Marine Weather and Tide Forecast for Altoona, WA

SHORT TERM
Now through Sunday Night...Early this morning satellite and radar imagery depict several consolidated bands of shower activity streaming south to north across Western Oregon and Southwestern Washington all thanks to the same synoptic feature which provided some excitement in the form of thunderstorm activity yesterday (Friday). This upper-level shortwave helping to drive the current precipitation will continue to eject northward over the next several hours with models indicating at least a noticeable decrease in shower activity by mid to late morning, especially across the central Willamette valley up through SW Washington.

If you're hoping for a repeat performance this afternoon in regard to thunderstorm chances, you may be in luck as the set-up today has several similarities to what occurred yesterday, albeit not as enticing in most aspects. Deterministic and ensemble models show another closed upper-level low pressure system over the far eastern pacific slowly marching towards the northern California coastline keeping us under broad and moist southerly flow highlighted by anomalously high PWAT values around 0.8-0.9in. This keeps us in a flow pattern which traditionally gives us a better shot at thunderstorms west of the Cascade crests. While the overall synoptics are favorable, the most important and unfortunately most uncertain aspect of the forecast revolves around the amount of clearing we receive throughout the day - more clearing likely leads to higher CAPE values and thus deeper/strong convection. Generally, the HRRR and deterministic models like the GFS/NAM project MUCAPE values peaking in the 300-600 J/kg range later today which should be sufficient for thunderstorm development, although these values are a bit lower than what occurred yesterday. The amount of wind shear aloft to support storm organization is also a bit lower as well with effective shear around ~20-25 knots. With all this in mind the storm mode should generally fall into the "pop-up" variety with a 15-30% chance of this activity occurring from roughly the US-26 corridor southward. Highest relative chances are over terrain features like the Cascades, coast range, and attendant foothills where convection is more likely to initiate between 1pm to 8pm. Impacts include lightning, small hail, heavy rain, and locally gusty/erratic winds in any storms which do occur. It's worth pointing out given the marginal depth and fairly skinny CAPE profile shown in model soundings, a greater degree of cloud cover than anticipated will almost certainly keep activity from electrifying (you want CAPE to extend through the -10 to -20c mixed phased region of the atmosphere) thus leaving us with just efficient rainfall producing showers. Once we get towards sunset and daytime heating wanes, so to will convection with just lingering showers overnight.

On Sunday, the upper-level low pressure feature stirring up the unsettled weather tracks further east transitioning the region into more of a westerly flow pattern and effectively ending most thunderstorm potential outside of a slight chance (around 10-15%) over the far southern portion of the Lane County Cascades near the crest-line. Still, don't expect a return to completely dry weather as models are in moderate to good agreement showing showers persisting due to wrap-around moisture on the northern periphery of the broader circulation - PoPs of 70-95% over the Cascades and foothills and 30-60% elsewhere.
With the loss southerly flow near the surface, temperatures also cool back into the upper 50s to low 60s inland, closer to seasonal normals for early to mid April. -99

LONG TERM
Monday through Friday...Ensemble guidance indicates broad west-northwest to northwest flow aloft for early next week helping to steer a weak embedded shortwave into the region on Monday. As a result showers chances (50-90%) will be maintained. Beyond this shortwave feature shower activity decreases Monday night into Tuesday as a transient ridge briefly builds overhead. Our attention then turns to the next Pacific frontal system arriving from the Gulf of Alaska late Tuesday into early Wednesday morning. Confidence is high cooler temperatures aloft associated with this system will lower snow levels to near or below Cascade pass elevations, raising the concern for potential travel impacts returning. A stark change from the rather warm conditions we've seen the last couple of weeks. The latest NBM probabilistic guidance indicates a 60-90% chance of 6+ inches of snowfall at the Cascade passes (Gov Camp, Santiam, Willamette Pass) over the 48 hours between 5pm Tuesday and 5pm Thursday. As for 12+ inches of snowfall, there is a 40-60% chance during this same time period. Should these probabilities and current forecast snowfall amounts hold, snowfall related highlights would be needed for our Oregon and Washington Cascade zones.

Additionally, cooler and potentially clearer conditions behind the system may introduce renewed frost concerns across portions of the interior valleys by mid to late week. Wednesday night into Thursday morning is the first of these such nights with NBM guidance suggesting a 30-70% chance for areas frost (<36F with partial clearing) across most inland locations and a 60-80% chance for freezing temperatures in the Upper-Hood River Valley.
A similar set-up is anticipated Thursday night into Friday morning as well. At least the trend is to moderate back towards normal temperatures for the following weekend. -99/12

AVIATION
Mixed flight conditions are expected through Saturday as unsettled weather continues due to an upper level low pressure system off of the northern California coast. Scattered showers continue through the day moving from south to north. Showers have been causing periods of lower flight conditions to MVFR/IFR cigs and vis. MVFR cigs outside of showers are forecast to become predominately VFR by 19z Sat. Thunderstorm chances return after 18- 20z Sat, ending around 03z Sun. Best chances are in the central and southern Willamette Valley into the adjacent Cascades with a 25-30% chance, and a 15-25% chance elsewhere. Strong showers or thunderstorms could produce gusty outflow winds, brief heavy rain, small hail, and lightning. Outside of convective outflow, winds will be variable and mainly less than 7 kts.

Latest guidance suggests widespread MVFR conditions return between 09-14z Sun ahead of another round of shower activity.

KPDX AND APPROACHES...Mainly VFR conditions through Saturday, though isolated showers could lower flight conditions to MVFR/IFR. There's a 15-20% chance for thunderstorms, mainly between 23z Sat to 04z Sun. However, confidence in timing and placement remains only low.
MVFR conditions forecast to return after 09-12z Sun ahead of another round of widespread showers. -03

MARINE
Headed through the weekend conditions remain rather benign as far as the sea state and winds are concerned with significant wave heights holding in the 4 to 7 foot range with winds gusts below 20 knots across the inner and outer waters.
We'll likely see south/southwest winds slowly become more north/northwest today into this evening before persisting through at least Monday night. Looking towards the middle of next week in the Tuesday and Wednesday time period a decently robust frontal boundary will likely (75-85% chance) result in Small Craft Advisory wind gusts and seas building to around 7 to 10 feet. Wave heights and winds then slowly decrease into the end of the week. -99

PQR WATCHES/WARNINGS/ADVISORIES
OR...None.
WA...None.
PZ...None.