Area Forecast Discussion

FXUS66 KMFR 200306
Area Forecast Discussion
National Weather Service Medford OR
806 PM PDT Mon Mar 19 2018
.UPDATE...A weakening upper level low just offshore will 
gradually slide to the east this evening and tonight. The energy 
aloft is producing little more than virga and increased 
cloudiness this evening, but as the low inches closer and passes 
overhead, increased lift along the Siskiyous and Cascades could 
kick off a few heavier showers, and precipitation form these 
should be able to reach the ground. Prevailing winds will push 
these showers to the north and east, so northern Lake and Klamath 
Counties could see some light rain or snow overnight and into the
early morning as well. have made a few updates this evening to
bring the forecast in line with latest observations, adjusting 
slightly the locations of the best chances for showers this 
evening will be, and adding some slightly stronger winds to the 
Shasta valley and the coastal mountains. 
The main event holds off a bit more, arriving in the afternoon 
and evening tomorrow with more widespread and significant 
precipitation. For more information on this and the rest of the 
forecast, see the previous discussion below. 
.AVIATION...20/00Z TAF CYCLE...VFR conditions are expected for most 
locations through the TAF period.  Upper level clouds can create 
partial obscuration of mountains and higher terrain with CIGS around 
7000 to 15000 feet.  Look for some virga too as showers are showing 
up on radar.  Precipitation and a slight chance for some showers is 
expected to continue through the TAF period even though it's not 
included in some of the TAF forecasts. 
.MARINE...Updated 800 PM PDT Monday 19 Mar 2018....Relatively quiet 
weather will remain over the waters into Wednesday. South winds 
could increase over the southern waters Wednesday afternoon and 
evening, but much will depend on the exact location of an approaching 
front.  If nothing else, it should remain below small craft. 
The aforementioned front will move onshore late Wednesday night with 
winds shifting to the west to northwest. An upper trough will dig 
south over the waters Thursday afternoon. At the same time pressure 
gradients will increase resulting in increasing southwest winds late 
Thursday afternoon and evening. Small craft conditions are possible 
during this time, especially over the northern waters with steep 
wind driven seas.  
There is increasing confidence we'll have an increasing west to 
northwest swell arriving Thursday night into Friday. There is still 
some variations with the timing and wave height, however the the 
general consensus we could see wave heights between 11 and 13 feet. 
Keep in mind the details are likely to change, so watch for updates 
on this. 
Also cold air aloft will move over the waters Thursday night into 
Friday. Convective showers will be plentiful, but we'll also have to 
keep a close watch on isolated thunderstorms.  
Unsettled weather and elevated seas are expected to continue into 
.PREV DISCUSSION... /Issued 248 PM PDT Mon Mar 19 2018/ 
SHORT TERM...Tonight through Thursday evening...Current 
satellite observations are showing a few clouds moving inland from
the offshore cloud shield associated with that low pressure 
system. Although the RADAR is showing extremely light echos 
associated with it, precipitation does not appear to be falling at
the 2 O'Clock Hour. The short- term forecast, however, remains 
active as the next systems are poised to arrive.
A few light showers are expected to build in later this afternoon
as the shortwave moves through. These showers will be mainly
concentrated across the coastal range, the Umpqua divide and along
the Cascades near Crater Lake. These showers will continue to
build in number overnight as the next system approaches the west
coast. This atmospheric river will bring periods of moderate to
heavy precipitation over the next few days, but the bulk of the
Precipitation is expected to occur on Wednesday. Snow levels will
rise to about 5000 to 6000 feet, so only the highest elevations of
the Cascades, including the Crater Lake Rim, will see snowfall. 
The rain is not expected to be heavy enough to bring about any
flooding or flash flooding concerns. Additionally, although the
rain will be beneficial to our ongoing rainfall deficits, it will
not be a drought buster for us since the majority of the
precipitation will be falling south of our area. That being said,
the main focus on the precipitation will be across the southern
Oregon coast near Brookings, the coastal range, and western
Siskiyou County. For the East Side (especially in Modoc County), a
couple of the models are showing the possibility for a rumble of
thunder on Thursday. Traditional indicators (CAPE, LI) are modest
so it cannot be ruled out, but confidence is not high enough to 
add it into the forecast quite yet.
In addition, winds will begin to pick up along with this system.
The windiest places will be along the east side as well as the
Shasta Valley where advisory level winds will be possible. It is,
unfortunately, a bit too early for putting out wind advisories--
especially because confidence is lower now that this atmospheric
river continues to trend southward. Will need to continue
evaluating whether or not any headlines will be warranted. 
This system will transition into a more traditional frontal
boundary Thursday night, and thus the long-term discussion will
talk about the impacts associated with the second portion of this
storm. -Schaaf
LONG TERM...Thursday night through Monday...Indeed, by this time 
frame, we will have shifted into astronomical spring. However, 
winter will keep a firm grip on the weather as multiple disturbances 
swing through the area beneath a deep, anomalous upper trough. 
The first front will shift east of the area Thursday evening. The 
cold pool aloft offshore will arrive overnight at the coast, and 
then Friday morning west of the Cascades. One disturbance will move 
through the area Friday, then another Friday night, followed by one 
more on Saturday. The core of the cold air mass associated with this 
trough (-35 to -38C) is colder than the one that moved through the 
area late last week. So once again, we'll be looking at very low 
snow levels with the potential for snow on the valley floors Friday 
through Saturday (especially if the timing of precipitation is 
This time of the year, it's pretty hard for snow to accumulate much 
at the lower elevations given the warming of the ground and the 
higher March sun angle. This is especially true during the daylight 
hours, but if precipitation arrives at night or very early in the 
morning, there could be snow accumulation down as low as 500-1500 
feet in some areas. This is possible both early Friday and Saturday 
mornings. While each valley west of the Cascades seems to have its 
own microclimate, perhaps the most vulnerable to this type of 
situation is the Illinois Valley, where locally heavier 
precipitation can really drag down the snow level at times. Right 
now, it's too early to say for certain how much snow will fall, but 
this event will continue the late winter trend of building the 
mountain snow pack. This is good news with respect to water storage, 
since most areas are still running significant precipitation 
deficits for the water year. But, it's not so great news for 
travelers, as there'll likely be snow impacts, perhaps at low 
elevations, but especially over the higher passes. 
The expected cold weather also presents an issue for local 
agriculture, especially this weekend west of the Cascades as the 
upper trough lifts out. Cloud cover/showers will be the limiting 
factor for freezing temperatures, but if there's enough clearing 
Saturday night or Sunday night, temperatures could drop into the 20s 
in the typically colder spots.  
Model guidance shows another disturbance arriving in the NW flow by 
early next week, with perhaps another bout of precipitation for the 
forecast area Monday/Tuesday. -Spilde
Pacific Coastal Waters...None.