Excessive Heat in California, August 10 to 14, 2021

Name: Excessive Heat in California, August 10 to 14, 2021
Start: 2021-08-10T12:00
End: 2021-08-14T11:17
Location: California

August 10 to 14, 2021 · NWS MFR · 2 events in 2 counties and zones

A strong ridge supported a heat wave over northern California during this interval.

Where it happened

Every event with a place to put it; 2 filed by county or forecast zone are drawn at its center, in a lighter ring. Tap one for its entry below.

Tornadoes
Tropical storms and surge
Floods
Wind
Hail
Winter weather
Heat and drought
Cold
Fire, smoke and dust
Marine and coastal
Other

Open the live radar over this ground

The toll

Deaths: 0
Injuries: 0
Property damage: None recorded
Crop damage: None recorded
Events: 2

Deaths and injuries are direct and indirect together, as the offices filed them. Damage is property and crops in the dollars of the year, not adjusted for inflation, and only where an office estimated it.

What happened

The episode as NWS MFR wrote it up.

A strong ridge supported a heat wave over northern California during this interval.

What forecasters said that day

The local office's own words before the first event, as it sent them, from the Iowa Environmental Mesonet's archive of National Weather Service text.

NWS Medford, OR wrote its last area forecast discussion before the first event at Aug 10, 6:54 AM PST, 5 hours 6 minutes earlier.

Area forecast discussion, Aug 10, 6:54 AM PST, 5 hours 6 minutes before

Area Forecast Discussion
National Weather Service Medford OR
754 AM PDT Tue Aug 10 2021

.UPDATE...The forecast remains on track and no updates are needed
at this time. Current visible satellite indicates continued smoky
skies affecting much of the inland areas and marine stratus along
the coast. No significant improvement in smoke from area wildfires for
most areas. Heat, smoke, and thunderstorm potential continues to
be our main focus for the forecast period.
- Viken

&&

.AVIATION...10/12Z TAFs...Across inland areas expect areas of MVFR
cigs today and tonight. Otherwise, expect VFR conditions across
inland areas.

Along the coast, areas of IFR/LIFR conditions in marine stratus will
fill into the coastal valleys, especially north of Cape Blanco, but
possibly also from Gold Beach southward. These areas should clear by
late morning, but are likely to return in the evening. -CC

&&

.MARINE...Updated 800 AM PDT Tuesday 10 August 2021...The thermal
trough will gradually move inland and weaken tonight into
Wednesday night. Very steep hazardous seas and gusty north winds
will continue for portions of the waters south of Cape Blanco
through this evening with small craft advisory level winds and
seas elsewhere. Conditions gradually improve tonight into
Wednesday. However, advisory level conditions persist through
Wednesday morning. Conditions improve overall from Wednesday into
Thursday as winds decrease further and seas transition to
northwest swell and lowering below small craft advisory level.

High pressure aloft off the Pacific NW coast will build eastward
Thursday into the weekend, and this will keep the thermal trough
from reestablishing itself. As such, relatively calm conditions are
expected through the weekend. -CC/Viken

&&

.PREV DISCUSSION... /Issued 317 AM PDT Tue Aug 10 2021/

DISCUSSION...Heat, smoke, and thunderstorm potential are the
primary concerns for this forecast cycle. Strong high pressure
aloft will extend across the Pacific Northwest through Saturday
followed by a cooling trend as a trough develops over the region
early next week with temperatures back to near normal. The models
differ in the strength of the trough and subsequently the amount
of cooling so there is uncertainty in these details but a change
to cooler conditions will occur.

The wildfire smoke creates unhealthy air and more uncertainty in
temperatures for those areas downwind of the fires, such as the
Illinois Valley, Rogue Valley, Klamath Basin, and nearly all of
far northern California. High temperatures have been reduced a
few degrees in these smokey areas but unfortunately there will
still be a combination of smokey air and highs above 100 degrees.

Regarding thunderstorms, moisture is quite limited on Wednesday
and the SREF calibrated thunderstorm chances are 5% or less in the
region. On Thursday there's a slight uptick in moisture with lots
of instability and the SREF calibrated thunderstorm chances are
10-20%. Continued southerly flow aloft on the backside of the
advancing upper high will keep thunderstorm chances across many
inland areas Friday and Saturday.

MARINE...Updated 230 AM PDT Tuesday 10 August 2021...The thermal
trough will gradually move inland and weaken tonight into Wednesday
night.  Very steep hazardous seas and gusty north winds will
continue for portions of the waters south of Cape Blanco through
this evening with small craft advisory level winds and seas
elsewhere. Conditions gradually improve tonight into Wednesday.
However, advisory level conditions persist through Wednesday
morning. Conditions improve overall from Wednesday into Thursday as
winds decrease further and seas transition to northwest swell and
lowering below small craft advisory level.

High pressure aloft off the Pacific NW coast will build eastward
Thursday into the weekend, and this will keep the thermal trough
from reestablishing itself. As such, relatively calm conditions are
expected through the weekend. -CC

FIRE WEATHER...Updated 245 AM Tuesday 10 August 2021...

