Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 210005

Area Forecast Discussion
National Weather Service Medford OR
505 PM PDT Sat Jul 20 2019

Updated AVIATION Section

It`s another quiet afternoon with temperatures currently running
1 to 3 degrees warmer than this time yesterday, except in
Brookings, where temperatures last hour were 15 degrees cooler
than yesterday at this time.

One item to note this afternoon is a bit of stratus that has
developed in the Charleston and Cape Arago areas, and to a lesser
degree, in a small area southwest of Reedsport along the coast
and between Denmark and Cape Blanco. In these areas, a breezy to
windy northerly flow is providing enough lift on the terrain to
result in some cloudiness. This is likely to ebb and flow through
this afternoon, before the marine layer increases along and near
the coast and into the Umpqua Basin this evening through Sunday

Another item of note is some cumulus development across NorCal
and from the Cascades east. This is associated with increasing
moisture in the mid-levels, mainly at the 700MB level. This is a
sign of things to come for Sunday, possibly lingering into Monday
and Tuesday, as a shower and thunderstorm threat moves northward
into the area.

More specifically, we`re expecting isolated thunderstorms Sunday
afternoon and evening across portions of Modoc, Lake, and Klamath
counties, and a slight chance of thunderstorms across part of
Siskiyou County from the Trinity Alps, Scott Mountains, and Scott
Valley areas. The feature to watch shows up well as a low
pressure swirl on the GOES15 and GOES17 Water Vapor Channels this
afternoon west of Baja California and the Southern California
coast west of LA and San Diego. Model guidance suggests that a
shortwave trough at 500MB on the front side of this feature,
related vorticity advection, and increasing atmospheric moisture
should be enough to result in isolated thunderstorms in the
mentioned areas tomorrow afternoon and evening.

What needs to be watched is model trends with this feature, as
there isn`t as much data coming in from that area as there is
from overland areas. In other words, the models might not have the
best read on this feature at the current time, and the evolution
of it in the model fields in the next 24 hours will be telling. As
of now it appears that Sunday will bring a relatively minor
amount of lightning, on the order of 20 to 200 strikes, but the
vorticity maximum moves across our area Sunday night into Monday
morning. Thus, while the chance is currently small (<10%), it`s
not impossible that there could be some lingering thunderstorm
activity then east of the Cascades.

Monday into Tuesday this feature edges eastward leaving a slight
chance of thunderstorms across the east side, mainly over
southeastern Lake and Modoc counties.

A slight cool down Tuesday is likely to be followed by another
warming trend for the remainder of next week. There may be a
threat of thunder again from the Cascades east by next weekend.


.AVIATION...For the 21/00Z TAFS...For inland areas, VFR conditions
are expected to continue tonight through most of Sunday. However,
isolated thunderstorms may form over the higher terrain of the east
side (generally east of Klamath Falls) Sunday afternoon/evening.
There is a very slight chance of a thunderstorm near the
Scott/Salmon Mountains of southern Siskiyou County.

Along the coast and the coastal valleys, gusty north winds of 25 to
30 kt will ease this evening. Low clouds with patchy fog are
expected to redevelop along and near the coast, including North
Bend, and also into the lower Umpqua Basin west of Roseburg.
Overall, expect conditions to be IFR, primarily between the hours of
03Z and 18Z. Increasing onshore flow Sunday morning means break-up
time for the low clouds and fog is likely to be a little later than
it was today, but should occur by early afternoon. BTL/Spilde


.MARINE...Updated 230 PM PDT Saturday 20 July 2019...The thermal
trough along the coast will continue to weaken this weekend.
Only Small Craft Advisory winds are expected tonight and Sunday.
Seas will remain hazardous to smaller craft in all the waters
through Sunday. However, they will drop below advisory criteria
north of Cape Blanco by Sunday afternoon. Calmer winds and seas will
cover all waters by Monday morning and continue through Tuesday

A strong thermal trough will begin to build towards the end of next
week.  Gale Force winds and hazardous seas will probably occur
towards the end of the week when this strong thermal trough is in
place. -Sargeant


.FIRE WEATHER...Updated 200 PM PDT Saturday 20 July 2019...North to
northeast winds will develop tonight and persist through Monday,
bringing moderate RH recoveries to the west side ridges. Inland
temperatures will warm to five  degrees above normal values Sunday
and humidities will drop. The thunderstorm forecast is interesting
as we feel confident with the forecast for isolated thunderstorms
east of the Cascades on Sunday.  Western Siskiyou County is a little
more uncertain, but we decided to keep isolated thunderstorms in the
forecast for that area Sunday evening.  The thunderstorms east of
the Cascades and in Siskiyou County will probably be dry. There is
also a fair amount of potential instability that persists Sunday
night just west of the Cascades. One model continues the trend of
keeping the Cascade crest and areas just west of the crest dry for
Sunday night.  We continue to see ingredients supporting
thunderstorm development in that area Sunday night, but the weather
models just don`t want to bring thunderstorms into the forecast.

Otherwise, the slight chance for thunderstorms in Lake and Klamath
continues Monday afternoon and Monday evening as upper level energy
keeps moving through the area.  We also have high confidence in a
warming and drying trend next weekend as models suggest high
pressure will build into the Pacific Northwest. -Smith




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


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