textproduct: Portland

SYNOPSIS

Our latest dry streak comes to an end on Tuesday as a frontal systems ushers in the return of widespread rain and breezy to locally gusty south/southwest winds - par for the course given it's late March. After the cold front passes to our east by Tuesday night, lingering showers persist through Wednesday with at least some (2-5in) mountain snowfall before tapering off by Thursday. High confidence in dry weather with a warming trend late week into the weekend. Chances increasing for another weather system late weekend into early next week, however, the exact timing of this later system remains nebulous.

DISCUSSION...Now through Monday

Dry conditions coupled with ample high-clouds continue this afternoon thanks to an upper- level ridge of high pressure over western CONUS beginning to depart further east, the same feature responsible for the rather pleasant conditions this past weekend. Our focus shifts to the northeastern Pacific tonight where an upper level low pressure system is expected to strengthen, eventually slingshotting a closed surface low pressure system on it's southern periphery into western Canada on Tuesday. The cold front associated with this surface low stretched south from western Canada to offshore of northern California swings inland as Tuesday progresses producing widespread rain and breezy winds for NW OR and SW WA. Expect periods of moderate to locally heavy rain at times, mainly in midday and afternoon hours, but overall total rainfall amounts are not expected to produce any flooding impacts. 24 hour rain amounts from 5 AM Tuesday to 5 AM Wednesday are around 0.25-0.50 inch for the interior lowlands, 0.5-1 inch along the coast, 0.75-1.5 inch for the coast range, Willapa Hills, SW WA Cascades, and 0.5-1.0 inch for the Oregon Cascades.

While rainfall doesn't look particularly impactful, we'll have to keep a close eye on winds which are expected to peak along the coast between 8 AM to 5 PM, and inland between 11 AM to 6 PM. The strongest winds will be along the coast, Coast Range, and Willapa Hills where the latest NBM suggests a 65-90% chance for maximum wind gusts to exceed 40 mph but only a 5-20% to exceed 50 mph. However, the latest high resolution guidance like the 12z HREF/REFS produces much higher gusts comparatively (most members in the 55-60 mph range) right along the coast, and while the HREF/REFS does tend to run "hot" when it comes to wind gusts in general, winds near the top of the boundary layer at ~850mb peak around 50-65 knots Tuesday morning before rapidly decreasing behind the front Tuesday afternoon. Deterministic model soundings from the NAM/GFS/ECMWF reveal a fairly stable temperature profile above the surface which would work against broader vertical mixing of these winds aloft to the surface. That said, more convective elements right along the cold front may provide enough forcing to punch these stronger winds to the surface, albeit briefly/non-uniformly. Would not be surprised to see a observation or two gust around 50-55mph+ Tuesday, particularly Clatsop County northward along the south WA Coast where this frontal forcing is maximized, but the lack of confidence among guidance (ECMWF/EPS not biting on winds to this magnitude and low (0-15%) NBM probs), limited duration, and only a marginal/conditional set-up leave much to be desired when looking to potentially issue any wind related highlights. Shifting inland, probabilities for maximum gusts to exceed 40mph are only around a 5-20% chance, highest in the central Willamette Valley south of the Portland metro area to the Salem area. So most likely scenario is peak wind gusts of 25-35 mph for inland locations with an isolated gust or two closer to 40 mph. High confidence winds drop off at the coast and inland come Tuesday evening bringing an end to this concern.

A secondary upper-level shortwave axis moves eastward behind the front Wednesday into early Thursday while we sit in a convective post-frontal airmass. Thus showers will continue on Wednesday, most frequent over the Cascades and coast/coast range, before slowly tapering off Wednesday night into early Thursday. While elevated initially, snow levels finally drop below the passes early Wednesday morning when the heaviest of stratiform precipitation has ended, so the snow forecast continues to diminish over the Cascades. The NBM probability of at least 6 inches of snow over the Oregon Cascades is less than 5-15%, even for the highest elevations. This increases slightly to 15-45% for the higher reaches of the SW WA Cascades. Snow levels lower to 1500-2000 ft Wednesday night into early Thursday morning, with a 10% chance of snow levels falling near valley floor - would need training showers with clearing beforehand. However, precipitation probabilities along with amounts are so low by this point that even if some wet snow does mix in, it would not accumulate or lead to any impacts, especially as road surfaces would be too warm.

