Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 010451
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
951 PM PDT Sun Apr 30 2017

.DISCUSSION...The forecast was updated earlier this evening to
add the vicinity of Fort Jones into the area with a
continued slight chance of thunderstorms from Thursday afternoon
into Thursday evening and introduce a slight chance of
thunderstorms east of the Cascades Friday afternoon and evening.

Besides the aforementioned risk of thunderstorms later in the
week, the other most noteworthy aspect of the forecast is a strong
warming trend into the middle of the week with highs approaching
but not likely to reach record levels.

Conditions on Monday will resemble those from this afternoon. But,
a warm front will approach the coast late Monday and bring a
chance of light showers north of the Umpqua Divide Monday night
into Tuesday.

With heating, an increase of mid level moisture but also a stable
ridge aloft on Wednesday... low topped cumulus clouds are likely
to form over the higher terrain.

Instability is likely to peak on Thursday afternoon but continue
on Friday. A cooler and continued showery pattern is likely next
weekend.

&&

.AVIATION...01/00Z TAF CYCLE...VFR Cigs/VIS will continue during
the next 24 hours at all the TAF sites. Gusty North winds up to 20
knots will continue until around 04Z. /FB

&&

.MARINE...Updated 800 PM PDT Sunday, 30 April 2017...A thermal
trough will reach peak strength this evening and produce gusty
north winds and steep seas through into early Monday morning
...affecting areas from about 3 to 40 nm from shore, but with
the highest winds and steepest seas south of Gold Beach. A weak
system will disrupt the thermal trough Monday night, but the
thermal trough will redevelop on Wednesday with gusty north
winds and steep seas.

A cold front will move through the coastal waters on Thursday with
northwest flow developing over the coastal waters into the
weekend. This is a change from previous thinking which called for
the thermal trough and northeast flow to develop. While we expect
winds to increase toward next weekend, it does not appear as
strong as was previously thought. -FB/DW

&&

.PREV DISCUSSION... /Issued 105 PM PDT Sun Apr 30 2017/

DISCUSSION...30/12Z NAM/GFS/EC in.

A short wave riding over the offshore long wave ridge pushed a
weak front onshore earlier today. That front is now washing out as
the upper level support exits the area. In terms of precipitation
this front is not amounting to much. Precipitation totals thus
far are less than 0.25 inches along the north coast and in the
Umpqua Basin. Amounts are around 0.05 inches or less along the
south coast and in the adjacent Coast Range. Other than that...
just a hundredth or two here and there from the Cascades west.

High temperatures will be around 5 degrees below normal along the
north coast and in the Umpqua Basin...near normal over the south
coast...Oregon west side and the Oregon/California east side...and
around 5 degrees above normal over Western/Central Siskiyou
County.

Another short wave riding over the ridge will push a front onshore
Monday afternoon and evening. This front will look a lot like the
current system with the bulk of the associated precipitation
falling along the coast and in the Umpqua Basin. High temperatures
will be near normal along the coast and in the Umpqua Basin...
around 5 degrees above normal over most of the rest of the area...
and 5 to 10 degrees above normal over Western/Central Siskiyou
County.

After that system...the ridge will strengthen as it approaches the
coast and moves onshore Wednesday. This will make for dry and
warmer weather Tuesday. High temperatures will be around 5 degrees
above normal along the coast and in the Umpqua Basin...and 5 to 10
degrees above normal over the rest of the area.

As the ridge moves onshore Wednesday...the thermal trough will be
forced inland over the west side. This will make that day the
warmest of the coming week over the west side. High temperatures
will be 5 to 10 degrees above normal along the coast and 15 to 20
degrees above normal over the rest of the area. The Medford high
may hit 90 degrees Wednesday.

EXTENDED DISCUSSION...Thursday through Sunday...The models are
coming into better agreement with the evolution of the upper trough
and overall pattern for the forecast period. The upper trough will
approach the forecast area Thursday and the models are in pretty
good agreement convective precipitation will break out Thursday
afternoon into Thursday night. At this time, thunderstorms should
initiate along the Cascades and Siskiyous, then migrate north to
northeast in the afternoon and early evening Thursday with the 700mb
flow from the south to southwest. Keep in mind it`s still a ways
out, so the details on the exact location could change. Therefore
watch for updates. For now kept in a slight chance mention and if
confidence increases, then they could be upgraded to chance.

Temperatures Thursday will be cooler then Wednesday west of the
Cascades. Meanwhile they will be very similar to Thursday east of
the Cascades, but mush of that will depend on the amount of cloud
cover.

An upper trough will replace the ridge Friday through Sunday with
cooler temperatures, lower snow levels and a chance of showers. The
trough axis will still be offshore Friday with a southerly flow
aloft, but instability is marginal at best, so for now kept any
mention of thunderstorms out, but this could change.

The upper trough will move over the area Saturday, then split with a
cutoff low developing to our south which could bring drier
conditions a week from Sunday. However, the details on exactly where
the cutoff low will reside at is not clear. For example the ECMWF
has the cutoff low in central Cal and the GFS in southern Nevada.
The Canadian shows a cutoff low developing sooner in southern Cal.
-Petrucelli

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...None.

CA...None.

Pacific Coastal Waters...Small Craft Advisory until 2 AM PDT Monday
   for PZZ356-376.

$$

DW/JRS/FJB


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