FXUS66 KMFR 012138

Area Forecast Discussion
National Weather Service Medford OR
238 PM PDT Wed Apr 1 2020

.DISCUSSION...The latest visible image clearly shows a rotation
associated with a shortwave rounding the base of an upper low just
north of our forecast area. Showers are on the increase over Douglas
County which the high res guidance has been showing the past several
hours. Showers will increase in coverage, mainly north of the Umpqua
Divide and northern Cascades as time goes on into early this
evening. Instability is marginal enough for a few lightning strikes
later this afternoon onto this evening in eastern Douglas County and
portions of Klamath and Lake County, thus we'll keep a slight chance
mention in the forecast.

The shortwave will move southeast through the region tonight and
showers will diminish early this evening with dry conditions
tonight. The main question will be the amount of clearing, but we
think there should be enough breaks in the cloud cover for
temperatures to drop fairly quickly after dark and will drop to near
or below freezing for several locations west of the Cascades,
especially in the valleys. Also near freezing temperatures are
possible near and at the coast. Therefore the Freeze Watch has been
upgraded to a Freeze Warning for portions of Douglas, Jackson and
Josephine Counties. The Frost Advisory still remains in effect.
Please see NPW for more details on the areas that will be impacted
by the Freeze Warning and Frost Advisory.

Another shortwave embedded in the northwest flow is expected to move
into the northwest part of the forecast area Thursday afternoon and
could be enough to kick off some showers mainly north of the Umpqua
Divide and northern Cascades. However QPF amounts will be light and
were not expecting any snow concerns.

Friday looks dry, then all signs are pointing towards a cool and
unsettled weekend.

Low pressure system is on track to arrive late Friday night into
Saturday morning. The operational models are in better agreement
with the timing, but there's still some varying solutions with
respect to the extent and location of heaviest precipitation. The
GFS is more robust in bringing precipitation inland, but the ECMWF
and NAM are not too far behind. This will heighten the concern for
snowfall during this time period between 3000 and 4000 feet which
includes Siskiyou Summit.

The surface low and associated upper trough will move into the area
Saturday, although the GFS still shows a faster solution and
actually has the upper trough moving out of our area by Saturday
evening while the ECMWF and NAM have the upper trough pretty much
over our area. Snow levels Saturday are expected to be between 4000
and 4500 feet, but with a stronger early April sun, road snow
concerns should be low and most of the time roads will be wet since
the heaviest precipitation should remain south and west of these

The upper trough moves east of the area Saturday night, but any
break in precipitation will be brief, if at all as another and
stronger upper low will be on the heals of the upper trough and will
reach the Oregon/California coast early Sunday morning. The forecast
will lean towards a continued precipitation. Snow levels are
expected to drop down between 2500 and 3000 feet which should
provide a window of opportunity for some slick spots during that
time period on Interstate 5 over Siskiyou Summit, in the mountains,
and across the east side. Once it becomes daylight, the threat for
road snow will be diminished and could end up wet most hours. The
one piece of good news is that it will help to alleviate the below
average snowpack and help to prolong spring run-off in area

The upper low will be a slow mover and will linger around the rest
of Sunday through Sunday night, then slowly move south of our area
Monday. The GFS is quicker to move the upper low south which is
suspect given it could end up getting cutoff from the main flow in
which case will be slower to exit. The operational ECMWF solution
looks more reasonable and is slower. At any rate, Monday will be a
transition day with precipitation generally decreasing from north to

After Monday, it should remain dry for most locations Tuesday and
Wednesday, however since the ECMWF is slower to move the upper low
out of our area, we'll have to be on the lookout for showers wrapping
around the upper low in northern California both days. -Petrucelli


.AVIATION...For the 01/18Z TAF Cycle...A weak upper level trough
will pass over the region today, producing showers and the potential
for isolated thunder through this afternoon. Heavier showers may
also produce small hail and/or periods of MVFR conditions. Terrain
obscuration may also be a concern, especially along north-south
trending ridges and any other upwind slopes. Otherwise, VFR will
prevail for most of the area through today, although areas of MVFR
ceilings are expected to develop this evening and tonight along the
coast and into the Umpqua Basin. -BPN


.MARINE...Updated 200 PM PDT Wednesday 1 April 2020...Showers will
continue through this evening as a weak system moves through the
waters, then high pressure will return for Thursday and Friday. Seas
will subside tonight and remain calm through Friday night, but winds
may become gusty Thursday afternoon, especially south of Cape
Blanco. The next front will move onshore Friday night into Saturday,
followed by several more weak systems through the weekend. Winds and
seas will increase during this time, but conditions should remain
below advisory thresholds until early next week. -BPN


OR...Frost Advisory from 10 PM this evening to 9 AM PDT Thursday for
Frost Advisory from 10 PM this evening to 9 AM PDT Thursday for
Freeze Warning from 10 PM this evening to 9 AM PDT Thursday for
Freeze Watch from Thursday evening through Friday morning for
Freeze Watch from Thursday evening through Friday morning for
Freeze Warning from 10 PM this evening to 9 AM PDT Thursday for


Pacific Coastal Waters...Small Craft Advisory until 11 PM PDT this evening for

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