By Thursday afternoon, the shortwave moves east of the area with upper level flow becoming more zonal behind it. Generally zonal flow with slight ridging continues Friday into Saturday, bringing dry conditions with clearing skies. Expect a warming trend with daytime temperatures in the mid to upper 50s on Thursday, the low to mid 60s for Friday, and upper 60s/near 70 on Saturday/Sunday for the interior lowlands. During the peak of the warming trend (latter two days) the NBM indicates a 30-50% chance for high temperatures to meet or exceed 70 degrees across the Portland metro area and Willamette Valley with slightly higher probabilities (50-65%) south of Salem near Eugene.

Both deterministic and ensemble guidance indicate a deep Pacific trough moving east through the Pacific and approaching the West Coast later on Sunday into Monday, bringing increasing chances of precipitation. However, this far out, there is quite a bit of uncertainty in regards the speed/amplitude of the incoming feature with the latest mean ensemble guidance delaying the arrival of the trough another ~6hrs compared to prior runs. Still, this uncertainty has lead to a large NBM 25th-75th percentile temperature spread, showing highs from the low 60s to the mid 70s are all in play for the Willamette Valley/Portland Metro Sunday afternoon. Should ensemble guidance trend even slower with the aforementioned trough, Sunday may end up being the warmest day of the upcoming late week/weekend time period. Wee'll have to see how forecast solutions converge in the coming days. -99/03

AVIATION

West to southwest flow aloft with increasing moisture will bring mid to high level clouds streaming across the area today through tonight. VFR conditions expected through at least 15z Tuesday, with conditions likely remaining VFR inland through Tuesday morning. Dry conditions prevail through tonight with generally light and variable winds. A frontal system will approach the coast early Tuesday morning bringing light rain and increasing southerly winds by 12z Tue. Chances for MVFR developing at the coast increases to around 30-50% after 15z Tue. Widespread rain likely pushes inland by 14-16z, but chances for conditions falling to MVFR is less than 10% across inland terminals, until after 21z Tue. Southerly winds are expected to gust up to 35 kt along the coast, peaking late Tue morning into Tue afternoon. There is also potential for LLWS to occur inland with SSW winds of around 40 kt at 2000 ft around the same time the rain begins.

KPDX AND APPROACHES...VFR expected through at least 18z Tuesday with mid to high level clouds increasing through tonight. Light winds become southeasterly by 06z Tuesday. Light rain will bring lowering CIGs by 15z Tue, but conditions expected to remain VFR. LLWS possible after 15z Tue as well as SSW winds increase at around 2000 ft. /DH

MARINE

High pressure weakens over the coastal waters today allowing northerly winds around 10-15 kt to further decrease through this evening, while seas gradually subside to around 6 to 7 ft tonight. Winds turn southerly overnight as a warm front lifts north across the waters. The strengthening surface low will remain offshore as it quickly moves NE toward Vancouver Island, but southerly winds are expected to increase across the coastal waters through Tuesday morning as the associated gale force front approaches. Widespread gusts of 34 kt or greater are expected across all waters, with a 30-50% chance of peak gusts reaching 45 kt. Have issued a Gale Warning for all coastal waters, including the Columbia River Bar, from Tuesday morning through Tuesday afternoon.

Seas are also expected to build on Tuesday in response to the increasing wind waves followed by a westerly swell behind the frontal passage. Combined seas will likely build to at least 13 to 15 ft, peaking by late Tue afternoon into Tue evening. Chances for seas exceeding 18 ft remain low (5% or less). Breezy southwest to westerly onshore winds will continue behind the frontal passage through Wednesday, while seas are expected to gradually subside to around 9 to 12 ft on Wednesday. High pressure then rebuilds over the waters later this week returning northerly winds later Thursday into Friday. /DH

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Gale Warning from 6 AM to 5 PM PDT Tuesday for PZZ210-251>253- 271>273.