Area Forecast Discussion
Issued by NWS Medford, OR

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000
FXUS66 KMFR 180313
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
813 PM PDT Tue Oct 17 2017

.DISCUSSION...

Made a few changes to the short term forecast again by adding fog
along the coast in addition to areas of drizzle. Have noticed in
the past when the NAM puts spotty QPF along the coast, Brookings
and North Bend seem to see drizzle in the dense fog. Aside from
that, modified the low temperatures in populated areas tonight to
the lows in the NAM MOS guidance bullitens, which seem to be
quite accurate in this pattern. For more information on the
upcoming storm systems, read the discussion below. Also note
the Hydrology section in this AFD. -Smith

&&

.AVIATION...18/00Z TAF CYCLE...Expect MVFR to return to the Coos
County coast this evening with IFR at OTH again around 0500.
Expecting overcast skies to develop over the Umpqua valley tonight
with the possibility of fog over RBG.  Left fog out for now as fog
barely formed last night.  -Smith

&&

.MARINE...Updated 200 PM PDT Tuesday 17 October 2017...Relatively
mild conditions will continue tonight into early Wednesday as high
pressure remains over the area. Things are on track for the next
front to bring stronger South winds to the marine waters Wednesday
afternoon. Confidence is approaching moderate for south winds to
reach gale Wednesday night and Thursday from Reedsport south to
around Port Orford. Winds are expected to decrease below gale
Thursday afternoon and turn more westerly behind the cold front.

Long period and large northwest swells (around 20 feet at 17
seconds) build into the waters late Thursday into Friday behind the
front, creating dangerous surf and bar conditions. The front is
expected to move back as a warm front, and winds are expected to
increase again Friday night and Saturday with guidance showing Gale
Force Winds again. /FB

&&

.HYDROLOGY...Updated 200 PM PDT Tuesday 17 October 2017...
There will be some hydrologic concerns with the Wednesday night
into Friday system. Currently, soils are dry and rivers are low,
but there will be enough rain to create possible debris flows near
recent burn scars. The river levels will not be an issue itself,
but the debris may obstruct river flows at times, creating
elevated river levels upstream of the block at first, then
downstream of the block if and when it breaks free. If you come
across a flooded road, find an alternate route. Remember to turn
around, don`t drown.

Debris floating down the river combined with high surf may create
dangerous bar conditions near the Port of Brookings. -Stockton

&&

.PREV DISCUSSION... /Issued 216 PM PDT Tue Oct 17 2017/

DISCUSSION...17/12Z NAM/GFS/EC in.

The northern hemispheric map is a bit amorphous this afternoon...
just a blob of cool air centered at the pole with a hard-to-
discern wave number...four maybe? However, this will change later
this week as the pattern transitions to a progressive wave
pattern. This will correspond with a transition to an active
weather pattern for the Pacific Northwest.

For now, flat ridging is in place over the west coast. A short
wave embedded in the flow has moved onshore but it hasn`t amounted
to much. No precipitation has been reported at this time. The main
frontal band remains offshore but it doesn`t look impressive
either. There are some gusty winds over the ridges and east side
though with peak gusts around 25 mph.

The ridge, what there is of it, will break to the east of the CWA
Wednesday and southwest flow aloft will increase over the area as
a long wave trough deepens offshore. This will open the storm
door, and short waves ejecting from the offshore trough will
barrel through it.

The first few waves will be weak and the impacts will be similar
to the system moving onshore today. However, a much stronger one
will move onshore Thursday into Thursday night, and this will
support a wet and windy front which will move onshore just ahead
of the trough Thursday.

The mid shift summarized the impacts of this system well and they
are listed below:

*Impacts: Ponding on roadways, debris flows near burn scars near
 the coast and western Siskiyou County.
*Slippery, snow-covered roads over the higher passes in the
 Cascades near Crater Lake and Diamond Lake. Light snow on highway
 140 near Lake of the Woods.
*Travel may become difficult for high profile vehicles due to
 strong southerly winds along the coastal headlands, Shasta
 Valley, and east of the Cascades.
*Isolated trees may fall near burn scars due to high winds.

Snow levels will start off above 8000 feet on Thursday and will
drop to 4000 to 4500 feet by Friday evening as the precipitation
comes to an end. Some of the higher passes could see some moderate
snow, particularly near Crater Lake and Diamond Lake; but lower
passes like highway 140 near Lake of the Woods will only see light
snow. Siskiyou Summit may see a few flurries on Friday, but it is
more likely that the precipitation will end before it changes
over to snow.

Precipitation totals will be significant, especially at the
coast. Rain rates will be less than a quarter of an inch an hour,
but some coastal areas could see 1.5 inches in 6 hours and some
inland areas will get 1 inch in 6 hours. Rivers are still low and
soils are dry from the summer, but a few impacts could still be
felt, particularly for coastal rivers.

This will be one of the first wind events in our forecast area
for the season. The strongest winds will occur at the coast
Wednesday night into Thursday with Shasta Valley and East Side
winds ramping up Thursday into Friday morning. This could create
some travel difficulties, especially for high profile vehicles.
There may be some treefalls due to the winds, particularly near
recent burn scars.

The long wave upper trough will move onshore Friday, and short
waves will continue to move through the trough. This will support
widespread shower activity into Friday. The offshore upper level
ridge will amplify as it approaches the coast, and this will cause
the front to lift back north as a strong and quite wet warm front
Saturday, followed by a weaker trailing cold front Saturday night
into Sunday.

The offshore ridge will be quite strong by this time just off the
coast. Low level flow will turn easterly Sunday night into Monday,
so the area will dry out quickly at that time. Easterly flow will
weaken Monday into Tuesday, but it will remain dry with no
incoming systems initially. A weak front may bring some
precipitation to the north coast and Umpqua Basin Wednesday.

-Stockton




&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...None.

CA...None.

Pacific Coastal Waters...Gale Watch from Wednesday evening through Thursday morning for
     PZZ350-356-370-376.
     Hazardous Seas Warning from 11 AM Thursday to 11 PM PDT Friday
     for PZZ350-356-370-376.
     Small Craft Advisory from 2 PM to 11 PM PDT Wednesday for
     PZZ350-370.

$$

CZS/JRS/FJB



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