Area Forecast Discussion
Issued by NWS Medford, OR

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000
FXUS66 KMFR 130417
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
817 PM PST Tue Dec 12 2017

.UPDATE...Persistence has been the best forecast for the past
several days and will continue to be for the next 48 hours. So,
have made some adjustments to the min/max temperature grids to
reflect only minor variations from what has been observed during
the past 24-48 hours. The rest of the short term forecast is
unchanged. Dense fog and freezing fog advisories are out for some
of the valleys west of the Cascades, details of which can be
viewed at NPWMFR.

Precipitation will be hard to come by with the weak front
expected to come through the ridge on Friday, but it should cause
some mixing to temporarily break the large inversions of the past
week. Models are showing the ridge rebuilding this weekend (albeit
not as strongly as it was this past week) with the storm track
off to our north. So, we may still be dealing with some low
clouds/fog into early next week in the valleys west of the
Cascades. More details about the extended range forecast follows
in the previous discussion below. -Spilde

&&

.AVIATION...13/00Z TAF CYCLE...Fog and freezing fog will return to
the same areas this evening and persist into Wednesday morning. VFR
conditions will prevail elsewhere, including the East Side, coast,
and coastal waters. Dense freezing fog and very low stratus will
return to the Medford area late tonight. breaking out to VFR again
mid to late morning. Sven


&&

.MARINE...Updated 800 PM PST Tuesday 12 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 over the
southern waters, but they are expected to remain below small craft.
Moderate west swell will build late Wednesday night through Thursday
and subside Friday, but the general consensus is seas will 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 in the southern
waters. -Sven

&&

.PREV DISCUSSION... /Issued 244 PM PST Tue Dec 12 2017/

SHORT TERM...

Persistence is the key yet again looking ahead until Friday. Went
ahead and issued a freezing fog advisory and dense fog advisory
for the exact same areas as last night. The only difference with
tonight vs last night is some upper level energy is moving through
and might destabilize the atmosphere a bit, but the inversion is
looking like it will be quite strong yet again with lows in the
lower 20`s in the valleys. Would like it to be noted that driving
can be hazardous with freezing fog. Co-workers reported a crash on
McAndrews road in Medford this morning, which may have been due
to freezing fog and associated slippery conditions.

Also not a huge impact, but added haze to the weather forecast
since it`s looking rather hazy outside. This is probably a
combination of the pollution in addition to the smoke from
wildfires and perhaps some other burns around the region.

The temperature forecast should be completely persistent. The only
other possible change is slightly cooler for daytime highs given
the increase in haze and the possibility of some high clouds
getting into the area Thursday. RH around 300 mb is showing some
brief periods around 60-70%, so high clouds are not out of the
question on Thursday.

Finally, a trough moves through Friday and models have been pretty
consistent bringing it through, the only question now is the
moisture. Thinking the latest dry trend seems reasonable and the
NAM is, yet again, the wettest solution. So went with a slight
chance for rain with only a few hundreths of accumulation around
terrain. The other thing we wanted to note is that the mixing
height on Friday evening will reach 3500 feet, which will meet
criteria to cancel air stagnation.

-Smith

LONG TERM...Sat (12/16) through Tue (12/19)...The models are
generally in poor agreement in the extended period (both run to
run and model to model) , and this results in lower than normal
confidence, at least in the later part of the extended forecast.
Looking back over the last four runs of the European Center (EC)
model, it has uncharacteristically shown the worst run to run
consistency, especially regarding the potential formation of a
cutoff low over or offshore of the PacNW early to mid next week.
Over these runs, the EC has gone from a progressive trough moving
quickly into the Rockies early next week to a cutoff low over the
Great Basin to a huge cutoff low over California and finally, at
12Z this morning, a huge low way off California. For this reason,
I have little confidence in the EC.

The GFS had an offshore cutoff low solution earlier as well, but the
last three runs have been largely consistent in showing a trough
coming ashore and then cutting off over the PacNW by Tuesday. It`s
issues are largely timing rather than strength or placement. It
should also be noted that while I haven`t looked at previous runs of
the Canadian, it doesn`t look much like either the GFS or EC. As a
result, I haven`t changed the forecast in the extended much, but
where I have, it`s been toward the GFS. That said, it wouldn`t be
surprising (or unprecedented) to have the models do a complete 180
on the next run and render all this moot.

Despite the differences, there is at least agreement that our area
will be dry this weekend into Monday with the exception of far
northern areas that may catch some light rain at times from a warm
front well to our north. Of course, we will still have night time
fog issues over the valleys. Regarding Tuesday and beyond: there is
a chance for some cooler and wetter weather, especially if the
currently more consistent GFS ends up being right, but again,
confidence in this is quite low.

-Wright

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Air Stagnation Advisory until 10 AM PST Friday for
     ORZ023-024-026-029>031.
     Dense Fog Advisory from midnight tonight to 9 AM PST Wednesday
     for ORZ023.
     Freezing Fog Advisory from midnight tonight to 9 AM PST
     Wednesday for ORZ024-026.

CA...None.

Pacific Coastal Waters...None.

$$

MAS/CZS/TRW/SBN



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