FNUS28 KWNS 092121

Day 3-8 Fire Weather Outlook  
NWS Storm Prediction Center Norman OK
0420 PM CDT Sun Aug 09 2020

Valid 111200Z - 171200Z

Mostly zonal mid-level flow is forecast over the northern US through
the extended forecast period. A significant shortwave trough is
expected to move through the Pacific Northwest early Day3/Tuesday
bringing gusty winds, a dry cold front, and thunderstorm chances to
portions of the West. The ridge of high pressure over the Southwest
will likely begin to strengthen by midweek, increasing temperatures
and the potential overlap of warm dry and windy conditions across
the Great Basin. Elevated fire weather potential will exist across
several areas with the potential for ignitions from dry
thunderstorms as well. 

...Pacific Northwest into southern Idaho...
A dry cold front is forecast to move through the area Day3/Tuesday
bringing gusty winds and dry surface conditions to much of the
interior Pacific Northwest. Flow aloft from the approaching
shortwave trough will translate to the surface where 15-25 mph winds
are expected. Downslope drying and warm conditions ahead of the
front should keep RH to between 15-20%. Fuels across the area are
mostly receptive to fire spread and the increase in dry and windy
conditions warrants the inclusion of 40% of critical probabilities. 

...Northwestern Nevada and the central/eastern Great Basin...
Dry and windy conditions may develop across the area on the southern
fringes of the shortwave trough passing through the Pacific
Northwest Day3/Tuesday through Day5/Thursday. A potential
complicating factor will be the impact that thunderstorm development
has on mixing and the transport of stronger flow aloft to the
surface. If storm coverage remains Isolated and mid-level flow
strengthens sufficiently south of the trough, elevated to near
critical fire weather conditions may develop. Favorable fire weather
conditions appear slightly more likely on Day4/Wednesday as
convection is forecast to move to the east with the passing trough.
However, winds aloft may begin to weaken limiting the potential for
more robust boundary-layer flow. Uncertainty on overlap between
dry/windy conditions and thunderstorm potential remains too high at
this time to include any critical probabilities, though this will
likely be updated in coming outlooks.

Dry and gusty conditions will be possible beyond Day6/Thursday as
ridging begins to build across the Southwest. Increasingly warm
temperatures and zonal mid-level flow may overlap favorably to
support elevated fire weather potential as early as Day7/Friday and
through next weekend. Ensemble member divergence and the lack of
consensus between deterministic solutions suggests predictability is
too low at this time to introduce probabilities. 

...Dry Thunderstorms...
Thunderstorm chances will increase Day3/Tuesday through
Day4/Wednesday as lift from the approaching shortwave trough
overspreads the Great Basin and southern half of the Pacific
Northwest. A plume of precipitable water ranging from 0.4 to 0.8
inches is forecast to move into the area ahead of the cold front.
Model soundings show 250-500 J/kg of MLCAPE developing atop a deep
and very well-mixed boundary-layer extending to around 550 mb. The
degree of dry air in the low levels will almost certainly support
dry thunderstorms despite the relatively slow storm speeds. There is
some potential for storm coverage to reach scattered levels
Day3/Tuesday owing to the degree of buoyancy and sufficient lift
from the approaching trough. However, increasing moisture in the
lower atmosphere will likely limit the potential for dry
thunderstorms to isolated coverage.

..Lyons.. 08/09/2020

...Please see www.spc.noaa.gov/fire for graphic product...