is weaker, has more of an open trough and further north. There will be a trough and rain either way, but the exact position and strength of this system will determine impacts as we go into the Memorial Day weekend, and the models are showing considerable disagreement at this time. QPF with the Wednesday system looks decent, with around three- quarters of an inch in the Blue Mountains and Wallowas and one-half to three-quarters of an inch in along the crest of the Cascades. In the lower elevations, there will be considerably less. Along the Foothills of the Blue Mountains, generally around one-quarter inch, and most other places just a few hundredths. The ECMWF EFI shows an area of 0.7 to 0.8 in the northern Blue Mountains for QPF on Wednesday. The second low looks to bring less precipitation, Generally up to one-quarter inch in the mountains, with some locally higher amounts and hundreds to around one tenth elsewhere. However, it will ultimately depend on the track and strength of the low. The ECMWF solution is supported by about 40 percent of the ensemble clusters as we approach Saturday, and the ECMWF ensemble mean is in better agreement with its deterministic run, giving more support to its solution. However, there is always natural variability in solutions at this time scale. Wednesday into Wednesday evening also looks to be breezy with the trough moving through, especially across the lower Columbia Basin, with wind gusts 25 to 35 mph, possibly as high as 40 mph. The ECMWF EFI keys in on a large portion of eastern Oregon with values of 0.7 to 0.8 during this time. High temperatures will be 5 to 10 degrees below normal on Wednesday, with the trough moving through, but will return closer to normal Thursday, except central Oregon. By Friday, most areas should be close to normal. && .PRELIMINARY POINT TEMPS/POPS... PDT 63 39 69 42 / 0 10 0 0 ALW 66 43 72 47 / 10 10 10 0 PSC 70 47 77 50 / 0 10 0 0 YKM 67 41 75 45 / 0 0 0 0 HRI 69 43 75 48 / 0 10 0 0 ELN 63 40 71 44 / 10 0 0 0 RDM 56 31 65 36 / 0 0 0 0 LGD 57 36 64 38 / 10 10 20 0 GCD 57 34 64 37 / 0 10 10 0 DLS 65 45 72 48 / 0 0 0 0 && .PDT WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. && $$ SHORT TERM...74 LONG TERM....77 AVIATION...83