textproduct: Portland

SYNOPSIS

A significant weather pattern change begins today as a frontal system swings through, bringing cooler temperatures and widespread chances for rain tonight through Saturday. There is also a 10-15% chance for thunderstorms Friday afternoon and a 15-25% chance for thunderstorms Saturday afternoon. Conditions trend drier and slightly warmer Sunday through early next week, but we'll maintain slight chances for light rain showers across the Coast Range and Cascades.

DISCUSSION...Today through Wednesday

Today marks the beginning of a large-scale pattern change to cooler and wetter conditions as an upper-level low pressure system remains on track to progress into the Pacific Northwest from the Gulf of Alaska. Satellite imagery as of early Thursday morning depicts widespread marine stratus along the coast with increasing high level clouds across the broader region ahead of this next weather system.

Temperatures will cool to near normal with mid 70s across the interior lowlands. High resolution model guidance has rainfall via a cold-frontal boundary reaching the coast by late this afternoon/evening and then spreading into the I-5 corridor by late tonight into Friday morning. We'll remain in this cooler and wet pattern through at least Saturday, but at least total rain amounts at this time appear beneficial and generally non- impactful. Looking at the latest NBM, chances for 48-hour rain amounts exceeding 1 inch from 5 PM Thursday to 5 PM Saturday has fallen slightly since yesterday for locations west of the Cascades: 30-60% for the coast and Coast Range, 5-15% along and east of the I-5 corridor. Meanwhile, chances remain around 70-80% or higher across the Cascades due to westerly flow bringing orographic enhancement. Will note that the western-most parts of the Willamette Valley (west of I-5 including McMinnville and Corvallis) have a less than 5% chance of exceeding 1 inch during this timeframe due to potential rainshadowing from the Coast Range. The decrease in these chances for 1 inch of rain may be due to the fact that we're in such a dry air mass. This dry air mass may sacrifice some of the initial rain before the atmosphere becomes completely saturated enough for rain to hit the ground.

By Friday afternoon, the upper-level trough continues to move south and eventually centers itself right over-top the Pacific Northwest as a closed low on Saturday. Temperatures will further cool aloft allowing for increasing atmospheric instability during the daytime hours and facilitating CAPE values of 50-100 J/kg Friday afternoon for SW Washington and the Portland/Vancouver Metro, and 100-200 J/kg late Saturday morning into the afternoon across the entire area. This instability plus sufficient lift from the low will support a 10-15% chance for heavy showers and thunderstorms across NW Oregon and SW Washington Friday afternoon, and a 15-25% chance for thunderstorms Saturday afternoon. For now, any convection appears rather weak and unorganized (our typical post-frontal pop-up storms). High resolution guidance suggests effective shear remains below 35 kt; therefore, chances these thunderstorms become severe are unlikely (<5%). Otherwise, any strong shower or passing thunderstorms may produce brief heavy rain, gusty and erratic winds, and a few lightning strikes. Also can't rule out a cold air funnel cloud or two given the time of year, especially on Saturday.

The latter half of the weekend into early next week, the majority of ensemble members are showing the broader upper- level low feature shifting eastward and lingering along the interior mountain west, returning relatively drier conditions and a warm-up of a few degrees to our area. This will put high temperatures just below the seasonal normals with low 70s for the interior lowlands and upper 50s to low 60s for the coast). Will still maintain precipitation chances across the Coast Range and Cascades as westerly flow continues to support orographic enhancement of precipitation, though any additional rain amounts appear very light. Still, we'll have to keep an eye on subtle shortwave features rotating into the region north to south on the back side of the upper-level low which may expand the coverage of showers temporarily - low confidence on the placement/timing of these smaller shortwave features. Ensemble members then suggest another upper-level trough dropping down into the area on Wednesday, however, there is also very high uncertainty with how much precipitation we get, if any. Westerly winds throughout the week will be breeziest along the central Columbia River Gorge and across Upper Hood River Valley, with chances for isolated wind gusts exceeding 45 mph around 10-35% from Friday through early next week, with the highest chances from Sunday to Tuesday. -10/03/99

AVIATION

Satellite imagery as of early Thursday morning depicts IFR marine stratus along the coast and increasing high clouds across the region ahead of the next frontal system. Expect IFR conditions along the coast through at least 17-18z Thu, before improving to MVFR between 18z Thu-02z Fri along the north Oregon coast (KAST). Meanwhile, IFR stratus likely (50-70%) hold throughout the day at KONP. Will note that there is a slight chance (15-25%) that KAST doesn't improve to MVFR and remains IFR. For inland terminals, predominately VFR conditions throughout the TAF period with CIGs gradually falling. After 00z Fri, rain starts at the coast and gradually spreads inland throughout the evening. At the same time, CIGs will drop to low-end VFR for inland terminals with a 15-30% chance for MVFR CIGs at any given hour through 12z Fri. West- northwesterly winds around 5 kt or less through 18z Fri, turning more southwesterly this afternoon and increasing near 10 kt with gusts up to 20 kt at KONP.

KPDX AND APPROACHES...VFR conditions through the majority of the TAF period. Increasing high clouds around 25 kft and gradually falling throughout the day. Low-end VFR CIGs this evening as the frontal system moves in and brings rain. 10-30% chance for MVFR CIGs at any given hour between 05-12z Fri. Northwesterly winds under 5 kt through 18-19z Thu, turning west-southwesterly 6-10 kt in the afternoon/evening. -10

MARINE

High pressure will maintain northwest winds across the coastal waters through early this morning. Winds are generally expected to remain under 20 kt. Weakening high pressure will give way to a frontal system approaching the area on today. Winds are expected to back west to southwesterly Thursday afternoon before the cold front pushes across the coastal waters Thursday night into Friday. As the front moves in, pressure gradients will tighten and result in brief small craft conditions with southerly wind gusts up to 25 kt across the inner coastal waters north of Cape Foulweather and up to 10 NM offshore. Therefore, a Small Craft Advisory has been issued for these waters including the Columbia River Bar between from 11 PM Thursday to 5 AM Friday.

West to northwest winds will persist behind the front later Friday and into Saturday as weak low pressure lingers near Vancouver Island. High pressure gradually builds across the waters late weekend. Seas around 4 to 5 ft through Friday build to around 7 to 9 ft this weekend as a west-northwesterly swell moves through the waters. -10/DH

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory from 11 PM this evening to 5 AM PDT Friday for PZZ210-251-252.