Scientific Forecaster Discussion
fxus66 kmfr 201841
Area forecast discussion
National Weather Service Medford or
1140 am PDT Sat Jul 20 2019
The aviation section has been updated...
an update to the forecast has been made for sky coverage today,
through late this evening.
the main reason for this morning's update is to better depict the
marine layer, especially over Coos County. Additionally, we're
likely to see cumulus this afternoon over and near the coastal
range of Coos County, and the marine layer is likely to begin to
form along and near the coast this evening.
Cumulus is also expected over northern California and portions of
the east side this afternoon into early this evening due to 700mb
moisture advection. In fact, there's a little across the east side
and norcal this morning. Some of this moisture will drift over
the interior west side this evening through tonight before getting
shunned back to the east side Sunday morning, but will only result
in some clouds.
Today we'll continue to focus on refining the thunderstorm
forecast for Sunday afternoon and evening, as we are expecting
some activity in our forecast area, primarily over portions of
Modoc, Klamath, and lake counties, though portions of western
Siskiyou County could see an isolated thunderstorm, too. Btl
Aviation...for the 20/18z tafs...VFR conditions are expected to
prevail through this afternoon. Gusty north winds will impact the
coast, including koth this afternoon and evening, with gusts peaking
in the 25 to 30kt range. Fairly typical Summer north to northwest
breezes will also affect inland sites this afternoon and evening.
This evening into tonight low clouds with patchy fog are expected to
reform along and near the coast and into the lower Umpqua basin,
generally west of Roseburg. These low clouds with patchy fog are
expected to be of the IFR variety, 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. Btl/jrs
Marine...updated 855 am PDT Saturday 20 July 2019...the thermal
trough along the coast will gradually weaken through this weekend.
Gale force winds have diminished this morning. High and very steeps
seas will generate hazardous conditions to all crafts south of Gold
Beach today. Seas will remain hazardous to smaller crafts in all the
waters through Sunday. Then a period of calmer winds and seas will
develop Monday and 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
Previous discussion... /issued 340 am PDT Sat Jul 20 2019/
Discussion...today will start with northeast winds over much of
our forecast area and the upper level trough shifting east, and a
zonal flow pattern over the region this morning. As the upper
level ridge then deepens well offshore today, the thermal trough
will shift inland today. The combination of the early offshore
flow and the thermal trough moving inland will bring a four to
five degree warm up today from yesterday. Increasing southerly
winds at ridge levels will begin to feed moist unstable air at the
ridge tops to mid levels of the atmosphere into Sunday.
As the upper low deepens off of Vancouver Island Sunday the upper
ridge moves over the Great Basin and the upper trough starts to
approach the coast. This will increase the feed of moist an
unstable air mass into the region at the ridge to mid levels and
by Sunday afternoon we will see an area of instability, combining
with sufficient mid level moisture, extending from northern
Klamath County to the southeast into the Warners in Modoc County
Sunday afternoon into the evening. The nam12 is also indicating an
area of slight chance thunderstorms in central to western
Siskiyou County for this time frame as well. The GFS is not
indicating this, but the nam12 does a decent job in his area and
south into the yolla bolly's so have a little more confidence in
spite of the model differences.
As we head into the Monday time frame the are of instability
shifts north and east to the Cascades from The Crater lake area
northward, but the mid level moisture is very low and not high
enough to allow any decent vertical development. On the fringes
of eastern Modoc and lake counties the southwest steerage flow is
strong enough to detach any development from the ridges and any
cells that develop should move out of the forecast area before
putting anything down. -Sven
Fire weather...updated 200 PM PDT Friday 19 July 2019...north to
northeast winds will develop tonight and persist through Monday,
bringing moderate relative humidity recoveries to the west side ridges. Inland
temperatures will warm to five to ten degrees above normal values
Sunday and humidities will drop. There will be a slight chance of
thunderstorms over lake and Modoc counties Sunday afternoon and
evening, and lightning from any storms that do form will be dry.
Inland temperatures will cool down to 0-5 degrees above normal
values Tuesday and remain there through Thursday. It may heat up
again Friday into next weekend.
Pacific coastal waters...
- Small Craft Advisory until 11 PM PDT Sunday for pzz356-376.
- Hazardous seas warning until 11 PM PDT this evening for