Area Forecast Discussion
Issued by NWS Medford, OR
FXUS66 KMFR 262345
Area Forecast Discussion
National Weather Service Medford OR
445 PM PDT MON SEP 26 2016
.Updated Aviation Discussion.
.DISCUSSION...An upper trough over the Gulf of Alaska is
flattening the upper ridge that has been bringing the very warm
to hot weather across SW Oregon and northern California the last
couple of days. Energy will continue to dive into the trough and
by late this week into this weekend, much of that energy will
careen into the Pacific Northwest. As a result, we expect big
changes from the summer-like pattern we`re currently experiencing
to one more representative of mid-late fall Friday through next
Monday. We`re most confident in the significant drop in
temperatures expected during this time frame and a little less
confident in the details with respect to timing and strength of
frontal systems arriving onshore. The trend, however, will be one
of appreciable cooling and also increasing rain chances. Albeit
slightly slower than previous extended guidance indicated, models
continue to suggest widespread wetting precipitation moving into
the area over the weekend and continuing into early next week.
Tonight, a marine push is expected along the coast and also into
the Umpqua Valley, where low clouds will return. Some spits of
drizzle are also possible toward morning. However, the marine
layer won`t be deep enough to traverse the Umpqua Divide, so it
will remain clear here in the Rogue Valley as will all areas south
and east of the Cascades.
Tuesday and Wednesday will remain dry and quite warm with inland
highs about 10-15 degrees above average.
On Thursday, the upper level trough will move closer to the area.
Southwest flow aloft will increase with some gusty winds expected
to develop east of the Cascades in the afternoon. Isolated showers
could develop over the Cascades, especially north of about Lake of
the Woods, but instability parameters still look marginal, so have
not included thunder.
Friday, the first significant short wave associated with the
trough approaches and moves into the area. This will likely bring
some showers to the coast with precipitation chances waning to
slight chance or nil east of the Cascades. Models are showing some
CAPE and negative LI moving into the coast north of Cape Blanco
and also the lower Umpqua Valley, so have added a slight chance of
thunder there during the afternoon/early evening. Shower chances
persist Friday night at the coast due to onshore flow, but it will
likely get quite chilly east of the Cascades where skies are
expected to clear for a while.
By this weekend, models diverge in the timing of the upper trough
as it drops southward along the coast...quite likely due to the
development of a strong jet (130-140kt) upstream over the North
Pacific. Despite the variations in the models, still expect much
cooler conditions with temperatures 5-15 degrees below average and
multiple chances at wetting rainfall. Right now, it appears the
best chance will occur with a disturbance riding along the jet
Sunday night into Monday. Details are likely to change though, so
stay tuned! -Spilde
.AVIATION...27/00Z TAF CYCLE...The latest visible image shows
marine stratus in the vicinity of North Bend and MVFR and occasional
IFR cigs could move in the area around 1z. Current TAF reflects
this, but confidence is low to medium on the timing and it could
move in earlier. So watch for updates on the TAF. The models suggest
the marine stratus could push far enough inland to include the
Umpqua Basin and the latest LAMP guidance and high resolution
moisture fields supports this idea towards 14z. Therefore we`ll
introduce MVFR CIGS into the TAF at Roseburg. There`s a chance these
conditions could come in sooner, so later shifts will need to
reevaluate. Elsewhere, VFR conditions will continue through the TAF
.MARINE...Updated 230 PM PDT Monday 26 September 2016...North winds
are already starting to increase as the thermal trough is in the
process of redeveloping. This will continue to strengthen tonight
and persists through Wednesday evening. This will lead to winds and
seas once again reaching small craft advisory levels tonight and
tomorrow. Winds will rise to gales in the expansion fan south of
Cape Blanco this evening and tonight with small craft conditions
elsewhere. The trough weakens and moves inland Wednesday and
Thursday with conditions gradually improving. -Petrucelli
.FIRE WEATHER...Updated 100 PM PDT Monday 26 September 2016...Hot
and dry weather is peaking over inland locations this afternoon as
the surface thermal trough moves inland. Fortunately, winds will not
be a problem, so red flag conditions are not expected. After a hot
and dry day at the coast yesterday, humidity is increasing due to
winds shifting to onshore.
Tuesday and Wednesday will feature gradually cooling conditions as
the upper ridge moves east and loses its influence over the region.
As an upper trough to the west moves closer, temperatures will drop
more noticeably on Thursday. This upper trough will bring
increasing winds Wednesday and moreso on Thursday, with 30mph gusts
expected. Humidities will increase some Wednesday into Thursday, but
we will still be monitoring for the potential for critical fire
weather conditions on both of these days, especially Thursday.
The cooling will continue into the weekend as an upper low sends a
series of systems into the area. A system may bring a wetting rain
to the coast and Umpqua Valley Friday then to much of the area over
the weekend or early next week. It is too early to know exactly how
much rain falls or where it falls, but confidence is fairly high
that the pattern will shift to be much cooler and wetter late this
Pacific Coastal Waters...Small Craft Advisory from 11 AM Tuesday to
2 AM PDT Wednesday for PZZ350-356-370-376.
Small Craft Advisory for hazardous seas from 8 PM this evening
to 11 AM PDT Thursday for PZZ350-356-370-376.
Gale Warning from 5 PM Tuesday to 2 AM PDT Wednesday for PZZ376.