Wildfire in Idaho, August 2 to 31, 2018

Area Forecast Discussion
National Weather Service Pocatello ID
244 PM MDT Thu Aug 2 2018

.DISCUSSION...Today Through Next Thursday.
The two main weather concerns this afternoon and evening will be
critical fire weather conditions and potential for strong t-storms.

CRITICAL FIRE WEATHER: Red Flag Warnings remain in effect until
midnight tonight across the Central Mntns, for a combination of dry
surface fuels, dry low-level air with RH values of 10-15%, and
strong winds gusting 25-30 MPH with higher gusts near peaks and
ridges. Isolated t-storms will also be possible in eastern
portions of the Central Mntns, with threats for lightning and
strong/erratic wind gusts in excess of 50 MPH. See the FIRE
WEATHER section below for more details.

STRONG THUNDERSTORMS: Yet another day of isolated to scattered t-
storms is anticipated, with the overall threat focused on the
eastern Central Mntns and Island Park region, along and east of
I-15, and along and south of I-86. HREF ensemble CAMs light up
the MT border region and the Southern Highlands into the Bear Lake
region for the most widespread activity, with coverage perhaps
just slightly less than yesterday. Atmospheric parameters are very
similar to yesterday, with NAM forecast soundings again
displaying inverted-V signatures with plenty of sub-cloud dry air
and fairly impressive DCAPE values of 1,100 to 1,600 J/kg, again
suggesting that t-storms will be capable of producing strong
downdraft/outflow winds in excess of 50 MPH. Latest SPC
mesoanalysis confirms these DCAPE values and also shows MUCAPE of
500-1,000 J/kg and slightly negative LIs. Deep-layer shear values
in excess of 30 knots (effective bulk) are largely confined to the
north, which may assist with storm organization/longevity in the
Island Park region. As we saw yesterday, isolated severe t-storms
producing damaging winds in excess of 60 MPH cannot be ruled out
anywhere. PWATS have backed off just slightly since yesterday,
running around 0.75, and temp-dew point depressions are fairly
significant in the low-levels, so generally heavy downpours should
be confined to the strongest cells and should be short-lived as
some of the precip evaporates on the way down. Overall instability
and shear values, particularly a lack of MUCAPE and modest CAPE
profiles in the hail growth zone, do not strongly favor large
hail, but cannot rule out small hail with the strongest updrafts.
The combination of frequent lightning and gusty winds will be of
concern to fire weather partners across the region, although
t-storm coverage was not quite high enough to justify a Red Flag
Warning south and east of the Central Mntns.

The trough brushing by the region to the north may be slow to leave,
and we carry low PoPs for some additional showers/t-storms Fri
afternoon along the MT and WY borders, and across the Bear Lake
region. Otherwise, fairly low RH values and gusty winds Fri-Sun
may again push portions of the region close or into critical fire
weather thresholds. Additional Red Flag Warnings are possible. In
general we should be dry from Sat right into next week as a strong ridge
of high pressure builds across the western US, centered over NV
and UT. A weak disturbance moving overhead Sun could produce a
shower or t-storm, but so far the models are spitting out very
little precip and confidence is low. High temps will cool by 5-10
degrees over the weekend in the wake of the trough, but will
progressively warm once again next week back into the 90s by Mon
or Tues, and potentially mid to upper 90s by Wed or Thurs in lower
elevations from the Snake Plain south. - KSmith/TW

&&

.AVIATION...
Upper level high pressure ridging continues to moves east as a
mostly dry cold front/trough moves through the area. Expect breezy
winds this afternoon and vicinity thunderstorms particularly at
KDIJ and to a lesser degree KIDA and KPIH. Expect mostly VFR
conditions though there will continue to be some smoke/haze
particularly for KSUN and KIDA. Winds will increase slightly more
tomorrow afternoon after the mostly dry cold front/trough moves
through. - TW

