419
FXUS66 KMFR 172150
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
250 PM PDT Wed Jul 17 2024

.DISCUSSION...Aside from a bit of coastal stratus, skies are
clear over the warning area this afternoon with nothing on the
KMAX 88-d. The upper trough that was the instigator for the
thunderstorm activity over the last couple days has moved north
into Portland`s and Pendleton`s areas with some ongoing lightning
up there. Plenty of new fire detections seen in satellite imagery
as well from Near Diamond Lake northward. Another hotspot seen in
Modoc county off Hwy 395 in the Warners but little if any smoke
there. Columns on the new area fires tilting pretty good in gusty
W-SW upper winds in the wake of the departing trough.

Southerly flow aloft will continue through Thursday and Friday as
high pressure remains inland centered near the 4-corners, nudging
westward, and a trough remains offshore. A short wave in the
south flow will ride up the CA coast Friday night into Saturday
with some increasing moisture and instability over the region by
Saturday afternoon. Afternoon highs should rise several degrees
warmer Saturday. Have collaborated with neighboring offices to
introduce a slight chance (10-20%) of thunderstorms over northern
California and the east side and northern California then,
spreading into SW Oregon Saturday night. Thunder threat from this
shortwave should clear to the east by later Sunday with temps
cooling 4-8 degrees. Little change into mid week. Stavish

&&

..AVIATION...17/18Z TAFS...VFR conditions in all locations with the
exception of MVFR ceilings over Coos county and extreme western
Douglas county. A tightly wound up cloud vortex just offshore of
Cape Blanco (K5S6) is moving northeastward toward the coastline and
may keep MVFR ceilings persistent through the afternoon along the
coast from K5S6 northward.  At least model guidance seems to think
so. Have kept MVFR ceilings for said coastal areas for the duration
of the TAF forecast period. Otherwise, VFR will persist with mainly
clear skies and breezy afternoon winds. -Sargeant

&&

.MARINE...Updated 230 PM Wednesday, July 17, 2024...Weak low pressure
offshore has disrupted the thermal trough pattern. Weak winds and
low seas will persist into Friday morning. A weak thermal trough is
expected to return Friday, then strengthen this weekend, with the
strongest winds south of Cape Blanco. Advisory strength northerly
winds and steep seas are possible Saturday, then gales and steep to
very steep seas are possible early next week. -DW/Sargeant

&&

.FIRE WEATHER...Issued 200 PM Wednesday July 17, 2024...A dry
and stable southwest flow sets up over the area today through
Friday with no concerns for thunderstorms. However, afternoon
relative humidities will trend lower for most interior locations
with values in the single digits for portions of the East Side
through Friday along with the typical gusty afternoon and early
evening breezes. This will result in near critical conditions for
portions of Fire zones 624, 625 and 285, but at this time were not
expecting the combination of wind and RH to result in Red Flag
conditions. Were already seeing signs of that from an emerging
fire called the Trail Fire which is located near Diamond Lake just
north of the highway 130 and highway 230 junction.

We`ll have to keep a close watch on thunderstorm potential this
weekend. Saturday, an upper trough will move up from the south just
off the California coast and could tap into some monsoonal moisture
on Saturday with a southeast flow aloft. At the same time, the upper
trough could become negatively tilted Saturday afternoon into
Saturday night. Based in part on pattern recognition, this could be
problematic in that we could see storms break out mid to late
Saturday afternoon in portions of fire zones 280, 281 and 284.

Saturday night, there`s good agreement the axis of the upper trough
is still south of our area which heightens the concern for nocturnal
storms west of the Cascades Saturday night. Many times the models
will not show signs of unstable conditions with this type of
pattern, then "wham", something breaks out. It could be a situation
in which the instability is elevated, and thus the convection will
be elevated as well. Not saying this is a slam dunk, but at the same
time it cannot be discounted. Of note, some evidence does point
towards increasing mid level moisture moving into the westside
Saturday night.

The upper trough axis shifts north during the day Sunday. Any threat
for storms could be confined to the early morning hours over the
northwest part of our area, then shifting north late Sunday morning
through Sunday afternoon with a drier and stable southwest flow
moving into the area. The exception could be in portions of Fire
zone 625 where there is a low end chance (12%) or less of an
isolated storms late Sunday afternoon, but mid level moisture and
trigger is lacking, thus odds are nothing will happen there.

Monday, a dry and stable southwest flow returns with an upper trough
north of our area. It will be cooler with gusty afternoon and early
evening breezes for the interior westside valleys and east of the
Cascades. -Petrucelli

&&

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

CA...None.

PACIFIC COASTAL WATERS...None.

&&

$$