textproduct: Medford

UPDATE

I would be remiss if I did not mention that snow has begun at Diamond Lake and is also about to spread to the Willamette Pass area. This snow could accumulate up to 3 inches during the next 2 or 3 hours, before changing to rain.

PREV DISCUSSION

/Issued 839 PM PST Sat Dec 20 2025/

DISCUSSION...An upper level trough, now centered offshore from southeast Alaska, will be the dominant feature driving our weather at least through Friday. The level of activity, particularly the frequency of precipitation being produced in our area, from now through Friday looks to exceed the impact of that weather. Most of our major passes are around 3500 to 4500 feet MSL, while snow levels look to be 6000 to 7000 feet MSL tonight through Sunday, then spend most of the week around 5000 feet MSL. A Winter Weather Advisory is in effect for Sunday through Sunday night for the southern Oregon Cascades, with the falling snow levels expected to bring several inches of snow in the Willamette Pass and Diamond Lake areas during Sunday night. Given the relatively small change in storm characteristics needed to increase the impactfulness, we will be paying close attention for updates as each of the several frontal systems approaches. For now, the main potential impacts look to arise from heavy rainfall in Siskiyou County. The heaviest rain looks to occur in southern Siskiyou County, from Mt. Shasta to Dunsmuir and eastward to McCloud. It is too early for fine details, but a range of 5 to 12 inches of rain from now through Friday is plausible. Please see the previous discussion below for more information.

AVIATION...21/06 TAFs...A front will move across the area tonight through Sunday. Ceilings are mostly VFR this evening. Rain will spread northeast into the area after 03Z, with gradually increasing coverage of MVFR and mountain obscuration overnight. MVFR and patchy IFR will persist late tonight through Sunday into Sunday evening, with the highest precipitation amounts in Siskiyou County. Meantime, freezing levels around 8200 feet MSL tonight will fall to 7000 feet MSL during Sunday, before dropping further to around 5000 feet MSL during Sunday evening.

MARINE...Updated 800 PM PST Saturday, December 20,2025...Relatively calm conditions with light winds, rain, and areas of fog are expected through Sunday. Winds and seas increase ahead of a front Sunday night into Monday, with very steep seas and isolated gale force gusts expected by Monday afternoon. Conditions gradually improve into Tuesday, then a series of systems will produce periods of gusty winds and steep to very steep seas through the remainder of the week. -BPN

PREV DISCUSSION... /Issued 214 PM PST Sat Dec 20 2025/

SYNOPSIS...

The northern edge of an atmospheric river will produce minor travel impacts this weekend. Our region will likely see accumulating snow over higher terrain through Christmas. The bulk of the snow will fall over northern California next week.

DISCUSSION...

Precipitation has started to enter the area from the south, with cloud coverage increasing quickly across the entire region. Precipitation coverage will spread to the north and rates will pick up as we move through the day, and especially towards the overnight hours in northern California and some of the higher terrain. Integrated water vapor transport(IVT) will rise to between 250 and 400, which is considered a fairly weak atmospheric river(AR) compared to the 750 to 1000 IVTs we can see during these winter storms. In addition, the extreme forecast index for QPF is showing some values around 0.8 in our northern California areas, which suggests decent rain and snow fall, although nothing extreme with rain or snow.

Therefore, we decided to stay away from a flood watch for our forecast area for today into Sunday night. Snow accumulation suggests some decent accumulation above 5000 feet. We're forecasting up to 12 inches around Crater Lake and 5 to 9 inches around Willamette Pass tonight through Sunday night. Therefore, impacts will be somewhat minor and concentrated in the higher Cascades around Diamond Lake and highway 62 going to Crater Lake. Given that holiday travel has begun, have opted to issue a Winter Weather Advisory for areas above 5000 ft in the Cascades north of Mt McLoughlin. Details on this can be found at PDXWSWMFR. Snow levels to the south will be too high to result in any significant winter weather concerns.

A warm front and the leading edge of the next AR will push into northern California and southern Oregon sometime around Sunday night, so look for snow levels to shoot higher in California and stay relatively lower farther to the north. For example, snow levels will reach around 6500 feet around Modoc County, although remain around 4000 feet over most of southern Oregon Monday night

Tuesday will kind of be a transition day as the AR will weaken over our forecast area and mean sea level pressure begins to build just briefly. Showers will likely linger around the forecast area, although accumulation should be pretty limited during the day.

By morning of Christmas Eve, the forecast looks a little more uncertain. Ensembles are showing a deep trough in the Pacific. There is a small subset of ensembles members(~20%) showing a meso low spinning up off the northern California coast and bringing quite the snow storm down to the valley floors to our region. However, a larger set of ensemble members keep us much drier and postpone the snow to later around Christmas and at a much higher elevation. So that will be something to look for in the next set of ensemble runs to see where ensemble members fall in these scenarios. Differing scenarios also point to periods of strong winds along the coast, over higher terrain, and across the East Side, while others keep winds clam to just breezy. Again, until guidance gets a better handle on the situation, the details will be hard to discern.

The one common theme in all this is the ski resorts and high terrain locations will see at least some snow through Christmas break. Shasta Ski Part might be the exception here as northern California will likely see a fair chunk of snow through Christmas. A 50th percentile 72 hour NBM forecast ending Thursday evening shows 2 feet of snow around 5000 feet in northern California. Farther north, the totals drop to around 9 inches in the Siskiyous and high central Cascades over Crater Lake.

Towards next weekend, conditions will likely trend drier as the probability of precipitation is about 10 to 20 percent with 40 percent along the coast.

MFR WATCHES/WARNINGS/ADVISORIES

OR...Winter Weather Advisory from 4 AM Sunday to 4 AM PST Monday for ORZ027-028.

CA...None.

PACIFIC COASTAL WATERS...Small Craft Advisory from 10 PM Sunday to 10 AM PST Monday for PZZ350-370.

Hazardous Seas Warning from 10 AM to 10 PM PST Monday for PZZ350- 356-370-376.

Small Craft Advisory from 10 PM Sunday to 10 PM PST Monday for PZZ356-376.