textproduct: Portland

SYNOPSIS

Lingering showers through this afternoon, with snow mainly above 1500 ft. The cool upper level trough will exit the region later today as weak high pressure returns on Friday. As the end of the week and weekend approaches, conditions look to generally warm toward seasonable normals and dry. However, a more active pattern looks to return to the region by the end of the weekend into the start of next week.

DISCUSSION...Now through Wednesday

A brief but calmer pattern is expected tonight through Saturday night. Temperatures tonight into Friday morning will likely cool into the upper 20s to lower 30s. Thus, any remaining moisture would freeze, resulting in slick conditions on most surfaces. As we move further into the end of the week and into the weekend , mostly dry and warm conditions return as a weak, upper level ridge builds over the area. The exception to the mostly dry conditions could be along the Coast and in the Cascades as some lingering, light showers are expected.

Looking into the weekend and the start of next week, WPC 500 mb cluster analysis remain in decent agreement that the aforementioned upper level ridge shifts eastward towards the Rockies on Sunday. As this ridge builds and moves eastward, 850 mb temps are expected to warm towards 0 to 2 degrees C by Saturday and 2 to 4 degrees C by Sunday. This will result in snow levels rising towards 3500-4500 ft by Saturday. Also of note, offshore gradients are expected to increase Saturday and Sunday as low pressure systems moves into the eastern Pacific. Models are currently forecasting a -6 to -8 mb gradient from KDLS to KPDX, if this does occur, expect easterly gusts up to 45 mph within the western part of the Columbia River Gorge which could bring easterly gusts up to 30 mph to the Portland/Vancouver metro area.

In addition to the warming pattern, models and their ensembles also bring precipitation back into the forecast via a weak atmospheric river by the start of next week. However, there is still high uncertainty in regards to precipitation amounts and location of the heaviest rainfall, but the majority of model and ensemble guidance shows up on the north/northeast flank of this weak atmospheric river. Currently, the highest probabilities of enhanced rainfall are pointed at portions of southern Oregon and California. While this is still well into the future, depending on precipitation amounts as well as snow melt, there could be hydrological concerns as the start of next week approaches. But if the forecast remains similar to what has been described, then any hydrological concerns will be minimal. Again, there is still a significant amount of uncertainty in the forecast in the extended. Nevertheless, we will continue to monitor conditions as they develop. As the middle of next week approaches, weak ridging to zonal flow looks to make a return for the Pacific NW. Which would result in a return to rather benign weather conditions. -42

AVIATION

CIGs have struggled to clear today and thus, MVFR CIGs have lingered. Will see very little improvement through the day, though the overcast skies may improve to low-end VFR for some locations. Coastal terminals would be the most susceptible to this trend.

Overnight winds will ease and, if skies are able to clear enough, radiational cooling will occur. The combination of this cooling with light winds will promote the formation of some patchy fog. Areas at the most risk will be within the Willamette Valley as winds will be southerly. One limiting factor though will be clouds. If terminals remain overcast with no breaks, then fog is less likely. Models suggest anywhere from a 10-40% chance of fog around KPDX, and KEUG therefore have hinted at it towards the end of the forecast.

KPDX AND APPROACHES...Light winds with a slight northwesterly component. VFR conditions to prevail over the next several hours. After 09Z Fri, if skies clear or partially clear, there is around a 20-30% chance of fog formation. Confidence is low and likely would be patchy. However will continue to watch satellite through the afternoon. -27

MARINE

The marine environment will be driven by a northwesterly background swell for the next 24-36 hours before becoming wind wave dominate. Weak high pressure over the area will maintain stable conditions of winds of 10-15 kt with gusts around 20 kt, and seas of 6-9 ft at 10 seconds. On Saturday, a much stronger low pressure system will move in causing winds to shift to the south and they will increase considerably. At this point have a Gale Watch in effect for the outer waters, but confidence is high in those conditions manifesting.

The challenge will be the orientation of the frontal system and associated low. In it's current position, there is a band of potential storm force winds in the south, with a slice moving into the outer waters of PZZ273 west of 30NM. Chances of these storm force winds becoming more widespread is around 20-30% within the southern waters. The norther waters appear to escape these types of speeds. Another consideration is the impact these winds will have on seas. Wave heights will be predominately wind wave driven with a weak 4 ft at 13 second northwesterly background swell. There is a slight southerly fresh swell which will amplify the overall wave heights. Will continue to monitor for the potential storm force winds. For the inner waters, a Hazardous Seas Watch is in effect. -27

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Hazardous Seas Watch from late Friday night through Sunday morning for PZZ251>253. Small Craft Advisory until 7 PM PST this evening for PZZ271>273. Gale Watch from late Friday night through Sunday morning for PZZ271>273.