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FXUS66 KMFR 281754 AFDMFR

Area Forecast Discussion National Weather Service Medford OR 954 AM PST Tue Nov 28 2023

DISCUSSION
The latest visible image shows low clouds and fog in portions of the Umpqua Basin and around Grants Pass. Elsewhere, most areas are clear with the exception of a thin layer of mid level clouds moving from south to north. the main change to the forecast was to adjust the cloud cover and patchy fog to better reflect the latest satellite image and observations.

The weather will remain quiet today through Wednesday with an upper low off the northern California coast moving south towards the Bay area and weakening over the next 24 hours.

A cold front will move towards the coast Thursday morning, then moving onshore during the day Thursday with a cooler air mass to follow Thursday afternoon and night. Please see the previous discussion for details on Thursday's front. -Petrucelli

AVIATION
28/18Z TAFs...LIFR ceilings are still persisting around the Umpqua Valley this morning. Based on the latest satellite imagery around 1730Z, the clouds are not burning off, so these ceilings should last into the early afternoon. We're anticipating more LIFR ceilings and fog later tonight.

We also considered adding fog into the Rogue and Illinois valleys later tonight, however we kept things clear in those areas tonight. Dew points don't change much and they should max out in the upper 20's like the last few nights. If there is a little more moisture sneaking in today, then it's a 50/50 chance of fog forming later tonight around MFR.

For other areas, VFR conditions are expected to continue for most areas through the TAF period under continuing stable atmospheric conditions.

-Smith

MARINE
Updated 800 AM Tuesday November 28, 2023...Seas remain rather low near the buoys closer to shore, however some higher seas are possible in the outer waters.

Otherwise, conditions ease later tonight and remain relatively quiet into early Thursday. Then, active weather returns with multiple fronts expected to pass through the region Thursday through the weekend into early next week. Expect periods of gusty south winds with building swell and wind seas. -DW/Smith

PREV DISCUSSION
/Issued 410 AM PST Tue Nov 28 2023/

SHORT TERM...A strong high pressure ridge is centered over the area today. A low pressure system will move under the ridge and approach northern California tonight. This low is forecast to move south along the California coast Wednesday. There is a slight chance for showers over southwest portions of the area (southern Curry, southern Josephine, and Siskiyou counties) tonight into Wednesday morning as this low slides south. Otherwise, expect continued dry weather and stable conditions under high pressure through Wednesday night.

With the stable air mass, light winds and strong inversions are expected through Wednesday night. This will continue to limit vertical mixing and trap pollutants, especially in inland valleys.
Due to the potential for continued health impacts, an air stagnation advisory remains in effect through Thursday morning for inland valleys. Additionally, Oregon Department of Environmental Quality has issued an air pollution advisory for Jackson and Josephine Counties and Klamath Falls (Klamath County) for air quality conditions. People are encouraged to take precautions including following local burn restrictions to prevent deteriorating air quality, avoiding strenuous activity during periods of poor air quality, using High Efficiency Particulate Air (HEPA) filters, and staying indoors during high smoke or particulate levels (especially for young children and those with health conditions who are vulnerable). For details, please see the AQAMFR and NPWMFR for details.

