Area Forecast Discussion
Issued by NWS Medford, OR

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573
FXUS66 KMFR 230417
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
917 PM PDT Sun Jul 22 2018

.DISCUSSION...A few thunderstorms are continuing across northern
California, although they should be diminishing in strength
relatively soon now that the sun has set. Have gone ahead and
cancelled the red flag warning for the Oregon fire weather zones,
and should be able to allow the red flag warning over the
California zones expire as planned.

The tricky part of the forecast is going to continue to be how
smoke affects both temperatures and thunderstorm chances. It
appears as if things become a little more unstable farther north
than today, but the smoke could hinder formation of thunderstorms.
The current forecast seems to be mostly on track, so have not
made any changes at this point. -Schaaf

&&

.AVIATION...For the 23/00Z TAFs...Wildfire smoke continues over
most of the inland valleys west of the Southern Oregon Cascades.
MVFR visibilities/cigs and local IFR conditions will exist near
these fires tonight and Monday, especially in Jackson and
Josephine counties.

At the coast north of Cape Blanco, IFR/LIFR conditions are expected
tonight into Monday morning, gradually improving to VFR in the late
morning on Monday.

East of the Southern Oregon Cascades and inland north of the fires,
VFR will prevail for the TAF period, though some light MVFR smoke is
possible closer to the Cascades and in the Umpqua basin. On Monday
afternoon , expect isolated thunderstorms to develop for areas east
of the Cascades.

Across northern California, expect mainly VFR conditions with areas
of patchy smoke. Gusty winds and local MVFR cigs/vis are possible
near thunderstorms this evening and Monday afternoon. -CC

&&

.MARINE...Updated 800 PM PDT Sunday 22 July 2018...The weakened
northerly winds will continue tonight. These winds combined with
the northerly fresh swell will generate steep hazardous seas for
small craft. Steep seas due to combined fresh swell and wind seas
are expected to linger through at least Tuesday, mainly affecting
the waters beyond 5 nm from shore and near capes. North winds will
strengthen slightly Monday afternoon into Monday evening for
areas south of Cape Blanco beyond 10 nm from shore and from Cape
Blanco to Gold Beach within 10 nm of shore. These winds may near
small craft advisory levels with occasional gusts to 25 kt. A
weaker thermal trough will keep these winds going through Tuesday.
Of note, for areas from Brookings south within 10 nm, weak winds
are expected through Tuesday. Winds and seas may increase slightly
Wednesday into Thursday. So small craft advisory level conditions
are possible through late in the week. -CC

&&

.PREV DISCUSSION... /Issued 240 PM PDT Sun Jul 22 2018/

DISCUSSION...The latest visible image shows marine stratus along
the coast, north of Cape Blanco. The upper flow has shifted
southwest and the smoke plumes from the fires in southwest Oregon
have shifted from north to northwest to south to southwest towards
Douglas County. Despite this, plenty of smoke remain in the westside
valleys and surface winds are expected to become west to northwest
pushing it back in later this afternoon. This is being supported by
the high res smoke models. This could put a cap on the amount of
heating, but we should still end up a few degrees warmer than
yesterday at this time for these areas.

Meanwhile, the mid level clouds that were moving up from the
southwest earlier this morning continue to move northeast into
southeast Lake and Klamath Counties. This has brought increasing
instability over northern California and southern Lake County as
evidenced by increasing and building cumulus. So far one lightning
strike has been observed in Shasta County, south of our forecast
area. Meanwhile a cell has popped up northwest of Tennant, but no
lightning. Yet.

There`s good agreement most of the instability will be confined in
northern California from the Marble Mountains east-northeast into
Medicine Lake and Northwest Modoc County. Earlier the thinking was
there could be an isolated thunderstorms in the Siskiyous, but the
short range high res and ensemble high res models keep it out.
Instability is only marginal at best in this area and there`s little
or trigger. So isolated thunderstorms were removed.

Additionally, the model soundings show a dry sub-layer and there`s a
chance any little if any rain that comes from any storms will not
reach the ground gusty winds are possible near any storms. Therefore
the concern is heightened for new starts from lightning.

Isolated storms will fizzle out later this evening with dry
conditions overnight. The GFS hangs on to some QPF tonight in
Northern California while the NAM and short range high res models do
not. Given the mid level moisture is decreasing, loss of daytime
heating and little or no trigger, suspect the GFS is overdone, so
we`ll go with dry conditions tonight.

The threat of thunderstorms exist again Monday and Tuesday
afternoons as a piece of the monsoonal moisture from the Sierra
Nevada moves north into our area. It looks like the best chance for
thunderstorms will be in the Sierras. Still could not rule out
something in northern California and east of the Cascades late in
the afternoon and evening both Monday and Tuesday. This is because
mid level moisture will increase along with marginal instability and
weak embedded shortwaves.

The rest of the week from Wednesday onward into next weekend will be
dry with no threat of thunderstorms. However Wednesday will be a
close call. The GFS shows marginal instability and moisture
Wednesday afternoon and evening in the Marble Mountains towards
medicine Lake, but the NAM is drier and not as unstable. Therefore
the forecast for Wednesday could change should future model runs
show more instability.

The other concern this week will be temperatures. Were looking at
hot temperatures just about everywhere with near or triple digit
values for the Rogue, Illinois, Applegate, Shasta, Scott and Klamath
River valleys starting tomorrow and lasting into next weekend. There
will be some, but not a lot of relief in the overnight period Monday
and Tuesday night. Earlier we had a heat watch out, but due to a
combination of slightly cooler overnight lows and smoke putting a
cap on the amount of heating during the day. Decided to issue a heat
advisory. Please see NPWMFR for more details. The coast will be the
only place where temperatures will be near normal with a persistent
weak onshore flow. -Petrucelli

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Heat Advisory from 11 AM Monday to 11 PM PDT Wednesday for
     ORZ024-026.

CA...Red Flag Warning until 11 PM PDT this evening for
     CAZ280>282-284-285.
     Heat Advisory from 11 AM Monday to 11 PM PDT Wednesday for
     CAZ080>082.

Pacific Coastal Waters...Small Craft Advisory for hazardous seas until 11 PM PDT Tuesday
     for PZZ350-356-370-376.

$$

BMS/CC/MAP



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