textproduct: Portland

SYNOPSIS

A more typical Spring weather pattern will bring cooler temperatures and periods of showers through the remainder of the week, with a more robust system expected late Friday/early Saturday. As Sunday approaches, temperatures moderate and at least temporarily dry out into the middle of next week as high pressure builds over the area.

DISCUSSION...Today through Wednesday

Upper level, zonal flow looks to develop over the region through the latter part of the week. A much weaker shortwave trough, embedded within the upper level zonal flow will slide eastward across the Pacific Northwest today and result in a dying front sliding into western Washington and northwest Oregon. Rain chances and amounts will generally be highest closer to the coast and the farther north you go with the prospect for any measurable rain very much in question for areas farther south towards Linn and Lane Counties.

A trough is slated to dive down out of the Gulf of Alaska and slide across the northern Pacific Northwest late Friday into Saturday. While there is high confidence in the overall setup and that the shortwave trough will skirt the northern portion of our CWA, subtle amplitude differences and exact tracks between ensemble members does result in some uncertainty when it comes to shower/thunderstorm intensity and coverage on Saturday. Given the convective nature of the precipitation, there will be variability in rainfall totals over short distances, but certainly areas farther north and closer to the Cascade foothills will stand the highest chances for at least 0.25" of rain falling late Friday into Saturday. For example, the NBM suggests a 95% chance for these amounts across the south Washington Cascades while the odds of these amounts occurring are down around 10% or less in areas across coastal Lane County.

The most impactful weather aside from the 15-20% chance at any given location of a short lived thunderstorm occurring Saturday afternoon, will be snow falling across the Cascades above 3500-4500 ft Friday night and Saturday. Considering many of the forest roads are free of snow with the early melt-out and access to unmaintained roads is growing, snow levels lowering to 3500-4500 feet Friday night into Saturday could catch people off-guard if they are unprepared for the cold, wet and even snowy conditions above those elevations. While NBM guidance only suggests a 15-25% chance of 2" or more snow falling across the Cascades, the actual probability of at least a couple inches of snow falling across the Cascades is probably quite a bit higher given the the aforementioned probabilities for at least the 0.25" of precipitation falling.

There is general consensus among the global ensemble members that ridging will return to the northeast Pacific late in the weekend into early next week. The end result is that temperatures should moderate across the region with the probability for precipitation dropping to 10% or less. Uncertainty in the large scale weather pattern does grow a little bit towards next Tuesday and Wednesday. Roughly 75-90% of the global ensembles show dry and mild weather continuing while while 10-25% of the ensembles show a troughing pattern that would bring at least some light precipitation back to the region.

AVIATION

Predominately VFR conditions with pockets of MVFR conditions expected across the airspace through the TAF period. However, a weak upper level shortwave will result in very light showers for coastal locations as well as for KPDX and KTTD. These locations could see a mixture of low-end VFR to high-end MVFR as a weak frontal passage will bring light precipitation across the northern portion of the airspace through the majority of the TAF period. Confidence is low regarding the exact timing of the precipitation and lowered flight conditions. Still, start time for any precipitation will likely (65-80%) be around 20Z Thursday through around 00Z Friday, with KAST being towards 20Z Thursday and KONP, KPDX and KTTD being closer to 00Z Friday. Have included a PROB30 group when the probabilities are the highest according to current guidance. Southwest to west winds restrengthen between 22Z Thursday and 00Z Friday to around 8-11 kt as the aforementioned, weak frontal system moves inland. Showers are expected to cease after 03Z-06Z Friday, with predominately VFR conditions persisting.

KPDX AND APPROACHES...VFR conditions expected through the majority of the TAF period. However, a weak frontal passage starting around 00Z Friday will bring increasing westerly winds, light precipitation and periods of MVFR flight conditions. Confidence is low regarding the exact timing, but have included a PROB30 group when the probabilities are the highest according to current guidance. Showers expected to cease around 03Z Friday, with predominately VFR conditions persisting. /42

MARINE

Conditions remain benign overall as zonal flow persist within the upper level of the atmosphere. However, a series of weak disturbances pass over the waters through late tonight, bringing periods of marginal Small Craft Advisory wind gusts up to 25 kt, increasing seas, and scattered showers to the waters. There is a 10-15% chance for Small Craft Advisory criteria gusts tonight over the central and northern outer waters.

Friday, probabilities increase and expand a bit to 15-25% for all waters to include the Columbia River Bar as a more robust system moves out of the Gulf of Alaska and into the Pac NW. This system shows the best chance produce Small Craft Advisory conditions as a fresh swell will result in seas building towards 10-12 ft. As a result, have issued a series of Small Craft Advisories starting in our northern zones and spreading southeast through Saturday. Also, bars will also be impacted by strong and very strong ebbs, so have also included a Small Craft Advisory for the Columbia River Bar starting late Friday night. /42

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory from 5 AM Saturday to 2 AM PDT Sunday for PZZ210-252-253-273.

Small Craft Advisory from 8 PM Friday to 2 AM PDT Sunday for PZZ251-271-272.