&&

.FIRE WEATHER...
The upper level high pressure ridge has continued to move east
today with a surface front/trough located around the Arco Desert
area. Drier air will continue to move in from the northwest
through the day. A Red Flag Warning remains in effect for the
Salmon- Challis and Sawtooth Forests. The surface front/trough
will move through southeastern Idaho on Friday pushing surface
winds stronger and lowering the relative humidity even more. Zone
427 south of Burley may be spared some of the stronger winds and
there is some uncertainty in Zone 413, otherwise winds and low
humidity will combine to create a critical day for the rest of the
area (outside of Zone 411 due to non-critical fuels) warranting a
red flag warning. Winds will decrease on Saturday in most areas,
but the Arco Desert may still be strong enough to consider the
area as critical. There will be a weak trough move through early
Sunday bringing very light precipitation/thunderstorm chances,
though models keep bringing this system through a little later
each model run. Models are starting to agree on high pressure
rebuilding in late Monday and remain through most of next week.
Expect well above normal temps to return by mid next week. - TW

&&

.CLIMATE...
Multiple heat-related records were set/broken during the month of
July 2018 among our SE Idaho climate sites. Pocatello Airport (KPIH)
spent a 28 day run from July 4th through July 31st at or above 90
degrees, shattering their previous all-time record for longest
streak of consecutive days with high temps at or above 90 degrees
which was 22 days ending August 10th, 2000. That streak continues
through today, now at 30 consecutive days. Pocatello also ended
the month of July in 10th Place for highest mean average
temperature, at 73.5 degrees. Records at KPIH date back to
January of 1939.

Burley Airport (KBYI) spent 20 days at or above 90 from July 12th
through July 31st, a streak that continues at 21 consecutive days
through yesterday (08/01). This ties KBYI in 3rd Place for
longest streak of consecutive days at or above 90. Burley also
ended July in 7th Place for highest mean average temperature, at
74.0 degrees. Records at KBYI go back to August of 1948.

Challis Airport (KLLJ) also impressively broke its all-time record
for longest streak at or above 90 since records began there all the
way back in January of 1895, reaching 28 consecutive days as of
yesterday (started July 5th). July 2018 was the hottest July on
record ever at Challis for mean average temperature, at 73.8
degrees. Challis also tied or broke 2 daily record high
temperatures during the month (July 9th and July 24th). Stanley
Ranger Station (KSNT) also tied 2 daily record high temperatures
during the month of July (the 9th and 25th).

There is good news on the horizon for those tired of the heat. High
temps Fri may be just cool enough to threaten these ongoing heat
streaks, but it will be close. By Sat, our confidence increases
significantly that our streaks at or above 90 will come to an end
at all climate sites, with high temps projected to hold in the
mid 80s. - KSmith/TW

&&

.AIR STAGNATION...
Varying degrees of degraded air quality will continue across SE
Idaho for the next couple days, as numerous wildfires burning across
Idaho and the Pacific Northwest produce smoke and haze. The Idaho
Department of Environmental Quality (ID DEQ) continues to rate air
quality "Moderate" or Yellow across the entire region, meaning that
air quality is acceptable, however, for some pollutants there may be
a moderate health concern for a very small number of people who are
unusually sensitive to air pollution. We have maintained areas of
smoke in the weather grids for the entire CWA through Sat morning.
Winds are expected to increase this afternoon, and again Fri
afternoon as a trough brushes by the state. This may help to
scour out some of the smoke and haze entrenched across the region
and trend us toward improved air quality. However, confidence is
low, as these winds could just as easily continue to advect in
smoke from large fires burning in northern CA and southern OR. A
review of the latest HRRR smoke model guidance does hint at some
possible improvement tomorrow, but also shows a fresh plume moving
NE toward the region out of NV. It is possible that significant
improvement may not come until additional progress can be made on
the source fires. -KSmith/TW

&&

.PIH WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning until midnight MDT tonight for IDZ422-475-476.

&&