948
FXUS66 KMFR 201610
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
910 AM PDT Sat Jul 20 2024

.DISCUSSION...Satellite imagery is showing some marine stratus
along portions of the Coast and inland Coos County. It is also
showing some smoke from some of the area wildfires. This is
indicative of our ridge of high pressure. A few changes were made
to the morning update to push thunderstorm potential at the coast
to a little later in the afternoon and evening. The forecast
remains somewhat dynamic with thunderstorms possible. We recommend
you read the previous forecast below for more information. -Schaaf

&&

.PREV DISCUSSION... /Issued 508 AM PDT Sat Jul 20 2024/

DISCUSSION...A high amplitude ridge remains in control of the
western third of the country, and with a trough to the northwest
over the Gulf of Alaska, this places southern Oregon and far
northern California under relatively dry and stable southwest flow
aloft. Meanwhile, a thermal trough has developed over central
California.

A shortwave impulse now lies along the southern periphery of the
trough to our southwest, and this feature will approach the area
through the day today. The southerly flow ahead of the wave will
push hot air into our area today, resulting in very warm
temperatures across much of the inland areas. Most inland valleys,
both east and west of the Cascades, will easily reach upper 90s to
low 100s for the daily high today. This will be just enough above
normal to trigger moderate to major Heat Risk for populated areas
across northern California and the East Side. Heat Advisories
have been issued for those areas for today, and details can be
found at NPWMFR.

The shortwave will also provide instability and additional moisture,
while also acting is a sufficient lift mechanism to produce
widespread slight chance to chance (generally 10-30%) for
thunderstorms late this afternoon into Sunday, including during the
overnight hours as the wave tracks through overhead. This event
looks very similar to the one from earlier this week, both in
overall pattern and in the dry lower layers of the atmosphere. There
are differences, however, in that this shortwave trough is coming
through overnight and will pass by farther offshore compared to
earlier in the week. We remain concerned about lighting and gusty
outflow winds once again, especially given that any storms that
develop will produce very little, if any, precipitation. However,
given the farther offshore trajectory, this should limit how strong
winds get this afternoon in advance of the shortwave. Also, we don`t
expect sufficient moisture for thunderstorm development to arrive
until late this afternoon and evening. So for today, thunderstorm
chances are focused over western Siskiyou County and over the East
Side, then transitioning to areas west of the Cascades this evening
and then to areas north of the Rogue-Umpqua Divide, including the
coast north of Cape Blanco, late tonight into Sunday morning. There
will be a relative break in thunderstorm activity during the late
morning hours, then the focus of convection concerns will shift to
the East Side on Sunday as the wave exits the region.

For the remainder of the week, the pattern will very slowly progress
to the east, with the Gulf of Alaska trough gradually moving inland
into Canada and Washington. This will push the ridge axis farther
east and, as a result, we are expecting temperatures to "cool"
slightly, returning to near normal values for this time of year.
We`ll need to monitor the potential for gusty winds as this trough
pushes inland, otherwise, relatively dry west to southwest stable
flow should continue through the week. Though conditions should
remain dry, there are some indications that some unstable monsoonal
air could skate by our far southeastern areas (eastern Modoc/Lake
counties) so we`ll need to monitor these trends as well. -BPN/BR-y

AVIATION...20/12Z TAFS...Along the coast, IFR marine stratus has
moved into the coastal valleys. The IFR is expected to linger
significantly longer than it did on Friday, through the morning with
breaks in the stratus during the afternoon. It will also quickly
return (around 00Z at North Bend) by the start of Saturday evening.
During the evening, the approach of a low will lift the coastal
stratus, with a mix of IFR and MVFR likely, but also a slight chance
of thunderstorms.

Inland, VFR conditions will prevail through this afternoon, with an
exception. The exception will be in the vicinity of the Trail fire
and various other fires in the vicinity of the Trail Fire. In
eastern Douglas County and across northern portions of Klamath and
Lake counties, visibility could end up between 3-5 miles. Visibility
could also be reduced southeast of the fires into northeast Jackson
County and the remainder of Klamath County, including possibly down
into Klamath Falls late tonight resulting in MVFR visibility.

