Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 190353
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
853 PM PDT Wed Apr 18 2018

.DISCUSSION...The showers have begun to dissipate over the coastal
range, but RADAR still shows plenty of shower activity across the
Cascades and in Siskiyou County. These showers are not moving very
quickly, but they also aren`t dumping a lot of rain. Overall, the
threat of thunder has decreased. The showers will continue
overnight, but become more concentrated along the Cascades and
east before an area of high pressure moves in over the area. Have
not made any additional changes to the forecast, and you can read
the previous discussion in its entirety below.

&&

.MARINE...Updated 800 PM PDT Wednesday 18 April 2018...High
pressure will build into the area tonight into Thursday. Northerly
winds will increase to small craft advisory levels in the
southern waters Thursday afternoon resulting in steep, short
period seas fairly close to shore south of Port Orford. Winds and
seas should subside as a weak front approaches the waters Friday,
then dissipates near the coast late Friday night. A thermal trough
will develop this weekend. This will bring strong north winds,
likely gale force, to areas south of Cape Blanco, while areas
farther north have winds and seas hazardous to small craft. -FB


&&

.PREV DISCUSSION... /Issued 558 PM PDT Wed Apr 18 2018/

UPDATE...The scattered showers will continue for the next few
hours, and we`ve seen a few strikes near the border of Douglas and
Coos Counties in the Coastal Range. Have also seen a few cloud
flashes in eastern Coos County. Have added a slight chance of
thunderstorms to the forecast for the Coastal Range. The threat
for thunder will largely dissipate shortly after sunset. -Schaaf

DISCUSSION...18/12Z NAM/GFS/EC in.

The northern hemispheric map looks very spring-ish. There is a
five wave progressive pattern around the globe with the jet stream
mostly north of 40N, though it dips a bit below in several
locations.

Closer to the Pacific Northwest, there are two events in the near
term. A trough is now digging off the coast, and it will shear
later today. The cutoff low will then dive into northern
California tonight, then move off to the east Thursday.

Ahead of the trough, there is a difluent area aloft that is
currently over the Medford CWA with an embedded short wave moving
through. This is supporting shower activity over the area. More
short waves will follow, so expect this activity to continue
through today. Analysis of model soundings indicate that none of
the showers will develop into thunderstorms. It will be cool with
inland high temperatures 5-15 degrees below normal.

Tonight, the wraparound moisture associated with the cutoff low
will move into the area. This will affect areas from the Cascades
east for the most part. These showers will gradually move off to
the east Thursday with the cutoff low. Inland temperatures will
warm up quite a bit, with the highs just a bit below normal over
the west side and about 5 degrees below normal east of the
Cascades.

Once that system moves out, upper level ridging with dry northerly
flow will bring mostly dry conditions to the area Thursday night
into Friday. A thermal trough will form near the coast and marine
north winds will increase offshore and along the coast. The ridge
will break to the east Friday and flow aloft will turn southwesterly.
The thermal trough will break down and move inland as this
occurs. That will make Friday the warmest day in the near term
west of the Cascades. Highs will be near normal along the coast
and over the Umpqua Basin, and 5 to 10 degrees above normal over
the rest of the area.

The next system to affect the area will move onshore Friday
night in the form of a short wave with most of the energy directed
into Washington state and northern Oregon. The Medford CWA will
catch the southern end of the associated surface front, and this
will bring the possibility of some light rain to the coast, Coast
Range, and Umpqua Basin Friday night into Saturday. Amounts will
be quite light. The thermal trough will be forced to the east side
Saturday, making that the warmest day in the near term over there.
Compared to the Friday highs, it will be a few degrees cooler over
the west side and a few degrees warmer east of the Cascades.

Long term discussion...5 AM PDT Sunday 22 Apr through Wednesday 25
Apr. Both models are showing a short wave moving through the
region during Sunday morning. However, the airmass will be rather
dry and we`re not expecting any precipitation out of this short
wave. So a very Spring like day can be expected with lots of
cumulus clouds and highs in the upper 60s.

This ridge pattern will continue for the next few days.  According
to our anomalies tables, there isn`t much that is unusual for the
upcoming week ahead.  Maybe a bit warmer than average with highs in
the upper 60s and perhaps lower 70s at times. We are forecasting
a high of 80 on Wednesday with the thermal trough kicking into
full gear.

Eventually the GFS is expecting a low to approach our coast during
Tuesday night into Wednesday.  The ECMWF on the other hand wants to
keep things dry, so the extended models are not agreeing on the
chance for some thunderstorms during Tuesday evening next week.  We
personally felt that the ECMWF was not handling the low approaching
our coast correctly.  Over the last week, the pattern has been very
active and short waves have been moving through fairly easily.
Therefore, sided with a more GFS-based forecast for precipitation
potential Tuesday into Wednesday.

AVIATION...19/00Z TAF CYCLE...Low pressure will bring showers to
the area through this evening, mainly over the mountains. Overall,
expect VFR to prevail, though local MVFR and partial terrain
obscuration can be expected, especially in any of the heavier
showers. Expect areas of MVFR and local IFR to develop in the west
side valleys and perhaps along the coast toward morning. Conditions
will gradually improve to VFR by late Thursday morning with gusty
northwest winds developing east of the Cascades. Breezy north winds
are expected and could be as high as 15-20 knot in the Klamath Basin
and LMT. -FB

&&

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

CA...None.

Pacific Coastal Waters...Small Craft Advisory from 1 PM to 11 PM PDT Thursday for
     PZZ356-376.

$$

BMS/FJB/JRS



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