Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 241552
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
852 AM PDT Mon Jul 24 2017

.DISCUSSION...The latest visible image shows marine stratus along
the coast north of Cape Blanco and in portions of the Coquille
Basin. Meanwhile mid and high clouds are moving north along and
east of the Cascades. Isolated thunderstorms are already
developing just south and southeast of Modoc County and there`s a
chance some of these storms could make there way into Modoc County
this morning. So kept a slight chance of storms there. Made a few
adjustments to the forecast this morning and afternoon mainly to
add detail to the timing and location of thunderstorms.

The latest HRRR shows convection initiating in Modoc County later
this morning, then we`ll see more action in Modoc, Lake and
Klamath County this afternoon and evening. The northern Cascades
could also get into the action later this afternoon. There have
been some reports of starts in Modoc County from yesterdays
afternoon and nights storms, but mostly grass and brush. The main
focus will be thunderstorms the next few days as the upper low
lurks just off the California Coast into Tuesday afternoon, then
slowly moving inland Tuesday night through Wednesday.

Please see previous discussion below for details on the weather
for the next few days. -Petrucelli


&&

.AVIATION...24/12Z TAF CYCLE...Satellite imagery at 0500 PDT shows
low clouds hugging the coast line but have not moved inland. Short
term model bring IFR cigs into KOTH around 13Z. confidence is only
moderate at this point, but with N-NW winds there is a reasonable
chance these low clouds could be pushed inland. Strong north winds
will occur again at the coast and even KRBG could see gust to 20kt
this afternoon.

Thunderstorms will be an issue this afternoon, especially east of
the Cascades. KLMT could see CB in the area around 22Z but there is
enough instability to keep thunderstorm clouds in the area until
around midnight. At KMFR thunderstorm clouds could be visible around
23Z as well but upstream winds are quite light even if steering flow
is from the SE which is favorable for MFR. There may instead be
several storms in the Yreka area but confidence is not high that
they will have enough forcing over the Siskiyous. /FB


&&

.MARINE...Updated 830 AM PDT Monday 24 July 2017...A thermal trough
will remain near the coast along with offshore high pressure. Gusty
north winds and steep seas will continue with gales south of Cape
Blanco through Tuesday. Very steep and hazardous seas will continue
to nearly all areas by late tonight due short period fresh swell.
The thermal trough will weaken 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.


&&

.PREV DISCUSSION... /Issued 339 AM PDT Mon Jul 24 2017/

DISCUSSION...An upper level low pressure system spinning off the
coast of northern California will continue to shape the weather
across the area over the next few days, bringing the potential for
several rounds of lightning and increased fire risk through mid
week. The position of the low is favorable for a continued influx
of warm, moist air today and Tuesday. Due to the expected slow
movement of the upper low, SW Oregon and northern California will
remain in diffluent flow aloft with several disturbances expected
to rotate around the low and into the region. One such disturbance
moved through yesterday afternoon and evening, resulting in about
300 cloud to ground lightning strikes, primarily from the Modoc
northward into eastern Klamath and Lake Counties. Most of the
strikes were in Lake County. That disturbance shifted northeast of
the area earlier (around 2 AM PDT), and while there can be a
shower or isolated thunderstorm here or there this morning,
overall, most locations will be dry.

That will change as another disturbance is forecast to arrive in
the same general area (Modoc/Lake/Klamath Counties) around peak
heating this afternoon, but perhaps a touch earlier than
yesterday. Latest HRRR runs are showing convective initiation
around 20-22Z (1-3 PM PDT) with scattered storms expected to
continue into the evening east of the Cascades. Models are
showing a bit wider coverage area of storms today than yesterday,
with some extending westward to the Cascades this evening, most
likely after 00z. Model CAPE fields show lots of instability even
here in the Rogue Valley (>1000 J/KG). While we can`t completely
rule out an isolated storm on the west side, model soundings also
show significant CIN (convective inhibition) and this will be the
main limiting factor to any convection west of the Cascades.
Storms today should be a bit wetter with PW values everywhere in
the 0.75-1.00" range this afternoon, increasing to 1.00-1.25" by
this evening. Showers and isolated thunderstorms could linger
through the nighttime hours, but most should dwindle again with
the loss of instability.

The upper low will ever so gradually move into northern California
near Fort Bragg by Tuesday morning. This will keep us under a
southeast to east flow of moist air likely generating another
round of showers and thunderstorms Tuesday afternoon/evening. Once
again, the idea is that most of the thunderstorm activity will be
across NE California and over the East Side. Due to more of an
easterly component to the flow, this is probably the best chance
for a shower or thunderstorm to move out over the Rogue Valley,
but again CIN is high, so we may miss out again. Nocturnal
showers/isolated thunderstorms are still possible Tuesday night.

On Wednesday, the upper low will eject northeastward across
NE California and into southern Oregon where it will open up into
a trough. There is still a fair amount of instability, so we
expect one more round of showers and thunderstorms from the
Cascades eastward, mainly in the afternoon/evening.

A primarily dry southwest flow aloft will take hold Thursday into
next weekend which will end the thunder threat for a while.
Overall, temperatures this week will be above normal with daily
highs in the mid to upper 90s in the Rogue Valley. -Spilde


&&

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.

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Red Flag Warning from 2 PM this afternoon to 11 AM PDT Tuesday
     for ORZ624-625.
     Fire Weather Watch from Tuesday morning through Tuesday evening
     for ORZ617>625.
     Red Flag Warning from 2 PM this afternoon to 11 AM PDT Tuesday
     for ORZ617-622-623.

CA...Fire Weather Watch from Tuesday morning through Tuesday evening
     for CAZ280-281-284-285.
     Red Flag Warning from 2 PM this afternoon to 11 AM PDT Tuesday
     for CAZ284-285.

Pacific Coastal Waters...Small Craft Advisory until 11 PM PDT this evening for
     PZZ350-356-370-376.
     Small Craft Advisory for hazardous seas from 11 PM this evening
     to 11 AM PDT Tuesday for PZZ350-356.
     Hazardous Seas Warning until 11 AM PDT Tuesday for
     PZZ356-370-376.
     Hazardous Seas Warning until 11 PM PDT this evening for PZZ350.
     Gale Warning until 11 AM PDT Tuesday for PZZ376.

$$


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