Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 181018
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
318 AM PDT Fri Aug 18 2017

.DISCUSSION...

Warming temperatures, smoke from the fires and a thermal trough
over the coast are the three big stories for this weekend. To
start, there is a pretty nice tilted ridge off the Pacific
extending into our warning area today and into the weekend. This,
along with the thermal trough along the coast, will bring back
warmer than average temperatures across the region, specifically
south of Douglas county into northern California Friday.

The flow around the thermal trough really kicks into high gear on
Saturday and Sunday with 850mb flow near Curry and Josephine
county reaching around 25 to 30 kts at 40 to 60 degrees on the
NAM. Therefore, we should see gusty winds along ridge tops close
to the coast and in western parts of Siskiyou county.

The other thing to note is that the Chetco effect should be in
full effect through the weekend with upper level winds going right
down the Chetco river. Kept the Brookings temperatures warmer
than all of our guidance and thinking lower 90`s is probable for
highs through Sunday.

The other talking point is thunderstorm potential as a cutoff low
positions it`s self off the southern California coast on Sunday
night. This low will sit there through Thursday. Models are
pulling up some moisture and initiate convection along the Sierra
Nevada range and northern Sacramento valley Monday night and
Tuesday night. We can see this low destabilizing the atmosphere
in northern California, it just seems pretty unlikely farther
north near Douglas and northern Lake and Klamath counties for both
days. Bottom line is fell pretty good about the slight chance in
Siskiyou and Modoc, but not as good about thunderstorms farther
north for both Monday night and Tuesday night.

Finally, a more well developed low and cold front will push
towards the Oregon coast near the end of the forecast period next
week Wednesday night into Thursday morning. This should give us
some thunderstorms on the east side and the upslope regions of the
Cascades. Right now we put in slight chance, but that will
probably be upgraded in the future as forecasters gain more
confidence of a more strongly forced event. -Smith

&&

.AVIATION...18/12Z TAF CYCLE...Over the coastal waters and along the
coast north of Cape Blanco...Areas of IFR cigs offshore will move
onshore this evening...persisting into Saturday morning. The lower
conditions may make it into some coastal valleys...but overall it
will not make much inland progress. Over the remainder of the
area...VFR conditions will prevail into Saturday with the exception
of smoke-reduced visibilities. Wildfire smoke will degrade
visibility and could create partial terrain obstruction, affecting a
broad section of the forecast area. -Stockton



&&

.MARINE...Updated 200 AM PDT Friday 18 August 2017...High pressure
centered offshore and a thermal trough along the coast will support
moderate to strong north winds and steep to very steep seas through
the weekend. Conditions will at least be hazardous to small craft,
but warning level winds/seas will occur beyond 5 NM from shore from
Gold Beach south. Winds and seas will be highest during the
afternoons and evenings each day through Sunday, easing a bit during
the overnight and early morning hours. The thermal trough will
weaken early next week, and winds and seas will diminish then.
-Stockton


&&

.FIRE WEATHER...Updated 300 AM Friday 18 August 2017...High
pressure centered over the eastern Pacific and low pressure over the
Northern Rockies will continue to maintain NW flow aloft through
Friday. At the surface, this will lead to NE winds and moderate to
poor relative humidity recoveries each night and morning through
Sunday morning along and near the coastal ranges, specifically in
FWZ`s 618, 619, 620, 621 and 280. Winds this morning are not very
strong...remaining well below Red Flag Warning criteria. Winds look
to be stronger Saturday night though.

Otherwise, it will be dry and hot each afternoon, driest and hottest
this afternoon. Dryness and instability on Friday will lead to a
moderate to high probability of active wildfires becoming plume
dominated as the Haines Index reaches 5 at most places. It should be
noted that there isn`t much difference between a Haines 5 and 6 once
a fire starts putting up a column of sufficient size - that is
pushing up to around 10,000 feet off the ground. Thus, large
uncontrolled fires stand the greatest chance of being problematic
Friday afternoon and evening.

Early next week the wind flow aloft will become more southerly,
leading to increasing probabilities of thunderstorm activity Monday
into Tuesday. -Stockton

&&

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

CA...None.

Pacific Coastal Waters...Small Craft Advisory until 11 PM PDT Sunday for
     PZZ350-356-370-376.
     Gale Warning until 11 PM PDT Sunday for PZZ356-376.

$$

CZS/JRS/NSK



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