Nws Portland - Briefing on freezing rain potential

Welcome to todays weather briefing from NWS Portland to talk about the potential for freezing rain as the next Pacific weather system moves into the region on Thursday, December 12. A cold air mass remains over the region where many valley temperatures have remained below freezing. A developing front will move into the area Thursday that will bring precipitation to the region. A few areas in the valley may get a short period of freezing rain late Thursday morning into the early afternoon before the cold air is scoured by the south winds with the front. I want to emphasize this will not be a wide-spread freezing rain event, but rather the potential for a few areas to get freezing rain with the onset of precipitation. Potential impacts are hazardous travel from icy roads and the potential impact to airfield operations. Here’s the satellite imagery from this morning, Wed, Dec 11, showing a developing front that will move into the region on Thursday, Dec 12. Also, shown is the low temperature forecast for Thursday morning. Low temperatures will be in the 20s in the valley, but expected to rise above freezing during the day. The front and associated precipitation will move into the coast early Thursday, then inland during the late morning hours into the afternoon. A few areas in the valley may get a short period of freezing rain before the cold air is scoured by strong south winds that accompany the front. This will not be a wide spread freezing rain event, but some areas will be prone to get a short period of freezing rain where cold air remains trapped during the late morning to early afternoon hours. The cold air will eventually get scoured out by the south winds associated with the front. The Columbia River Gorge will likely be the last area to scour out late Thursday night. Some uncertainties with this situation is the timing of the onset and duration of the freezing rain. Also, the effect the snow cover in Benton and Lane counties will have on retaining the cold low-level air. At this time, we do expect the cold air to scour out in these areas by midday Thursday. These are the areas prone to freezing rain. I want to emphasize this will not be a wide-spread freezing rain event. There is a moderate risk of freezing rain in the northern part of the forecast area stretching from the Kelso-Longview area southward into the Portland Metro area and Columbia River Gorge from mid-morning through mid-afternoon Thursday before the cold air scours out. The Columbia River Gorge will likely not scour out until Thursday night. There is a slight risk of pockets of freezing rain in the central and southern Willamette Valley from Salem to Cottage Grove for a few hours late Thursday morning into the early afternoon. At this time, we expect the cold air to scour out fairly quickly in this area with the south winds associated with the front. Here is a summary of impacts and forecaster confidence levels. Forecaster confidence is low to moderate for possibly freezing rain in the northern part of the forecast area from around 10 am to 3pm Thursday. Confidence is lower in the central and southern Willamette Valley, but pockets of freezing rain is possible for a few hours late Thursday morning into the early afternoon. Any freezing rain that falls will create hazardous driving conditions.