Debris Flow in California, July 27, 2021

Area Forecast Discussion
National Weather Service Medford OR
752 AM PDT Tue Jul 27 2021

.DISCUSSION...Radar is showing a few bands of showers moving
across portions of southern Oregon and northern California. These
showers are producing measurable rain, albeit the rain measured
has been extremely light. Since 11 pm PDT last night, the area has
seen 52 lightning strikes with the most active cell occurring in
Douglas County around the Jack Fire. These showers will continue
today, and a few of those showers could produce lightning--
although storms will be isolated today. The overall forecast looks
to be well enough on track, so no changes are needed for the
forecast for this morning/afternoon; but will be taking a more
extensive look at thunderstorm chances over the next couple of
days as the situation seems to be evolving. Please read the
previous discussion below for more information. -Schaaf

&&

.PREV DISCUSSION... /Issued 440 AM PDT Tue Jul 27 2021/

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

Lots of clouds streaming from south to north over the Medford
forecast area this morning. Radar shows returns, but very little
in the way of precipitation is reaching the ground due to very dry
conditions in the lower atmosphere. The 27/00Z KMFR sounding
showed precipitable water of 1.31 inches, which is close to the
daily maximum recorded value for this location and date. However,
most of it is between 700 mb and 500 mb.

The most likely scenario for the remainder of Monday night is
light showers with mostly virga falling from them. However, some
sites, especially the higher elevation ones, may record measurable
precipitation. Isolated thunderstorms remain a possibility, and
some strikes were recorded over north central Douglas county
earlier this morning.

A long wave trough is west of southeast Alaska and a strong long
wave ridge is over the central U.S. The pattern will change little
over the coming week, so the southerly flow aloft will continue
to bring monsoonal moisture into the area.

As for Tuesday, short waves will move through the area today, and
this, combined with surface heating and increasing moisture, will
support showers and embedded thunderstorms over the forecast
area. Wetting rains are possible primarily east of the Cascades,
but some west side locations, mostly east of I-5, may get some as
well. Quarter inch rainfall totals aren't out of the question,
and much of the data places the heaviest rainfall right over the
Bootleg Fire. Rainfall rates will be insufficient for flooding
over the burn scars though.

The ridge will build to the west over the latter half of the
week. This will bring the heat back to the area. It will be very
hot inland Wednesday through Saturday. The hottest day looks to be
Friday with inland west side highs around 15 degrees above
normal. Cloud cover will keep the east side cooler with forecast
highs around 5 degrees above normal. There will be enough moisture
for isolated to scattered afternoon and evening thunderstorms
every day during this interval, mostly from the Cascades east and
Siskiyous south.

There are some indications of a pattern change late Sunday or
early next week to slightly cooler, but still above normal,
temperatures and more stable conditions, but confidence is low on
this. Afternoon and evening thunderstorms remain in the forecast
through Monday.

AVIATION...For the 27/12Z TAF Cycle... LIFR/IFR conditions are
expected along the immediate coast and into coastal valleys  through
this morning.  Also, weak storms are also ongoing this morning west
of the Cascades.  Some of those storms might come close to MFR later
this morning.  We'll monitor radar and see if we need to add VCTS or
TS to the TAF.  Otherwise,  the environment appears to destabilize
further this afternoon.  Look for more thunderstorms across the
region later this afternoon and evening.  Some storms may be strong
over the Cascades. Prepare for lower ceilings and poor visibilities
near thunderstorms throughout the day.  If storms are strong enough,
localized IFR visibilities are likely.

Other than the storms today,  we'll see the LIFR ceilings move over
the coast later tonight and Wednesday morning.
-Smith

MARINE...Updated 200 AM PDT Tuesday, 27 July 2021...Seas will
remain steep south of Cape Blanco through this afternoon as fresh
swell moves through the waters.  A weak thermal trough will persist
through the next few days creating some gusty 20-25 north winds in
the outer waters Tuesday and again on Wednesday. It seems the
strongest winds will occur farther offshore and near Port Orford
with weaker winds closer to shore to the south near Brookings.
These gusty north winds will persist later this week into the
weekend.

FIRE WEATHER...Updated 200 AM Tuesday 27 July 2021...There is an
isolated thunderstorm threat over a fairly large portion of the
forecast area through Tuesday.

A short wave will arrive from the south Tuesday. PW values will
be 1.00-1.50 inches, which is a lot for this time of year. However,
cloud cover will limit surface heating, so instability will be
reduced. This looks to be a widespread shower event with embedded
and isolated thunderstorms, though any thunderstorms will be
effective rain-producers. Chances of wetting rain (>0.10") are
highest from about eastern Siskiyou County north-northeastward
into Klamath County and western Lake County, up to and including
the Bootleg fire.

Wednesday through Saturday, it will heat up again with moist
southerly flow persisting over the area. Instability will develop
each afternoon/evening during this time period, and convective
activity will be dictated by short waves embedded in the southerly
flow. Short waves will be more active Thursday, so that will
likely be the busiest day for thunderstorms.

&&

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

CA...None.

Pacific Coastal Waters...Small Craft Advisory until 11 PM PDT this evening for PZZ356.
     Small Craft Advisory until 5 PM PDT Wednesday for PZZ376.