FXUS66 KMFR 212142

Area Forecast Discussion
National Weather Service Medford OR
242 PM PDT Fri Jul 21 2017

.DISCUSSION...The latest water vapor image shows a west to southwest
flow over our area and this will continue for at least the next 24
to 36 hours. Also of note is a weak upper low centered near
35N/135W. This is of particular importance because this feature is
expected to move east to northeast over the next 36-48 hours setting
up off the northern California coast by late Sunday morning. it will
be at this point where we will be looking at an increased threat for
thunderstorms. More on that to follow below. Until then we'll be
dealing with much of the same this afternoon through Saturday. A
continuation of clear, dry and hot weather away from the coast.
Tonight, the models suggest the marine stratus will get close to the
coast, but the flow will become offshore which should keep the
marine stratus offshore. Could not rule out localized and shallow
low clouds near and at North Bend tonight, but confidence is not
high enough to put cloud cover in the forecast.

Saturday will be much like today, except we'll be 3-5 degrees warmer
on average for inland areas. Temperatures at the coast will be
similar to today, except for the south coast, in particular
Brookings where the Chetco effect will set up allowing temperatures
to push into the mid to upper 80s or even higher. Current forecast
high for Brookings is 86 for Saturday, but would not be surprised if
this end up higher.

Saturday night into Sunday evening, the aforementioned upper low will
be lurking off the Northern California coast. The upper low will tap
into some monsoonal moisture with the upper flow becoming southeast
and this in combination with increasing instability and hotter
temperatures will bring an increased risk for thunderstorms Sunday
afternoon through Sunday evening. Right now, we think the storms
will first develop over the Cascades, Siskiyous and portions of the
eastside, then expand in coverage by Sunday evening. Since the 700mb
flow is expected to be easterly and 700-500mb flow southeast, there
is a chance that any storms that form over the Cascades and
Siskiyous could drift into portions of the westside, including the
Rogue, Illinois, Applegate and eastern Douglas County. Right now we
have a slight chance for these areas, but the details on this could
change, so watch for updates on this. Of note, the models show
greater instability in the northern Cascades so we have a chance for
thunderstorms there.

The change for thunderstorms diminishes Sunday night, but they still
exist mainly along the Cascades, so kept a slight chance mention,
Elsewhere, we have a slight chance of showers.

The upper low will remain just off the California coast Monday and
will tap into more moisture and instability will be greatest along
and east of the Cascades. Thus we think Monday could be more active
not only in terms of thunderstorms, but there may be a better chance
for wetting rains as PWATS are expected to increase in these areas
Monday afternoon. Also winds at 700mb and 700-500mb layer will be
lighter, therefore storms could be slow movers.

The upper low will move inland Tuesday afternoon, but the models
differ with the position of the low which could affect where the
best chance for and coverage of thunderstorms will be. For example
the ECMWF has the upper low moving over our area while the GFS has
the low moving near Redding California. It's likely the details on
the progression and track of the upper low as it moves inland will
change in the coming days, thus the details on the thunderstorm
coverage could change.

After Tuesday, the threat for thunderstorms shifts east of our area
and we'll see a stable southwest flow return. Temperatures will cool
down slightly Wednesday and Thursday, then could heat up again late
next week into next weekend. -Petrucelli


.AVIATION...21/18Z TAF CYCLE...VFR conditions are being observed
almost everywhere, with some lingering low clouds near Brookings
that should clear off this afternoon. Confidence is moderate that
ceilings will make it into KOTH tonight based on mixed signals in
the guidance, but if they do, conditions will be LIFR because of a
shallow marine layer. Over the remainder of the area...VFR
conditions will prevail through Saturday morning. SK


.MARINE...Updated 200 PM PDT Friday 21 July 2017...Winds and seas
will remain relatively calm through this morning as high pressure
builds to the west. The thermal trough will strengthen late this
afternoon through the weekend, and by Saturday morning will be
producing gusty north winds and steep seas south of Cape Blanco.
Winds and seas will strengthen even more by Saturday night and
Sunday, with Gales and very steep and hazardous seas developing over
much of the area. The thermal trough will persist into early next
week, though it will weaken Tuesday and Wednesday. -FB/BPN


.FIRE WEATHER...Updated 200 PM Friday 21 July 2017...There are 3
main items of interest in the forecast: easterly winds with moderate
ridge top humidity recoveries tonight including a moderate to
strong Chetco effect near Brookings on Saturday and Sunday, hot
inland temperatures Saturday through Monday, and especially a slight
chance to chance of thunderstorms Sunday afternoon through Tuesday
evening including the possibility of overnight thunderstorms.

Humidity recoveries will be at their worst tonight, but not so poor
as to warrant issuing any warnings. Northeast winds will become
breezy tonight and will be at least as strong on Saturday night.
But, recoveries will not be as poor due to increasing dew points.

The hotter temperatures and increasing moisture are accompanying the
approach of an upper trough to the northern California coast. This
is a classic pattern for the development of thunderstorms in our
area. Mid-level moisture will increase from the southeast. The
slight chance of thunderstorms begins on Sunday afternoon...except a
chance in the vicinity of Crater Lake. There is also a slightly
enhanced risk for the higher terrain from Modoc County northwestward
across south central Oregon fire weather zone 624. Thunderstorms on
Sunday are likely to be neither especially dry or especially wet.
There is a lingering slight chance of showers and thunderstorms
overnight into Monday with a shortwave disturbance or two likely to
track across our area.

The probability and coverage of thunderstorms are expected to be at
a peak on Monday evening. The increase in instability will be
accompanied by an increase in moisture. There is substantial model
difference in which areas will have the highest probability of
thunderstorms, but general agreement on widespread coverage. With
the upper low forecast to still be centered over the northern
California coast, there is a high probability of isolated nocturnal
thunderstorms Monday night. The risk will continue on Tuesday though
perhaps with the highest probability east of the Cascades as the low
pushes slowly eastward. Temperatures will return to close to normal
at mid-week with a drier westerly flow behind the trough. -DW




Pacific Coastal Waters...Small Craft Advisory from 5 AM Saturday to 11 PM PDT Monday for
Hazardous Seas Warning from 5 AM Sunday to 11 PM PDT Monday for
Small Craft Advisory from 5 PM Saturday to 5 AM PDT Sunday for
Small Craft Advisory from 5 AM Saturday to 5 AM PDT Sunday for
Gale Warning from 5 PM Saturday to 11 PM PDT Monday for PZZ376.
Small Craft Advisory from 5 PM Saturday to 11 PM PDT Monday for

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