textproduct: Portland

SYNOPSIS

A transition to a cooler, more progressive pattern is underway this afternoon as flow aloft gradually turns west to northwest. Scattered showers continue at times through early next week. Confidence remains high in a stronger frontal system arriving late Tuesday into Wednesday, bringing widespread precipitation and a shift toward Cascade snow impacts. A Winter Storm Watch has been issued for the Cascades from late Tuesday into early Thursday as a result of increasing confidence. Cooler overnight temperatures follow, with increasing frost concerns by mid to late week.

LONG TERM...Tuesday night through Saturday

Ensemble and deterministic guidance remain in strong agreement that a cold front will sweep through the region Tuesday night into Wednesday morning (arriving any time between 11 PM Tue and 11 AM Wed), bringing widespread precipitation. Confidence has increased in moderate rainfall amounts, with 24-hour totals generally ranging from 0.7 to 1.2 inches across the Willamette Valley and Portland metro (10-30% chance to exceed 1.2 inches), 0.9 to 1.7 inches along the coast and Coast Range (5-15% chance to exceed 1.7 inches), and 0.9 to 2.0 inches across the Cascades (10-20% chance to exceed 2.0 inches).

Cooling temperatures along and behind the front will lower snow levels to near or below Cascade pass elevations, increasing the likelihood of winter travel impacts. Current probabilistic guidance indicates a 60-90% chance of 12+ inches of snowfall at the Cascade passes between Tuesday afternoon and Thursday afternoon (48 hours). As a result, a Winter Storm Watch has been issued for heavy snow possible above 3500 ft in the South Washington Cascades, North Oregon Cascades, Cascades of Marion and Linn Counties, and Cascades of Lane County from 5 PM Tuesday through 5 AM Thursday. Total snow accumulations between 9 to 18 inches possible, with the highest amounts in the high Cascades from Marion to Lane County. Roads, and especially bridges and overpasses, will likely become slick and hazardous, and winds may gust as high as 45 mph.

In addition to Cascade snowfall, cooler and potentially clearer conditions behind the system will bring renewed frost concerns to the interior valleys. The first opportunity appears Wednesday night into Thursday morning, with a 60-80% chance of frost across much of the interior lowlands, except for a 25-45% in the metro and at the coast. Additionally, there is a 60-90% chance of freezing temperatures in the Upper Hood River Valley. Similar conditions may develop again Thursday night into Friday morning, though with slightly lower probabilities (10-20% lower). While frost potential starts for most areas Thursday morning, the Upper Hood River Valley will have frost potential sooner and longer, with a 25-65% chance of frost Wednesday morning, 85-95% Thursday and Friday morning, and 60-80% Saturday morning. Chances of frost lower to 10-20% by Sunday morning for the Upper Hood River Valley. Temperatures gradually warm into the weekend, reducing frost concerns. ~12

AVIATION

Light showers continue to taper off as an upper level low shifts eastward. Conditions have trended toward VFR as CIGs increase above 3000 feet at most terminals but aren't expected to persist through the night as a shortwave drops southeast over the PacNW. Shower activity is expected to increase between 8-12z Monday as this disturbance passes overhead. Scattered showers persist through most of Monday but will be light. This disturbance will also increase probabilities for sub- VFR conditions for all terminals. Inland terminals have a 40-70% chance for MVFR CIGs until 12z and coastal terminals have a batter chances at 60-80% through 20z. Winds remain west to northwest through the period.

KPDX AND APPROACHES...VFR conditions expected to drop back to MVFR as a disturbance passes overhead. Shower activity also expected to increase but will be scattered enough to not be in a prevailing group at this time. Light westerly winds expected through the period. -19

MARINE

North to northwesterly winds are expected to continue into Monday, albeit not particularly strong with gusts staying below 20 knots. Significant wave heights hold in the in the 4 to 7 foot range through this time period as well. Focus then turns to the next weather system that will caused increased winds and seas Tuesday and Wednesday. There is high probability (85-95% chance) in Small Craft Advisory wind gusts greater than 21 kts with a 10-25% chance of a brief period of Gale Force gusts over 34 kts as a robust cold front passes through the waters Tuesday afternoon and evening. An increasing westerly swell along with the winds will cause seas to build to around 8-11 feet at 9-10 seconds. Winds begin decreasing Wednesday while the seas remain elevated, decreasing on Thursday. -03

PQR WATCHES/WARNINGS/ADVISORIES

OR...Winter Storm Watch from Tuesday afternoon through late Wednesday night for ORZ126>128.

WA...Winter Storm Watch from Tuesday afternoon through late Wednesday night for WAZ211.

PZ...None.