Area Forecast Discussion
Issued by NWS Portland, OR
Issued by NWS Portland, OR
450 FXUS66 KPQR 152310 AFDPQR Area Forecast Discussion National Weather Service Portland OR 410 PM PDT Mon Jul 15 2024 .SYNOPSIS...Above normal temperatures will persist for inland portions of SW Washington and NW Oregon through the weekend, though onshore flow should help to guard against more extreme temperatures. Flow aloft will turn more southerly Tuesday, potentially carrying moisture and instability northward from California. This will lead to about a 10-20% chance of thunderstorms Tuesday afternoon into Tuesday night, best chances over the higher terrain. && .SHORT TERM...Monday through Wednesday...Temperature observations as of 2PM Monday are around 4 to 10 degrees cooler then this time yesterday for most low lying inland locations, and similar to a few degrees cooler for the rest of the region. The area of most cooling is the northern Willamette Valley as well as along and north of the Columbia River due to the overnight and early morning cooler marine air intrusion along the Columbia River. Still, temperatures are expected to peak in the upper 80s to low 90s this afternoon for the Willamette Valley and only low 80s along the I-5 corridor in SW Washington north of Clark County. Temperatures are expected to heat up again on Tuesday as the flow aloft becomes more southerly and 850 mb temps jump to around 23-25C, with the potential of a few locations within the interior valleys reaching triple digits. Latest NBM guidance has increased chances for 100 degree temps for the Willamette Valley to 40-60%, except a 60-70% chance from Salem to Canby. The Portland metro area, as well as the Columbia River Gorge and Hood River Valley, remains in the Major HeatRisk category for heat related illness. The complicating factor is flow will be turning southerly, which brings the potential for additional high level cloud cover, but even if temperatures are a few degrees cooler than currently forecast, the relief would likely be minimal as the influx in moisture would result in slightly more humid conditions than we are accustomed to in the Pacific Northwest. The other forecast challenge will concern a cutoff low near the California coast as models continue to depict it becoming dislodged by the amplification of the pattern and moving northward as a negatively tilted shortwave trough Tuesday into Tuesday night. This will bring concern for lightning strikes as mid level moisture and instability are advected northward into our area, yielding surface based convection Tuesday afternoon transitioning to more of an elevated nocturnal threat overnight Tuesday into Wednesday morning. Ensemble based probabilistic guidance is still somewhat underplaying the thunder threat as it tends to struggle with elevated convection concerns. However, soundings indicate some CAPE in the 700-500mb elevated layer tomorrow afternoon and into Wednesday morning. The best location for convection is across the Oregon and south Washington Casacdes and the coast to the Coast Range, but we are monitoring the potential for elevated instability over the inland valleys tomorrow night, as well. Any lightning will obviously present heightened fire weather concerns given the recent prolonged stretch of hot and dry weather. In the wake of the upper shortwave trough, there is good agreement of a stout southerly marine push at the surface late Tuesday into Wednesday. The upper disturbance will likely deepen the marine layer with substantial onshore flow bringing the potential for increased marine stratus and somewhat cooler temperatures for Wednesday, albeit mid-to upper 80s for highs, with at least a 50% chance of temps exceeding 90 degrees for Salem to Portland. DH/HEC .LONG TERM...Thursday through next Monday...Warm and dry conditions expected through the remainder of the week and into the weekend. Ensembles as well as WPC 500 mb clusters show good agreement of upper level ridging building back westward with 500 mb heights climbing towards towards 595 dam for the latter part of this week, through the weekend and into Monday of next week. An upper level trough off the coast towards the middle of next week could provide a bit of a reprieve for the coast, but there is a relatively low probability at this time. Expect inland valley highs remaining around 85-95, the coast around 65-75 and the Cascades 75-85. /42 && .AVIATION...High pressure will maintain VFR conditions under clear skies through the rest of the evening with northerly to northwesterly winds. Expect wind gusts up to 25 kt along the coast, and gusts up to 20 kt inland. Winds weaken below 10 kt tonight. High confidence for marine stratus returning along the coast tonight after 03-05z Tue, returning IFR/MVFR conditions or lower. Guidance also suggests a 15-30% chance of MVFR stratus returning to the Portland/Vancouver Metro Area again after 12z Tue. These probabilities may be higher as MVFR stratus has been observed at KPDX/KTTD in the mornings even though previous chances have been low. Thus, MVFR cigs were included in the recent TAF package for KPDX/KTTD for tomorrow morning. Any low stratus that develops inland should clear up by 17-18z Tue, and by 21-22z Tue for coastal locations. PDX AND APPROACHES...VFR conditions under clear skies through most of the TAF period. Chance for low stratus returns for the Portland-Vancouver Metro Area around 12-16z Tue. Northwesterly winds through the rest of the evening around 7-9 kt, further weakening tonight. -Alviz && .MARINE...High pressure over the NE Pacific will maintain gusty northerly winds through Tuesday morning. Expect gusts up to 25 kt through late evening, mainly for the waters south of Cape Falcon. Tonight, winds will weaken below small craft conditions as winds turn slightly offshore along the immediate coast. Therefore, the Small Craft Advisory for the inner waters (out 10 NM) between Cape Falcon and Florence, OR will only be valid through 2 AM PDT Tuesday. Meanwhile, small craft conditions will continue over the outer waters (10-60 NM) through mid-morning, with the Small Craft Advisory for these waters valid through 9 AM Tuesday. Seas will mainly be wind-driven and choppy at times, between 4-6 feet. Note that a weak low pressure system off the California coast will lift north along the Oregon coast Tuesday night into Wednesday morning, resulting in a slight chance (15-20%) of thunderstorms over the waters. Lightning and/or localized gusty winds (up to low-end Gales) could impact the waters if thunderstorms do develop. -Alviz && .FIRE WEATHER...A combination of environmental factors has prompted a fire weather watch to be issued for areas east of the Coast Range, including the Willamette Valley, the Cascade Foothills and portions of the Oregon and Washington Cascades. Hot and dry conditions are expected Tuesday afternoon with highs reaching into the mid to upper 90s across the Willamette Valley and upper 80s to low 90s in the Foothills and Cascades. RH values will drop below the critical threshold with values expected in the 15-30% range during the afternoon. While wind is not expected to be a factor tomorrow, a surface low riding north/northeast along the Oregon Coast will bring an influx of mid-level moisture to the region along south/southeast winds. This influx of moisture will support elevated instability which could support convective initiation across the Cascades and into the Willamette Valley. Given thunderstorms are expected to be elevated in nature, not much precipitation will be associated with them at the surface which will not help improve the dry conditions. Fuels in the Willamette Valley are the driest but fuels are also conducive for wildfires in the Foothills and Cascades. The main threat will be fires started by lightning strikes associated with thunderstorms. The thunderstorm threat will be greatest Tuesday evening and into the overnight hours, mainly from 00-12Z and will linger into Wednesday afternoon and evening mainly across the WA634/635 zones. -Batz && .PQR WATCHES/WARNINGS/ADVISORIES... OR...Heat Advisory from 11 AM Tuesday to 8 AM PDT Wednesday for ORZ111-112-120-122. WA...Heat Advisory from 11 AM Tuesday to 8 AM PDT Wednesday for WAZ206-207-209-210. PZ...Small Craft Advisory until 2 AM PDT Tuesday for PZZ252-253. Small Craft Advisory until 9 AM PDT Tuesday for PZZ272-273. && $$ www.weather.gov/portland Interact with us via social media: www.facebook.com/NWSPortland www.twitter.com/NWSPortland