Excessively hot, very unstable and dry air becomes the main
concern across southern Oregon and northern California through the
rest of the week. The red flag warning continues for Haines 6
on Wednesday within FWZ 616 and 617. The mixing heights and lapse
rates weren't impressive enough over the Oregon fires today, but
they should reach up to 12000 feet and 10C/km over northern
California with enough surface heating. Thick enough smoke would
limit mixing heights and low to mid level lapse rates.

By Wednesday, we continue to feel more confident with Haines 6
over FWZ 616 and 617 as surface heating and instability build
according to the models. The Hot-Dry-Windy Index (HDWI), which
was developed as a better predictor of fire plume growth instead
of Haines Index is also predicting a high chance of exceeding the
90-95th percentile on Wednesday and even a greater chance on
Thursday. Therefore another red flag warning will likely be needed
on Thursday.

The last thing worth mentioning are the thunderstorms on Thursday
into Friday.  Some of the potential instability may need sufficient
surface heating to develop. However, we felt potential instability
would be present without ample surface heating as there is enough
moisture aloft, so smoke will likely not play a big factor in this
event.

With all that in mind, we're forecasting a 10-20 percent chance
of thunder across Siskiyou County and the high Cascades in Oregon.
We also felt these storms would be on the drier side given the
large inverted V moisture profile reaching up to 600 mb over KMFR
and KMHS forecast soundings Thursday.

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Heat Advisory from 1 PM Wednesday to 9 PM PDT Friday for
     ORZ029>031.
     Excessive Heat Warning from 1 PM this afternoon to 11 PM PDT
     Saturday for ORZ023>028.
     Red Flag Warning from 2 PM to 11 PM PDT Wednesday for
     ORZ616-617.
     Small Craft Advisory from 11 PM this evening to 11 AM PDT
     Wednesday for ORZ021-022.

CA...Heat Advisory from 1 PM Wednesday to 9 PM PDT Friday for
     CAZ083>085.
     Excessive Heat Warning from 1 PM this afternoon to 11 PM PDT
     Saturday for CAZ080>082.

Pacific Coastal Waters...Small Craft Advisory until 11 AM PDT Wednesday for
     PZZ350-356-370-376.
     Hazardous Seas Warning until 11 PM PDT this evening for
     PZZ356-376.

The radar

The national radar mosaic for the moment the first event began.

Open the radar archive at this moment

Event by event

2 excessive heat. Local standard time, as filed.

Excessive Heat, Central Siskiyou County

When: August 10, 12:00 PM PST to August 14, 11:17 AM PST
Where: Central Siskiyou County forecast zone
Reported by: Official NWS Observations
Coordinates: 41.593, -122.540 (the zone’s center; the record gives no point)

On 08/10/21, the reported low temperatures in this zone ranged from 56 to 57 degrees and the high temperatures ranged from 100 to 102 degrees. On 08/11/21, the reported low temperatures in this zone ranged from 57 to 60 degrees and the high temperatures ranged from 104 to 108 degrees. On 08/12/21, the reported low temperatures in this zone ranged from 62 to 65 degrees and the high temperatures ranged from 103 to 104 degrees. On 08/13/21, the reported low temperatures in this zone ranged from 66 to 66 degrees and the reported high temperatures ranged from 99 to 103 degrees. On 08/14/21, the reported low temperatures in this zone ranged from 64 to 65 degrees and the high temperatures ranged from 101 to 104 degrees. On 08/15/21, the reported low temperatures in this zone ranged from 65 to 68 degrees and the high temperatures ranged from 103 to 105 degrees. On 08/15/21, Montague recorded a high temperature of 105 degrees, tying the record high for that date.

Excessive Heat, South Central Siskiyou County

When: August 10, 12:00 PM PST to August 14, 11:17 AM PST
Where: South Central Siskiyou County forecast zone
Reported by: Official NWS Observations
Coordinates: 41.593, -122.540 (the zone’s center; the record gives no point)

On 08/10/21, the reported low temperatures in this zone ranged from 49 to 56 degrees and the high temperatures ranged from 95 to 98 degrees. On 08/11/21, the reported low temperatures in this zone ranged from 51 to 55 degrees and the high temperatures ranged from 98 to 99 degrees. On 08/12/21, the reported low temperatures in this zone ranged from 50 to 58 degrees and the high temperatures ranged from 95 to 99 degrees. On 08/13/21, the reported low temperatures in this zone ranged from 58 to 59 degrees and the only reported high temperature was 94 degrees. On 08/14/21, the only reported low temperature in this zone was 60 degrees and the only reported high was 99 degrees. On 08/15/21, the only reported low temperature in this zone was 61 degrees and the high temperatures ranged from 96 to 103 degrees.

What these are

Excessive Heat: Heat at or above the local excessive heat warning criteria, or heat that caused deaths.

After NWS Directive 10-1605, the rules offices file Storm Data by.

The same day elsewhere

Every storm in California in 2021 Every state in 2021 This day in weather history Warnings in California, August 2021

Sources

NOAA National Centers for Environmental Information, Storm Events Database, compiled from the National Weather Service’s monthly Storm Data. Complete through 2026-06; NCEI adds each month about 75 days after it ends and revises past years a few times a year. Rebuilt 2026-09-24. Each event’s entry at NCEI: 967006, 967007.