A slight chance to chance of thunderstorms will develop across the
area later this afternoon/early Saturday evening and continue
Saturday night. -DW/Hermansen

MARINE...Updated 245 AM Saturday, July 20, 2024...A thermal trough
will return today, then strengthen tomorrow into Tuesday. Moderate
north winds and steep seas are expected south of Port Orford today.
Expect conditions hazardous to small craft to expand to all areas
tomorrow with the strongest winds and highest seas south of Pistol
River. DW/Hermansen

FIRE WEATHER...Issued 400 AM Saturday July 20, 2024...The main
fire weather concern in the near term is the potential for
thunderstorms late this afternoon into Sunday evening. Much like
the pattern earlier in the week, a shortwave trough will approach
the region today and pass through the area overnight tonight into
Sunday. There are some differences, however, in that the trough is
passing by farther offshore compared to earlier in the week,
which should limit the extent of gusty south to southwest winds in
advance of the trough this afternoon. Additionally, this trough
will pass overhead during the overnight hours, which will also
limit the extent of gusty winds since there won`t be the daytime
mixing component in play. Because of this, we aren`t anticipating
any winds/RH concerns in advance of this trough today or Sunday.

One last difference is that sufficient mid-level moisture for
thunderstorm development won`t arrive until late afternoon/early
evening. This is different from the previous pattern as there was
already sufficient moisture present for a few days prior to that
shortwave passing through. This last difference will mean that
thunderstorm activity will be focused during the evening and
overnight hours today/tonight into Sunday morning. Based on pattern
recognition, this could be problematic in that we could see storms
break out mid to late afternoon today in portions of northern
California and east of the Cascades. Thunderstorm potential spreads
westward this evening and isolated to scattered storms are possible
over much of the West Side through late this evening and even
overnight. As the trough axis swings through late tonight, nocturnal
convection remains a concern, especially north of the Rogue-Umpqua
Divide where high resolution guidance indicates the focus of
overnight activity. A Fire Weather Watch remains in effect for this
thunderstorm potential today and overnight and have included Fire
Weather Zone (FWZ) 624 for late this afternoon and evening.
Despite the differences in the patterns, we remain concerned
about lighting and gusty outflow winds once again, especially
given that any storms that develop will produce very little, if
any, precipitation.

The upper trough axis shifts north during the day Sunday. Any threat
for storms over the northwest part of our area could be confined to
the early morning hours, then shifting north late Sunday morning
through Sunday afternoon with a drier and more stable southwest
flow moving into the area west of the Cascades. East of the
Cascades, however, sufficient mid-level moisture will linger
through the afternoon/evening for another round of thunderstorms
over much of FWZ 625 and eastern portions of FWZ 624. This
warranted an additional change to the Fire Weather Watch to
include zones 624 and 625 for Sunday afternoon and evening.

A dry and stable west to southwest flow will set up on Monday with
any chance for storms shifting east and northeast out of the area.
Low pressure over the Gulf of Alaska will slowly trek eastward into
Canada, bringing slightly cooler temperatures with gusty afternoon
and early evening breezes. The rest of next week is pointing towards
dry weather, though there are some indications that some monsoonal
moisture could skate by our southeastern areas (Lake/Modoc counties)
at times next week, so we`ll have to monitor those trends.
Additionally, we could see some winds/RH concerns around mid-week as
the deep trough to the north continues to push eastward, resulting
in some enhanced afternoon breezes that could combined with low
afternoon RH`s and result in critical fire weather conditions. /BR-y

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Heat Advisory until 11 PM PDT this evening for ORZ029>031.

     Fire Weather Watch from 5 PM PDT this afternoon through Sunday
     morning for ORZ615>620-622-623.

     Fire Weather Watch from 5 PM PDT this afternoon through Sunday
     evening for ORZ624.

     Fire Weather Watch from Sunday afternoon through Sunday evening
     for ORZ625.

CA...Heat Advisory until 11 PM PDT this evening for CAZ081>085.

PACIFIC COASTAL WATERS...Small Craft Advisory until 11 PM PDT this evening for PZZ350-356-
     370-376.

     Gale Watch from Monday afternoon through Tuesday evening for
     PZZ356-376.

&&

$$