


Area Forecast Discussion
Issued by NWS Portland, OR
Issued by NWS Portland, OR
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848 FXUS66 KPQR 022148 AFDPQR Area Forecast Discussion National Weather Service Portland OR 248 PM PDT Wed Apr 2 2025 .SYNOPSIS...Showers persist today dissipating overnight as high pressure builds. Clear days and mostly clear evenings will set the stage for fog/frost development tonight and Thur night/Fri morning. Increasing temperatures on Saturday will be above normal. Frontal system arrives Sunday through early next week. && .SHORT TERM...Now through Friday Night...Satellite shows patchy skies associated with post frontal showers. These showers have brought periods of heavy rain and small hail to the region. A dry slot will be moving inland later this evening which will cause skies to clear or become partically clear. The clear skies combined with colder air will promote fog development throughout the Willamette Valley, and coastal valleys especially. There are some limiting factors though, the first of which being cloud cover. If the clouds are too dense, radiational cooling will not be enough to reach the dewpoint. However, the amount of available moisture from the previous day`s showers will provide the moisture needed fo saturation. High pressure persists through Saturday with clearer skies each day. The next weather factor to consider occurs Thursday night into Friday morning. Overnight temperatures will dip into the 30s. This, combined with light winds and drier conditions, can produce frost through the area. This would be the first frost of the growing season so it is something we are watching closely. In order to get frost we need a few ingredients: calm winds, temperatures between 33-38 deg F, a drier atmosphere initially, then temperatures reaching the dewpoint temperature (the temperature an airmass has to cool to inorder to become saturated). Light easterly winds during the day will create the decreased moisture and cooler air filtering in; winds are expected to then go calm. Probabilitistcally there are a few concerns. First: temperatures are sitting on the upper threshold for frost formation closer to 37-40 deg F overnight. While the 10th percentile would be cold enough, any fluctuation to warmer would not. In addition, the spread between the dew point depression is around 3 degrees which may be just large enough to prevent frost. Ultimately, it`s not a bad idea if you`re below 1500 ft in the valleys (coastal, inland, Cowlitz, and Gorge) to prepare for patchy to widespread frost. Areas most at risk will be rural areas and the southern Willamette Valley. Once the sun rises, any frost that did form will melt off and conditions will warm to a spring-like day. -Muessle .LONG TERM...Saturday through Wednesday...Saturday will be anyother springy day giving us a break before the next rain arrives on Sunday. High pressure will usher in warmer air from the south aloft, which will mix down causing highs to increase into the upper 60s to lower 70s. Along the coast, will remain seasonable, but sunnier than we have seen the last week. Inland areas will be warmer than normal. Models still have some discontinuity with the overall high temperature forecast. For example, the NBM ranges from 70 deg F to nearly 76 deg F (the 90th percentile). The deterministic forecast though sat right along the 25th percentile and in some areas even below this value. Decided to incorporate and nudge to the 50th percentile in the forecast in the Willamette Valley, Clark County, and the lowlands of Cowlitz county. If easterly winds are elevated, these tempeatures may be less if close to the Cascades or the Gorge. Enjoy the sun as a frontal system will move in Sunday night with a series of shortwaves through Wednesday. Overall troughing is not necessarily well realized by models at this point, and precipitation amounts will be low in comparison to what we typically see this time of year. The coast though will be the exception as the ECMWF is depicting anywhere from a quarter of an inch in 24 hours, to some of the ensembles of greater than 1.25 inches. The GFS is less wet with much lower amounts (closer to 0.25 inch). -Muessle && .AVIATION...Radar, satellite and surface observations continue to show mostly generally VFR conditions with scattered showers moving into the airspace from the northwest. These showers will persist throughout the day and will result in predominately VFR conditions with FL030 to FL050. Showers could result in brief periods of MVFR conditions (15%-30% probability) through 00Z-03Z Thursday. For inland locations there is also a 20% probability for thunderstorms through 03Z Thursday as well. Any thunderstorm that does develop could produce small hail as well as gusty and erratic winds. Afterwards, conditions will gradually dry up and result in widespread VFR. However, as we clear out, there will be a 20% probability of fog development within the Willamette Valley starting around 13Z Thursday. Expect westerly to northwesterly winds across all terminals around up to 10 kt. KAST anemometer is still inoperative. Therefore, TAFs limited to CIG and VIS. PDX AND APPROACHES...Predominately VFR conditions with scattered showers through 03Z Thursday. Any showers could result in brief periods of MVFR conditions (15%-30% probability) through 03Z Thursday. Afterwards, widespread VFR. Westerly winds become more northwesterly, generally less than 10 kt. -42 && .MARINE...High pressure over the northeast Pacific and lower pressure inland will maintain northerly to northwesterly winds under 15 kt through the end of the week. The next front arrives Saturday evening into Sunday and that will bring a return to breezy southerly winds across all waters. As the front approaches, there is a 15-30% probability for Gale-force southerly wind gusts up to 40 kt on Sunday. Seas generally 3 to 6 ft through Sunday. As the front approaches the waters, seas will build towards 10 to 12 ft as a fresh westerly swell pushes into the waters. Another strong ebb is expected Thursday morning; however, seas with this ebb are forecast around 4 to 5 ft. -42 && .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