Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 210410
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
910 PM PDT Sun May 20 2018

.DISCUSSION...An evening update to the forecast is not necessary.
The 00Z GFS data has just begun to arrive, and the 18Z data
supports the inherited forecast. Temperatures will remain above to
well above normal this week, but an active pattern will continue a
day-to-day varying risk of showers and thunderstorms.

The coverage of showers and thunderstorms has now begun to
diminish with the end of day-time heating. The short term models
did a fine job in indicating the large majority of the activity in
our area from central Siskiyou County eastward and east of the
southern Oregon Cascades. The intensity of storms stayed largely
weak, though any thunderstorm can be dangerous and had they
occurred during summer with dry, receptive fuels the roughly 400
lightning strikes in the area since late morning would likely have
been far more impactful.

Of every day this week, Monday will be dry for by far the widest
swath of the area. A ridge offshore will build inland and further
stabilize the west side while a trough moves farther southeast
into southern California. There is a slight chance of late day
showers and thunderstorms only from near Mount Shasta city east-
northeast across much of Modoc and southeast Lake Counties.

Activity on Tuesday is expected to roughly resemble what happened
today with weak diffluence aloft and instability across much of
the area east of the Cascades as the trough moves east-northeast
into Nevada...back closer to our area.

Available moisture will increase and the flow aloft will turn
southerly on Wednesday with a shortwave trough likely ahead of the
next broad trough approaching the west coast. The increase in
cloud cover on Wednesday may lower the risk of thunderstorms, but
it will rival Friday and Saturday for the title of wettest day of
the week. Confidence is only moderate at best beyond Wednesday but
a model blend leans toward at least a modest decrease in shower
and thunderstorm activity on Thursday with the influence of
shortwave ridging. A cooler, but drier pattern may develop beyond
Saturday.

&&

.AVIATION...21/00Z TAF CYCLE...Along the coast, MVFR from lower
clouds will return late this afternoon or early this evening.

Inland, VFR to MVFR prevails for the next 24 hours. Scattered
showers and thunderstorms east of the Cascades will continue into
the early evening. This could lead to temporary MVFR and gusty and
erratic winds. Stratus should return to similar locations and
heights as this morning.

&&

.MARINE...Updated 800 PM PDT Sunday 20 May 2018...North
winds south of Cape Blanco are peaking this evening but will remain
strong overnight. Winds will be strongest south of Cape Blanco and
seas will be steepest south of Gold Beach. Winds and seas will
increase again Monday afternoon as the thermal trough peaks in
strength, gradually subsiding late Tuesday afternoon into Wednesday.
-FB

&&

.PREV DISCUSSION... /Issued 509 PM PDT Sun May 20 2018/

DISCUSSION...From the Trinity Alps ,through Mount Shasta, north
through the Cascades, and all points east, the RADAR has lit up
with numerous showers and thunderstorms. These cells are
progressing slowly to the east and northeast, producing light to
moderate rain and a few rumbles of thunder. Expect convective
activity to peak later this afternoon, then taper off through the
evening.

The open upper level trough overhead, responsible for producing
the cooler and more unstable weather today, will close off and
slide to the south, taking up temporary residence over southern
Nevada and California. This will keep precipitation chances out
of all but the far southeastern portions of Lake and Modoc
counties Monday, as ridging will be allowed to nudge in from the
west. This will also allow for the development of the thermal
trough to our south, which in turn will produce warming
temperatures and dry east flow.

The trough will then meander north Tuesday, while the thermal
trough extends northward and pushes inland. This looks to be the
warmest day for much of the area, but at the same time, shower and
thunderstorm chances will increase along and east of the Cascades
and along and south of the Siskiyous as moisture and instability
return along with the upper level trough. The trough is expected o
open and join back up with the main belt of westerlies Tuesday
night, and move to the north by Wednesday morning. Meanwhile, a
second, much more robust upper level low over the eastern Pacific
will approach the coastline. This pattern is expected to produce
plenty of moisture inflow, as well as increased diffluence aloft,
and lift in the mid levels. These are the prime ingredients for
another round of afternoon and evening thunderstorms, and given
the amount of moisture possible, some of this convection could
produce a good deal of rain. It is worth noting, that given the
quick transition between troughs Tuesday and Wednesday, we could
see thunderstorms continue through the night Tuesday and into
Wednesday morning.

Overall, have bumped temperatures up slightly, especially for the
south coast Monday and the inland valleys Tuesday, and increased
the chances for precipitation throughout the first half of the
week, except for Monday which should trend mostly dry. -BPN

Extended Term...Thursday through Sunday...There remains a
significant degree of uncertainty in the extended forecast, but
not as much as yesterday. It all depends on where a persistent
cutoff low sets up and the model solutions, while still in
disagreement, have converged a bit since yesterday.

The 20/00Z GFS and EC ensembles help to clarify this issue a bit.
Both are advertising the cutoff setting up off the central
California coast but the EC solution has a tighter cluster of
ensemble members where the cutoff is located.

Given this, will continue to favor the EC solution due to greater
consistency, both run-to-run and within the same run.  The EC
solution keeps thecutoff closer to the Medford CWA and this means
a forecast of showers and thunderstorms Thursday through
Saturday.

On Wednesday afternoon, the cutoff will be near 35N 135W with
weak southerly flow aloft over the forecast area. The low will
move slowly northeast to around 38N 127W Thursday afternoon with
flow aloft becoming southeasterly and increasing a bit. The low
won`t move much into Saturday afternoon.

This pattern will advect a steady flow of moisture into the area
through Thursday through Saturday. Precipitable water values are
not available from the EC model, but the GFS shows quite a bit of
moisture Thursday...0.75 to 1.00 over the entire area. Friday
overall is just a bit drier, but with a higher range from around
0.66 inches to just over an inch. Saturday is forecast to be very
wet indeed with 1.00-1.50 inches over the west side and 0.75 to
1.00 inches east.

As the low moves closer, cooling aloft will enhance the
convective environment. The EC 850mb lifted indices lack
resolution, but they still depict significant lifting over most of
the inland portions of the CWA, the Umpqua Basin excepted,
Thursday evening. The GFS is less bullish on convection, which
makes sense as it places the cutoff farther offshore. On Friday,
the EC goes with an east side convective scenario. The GFS lifted
indices go a bit bananas this day and looks anomolous with very
high LIs over the west side. On Saturday, both models shows
broader coverage over the east side and Siskiyou county with some
intrusion into the west side.

Sunday looks to be the transition day, as the low is forecast to
lift back into the westerlies at that time, weakening and moving
through the Medford CWA as it does so. The air mass stabilizes
once the low moves through. This is a pretty dicey forecast this
far out and forecast confidence is low.

If this forecast pans out, it`s going to be pretty busy around
here for the second half of the week.

As for temperatures, model output now suggests Thursday will be
the hottest day in the extended period with inland highs 5-10
degrees above normal. Temperatures will then cool to around 5
degrees above normal Friday, though the east side will be slower
to cool. After that, inland temperatures will generally range from
normal to around 5 degrees above normal. Coastal temperatures
will not stray far from normal. -JRS

&&

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

CA...None.

Pacific Coastal Waters...Small Craft Advisory until 5 AM PDT Wednesday
     for PZZ356-376.
     Small Craft Advisory for hazardous seas until 5 AM PDT Wednesday
     for PZZ350-356-370-376.

$$

DW/JRS/FJB


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