Area forecast discussion
National Weather Service Medford or
210 PM PDT Sat may 25 2013
Discussion...25/12z NAM/GFS in.
Mostly clear skies so far this afternoon...except for some
stratocumulus developing over northern California and southeast
Lake County...and stratus near the coast and offshore. The area of
stratocumulus lines up fairly well with the model stability
fields...and those fields don't look favorable for thunderstorms
later this afternoon.
A long wave trough remains over the Pacific northwest this
afternoon...and a short wave rotating around the low will move
onshore late tonight. This one is stronger than the one that just
passed through...so it will have more associated precipitation...
which will develop at the coast this evening and spread inland
tonight. The bulk of it will fall along the coast...Coast
Range...and Umpqua basin. Showers will persist into Sunday as the
trough will remain in the area. After evaluating the
stability...have limited the Sunday afternoon/evening
thunderstorms mostly to Lake County with the area of activity
migrating east through the period.
Both the short wave and the parent trough will move out to the
east Sunday evening and a weak short wave ridge will briefly build
into the area Sunday night. This will break down quickly and
another trough will move onshore Monday. This system will be the
biggest rain maker of the week. Rain will develop along the coast
early Monday morning...spreading inland during the day. Unlike the
previous system...just about the entire Medford County Warning Area will see some
precipitation from this system.
The upper trough will move out to the east Monday night...though
some showers will persist through Tuesday due to short waves
moving down the back side of the trough. Roughly zonal flow aloft
will develop in its wake and one more embedded short wave will
move onshore Tuesday night into Wednesday morning for another
round of showers.
Long term...Tuesday night through Saturday...the long wave trough
will continue to impact the area at the beginning of the
extended as another shortwave moves in on a fast 140 knots westerly
jet. Rain will arrive at the coast later Tuesday night, then push
inland Wednesday and become more showery Wednesday afternoon. The
operational GFS lies on the fast side of the guidance with this
feature, so have maintained high probability of precipitation all day Wednesday. Models
still showing this system slowing/amplifying a bit Wednesday night
into Thursday as it moves into eastern Washington/or then into Idaho, so
have kept probability of precipitation higher during this time to account for this. This
system will also keep temperatures cooler than the seasonal averages
by several degrees, which for Medford are in the middle to upper 70s
for highs and in the upper 40s for lows. The tendency in the
ensembles is to build heights along the West Coast Friday into
Saturday with a drier northerly flow aloft, gradually turning
offshore. The net result would be a warming and drying trend with
temperatures getting back to or even above the seasonal norms.
However, we'll have to keep an eye on the trough to the east over
the intermountain west and energy moving into western Canada. This
may allow the mean trough position to retrograde back over the
northwest sometime over the weekend. Spilde
&&
Aviation...based on the 25/18z taf cycle.
VFR will prevail along the coast late this afternoon, but MVFR ceilings
and -ra will develop quickly after 01z this evening as low pressure
moves in. Isolated/scattered -shra will develop ahead of this system across the
east side this evening, but VFR will persist most areas. MVFR ceilings
and -ra will spread inland west of the Cascades tonight resulting in
widespread terrain obscurations. Ceilings may flirt with MVFR at kmfr,
but probably remain VFR even with a little -ra around daybreak.
Reduced ceilings should improve to VFR Sunday afternoon in most
locations. Spilde
&&
Mfr watches/warnings/advisories...
or...none.
California...none.
Pacific coastal waters...none.
$$
15/18/10
