Gresham, OR 97080 Weather

Updated Aviation discussion

Synopsis

Seasonably cool and showery conditions continue through the workweek as repeated weather systems support persistent chances for rain. A drying trend beginning Friday may bring better trick-or-treating conditions by late Saturday. Could see benign conditions into early next week before the pattern becomes more unclear by the middle of next week.

Short term

Now through Wednesday Night: The wet and cool pattern is expected to continue through the work week as several weather systems impact the region. Stratiform precipitation earlier today has transitioned to showers with the passage of a cold front. Rainfall will vary across the forecast area due to the showery nature of the precipitation, wettest spots could see 0.10-0.20 inches through the evening. Shower activity is expected to decrease this evening before the next system moves in tonight. Ensemble models are in good agreement that an upper level low over western Canada will slowly drop south toward the PacNW. Models have this low centered just offshore of WA and OR but several embedded shortwaves are expected to pass overhead. Similar to today, expecting the bulk of the rain late tonight into Wednesday morning before transitioning to showers Wednesday afternoon. Rainfall totals from 5 AM Wednesday to 5 AM Thursday are expected to be between 0.10- 0.60 inches, highest along the coast and in the higher terrain and lowest south of HWY 20. There is a 20-50% chance that areas in the SW WA and N OR Cascades could receive and inch or more of rain during this time frame.

850 mb temperatures are expected to drop through Wednesday as the upper level low brings cooler air with it. This will drop snow levels to between 4000-4500 feet later this evening and into Wednesday morning. This could bring snow to some of the mountain passes which could create dangerous travel conditions through the Cascades. For travel around Mount Hood, including Government Camp, chances for 1 inch or more of snow from early Wednesday through Thursday morning are 40-50%. During the same period, chances are highest for Santiam Pass at over 90%. Snow levels wednesday night into Thursday morning drop closer to 2500-3000 feet over the Cascades. However, this is expected to be a relatively dry period so impactful snow over the passes is not expected. -19

Long term

Thursday through Tuesday: Generally wet weather is expected to continue Thursday and Friday as the upper level low pinches off from the main flow and churns just off the coast of WA and OR. QPF during this time period is quite low so looking at a high PoP, low QPF set up which means we are likely to see rain but it will be light. 48 hour totals across the area from Thursday morning through Saturday morning likely 0.10- 0.30 inch range. Locations along the coast and higher terrain could push 0.5 inches with a 20-40% chance. Ensembles and cluster analysis point towards a drier pattern by the weekend and into early next week. Models show the pattern becoming more progressive with the closed low becoming an open wave and moving east Saturday into Sunday with a shortwave ride building into the area by late Sunday. Exact timing and details of this progression still vary among the models. -19

Aviation

Through 06Z Thursday: Northwest flow aloft becomes westerly on Wednesday while scattered to numerous showers continue across the area. After a brief lull in shower activity this evening, conditions will likely deteriorate from VFR to MVFR at times as another round of rain showers moves across the area tonight. There is around a 30-40% chance for MVFR CIGs at any given hour between 08-18z Wed. Conditions then expected to trend toward predominately VFR after 19z Wed as showers decrease again.

Winds are expected to increase along the coast and inland this evening out of the S-SW with gusts up to 15-25 kt into Wednesday morning before winds gain more of a westerly component and moderate Wednesday afternoon.

KPDX AND APPROACHES: VFR as of 05z Wed, but chances for MVFR increase again after 07z to around 30-40% as another round of rain showers move across the area. VFR likely again by 19z Wed. Southwest winds expected to peak between 07-12z with gusts up to 20 kt. /DH

Marine

A low pressure system will keep conditions active across the waters through the middle of the week. Currently an upper-level low is dropping out of the Gulf of Alaska with winds and seas increasing this evening out of the west to southwest. Confidence is moderate to high for gusts to exceed 21 knots across the inner and outer waters, especially north of Cape Foulweather, thus a Small Craft Advisory remain in effect including the Columbia River Bar as well. Seas likely peak around 7-10 ft at 8-10 seconds this evening through Wednesday evening. Then, conditions slowly subside Thursday into Friday with gusts below 15-20kt and seas subsiding towards 4-6 ft at 10-12 seconds through the weekend. Looking towards the start of next week, another low pressure system from the Gulf of Alaska looks to bring building seas and winds that could result in Small Craft conditions for the start of next week yet again. -99/42

Watches, warnings and advisories

OR: None. WA: None. PZ: Small Craft Advisory until 11 AM PDT Wednesday for PZZ210-251- 252.

Small Craft Advisory until 8 AM PDT Wednesday for PZZ253-273.

Small Craft Advisory until 5 PM PDT Wednesday for PZZ271-272.