Wildfire in Utah, September 6 to 30, 2018

Area Forecast Discussion
National Weather Service Salt Lake City UT
926 AM MDT Thu Sep 6 2018

.SYNOPSIS....A weak trough will cross the area today, allowing
for a warming and drying trend Friday into the weekend.

&&

.DISCUSSION...
Impacts...
* A few storms may contain locally heavy rainfall this afternoon

Antecedent Conditions...
Exceptional Drought is occurring centered across San Pete county
and near the Four corners. Extreme drought continues across the
remainder of central and eastern Utah, with Severe drought along
the I-15 and Highway 40 corridors. Drought conditions scale down
to Moderate Drought along the Nevada, Idaho and Wyoming borders.

Big Picture...
During the past 5 days, the pattern has amplified across the
Pacific with a closed low south of the Gulf of Alaska and a ridge
off the southern California coast. A weak trough is ejecting
northeast across Nevada with a stronger closed circulation over
northeast Utah.

Water Vapor satellite and MDCARS aircraft observations show split
flow across the west, with a trough overhead and westerly flow
across southern Canada.

Local Observations...
12Z KSLC RAOB shows a well mixed dry layer above the nocturnal
inversion to about 500mb. Mid and upper levels show some areas of
near saturation. Winds are very light below 300mb.

Precipitable Water values range from 0.30"-0.40" northern
mountains to 0.75"-1.00" southern and eastern valleys.

A 1012mb surface cyclone resides near Boise with a 1022mb surface
anticyclone across southwest Colorado. This is creating a 6mb
southeasterly pressure gradient across the state.

24 hour trends:
* Precipitable Water 0.10"-0.20" higher across the north
* Temperatures are 5-15F cooler outside of the Wasatch Front
* Dewpoint Depression 5-15F lower (moister) outside of the Wasatch
  Front

Forecast...
Ahead of the approaching trough, experimental CIRA Advective
Precipitable Water shows significant moistening across the Great
Basin in the 700-500mb layer, with some moistening also evident
above 500mb and below 700mb.

This moisture will support modest destabilization per SREF and
FV3 BUFKIT forecast soundings. Latest Convective Allowing Models
show good storm coverage across the higher terrain, which may
bleed into the Western Uinta Basin and Castle Country on a light
5-10kt NW flow. Also added some PoPs near the Nevada border as
upstream development takes place along the trough axis and higher
terrain of Nevada.

Dry microbursts are possible with a DCAPE greater than 1000J/kg
today. Also cannot rule out a few spots of locally heavy rainfall,
despite a LCL near 600mb, as dewpoints are climbing to or
exceeding 40F across many valleys. Warm cloud layer only appears
to be 1-2kft thick. Suspect most or all of these areas of better
precipitation rates to take place in the mountains.

&&

.PREVIOUS DISCUSSION...
Weakly defined trough continues to lift northeast out of the
forecast area this morning. What remains of this system will move
out by this afternoon, turning the flow more northerly in its
wake. The airmass has already started to dry somewhat, and PWs
should predominately be in the 0.5 to 0.7 range by late afternoon.
With a small amount of lingering moisture and instability, some
showers and thunderstorms should develop again this afternoon,
primarily over the higher terrain. With the airmass continuing its
drying trend, coverage of convection should decrease by early
evening.

Maxes today will run near to just above seasonal normals, and will
see a subtle warming trend through Saturday as high pressure
builds off the California coast. This will also continue the
drying trend, limiting convective development for the remainder of
the short term period. A rather weak trough grazing northern Utah
on Saturday may allow for a few showers and thunderstorms to
develop over the higher terrain of northern Utah Saturday
afternoon, but the airmass will be too dry to support much. The
trough is also anticipated to cool temperatures slightly for
Sunday.

The longwave pattern across the western CONUS during this period
will be dominated by an increasingly amplified trough off the west
coast. High confidence exists that through midweek a dry and
increasingly strong southwesterly flow aloft will persist across
the eastern Great Basin downstream of this, with warming trend
temps and increasingly gusty conditions likely across the local
area. Fire weather conditions look heightened through the period
as a result.

Some questions arise regarding the evolution of this trough as it
begins filling during the latter half of the week, specifically the
likelihood of the southwest flow coupling with subtropical moisture
from the desert southwest which could allow for focused moisture
advection across namely eastern portions of the area. Subtle
differences in trajectories regarding said trough evolution will
make all the difference. At this time have maintained a dry forecast
through day 7, but this remains worth watching moving forward.

&&

.AVIATION...
Winds are expected to shift to the northwest at the SLC terminal
between 18-20Z.

&&

.FIRE WEATHER...
ERC values are running between the 80th and 96th percentile
across northern Utah, trending closer to the 50th percentile
across the south. ERCs have been either trending lower or steady
during the past several days.

An increase in moisture ahead of an approaching trough will
support a good chance of seeing wetting rain in the mountains this
afternoon and evening. This activity may also bring gusty/erratic
winds and cloud to ground lightning.

A drier westerly flow should dominate into the weekend, allowing
temperatures to warm and RH to trend lower. A weak cool front may
bring cooler temperatures to the north on Sunday.

Looking ahead to next week, a trough may develop near the West
Coast which would set up a dry and gusty southwest flow across the
region by mid week.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...None.
WY...None.
&&

PUBLIC...10/Traphagan/Merrill
AVIATION...Kruse
FIRE WEATHER...10

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