textproduct: Portland

SYNOPSIS

Scattered showers continue across the region today, becoming isolated and limited mainly to far northern Oregon and SW Washington Wednesday through Friday. Rain chances then shift to central Oregon Saturday into Sunday as a weather system moving into northern California brushes central Oregon, but confidence is low in details. Dry weather then returns early next week. Conditions likely remain within typical climatological normals through the forecast period with no significant weather impacts on the horizon.

DISCUSSION...Tuesday afternoon through Monday

The atmospheric river that impacted NW Oregon and SW Washington through last night shifted into S Oregon and N California this morning as the surface front shifted just south of Lane County, leaving scattered showers across the area in the wake of the front. Satellite imagery Tuesday afternoon shows the upper trough that funneled the atmospheric river into the PacNW has pinched off into a cut off low in the eastern Pacific west of California, and ensemble guidance indicates this low will retrograde west before pushing back east towards the West Coast later this week. In the meantime, scattered showers continue across the region through Wednesday as a weak shortwave moves from the north through Washington and Oregon. Snow levels will lower tonight with the incoming shortwave, falling to 2500-3500 ft north of Lane County and 3500-4500 ft in Lane County by early Wednesday morning. However, with showers being scattered and limited in precipitation, not much snow is expected over the Cascades, maybe up to 2 inches in the early morning hours Wednesday at Willamette Pass.

Ensemble guidance indicates flow becomes zonal Thursday into Friday through the weekend. Low precipitation chances continue across the area through Friday morning, mainly limited to the north Oregon coast and Coast Range north of Lincoln City, far northern Oregon Cascades, and SW Washington. Limited accumulation is expected. Colder air is expected to be funneled into the PacNW behind Wednesday's shortwave. This along with clearing skies across the majority of the interior lowlands will lead to colder overnight temperatures Wednesday night with a 60-90% chance of low temperatures Thursday morning falling below freezing in the central and southern Willamette Valley and a 40-60% chance for the SW Washington lowlands. However, if skies don't clear as expected, locations with remaining cloud cover will not cool quite as much. There's an off chance (~5-10%) an isolated shower wanders off the coast range or Willapa Hills into the southwest Washington lowlands during this period, which would have the potential to be a rain/snow mix if temperatures do lower near or below freezing. However, a lot of variables would have to perfectly align for this to occur. So while the forecast may introduce a mention of a slight chance (15%) for rain/snow late Wednesday night into Thursday morning, it is extremely unlikely you'll see any impacts below 1500-2000ft - in fact you'll be lucky to see anything at all.

Mostly dry conditions return Friday afternoon through Saturday with inland high temperatures in the 50s and low temperatures in the mid 30s. Uncertainty increases in the forecast late Saturday into late Sunday. Ensemble guidance is in high agreement that the closed low remnant from the current longwave trough is slated to move inland along the California during this timeframe. However, uncertainty remains in how far north along the California coast this will occur. A track further north would yield better chances for precipitation. Latest forecasts indicate precipitation chances limited to south of Highway 20 in Oregon. Additionally, probabilities for overnight temperatures falling below freezing in the interior lowlands remain low to moderate, mainly around 30-60%, due to the uncertainty of this closed low. A bit more of a northern track will likely keep low temperatures above freezing due to cloud cover, but a more southern track could keep the area from being impacted by the closed low and allow for the jet stream to dip into Oregon and bring colder temperatures. Either way, the closed low is expected to move east by Monday, leading to increasing confidence of dry weather later Monday into Tuesday. -03/99

AVIATION

A frontal system associated with an atmospheric river remains stalled over the region this afternoon with very little movement. Because it has yet to shift southward as models suggested, have seen a trend towards more widespread IFR conditions with pockets of LIFR in passing rain showers. VIS has become much lower than previously forecast with visibility less than 2 SM throughout the area. Based on how this pattern is progressing (or a lack thereof), will see minimal change over the next 12 hours. While models want to attempt to clear or lift CIGs overnight, the amount of moisture in the atmosphere combined with a lack of mixing and light northerly winds, it will be quite difficult to do. In fact, based on the overall pattern beginning to lean towards a much more degraded forecast than previously thought. However, there remains around a 40% chance that skies do lift. If they do, and enough cooling occurs, cannot rule out dense fog.

KPDX AND APPROACHES...IFR CIGs have dominated the region and based on area PIREPs, the cloud deck appears to be quite dense. With this being the case, unless significantly drier air moves in we will see limited improvement. However, the drier airmass is sitting just to the north so as the front shifts southward, that air should move in. If this does occur, and skies improve, there will be a trend towards dense fog development due to encouraged diurnal cooling. Based on ensemble models, there is around a 30% chance of LIFR VIS, and a 65% chance of at least MVFR VIS after 10Z Wed. Based on the current observations, there is a stronger lean towards MVFR VIS with IFR CIG overnight. Confidence remains low at this time.-27

MARINE

Seas and winds have eased as the main frontal band has shifted southward. Northerly winds persist around 8-13 kt with isolated gusts up to 20 kt. These conditions have not been widespread and have slowly eased through the afternoon. Seas too have fallen below 8 ft for most buoys with most sitting in the 5-7 ft at 10 second range. These conditions will persist through Friday. Local guidance shows a mixed bag of swells with a primary northwesterly swell and a southerly fresh swell that then transitions into a true secondary swell. While a complicated swell organization, will see widespread seas rise to or above 10 ft but there is a low probability of seas exceeding that 10 ft mark. -27

PQR WATCHES/WARNINGS/ADVISORIES

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