Marine Weather and Tide Forecast for Cascade Locks, OR

SHORT TERM
Now through Wednesday night...Current radar and satellite observations this afternoon show widespread cloud cover with rain moving east and south across our CWA The source of the rain and clouds is a low pressure system, that is slowly moving southeastward from the Gulf of Alaska and towards the Pac NW. As this system continues to trek towards the region, precipitation as well as cooler conditions will slowly start to take hold. This will result in widespread rain for the lower elevations and the coast, along with wet snow for the Cascades.
QPF values have changed very little over the past 24 hours and generally show a 36 hour QPF totals through Wednesday night of 0.75 to 1.25 inches across the Willamette Valley and Portland metro, 1.00 to 2.75 inches along the coast and Coast Range, and 1.20 to 3.00 inches across the Cascades. However, for the latter region, especially at pass level, the precipitation will likely be falling as snow for a good chunk of this event.

So, lets talk about snow. The incoming system looks to push snow levels down towards 2000-2500 ft by very early Thursday morning and given that a robust slug of moisture will accompany the cold air, that means that snow is in the forecast for the Cascades and parts of the Cascade Foothills. Focusing on the Cascades above 3500 ft, roughly 12 to 22 inches of snow accumulation is expected. This looks to start around 5PM this afternoon through early Thursday morning. Highest snowfall totals are expected mainly above 3500-4000 ft. As for the Cascade Foothills, the primary area in question is the Clackamas County Cascade Foothills. 1 to 4 inches of snow accumulation is expected, with the highest accumulations above 1500 ft, this includes Mt. Hood Village and Zigzag, OR. 1 inch or less for locations around 1000 ft, which includes Sandy, OR. The time of concern for the Clackamas County Cascade Foothills is from 3AM to 11AM Wednesday.

The latest NBM probabilities show a 75-90% probability of 12 inches or greater snowfall over a 48 hour period from this afternoon through Thursday afternoon at Santiam level and Willamette Pass, slightly lower (35-45%) at Government Camp.
Overall, the highest amounts likely fall in the high Cascades from Marion to Lane County. The period to watch as far the greatest travel impacts are will be from 3AM to 11AM on Wednesday when snowfall rates in the heavier precipitation bands right along the front may exceed 1-1.5 in/hr. So, if you are planning to travel over the Cascades passes, particularly Wednesday morning, please prepare for winter-like travel conditions.

One other interesting facet of the forecast we're watching is the potential for 1-3 hr period of dynamic cooling temporarily punching snow levels into the 500-1500ft range near sunrise Wednesday morning. This has been a feature hinted at by the last several runs of the higher resolution guidance like the HRRR and NAMNEST right on the back edge of the cold-frontal boundary where persistent heavier precipitation combined with cold air just above the surface (-5C at 850mb) could facilitate this process, assuming all the proper variables align. The areas of particular interest are southwest Washington into the Portland Metro and the higher coast range/Cascade valleys. If this were to occur elevations as low as 500ft could see a rain/snow mix or "chunky rain" for an hour or two with no impacts. The chances for a light slush-up in the grass gets higher once you reach 1500ft, particularly in the Cascade foothills/valleys, but due to the warm antecedent conditions most impacts remain 2000ft+.

Progressing through Wednesday a showery post frontal environment quickly overtakes the region coupled with increasing instability during the afternoon as the core of the upper level low moves over the region. Most models show Convective Available Potential Energy (CAPE) values peaking around 200 to 500 j/kg, with very little Convective Inhibition (CIN) around -5 to 10 j/kg. These conditions are resulting in a 15-30% chance for thunderstorm development within the post-frontal environment across the CWA The main time period of focus is 11AM to 7PM on Wednesday when heating between shower bands may be maximized.
Convective Allowing Model (CAM) soundings are showing a skinny CAPE profile and along with cooler temperatures aloft, this does support tiny hail/graupel development with any thunderstorms that develop and given that when spring time thunderstorms develop within our CWA, they have a tendency to produce a lot of tiny 0.10-0.25" diameter (pea size) hail. Also, while hail/graupel might blanket surfaces, it will typically not linger, once the cell that produced it moves out of the area.

