![National Oceanic and Atmospheric Administration](/build/images/header/noaa.d87e0251.png)
![National Weather Service](/build/images/header/nws.4e6585d8.png)
![United States Department of Commerce](/build/images/header/doc.b38ba91a.png)
Area Forecast Discussion
Issued by NWS Pendleton, OR
Issued by NWS Pendleton, OR
Versions:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
192 FXUS66 KPDT 150431 AFDPDT Area Forecast Discussion National Weather Service Pendleton OR 931 PM PDT Wed Aug 14 2024 .EVENING UPDATE...Water vapor imagery reveals a closed low pressure system over southwest Oregon, moving slowly inland. Southerly flow aloft is advecting moisture northward into central Oregon, and radar shows a cluster of showers and thunderstorms rumbling slowly northward across south-central Oregon, predominantly in Klamath and Lake counties. While the 00Z MFR sounding recorded a modest 207 J/kg of MUCAPE, these storms have produced prolific lightning, including cloud-to-ground lightning, with GLM flash density rates as high as 97 flashes/5min. Heavier precipitation cores have yielded wetting rain in areas with surface observations picking up over a half inch. The question remains, how will this convection evolve as it propagates into our CWA? RAP-based mesoanalysis suggests the best 700-500 mb (mid-level) layer-average omega is over southeast Deschutes and Crook counties. Additionally, 00Z global guidance indicates negative mid-level theta-e lapse rates in this region, and 00Z HREF forecast soundings continue to advertise modest MUCAPE of 50-200 J/kg through the night. This instability coupled with ample moisture through the column and strong synoptic-scale forcing from the vorticity maximum lifting northward should support showers and thunderstorms into central and eastern Oregon. As far as forecast updates, have adjusted PoPs and thunder coverage based on satellite and radar trends and 00Z NWP guidance. Forecast morning lows were also adjusted based on the assumption that many areas will see precipitation and cool to near forecast wet bulb temperatures. Red Flag Warnings remain in effect tonight and Thursday due to forecast abundant lightning. Plunkett/86 && .AVIATION...06Z TAFs...Cloud cover will increase across central and eastern Oregon overnight due to ongoing convection in south- central and central Oregon. BDN/RDM will likely (50-60% chance) see rain showers between 07Z and 11Z Thursday with a chance (25-30%) of thunder. Elsewhere, precipitation chances are low (30% or less) through the period, highest at PDT between 15Z and 18Z, but mid- and high-level clouds will increase through Thursday for all sites. Aside from the potential for gusty outflow winds with any showers and thunderstorms, diurnally driven gap winds are forecast, strongest at DLS Thursday afternoon and evening. Any showers and thunderstorms that pass over terminals will be capable of dropping CIGs/VSBYs temporarily to MVFR. Plunkett/86 && .PREV DISCUSSION... /issued 501 PM PDT Wed Aug 14 2024/ SHORT TERM...Today through Tomorrow Night...A transient ridge is crossing the region today. Mostly clear skies and near normal high temperatures today. However, few cumulus clouds are developing within Wallowa county, the crest of WA Cascades including south central OR, observed from satellite imagery. Confined smoke aloft still remains in central OR and Ochoco-John Day Highlands from local wildfires. Winds will be around at 20 mph or less across the forecast area this afternoon. Tonight through tomorrow afternoon, an upper level trough makes its way through the PacNW which is now offshore the OR/CA border. There are chances (40-60%) of thunderstorms starting late tonight over central OR that then ramp up tomorrow afternoon and evening in terms of both coverage and lightning potential. A bit more spread/uncertainty in terms of elevated instability overnight that spreads across south central OR northward. Best signal is focused across northern Klamath-Lake. Thereafter, environment becomes more conducive for storms tomorrow. Model soundings indicated MUCAPEs between 500-1000 J/kg, Lifted indices (LI) of -3, and lapse rates 6.5-8 C/km across central OR to eastern OR. Thus a moderately unstable environment. Of note, HREF probability of thunder is forecast around 30-60% overnight that then ramp to greater than 80% across portions of eastern OR. Of note, a