Area Forecast Discussion
Issued by NWS Medford, OR

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
FXUS66 KMFR 182154

Area Forecast Discussion
National Weather Service Medford OR
254 PM PDT Sun Mar 18 2018

.SHORT TERM...Today through Wednesday, March 21st...
It`s a mix of clouds and sun this afternoon as weak high pressure
resides over the area. Brisk west flow aloft and southwest flow in
the lower levels under March sun is combining with the moist
ground causing cumulus clouds to blossom this afternoon,
especially over the higher terrain. Isolated to scattered showers
are still expected to develop this afternoon and evening, mainly
along and near the Cascades, translating eastward as they
dissipate this evening. Amounts are also still expected to be

Cloud cover will increase from the southwest tonight, lingering
into early Monday morning before diminishing during the day on
Monday. More sun on Monday should allow temperatures to be near
normal on the West Side, but still about 5 degrees below normal
east of the Cascades. This will result in highs of 55 to 60 in the
Medford area and 50 to 55 for the valleys east of the Cascades.

Numerical model guidance has been consistent in developing light
precipitation over portions of forecast area Monday night into
Tuesday, but have wavered on the location of where this
precipitation will develop. In short, a warm frontal boundary is
expected to develop over the area during this time under broad,
increasing southwest flow aloft.

Tuesday evening southerly winds are expected to increase across
the forecast area, likely peaking Wednesday afternoon into
Wednesday night as a wave of low pressure arriving on the
southwestern Oregon coast from the southwest drives a cold front
inland. Advisory level winds are expected in the Shasta Valley
and across portion of the East Side. This front is much like the
front that we saw ahead of the cold upper level trough last week
in that, an occasional model run suggests stronger winds such that
watches/warnings could be needed in the mentioned locations, but
the consensus of the guidance suggests advisory level winds are
more likely. This is because winds appear to be focused more off
of the surface up around 700MB and surface pressure gradients are
not too strong (MFR-RDD: ~6MB). Additionally the initial
shortwave trough in the flow that gets precipitation going Monday
night into Tuesday should result in enough frictional drag from
precipitation to prevent strong winds at valley locations. The one
factor that could change this would be if the system gets
convective enough to cause thunderstorms, which is something we
always need to be on the lookout for in March. However, at this
time it appears that, while some convection is expected within
this weather system, it does not look so unstable that
thunderstorms will be more than a slight concern.

Precipitation amounts will be light until Tuesday night, when the
Mount shasta area and Curry County could see a quarter to a half
inch of water. Snow levels Tuesday night in the 5kft to 5.5kft
range are expected to rise to the 6kft to 7kft range Wednesday
into Wednesday evening before falling.

From Tuesday evening through Saturday morning water amounts in
the Mount Shasta area look to be in the 3 to 4.5 inch range, 2 to
4 inches in the Cascades, Siskiyous, and mountains of western
Siskiyou County, and 3 to 7 inches across Curry County. Coos and
Douglas counties should get 1 to 3 inches with local amounts up to
4 inches. For the Rogue Valley, amounts around an inch are
expected, while 0.50 to 1 inch amounts are expected across the
drier valleys from the Shasta Valley eastward, except lighter
from Silver Lake and Summer Lake northeastward. This is likely to
equate to a lot of beneficial snow in the mountains. However,
major roadways are not likely to be impacted by significant
snowfall until Thursday onward. To get an idea of potential
snowfall amounts above 6kft in inches, multiply the amount of
water expected in the ranges above by 6 to 10. BTL

.LONG TERM...Wed night through Sun, March 25th...
The extended forecast is still showing a closed low approaching from
the northwest and another smaller low approaching from the
southwest. Snow levels are still expected to start out in the
6000-7000 foot range early Wednesday evening, dropping quickly
down to 4000 feet Thursday morning. Snow levels eventually fall to
2000 feet Thursday evening and night as more cold air settles in.

Precipitation well be heavy at times along and ahead of the front.
The south winds ahead of the front will be strong creating upslope
flow over northern part of the Sacramento valley. In these
scenarios some weird weather can occur in Mt. Shasta City with
snow levels falling much lower than forecasted because it
precipitates so hard there. Tried to compensate for that Thursday
as we lowered snow levels about 1000 feet below what the models
were going with. Therefore, the forecast is showing a few inches
of snow late Thursday morning into Thursday evening.

Behind the front, 850mb temperatures are still showing about -5C
around our area Friday morning. So the valley floors and
elevations above 1500 feet are still under the gun for some snow
Thursday night into Friday morning. Right now, we`ll go with 1
inch of wet snow is possible overnight on the valley floors.

As this low continues to hover off the coast, some embedded
energy and another surface low will hit the area Friday evening
into Saturday morning. Snow levels will rise because of day time
heating and we`ll also be in the warm sector of this wave Friday
evening. Then the snow levels are expected to fall again as
another cold front moves through. This front won`t be as intense,
but look for another round of precipitation Friday night.

Overall, storm total snow is showing at least 1-2 feet of snow
over Mt. Ashland and the Crater Lake area from Wednesday evening
through Saturday evening with low level snow on the valley floors
remaining a possibility. Windy conditions can also be expected in
the Shasta valley during Thursday as south winds funnel through
the valley. In the past we`ve seen the low approaching from the
southwest muck things up in the valleys, but we`ll still keep an
eye on the winds Wednesday into Thursday.


.AVIATION...18/18Z TAF CYCLE...VFR conditions are expected most
hours through the TAF period. However could not rule out partial
mountain obscurations in showers along and north of the Umpqua
Divide this afternoon into early this evening. There`s a chance for
IFR cigs to develop in the Umpqua Basin, including Roseburg towards
daybreak Monday due to sufficient low level moisture. This is
reflected in the TAF at Roseburg. -Petrucelli


.MARINE...Updated 200 PM PDT Sunday 18 Mar 2018....Relatively calm
conditions will persist as weak high pressure remains over the area
through Monday night. Winds will generally be below 20 knots with
light seas at or below 6 feet through Tuesday night. A weak upper
low north of the waters could bring a few showers mainly north of
Cape Blanco through tonight.

The models are in better agreement with how they handle the upper
trough to the north and the weak upper low southwest of the waters.
The upper trough is expected to dig south towards the waters
Wednesday and draw the aforementioned upper low northeast.
Differences still exist between the ECMWF and GFS and suspect the
details will change again in future model runs. Right now, the GFS
shows a surface low weakening as it moves southeast towards the
waters late Wednesday afternoon and evening. This solution lessens
the treat for gales, but small craft conditions are still possible
late Wednesday night.

Swell height and total wave height still looks like a concern late
Thursday afternoon through at least Friday morning. There is fairly
good agreement on the total wave height between the wave watch
bulletin and the ECMWF.

Despite the above mentioned reasoning, confidence is still low on
the details, therefore mariners should stay tuned for latest updates.




Pacific Coastal Waters...None.


BTL/CZS/MAP is the U.S. government's official web portal to all federal, state and local government web resources and services.