National Weather Service (2024)

Area Forecast Discussion
Issued by NWS Portland, OR

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-- Discussion containing changed information from previous version are highlighted. --604FXUS66 KPQR 171052AFDPQRArea Forecast DiscussionNational Weather Service Portland OR352 AM PDT Sat Aug 17 2024.SYNOPSIS...
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A strong low pressure system offshore will bringwidespread wetting rain to the area this afternoon into tonight,with concerns for localized flash flooding near ongoingwildfire burn areas. There is also a chance of thunderstormsacross the area today, a few of which could produce severe windgusts and large hail from the Cascades west to the I-5corridor. Showers taper off on Sunday, yielding a return of mildonshore flow through much of next week.
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Now through Sunday Night...Water vapor satelliteimagery reveals a large and well defined upper level lowoffshore of the Pacific Northwest this morning, as has beendepicted by models for the past several days. Diffluentsoutherly flow aloft is present across western Oregon andWashington on the periphery of this feature, with numerousshowers already being picked up by regional radars over theoffshore waters as of 3 AM Saturday. Southerly flow will pivota bit more southeasterly over the course of the day, helping toadvect rich mid level moisture and enhanced instability into theregion to set the stage for widespread wetting rain showerswith embedded thunderstorms across the area this afternoon andevening. The trigger for this activity will be a potent,negative-tilt shortwave trough which will lift northward acrossthe area this afternoon, coincident with peak heating.Temperatures will warm into the low to mid 80s in the interiorvalleys and into the 70s in the higher terrain today prior tothe arrival of this feature, contributing to destabilizationcharacterized by MUCAPE values of 500-1500 J/kg across much ofthe area away from the coast this afternoon. Additionally,strong mid and upper level wind fields will provide ample shearvalues to support organized updrafts for any thunderstorms thatdevelop. Finally, all of this will occur within the left exitregion of a 90 kt jet streak rounding the base of the largescale trough, providing additional forcing for ascent acrossnorthwest Oregon and southwest Washington.Given all of the favorable factors described above, SPCmaintains a rare slight risk for severe thunderstorms acrossthe Cascades today, with a marginal risk extending westward tothe I-5 corridor. For some historical context, we have to goback nearly four years to September 2020 to find the last timeany portion of our forecast area was included in an SPC Day 1slight risk. The primary severe threat will likely be damagingthunderstorm wind gusts of 60+ mph given a well mixed sub cloudlayer contributing to DCAPE values of 700-1100 J/kg. That said,can`t rule out the possibility of some large hail to the size ofquarters, especially farther south over Lane County where cellsmay tend to be more discreet early in the event. SPC maintainsa 15 percent chance of both severe modes for areas in the slightrisk, and a 5-15 percent chance in the marginal areas. Thismeans that any point in the Cascades slight risk area will havea 15 percent chance of a severe thunderstorm occurring within 25miles today, with a corresponding 5-15 percent chance within 25miles of any point in the marginal risk area. In terms oftiming, hi-res guidance has been fairly consistent over the pastseveral runs in depicting showers and thunderstorms popping upacross Lane County around 1-2 PM this afternoon and thenspreading northward and reaching the Mt Hood and Portland areasapproximately 4-5 PM. Activity then continues into the eveningand starts to ramp down in intensity after sunset through aboutmidnight.Shifting gears to the heavy rain/potential flooding aspect oftoday`s activity, the forecast remains largely on track withwhat has been depicted in previous model runs. Yesterday`ssounding from SLE showed a precipitable water value over 1.1",with additional moisture streaming into the area today lookingto send PWATs above 1.25" and perhaps as high as 1.50" accordingto the higher end of guidance, which would be right around themaximum values of mid August climatology. Given the amplemoisture and