118
FXUS66 KMFR 261941 CCA
AFDMFR

Area Forecast Discussion...CORRECTED
National Weather Service Medford OR
1241 PM PDT Sat Apr 26 2025

.DISCUSSION (Today through Friday)...

Overview:

Main weather hazards for this forecast package include the threat
of an isolated thunderstorm this afternoon across the eastside,
as well as areas across northern California. Not expecting severe
thunderstorms, but lightning will remain a threat over these areas
through this evening. Lingering showers tomorrow will end and
thereafter we will go dry until Tuesday with low end chances for
showers. Then a much better chance for showers and thunderstorms
will come Friday, but its too early to say with much confidence
the level of impacts we may (or may not) have late next week.

Further Details:

An upper level (H5) low currently off the coast of California
west of San Fransisco will slowly move on shore today, and showers
will follow suit across our forecast area on the poleward side of
the low. These showers are expected to slowly move southwest and
south through this evening and linger tomorrow morning before
dissipating. MUCAPE values today are on the lower end (100-450
J/kg) for areas mainly east of the Cascades. Cannot rule out a
thunderstorm today, but we are not expected any severe weather,
but lightning will be a threat for anyone with outdoor actives
today. Not too worried about gusty winds with momentum transfer
due to weak mid-level wind speeds. Lingering showers tomorrow will
lack any noteworthy instability, so we are not anticipating any
lightning tomorrow before showers dissipate.

We will transition to northerly flow in the wake of the
aforementioned low as an area of high pressure builds over the
Pacific. Then on Tuesday we briefly go zonal before a weak
embedded shortwave transitions across the forecast area. We are
lacking any noteworthy instability, so not expecting any
thunderstorms, but there will be a chance (20-40%) for light
showers (QPF < 0.10") across northern and eastern parts of the
forecast area.

Then a dry pattern will develop until Friday. A deeper negatively
tilted trough is anticipated to develop with the trough axis
extending down from British Columbia. The base of the trough and
vorticity maximum will pass over the forecast area. This coupled
with with widespread ample CAPE (upwards of 600 J/kg) will likely
lead to showers and thunderstorms. Bulk shear values around 40-50
knots would allow for updrafts maintaining, so we could have a
couple strong storms, but its too early to say with much
confidence the coverage of these thunderstorms. DCAPE values are
on the lower end, and this is evident with forecast soundings
indicating moist low levels. In fact, extended range forecast
soundings show a "tropical" like soundings with a very moist
column. This would inhibit strong downburst potential, so the main
threat could end up being lightning with small hail. Will
continue to monitor this because Friday could be impactful from a
rain and lightning point of view. PWATs are around an inch in some
areas, so we will need to also keep an eye on heavy rainfall
chances. Early indication suggest the system is progressive, and
the mean flow could result in fast moving cells which would limit
rainfall amounts over one area. Stay tuned as we refine the
details over the coming days.

-Guerrero

&&

.AVIATION...26/18Z TAFs...Showers are starting to move towards the
south this afternoon with skies also starting to clear a bit towards
the north east and in central Oregon. We`ll see plenty of MVFR
ceilings through the afternoon hours with IFR ceilings reported east
of the Cascades. Some areas will see ceilings lift or simply clouds
clear out later this evening into tonight.

However, IFR ceilings will persist in northern California and areas
east of the Cascades as the upper level low pushes towards the east.
We`ll see more widespread VFR conditions towards Sunday evening as
this low departs the area.

-Smith


&&

.MARINE...Updated 800 AM Saturday, April 26, 2025...Conditions
improve some tonight into Sunday as winds ease a bit, though steep
seas are likely to persist south of Cape Blanco into early next
week. A thermal trough will maintain gusty north winds and steep
seas, highest south of Cape Blanco, into much of next week, with
peak winds and waves expected in the afternoons and evenings.

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...CA...None.

PACIFIC COASTAL WATERS...Small Craft Advisory until 5 AM PDT Sunday
     for PZZ350-356-370-376.

&&

$$