National Weather Service Raw Text Product

816 
FXUS66 KMFR 252332
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
332 PM PST Mon Nov 25 2019

.SHORT TERM...Today will be the calm before the storm. A rapidly 
rapidly deepening "bomb" cyclone will take aim for the Southern 
Oregon Coast on Tuesday. The main message today is to prepare today 
ahead of this storm and if you have travel plans, now would be the 
time to do it. Weather conditions will deteriorate rapidly on
Tuesday with strong winds and heavy snow impacts expected over
much of the area. 

This low is unprecedented in its strength and track. It's also 
historic because storms of this magnitude have not been observed on 
this track in the last 40 years or more!

Keep in mind, the location of the highest winds will largely depend 
on the exact track of the low. Small shifts in the track will make a 
difference in which areas see very strong winds. So have a way to
stay up to date on the forecast and watch for updates. Overall, 
the majority of the models and ensembles bring a 974 mb low into 
the Curry Coast on Tuesday, then move the low onshore near the 
California-Oregon border Tuesday evening and inland over the area 
Tuesday night into Wednesday morning. Ahead of the low center, 
expect very strong winds at the coast and over some inland western
areas along with heavy snow for the mountain passes (including 
Sexton Summit, Siskiyou Summit, pass over the Southern Oregon 
Cascades, and areas near Mount Shasta). As the low moves inland, 
expect wrap around showers to affect the area Tuesday night into 
Wednesday. Snow levels will lower to some western valley floors 
during this time bringing the potential for valley snow impacts. 
High winds and moderate to heavy snow will be the main impacts 
from this storm. Please see details below. 
-Petrucelli

WIND...Low pressure out around 48N and 146W will rapidly 
intensify tonight into Tuesday. The pressure of this developing 
storm this evening at 4 pm PST will be around 1020 mb. The storm 
will "bomb" out, with the pressure lowering to below 980 mb by 4 
pm PST on Tuesday. It will move to a position near 42N and 125W, 
just off the SW coast of Oregon (or about 50-100 miles west of 
Gold Beach). This is a greater than 40 mb drop in just 24 hours. A
"bomb cyclone" is defined as a 24 mb drop in 24 hours, so this 
one is almost double that and wind impacts will be felt across the
entire area. 

What makes this storm unique is the trajectory that it will take 
as it deepens and moves inland. Rather than a deepening system 
moving from SW to NE inside 130W, which typically bring our 
strongest coastal and inland wind storms in SW Oregon and northern 
California (and onshore gradients ALL along the coast), this one 
will be moving in from NW to SE as it moves onshore Tuesday evening. 
This will focus the core of the strongest winds (and onshore 
gradient) for the headlands/beaches from Cape Blanco southward. 
Winds in those places will likely gust in excess of 75 mph with 
gusts in excess of 100 mph possible at exposed areas and headlands. 
This is a dangerous storm and we don't want to downplay the winds 
for areas farther north, but an easterly (offshore) gradient 
usually results in the winds getting blocked somewhat by the 
terrain. We are still expecting gusts of 45 to 55 mph at North Bend, 
but that's the reason it should be somewhat lower there. 

Inland, strong winds should spread inland Tuesday afternoon, 
impacting the higher terrain first, but also the south-north 
(Shasta), and southeast-northwest (Rogue) oriented valleys. The 
pressure gradient on the guidance RDD-MFR continues to be 10-12mb. 
Studies have shown that this strong of a gradient, combined with an 
alignment of the strong (850mb) winds with the valleys, has caused 
high winds in the Shasta, and also the Rogue Valley. Gusts could be 
in the 60-70 mph range. 

High wind warnings or wind advisories are in place for all areas and 
can be viewed at NPWMFR. The low will move inland Tuesday night and 
the strong winds will end. -Spilde

SNOW...Updated 230 PM PST Monday, 25 November 2019...
Showers will continue under northwest flow this afternoon and 
evening, primarily along and near the Cascades westward. Snow levels 
currently near 3500 feet are expected to gradually fall to between 
2000 to 2500 feet overnight. Showers are likely to gradually taper 
off overnight. Additional snowfall amounts through the morning are 
expected to 1 to 3 inches above 3500 feet, though 3-7 inches is 
expected on the higher peaks of the Cascades above 5kft, to include 
on Willamette Pass.

Tuesday morning light precipitation is expected to gradually 
overspread the west side with snow levels in the 1700 to 2700 foot 
range. This begins to present some concern for travel for the higher 
elevations of the coastal mountains, to include the Sexton Summit 
area on Interstate 5. 