To summarize, any of these pop-up thunderstorms may produce moderate to heavy rain, infrequent lightning, small hail/graupel, along with gusty and erratic winds. So, when thunder roars, go indoors or when you see a flash (of lightning), dash inside. With the loss of daytime heating thunderstorm chances decrease Wednesday night although showers likely persist. /42-99

LONG TERM
Thursday through Monday...By Thursday, the upper level low and associated front have left the CWA, but will leave the region under cooler conditions with increasing clearing.
Thursday into Friday looks to have overnight lows below normal bottoming out in the mid 30s across the lowlands and spelling a potential for frost. Below freezing temperatures are even forecast for the higher coast range/Cascade valleys, including the Hood River Valley. The latest NBM probabilities for temperatures below 36 degrees F Wednesday night into Thursday morning for the Portland/Vancouver Metro is 10-30% chance. From the Portland/Vancouver Metro areas southward towards Salem and Eugene, OR, expect a 15-75% chance. For locations north of Vancouver, expect a 15-60%. Additionally, there is a 80-95% chance of at least freezing temperatures in the Upper Hood River Valley for Wednesday night into Thursday morning. Similar conditions may develop again Thursday night into Friday morning, though with similar probabilities.

Looking towards the weekend, temperatures gradually warm, reducing these morning frost concerns. Ensembles show a shortwave ridge of high pressure moving quickly overhead on Saturday, but being quickly followed by another low pressure system on Sunday originating from the Gulf of Alaska. Now, beyond Sunday and into the start of next week, WPC Cluster Guidance is favoring this low holding just off the OR/WA coast or moving over the inland Pac NW, which means that cooler and moister conditions are favored for the start of next week. /42-99

AVIATION
Westerly flow aloft ahead of an upper level trough today as a cold front pushes across the area with rainfall increasing later this afternoon through tonight. As of 20z, conditions remain a mix of MVFR and VFR with CIGS around FL025-FL035, along with rain reducing visibility at the coast to around 3-5 SM. Chances for widespread MVFR inland quickly increase to 50-70% later this afternoon/evening, after 00z Wed; while chances for IFR at the coast increase to around 30-40% around the same time. Southerly winds are also expected to remain breezy through this evening, with gusts up to 30 kt likely. The cold front is expected to slide southward across the area overnight, with winds behind the front shifting to the WNW and steady rain quickly decreasing, with the front exiting the area by 18z Wed. Conditions are likely (70-80% chance) to return to VFR by around 15z Wed, followed by increasing showers later Wed morning.

KPDX AND APPROACHES...As of 20z, lower VFR CIGs with light rain.
Conditions are expected to deteriorate as a cold front pushes increasing rain across the area. Probs for MVFR increase to around 60-80% by 00z Wed, and continue through at least 10-12z Wed.
Southerly winds are expected to remain breezy with gusts up to 25-30 kt through at least 08z tonight. Around 10-12z Wed, expect the front to push through with west to northwest winds and a return to VFR conditions likely. /DH

MARINE
Gusty southwest winds continue to increase this afternoon as a strong cold front approaches the coastal waters.
Latest guidance increased winds enough to warrant a Gale Warning for the northern coastal waters, as well as the inner central waters and the Columbia River Bar. Expect southwest wind gusts up to 35 kt there, while elsewhere, widespread gusts to 30 kt is expected, with isolated gusts up to 35 kt possible, especially over the inner waters as the front impinges along the coast. The increased wind waves are expected to build seas to around 8 to 10 ft with a period of around 8-10 seconds. Seas remain steep and choppy through tonight.

Behind the frontal boundary, expect an abrupt shift to the northwest as the front drops south overnight. Breezy northwest winds with gusts up to 20-25 kt continue through Wednesday as an area of low pressure drops along the Canadian coast toward the Puget Sound. This will also bring a 15-25% chance of thunderstorms over the waters through Wed evening. Seas are expected to persist at around around 8 to 10 ft on Wednesday, building slightly to around 10 to 12 ft Wed night as a northwest swell moves through the waters. The Small Craft Advisory remains in place into Thursday morning. High pressure then builds over the waters later this week as northerly winds return and seas gradually subside to around 3 to 6 ft Friday into Saturday. There is potential for another weather system to impact the waters later this weekend, possibly increasing winds and seas. /DH

PQR WATCHES/WARNINGS/ADVISORIES

OR...Winter Weather Advisory from 3 AM to 11 AM PDT Wednesday for ORZ123.

Winter Storm Warning from 5 PM this afternoon to 5 AM PDT Thursday for ORZ126>128.

WA...Winter Storm Warning from 5 PM this afternoon to 5 AM PDT Thursday for WAZ211.

PZ...Gale Warning until 8 PM PDT this evening for PZZ210-251-252-271.

Small Craft Advisory from 8 PM this evening to 5 AM PDT Wednesday for PZZ210.

Small Craft Advisory from 8 PM this evening to 5 AM PDT Thursday for PZZ251-252-271.

Small Craft Advisory until 5 AM PDT Thursday for PZZ253-272-273.