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Area Forecast Discussion
Issued by NWS Pendleton, OR
Issued by NWS Pendleton, OR
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126 FXUS66 KPDT 151155 AAA AFDPDT Area Forecast Discussion...Updated National Weather Service Pendleton OR 455 AM PDT Thu Aug 15 2024 Updated aviation discussion .SHORT TERM...Today through Saturday...Overnight satellite imagery showing a low pressure system centered over south central Oregon, currently inching north and east, bringing with it a shield of moisture with light showers currently impacting our central Oregon zones. Earlier in the evening, thunderstorm activity had begun to ramp up over Lake and Klamath Counties, however as the cluster of storms approached our forecast area, its strength tapered off, leaving us with light showers instead. Upper-level support seems to be better situated over the southernmost counties of Oregon, however we could still see some lightning strikes over the southernmost portion of our forecast area as this low continues its northeastward push through the Interior Northwest. Based on latest CAMs, the current disorganized band of showers will move toward the eastern mountains by daybreak, before models suggest another cluster of convection developing primarily over the Strawberries and Grant County, likely due to the center of the low being overhead at the time, thus maximizing lift. Confidence is on the lower end (20-30%) as to whether or not what the CAMs are suggesting for the afternoon will verify, which does not bode well for our current Red Flag Warning, but plans for now are to keep it in effect in case convection starts to ramp up across our forecast area with the low overhead. The trend for Friday continues to be that of drier conditions as mid- level moisture falls off quite a bit. The current low lifts northward as is eventually absorbed by a larger, developing low that looks to center itself just off the PacNW coast, staying in place due to a blocking pattern. Models suggest this system will dominate our weather through as long as the midweek next week. Greatest forecast challenge heading into the weekend will be where exactly the strongest axis of moisture advection sets up aloft, with models right now suggesting a more SSW orientation. This would maximize PoPs and storm chances along the Cascades, while drier, hotter desert air from the Great Basin moves in over the eastern half of the forecast area. Saturday thus looks to be warmer east of the Cascade east slopes, with scattered shower and thunderstorm chances elsewhere. Whether or not these storms will warrant any fire weather headlines is to be determined once this ongoing low moves out of our area. Evans/74 .LONG TERM...Sunday through Thursday...Models are in very good agreement in having an upper low and trough over the Pacific Northwest through the long term period though there are differences as to the strength and location of the trough axis. In general, temperatures will be a few degrees below normal and despite a southwest flow over the area through the period, light showers and a few thunderstorms will be confined to the Washington Cascade crest. Sunday through Tuesday, models are in good excellent agreement in having the trough centered just offshore with a closed low wobbling about the base of the trough, moving from the Washington coast on Monday north to Vancouver Island on Tuesday. By Wednesday, the GFS favors the trough off of the Oregon/Washington coast while over half of the ECMWF ensemble members have a weaker trough further offshore though this should not change the forecast significantly. On Sunday, there is significant uncertainty in the temperatures as the NBM has a 6-11 degree temperature spread between the 25th and 75th percentiles. Looking at the individual model ensemble members, The GFS is cooler with highs in the mid 70s to mid 80s while the ECMWF is mainly in the 80s. The NBM determinant temperatures favor the cooler GFS temperatures and given the trough overhead, have accepted those temperatures but confidence is not high. This yields temperatures in the mid 70s to mid 80s over the area. Monday warms up a couple of degrees to the upper 70s and 80s and then on Tuesday cools back down to Sunday`s temperatures. For each day, the NBM has a greater than 85 percent chance of reaching 80 degrees in the lower elevations, but the chance of reaching 90 degrees generally less than 30 percent, except in the lower Columbia Basin where the Tri- Cities has a 40-60 percent chance each day. Instability is nearly nonexistent Sunday through Tuesday aside from some weak instability along the Washington Cascade crest Sunday and Tuesday afternoon so have a limited area with a slight chance of showers and thunderstorms there. Rain amounts will be just a few hundredths of an inch at most. Winds will be fairly light aside from breezy afternoon winds of 10-20 mph through the Cascade gaps on Monday and Tuesday. By Wednesday, models begin having bigger differences. The ECMWF favors a shallower weaker trough with the closed low north of Vancouver Island. The GFS favors a stronger trough with the low off the Washington coast and the Canadian is split between the two with no solution being dominant. The GFS does have some instability over the Washington Cascades and the NBM does put a slight chance of mainly showers all along the Washington and Oregon Cascade crest in the afternoon. Temperatures remain several degrees below normal in the mid 70s to mid 80s. On Thursday, the GFS continues to favor a trough off the Pacific Northwest coast though the deterministic run sends the closed low into British Columbia and weakens the trough, while the ECMWF and Canadian have a stronger trough as a reinforcing low arrives out of northern British Columbia but both struggle with the location of the trough with it either well offshore, near the coast or directly overhead. With models favoring a deeper trough, temperatures cool a degree or so from Wednesday. A slight chance of light rain is once again confined to the Washington Cascade crest in the afternoon. Perry/83 && .AVIATION...12Z TAFs...An upper low is moving northeast into the area this morning with a band of showers moving north through central Oregon and the eastern Oregon mountains. There are isolated thunderstorms moving towards of the mountains but further west, thunderstorms ended a few hours ago. Showers should end at KBDN and KRDM in the next couple of hours but BKN-OVC clouds will continue to move north this morning at about 10K feet AGL. Showers and perhaps a few thunderstorms will develop and affect all TAF sites from 20Z- 03Z. Due to low confidence have limited mention of precipitation to a mix of VCSH and PROB30 -SHRA at all TAF sites. Showers will end and skies will start clearing after 04Z. Winds at KDLS will increase to northwest at 15-20 kts with gusts to 30 kts from 17Z-06Z. Other TAF sites will increase to west to northwest at 10-15 kts with gusts to 20 kts from 22Z-04Z. Perry/83 && .PRELIMINARY POINT TEMPS/POPS... PDT 80 53 81 53 / 20 10 0 0 ALW 84 59 85 57 / 10 20 10 0 PSC 87 62 86 59 / 10 10 0 0 YKM 85 54 84 55 / 10 20 0 10 HRI 85 59 85 56 / 10 10 0 0 ELN 85 56 83 54 / 10 10 10 0 RDM 74 47 80 49 / 40 10 0 10 LGD 80 49 82 51 / 40 40 20 10 GCD 80 49 83 53 / 60 40 10 10 DLS 80 58 83 60 / 10 0 0 10 && .PDT WATCHES/WARNINGS/ADVISORIES... OR...Red Flag Warning from 11 AM this morning to 11 PM PDT this evening for ORZ644-645. Red Flag Warning until 11 PM PDT this evening for ORZ611-640-642. WA...None. && $$ SHORT TERM...74 LONG TERM....83 AVIATION...85