textproduct: Portland

SYNOPSIS

The next seven days are highlighted by below average temperatures for this time of year and a good deal of cloud cover. The heaviest rain is expected Saturday afternoon when widespread rain showers and isolated thunderstorms are expected from the coast to the Cascades. The strongest showers and storms will produce heavy downpours. Mainly dry and cool Sunday through Wednesday, aside from isolated light showers mainly in the mountains.

DISCUSSION...Friday afternoon through Thursday night

Cool and moist onshore flow remains in place today, with radar observations from early Saturday morning depicting scattered rain showers across northwest Oregon and southwest Washington. A band of heavy showers moved across the greater Portland/Vancouver metro area between 1:00-3:15 AM PDT Saturday, with some locations observing heavy downpours. In fact, heavy downpours hit the NWS Portland office between 2:00-2:45 AM PDT and produced 0.33 inches of rain within 45 minutes. Since radar observations showed even heavier cells nearby the office, some locations inevitably picked up even more than that. Given the showery hit-or-miss nature of this precipitation, observed rain amounts vary significantly from location to location with some locations picking up only a trace to few hundredths of an inch thus far this morning.

These early morning downpours are a taste of what is to come later today, particularly this afternoon once daytime heating begins. This is when models and their ensembles continue to show the center of a closed upper level low moving directly overhead, which will bring increasing moisture, lift, and instability. REFS ensemble mean soundings continue to suggest surface-based CAPE values will increase towards 250-400 J/kg for inland areas this afternoon, while the NBM mean suggests the same. PWAT values are forecast to reach 0.7-0.8 inches with the depth of the warm cloud layer reaching an impressive 3500 meters. This means all of the ingredients are in place today for widespread showers and isolated thunderstorms with heavy downpours expected with the strongest showers and storms. NBM probabilities for thunderstorms peak near 20-30% for inland areas and 15-20% at the coast this afternoon. Given weak cloud-layer winds/slow storm motion, heavy downpours will have the potential to last long enough over any given location to produce upwards of 0.25-0.50 inches of rain in one hour or less. Rain rates of this magnitude are high enough to result in ponding of water in urban areas and increase the risk of short-lived minor flooding in areas with poor drainage. Heavy downpours will also pose a safety hazard for motorists due to an increased risk of hydroplaning, as well as reduced visibilities. In fact, heavier showers that moved directly over the Portland International Airport around 2:15 AM PDT Saturday lowered visibilities down to 1.75 miles, which is a good proxy to work with. It is also worth mentioning that isolated non-mesocyclonic cold air funnel clouds are occasionally observed in this type of weather pattern, however the rotation with these typically fails to lower to the ground and thus damage does not occur. If rotation with one of these were to lower to the ground, it would be considered a weak landspout, and damage tends to be extremely isolated if any occurs at all.

Rain amounts trend much lighter on Sunday as the upper level low shifts eastward into Idaho and showers become lighter and more isolated. Most of the showery activity on Sunday will be confined to the Cascades, Cascade foothills, and Coast Range. Sunday will also be another cool day with a good deal of cloud cover and highs in the 60s across the lowlands. General longwave troughing over the western US continues Monday through Wednesday, keeping temperatures cooler than average. However, conditions are trending mainly dry Monday through Wednesday, aside from isolated light showers in the mountains from time to time. Rain chances increase to 30-40% for both the lowlands and the mountains on Thursday with temperatures remaining cool. -23

AVIATION

Cool and moist onshore flow will persist across the airspace through the TAF period, bringing widespread cloud cover and periodic rain showers from the coast to the Cascades. Showers will be most widespread and heaviest between 18Z Saturday and 00Z Sunday, which is also when there is a 20-30% chance of thunderstorms at any given hour for inland TAF sites. Probabilities are relatively lower at the coast at 15-20%. Stronger showers and thunderstorms today will bring the potential for heavy downpours with surface visibilities as low as 1 1/2 to 2 SM for brief periods of time. Outflow winds associated with convection are not expected to be particularly strong, however abrupt shifts in the wind direction are likely near passing showers and storms.

Expect a mixture of MVFR to VFR ceilings throughout the TAF period both inland and at the coast, with MVFR ceilings mainly occurring with passing showers or thunderstorms.

KPDX AND APPROACHES...Expect mainly VFR ceilings throughout the TAF period, aside from occasional drops to MVFR ceilings with passing showers or thunderstorms. Thunderstorm probabilities peak near 30% between 18Z Saturday and 00Z Sunday. Stronger showers and storms will bring heavy downpours with surface visibilities briefly falling as low as 1 1/2 to 2 SM. Outflow winds are not expected to be strong with passing showers and storms, however abrupt shifts in the wind direction are likely. -23

MARINE

Winds and seas will increase Saturday into Sunday as a pair of fronts move southward over the coastal waters and persistent northwesterly winds allow for the development of a fresh northwest swell upwards of 7-9 ft at 9 seconds, highest over the outer waters beyond 10 NM offshore. Given seas will become steeper and winds will gusts up to 20-25 kt over the outer waters, a Small Craft Advisory remains in effect for the outer waters from Saturday morning through Sunday night. Although winds are a bit weaker over the inner waters, seas will still become steep enough to pose a hazard for small craft Saturday afternoon into Sunday. Therefore, the Small Craft Advisory has been expanded to include the inner waters.

Although weak high pressure builds over the waters on Monday, northwest winds will persist and will maintain a fresh northwest swell with significant wave heights peaking near at least 8-11 ft. There is a 20-40% chance significant wave heights will peak over 11 ft beyond 30 NM offshore on Monday, mainly to the north of Cape Falcon. Northwest winds and a fresh northwest swell then continues Tuesday through Wednesday, maintaining steep seas and hazardous conditions for small craft. -23

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory from 2 PM this afternoon to 11 PM PDT Sunday for PZZ251>253.

Small Craft Advisory from 5 AM early this morning to 11 PM PDT Sunday for PZZ271>273.