textproduct: Portland

SYNOPSIS

Very few weather concerns through Wednesday as high pressure dominates the area. A messy series of fronts will move inland as early as Thursday and persist through the remainder of the week.

SHORT TERM...Now through Wednesday

A broad area of high pressure consumes the west with a digging trough well offshore. Current pressure as been measured around 1035 mb and will continue to intensify through the next few days. This pervasive ridge will lead to clearing and stable conditions as air density decreases due to cooler air settling in. This stable airmass will cause mixing to be nearly non-existant at night and therefore increasing chances for fog and frost Monday and Tuesday morning. The caveat to fog formation will be the timing of cold air to saturation. If temperatures drop to or below freezing before water droplets can form and the wet bulb temperature is below freezing, then frost is more likely to form. Now if saturation occurs before freezing then there is a higher probability for fog. The challenge lies with cloud cover. The cloud cover, especially in the southern Willamette Valley where it is pervasive, may insulate enough that temperatures do not drop nearly as low as models are attempting. Therefore, fog would be a more likely solution. Winds will remain generally north to northwesterly except through the Columbia River Gorge where diurnally driven drainage winds will dominate. There is some level of uncertainty in wind speeds within the Gorge, but overall the pressure gradient is the strongest in the morning hours, and especially on Tuesday as the ridge is at it's peak and the strongest temperature change.

Another factor to consider is the potential for air stagnation concerns. The stable atmosphere for this long and cold temperatures can limit mixing and cause mixing heights to lower. In the forecast, we are seeing heights around 1000 ft or lower overnight. However, transport winds are greater than 5 kt which would bode for just enough mixing. It is something to consider though.

Overnight temperatures will fall below freezing throughout much of the forecast area. For those with bulbs still blooming, these temperatures may be cold enough to cause a "killing freeze". Areas most susceptible will be within the southern Willamette Valley, rural portions of Clark County, the Upper Hood River Valley and rural outskirts of the I-5 corridor. There is around a 30% chance of overnight lows of 30 degrees F or less tonight into Monday and around a 15% chance of similar temperatures on Tuesday morning within the southern Willamette Valley based on hi-resolution ensemble models. Lower probability elsewhere except in the aforementioned locations. Within the Upper Hood River Valley near Odell and Hood River, there is around a 5% chance of temperatures lower than 25 degrees F on Monday and Tuesday morning.

The ridge appears to attempt to break down on Wednesday; though based on the strength of the system it will be a slow decay. Temperatures -27

LONG TERM...Wednesday Night through Sunday

Looking into the middle and latter part of the upcoming week, models are suggesting a pattern change for the region. However, there continues to be a great level of uncertainty in the long-term pattern due to the aforementioned ridge. The EPS ensembles are showing a weak closed low in the southeast Pacific that will slowly advect northward on Thursday. A frontal system stemming from a broad and very messy low pressure system in the Gulf of Alaska is dropping down at nearly the same time. The combination of these two features is making the overall pattern incredibly messy and uncertainty high. The NBM is showing rain initiating as early as Thursday where most of northwest Oregon and southwest Washington see around a 15-30% chance of rain. However, the GEFS and GEPS show rain starting that early afternoon. The ENS appears to be the faster of the solutions while the GEPS with the highest rain amounts. With all of this said, precipitation totals will be non-impactful and fairly typical for this time of year. Some models are attempting to show a secondary frontal band moving in on Saturday into Sunday but it is incredibly unorganized so it's nothing to quite address at this point. -27

AVIATION

Satellite imagery and terminal observations as of late Sunday evening depicts VFR conditions with high clouds across northwest Oregon. Due to persistent cloud cover, there is low confidence for fog development overnight. Most places across the Willamette Valley have a 10-15% chance for LIFR conditions due to fog. An exception would be the southern Valley where conditions remain saturated. Guidance suggests a 30-50% chance for LIFR conditions at KEUG due to fog/low stratus between 06-18z Mon. Along the coast, VFR conditions prevail as offshore flow continues. If any fog develops, it should dissipate by 18-20z Mon with daytime heating. Moderate confidence for frost development overnight as temperatures are forecast to fall into the low to mid 30s, however cloud cover may also prevent frost. Light and variable winds overnight, becoming more northerly across the Valley on Monday. Easterly winds increase through the Columbia River Gorge, bringing gusts up to 20 kt to the eastern Portland Metro.

KPDX AND APPROACHES...Predominately VFR conditions with a 10-15% chance for LIFR VIS and CIGs from fog/low stratus. Light northwest winds around 5 kt or less overnight, increasing and becoming easterly on Monday. -10

MARINE

High pressure remains in control of the region through the middle of the week. Winds expected to be northerly to northeasterly as a weak thermal trough develops at the surface. Winds 10 kts or less. Seas start out around 3 to feet but a westerly swell moves into the waters Monday morning, pushing seas to 7 to 8 feet at 11 to 13 seconds. There is potential for the pattern to become more active toward the end of the week but details are unclear at this time. /19

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...None.