Area Forecast Discussion
Issued by NWS Medford, OR

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000
FXUS66 KMFR 140444
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
844 PM PST Wed Dec 13 2017

.UPDATE...With high pressure remaining in control tonight through
Thursday and the forecast well on track, will not be making an
update to the near term grids this evening. Still expect fog in
the valleys west of the Cascades with areas of freezing fog, which
could result in frost and some slippery spots for the Thursday
morning commute.

New 00z models this evening, however, have continued their
stronger/wetter trend for Friday afternoon/evening`s frontal
system. So, have further increased precipitation probabilities
and associated precipitation amounts then. The front will move
through fast with timing of precipitation mostly Friday afternoon
west of the Cascades and Friday evening over the east side. Post-
frontal showers could still linger briefly in the evening along
and west of the Cascades. The main impact from this front will be
to finally mix out the steep inversions that have been over the
area for the last week or so. With respect to precipitation -
liquid amounts for most areas will be less than 0.10 of an inch,
but could be isolated amounts of around 0.20 of an inch, near the
Cascades/Foothills near and/or north of Diamond Lake. Snow levels
will drop quickly and there will be a changeover from rain to snow
near the Cascades later Friday afternoon and over the east side
Friday evening. Even so, snow amounts there will likely be less
than an inch and mainly above 5000 feet. This system will exit the
area to the east late Friday night. Details from Saturday through
next week can be found in the previous discussion below. -Spilde

&&

.AVIATION...14/00Z TAF CYCLE...Going with a persistence forecast,
current LIFR conditions are expected to continue throughout the TAF
period for portions of the Umpqua Basin, including KRBG, and the
lower portions of the Rogue Valley around Grants Pass. LIFR
conditions at KMFR are expected to return around sunrise, then
clear by late morning.

VFR conditions will prevail elsewhere, including the East Side,
coast, and coastal waters. Similar fog coverage and duration is
expected tonight into Thursday morning with a similar large-scale
pattern persisting. Sven

&&

.MARINE...Updated 400 PM PST Wednesday 13 Dec 2017...Relatively
light winds for this time of year will persist through the rest of
the week. North winds will persist and will be strongest south of
Cape Blanco, but they are expected to remain below small craft
advisory level. Moderate west swell will build this afternoon
through Thursday and subside Friday, but seas should remain below 10
feet. A weak front will move onshore Friday, followed by offshore
high pressure Saturday. North winds will increase Saturday and could
be strong enough to reach small craft advisory levels in the
southern waters. -Sven

&&

.PREV DISCUSSION... /Issued 318 PM PST Wed Dec 13 2017/

SHORT TERM...Today through Saturday, Dec 16th, 2017...
Little change has occured in the past 24 hours at most locations
today, with a strong inversion remaining across the area. Some mid
elevation ridges on the west side in the 3,000 to 5,000 foot
elevation range have indicated temperatures in the 60 to 70 degree
range, with highs in the 50s all the way up to 8,000 feet on Mount
Shasta and Crater Lake Rim. Meanwhile, cooler conditions remain in
the valleys, with fog and low clouds continuing in and around
Roseburg and Grants Pass.

In the next 24 hours the surface pressure gradient increases from
about 10.5 millibars to 14.5 millibars from Lake County to the
coast. This is likely to result in some increase in east and
southeast wind across the forecast area, mainly at higher
elevations and to the lee of the Cascades and Siskiyous.
Temperatures have raised a little for tonight and, moreso, for
tomorrow due to this increase expected in winds. There is an
outside chance some that some valley locations such as Ashland
could reach 60F Thursday afternoon, though we`re going with the
more likely value of 56F.

Model guidance, mainly the GFS, has trended wetter with the cold
front due in on NW flow Friday afternoon and evening.
Precipitation probabilities have been expanded some based on this
recent trend. Snow levels have also been adjusted more toward wet
bulb zero, as models indicate most of the precipitation with this
system being behind the cold front. Altogether, amounts are
likely to be light, probably on the order of a tenth of an inch or
less of water. The effect of this system will be to clean out the
air in the valleys and change the temperature regime to more
normal, with temperatures cooler in the mountains than in the
valleys.

We expect that the high pressure ridge will rebuild in a weakened
state this weekend, but it`s likely to take until Sunday or
Monday for inversions to become established enough that air
stagnation will be a concern again. BTL

LONG TERM...Sun (12/17) through Wed (12/20)...
It looks like we chose wisely yesterday as the 13/12Z European
Center (EC) model has trended toward the more consistent GFS
solutions in the long range. While there are still significant
differences in the details, the overall pattern evolution looks
similar between the two models through the current forecast
period. Not only that, but both deterministic solutions are
becoming more similar to their respective ensemble solutions,
which is good news.

There is the potential for a significant pattern change late next
week as ensembles show the long wave ridge that has kept us dry
seemingly forever retrogrades offshore and opens the door to more
northerly flow aloft over the PacNW. This pattern would lend itself
to cold air intrusions into the Great Basin which would affect our
east side locations. This pattern would tend to be dry and keep cold
air bottled up east of the Cascades except if and when shortwaves
buckle the flow enough to drive cold air westward and produce any
over water trajectory to moisten things up. Ensembles give us
confidence in the long wave pattern (e.g. cold air on the east
side), but they average out important details like shortwaves (e.g.
cold air moving westward and snow potential). Deterministic models
can often help us with the important details, and that gets better
as we get closer in time.

Bottom line: confidence is growing in a westward shift in the upper
ridge axis and a trend toward colder weather, especially on the east
side toward the latter half of next week. The details on whether any
cold air gets over to the west side or shortwaves that could produce
snow are uncertain, but should get better with time. Prior to that,
weak upper ridging will likely keep our area dry from Sunday through
early Wednesday. Valley inversions are likely to produce stagnant
air in the valleys again during that timeframe. The forecast has
been trended in this direction with the removal of pops, adjustments
to temperatures, and the addition of night time fog. Guidance all
shows a frontal system bringing some chance for precipitation into
the area later Wednesday, and pops have been raised. Although it`s
beyond the current forecast period, after Wednesday, the ridge
likely retrogrades bringing colder air to the east side. We`ll have
to wait on any more specific details than that. -Wright

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Air Stagnation Advisory until 10 AM PST Friday for
     ORZ023-024-026-029>031.

CA...None.

Pacific Coastal Waters...None.

$$

MAS/BTL/TRW/SBN



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