textproduct: Portland

SYNOPSIS

Seasonably cool and dry weather continues though at least Wednesday night, before a frontal system moves onshore Thursday into Friday, bringing widespread chances for rain. Dry weather returns Saturday, with increasing uncertainty in the forecast Sunday into next week.

LONG TERM...Thursday through Tuesday

A fall-like frontal system will affect the region Thursday into Friday as an upper shortwave rounding the base of the trough acts to amplify the pattern and supports the deepening of a surface low over the Northeast Pacific. An initial period of steady rain will reach the coast most likely early Thursday afternoon, before pushing inland to the I-5 corridor by the mid-afternoon. Steady rain will give way to a hit-or-miss distribution of rain showers late Thursday evening into Friday morning before conditions tend drier through Friday afternoon and evening. Temperatures will fall to some 10 degrees below normal behind the frontal passage on Friday, when locals on the coast may struggle to reach 60 degrees and inland areas the mid 60s in the afternoon.

Average forecast rainfall amounts from 5 PM Thursday through 5 PM Friday are generally 1-1.25 inches along the coast, in the Coast Range and Willapa Hills, and in the Cascades, with 0.75-1 inches in the Willamette Valley and 0.5-0.75 inches east of the Cascade crest in Hood River County. These ranges do not capture the full uncertainty in potential rainfall amounts, however, as there is a 10% chance of less than 0.3-0.4 inches as well as a 10% chance of 1.5-2.5 inches across portions of the region. Some of this variance may be due to uncertainty in the position of the surface low as it weakens and moves onshore; the majority of mid-range ensembles take a track into the central Oregon coast, but a minority instead show a track toward the Columbia River, which could allow for a longer duration of moisture transport and convergence overhead. Ensemble means support an IVT values near the weak/moderate atmospheric river threshold, however given the progressive overall nature of the system and the largely dry antecedent conditions, this early-season AR is unlikely to yield any flooding concerns. The uncertainty in low locations also introduces the potential for locally strong winds along the coast if the low center tracks nearby, specifically 15-25% chances for winds reaching 40 mph at some point on Friday behind the frontal boundary. These winds would not typically result in impacts to coastal communities, but the early-season timing will see more leaves on trees, which could allow some trees or branches to come down where the winds are the strongest.

Cool air aloft behind the frontal boundary may also support shallow convection on Friday, although the occurrence and coverage of any thunderstorms may hinge on how quickly skies clear Friday, with earlier or more widespread sun breaks allowing for more instability to develop by peak heating in the afternoon. Mean surface-based CAPE of around 100 J/kg is expected at this point, but there is a 10% chance of CAPE as high as 200-250 J/kg which could support more widespread thunder chances. Given this uncertainty, thunderstorm chances are currently assessed to be 15-25% across the region, peaking in the afternoon on Friday, with storms capable of producing locally heavy rainfall rates and gusty, erratic outflow winds.

Behind the departing system, there is good agreement of drier and marginally warmer weather returning on Saturday as brief shortwave ridging passes overhead. There continue to be low chances for morning low temperatures of 36 degrees or cooler in the Upper Hood River Valley on Saturday and Sunday mornings, specifically around 5% near Odell and 25-35% near Parkdale. Additional troughing which may move onshore Sunday into early next week will maintain persistent low chances, 5-25%, for light rain across the region. -36

AVIATION...Through 00Z Thursday

West to southwest flow aloft will persist through Wednesday. Predominately VFR conditions expected through this evening. Marine stratus likely returns to the coast with chances for MVFR to IFR CIGs increasing over 50% by 8-10z Wed. Probs for MVFR stratus inland late tonight into tomorrow morning have decreased slightly, but will maintain a few hours of MVFR CIGs at inland terminals between 12-18z Wed. A decaying front will approach the coast early Wed morning, which could bring some light rain or drizzle to KAST between 12-18z Wed. Otherwise, expect increase mid to high level clouds streaming across the airspace tonight into Wednesday. Diurnally driven winds generally remain northwest around 5-10 kt or less.

KPDX AND APPROACHES...VFR conditions expected to persist through at least 12z Wed with scattered clouds. There is around a 30% chance for MVFR stratus to return, mainly between 13-17z Wed. Otherwise, expect VFR to persist with increasing mid to high level clouds. Northwest winds expected around 3-5 kt through the TAF period. /DH

MARINE

High pressure will maintain predominately northerly winds across the waters through Thursday afternoon. An exception would be Tuesday afternoon to Wednesday morning, when winds may turn more westerly to southwesterly for the waters north of Cape Falcon with the passage of a weak front. A relatively stronger frontal system will then move over the coastal waters late Thursday into Friday, bringing increasing northwesterly winds and rain. Chances for small craft advisory level wind gusts of 21 kt or stronger are around 60-80% with this system with the highest probabilities over the northern waters to the north of the Columbia River Bar. There is also a 15-25% chance for max wind gusts of 34 kt or stronger, as some guidance suggests a stronger area of low pressure is possible over the coastal waters with this system. However, the most likely outcome is for a relatively weaker low pressure system with peak gusts somewhere in the 20-25 kt range.

Seas will most likely range between 6 and 9 ft through Friday, but are not expected to be particularly steep at any point in time due to dominant wave periods of 10 to 13 seconds. However, there is a 15-20% chance seas will peak at 10 ft or higher late Thursday night into Friday if the aforementioned area of low pressure comes in stronger than what most guidance suggests. Seas decrease to around 5 to 6 ft at 9 to 10 seconds for the weekend. -03/23

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...None.