Scientific Forecaster Discussion
fxus66 kmfr 201602
Area forecast discussion
National Weather Service Medford or
902 am PDT sun may 20 2018
Update...clouds have overspread a significant portion of the
forecast area this morning, as a result of a rather robust marine
layer and the inland progress of the upper level trough.
Temperatures will be cooler today for nearly all areas, but there
will also be more instability across northern California and the
east side. West of the Cascades, the influence of marine air and
the extensive cloud deck are likely to keep conditions much too
stable for anything more than weak localized convection.
That being said, the forecast appears right on track this
morning, and no changes are necessary. For more information, see
the previous forecast below. -Bpn
Aviation...20/06z taf cycle...MVFR cigs continue along the coast
and have made it inland to krbg. Cigs in the Umpqua basin should
improve to VFR by late morning. Along the coast, there may be a
brief period of clearing this afternoon as gusty winds help scatter
out the marine layer. However, the lower conditions will return this
Inland, VFR prevails for the next 24 hours. Scattered showers east
of the Cascades will increase in coverage and intensity this
afternoon. Thunderstorms are also possible this afternoon, mainly in
northern California and east of the Cascade crest. This could lead
to temporary MVFR and gusty and erratic winds. /Br-y
Marine...updated 830 am PDT Sunday 20 may 2018...light seas and
moderate northerly winds will persist through this morning. North
winds increase this afternoon, leading to steep wind driven seas.
Winds will be strongest south of Cape Blanco and seas will be
steepest south of Gold Beach. Winds and seas peak Monday afternoon
as the thermal trough peaks in strength, gradually subsiding
late Tuesday afternoon into Wednesday. /Br-y/sven
Previous discussion... /issued 348 am PDT sun may 20 2018/
Scattered showers continue this morning on the Eastside as this
upper level energy lingers. We'll see this trough start to dig in
more this morning. Convection is expected east of the Cascades
and in northern California again. Saturday night we saw around 20
- 30 lightning strikes, but we're expecting a bit more given this
low coming in Sunday afternoon and evening is stronger. Cape
varies from 400-1000 j/kg depending what model you look at and
bulk shear is around 25 to 30 knots. Not expecting anything severe
to come out of that environment, but some stronger wind gusts are
still possible in northern California.
The digging low will eventually give way and wander south into
Southern California. We'll see subsiding air, so our County Warning Area should
dry out substantially into Monday. There is a slight chance for a
storm or two in Modoc County.
Tuesday is pretty interesting as models want to initiate some more
moist convection in an environment with 10 knot 0 - 6 km shear.
This basically means that any updraft that gets going should get
knocked down fairly quickly. Cape is also quite low only around
400- 600 j/kg. If we do see lightning, northern California is the
most likely place Tuesday.
High temperatures Tuesday should be in the upper 80's with 90's
possible in the northern California valleys west of the Cascades.
Weak offshore flow develops and this is usually a sign of a decent
to significant warm up. Temperatures will be around 10 to 15
degrees above average for late may.
Finally, Wednesday is another interesting day as models are
generating significant amounts of potential instability along
mountainous terrain with 1000 j/kg to 2400 j/kg of cape. It's
also interesting how there is yet again a complete lack of upper
level flow with 0-6km shear only around 5 knots. It's still a
little puzzling where the models are coming up with significant
more moisture compared to Tuesday. The only explanation that we
can think of right now is the low coming up from Southern
California is adding a little more moisture to the equation in
addition to the stronger upper level divergence as the two lows
approach from the Pacific. Both the European model (ecmwf) and GFS are pretty
bullish with moist convection Wednesday afternoon and evening.
This seems reasonable, but storms should be short lived yet again
because of the complete lack of shear.
Long term Wednesday through Saturday...
/issued 246 PM PDT Sat may 19 2018/
There is a high degree of uncertainty in the extended forecast.
It all depends on where a persistent cutoff low sets up and the
models disagree on this point. However, the 00z GFS and ec
ensembles help to clarify this issue. The 19/00z GFS/ec solutions
actually agreed fairly well with the position of the cutoff low,
at least in the Wednesday/Thursday time frame. The 19/06z GFS had
a very different solution. Now the 19/12z GFS is trending back to
the ec position. Also, the ec ensemble members are fairly tightly
bunched with respect to the position of the cutoff while the GFS
shows a wide range.
Given all this, have decided to favor the ec solution due to greater
consistency, both run-to-run and within the same run. The ec
solution keeps the cutoff closer to the Medford County Warning Area and this means a
forecast of showers and thunderstorms through the extended period.
On Wednesday, the cutoff will be near 40n 135w with south to
southwest flow aloft over the forecast area. The cutoff will move
east-southeast to around 37n 130w, and the flow aloft will come
around to southeast to south. On Friday morning, the low will be
just off the central California coast with stronger southeast flow
aloft over the Medford County Warning Area. Then the low will move very slowly east
to be over central California Sunday morning. By this time the flow
aloft over the Medford County Warning Area will be easterly.
This pattern will advect a steady flow of moisture into the area
through the extended period. Precipitable water values are not
available from the ec model, but the GFS shows quite a bit of
moisture Wednesday into Thursday...1.00 to 1.25 inches over the west
side and 0.75 to 1.00 inches over the east side. Friday is about
0.25 inches drier, but the higher values return Saturday.
As the low moves closer, cooling aloft will enhance the convective
environment. The ec 850mb lifted indices lack resolution, but they
still depict significant lifting over from the Siskiyous south and
Cascades east, especially in the afternoons and evening. The
surface based lifted indices show a similar pattern with instability
spreading to the west side near the Cascades in the afternoons and
If this forecast pans out, it's going to be pretty busy around here
for the second half of the week.
OH yeah, temperatures. It's going to be warm Wednesday, getting
warmer through Friday. Inland Wednesday highs will be 5-10 degrees
above normal, rising to around 10 degrees above normal Friday.
Saturday will be near 5 degrees cooler. Along the coast, highs will
be near normal through the extended period.
Pacific coastal waters...Small Craft Advisory until 5 PM PDT Tuesday for pzz356-376.
Small Craft Advisory for hazardous seas until 5 PM PDT Tuesday