Grants Pass, Oregon 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

584
FXUS66 KMFR 262058
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
158 PM PDT Thu Jun 26 2025

.DISCUSSION...The overhead trough has once again provided support
for a strong marine push along the coast this morning, where
stratus and fog pushed into all the coastal valleys and into the
Umpqua Basin all the way to Roseburg. A very similar push is
expected again tonight into Friday morning, although this one
may be a bit weaker and make less headway inland.

Temperatures will remain around or just above normal for this
time of year through Friday. Daily marine layer pushes will
continue at the coast and coastal valleys as well. Ridging will
build into the area beginning Saturday, along with the
redevelopment of the thermal trough over California and the
southern Oregon coast, and as a result, we will see temperatures
rise by about 5 to 10 degrees Saturday, and again on Sunday.
Sunday will be the warmest day of this warm spell for areas west
of the Cascades, then Monday could be the warmest for the East
Side (although cloudiness from thunderstorms and monsoonal
moisture inflow could limit high temperatures Monday, more on
that below). The West Side valleys on Sunday will see highs in the
upper 90s to low 100s, while the highs on Monday for the East
Side will peak out around 90 to 95. Temperatures will then cool
slightly heading further into next week, but will remain above
normal.

As is typical with periods of heat in the region, it will come to
an end with the threat of thunderstorms. With the ridge passing
just to our east and strengthening as it moves over the Rockies, a
trough develops off the coast of California, which then attempts
to move onshore around Monday or Tuesday, taking on a negative
tilt as it does so. This is a classic pattern for convection here,
with the trough tapping into monsoonal moisture from the south,
and taking advantage of the warm surface temperatures and cooler
temperatures aloft to produce instability across the inland
portions of the forecast area.

Convection may begin as early as Sunday afternoon over far
southern portions of Siskiyou and Modoc counties, depending on
how far north the moisture will get by the peak heating hours of
the afternoon. Confidence on thunder Sunday afternoon is low, and
wording has been left out of the forecast, but the chance is
there. Most likely, Sunday will be the "priming-of-the-pump" day,
with little convection but definite signs of increasing moisture.
As moisture continues to push north into our area, and with some
support from weak energy aloft, more widespread convection could
then begin as soon as early Monday morning (very low probability),
although the most likely scenario has convective initiation
beginning Monday afternoon. I suspect that the area will be
greeted by altocumulus clouds and a feeling of "high" humidity
Monday morning, showing the arrival of the moisture and potential
instability of that afternoon.

Given the pattern, thunderstorms could form up anywhere east of
the coastal ranges Monday, with the marine layer limiting
development along the coast and perhaps into the Umpqua Basin as
well. Most convection should be concentrated along the Klamath,
Siskiyou, and Cascade mountains, as well as the East Side, with
cells then drifting off the terrain and into the valleys.
Steering flow appears weak, so storms may be slow moving, leading
to more hit-or-miss "popcorn" style convection.

Some models then continue shower and perhaps thunderstorm
activity overnight, although confidence on that is low. There is
much more confidence on thunderstorms reforming Tuesday afternoon,
although with the eastward progression of the trough, the best
dynamics and support would then be over northern California and
along and east of the Cascades. Other than location, storm
behavior and character should be very similar to Monday. Storms
could then continue on the East Side into Tuesday night, with
some models even keeping some form of convective showers over Lake
County into Wednesday afternoon.

In short, it is nearly a certain thing that there will be
lighting across the majority of the forecast area both Monday and
Tuesday afternoons. Based on current guidance, we do not expect
very strong or severe thunderstorms, but small hail and gusty
winds are possible. Also, given the high moisture content in the
atmosphere that the models are depicting, these storms will be
wet, and could produce periods of moderate to heavy rainfall. As
always with thunderstorms, lightning will be a threat, not only in
regards to new wildfire starts, but also to anyone attempting to
enjoy outdoor recreation in the area.

One caveat to this scenario will be cloud cover. With this much
moisture entering the area, and with the widespread convection
expected Monday, there may be extensive cloud cover over the area
Tuesday limiting incoming solar energy, keeping temperatures
lower, and limiting instability. This has occured with events
like this in the past, and has significantly reduced thunderstorm
development and blown the forecast on the second or third day,
and this may be another example. Keep up to date with the latest
forecasts as the details regarding this thunderstorm threat become
more clear over the next few days.

