FXUS66 KPDT 171220 AFDPDT Area Forecast Discussion National Weather Service Pendleton OR 520 AM PDT Wed Jul 17 2024 .AVIATION...12Z TAFs...VFR conditions are forecast through the period for all sites, though a low pressure system transiting the region will facilitate widespread mid- and high-level clouds as well as shower and thunderstorm chances across the region today. Ongoing shower and thunderstorm activity will continue to lift north-northeast across north-central Oregon into south-central Washington through the morning. Confidence in lightning activity at terminals is too low to include mention in TAFs at this time. This afternoon, there is more uncertainty with regard to convection across the Columbia Basin and foothills of the Blue Mountains; current thinking places a 15-20% chance of isolated thunderstorms in the vicinity of PDT/ALW/PSC/YKM, but confidence in timing and location precludes mention in TAFs. Gusty outflow winds of up to 45 mph have been observed with ongoing convection and will be possible with any afternoon activity. Plunkett/86 && .PREV DISCUSSION... /issued 418 AM PDT Wed Jul 17 2024/ SHORT TERM...Today through Friday...Multiple weather concerns over the next 12-24 hours revolving around isolated thunderstorm potential and heat. Initial concerns revolve around isolated thunderstorms capable of infrequent CG lightning and strong outflows, the former promoting potential for new fire starts. Isolated storms have been confined to central OR and west of the Cascades overnight so far with predominantly very infrequent lightning (5 or less CG strikes/5 mins) with the activity over the last three hours. Some gusts this evening have been notable with some sites seeing above 45 mph from thunderstorms. Otherwise, heat products remain unchanged with existing heat advisory areas outside the Lower Columbia Basin, Yakima Valley, and Kittitas valley dropping off this evening. The aforementioned areas will see an additional day of heat-impacts with the head advisory and excessive heat warning ending tomorrow/Thursday evening. Latest water vapor imagery shows dry slot moving across central OR as a shortwave trough is lifting north northeast across western OR. Isolated thunderstorms have been seen to be forward of the tropopause anomaly where there is strong broad ascent. Very limited instability but a primed environment with steep mid-level lapse rates and decent mid-lower level moisture over 700-500 mb as moisture continues to move north and east. This combined with enhanced orographic lift will keep a risk for isolated thunderstorms in place. Of which, the risk will be spreading and shifting north and east through the morning becoming more confined to south central WA today. That said, there is a low chance (15-20%) for isolated storms across the Blues this afternoon as well. A decent signal is seen across the WA Cascades with the latest HREF when it comes to 4-hr max 1-km dBZ > 40 dBZ (which exceeds 50%), alongside 4-hr lightning probs that exceed 60%. This increases confidence in the RFW for abundant lightning with lower chances then extending farther east into the Lower Basin of Washington and Oregon. The threat remains sufficient across the Lower Basin so no change to the Red Flag Warning. Meantime, highs will be 2 to 4 degrees warmer outside the Cascades with heat headlines continuing today owing to NWS HeatRisk of high-end moderate to major. Greatest risk for heat impacts tied to the Kittitas and Yakima Valleys driven by mild morning lows this morning and overnight tomorrow. Little to no change in highs tomorrow across the Lower Columbia Basin and the Yakima and Kittitas valleys. As such, the headlines extend one day further in those areas. Of which, highs around 15 to approaching 20 degrees above normal today and tomorrow. Looking ahead, shortwave ridge builds in behind the exiting disturbance Thursday with southwest flow in place. Highs and lows still above average but are sufficient to lower the risk of impacts from heat. The dominant upper ridge across the desert SW shifts west by Friday with its ridge axis extending north across the Interior NW promoting dry and fair weather. However, triple digit temperatures more likely than not across the lower elevations. 60-80% for exceeding 100 F on Friday across the Lower Basin with lower chances/lower confidence elsewhere. LONG TERM...Friday night through Wednesday...Confidence is high (>95% chance) in continued above-normal temperatures Saturday through Monday as an amplified mid- to upper-level ridge remains over the West. While ensemble guidance does showcase some spread in temperatures over the weekend, the current forecast is supportive of heat highlights for most of our forecast zones. Additionally, low- elevation population centers in the Columbia Basin and adjoining foothills and valleys have a 20-60% chance of exceeding 105 degrees on Saturday and Sunday. More uncertainty surrounds chances of convection. Analysis of ensemble clusters reveals over 60% of members track some form of a vorticity maximum into the Pacific Northwest sometime between late Saturday through Sunday. Timing of the shortwave currently favors convection the most on Sunday. Monday through Wednesday, ensembles suggest a potential ridge breakdown as a low pressure system in the Pacific attempts to push onshore. While significant variance among ensemble members exists with regard to pattern details, a ridge breakdown would facilitate shower and thunderstorm chances as well as cooler temperatures and a return to breezy winds through the Cascade gaps. Ensemble clusters are roughly split on whether the low will be able to displace to ridge or if the ridge will divert the low northwest into Canada. Plunkett/86 && .PRELIMINARY POINT TEMPS/POPS... PDT 100 65 98 64 / 20 10 0 0 ALW 102 69 101 67 / 20 10 0 0 PSC 103 69 103 68 / 20 10 0 0 YKM 100 65 101 63 / 20 10 0 0 HRI 104 69 103 67 / 20 10 0 0 ELN 99 66 99 65 / 20 20 0 0 RDM 96 56 95 55 / 10 0 0 0 LGD 95 62 96 61 / 20 10 0 0 GCD 98 60 97 60 / 20 0 0 0 DLS 97 65 94 67 / 20 0 0 0 && .PDT WATCHES/WARNINGS/ADVISORIES... OR...Heat Advisory until 10 PM PDT this evening for ORZ041-505-507- 508-510. Red Flag Warning until 5 PM PDT this afternoon for ORZ641. Heat Advisory until 10 PM PDT Thursday for ORZ044. Red Flag Warning until 9 AM PDT this morning for ORZ610-611-639- 640. WA...Heat Advisory until 10 PM PDT this evening for WAZ024-029-521. Red Flag Warning until 5 PM PDT this afternoon for WAZ690-691- 694-695. Heat Advisory until 10 PM PDT Thursday for WAZ028. Excessive Heat Warning until 10 PM PDT Thursday for WAZ026-027. && $$ SHORT TERM...80 LONG TERM....86 AVIATION...86