LIGHTNING and DRY THUNDERSTORMS (thunderstorms that produce less than 0.10 inch of rain)
SPC Day 1 and Day 2 Fire Weather Outlooks are issued ONCE each day at 4 AM local.
***NEW***The SPC Convective Day 1 and Day 2 Outlooks are linked to the Fire Weather Web page and are UPDATED when they are issued during the day. The erratic nature of all thunderstom winds (changes in direction and speed;/gusts) plus lightning make all thunderstorm winds potentially dangerous for fire fighters.
***NEW***To aid fire weather forecasts of dry thunderstorm during Day 1, after the Fire Weather Forecast is issued at 4AM, an AUTOMATED MODEL DERIVED product set designed specifically for dry thunderstorm forecasting in the western United States will update on the Web. The product is designed to update with a forecast out to 12 hours every three hours during Day 1 (for times...see below).
COMPONENTS of the AUTOMATED MODEL DERIVED PRODUCT
Each graphic has two parts. The left half of the graphic displays 1) the Convective Available Potential Energy (CAPE), 2) Convective INHibition (CIN or CINH) and 3) AVeraGe RELative Humidity (AVG RELH), the humidity averaged over the lowest 2000 ft (60 mb) above the RUC 2 model surface. The CAPE is derived at each model grid point by searching the lowest 6000 ft (180mb) for the most unstable parcel. Studies over the past year have shown that thunderstorms can develop IN or NEAR areas where the CAPE is 50 Joules/kg or greater (shaded light green on the graphic). The CIN is a measure of the capping inversion that acts to keep the thunderstorms from developing. Experience has shown that if the CIN is greater tham |-50| Joules/kg (shaded dark red), thunderstorms are much less likely. The most likely area for thunderstorms to develop would be in or near the light green shaded areas.The areas with the dark red shading are areas with CAPE, BUT these areas would be MUCH LESS LIKELY to have thunderstorms due to the capping inversion (because the CIN is more negative ("stronger") than -50 Joules/kg). The AVG RELH represents the average humidity in the lowest 2000 ft above the model terrain (chosen to reflect a sufficiently deep layer away from cloud base where precipitation could potentially evaporate). If thunderstorms develop(see CAPE); the lower the AVG RELH, the better the chance of dry thunderstorms due to evaporation of precipitation.
The right half of each graphic is designed to highlight any upper level triggering mechanism. It displays the Potential Vorticity (PV) in the 200 to 400 mb layer (contoured in 0.5 "PV units" at 1.0 and above) and 300 mb STREAMLINES to show the upper level flow in the same layer as the PV. The shaded areas of PV above 1.0 "PV units" often highlight weak upper-level disturbances that can trigger thunderstorms where the CAPE is >= 50 Joules/kg.