Wildfire in California, September 5 to 22, 2019

Area Forecast Discussion
National Weather Service Medford OR
1054 AM PDT Thu Sep 5 2019

.Updated Aviation Discussion.

.DISCUSSION...Plenty of lightning occurred in the overnight hours
with a total of 774 lighting strikes over our forecast area in
the last 24 hours. of those 774 occurred in the last 12 hours.
Thunderstorms in the overnight period did produce some rainfall
and did vary depending in location. For example rainfall amounts
in Jackson County varied from northing to 0.20 of an inch. Plenty
of low level moisture exist this morning due to thunderstorms and
precipitation. Dewpoints are higher, especially where it rain.
Medford the dewpoint is currently 62 and Grants Pass is 61.

So plenty of low level moisture exist and the fact that
thunderstorms gave off some latent heat could provide a spring load
for a very active day late this morning through the afternoon.
Basically once that spring gives way, clouds will quickly build
followed by thunderstorms.

The latest satellite image shows a lot of clearing in the
southern part of our forecast area. However, don't be fooled by
that. This will only set the table for increasing instability as
the day progresses. There's good agreement among the convective
allowing models (CAM) for thunderstorms to fire up early to mid
afternoon, then increasing in coverage late this afternoon into
this evening. Could not rule out a few strong to possibly severe
storms late this afternoon and evening, mainly along and east of
the Cascades, westside near the Cascades and Modoc County in
California. Also could not rule out strong storms in some of the
west side valleys, including the Rogue valley and Umpqua Basin.

The focus for today will be close monitoring of thunderstorms
through this evening.

This evening and tonight, the focus for thunderstorms will shift
north and east of the area as the upper trough moves through the
area. -Petrucelli

&&

.AVIATION...For the 05/18Z TAFs...The main hazard today will be
isolated to scattered thunderstorms, some of which could be strong
producing gusty and erratic winds and hail. An unstable air mass
remains over the area and clearing that has developed for most
inland locations will only set the table to increasing instability
as the day progresses. The general consensus is for thunderstorms to
develop between 21-22z and peak in coverage and intensity between 23-
1z this evening. However confidence on the timing is low, so updates
to the TAF's are likely. VFR ceilings are expected to be the
predominate condition into this evening, except for MVFR ceilings
and visibility in thunderstorms.

Meanwhile, coastal IFR/LIFR stratus/fog will persist this morning.
The marine stratus could break up this afternoon at North Bend and
along the coast as instability increases, but confidence on the
timing and duration of VFR ceilings are low. Any improvement will
not last long with lower conditions returning tonight.
-Petrucelli

&&

.MARINE...Updated 905 AM PDT Thursday 5 September 2019...Winds and
steep seas will cause conditions hazardous to small craft this
morning, then both will diminish this afternoon then remain low
through the weekend as weak low pressure moves through the waters.
Northwest swell will increase modestly early next week as another
low moves toward the Pacific Northwest. -Spilde/Petrucelli

&&

.PREV DISCUSSION... /Issued 319 AM PDT Thu Sep 5 2019/

DISCUSSION...

We've seen a fair amount of lightning across the region this
morning with 300 cloud to ground strikes reported in the Medford
county warning area. Most of these strikes occured in Siskiyou
county closer to the north east quadrant of the low. Looking at
current radar imagery, the air appears to be more stable to the
north near Roseburg and Crater Lake. So, we're not expecting that
excessive lightning to spread north until later this afternoon.
The other talking point is that these storms have been on the
drier side of things. Most of the surface observation stations
have reported under 0.1 inches of rain under these cells this
morning.

As for the forecast later today, highs will push up into the mid
90's again with scattered thunderstorms across most of the area.
Potential instability will increase with stronger winds aloft.
This should result in some stronger storms producing gusty winds
across south central Oregon. The increase in potential instability
will also increase the occurrence of cloud to ground lightning
this afternoon and evening. The good news here is that the
increase in moisture should allow these storms to gradually become
wetter than what we saw this morning. Regardless, they will still
pose a fire weather risk and we'll continue the red flag warning
through this evening.

Eventually, this low will depart the region on Friday and we'll
see high pressure build in it's wake. Temperatures will still
reach the lower 90's in the west side valleys and 80's for most
other locations with 70's along the coast.

There are still some uncertainties when the next low will arrive,
but it's looking like Saturday afternoon and evening as of now.
The best chances for rain will be east of the Cascades. It is
still a little strange why the guidance is so dry west of the
Cascades Saturday afternoon. Another low will follow pretty
quickly on Sunday keeping the chance for rain in the forecast for
the weekend. With these lows moving through, high temperatures
will trend much cooler compared to what we've seen over the last
week or so. Highs will struggle to move into the lower 80's on
Sunday.

This troughy weather pattern is expected to continue into the
start of next week with the ensemble models showing deep troughing
over northern California Tuesday into Wednesday. The good news
here is that if any fires are able to start and get going today,
then the cooler weather, with a little bit of rain, will help the
fire fighting efforts over the next seven days.

-Smith

&&

FIRE WEATHER...Updated 230 PM PDT Wednesday 4 September 2019...The
main concern remains Thursday into Thursday evening with the
potential for abundant lightning with dry fuels.

Isolated thunderstorms will develop tonight with little to no
rainfall, mostly over northern California and the Cascades.
Thunderstorms will increase in coverage Thursday, peaking late
Thursday morning into Thursday evening. The heaviest activity will
occur from the Cascades east. The thunderstorms Thursday will mostly
be wet, but lightning can occur outside of the rain core, so the
risk of new fire starts will remain high. The thunderstorms will end
Thursday night and Friday will be warm and dry as high pressure
briefly builds over the area. A cool and showery regime will develop
Saturday as an upper trough moves to the coast, and this regime will
persist into next week.

-Stockton

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Red Flag Warning until 11 PM PDT this evening for
     ORZ616-617-620>625.

CA...Red Flag Warning until 11 PM PDT this evening for
     CAZ280-281-284-285.

Pacific Coastal Waters...Small Craft Advisory until 11 AM PDT this morning for
     PZZ370-376.