Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 240327
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
827 PM PDT Sun Jul 23 2017

.SHORT TERM...An upper low will remain located just off the
Northern California coast tonight and Monday. This will bring
moisture into the area which combined with instability and forcing
will result in showers and thunderstorms. Meanwhile a strong
thermal trough at the surface will continue tonight and tomorrow,
resulting in strong gusty north winds along the coast in the
afternoon and evenings and strong gusty northeast winds over the
coastal mountains.

Isolated to scattered thunderstorms have developed east of the
Cascades this evening and storms occurred earlier over Modoc
county. Currently storms are located over far eastern Klamath
county and over Lake county. These storms have had gusty winds
associated with them along with periods of rain and the potential
for small hail. Storms have not developed over the Southern Oregon
Cascades. Models continue to show additional showers and
thunderstorms possible through tonight east of the Cascades.
Showers and thunderstorms may also develop over the Southern
Oregon Cascades late tonight.

On Monday, models remain on track with scattered storms developing
from the Southern Oregon Cascades east and over Modoc county. The
GFS and SREF also continue to show a risk for isolated to
scattered thunderstorms west of the Cascades, especially in
western Siskiyou county and northward into portions of southwest
Oregon.

The upper low is forecast to remain in place Tuesday, drifting
slightly northeast. So expect another day of isolated to scattered
thunderstorms. On Wednesday, the low moves inland as an upper
trough and shifts eastward. This will result in the focus for
thunderstorms becoming more over eastern portions of the area,
mainly east of the Cascades. There remains some uncertainty on how
much and where storms will develop as this low moves inland though
due to model variability on the track and timing of this
disturbance as it moves east-northeast.

&&

.AVIATION...24/00Z TAF CYCLE...VFR conditions currently prevail over
the area right now, including the waters, and will continue to do so
throughout the TAF period. The exception will be along the coast
where IFR conditions are expected to return this evening and persist
into Monday morning.

Isolated thunderstorms have developed over eastern Modoc and
southeast Lake counties. Near term models keep thunderstorm activity
restricted to this area, progressing it north into Lake county
during the evening and diminishing by midnight in this area.
However, convection will still be possible overnight, focused along
the Cascades. Expect gusty outflow winds in the vicinity of any
thunderstorms. Monday will feature more widespread thunderstorm
activity. /BR-y

&&

.MARINE...Updated 800 PM PDT Sunday 23 July 2017...A thermal
trough will remain near the coast along with offshore high
pressure. This combination will produce gusty north winds and
steep seas, with gales continuing south of Cape Blanco through
Monday night. Very steep and hazardous seas will develop south of
Reedsport this evening, expanding to nearly all areas by late
tonight. The thermal trough will persist into early next week,
though it will weaken Tuesday and Wednesday. Winds will diminish
slightly, but seas will remain elevated as fresh swell remains in
the area. Hazardous seas and small craft conditions will likely
remain in the area through at least Wednesday. /CC


&&

.PREV DISCUSSION... /Issued 227 PM PDT Sun Jul 23 2017/

DISCUSSION...The latest water vapor image clearly shows the upper
low off the California coast which has been advertised to happen the
last couple of days. The upper low is already tapping into some
monsoonal moisture and the result is mid level altocumulus clouds
that are moving from south to north into our area. Typically
altocumulus clouds are a indicator of increasing instability and
moisture and in some cases could be a precursor to thunderstorm
activity. PWATS are higher with this mornings sounding showing 1.02
inches.

As previously mentioned in earlier discussions, the short range high
res HRRR model continues to show first storm development showing up
in Alturas County later this afternoon and migrating north into Lake
and southeast Klamath County early this evening. This has been
consistent with the last several model runs (every hour).
Instability will be greatest over the northern Cascades this
afternoon. 700mb moisture is not as impressive, but PWATS are
expected to increase this afternoon, so still could not rule out
isolated storms late this afternoon.