Broad cyclonic flow and lower heights then continue over the
area for the remainder of the forecast term (midweek next week).
This should keep temperatures at or just above normal, with
mostly dry conditions. -BPN


&&

.AVIATION...26/18Z TAFs...Marine MVFR ceilings are retreating to the
coast, and most areas should be VFR for at least a few hours this
afternoon, although lower conditions may linger longest along the
coast. The marine layer will return again this evening through
tonight, with ceilings and visibilities expected to be very similar to
those of last night into the morning.

Elsewhere, VFR will prevail through this evening, though scattered
afternoon cumulus buildups are expected again this afternoon and
evening, mainly from the Cascades eastward and in Siskiyou County.
Expect the typical increase in afternoon/evening winds, though
should be slightly weaker than they were on Wednesday and closer to
seasonable values of 15 to 25 kt. -BPN

&&

.MARINE...Updated 200 PM PDT Thursday, June 26, 2025...Relatively
calm conditions will continue into this evening with light winds and
low seas. The exception will be south of Gold Beach where a
developing thermal trough will result in winds increasing mainly
south of Gold Beach late this afternoon and tonight. However, winds
will remain below advisory levels. The thermal trough strengthens on
Friday and even more so over the weekend. Conditions hazardous to
small craft will develop Friday afternoon from Cape Blanco south as
north winds increase and seas steepen. Winds will be strongest south
of Gold Beach.

Winds will increase further Saturday as advisory level winds and
seas likely spread north of Cape Blanco, with gales and very steep
seas possible south of Cape Blanco. The outlook for next week is for
the thermal trough to remain strong and maintain hazardous
conditions, especially south of Cape Blanco. -Petrucelli

&&

.FIRE WEATHER...Updated 200 PM PDT Thursday, June 26, 2025...Mostly
low impact fire weather conditions are expected the rest of this
week with fairly typical diurnal (afternoon/evening) breezes,
seasonable humidity and near to above normal warmth.

A strengthening upper ridge will bring hotter weather to all but the
immediate coast this weekend with continued drying. Even the south
coast could have highs in the 70s to near 80F Saturday (maybe Sunday
too?). A brief period of enhanced E-NE winds at the mid slope/ridge
level is expected Friday night and again Saturday night. However
overnight recoveries Friday not don`t look all that bad. Saturday
overnight recoveries will be moderate, but probably not enough to be
concerned about any watches/warnings. Temperatures increase across
the interior Saturday with widespread highs in the 90s on Sunday.
Monday will be the hottest day, with afternoon temperatures near or
at triple digit values for the interior westside valleys.

At the same time, an upper low will consolidate off the California
coast Sunday. The upper ridge shifts to the Four Corners region and
this pattern sets up south to southeast flow aloft from California
into southern Oregon. This will begin to tap into some monsoonal
moisture that will slide up into northern California Sunday
afternoon and evening. The most likely scenario Sunday afternoon
into Sunday night will be building cumulus over the mountains in
northern California, because there is little or no trigger, mid
level moisture is lacking and instability is marginal at best. Worst
case scenario will be a couple of isolated storms near the Trinity
Horn and points northeastward towards the Shasta Valley late in the
afternoon and early evening hours Sunday. In summary, Sunday will be
the day in which the pump is primed for whats expected for Monday.

Monsoonal moisture will increase late Sunday night through Monday
and at the same time mid level moisture and trigger will increase.
This will set the table for resulting in thunderstorms over most of
the area inland away from the coast, with storms developing as early
as Monday morning. Right now, the expectation is for thunderstorms
to be isolated Monday morning, then the shear number and areas
affected increasing Monday afternoon into Monday evening.

It`s worth noting the risk for nocturnal storms are next to, but not
zero Sunday night, with an elevated risk for lightning Monday and
Tuesday night. The main storm threat shifts to the Cascades and
points south and east Tuesday into Wednesday. As fuels continue to
dry out and the calendar shifts from June to July, fire danger will
increase. So, be on the lookout for potential Fire Weather
Watches/Red Flag Warnings as we head into next week. -Petrucelli

&&

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

PACIFIC COASTAL WATERS...Small Craft Advisory from 2 PM Friday to 11
     AM PDT Saturday for PZZ356-376.

&&

$$