Precipitation will increase in intensity late Tuesday morning 
through Tuesday afternoon. Overall, numerical model guidance 
suggests that snow levels will change very little during this time 
period as ice loading from precipitation aloft evaporatively cools 
the lower levels. This will effectively off-set daytime solar 
surface heating. One particularly tricky area will be those 
locations such as the Rogue and Shasta valleys, where downslope 
warming is likely due to the increasing southeast winds. As the 
afternoon wears on in those areas, we expect snow levels to nudge 
into the 2kft to 2.5kft range. However, meanwhile, as the rapidly 
deepening surface low nears the coast, snow levels are likely to 
sink lower and accumulations are likely to increase on the Sexton 
passes, to near 1700 feet. 2 to 4 inches of snow is possible in the 
afternoon there and on Siskiyou Summit, as well as across the 
Cascades near Crater and Diamond Lakes.

Overall, the highest snowfall totals will be in the Mt. Shasta
area where 18 to 24 inches of snow could fall in a 24 hour period.
Additionally, Interstate 5 at Siskiyou Summit could see up to a 
foot of snow during the same time frame. This snow with the gusty
winds could create some near white-out conditions on these
summits. This combination will also work to reduce visibilities
along all of the higher passes if not near "white-out" conditions.

Tuesday evening and night we expect snowfall impacts will rapidly 
increase as the core of the low pressure system tracks along or near 
the Oregon-California border. Snow levels in the 1.5kft to 2.5kft 
range are expected to fall to between 500 and 1500 feet through 
Wednesday morning. Snowfall rates are expected to maximize Tuesday 
evening as snow levels fall and the low translates eastward. It is 
in this time frame that valley accumulations are expected on the 
west side, mostly in the 1 to 4 inch range for valley locations 
between 1kft and 2.5kft on the west side. BTL

.PREVIOUS LONG TERM...(Wednesday through Sunday)...The low 
pressure system will remain over the area Wednesday resulting in a
cold showery air mass with light to moderate showers. Moderate 
shower activity is expected to be focused over the higher terrain 
from the Cascades east and across Northern California. As the low 
pressure system moves east to southeast, showers will decrease 
over the area late Wednesday through Wednesday night. Expect a 
cold air mass to settle over the area Wednesday night and remain 
in place Thursday into Friday. Morning lows on Thursday and Friday
may be in the 20s across western valleys and in the teens to 
single digits for east side valleys. Additionally for Wednesday 
night and Thursday morning, any snow that melted during the day 
Wednesday in western valleys may refreeze on roadways Wednesday 
night. So expect slippery roadways possible. From the Cascades 
east and across northern California, light isolated snow showers 
may linger Wednesday night and Thursday morning. 

On Thursday and Friday, expect mainly dry and cold conditions 
over the area. Friday morning may be particularly cold with low
temperatures in the lower to mid 20s in western valleys and in 
the single digits across eastern valleys. This will bring a very 
cold start to the holiday shopping period. Then Saturday and 
Sunday, another frontal system will approach the area. Snow levels
are expected to rise some ahead of this low but confidence is low
on the details as models continue to show variability on the 
strength and track of this system. Overall, this system does not 
look as strong as the Tuesday system but still expect another 
period of possible wet weather. As this will occur at the end of 
the holiday travel week, we will be monitoring this closely and 
updating when details become more clear. 


&&

.AVIATION...For the 25/18Z TAFs...The next 24 hours will be the 
relative calm before the storm, but still won't be without 
aviation impacts. Northwest flow aloft will continue to bring moist 
onshore flow and some showers, MVFR cigs and terrain obscuration to 
areas along and west of the Cascades today and tonight. Showers will 
be primarily focused on north and west facing terrain and also in 
the Umpqua Valley/Coast where showers are typically less inhibited 
by the terrain. Showers will be much more sparse east of the 
Cascades and in northern California. Winds can be locally 
gusty from the northwest at North Bend and Klamath Falls this 
afternoon. Tonight into Tuesday morning, a period of lower 
ceilings/visibility is possible in the valleys west of the Cascades.

