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

As of 04Z Sun, most of the rain showers have shifted east of the inland terminals. Cool and moist onshore flow will continue to support cloud cover and periodic rain showers through most of the TAF period. Rain showers will likely taper off completely by 00-06Z Mon. Expect coastal terminals to maintain MVFR CIGs through the TAF period, with the exception of brief VFR CIGs before 12Z Sun and after 00Z Mon for KONP. Inland terminals will see mainly VFR conditions through the TAF period, however there is a 30-60% of MVFR CIGs from now until 19Z Sun. While most inland terminals are VFR as of 04Z Sun, expect a trend towards widespread MVFR by 12Z Sun. Westerly to northwesterly winds will continue through at least 06Z Monday, generally ranging from 5-10 kt at inland terminals and 8-12 kt at coast terminals. Expect the strongest winds during the afternoon, with gusts up to 18 kt possible at coastal terminals and a few inland terminals.

KPDX AND APPROACHES...Expect VFR conditions through most of the TAF period, with a 30-60% chance of MVFR CIGs from now until 19Z Sun. Guidance currently suggests MVFR CIGs between 12-18Z Sun. Westerly winds will shift more northwesterly around 00Z Mon, generally staying between 5-10 kt through the period. Gusts up to 18 kt possible during the afternoon. ~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 PZZ251-271>273.

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