Area Forecast Discussion
Issued by NWS Portland, OR
Issued by NWS Portland, OR
852 FXUS66 KPQR 251717 AFDPQR Area Forecast Discussion...UPDATED National Weather Service Portland OR 1014 AM PDT Wed Sep 25 2024 Updated aviation discussion .SYNOPSIS...Cold front is moving inland today which will bring widespread rainfall and breezy winds. Thursday will bring a brief lull in active weather, aside from lingering fog, Thursday afternoon. Another weaker front arrives overnight into Friday late Thursday into Friday morning. High pressure returns over the weekend. More active weather expected mid-week. && .SHORT TERM...Now through Friday...Satellite and radar show the frontal boundary along -125W latitude this morning, and it is very slowly moving eastward. This front is associated with a cold air wrapped low that has dropped down from the Gulf of Alaska. It is weakening as it fills, but ample cold air and moisture accompanies it. The front will move inland late this morning, with the coast seeing a few showers earlier. It will then slowly move inland through the day becoming showery by 2200. Rainfall accumulations will be variable both by terrain and latitude as the low is positioned to the north of the forecast area. Because the area is sitting on the southern edge of the front, a bulk of the precipitation will fall along the coast, Coast Range, and the southern counties of Washington. A bit of a rain shadow will keep the valley drier, then orographic lift will aid heavier rain over the Cascades. This front is ushering in air at around 4 degrees C at 850 mb (5500 ft). This, combined with the orographic lift over the Cascades, southwesterly flow, and warm temperatures from the previous day, will increase the probability for thunderstorms. There is a 15-20% chance of thunderstorms over Skamania, Cowlitz, Clark, Hood River, Multnomah and northern half of Clackamas Counties. As the front exits the area to the east, zonal flow will take over with dry air shifting in. Will see moist conditions overnight with clearing skies and radiational cooling. This will make the environment suitable for fog formation Thursday morning. Overnight Thursday though, yet another shortwave trough will advect over the Pacific Northwest which is associated with a 980 mb low at 51.2N -134.3W. This low will move northeast over the Alaska Peninsula through Friday morning putting the forecast area in the southern end of the front. Because of this, precipitation will be reserved for the northern counties in northwest Oregon and southern counties of Washington. Accumulation will be minimal and will dry in the afternoon as high pressure develops once again. -Muessle .LONG TERM...Friday Night through Tuesday...For those with weekend plans, prepare for another sunny and dry Saturday! High pressure will reform late Friday into Saturday and persist through late Monday/early Tuesday. Based on the pressure gradient composition, winds will generally be diurnally driven based on the position of the ridge and thermal trough axis. Overnight, winds will be generally easterly/drainage then become westerly/northerly in the afternoons. The pressure gradient is around 4 mb during the day so the gustiest winds will occur in the E-W aligned terrain like the Columbia River Gorge. Even with this high pressure in place, not seeing significant temperature rises as highs will remain near normal in much of the long-term forecast. Interestingly, on Sunday at the mid-levels, 500 mb vorticity advection and heights are showing an energetic trough moving through. However, it is not robust enough to bring rainfall to the area that will overcome the ridge from Saturday. Instead, it will be cooler than highs on Saturday. High pressure rebuilds on Monday and Tuesday with it reaching it`s most amplified ridging. Temperatures will rise on the "high side" of normal on these days with the spread being around 5 degrees C between the 10th and 90th percentile. Late Tuesday yet another front nears the coast which could bring another round of rainfall. Confidence is low at this time though as the pattern is very unorganized and the system would have to overcome the depth of dry air from the previous days. -Muessle && .AVIATION...Southwest flow aloft as a front moves across the area today. As of 9 AM radar shows leading edge of rain reaching the south WA & far north Oregon coast. MVFR prevails around KAST where there is more mixing, but may see IFR conditions as the front moves through. Now, south of about KTMK fog/stratus continues this morning with LIFR conditions. Expect conditions to improve there as the front moves ashore, roughly 21-23Z Wednesday. Inland areas - there is a mix of VFR and MVFR which is expected to continue through about 00-02Z Thu. For tonight expect mainly VFR conditions at inland TAF sites, but may see some MVFR cigs back build from the Cascade foothill to valley terminals. While at the coast more of a mix of VFR and MVFR through the night. PDX APPROACHES...A mix of VFR and MVFR with rain chances with the incoming front peak around 20-02z this afternoon. Southerly winds expected to increase to around 6-10 kt by 20Z Wednesday before abruptly shifting to the west later this afternoon, around 22z-00z. Expect mainly VFR overnight, but HREF indicating around a 30-50% chance for MVFR cigs after 10Z Thu. /mh /DH && .MARINE...A quick moving cold front will move across the coastal waters today, pushing onshore this afternoon. Southerly winds ahead of the front are expected to gust up to 25 kt during a brief period of 2 to 4 hours north of Cape Foulweather. Seas are also expected to build to around 8 to 10 ft, therefore have issued a Small Craft Advisory for the combined potential impacts through early this afternoon. Winds will shift to the northwest with the frontal passage, becoming light overnight. Winds turn southerly again on Thursday ahead of another frontal system. Low pressure will rapidly intensify across the far NE Pacific and move toward Haida Gwaii, while pushing a front toward the coastal waters later Thursday. The strongest winds associated with this system are expected across the northern waters. In fact, model guidance continues to increase the probability of Gale Force southerly wind gusts exceeding 33 kt to around 30-50% Thursday afternoon and evening. The trailing cold front will likely weaken as it moves across the coastal waters Thursday night. Then, a westerly swell moving in behind the front is expected to build seas to around 12 to 14 ft on Friday. High pressure also rebuilds on Friday, along with a strengthening thermal trough along the southern Oregon coast. This will result in a return to gusty northerly winds. Northerlies are likely to continue through the weekend, while seas settle to around 8 to 10 ft. /DH && .PQR WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. PZ...Dense Fog Advisory until 11 AM PDT this morning for PZZ210- 251>253-271>273. Small Craft Advisory until 1 PM PDT this afternoon for PZZ251- 252-271-272. && $$ www.weather.gov/portland Interact with us via social media: www.facebook.com/NWSPortland x.com/NWSPortland