Area Forecast Discussion
Issued by NWS Medford, OR

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000
FXUS66 KMFR 271607
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
907 AM PDT Sat May 27 2017

.DISCUSSION...An update to the short term forecast is not
necessary this morning. The forecast for the holiday weekend looks
on track. The three main items of interest for the weekend are
coastal low clouds and fog, warm to hot inland high temperatures,
and the initially very weak but trending higher afternoon and
instability over the west side and far western Klamath County.

The low clouds and fog will retreat to the beaches through the
early afternoon with a few hours of afternoon sun possible at the
beaches. Another surge of low clouds and fog tonight will be
deeper and may reach Roseburg early Sunday morning. If so, it is
likely to dissipate there by mid-morning but the coastal stratus
is likely to be longer lasting and may remain all day at the
beaches. Conditions on Sunday night into Monday are likely to
closely resemble those of tonight into Sunday.

Elsewhere, it has been long advertised that it will be Sunny today
then a gradual trend of increased afternoon and evening cumulus
clouds...mainly over the higher terrain from east of the coastal
range to the Cascades. High temperatures for the weekend will be
around 15 degrees above normal...generally the mid 60s to upper
70s for the mountains, upper 70s to lower 80s for the east side,
and 80s to lower 90s for west side valleys. Low temperatures will
also be a bit above normal on the west side, but near the normally
chilly levels in the upper 30s to lower 40s on the east side of
the Cascades.

Insufficient mid-level moisture will be a strong limiter on any
potential for thunderstorms this afternoon. A few low-topped cumulus
clouds are likely to pop up west of Happy Camp. There will be a
modest increase in instability and available moisture on Sunday.
This is well represented in the current forecast of a slight
chance of afternoon and evening thunderstorms for far western
Siskiyou County to the Siskiyou Mountains along the Oregon border,
and also from around Lake of the Woods northward along the spine
of the southern Oregon Cascades. The 12Z data will be scrutinized
for the possible need of an areal expansion of that risk.
The risk of thunderstorms will be higher on Memorial Day. A slight
chance to chance is forecast for most of the area east of the
coastal range...excepting eastern portions of Lake and Modoc
Counties. The latest data strongly supports the current forecast
with the highest probability from around Lake of the Woods
northward and nearly as high for far western Siskiyou County.

The 06Z GFS continues to indicate a change to a noticeably cooler
and wetter trend than the 00Z ECMWF for Tuesday onward.

For the afternoon package, the dual focus will be on fine tuning
the short term details and then looking for the potential development
of model consistency regarding the forecast for Tuesday through
the remainder of the week.


&&

.AVIATION...27/12Z TAF CYCLE...Along the coast and over the
coastal waters...MVFR cigs with areas of IFR cigs/vsbys will
prevail through this morning, clearing to VFR along the immediate
coast for a short period this afternoon, before MVFR returns this
evening. In the coastal valley and lower Umpqua basin, areas of
MVFR/IFR low clouds and fog this morning will burn off by this
afternoon. Over the remainder of the area, VFR conditions will
prevail through the TAF period. -BPN


&&

.MARINE...Updated 200 AM Saturday 27 May 2017...Fog and low stratus
will persist with light and variable winds through the weekend. The
thermal trough will redevelop Sunday into Monday and gradually
strengthen through the first half of next week, bringing increasing
north winds to the area. Small craft advisory winds and seas will
likely develop south of Cape Blanco by the middle of next week.
-BPN

&&

.PREV DISCUSSION... /Issued 307 AM PDT Sat May 27 2017/

DISCUSSION...Upper level ridging will strengthen over our area
today into Sunday and then slide just to our east Monday. A
surface thermal trough has already developed and its axis is
oriented north-south approximately right over Medford at this
hour. It will remain over us today and then gradually shift east
in tandem with the upper ridge axis. The majority of us will see
sunny and warm to hot weather as a result of these features
through Monday. The exception is at the coast where onshore flow
will continue to pull marine stratus in over the area, especially
at night.

We do not expect any precipitation today, however, as the heat
builds, so will the instability, and it will eventually be
released in the form of thunderstorms. The instability will remain
capped by warm air aloft through today and probably much of
Sunday, and there really isn`t much to trigger any convection.
However, we could see a thunderstorm or two pop up over the
Cascades and/or Siskiyous later Sunday.

The better chance for thunderstorms still looks like Monday.
Moisture and corresponding instability become rather extreme by
Monday, and while the weak link still looks like the trigger, an
approaching trough will probably provide enough cooling aloft and
lift in the form of upper diffluence to get some storms going.
Furthermore, as mentioned by previous shifts, models are tending
to show a negative tilt to the trough as it approaches, and the
resulting steering flow being slightly east of due south could
bring a storm out over the west side. The adjustments made by
previous shifts to account for these factors, look good, and few
adjustments were made this morning. Given the extreme instability,
storms that do form will have the potential to become strong to
severe with large hail likely being the main threat.

By Tuesday, the incoming trough will have probably stabilized
things enough to preclude any thunderstorms from the Cascade crest
westward, but thunderstorms remain possible on the east side.
It is notable that the GFS is an outlier in showing a very
negatively tiled trough approaching late Tuesday and a closed low
going right over us on Wednesday. As a result, the GFS solution
could bring thunderstorms back out over the west side Tuesday and
certainly would produce much cooler, wetter weather on Wednesday
than other solutions. That`s not to say it`s wrong, just that the
ECMWF and Canadian models show a more open wave further north. For
what it`s worth, although the NAM doesn`t go out past Tuesday
morning, it appears supportive of the GFS idea. I`ve bumped
temperatures down for Wednesday, but I did not raise pops to
anywhere near what would be warranted if the GFS is right. -Wright

&&

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

CA...None.

Pacific Coastal Waters...None.

$$

DW/SBN/BPN


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