The upper flow becoming southeast and this in combination with
increasing instability and hotter temperatures will bring an
increased risk for at least isolated thunderstorms mid to late
Sunday afternoon through Sunday evening. Right now, we think the
storms will first develop over the Cascades, Siskiyous and portions
of the eastside, then could drift into portions of the westside,
including the Rogue, Illinois, Applegate and eastern Douglas County
towards Sunday evening. This is because the 700mb flow will be
easterly and 700-500mb flow southeast. So we`ll keep a slight chance
for these areas, but confidence on exact location of storms in these
areas remain low. Meanwhile, the models show greater instability
along the northern Cascades Sunday afternoon and expanding south
Sunday evening, so the chance for thunderstorms are higher.
Confidence is not super high that nothing will happen over the west
side where CAPES exceed 1000 J/kg but inhibition should also remain
somewhat high. Storms may be quite dry over the east while anything
to develop across the west will have much more moisture to work
with. Activity tonight should diminish toward midnight but there`s a
good chance for isolated storms to continue overnight mainly along
the Cascades and portions of the eastside. Of note, SPS lightning
progs show a higher probability of 10 strikes stretching west into
the Rogue Valley. This looks reasonable given the models are showing
some weak shortwaves moving north to northwest around the upper low.
Therefore have expanded the slight change coverage to include the
Rogue Valley. Elsewhere, we have a slight chance of showers.

The models are in general agreement Monday and Tuesday will be the
most active days in terms of thunderstorm coverage and chances. The
upper low will remain just off the California coast, but it will tap
into more moisture on Monday. Instability is still greatest along
and east of the Cascades, but there is some evidence portions of the
westside, including the Rogue and Illinois Valleys could see more
frequent storms. Also there is a better chance for wetting rains as
PWATS are expected to increase with a swath of 1+ inches in these
areas Monday afternoon. Also winds at 700mb will be lighter,
therefore storms could be slow movers.

Nocturnal storms again are possible along with lingering overnight
showers and some remaining cloud cover Monday night as the upper low
slowly moves onshore and multiple shortwaves rotate ahead of it.
Right now it`s unclear how the area of storms shapes up, but we
could see a fair amount of activity.

Tuesday will be another active day with showers and thunderstorms
over most of the area as the upper low slowly drifts northeast into
Northern California. 700 mb winds are expected to be on the light
side (between 5-10 kts) so we`ll once again be dealing with slow
moving storms. Moisture is plentiful, therefore the potential for
wetting storms are high with locally heavy rain possible where
storms are at.

The threat for  nocturnal storms exist again Tuesday night as the
upper low moves over our area. Right now, we think the focus for
storms will be over most locations Tuesday evening, then shifting
east into Lake and eastern Klamath and Modoc Counties late Tuesday
night, but details on this could change, so watch for updates.

We do have some lingering thunder chances into Wednesday with
wavering confidence on the low actually moving fully out by then.
However the latest GFS ensembles mostly support a dry day with storm
activity likely off to the east.

Dry weather will return Thursday and could last through next
weekend. The models have come into agreement showing the ridge
building westward into our area in this time period with
temperatures heating back up Friday into next weekend with triple
digit readings in some inland westside locations and lower to mid
90s east of the Cascades. -Petrucelli

FIRE WEATHER...Updated 100 PM Sunday 23 July 2017...The main
concern over the next several days continues to be thunderstorms.
The current Red Flag Warning will be expanded into more of zone 625
this afternoon.  The Watch currently in effect for Monday afternoon
and evening will also be expanded in area as well as extended
through 18z Tuesday due to forecast nocturnal thunderstorm activity.
Finally, a watch as been issued for Tuesday afternoon and evening.

The probability and coverage of thunderstorms will peak on Monday
evening and Tuesday evening.  The increase in instability will be
accompanied by an increase in moisture and rainfall with the storms
could be near wetting rain.

Thunderstorm potential covers a much larger area than described in
the Fire Weather Warning/Watch. The area reflects forecasted LALs of
3 or more. The thunderstorm threat will diminish Wednesday afternoon
and evening as the upper low moves out to the east of the area and
westerly flow develops over the area. This however will likely lead
to breezy west winds during the second half on next week. -JRS

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Red Flag Warning until 11 PM PDT this evening for
     ORZ617-623>625.
     Red Flag Warning from 11 AM Monday to 11 AM PDT Tuesday for
     ORZ617-622>625.
     Fire Weather Watch from Tuesday morning through Tuesday evening
     for ORZ617>625.

CA...Red Flag Warning until 11 PM PDT this evening for CAZ285.
     Red Flag Warning from 11 AM Monday to 11 AM PDT Tuesday for
     CAZ284-285.
     Fire Weather Watch from Tuesday morning through Tuesday evening
     for CAZ280-281-284-285.

Pacific Coastal Waters...Small Craft Advisory until 11 PM PDT Monday for PZZ350-356-370.
     Gale Warning until 11 PM PDT this evening for PZZ356-376.
     Hazardous Seas Warning until 11 PM PDT Monday for
     PZZ350-356-370-376.

$$

CC/CC/CC



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