NWS Medford, OR wrote its last area forecast discussion before the first event at Jul 31, 8:10 PM PST, 3 hours 50 minutes earlier.
Area forecast discussion, Jul 31, 8:10 PM PST, 3 hours 50 minutes before
Area Forecast Discussion
National Weather Service Medford OR
910 PM PDT Sat Jul 31 2021
.UPDATE...
Pretty timid thunderstorms this evening as the atmosphere begins to
stabilize. The high resolution rapid refresh model shows
essentially no convection for the remainder of the evening, so
just look for some lingering isolated showers over southern
Oregon and northern California this evening. We also increased the
sky cover for tomorrow near and under the thunderstorm forecast.
Other than the storms, it will be another warm night with low
temps well into the upper 60's in many of the west side valleys
tonight. The upper level clouds should also help us stay on the
warm side tonight.
-Smith
&&
.SHORT TERM.../Issued 334 PM PDT Sat Jul 31 2021/
Tonight through Tuesday...We will remain in a pattern with
unusually high amounts of monsoonal moisture as the most notable
feature. The main item of interest in the short term is continued
isolated to scattered coverage of showers and thunderstorms over
portions of our area, with activity mainly during the afternoon
and evening hours...and mainly away from the coast and Douglas
County.
But, the first item of business was to extend the Heat Advisory
for valleys of Josephine, Jackson, and western and central
Siskiyou counties. It is now in effect through Monday evening,
with above normal high temperatures (by several degrees) trending
slightly higher...into the lower 100s, and also low temperatures
remaining much above normal (by about 15 degrees) in the mid 60s
to near 70. It will be a consistent story at the coast in the
short term with night and morning low clouds and fog accompanying
cool, near normal temperatures.
Showers today have been mainly east of the Cascades. Coverage and
intensity of showers will soon increase this afternoon into this
afternoon with scattered coverage expected over the higher terrain
of the Siskiyous and southern Oregon Cascades, extending across
south central Oregon. Coverage will be isolated over the rest of
the area, excluding the coastal counties and the Umpqua Valley.
In keeping with our unusually hot summer, this is a pattern with a
ridge that has built unusually far north along the Rockies with
substantial amounts of mid and high level moisture. The region of
highest moisture amounts is centered over Idaho, but its western
fringe is represented by the showers east of the Cascades. On
Sunday, the northward tracking stream of monsoonal moisture will
shift farther to the east. But, shortwaves will still be embedded
in a southerly flow aloft, the convective hindrance of high level
cloud cover will diminish, it will be a bit hotter, and 700-500mb
moisture amounts will still be supportive of isolated to scattered
showers and thunderstorms over roughly the same area as today.
The pattern will shift slightly on Monday with drier air moving
into California, but another late day episode of isolated showers
and thunderstorms from the southern Oregon Cascades eastward.
Model uncertainty increases for Tuesday. Heat will likely be at
similar levels as the previous few days, but the air mass will
start a drying trend. For now, we will follow the large majority
of model solutions (including the GFS and ECMWF) with a forecast
of a dry day. But, the NAM and SREF models are not quite as dry,
leaving open the door for the possibility of a few storms over
northern Klamath County and Lake County.
.LONG TERM.../Issued 334 PM PDT Sat Jul 31 2021/
Wednesday through Saturday...In a real shift, a
series of Pacific troughs will become the dominant drivers of our
weather. Temperatures are expected to be slightly less hot
Wednesday then trend down to near normal for the remainder of the
week. This change to an onshore flow pattern will bring gusty
afternoon and evening westerly winds, strongest east of the
Cascades. It will also bring an influx of low level moisture to
the coast with a slight possibility of light showers as early as
Wednesday afternoon, and a higher probability with a frontal
passage in the range of Friday night into Saturday morning.
Lastly, the timing and track of the troughs will be crucial in
determining the location and extent of any risk of thunderstorms.
This type of pattern usually creates a concern focused over
northeast Oregon and eastern Washington, more so than in our
area.
&&
.AVIATION...For the 01/00Z TAF Cycle...Over the coastal waters,
along the coast, and in the coastal valleys...Widespread IFR
cigs/vsbys in low clouds and fog will move back onshore Saturday
night, then burn back to the coast by Sunday afternoon. The lower
conditions will move back onshore Sunday afternoon and evening. The
lower conditions will then persist into Monday morning. One notable
exception: The coast and offshore up to a few miles from shore from
Cape Blanco to Cape Sebastian will remain mostly clear through
Sunday evening, and possibly through Sunday night.
Over the remainder of the area...VFR conditions will persist into
Monday morning, but there will be isolated to scattered
thunderstorms in the afternoons and evenings with gusty winds. Some
shower activity may persist through the night. There will also be
smoke from area wildfires, but not enough to drop visibilities below
VFR except possibly very near the fires.
&&
.MARINE...Updated 800 PM PDT Sunday 31 July 2021...Offshore high
pressure and a thermal trough near the coast will both persist
through Tuesday, bringing gusty north winds to the area. Steep
seas are expected south of Cape Blanco through Saturday night,
then will extend north into the outer waters from Florence to
Cape Blanco Sunday into at least Monday. Winds and seas will
remain highest south of Cape Blanco. Areas of marine stratus and
fog will also persist mainly from Cape Blanco northward and from
Brookings southward. North winds will weaken Wednesday as a front
approaches the coast. That front will move onshore sometime
Thursday or Friday.
&&
.FIRE WEATHER...Updated 800 PM Saturday 31 July 2021...Deep
southerly flow will continue to bring monsoonal moisture into the
area through the weekend. It will not be as hot over the inland
areas due to more cloud cover, but afternoon temperatures will still
be above normal, with much warmer than normal overnight low
temperatures.
Fire agencies have noted 14 new fire starts from lightning across
portions of southwest Oregon, with western Siskiyou County also
reporting numerous new fires. This occurred despite wetting rains
that occurred with many of these thunderstorms. So, any
thunderstorms are capable of creating new fires.
As for Sunday, instability remains relatively high, especially
along the Cascades and portions of fire zones 624, but NOT including
the Bootleg or Jack Fires. Therefore a Red Flag Warning is in
effect for these areas. See RFWMFR for details.
The location of thunderstorm activity will move east Monday and a
warmer, drier breezier, and more stable regime will develop in its
wake. A strong short-wave, as noted by ensemble means (this is
exceptional as ensembles tend to smooth out these features), will
approach northern California and southern Oregon Tuesday. This could
bring more hot, dry, windy, and unstable conditions, but no
thunderstorms.
Conditions will worsen on Wednesday as the short wave moves across
the area. Widespread Haines indices of 5 and 6 is expected over much
of the area and any existing fires could see some rapid spread,
especially east of the Cascades. Winds along the Winter Rim will
also be particularly breezy. There will also be a slight chance of
showers and thunderstorms over northern Lake and Klamath counties.
&&
.MFR WATCHES/WARNINGS/ADVISORIES...
OR... Red Flag Warning from 2 PM to 10 PM PDT Sunday for ORZ623-624.
Heat Advisory until 11 PM PDT Monday for ORZ024-026.
Red Flag Warning until 10 PM PDT this evening for ORZ621>625.
CA...Red Flag Warning until 10 PM PDT this evening for CAZ280.
Heat Advisory until 11 PM PDT Monday for CAZ080-081.
Pacific Coastal Waters...Small Craft Advisory until 11 PM PDT Monday for PZZ356-376.
Small Craft Advisory from 5 AM Sunday to 11 PM PDT Monday for
PZZ370.