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Area Forecast Discussion
Issued by NWS Portland, OR
Issued by NWS Portland, OR
856 FXUS66 KPQR 171758 AFDPQR Area Forecast Discussion National Weather Service Portland OR 1056 AM PDT Sat Aug 17 2024 Updated aviation discussion .SYNOPSIS...A strong low pressure system offshore will bring widespread wetting rain to the area this afternoon into tonight, with concerns for localized flash flooding near ongoing wildfire burn areas. There is also a chance of thunderstorms across the area today, a few of which could produce severe wind gusts and large hail from the Cascades west to the I-5 corridor. Showers taper off on Sunday, yielding a return of mild onshore flow through much of next week. && .SHORT TERM...Now through Sunday Night...Water vapor satellite imagery reveals a large and well defined upper level low offshore of the Pacific Northwest this morning, as has been depicted by models for the past several days. Diffluent southerly flow aloft is present across western Oregon and Washington on the periphery of this feature, with numerous showers already being picked up by regional radars over the offshore waters as of 3 AM Saturday. Southerly flow will pivot a bit more southeasterly over the course of the day, helping to advect rich mid level moisture and enhanced instability into the region to set the stage for widespread wetting rain showers with embedded thunderstorms across the area this afternoon and evening. The trigger for this activity will be a potent, negative-tilt shortwave trough which will lift northward across the area this afternoon, coincident with peak heating. Temperatures will warm into the low to mid 80s in the interior valleys and into the 70s in the higher terrain today prior to the arrival of this feature, contributing to destabilization characterized by MUCAPE values of 500-1500 J/kg across much of the area away from the coast this afternoon. Additionally, strong mid and upper level wind fields will provide ample shear values to support organized updrafts for any thunderstorms that develop. Finally, all of this will occur within the left exit region of a 90 kt jet streak rounding the base of the large scale trough, providing additional forcing for ascent across northwest Oregon and southwest Washington. Given all of the favorable factors described above, SPC maintains a rare slight risk for severe thunderstorms across the Cascades today, with a marginal risk extending westward to the I-5 corridor. For some historical context, we have to go back nearly four years to September 2020 to find the last time any portion of our forecast area was included in an SPC Day 1 slight risk. The primary severe threat will likely be damaging thunderstorm wind gusts of 60+ mph given a well mixed sub cloud layer contributing to DCAPE values of 700-1100 J/kg. That said, can`t rule out the possibility of some large hail to the size of quarters, especially farther south over Lane County where cells may tend to be more discreet early in the event. SPC maintains a 15 percent chance of both severe modes for areas in the slight risk, and a 5-15 percent chance in the marginal areas. This means that any point in the Cascades slight risk area will have a 15 percent chance of a severe thunderstorm occurring within 25 miles today, with a corresponding 5-15 percent chance within 25 miles of any point in the marginal risk area. In terms of timing, hi-res guidance has been fairly consistent over the past several runs in depicting showers and thunderstorms popping up across Lane County around 1-2 PM this afternoon and then spreading northward and reaching the Mt Hood and Portland areas approximately 4-5 PM. Activity then continues into the evening and starts to ramp down in intensity after sunset through about midnight. Shifting gears to the heavy rain/potential flooding aspect of today`s activity, the forecast remains largely on track with what has been depicted in previous model runs. Yesterday`s sounding from SLE showed a precipitable water value over 1.1", with additional moisture streaming into the area today looking to send PWATs above 1.25" and perhaps as high as 1.50" according to the higher end of guidance, which would be right around the maximum values of mid August climatology. Given the ample moisture and forcing, hi-res guidance continues to depict rainfall rates mostly topping out around 0.75" per hour, though latest HREF guidance does show a 5-10 percent chance of rates locally as high as 1 inch per hour starting to show up over some areas this afternoon. As such, will keep the Flash Flood Watch intact along the length of the Oregon and south Washington Cascades this afternoon into tonight. The primary areas of concern remain the recently burned areas associated with ongoing wildfires, but will have to also keep an eye on burn scars from past years give the slight uptick in probabilistic guidance for rainfall rates. Flood concerns in the inland valleys should remain more limited in scope to some localized ponding of water in areas that happen to see heavier rainfall rates over a prolonged period of time, but widespread flooding concerns are not expected in the lowlands. Showers will ramp down in intensity and eventually taper off from south to north through late this evening as the bulk of the energy associated with the upper level trough shifts north of the area into Washington. Could certainly see some lingering shower activity into the day on Sunday and perhaps some isolated thunderstorms into Sunday morning across southwest Washington, but any additional rainfall will be minimal. The region will start to settle back into a benign onshore flow regime by later Sunday afternoon, with highs remaining in the upper 70s to around 80 in the interior valleys. /CB .LONG TERM...Monday through Friday...Little change in the long term forecast as ensemble guidance continues to favor mean upper level troughing over the NE Pacific through much of next week. This will keep western Oregon and Washington in a stable onshore flow regime through the period, bringing seasonably mild temperatures with morning cloudiness and afternoon highs generally in the upper 70s. The proximity to the upper level trough will also yield continued chances for sporadic shower activity and perhaps some minimal thunderstorm potential at times, but no significant rainfall or strong thunderstorm potential is expected beyond this weekend. /CB && .AVIATION...VFR conditions prevail at 18Z Sat, but isolated thunderstorms that started around 14Z over parts of the Willamette Valley to the coast are expected to depart the area to the northwest shortly. Next shortwave to trigger more robust thunderstorms this afternoon into the evening hours. Hires models show most active thunderstorms over the Cascades during this time. But valley locations too have a chance for thunderstorms. Approximate timing for stronger batch of thunderstorms at TAF sites: KEUG 21Z Sat-01Z Sun, KSLE 22Z Sat-02Z Sun, KPDX 23Z Sat-03Z Sun. After that thunderstorm chances drop but not quite end until about 09Z Sun. Predominantly VFR conditions throughout, though passing thunderstorms could briefly reduce visibilities below 6SM due to heavy rainfall. Gusty erratic winds will be possible after 00Z as thunderstorms pass through, with a 5-14% chance of severe convective wind gusts up to 60 mph. Frequent lightning will also be possible through this time range. PDX AND APPROACHES...Thunderstorms and showers reach KPDX around 00z Sun through about 03Z Sun, then chances drop but not quite end until about 09Z Sun. Predominantly VFR conditions throughout, though passing thunderstorms could briefly reduce visibilities below 6SM due to heavy rainfall. Other impacts of thunderstorms covered above. /mh && .MARINE...Expect benign conditions over the waters through mid next week. Winds remain N/NW this morning but will turn SW late tonight/early Sunday as surface low pressure moves through the waters. Showers pass through all waters on Saturday, with around a 20% chance of isolated thunderstorms possible. Low pressure remains over the NE Pacific through mid-week, maintaining SW winds. Winds are expected to remain at 10 kts or less with gusts up to 15 kts; wind-driven seas of 2 of 4 feet. -Alviz/Sala && .PQR WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. PZ...None. && $$ www.weather.gov/portland Interact with us via social media: www.facebook.com/NWSPortland x.com/NWSPortland