212 FNUS28 KWNS 092121 FWDD38 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... $$
