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 this 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 Friday, aside from isolated light showers mainly in the mountains.

DISCUSSION...Saturday afternoon through Friday night

Radar imagery early Saturday afternoon shows showers widespread showers have formed across NW OR and SW WA as a closed low pushes south into the region. PWAT values on SPC's mesoscale analysis page are reaching 0.7-0.8 inches, and model soundings still indicate the depth of the warm cloud layer is around 3500 meters. This means showers this morning and early afternoon, even ones that seem small and benign on radar, are producing very decent rain rates. Some showers are becoming stronger as they encounter the western slopes of the Cascades and Coast Range due to orographic lifting, and these could produce very heavy rain.

The closed upper level low will continue moving directly overhead through the day, which will continue to 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. This means all of the ingredients are in place today for isolated thunderstorms along with the widespread showers. NBM probabilities for thunderstorms peak near 20-40% this afternoon. The main impact with the strongest showers and thunderstorms are heavy downpours. Because the warm cloud layer is so high, it will be very difficult for hail to form. 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. 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 20-40% for both the lowlands and the mountains for Thursday into Friday with temperatures remaining cooler. -23/03

AVIATION

Cool and moist onshore flow will continue to bring widespread cloud cover and periodic rain showers across the airspace through at least 18Z Sun. Showers will be most widespread and heavy until 00Z Sun. From now until 21Z Sat, there is a 25-45% chance of thunderstorms at any given hour then higher confidence for prevailing thunderstorms from 21Z Sat to 00Z Sun for inland TAF sites. Probabilities are a bit lower at the coast at 15-30%. Chances for thunderstorms then lower to 15-20% for all terminals between 00-03Z Sun. Stronger showers and thunderstorms today will bring the potential for heavy downpours with surface visibilities as low as 2 SM for brief periods of time. With convection today, outflow winds are not expected to be particularly strong but may cause abrupt shifts in wind direction as showers and/or storms pass by.

Expect a mixture of MVFR/VFR CIGs at all terminals throughout the TAF period, with MVFR CIGs mainly occurring with passing showers or thunderstorms. After 06-12Z Sun, guidance suggests the possibility (20-40% chance inland, 60-80% chance at the coast) of prevailing MVFR CIGs through at least 18Z Sun.

KPDX AND APPROACHES...Mainly VFR CIGs through at least 18Z Sun. Occasional drops to MVFR CIGs possible with passing showers or thunderstorms. Thunderstorm probabilities peak near 45% between 18Z Saturday and 00Z Sunday, with the highest confidence for prevailing thunderstorms between 21Z Sat and 00Z Sun. Chances for thunderstorms lower to 15-20% between 00-03Z Sun, then less than 15% thereafter. Stronger showers and storms will bring heavy downpours with surface visibilities briefly falling as low as 2 SM. Outflow winds are not expected to be strong with passing showers and storms, however abrupt shifts in the wind direction are likely. ~12

MARINE

Winds and seas increase today through 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 to 9 ft at 9 seconds, highest over the outer waters beyond 10 NM offshore. Monday, weak high pressure builds over the waters, however northwest winds will continue to build the northwest swell into wave heights around 8 to 11 ft. There is a 20 to 40 percent 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.

Given steep seas and wind gusts up to 20 to 25 kt over the outer waters, a Small Craft Advisory remains in effect for the outer waters through Monday evening. Additionally, a Small Craft Advisory over the inner waters remains in effect until late Sunday for steep seas. ~12/23

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory until 5 PM PDT Monday for PZZ251.

Small Craft Advisory until 11 PM PDT Sunday for PZZ252-253.

Small Craft Advisory until 5 PM PDT Monday for PZZ271>273.