Area Forecast Discussion
Issued by NWS Portland, OR
Issued by NWS Portland, OR
342 FXUS66 KPQR 202144 AFDPQR Area Forecast Discussion National Weather Service Portland OR 244 PM PDT Sat Jul 20 2024 .SYNOPSIS...Hot inland today, with bit cooler weather along the coast where morning clouds will give way to sunshine. Upper disturbance offshore will lift north across the region Saturday night and Sunday, with some light showers and a few thunderstorms Saturday night through Sunday afternoon. Bit cooler with mix of sunshine and clouds Sunday. Afterwards, increased onshore flow will maintain near normal temperatures for Monday and next week. && .SHORT TERM...Saturday afternoon through Monday... Key Messages: 1) Hot inland today, with highs in the 90s. 2) Scattered showers and a chance of thunderstorms Saturday night into Sunday morning, beginning in Lane county late tonight then spreading northward through Sunday morning. 3) Cooler air spreads inland Sunday night, with temperatures near what is expected for late July (lower 80s inland with 60s on the coast) for Monday into next week. Water vapor satellite imagery this afternoon shows our incoming shortwave situated off of the central California, slowly moving north, with dry air associated with weak high pressure over western Oregon and Washington. This high pressure is producing another day of above normal temperatures. As of 2pm, temperatures are in the upper 80s to low 90s across the inland valleys, and are forecast to peak in the mid 90s. Some hazy skies from wildfire smoke in the Cascades seem to be limiting temperatures somewhat in the southern Willamette Valley, so these areas may only reach the low 90s today. Ensemble guidance is in good agreement that the aforementioned shortwave trough will move north through NW Oregon and SW Washington tonight through Sunday. This shortwave will merge with monsoonal moisture currently situated over the Sierra Nevada as it moves north, which will allow enough moisture for the potential of showers over the area tonight through tomorrow afternoon. CAMs indicate the negative tilt of this trough along with the midlevel moisture will provide enough elevated instability for thunderstorm chances tonight, shifting to surface based thunderstorms along the Cascades tomorrow morning and afternoon. The locations with the best chances for thunderstorms are along the coast, Coast Range, and Cascades due to orographic lift, especially south of a line from Tillamook to Clackamas County, but a few thunderstorms could meander into the inland valleys from the terrain. Scattered showers and thunderstorms will begin in Lane County around 10 PM to midnight tonight, spreading north/northeast in a line from the northern Oregon/southwest Washington coast to the central Willamette Valley by daybreak, then continuing to spread northeast through the morning and afternoon hours. The majority of showers should move east of the Cascade crest by 5 PM tomorrow, with dry weather returning tomorrow night into next week. Cooler temperatures will follow this shortwave. Daytime temperatures tomorrow will drop to the low to mid 80s, then fall again to the upper 70s to low 80s for Monday for inland locations. Onshore flow brought behind the shortwave will continue into Monday due to a longwave trough situated over the eastern Pacific. -HEC .LONG TERM...Tuesday through Saturday...Ensemble guidance is in agreement that a longwave trough will remain situated over the eastern Pacific through most of next week, keeping high pressure east of the Pacific NW. NBM guidance indicates temperatures are expected to hover right around seasonal normals, in the low to mid 80s inland and 60s along the coast, with onshore flow and dry weather persisting. -HEC && .AVIATION...Marine stratus from the morning has cleared out of northern terminals, but the central/southern coast continues to see marine stratus. KONP is still seeing IFR cigs, and there is 70-80% chance of KONP remaining IFR throughout Saturday as stratus never fully clears. NNW winds at the coast will continue picking up going into the afternoon, with gusts up to 25 kt possible at times during Saturday afternoon and evening. Small chance of thunderstorms at times after 06-09z Sun beginning at the coast. The Willamette Valley will see chances beginning around 18z Sunday, with even lower chance than the coast. Should generally expect VFR conditions inland, though a high overcast deck is possible. PDX AND APPROACHES...VFR conditions expected through the forecast period. NW gusts to 20 kt possible Saturday evening, between 00-06z Sun. Very low chance of a stray thunderstorm at the end of the TAF period, around 18z Sunday. /JLiu && .MARINE...High pressure continues to dominate, and wind pattern is dominated by diurnal cycle. Northerly gusts up to 25 kt possible at times, mostly during afternoon and evening hours. Seas 3 to 6 ft range, with a mix of variable period swell, but with gusty winds, will have choppy, steep seas. Beginning around 8-9pm, slight chance of intermittent thunderstorms for all waters as a system passes through the area. Chances are fairly low, 10-20%, but threat exists for an isolated strike or two. /mh/JLiu && .FIRE WEATHER...Only minor changes in forecasts, but overall pattern remains on track. An upper level disturbance offshore will lift across the region late tonight and Sunday. As such, will see isolated to scattered showers, with a chance of thunderstorms. Generally, looks like a 15-30% of thunderstorms over the region, with highest chance being over the Cascades/foothills, Coast Range, and along the coast. These zones along with the southern Willamette Valley have been upgraded to a Red Flag Warning for the potential of abundant lightning. Even though there is a chance of similar conditions and a lightning ignition here and there across the north Oregon Coast Range and Willapa Hills, the forest fuels in those areas are not dry enough for significant fire spread. That is why those areas are not in the Red Flag Warning. Additionally, the central and northern Willamette Valley and Clark County zones have not been upgraded as the potential for abundant lightning is much lower, though a few isolated weak thunderstorms are still possible. -HEC && .PQR WATCHES/WARNINGS/ADVISORIES... OR...Heat Advisory until 11 PM PDT this evening for ORZ111-112-115- 117-118-120>122. WA...Heat Advisory until 11 PM PDT this evening for WAZ205>207-209- 210. PZ...Small Craft Advisory until midnight PDT tonight for PZZ251>253. Small Craft Advisory until midnight PDT Sunday night for PZZ271>273. && $$ www.weather.gov/portland Interact with us via social media: www.facebook.com/NWSPortland x.com/NWSPortland