Area Forecast Discussion
Issued by NWS Portland, OR
Issued by NWS Portland, OR
177 FXUS66 KPQR 010420 AAA AFDPQR Area Forecast Discussion...Update National Weather Service Portland OR 919 PM PDT Tue Apr 30 2024 .UPDATE...Latest KRTX Doppler Radar imagery shows snow showers becoming more confined to the Clackamas, Marion, and Linn Cascades this evening, but they are dwindling a little slower than expected. We were able to expire the Winter Wx Advisory for the S WA Cascades on time at 6 PM, but debris from convective activity over the Willamette Valley and Cascade foothills has maintained the snow showers into this evening between Mount Hood and the Three Sisters area. It appears most of this activity is aiming for the Marion/Linn Cascades, so we extended the advisory above 3000 ft for that zone until midnight. By then, subsidence associated with high pressure building in from the Pacific should be sufficient to bring an end to the showers. Those travelling across the Cascades should know that roads that were wet during the daylight hours are likely becoming icy tonight as temperatures fall. The Frost Advisory looks to be in good shape for some of our outlying valleys; skies are clearing across much of our interior lowlands. Given the short nights this time of year, frost should mostly impact outlying valleys and perhaps some suburbs Wednesday morning. Aviation and marine sections have also been updated below. Weagle && .SYNOPSIS...A cool late season storm will move east of the area overnight and result in showers and thunderstorms decreasing this evening. Very brief drying and warming arrives on Wednesday before the next system arrives Wednesday night/Thursday morning. Another wetter system will arrive Friday night/Saturday with unsettled weather continuing into early next week. && .SHORT TERM...Now through Thursday...Satellite and radar show scattered showers that have bubbled up as daytime heating has warmed inland locations under a cold air mass aloft. Isolated and short lived thunderstorms will primarily confined to the eastern Willamette Valley late this afternoon and early this evening. Between freezing levels near 3000 feet and surfaced based CAPE values of 500 J/kg as analyzed by the RAP, small hail will accompany the stronger showers and thunderstorms. Models are in good agreement an upper level shortwave trough and an attendant surface low pressure will shift east of the region overnight. This will decrease showers and thunderstorm chances from west to east through this evening. The current Winter Weather Advisory for the Cascades between the South Washington Cascades and Marion/Linn County Cascades remains in effect through this evening, as convection may bring heavy snow showers at times. Those traveling through the passes (highways 20/22/26) today should prepare for winter weather conditions. No plans to extend this Winter Weather Advisory, but be prepared for wet surfaced to freeze overnight across the mountain passes/Cascades. Tonight into tomorrow (Wednesday), conditions dry as this system pushes east and upper level high pressure builds. We will see some cloud breaks tonight, with low stratus and chances of fog (20-30%) in the Coast Range and valleys. With clouds breaking, radiational cooling can occur and thus cause frost development. NBM probabilities for Wednesday morning`s lows below 36 degrees are around 60-80% in the Coast Range and Willapa Hills, and 10-20% in the Willamette Valley. HREF suggests higher chances in the Tualatin Valley and mainly Yamhill and Polk counties so have opted to issue a Frost Advisory for these areas. Some uncertainty remains with frost development because there remains differences between models in how much clearing will take place overnight versus stratus backbuilding off the Cascades and Coast Range. Temperatures warm up slightly on Wednesday, with highs in the lower/mid 50s along the coast and Coast Range, upper 50s/low 60s for inland valleys, and 40s in the Cascades. The next system looks to arrive Wednesday night/Thursday morning, bringing another round of precipitation, including 1 to 4 inches of snow across the Cascades above 4000 feet. -JH .LONG TERM...Friday through Monday...The WPC clusters suggest weak shortwave ridging will traverse the PNW early Friday. While the day appears to start off dry, NBM PoPs will climb to 80-90% by Friday evening as the next warm front pushes towards the region. The NBM and our official forecast currently have between 0.50 and 1 inch of rain falling Friday night into early Saturday in the Willamette Valley and 1 to 2 inches in the Coast Range and Cascades. The EPS generally shows between 1 to 1.5 inches in the Willamette Valley and 2 to 3 inches in the Coast Range and Cascades. Given reservoirs are near full for water supply, the highest end scenario does bring some concern to hydrology issues along the Willamette River. Fortunately this still appears to be a less than 10% chance of occurring, but we will need to monitor if more ensemble guidance falls in line with the bulk of the EPS data. WPC cluster guidance suggests a low probability of less than 20% that the Saturday storm system drops far enough south that we dry out Sunday. The rest of the ensemble guidance generally suggests we will remain under the influence of the Saturday storm system and keep rain chances going into Sunday. There is good agreement systems that another storm system will enter the PNW Monday into Tuesday and keep the area cool and wet. -JH && .AVIATION...Conditions are predominantly VFR throughout the region as almost all precipitation has moved off to the east over the Cascades, and threat of thunderstorms has ended. The rest of the TAF period remains essentially dry barring brief drizzles due to weak high pressure building in. Winds will remain weak and variable for the rest of the night, with a slight increase in southerly winds after sunrise on Wednesday, though winds still generally remain under 6-8 kt. Uncertainty remains regarding the possibility of fog later Tuesday night. High resolution model guidance continues to trend towards increased probability of fog developing in the northern inland terminals, with around a 40% probability of LIFR visibility thresholds currently. However, other guidance is keeping the possibility of fog closer to 10-20%, with the final outcome heavily dependent on exactly which areas see clearing. For now, going with around a 25% chance of dense fog at KPDX, KHIO, KTTD, and KUAO. Additionally, there`s around a 20% probability that some clouds ahead of the next system move in around 12z Wed, bringing a broken deck that could reduce ceilings to MVFR. Have not included in the TAFs due to low probability. /JLiu Notes: The ASOS at KTTD is only available via the dial in option. There is no estimated time of full repair and restoration for this location. PDX AND APPROACHES...Conditions are currently VFR, but two possibilities exist that could deteriorate conditions throughout the TAF period. Chances are around 25% for dense fog to form in the latter half of Tuesday night (11z to 17z Wed) that could potentially reduce visibilities down to LIFR thresholds. Additionally, some clouds ahead of the next front on Wednesday could cause ceilings to drop to MVFR between 12-16z Wed, dropping ceilings to MVFR thresholds. These situations will likely be mutually exclusive; if cloud cover develops, then fog is unlikely to develop or become dense. /JLiu && .MARINE...Observations from buoy 46050 show wind gusts continuing to barely meet Small Craft Advisory criteria, with gusts just barely above 20 kt. Seas remain around 8-10 ft at 11 seconds through the afternoon, slowly decreasing going into the evening. High pressure continues building, and over Tuesday night, winds will become very weak and seas drop to 4-6 ft by the end of the night. More active weather then returns later Wednesday into Thursday as another low approaches the waters. /JLiu && .PQR WATCHES/WARNINGS/ADVISORIES... OR...Frost Advisory from midnight tonight to 8 AM PDT Wednesday for ORZ104-106-109-114. Winter Weather Advisory until midnight PDT tonight for ORZ127. WA...Frost Advisory from midnight tonight to 8 AM PDT Wednesday for WAZ203. PZ...Small Craft Advisory until 11 PM PDT this evening for PZZ210- 251>253-271>273. && $$ www.weather.gov/portland Interact with us via social media: www.facebook.com/NWSPortland www.twitter.com/NWSPortland