Severe Thunderstorm Warning 18, NWS Medford, OR

Area Forecast Discussion
National Weather Service Medford OR
301 PM PDT Tue Jul 28 2020

.DISCUSSION...

Radar is showing scattered thunderstorms east of the Cascades this
afternoon with a lot of cumulus build up in northern California.
We're still forecasting isolated storms in northern California
this afternoon as the 18z north american mesoscale(NAM) model is
predicting some convective initiation across northern California.
Given the buildups observed on satellite, this seems reasonable to
see a few cloud to ground lightning flashes in northern California.
Convective available potential energy(CAPE) will be between 200
to 600 east of the Cascades this afternoon into the early
evening. Therefore, we're not anticipating strong storms. Cloud to
ground(CG) lightning will likely be less than 500 CG flashes
observed yesterday. However, we're still expecting 100-300 CG
flashes in our county warning area, hence the red flag warning for
today.

It will also remain hot as high temperatures push into the lower
100's in some of the valleys west of the Cascades today and
through Thursday. Overnight lows will be abnormally warm and in
the lower to mid 60's for tonight. Our internal heat risk
algorithm is recommending a heat advisory this afternoon through
Wednesday. We decided not issue one given we've seen multiple
lower 100 degree days already this year and we are at the
statistically hottest time of year the region. The normal high for
Medford is 93.

The other talking point is wildfire smoke now through Friday.
Sadly, the high resolution rapid refresh(HRRR) smoke model is down
due to a hardware issue. So we tried our best estimating smoke
coverage over our warning area for the next few days. In general,
we'll be under southerly flow with the winds shifting to the west
to northwest at times through Friday. This will push a lot of
smoke from the Caldwell fire north into Klamath Falls. Because of
this, the Oregon Department of Environmental Quality issued an air
Quality Advisory for Klamath County. There is also the Salmon
fire , pumping out plenty of smoke and started last night , in
south western Siskiyou County. Both of these fires will contribute
to reduced air quality over most of southern Oregon. In the past,
fires from the south have trouble significantly reducing the air
quality in the Rogue, Illinois and Applegate Valleys. Therefore,
we're expecting moderate air quality to continue for the majority
of the time in the afroementioned valleys.

Heading into Thursday, an upper level short wave will swing
through southern Oregon Wednesday night and Thursday morning.
These lows have been known to cause trouble with nocturnal
thunderstorms. Models are quite dry as of now with 1000-500 mb
moisture around 20 to 30%, so the models aren't expecting any
thunderstorms over the region. However, they have been known to
miss these events in the past. So we did throw a 10-20% chance for
thunderstorms in the forecast over central Cascades.

For Friday, a deep closed will settle in to our west with the
four corners high building to our east. We'll also see a dry cold
front and relatively cooler air surge in from the ocean during
the day Friday. High temperatures will reach climatological
normals with highs in the lower 90's west of the Cascades and
upper 80's east of the Cascades. One could also analyze another
dry cold front moving in around Sunday. Take home point here is
that it will be breezy ahead of these cold fronts in addition to
cooler for weekend.

-Smith

&&

.AVIATION...For the 28/18Z TAF Cycle...Over the coastal waters and
along the coast...Areas of IFR cigs/vsbys will burn off to the coast
by afternoon, then return to the coast and coastal valleys this
evening. The lower conditions will then burn back to the coast by
Wednesday afternoon.  Over the inland areas...VFR conditions will
prevail through Wednesday, except for areas of MVFR vsbys in smoke,
especially near the more active fires like the July complex near the
Siskiyou/Modoc county boundary.  There will be isolated to scattered
thunderstorms this afternoon and evening from the Cascades east and
Siskiyous south.

Stockton

&&

.MARINE...Updated 230 PM PDT Tuesday 28 July 2020...Gusty north
winds and steep seas south of Cape Blanco will diminish tonight.
Relatively light winds and seas will prevail through this weekend.
Winds will shift to the south Thursday night and Friday as a weak
cold front moves across the area. Winds will be variable Friday
into Sunday, then become generally from the north Sunday. Stronger
north winds and Small Craft Advisory conditions may develop Monday.

Stockton

&&

.FIRE WEATHER...Updated 200 PM PDT Tuesday 28 July 2020...This weeks
represents a critical week for fire weather across much of the
forecast area. About 850 cloud to ground lightning flashes have been
observed since July 26 (and more expected today primarily east of
the Cascades), and now gusty winds, low humidity, and hot
temperatures are following up those lightning events. Conditions
will become less critical over the weekend as humidity increases
and temperatures decrease.

Today likely represents the last day for abundant lightning through
at least the weekend (though some storms possible between now and
then and more on that below). Storms are focused over northern
Klamath and Lake Counties now and will diminish this evening.

On Wednesday, winds will trend higher across most areas. That, in
combination with some very dry air, has prompted us to issue some
Red Flag Warnings for Fire Weather Zones 622, 284, 285, and 624. In
addition to these areas, gusty south winds will impact the Scott and
Shasta Valleys in Siskiyou County, not reaching Red Flag Warning
criteria, but certainly enough to warrant additional attention in
this discussion and other products.

Wednesday night features a bit of energy and instability moving
south to north, affecting the Cascades and westward, and we've
introduce a slight chance for storms in the Northern Cascades
Wednesday night. We'll keep an eye on the potential for storms west
of the Cascades, but right now there's not enough potential to bring
that into the forecast.

On Thursday, dry conditions continue but winds should trend slightly
lower. Friday will feature another round of gusty winds and critical
conditions are possible again.

Cooling conditions are expected late in the week into next week, but
it is not a sharp cooling trend, and temperatures will likely remain
near to above normal.

Keene

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Red Flag Warning until 8 PM PDT this evening for ORZ624-625.
     Red Flag Warning from 2 PM to 8 PM PDT Wednesday for ORZ624.
     Red Flag Warning from 2 PM to 8 PM PDT Wednesday for ORZ622.

CA...Red Flag Warning from 2 PM to 8 PM PDT Wednesday for CAZ284-285.

Pacific Coastal Waters...Small Craft Advisory until 5 AM PDT Wednesday for PZZ356-376.