textproduct: Portland

SYNOPSIS

A more typical Spring weather pattern will bring cooler temperatures and periods of showers through the first half of the weekend before temperatures moderate and conditions at least temporarily dry out Sunday into Monday.

DISCUSSION...Through Tuesday

Water vapor satellite imagery midday today reveals a rather potent and compact shortwave trough sliding eastward across Oregon. Large scale lift and instability with the approach of the upper level shortwave trough led to widespread elevated thunderstorms last night along the Interstate 5 corridor, and particularly across the Cascades. These storms have since shifted eastward into Idaho and Montana.

Current visible satellite imagery does a reveal a low to mid level circulation across central Washington that does not appear well analyzed by the RAP. Nonetheless, a deformation band wrapping around this feature is bringing steady light rain to much of the Washington Cascades currently. Given the speed with which the parent trough is moving this steadier rain should push east of the Cascades by later this afternoon. Shallow instability behind this storm system has led to a few showers coming onto the coast and into the Cascades. However, a rapidly lowering subsidence inversion with the departing upper level shortwave trough should result in most of these showers decreasing from southwest to northeast across the area this afternoon and evening.

Another, but much weaker shortwave trough will slide eastward across the Pacific Northwest Thursday 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 stronger shortwave trough is slated to 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 our northern CWA, subtle amplitude differences between ensemble members does result in some uncertainty in shower/thunderstorm intensity and coverage 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 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 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 80-90% of the global ensembles show dry and mild weather continuing while while 10-20% of the ensembles show a troughing pattern that would bring at least some light precipitation back to the region.

AVIATION

A shortwave trough traversing the region continues to support scattered showers over the forecast area along with southwest to west winds. Flight conditions remain mostly VFR with some pockets of sub-VFR conditions in the southern Willamette Valley and along the coast. Precipitation is expected to taper off through the afternoon with mostly dry conditions returning by 00z Thursday. Winds will be around 10-15 kt with gusts between 20-25 kt for most terminals.

KPDX AND APPROACHES...VFR conditions expected to continue with intermittent MVFR conditions as precipitation continues to stream into the region from the southwest through around 00Z Thursday. Winds 10-15 kt with gusts 20-25 kt through the afternoon. KTTD could see gusts closer to 30 kt. -19

MARINE

Conditions remain benign overall with weak upper level systems moving across the region through the weekend. Westerly to southwesterly flow persists in the wake of a cold front that passed through this morning. Winds 10-15 kt with gusts to around 20 kt. Could see some infrequent, isolated gusts of up to 25 kt through around 6 PM today. Seas remain around 7-8 feet at 10-11 seconds through the evening before dropping back towards the 4-6 foot range overnight.

A series of weak disturbances pass over the waters tomorrow through Saturday, bringing periods of borderline Small Craft Advisory wind gusts, increasing seas, and scattered showers to the waters. There is a 10-25% chance for Small Craft Advisory criteria gusts Thursday evening over the central and northern outer waters. Friday, probabilities increase and expand a bit to 10-30% for both inner and outer central and northern waters. The system moving through on Saturday shows the best chance produce Small Craft Advisory conditions with a 20-50% chance, highest across the northern waters and lowest over the southern waters. Seas are also expected to increase Friday night into Saturday to 8-10 feet at 12-14 seconds. High pressure builds into the region during the latter half of the weekend, shifting winds to the northwest into early next week. -19

PQR WATCHES/WARNINGS/ADVISORIES

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