mix of wet and dry storms are expected in this event with storms likely to be more wet than not across the Southern Blue and Strawberry Mountains north and east. Of which, HREF LPMM highlights this nature. A reasonable high-end is expected to be approaching one inch or more. HREF shows raw probs of exceeding 0.5" at around 40-50% in spots. Wetting rains are further supported by NBM probs. Of which, a tenth or more exceed 60% across a large area, roughly Crook county southward and northeast into Wallowa through 06Z Friday. While storms are wet there, the threat for new fires is sufficient between fuels and lightning activity. Thus Red Flag Warnings were issued for ORZ644-645. Elsewhere, the Fire Weather Watches were replaced with a warning with these zones having two windows of lightning potential, albeit storms more isolated-widely scattered in character compared to the storms in the east. Overall, models are in good agreement with trough returning over the PacNW with a cyclonic low for tomorrow. Breezy winds will also develop up to 20-30 mph tomorrow around the Cascade Gaps extending to Columbia Basin (>50% confidence). This is due to increasing surface pressure gradients up to 5-8 mb. Tomorrow night, the worst will come to an end as thunderstorms begin moving out of the area. Gusty winds will then return at 15-25 mph with the surface pressure gradient decreasing to 3-5 mb. Clear skies will return as well as clouds decrease. Temperatures will then steadily increase by a few degrees with relative humidity above critical levels. Confidence enhances with the Extreme Forecast Index (EFI) showing temperatures to be near- normal through the forecast period. Feaster/97/80 LONG TERM...Friday through Wednesday...The previous area of low pressure continues to lift northeast out of the forecast area on Friday morning. Lingering moisture will provide a slight chance (15-20%) for continued shower activity, mainly over Wallowa County in Oregon and Columbia County in Washington. This system should generally lift out of the forecast area by the late morning hours, with another area of low pressure moving over the Pacific Coast. Models continue to indicate instability, especially over the Eastern Mountains of Oregon. With atmospheric moisture over the area, showers and isolated thunderstorms (15-25% chance) will be possible through Friday afternoon. The best opportunity for thunderstorm development will be over eastern Union County, with showers otherwise. The previously mentioned area of low pressure off the Pacific Coast will advance east with moisture moving over Washington and Oregon between Saturday and Sunday. Ahead of the arrival on Saturday, temperatures will warm around 5 degrees across the area, with cooler conditions anticipated on Sunday. The best moisture will be found west of, and including, the Cascades, though moisture will spill over into the central portions as well (30-50% for favored locations). Guidance indicates the afternoon and evening hours to be the favored timeframe for shower activity . Instability is generally faced in the western reaches of the moisture with slight chance (10-20%) of development. The area of low pressure will drift far enough northward to lose influence over area weather by Sunday afternoon/evening. Drier conditions expected to persist through the remainder of the forecast period, though pockets of moisture will produce isolated showers and thunderstorms over the Washington Cascades through Wednesday. Temperatures through this period will remain near to slightly above normal. Overall, clusters are demonstrating fair agreement with the progression of these systems. Intensity differences to exist, as well as how far east the weekend system moves, but significant differences are not currently present. Branham/76 && .PRELIMINARY POINT TEMPS/POPS... PDT 54 80 54 81 / 0 30 20 10 ALW 57 84 59 85 / 0 20 30 10 PSC 59 87 62 86 / 0 10 20 0 YKM 55 85 54 84 / 0 20 20 0 HRI 59 85 59 86 / 0 10 20 0 ELN 57 85 56 83 / 0 10 20 10 RDM 52 75 47 80 / 60 40 20 10 LGD 52 80 50 83 / 0 50 60 20 GCD 54 80 50 83 / 40 70 40 20 DLS 61 80 59 83 / 10 20 10 0 && .PDT WATCHES/WARNINGS/ADVISORIES... OR...Red Flag Warning from 11 AM to 11 PM PDT Thursday for ORZ644-645. Red Flag Warning until 11 PM PDT Thursday for ORZ611-640-642. WA...None. && $$ EVENING UPDATE...86 SHORT TERM...97/80 LONG TERM....76 AVIATION...86