textproduct: Portland

SYNOPSIS

Post-frontal rain and mountain snow showers continue but will decrease coverage through Thursday night. Any heavier shower may produce small hail, while there is also a 15-30% chance of isolated thunderstorms this afternoon and evening. Accumulating snow continues above 2500 feet and Winter Storm Warnings remain in effect in the Cascades through tonight. The cooler airmass will also favor frost/freeze conditions tonight and possibly Thursday night.

LONG TERM...Saturday through Wednesday

An upper-level low or open trough tracking from the Gulf of Alaska over the Northeast Pacific will be the driver of sensible weather this weekend and into early next week, and the forecast remains sensitive to its low-confidence evolution. An upper low located nearby will favor cloudy skies and rainfall chances, while a position well to the west would see continued warm and dry weather. If it is located off the northern CA or southern OR coast, the pattern could allow sufficient moisture to move northward on its eastern flank to support convection along the Cascades. -36

AVIATION

Post frontal showers are expected to persist through the day with increasing chances for convection in the afternoon through at leas 01Z Thu. Highest probabilities lie along the Cascade foothills and to the north with around a 25-35% chance inland within the Willamette Valley. Have already observed thunder within the vicinity of some Willamette Valley terminals, but has yet to be widespread. Thunder is challenging here as often it may be one storm that pops up, while other times it can be a wide swath. In this case, because the flow is northwesterly (not as favorable) behind the previous system, would trend towards a more isolated thunderstorm scenario. Overnight, showers will slowly dissipate and clear. Dropping temperatures mean that there is a slight chance for frost formation over exposed surfaces. If skies remain cloudy though, those chances decrease significantly. Another limiting factor will be that winds are just slightly above what we would like to see for more widespread frost formation.

KPDX AND APPROACHES...Combination of MVFR and VFR conditions with showers. Around a 30% chance of thunderstorms through 06Z Thu. Thunderstorms will trend towards an isolated scenario. With passing storms, expect gusty outflow winds leading to LLWS type conditions. Note that atmospheric LLWS is not expected, but rather environment induced conditions. -27

MARINE

Post frontal showers will increase probability for lightning over the waters through around 1700 today. Small Craft Seas persist due to a westerly swell combined with a westerly wind wave. These small craft seas are hovering right around 9-10 ft at 8 seconds. Have extended the advisory through Thursday morning, though will see a steady decline in seas shifting from west to east. In addition, there will be a strong ebb early Thursday morning for the Columbia River Bar. The inner waters will see lowering seas as early as early Thursday morning. Winds will remain westerly through Friday morning, then shift to the north as a high pressure ridge sets up over the waters. This summer time pattern will bring increased winds to PZZ253 in the afternoon. While not quite a low- level jet, we could see a short burst of gusts up to 25 kt.

High pressure over the area will maintain more settled conditions through early next week. -27

PQR WATCHES/WARNINGS/ADVISORIES

OR...Frost Advisory from 1 AM to 9 AM PDT Thursday for ORZ104>110- 114>118-123>125.

Freeze Warning from 1 AM to 9 AM PDT Thursday for ORZ121.

Freeze Watch from late Thursday night through Friday morning for ORZ121-124-125.

Winter Storm Warning until 5 AM PDT Thursday for ORZ126>128.

WA...Frost Advisory from 1 AM to 9 AM PDT Thursday for WAZ203>205- 208.

Winter Storm Warning until 5 AM PDT Thursday for WAZ211.

PZ...Small Craft Advisory until 5 AM PDT Thursday for PZZ210.

Small Craft Advisory until 5 PM PDT Thursday for PZZ251>253- 271>273.