A major storm will then move in on Tuesday bringing high winds, 
rain and snow to the entire region. This will result in serious 
impacts to aviation. Winds will increase along the coast around 18z 
and then spread inland during the afternoon and evening. The main 
impact will be the strong, gusty winds, including the potential for 
low-level wind shear. But, rain and snow will also cause widespread 
ceilings/visibility reductions and total terrain obscuration. Cold 
air will also move in causing low freezing levels of 1000-1500 ft 
Tuesday night into Wednesday MSL west of the Cascades, which means 
snow will likely reach some valley floors. -Spilde


&&

.MARINE...Updated 200 PM PST Monday, 25 November 2019...
A Storm Warning is in effect for the coastal waters Tuesday and 
Tuesday night as a rapidly deepening, low pressure system moves 
into the area from the northwest and rapidly intensifies. Due the 
track and rapid intensification of this low, this storm system is 
expected to be unprecedented in its strength and a similar low has
not been observed in 40+ years. 

Before the storm arrives, high and steep seas will continue over 
the waters tonight into Tuesday morning. 

Conditions will deteriorate  rapidly late Tuesday morning and 
Tuesday afternoon as a rapidly deepening low moving into the 
waters. Winds will shift to south to southeast and rapidly 
increase late Tuesday morning into Tuesday afternoon as the low 
center deepens and nears the coastal waters from the west. Peak 
south to southeast winds are expected during the afternoon on 
Tuesday afternoon and into the early evening. Winds will likely 
reach 45 to 60 kt with occasional gusts to hurricane force. Very 
steep, dangerous and chaotic seas will build, reaching 20 to 32 
ft. Given the track of the low, moving towards Cape Blanco and 
areas just to the south, expect the highest seas to occur from 
Cape Blanco southward. However, very dangerous seas and very 
strong winds will also affect waters north of Cape Blanco. As the 
low moves onshore Tuesday evening along the south Curry coast, 
expect strong north winds to develop over the waters with winds 
reaching storm force. North winds will gradually lower Tuesday 
night but remain at gale force. Very high and very steep seas will
continue through Tuesday night and into Wednesday.

Mariners should seek refuge or safe  harbor and remain in port 
for the duration of this event. This is an unprecedented storm, 
the likes of which have not been seen in 40+ years. 

On Wednesday, winds are expected to gradually decrease, lowering 
to small craft levels during the morning. However seas will likely
remain high and steep to very steep Wednesday into Thursday 
morning. Further improvement is expected late in the week. -CC


&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory from 10 AM Tuesday to 10 AM PST 
     Wednesday for ORZ029>031. 
     Winter Weather Advisory from 4 PM Tuesday to 10 AM PST Wednesday 
     below 2500 feet in the for ORZ024>026. 
     High Surf Advisory from 1 PM Tuesday to 10 AM PST Wednesday for 
     ORZ021-022. 
     Wind Advisory from 1 PM Tuesday to 1 AM PST Wednesday for 
     ORZ023-025. 
     Winter Storm Warning from 10 AM Tuesday to 10 AM PST Wednesday 
     above 2500 feet in the for ORZ023>026. 
     High Wind Warning from 10 AM Tuesday to 1 AM PST Wednesday for 
     ORZ021-022. 
     Winter Weather Advisory from 4 PM Tuesday to 10 AM PST Wednesday 
     above 1500 feet in the for ORZ021-022. 
     High Wind Warning from 1 PM Tuesday to 1 AM PST Wednesday for 
     ORZ024-026. 
     Winter Storm Warning from 10 AM Tuesday to 10 AM PST Wednesday 
     for ORZ028. 
     Winter Weather Advisory from 4 PM Tuesday to 10 AM PST Wednesday 
     above 1000 feet in the for ORZ023. 
     Winter Weather Advisory from 10 AM Tuesday to 10 AM PST 
     Wednesday for ORZ027-028. 

CA...Winter Storm Warning from 10 AM Tuesday to 10 AM PST Wednesday 
     for CAZ082-083. 
     Winter Weather Advisory from 4 PM Tuesday to 10 AM PST Wednesday 
     above 1000 feet in the for CAZ080. 
     Winter Storm Warning from 10 AM Tuesday to 10 AM PST Wednesday 
     above 2500 feet in the for CAZ080. 
     Winter Storm Warning from 10 AM Tuesday to 10 AM PST Wednesday 
     for CAZ081. 
     Winter Weather Advisory from 10 AM Tuesday to 10 AM PST 
     Wednesday for CAZ084-085. 

Pacific Coastal Waters...Small Craft Advisory until 10 AM PST Tuesday for 
     PZZ350-356-370-376. 
     Storm Warning from 10 AM Tuesday to 7 AM PST Wednesday for 
     PZZ350-356-370-376. 
     Hazardous Seas Watch from late Tuesday night through Wednesday 
     evening for PZZ350-356-370-376. 

$$

CC