Frost Advisory 28, NWS Medford, OR

Area Forecast Discussion
National Weather Service Medford OR
1215 PM PST Wed Dec 6 2017

.UPDATED AVIATION DISCUSSION.

.DISCUSSION...The moderately strong south winds about 2000 feet
above ground is strong enough to prevent fog and log clouds from
forming in the Rogue Valley and elsewhere in northern California.
As a result, we have updated the sky grids to reflect more
sunshine today. Offshore flow at Brookings brought overnight lows
into the 50s. We also updated the lows for tonight in Curry
County. Expect the moderate South winds to continue into Thursday
but winds will be lighter Friday. Could see return of fog and low
clouds then.

&&

.AVIATION...06/18z TAF CYCLE...The main concern will be wind speed
and possibly directional shear at Medford through tonight. Surface
observations not far from the surface show gusty southeast winds and
guidance suggest this could continue through tonight. The TAF at
Medford has been updated to reflect this thinking.

IFR cigs will continue in the Umpqua Basin, including Roseburg
through at least midday. The TAF at Roseburg shows VFR cigs
developing around 21z, but there's a chance improving conditions
could be delayed by an hour. This will have to be watched closely,
so watch for updates on this. Elsewhere VFR conditions will continue
through the TAF period. The exception could be Medford late tonight.
The TAF shows IFR cigs developing around 11z, but confidence is not
high these conditions will develop. This is because the models are
showing continued southeast flow not far from the surface which
could be sufficient enough to prevent low clouds and fog from
developing. Later shifts will need to take another look at this.
-Petrucelli

&&

.MARINE...Updated 200 AM PST Wednesday, Dec 6th, 2017...
Unusually strong high pressure centered to our northeast will
continue to produce northeast winds across the coastal waters
through this morning. The high will shift to the southeast later
today into Thursday, resulting in mostly east winds this afternoon,
and then south winds Thursday and Friday. Meanwhile, long period
west-northwest swell will remain low and gradually subside. High
pressure is expected to remain in control for some time, keeping
conditions relatively calm through at least the middle of the
month. -BPN/BTL

&&

.PREV DISCUSSION... /Issued 220 AM PST Wed Dec 6 2017/

DISCUSSION...The pattern continues with high pressure residing
over the Pacific Northwest through next week. Rain chances will
remain negligent through the forecast period and into the extended
period as long as the high pressure remains. Current observations
support the continuation of the frost advisory which remains in
effect across inland portions of Coos County and extending toward
the outlaying areas of Coos Bay. Despite the overall forecast for
ultimately stable conditions, several challenges with this
forecast cycle remain.

The main challenge for each day will be winds and temperatures and
how it relates to the inversion that sets up each night. For
example, the inversion yesterday broke earlier than expected and
temperatures were warmer than forecast. As the inversion sets up
today, fog and low stratus will fill into the West Side valleys.
Some patchy fog may occur east of the Cascades, but overall
conditions are much drier, and east winds will help preclude
lasting visibility effects. These breezy easterly and
southeasterly winds do extend across much of the land portions of
the forecast area which could help break up the inversion
relatively early tomorrow. If the inversions break tomorrow
morning (as opposed to in the afternoon), the high temperature
forecasts will be closer to what our forecasts (and guidance) is
showing (mid to upper 40s) across the West Side. Warmer
temperatures are expected along and near the coast as the east
winds help to regulate the temperatures.

That being said, there's a chance that the winds will not be able
to mix below the inversion and the strongest winds will stay
aloft or along ridge tops. If this occurs, then temperatures could
be cooler than what is in the current forecast in the West Side
valleys. The tricky part that comes into play is going to be
whether or not the winds funnel into the Rogue Valley. The current
high-resolution guidance is showing that the winds will mix down
to the higher terrain near the south end of the Rogue Valley. This
means that Ashland and Talent could be somewhat breezy tomorrow
and tomorrow night. This wind could even funnel toward Medford,
although there is low confidence in Medford winds. These winds
would temporarily improve the air stagnation concerns across the
Rogue Valley. But with the low confidence in the winds and high
confidence that the air stagnation concerns will return as soon as
the winds stop, have decided to continue the Air Stagnation
Advisory across much of the forecast area, including the Rogue
Valley. See NPWMFR for details.

After Friday, the winds aloft will calm and the inversion, fog and
stratus will return with more emphasis. The inversion will likely
take longer to break out of, and thus we are expecting overall
cooler daytime temperatures. This pattern is expected to continue
into next week despite a slight westward movement of the high
pressure ridge as an area of low pressure moves around the high,
completely missing the southern half of the Pacific Northwest.
-Schaaf

AVIATION...06/12z TAF CYCLE... Increasing east winds across the
area will result in some low level turbulence at many TAF
locations in the next 24 hours. Low clouds and fog are also
expected to be lesser in duration and coverage. IFR ceilings in
the valleys of the Umpqua Basin, to include KRBG, are expected to
lower overnight resulting in LIFR fog until early afternoon. For
airfields further south at Grants Pass and Medford fog and low
clouds are again expected, but could take until around sunrise to
develop at KMFR. More recent forecast guidance indicates that the
going 06Z TAFS might be a bit on the pessimistic side, so, if the
forecast errs, conditions are likely to be better than forecast.
There is high confidence that southeast winds will result in
earlier clearing Wednesday than was observed Tuesday. BTL

MARINE...Updated 200 AM PST Wednesday, Dec 6th, 2017...
Unusually strong high pressure centered to our northeast will
continue to produce northeast winds across the coastal waters
through this morning. The high will shift to the southeast later
today into Thursday, resulting in mostly east winds this afternoon,
and then south winds Thursday and Friday. Meanwhile, long period
west-northwest swell will remain low and gradually subside. High
pressure is expected to remain in control for some time, keeping
conditions relatively calm through at least the middle of the
month. -BPN/BTL

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Frost Advisory until 9 AM PST this morning for ORZ021.
     Air Stagnation Advisory until 4 AM PST Wednesday for
     ORZ023-024-026-029>031.

CA...None.

Pacific Coastal Waters...None.

FJB/FJB/FJB