Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 171727
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
927 AM PST Sun Dec 17 2017

The Marine section has been updated...

.UPDATE...Current observations are showing clear skies across most
of northern California and portions of Southern Oregon with high
clouds to the north and east. Some lingering fog is still
occurring across the Umpqua Basin, but where fog has not formed,
temperatures have become very cold. That being said, the current
forecast looks mostly on track today, and will make no changes.

The overnight inclusion of valley snow in the forecast seems
reasonable, and will be the main focus of this forecast cycle
today. Have included the previous discussion below which details
the though process. -Schaaf

&&

.MARINE...Updated 900 AM PST Sunday, 17 Dec 2017...
An update was made to the forecast to increase winds and waves a bit
for today south of Cape Blanco. However, small craft advisory level
conditions are very unlikely to occur. High pressure centered
offshore and a coastal thermal trough will continue to support
somewhat gusty north winds south of Cape Ferrelo into this evening.

The trough will then weaken and winds gradually back to southerly.
Winds are then expected to increase Monday night ahead of a cold
front. The front will sweep through the waters Tuesday into Tuesday
evening with high seas and advisory to gale strength winds likely.
High and steep swell will follow Tuesday night into Wednesday night.
A thermal trough will develop Thursday, bringing moderate north
winds.

Overnight, the main change to the forecast was to increase expected
southerly winds, including gales likely north of Cape Blanco, with
the passage of a strong cold front on Tuesday. While not likely,
there is a possibility that gales may extend into the waters south
of Cape Blanco with strong winds continuing during Tuesday evening.
BTL/DW

&&

.AVIATION...17/12Z TAF CYCLE...West of the Cascades...Through mid-
morning and again late tonight...areas of LIFR cigs/vsbys with fog
in the Umpqua Valley and patches of valley LIFR in fog/freezing fog
elsewhere. Otherwise, VFR. From the Cascades east...VFR conditions
will prevail through tonight. -DW

&&

.PREV DISCUSSION... /Issued 348 AM PST Sun Dec 17 2017/

DISCUSSION...

Fog has developed in the Umpqua Valley and we were contemplating
on what to do with the Rogue and Illinois Valleys this morning.
Not much visibility reduction over the last couple hours, so
confidence is definitely low that fog will form this morning.
This is magnified by the fact that some upper level clouds are
making their way into the region. So decided to remove it from
the forecast in the Rogue Valley, but kept it in for Grants Pass
and the Illinois valley as the sensors and road cameras are
reporting fog in that area.

As for Sunday into Monday, high pressure will return to the
forecast and we`re expecting some offshore flow and the Chetco
effect in Brookings. Fog will again play a big role for the
Umpqua valley. The current dewpoint is 37 degrees and that
moisture won`t be going anywhere over the next few days. Other
areas are more uncertain, so it will likely be a day to day
decision.

The system on Tuesday night and Wednesday morning is still
expected to bring a large amount of precipitation to the region.
The NAM is the most aggressive model with the precipitation, and
we decided to go with it for this forecast as we should see that
moist west to north west flow hit the Cascades. Right now the
forecast is calling for 12 inches at Crater Lake over 24 hours and
in general 6 - 12 inches above 5500 feet during Tuesday night
into Wednesday morning. East of the Cascades we will see roughly
2 inches with some higher totals near Chemult and Crescent. The
snow levels during this event will start out around 5000 feet and
then fall to 1300 to 1700 feet during 12 to 18z Wednesday after
frontal passage. Felt that since we have lows around 34 to 35
degrees Wednesday morning, we should see snow near the valley
floor with accumulations under half an inch. Overall, have pretty
high confidence that we will at least see advisory level snow fall
over the Cascades.

The only other thing worth noting is that after this storm system
moves through, temperatures east of the Cascades will be quite
cold given the fresh snow pack on the ground. Tried to under cut
guidance for Lakeview and Alturas on Thursday morning and Friday
morning. -Smith

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Air Stagnation Advisory until 8 AM PST Tuesday for
     ORZ023-024-026.
     Air Stagnation Advisory until 8 AM PST Tuesday for ORZ029>031.

CA...None.

Pacific Coastal Waters...
- Gale Watch from Tuesday morning through Tuesday afternoon for
  PZZ350-370.

$$

BMS/CZS/BTL/DW


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