Weather Service Report


046 
FXUS66 KMFR 202304
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
404 PM PDT Fri Jul 20 2018

.DISCUSSION...The forecast has become increasingly complex and
impactful, now that we have multiple active fires, smoke and
unhealthy air quality, another heatwave on the way, and more 
chances for thunderstorms.

Temperatures today are running about 5 degrees lower than
yesterday for areas that have the heaviest smoke, and temperatures
are higher than yesterday by about 5 degrees in areas outside of
the smoke in the Umpqua Basin around Roseburg.

Tomorrow temperatures should trend upward by a few degrees, with
readings about 5 degrees above normal for this time of year. Just
enough moisture will creep up from the south to bring a slight
chance for thunderstorms into the southeast portion of Modoc
County. Latest model data suggests no thunderstorms, but after
observing the models going back and forth the past few days, we
decided to keep in the slight chance of thunder there. 

On Sunday, we'll have a combination of trigger, moisture, and
instability to support thunderstorm chances. The trigger will be
weak shortwave energy moving up from the south. This is noticeably
weaker than the energy that brought the thousands of cloud to
ground lightning strikes last Sunday, but it should be enough to
kick off some thunderstorms. The moisture is certainly sufficient
with PW values near an inch and a large area of H5-H7 humidity of
60% or greater. Lastly, H8 computed Lifted Index values of -2C to
-3C and CAPE values near 1000 J/kg suggest enough instability to 
support thunderstorms. Even areas that see isolated thunderstorm 
coverage could see a significant amount of new lightning starts  due 
to the very dry vegetation. Please see the fire weather  discussion 
below for more details.

Temperatures begin a significant trend upward on Monday, and the 
hottest conditions should last through Wednesday. Smoke will likely 
prevent many areas from experiencing the full extent of the heat, 
and we've kept temperatures slightly below guidance for now.
Tomorrow, we'll continue to evaluate and consider a heat advisory or 
excessive heat watch for areas. Instability does hang around Monday, 
and will continue to bring a slight chance for thunderstorms, though 
chances don't look as high as Sunday. 

Another hot day is expected Wednesday, and then there are some model 
differences in terms of when cooling will occur. The GFS suggests 
cooling by Thursday, while the EC maintains the heat. If past is 
prologue, we'll likely be in store for at least another very hot 
day. 

&&

.AVIATION...For the 20/18Z TAFs...Wildfire smoke continues over many 
of the inland valleys west of the Cascades. Lower visibilities along 
with MVFR Cigs will exist near these fires. The latest high res 
smoke guidance suggest smoke remaining in the valleys for at least 
the next 24 hours. Brief improvement is possible in the Rogue 
Valley, including the Medford Airport due to gusty northwest winds 
towards 0z, but confidence is not high enough to include it in the 
TAF. Also expect gusty winds at Roseburg and Klamath Falls late this 
afternoon through early this evening, then diminishing late this 
evening. 

At the coast, the main concern will be gusty north winds, north of 
Cape Blanco through this evening with gusts to 35 kts likely at 
North Bend. VFR cigs will continue through this evening. Guidance 
hints at marine stratus developing around daybreak Saturday with 
MVFR cigs, but more likely the cloud cover will be scattered and 
this is indicated in the TAF. Gusty north winds will be a concern 
again for similar locations at the coast. -Petrucelli

&&

.MARINE...Updated 200 PM PDT Friday 20 July 2018...High 
pressure offshore and a thermal trough at the coast will persist 
through Saturday night with gusty north winds and very steep wind 
driven seas. The steepest seas and strongest winds will occur into 
this evening, then again Saturday afternoon and evening. Gales and 
very steep seas will persist south of Cape Blanco beyond 5 nm from 
shore through Saturday night, with advisory conditions elsewhere. 

Winds and seas will then see a weakening trend late Saturday night 
into at least Wednesday. The weakening will be due to the thermal 
trough moving inland with high pressure building into the waters. 
Though conditions will be improved, steep seas are likely on Sunday 
and are also possible in the outer waters south of Cape Blanco on 
Monday. Northerly winds may increase on Thursday into Friday. 
-Petrucelli

&&

.FIRE WEATHER...Updated 330 PM PDT Friday 20 July 2018... 

A thermal trough pattern will continue to deliver north winds and 
dry conditions to the forecast area. Burning periods will gradually 
lengthen through Saturday at least. North to south valleys will be 
favored for the strongest winds into this evening, and this will 
combine with low humidity in the Illinois Valley portion of FWZ620, 
and a Red Flag Warning for that area continues to highlight these 
expected conditions. 

Saturday will see north winds trending lower but west winds on the 
East Side of the Cascades will increase some. Additionally, moisture 
and instability will increase some from the south and bring a slight 
chance for thunderstorms to eastern Siskiyou and Modoc Counties. 
Winds and RH may combine again in the Illinois Valley again Saturday 
afternoon, and will see if the next model runs continue to trend 
this way and issue a watch if they do.

Thanks to a disturbance to the north and some southerly flow to the 
ridge levels, the thermal trough and gusty north wind pattern 
weakens on Sunday. Models continue to show the thermal ridge moving 
inland into the Cascades Sunday. This would create a very dry and 
steep lapse rate near the ground. This will combine with cooler 
temperatures moving in aloft and an unstable air mass developing 
over much of the region. Each model run continues to creep the area 
of instability farther north into the west side of Oregon, and the 
current watch may creep farther north by the time we reach the 
Sunday time frame. These thunderstorm chances basically cover the 
Siskiyous southward and Cascades eastward. Although we are only 
expecting isolated thunderstorms over the west side Saturday, ERC's 
and 1000 hr fuels are at record levels and over the west side the 
almost every strike resulted in a start last Sunday. Instability 
weakens but remains over the region Monday, but with weak steering 
winds anything that develops should latch to terrain. Sven

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Fire Weather Watch from Sunday afternoon through Sunday evening 
     for ORZ621>625. 
     Red Flag Warning until 8 PM PDT this evening for ORZ620. 

CA...Fire Weather Watch from Sunday afternoon through Sunday evening 
     for CAZ280>282-284-285. 

Pacific Coastal Waters...Small Craft Advisory until 5 AM PDT Sunday 
for      PZZ350-356-370-376.      Gale Warning until 5 AM PDT Sunday 
for PZZ356-376.      Hazardous Seas Warning until 5 AM PDT Sunday 
for PZZ356-370-376. 

$$

Return to Home page


Western Regional Climate Center, wrcc@dri.edu