FXUS66 KMFR 191140
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
440 AM PDT Mon Aug 19 2024

.AVIATION...19/12Z TAFS...The coast has not seen the development of
marine stratus, and high clouds have been passing overhead tonight.
VFR low clouds are in the Umpqua Basin through this morning before
lifting. All inland locations are expected to keep VFR levels
through the TAF period. Lowering ceilings are possible tonight for
North Bend. -TAD/Hermansen

&&

.PREV DISCUSSION... /Issued 305 AM PDT Mon Aug 19 2024/

DISCUSSION...Model agreement is very good through Thursday, and
less precisely/more generally in agreement regarding the character
of conditions Thursday night through Friday.

Our weather this week will be driven by two upper level troughs.
The first trough has its center located offshore of Washington,
with the trough extending to far offshore from northern
California. It will track slowly eastward, moving inland on
Tuesday, then reaching the Rockies on Wednesday while the second
trough moves south along the coast of Canada. Confidence has
increased that this second trough will track across our area and
produce another episode of unseasonably cold, and wet weather
with a slight chance to chance of thunderstorms at least from
Thursday into Friday. These conditions could resemble those from
Saturday, with the addition of a much larger portion of the east
side also being included in the risk of showers and thunderstorms.

Conditions today through Wednesday afternoon will resemble those
from yesterday. This includes temperatures below normal by a few
to several degrees, breezy afternoon westerly winds with the
strongest gusts and lowest humidity east of the Cascades, and a
variable mix of low, mid, and high level clouds, but mostly plenty
of clear skies. High level cirrus clouds will be at a maximum of
coverage this morning, while low clouds will also be at a peak
this morning in Douglas County. Cumulus buildups are expected
across our northern tier, from eastern Douglas County across far
northern portions of Klamath and Lake counties both this afternoon
and Tuesday afternoon. The SREF and NBM each indicate about a 5%
probability of late day thunderstorms in that area on both days,
with more moisture available in the 700-500 mb layer today, and
the convective benefit of a passing shortwave on Tuesday. So, no
activity is expected but a lightning strike or two cannot be
entirely ruled out.

Wednesday can be considered a transition day with an influx of
high clouds arriving by evening, and a deep marine layer also
expected to reach the coast north of Cape Blanco early Wednesday
evening with a slight chance to chance of light rain.

The next phase of our weather looks to begin late Wednesday night,
after midnight Thursday, with a cold, wet, unstable air mass
extending inland through Friday. While the pattern is not an exact
replica, the weather on Thursday and Friday will resemble Saturday
in that the risk of fire starts from copious lightning would be
offset by very cool temperatures and substantial rainfall.

Uncertainty in the forecast takes a leap higher for Friday night
and Saturday with a wide range of solutions regarding the speed
and track of the upper low. Nearly 50% of ECMWF ensemble members,
the operational ECMWF, and typical model biases support a slower
movement of the low than that indicated by the GFS. As such, the
very cool temperatures and shower chances may linger beyond
Friday...continuing Friday night through Saturday, particularly
east of the Cascades.

The southern extent of the next (third) trough into the Pacific
Northwest early next week during the day 7 through 10 period is
uncertain. But, there is general agreement on a northward shift of
the storm track. This would result in a warming and drying trend.

MARINE...Updated 200 AM Monday, August 19, 2024...Calm conditions
will persist through the forecast period. South winds will persist
through Tuesday, then become northerly by late Tuesday afternoon.
Seas will remain at or below 5 ft. Another upper level low will move
into the region at the end of the week, bringing another round of
showers Thursday into Friday, and the return of south winds.
Conditions are expected to remain below advisory criteria through
the end of the week, if not longer. /BR-y/Hermansen

FIRE WEATHER...
Much quieter weather is expected through at least early Tuesday,
but it won't be devoid of some fire weather concerns. We expect
gusty SW winds (20-30 mph) again this evening for portions of the
east side, especially southern Lake and Modoc counties along with
RH values down to around 15%. These are near critical conditions,
but not quite there for a Red Flag warning. So, we'll continue to
headline that in the Fire Weather Planning Forecast.

The low that was responsible for the thunderstorms Saturday is
moving north and weakening offshore. Eventually, a short wave
trough will round the back side of that low and approach the coast
on Tuesday. Moisture and instability are fairly weak with this
system, so we aren't anticipating any widespread convective
activity, but models are showing potential for some cumulus
buildups near the Cascades (or just east) in northern Klamath and
also northern Lake county later Tuesday afternoon. Right now,
models are not too bullish on thunder probs (<15% chance), but
it's something to watch. Breezy pm/eve winds and low RH continue
for east side areas.

The upper trough swings through Tuesday night into Wednesday with
the trough axis shifting into the Great Basin by late morning. This
should once again preclude much chance at precip.

However, a much deeper/stronger closed low will come south along the
British Columbia coast Wednesday and eventually move down the PacNW
coast Thu-Fri. This will bring another cool down with
showers/thunderstorms likely, possibly with wetting rainfall in some
areas. Multi-model clusters are showing the closed low opening up
and moving into the Great Basin this weekend (Sat), but as with all
closed lows there are some uncertainties in just how fast it will
move inland. We'll keep an eye on it. -Spilde

&&

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

CA...None.

PACIFIC COASTAL WATERS...None.

&&

$$