Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Medford or
447 PM PDT Thursday Jul 28 2016

Updated aviation section

Discussion...little has changed over the past few days. We'll see a
continuation of hot and dry weather for inland locations with triple
digit readings for interior Westside valleys. Current surface
analysis shows the thermal trough positioned along the coast with
light onshore flow for the South Coast, including Brookings. You
don't have to go far inland before temperatures really heat up. For
example offshore flow is occurring at Red Mound with a temperature
of 93 degrees. Skies will remain clear over most locations through
Friday morning. The exception will be along the coast, north of Cape
Blanco where marine clouds will hug along the coast and like the
past few nights, will move inland into portions of the Coquille
basin. Also the marine stratus could sneak up into Brookings.

As we have mentioned in previous discussions the past couple of
days, our main focus will be the potential for thunderstorms or lack
thereof mid to late Friday afternoon and again Saturday afternoon
and evening. Right now, the models put most of the instability in
the Marble, Salmon, Shasta Trinity Alps and points northeast into
Medicine Bow Peak and southern Cascades Friday afternoon. Even then
the amount of instability is marginal at best and there is little or
no trigger. Additionally, mid level moisture is lacking around
700mb, but there is more around 500mb. Given the above mentioned
scenarios, if any storms were to develop, they would be isolated at
best. However, fire danger is high fuels are receptive in these
areas, so it would not take much to get something going. Also models
soundings show an inverted "v" which suggest any storms that develop
could be mostly dry and produce gusty outflow winds.

For now were keeping Friday night dry. However the NAM and GFS show
a weak vort lobe moving east to northeast into northern cal and the
trigger could be strong enough for isolated storms to continue into
the overnight period. The GFS in particular has been showing what
looks like convective feedback in the quantitative precipitation forecast field during the overnight
period, but the NAM does not. In contrast the ec has shown little or
no trigger the past few runs. The bottom line is were not confident
we'll get anything, but this is something that will need to be
watched carefully.

The focus for thunderstorms will shift east to northeast into
Southern Lake, Klamath, northeast Siskiyou and northwest Modoc
County as the aforementioned vort lobe moves east to northeast as it
gets caught up in the westerly flow as the upper trough in western
b.C. Begins to move south. The environment should be similar to
Friday, therefore we'll keep a slight chance mention. Gusty winds
could be a concern Saturday afternoon and evening east of the
Cascades.

After Saturday the threat for thunderstorms is pretty much over
with. An upper trough will swing north of the area Sunday morning
bringing a dry stable westerly flow over the area with the threat
for storms shifting well east of US. Temperatures will continue to
cool, but will still remain near or above normal. Another upper
trough will swing north on Monday bringing further cooling and
continued dry conditions. After Monday, weak troughiness will remain
over the area with dry weather and near normal temperatures.
-Petrucelli

&&

Aviation...29/00z taf cycle...VFR conditions will continue through
Friday morning at inland locations. IFR to LIFR marine stratus is
around 5-10 nm from shore south of Cape Blanco but hugs the coast
from Cape Blanco northward. These low clouds will move into the
Coquille Valley this evening and persist until around 16-18z Friday
morning before dissipating. Low stratus and/or fog may also
develop bear Brookings and Gold Beach late tonight before
dissipating Friday morning. Moderate to strong and gusty north
winds are expected for most areas along the coast and offshore,
though with a decrease during the early morning hours. Cumulus
buildups are expected over the higher terrain of northern
California into south central Oregon on Friday afternoon and
evening with a slight chance of thunderstorms. -Spilde

&&

Marine...updated 230 PM PDT Thursday, 28 July 2016...
a thermal trough along the coast and high pressure offshore will
continue to produce moderate to strong north winds and steep to very
steep seas through the weekend and into next week. Conditions will
be most hazardous north of Cape Blanco through Friday evening.
South of Cape Blanco the most hazardous conditions are expected to
be through Saturday evening. Northerly winds will be nearly as
strong Sunday into Sunday evening. Winds may drop below gale force
across the waters on Monday but winds are likely to strengthen again
on Tuesday then remain moderate to strong through the middle of the
week. -Spilde

&&

Fire weather...updated 200 PM PDT Thursday, 28 July 2016...
this spell of hot temperatures will peak on Friday. But, of greater
note is that weak instability will develop on Friday as very weak
low pressure aloft moves closer to the northern California coast and
the ridge over the Great Basin slides eastward. This will produce an
increase in 700-500 mb moisture and slight cooling aloft. Enough to
warrant continuing a slight chance of thunderstorms over much of
northern California...nudging into the southern Oregon Siskiyou
Mountains and far southern Klamath County. Coverage of thunderstorms
will not be more than isolated...most likely to be just a few cells
that develop. But, any thunderstorms that do form will not produce
much rainfall. There will be two tongues of moisture...one from the
Trinity Alps/Marble Mountains in extreme southwest Siskiyou County
east-northeast to northeast Siskiyou County/far southern Klamath
County, and another from the Sierra Nevada into far southeast
Modoc County. The probability is lower but the development of a
thunderstorm or 2 is not out of the question for the remainder of
the southern half of Klamath County and the southern half of Lake
County. We have a 5% to 10% probability of thunderstorms
continuing in that same area overnight Friday night into Saturday
morning as a weak shortwave moves through. On Saturday, there will
again be a slight chance of thunderstorms for much of the same
area as a weak upper trough and its associated surface trough move
inland. The focus for thunderstorms looks to shift slightly
northeast on Saturday...into Southern Lake County. A Fire Weather
Watch will not be issued due to the expected limited coverage of
thunderstorms...but with high fire danger, hot and dry conditions,
and limited rainfall from any thunderstorms, the probability of
any lightning starting fires is high.

Besides the convective risk, wind speeds this weekend look to be
slightly stronger than normal. They will be strongest with northerly
winds at the coast and westerly winds in lake and Modoc counties.

Another continued concern is moderate to locally poor humidity
recoveries (40-60%) with light to moderate east winds (gusts 10-20
mph) across the upper slopes and ridges of northern California and
the east side tonight through Friday morning. These winds will then
generally persist through the weekend into early next week with only
a meager improvement in humidity recoveries.

A stronger trough will move into the Pacific northwest on Monday.
Gusty winds will low humidity on Monday afternoon may reach critical
levels east of the Cascades. Conditions during Tuesday through
Thursday are likely to resemble those from early this
week...stable...breezy and not quite as hot though still hotter than
normal. -Dw

&&

Mfr watches/warnings/advisories...
or...
California...none.

Pacific coastal waters...Small Craft Advisory until 11 PM PDT Sunday for
pzz350-356-370-376.
Hazardous seas warning until 11 PM PDT Sunday for pzz356-376.
Hazardous seas warning until 11 PM PDT Friday for pzz370.
Gale Warning until 11 PM PDT Sunday for pzz376.

$$

Mas/map/dw

National Weather Service Glossary of Abbreviations