Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 192148

Area Forecast Discussion
National Weather Service Medford OR
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


.AVIATION...19/18Z TAF CYCLE...VFR conditions are expected for
most locations through the TAF period. Low clouds remain in
portions of the Umpqua Basin and Illinois Valley, but it`s burning
off. The TAF at Roseburg should see improving conditions around
19z and there some indications IFR CIGs could develop there again
around daybreak Tuesday and that thinking has also been reflected
in the TAF. -Petrucelli


.MARINE...Updated 200 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

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
Saturday. -Petrucelli




Pacific Coastal Waters...None.


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