Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 270300

Area Forecast Discussion
National Weather Service Medford OR
800 PM PDT Fri May 26 2017

.DISCUSSION...27/00Z NAM in.

Mostly clear skies prevail across the Medford CWA with the exception
of the coast and the adjacent coastal waters...where low marine
stratus dominates. There is also some dissipating shallow cumulus
from the Cascades east.

The mostly clear skies come courtesy of a strong long wave ridge
centered offshore along 130W. This is bringing dry northerly flow
aloft to the area. This ridge will move slowly east...moving
onshore Saturday and then to the east of the area Sunday afternoon
into Sunday evening.

Initially...dry and warm weather will prevail. Saturday highs will
be near normal along the coast...but 10 to 15 degrees above normal
over the rest of the area. Sunday highs inland will be 2 to 4
degrees higher than the Saturday highs except in the Umpqua
Basin. The encroaching marine layer will cool that area a few
degrees Sunday.

As the long wave ridge axis breaks to the east of the area, the
flow aloft will become southerly and this will open the door to
monsoonal moisture advecting into the area. Thus, thunderstorms
become a possibility as early as Sunday afternoon. However, the
moisture will still be very limited at that time, so the chances
of thunderstorm development is quite low.

Southerly flow aloft will gradually increase into Monday, and
conditions will become much more favorable for thunderstorm
development. Difluence aloft, increasing south to southeast flow
(15 knots or greater) due to an approaching slightly negatively
tilted upper level trough, increasing moisture (PWs increasing to
0.80 to 1.10 inches), and increasing instability will combine to
support showers and thunderstorms across much of the area,
centered on the Cascades and Siskiyous. One or two of these storms
could become severe.

As far as temperatures go...Monday highs will be about the same
as the Sunday highs.

Extended discussion from the Friday afternoon AFD...Tuesday
through Friday. The extended period begins active with southerly
low level flow continuing to feed unstable air into the region.
The GFS is indicating LI`s of -4 over SW Jackson/SE Josephine
counties and -7 over the Crater lake area. In addition to the very
unstable air mass and strong convergence in the Siskiyous and
Cascades with the southerly flow meeting the afternoon upvalley
flow a strong vort axis is also indicated to move through shortly
after max heating. Starting Wednesday the GFS and EC veer apart
with the GFS closing an upper low and bringing it south into
southern CA, and the EC indication an open trough. With this I do
not have high confidence in convection on Thursday as the EC is
indicating a weak ridge building over the region. Sven


.AVIATION...27/00Z TAF CYCLE...A surge of south winds and IFR/LIFR
fog has moved up the coast and coastal waters up to Lane county.
This low stratus and fog will remain in the coastal waters and move
inland at the coast this evening, remaining through the early

All other areas will remain VFR through the TAF period with some
fair weather cumulus building tomorrow afternoon -Sven/Schaaf


.MARINE...Updated 800 PM Friday 26 May 2017...Winds and seas will
diminish tonight into Saturday as the thermal trough weakens. Fog
and low stratus will persist through the weekend. The thermal
trough will redevelop Sunday into Monday and gradually strengthen
through the first half of next week...bringing increasing north
winds to the area. Small craft advisory winds and seas will likely
develop south of Cape Blanco by the middle of next week.


.FIRE WEATHER... A chance of thunderstorms is expected the afternoon
and evening of Memorial Day across the area following a period of
warm and dry weather. While most of the thunderstorms are currently
expected along, south, and east of the Cascades and Siskiyous,
moderate southeast flow across the area means that some areas west
of the Cascades could also be affected. The threat is expected to
continue into Tuesday. BTL




Pacific Coastal Waters...None.


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