forcing, hi-res guidance continues to depictrainfall rates mostly topping out around 0.75" per hour, thoughlatest HREF guidance does show a 5-10 percent chance of rateslocally as high as 1 inch per hour starting to show up over someareas this afternoon. As such, will keep the Flash Flood Watchintact along the length of the Oregon and south WashingtonCascades this afternoon into tonight. The primary areas ofconcern remain the recently burned areas associated with ongoingwildfires, but will have to also keep an eye on burn scars frompast years give the slight uptick in probabilistic guidance forrainfall rates. Flood concerns in the inland valleys shouldremain more limited in scope to some localized ponding of waterin areas that happen to see heavier rainfall rates over aprolonged period of time, but widespread flooding concerns arenot expected in the lowlands.Showers will ramp down in intensity and eventually taper offfrom south to north through late this evening as the bulk ofthe energy associated with the upper level trough shifts northof the area into Washington. Could certainly see some lingeringshower activity into the day on Sunday and perhaps some isolatedthunderstorms into Sunday morning across southwest Washington,but any additional rainfall will be minimal. The region willstart to settle back into a benign onshore flow regime by laterSunday afternoon, with highs remaining in the upper 70s toaround 80 in the interior valleys. /CB.LONG TERM...Monday through Friday...Little change in thelong term forecast as ensemble guidance continues to favormean upper level troughing over the NE Pacific through much ofnext week. This will keep western Oregon and Washington in astable onshore flow regime through the period, bringingseasonably mild temperatures with morning cloudiness andafternoon highs generally in the upper 70s. The proximity to theupper level trough will also yield continued chances forsporadic shower activity and perhaps some minimal thunderstormpotential at times, but no significant rainfall or strongthunderstorm potential is expected beyond this weekend. /CB
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Generally VFR conditions for all inland terminalsthroughout the period, with a 20-40% chance of MVFR CIGs beginning00Z Sunday. The coast will have lowering conditions around 00ZSunday as well, but with chances around 40-60% of MVFR, 20-30%of IFR, and 10-20% of LIFR. With a storm moving northward, theseconditions will be staggered. Highest probs of these loweredconditions begin at KONP around 03-06Z Sunday and at KAST around06-09Z Sunday.Main concern will be showers and thunderstorms moving northwardthrough the area Saturday. Showers/thunderstorms begin to moveinto southern terminals (Lane County) around 18-20Z Saturday,slowly pushing northward and reaching Portland metro areaterminals (KPDX, KHIO, KTTD) by 00z Sunday. Currently looking at30-40% chance of thunderstorms at all terminals, with 5-14%chance of damaging convective wind gusts up to 60 mph and up toquarter-sized hail possible with the strongest storms. However,threat moreso looks to be primarily wind, heavy downpours, andfrequent lightning.PDX AND APPROACHES...Thunderstorms and showers reach KPDX around00z Sunday, with around a 30-40% chance of thunderstorms from 00ZSunday through around 09-12Z Sunday. Predominantly VFR conditionsthroughout, though passing thunderstorms could briefly reducevisibilities below 6SM due to heavy rainfall. Gusty erratic windswill be possible after 00Z as thunderstorms pass through, with a5-14% chance of severe convective wind gusts up to 60 mph.Frequent lightning will also be possible through this time range./Sala/JLiu
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Expect benign conditions over the waters through mid nextweek. Winds remain N/NW this morning but will turn SW latetonight/early Sunday as surface low pressure moves through thewaters. Showers pass through all waters on Saturday, with around a20% chance of isolated thunderstorms possible. Low pressureremains over the NE Pacific through mid-week, maintaining SWwinds. Winds are expected to remain at 10 kts or less with gustsup to 15 kts; wind-driven seas of 2 of 4 feet.-Alviz/Sala
-- End Changed Discussion --&&.PQR WATCHES/WARNINGS/ADVISORIES...OR...None.WA...None.PZ...None.&&$$www.weather.gov/portlandInteract with us via social media:www.facebook.com/NWSPortlandx.com/NWSPortland