textproduct: Portland

SYNOPSIS

An unsettled airmass remains overhead into the weekend with lingering showers gradually diminishing day to day. Then conditions trend drier by the early to middle portion of next week as temperatures slowly claw their way back near normal for early September. However, uncertainty in the large scale pattern grows noticeably around the middle portion of next week.

DISCUSSION...Today through Thursday

A vertically stacked upper-level low continues to churn off the Oregon coast this afternoon with instability reaching just deep enough into the atmosphere to support occasional lightning strikes in some of the stronger convective cells. Given the placement of the low pressure and pockets localized low level shear partially induced by our terrain features, one or two showers/T-storms have exhibited rotation with at least one funnel cloud reported by a weather spotter near midday today. The threat for thunderstorms and very isolated cold-core funnel clouds will persist into the evening hours before the loss the daytime heating weakens remaining convection. Shower chances likely continue overnight.

Come Friday the core of this low pressure feature will finally begin to shift into western Oregon although not too much will change in regard to our overall conditions. That said, models like the HREF indicate the coverage of showers may be a touch lower than today (Thursday), and more likely in the southern Willamette Valley in addition to favoring terrain features like the Coast Range, and Cascades. Exploring deterministic model soundings from the GFS, NAM, etc. show just enough CAPE extending into the mix-phased region of the atmosphere for a lightning strike or two to be produced in stronger cells, however, a glaring lack of vertical shear means any storms which do develop will be short-lived and pulse-like. Infrequent lightning, heavy rain, and gusty outflow winds are the primary impacts. Any activity then quickly decreases after sunset into Friday night.

Over the weekend, the majority of deterministic and ensemble members continue to show the broad upper-level low pressure holding firm over the Pacific Northwest with lingering shower chances most likely during the daytime hours and over terrain features as diurnal heating likely plays a major role in shower generation. Confidence is high that temperatures remain below normal for this time of year by roughly 3-12 degrees (coolest in the Cascades). It might be worth packing a jacket/coat if you plan on venturing into the mountains this weekend. Early next week, the majority of ensemble members finally show the upper low beginning to accelerate eastward favoring the return of dry conditions and a gradual warming trend. But from this point forward ensembles begin to deviate from one another when trying to resolve the upper-level pattern, particularly in regards to a ridge of high pressure trying to develop over the Rockies into southeast Oregon while a trough amplifies over the eastern Pacific. Currently around 60% of ensembles favor a warmer/drier solution with highs above normal (mid 80s) by Wednesday/Thursday while the remaining 40% point to a somewhat cooler solution (low to mid 70s). We'll see how models trend during this mid-week period over the next several days as ensemble agreement hopefully increases. -99

AVIATION...Through 18Z Friday

Moist, southwest flow aloft will persist today as a low pressure system moves toward the Oregon coast. This system will maintain widespread rain showers across the area with around a 15-30% chance of thunderstorms through this afternoon, highest chances along the coast. Predominately VFR conditions are expected to prevail, except with the heavier showers which are likely to produce brief periods of MVFR to IFR reductions in visibility and CIGs. Rain is expected to gradually taper off from north to south through this evening. Probabilities for widespread MVFR or worse remains relatively low at around 20% or less, except at KONP where probs are closer to around 30-40%. Generally expected south to southeasterly winds with brief gusts possible up to 20 kt. Any thunderstorms that do develop will have the potential to produce lightning and gusty/erratic winds.

KPDX AND APPROACHES...Predominately VFR expected with brief and intermittent MVFR conditions possible with heavier showers. Generally less than a 20% chance for MVFR at any given hour through the period. There is also around a 10-20% chance for thunderstorms through this afternoon. South to southeast winds around 10 kt with gusts up to 20 kt expected to ease after 02z Friday. /DH

MARINE

A low pressure system centered approximately 100 NM west of the north Oregon coast will gradually weaken as it moves toward the southern OR coast through Friday. Breezy southerly winds continue through this evening with gusts up to 30 kt possible this afternoon. Buoy observations this afternoon show seas at around 10 ft at 11 seconds. Expect Small Craft Advisory conditions to persist across the coastal waters through 11 PM this evening. Winds over the coastal waters are then expected to ease turning more easterly overnight into Friday. Also, a 15-30% chance for thunderstorms exists across the waters, which could produce lightning and gusty/erratic winds.

Light offshore flow becomes northerly later Saturday and Sunday as weak high pressure rebuilds over the waters. Seas are expected to fall below 10 ft by early Friday morning, and continue to subside to around 4 to 5 ft by Saturday. Conditions look to remain rather benign into early next week. /DH

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory until 11 PM PDT this evening for PZZ210- 251>253-271>273.