273
FXUS66 KMFR 242119
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
219 PM PDT Tue Sep 24 2024

.DISCUSSION...

Overview:

Well above normal temperatures are still expected today. In fact, a
few westside valley areas could flirt with triple digits. We are
still expecting rain showers through the day tomorrow, and we could
start to see these chances increasing along the coast by late
morning (~10am-11am). We are not expecting these showers to go past
8p-9p tomorrow night. Generally light rainfall amounts expected for
areas on the westside, with areas along and near the coast having
the best chance to accumulate precipitation. That said, still not
expecting a lot of rainfall with this system, but some westside
areas could see amounts approaching 0.10" of rainfall. Relatively
quiet weather expected through the rest of the week.

Further Details:

A Rex Block pattern is evident over the western CONUS today. An area
of high pressure (closed contour) on the poleward side of a closed
low pressure system off the southern California coast is expected to
continuing sliding east through tonight. This high will eventually
be centered over the Four Corners region, and by Wednesday an Omega
Block pattern will be evident over the entire CONUS. The location of
the H5 high will allow for the trough over the Pacific to ride the
western periphery of the high into the PacNW. At this time, the once
closed low will now be an open wave as it passes through the region,
so it is expected to be a progressive system with rain showers
moving west to east through the day relatively quickly. Given the
quickness of the system, we likely wont have an opportunity for high
rainfall amounts given both the speed and lack of convective
elements (CAPE < 75 J/kg). If you draw a line from roughly Brookings
to Diamond Lake Oregon, areas to the north and west of this line
will have the greatest chance for rainfall accumulation. Elsewhere,
very little rainfall (trace to a couple hundreths) is expected. For
those areas west and north of that line, there is about a 10%-50%
probability for rainfall amounts >= 0.10" over 12 hours ending
Wednesday evening. This is one of those high PoP but low QPF
scenarios where rainfall in very likely along and near the coast,
but the amounts are just not going to be very high.

Thereafter, the forecast is relatively quiet with another system
likely going north of our area Thursday evening/night, but it might
clip parts of far northern Coos County and NW Douglas County with
rainfall chances. The biggest change for this forecast was the
increase in high temperatures later this week/weekend. Saturday, for
example, went up almost 10 degrees in some locations. The Omega
Block that sets up across the CONUS could make the long term
forecast difficult, especially once Tropical Storm Helene gets
absorbed into the mean flow. It will be interesting to see how the
Omega Block responds to tropical storm. In other words, we could see
some fluctuations in the extended if models don`t handle the
blocking pattern well with the addition of Helene.

&&

.AVIATION...24/18Z TAFS...VFR prevails over the area late this
morning. This will continue through early this evening, then marine
stratus/fog offshore will move into North Bend around sunset with
conditions deteriorating to IFR/LIFR. Most immediate coastal areas
will have some form of restricted visibility or lower ceilings
overnight into Wednesday morning. Inland, VFR continues through most
of the overnight, but a deepening marine layer could bring MVFR
ceilings into Roseburg toward morning. A frontal system then could
bring some light rain to coastal areas and other areas north of the
Rogue-Umpqua Divide Wednesday afternoon. This front will cause gusty
W-SW winds (25-35kt) Wednesday afternoon and evening east of the
Cascades and in NE California. -Spilde


&&

.MARINE...Updated 230 PM ,September 24, 2024... A west-northwesterly
swell has arrived in the waters and that swell will persist through
the week. A cold front will push through the waters on Wednesday,
bringing rain and some weak southerly winds before winds shift back
to northerly yet again.

The big change will be towards Friday when another cold front shoots
down the coast and a strong thermal trough will like develop around
Friday night and Saturday. The latest forecast has winds reaching
gale force in the southern outer waters. A more probabilistic
approaching shows these winds have a 50% chance of reaching gale
force. Even if they don`t reach gale force, the winds will still
likely be strong. We waited to issue a gale watch to gain a little
more confidence on the evolving pattern. Otherwise, plan for more
chaotic seas and strong winds towards the weekend. -Smith


&&

.FIRE WEATHER...Issued 200 PM Tuesday, September 24, 2024...
Onshore flow at the coast is bringing cooler air to the beaches
compared to just 24 hours ago when the thermal trough brought ENE
winds that sent temps into the 90s in Brookings. The thermal trough
is weakening as it moves inland today, but this is leading to a very
hot day for the valleys west of the Cascades. By late this
afternoon, most areas will be in the mid to upper 90s, even a few
triple digit readings are expected in the lower Klamath Valley of
western Siskiyou County. The good news is there isn`t much wind to
speak of this afternoon, other than the typical slope winds that
usually occur in the afternoons.

This will change Wednesday as an offshore upper trough moves
inland into the PacNW. Pressure gradients tighten and gusty W-SW
winds are expected from the Cascades eastward and in NE CA during
the afternoon and early evening. Still looking at sustained
winds of 15-20 mph with frequent gusts around 35 mph. And, local
RAWS guidance still indicates potential for peak gusts at the more
exposed sites (Coffee Pot, Summit, Summer Lake, Timber Mtn) in the
40-45 mph range. Humidity is also still the limiting factor
precluding the issuance of a Red Flag Warning (minimum RH values
are expected down around 15-20%). So, we`ll maintain the headline
in the Fire Weather Forecast. The front itself could bring a
little light rain to areas nearest the coast and also for areas
north of the Rogue-Umpqua Divide, but expected amounts will be
generally 0.10 of an inch or less. Probability of wetting rainfall
is low (generally 20% or less) in those areas (including the Bottom
Creek fire).

Thursday will be a transition day with fairly typical fall weather
and little or no fire weather concerns. Another weaker front will
skim by to the north of the area Thu night through Friday, but also
should have minimal fire weather impacts, though it will turn
warmer inland.

There is potential for a stronger upper trough to move through the
area on Saturday. Recent guidance has shown this trough to be
weaker than earlier runs, but there should be an uptick in winds and
lowering of humidity again preceding it. We`ll monitor this.

After that, models are showing another upper ridge building offshore
and allowing another thermal trough to develop along the coast. This
may lead to a period of offshore E-NE winds by early next week.
Overall, a dry fall pattern with some wind/rh concerns for the next
7 days. -Spilde

&&

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

CA...None.

PACIFIC COASTAL WATERS...None.

&&

$$

JWG/MAS/CZS