Models and ensembles remain in agreement that a low pressure system will move out of the Gulf of Alaska and towards the area early Thursday. This will result in the high pressure ridge breaking down over the area and precipitation chances increasing Thursday. There remains some timing differences between models on when the front associated with this low pressure system will bring precipitation into the area, ranging from precipitation spreading inland early Thursday morning to early Thursday afternoon.
Overall, expect increasing chances for precipitation through the day Thursday into Thursday evening. Precipitation will be mainly light with some moderate precipitation along the coast, coastal mountains and into the southern and south-central Oregon Cascades.
Snow levels are expected to be around 3500 to 4000 feet Thursday with snow accumulations likely in the southern and south- central Oregon Cascades. For Thursday through Thursday night, the National Blend of Models indicates a 50-60% chance for 6 inches or snow or more in the Cascades above 4000 ft elevation north of Highway 140. Lighter snow of around 1 to 4 inches is expected for the Cascades south of Highway 140, in the Siskiyous and for the mountains in Siskiyou County. East of the Cascades, very light snow is expected (generally under an inch), except for across northern Klamath including Highway 97 north of Chiloquin to Chemult and in the Warner Mountains in Modoc County where there is a 40-50% chance for around 2 inches or more of snow late Thursday and Thursday night. Behind this system, another low pressure system will move towards British Columbia and bring a moist front into the area Friday into Saturday. This may bring more precipitation impacts to the area. Heavy mountain snow will be the main concern. Snow levels are forecast in the 3500 to 4000 ft range during the heaviest precipitation late Friday into Saturday. The National Blend of Models indicates a 50-90% chance of 12 inches or more of snow within 24 hours (ending Saturday afternoon) in the Cascades above 4000 ft elevation from Highway 140 north and a 50% chance for 24 inches of snow at Crater Lake. We will continue to monitor this situation and the potential for heavy snow will additional model runs. For additional details on this system, please see the long term discussion below.

LONG TERM DISCUSSION.../Issued 300 PM PST Mon Nov 27 2023/

Thursday 11/30 to Monday 01/04...

A real pattern chance comes Thursday, when the first of several wet systems this week swings through the region. A trough and associated front dips down from the eastern edge of the Bay of Alaska/west BC Thursday morning, with the bulk of the precipitation occuring from the afternoon onwards. This system isn't very strong overall, with very little moisture making it past the Cascades. Considering the dry air mass at the surface, some of our western valleys will likely see a lot of evaporation initially, especially in the Rogue/Bear Creek Valley (particularly White City to Ashland). This will bring a cold air mass with it, with snow levels dipping towards 3,000 feet (perhaps just under)
through Thursday, and a cooling trend in daytime temperatures both Thursday and Friday towards below average. Generally light snow over the Cascades and western N. CA Siskiyou Mountains is expected, with a forecast 2-4" of snow above 5000 feet in these areas, as well as lighter snow across areas east of the Cascades (less than half an inch).

Periodic light rain/snow in showers will follow through early Friday before the next trough swiftly moves in Friday afternoon. This system looks wetter than Thursday's, with both more precipitation and a broader area receiving precipitation expected. That said, this trough doesn't dig as far south and will be slightly warmer, as snow levels rebound back towards 4,000 feet. Disturbances will continue to ride down into the area through the weekend, bringing yet more rain and mountain snow to the area. Precipitation will be greatest/heaviest along the coast, especially over the Klamath/Curry COunty mountains, but the Cascades are expected to see a healthy amount out of this as well. Weekend 48 hour probabilities for 12" or more of snow over the higher Cascades have risen in recent model runs to 70-80%, with a 50-60% chance for 18 inches. Mt. Ashland has about a 50% chance to see 6 inches of snow.

Sunday into early next week ensembles are suggest an atmospheric river type system setting up early next week, with snow levels subsequently rising to 6,000-8,000 feet.

-CSP

MFR WATCHES/WARNINGS/ADVISORIES
OR...Air Stagnation Advisory until 10 AM PST Thursday for ORZ023>026- 028>031.

Dense Fog Advisory until 10 AM PST this morning for ORZ023.

CA...Air Stagnation Advisory until 10 AM PST Thursday for CAZ080-081- 084-085.

PACIFIC COASTAL WATERS...Air Stagnation Advisory until 10 AM PST Thursday for PZZ356.

Small Craft Advisory until 10 PM PST this evening for PZZ370- 376.

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Brookings
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Tue -- 12:33 AM PST     6.09 feet High Tide
Tue -- 05:27 AM PST     3.41 feet Low Tide
Tue -- 07:22 AM PST     Sunrise
Tue -- 09:05 AM PST     Moonset
Tue -- 11:15 AM PST     7.98 feet High Tide
Tue -- 04:47 PM PST     Sunset
Tue -- 05:39 PM PST     Moonrise
Tue -- 06:32 PM PST     -1.21 feet Low Tide
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Brookings, Chetco Cove, Oregon, Tide feet

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