National Fire Danger Rating System

The process of forecasting LALs begins with a detailed assessment of the current synoptic scale weather pattern to determine if thunderstorm activity is possible in your forecast area during the next 12 hours. Stability indices, computed from morning 12Z soundings, are useful tools in predicting the possibility of thunderstorm activity later in the day. Pay close attention to calculated values of CAPE, 700-500 MB lapse rates, lifted index, K index, Showalter index, and the Total Totals index. On the synoptic scale, look for couplets of low level convergence and upper level divergence, mid-level triggers, theta-e ridges, frontal zone boundaries, jet streaks, etc to determine whether or not thunderstorms are likely in the next 12-24 hours. Also check the stability indices at upstream locations. Check RAMDIS images, or the AFOS lightning chart (GPHLDS), for lightning activity which is occurring at the present time, or for any activity which may have occurred overnight. Short wave troughs or cold, upper level lows in May and June will oftentimes produce thunderstorm activity across western Washington. With this type of pattern, thunderstorms usually form when mid- to late-afternoon surface heating destabilizes the air mass. However, this type of activity usually dissipates after sunset. The Puget Sound convergence zone can also be a favored location for thunderstorm activity this time of the year. With the above mentioned synoptic patterns, an LAL of 3 to 4 would be appropriate, since this type of thunderstorm activity is usually associated with locally moderate to heavy rainfall this time of the year.

STEP #2 - FORECASTING LIGHTNING ACTIVITY LEVELS (LAL)

FORECAST STABILITY INDEXES

Look at model forecast stability indices, Cape values, and 700-500 mb lapse rates to determine the potential for thunderstorm activity. If the air mass is forecast to become unstable and there is a threat of thunderstorms, forecast an LAL of 2 or 3 depending on the amount of ground strikes and rain you expect. Use the LAL guide on the Fire Weather homepage as tool for determining the correct LAL. If the forecasted indices indicate the potential for very unstable air, and you expect moderate to heavy rain, consider forecasting an LAL of 4 or 5. Remember to check your moisture progs to help determine the probability of moderate to heavy rain. If the air mass below 700 mb is expected to remain very dry (seldom the case in western Washington), there would be a threat of dry lightning. If dry fuel conditions have raised the fire danger to high or extreme, think about issuing a Fire Weather Watch or a Red Flag Warning. Remember...if you issue a Red Flag Warning for dry lightning, you must forecast a 6 for the LAL.

FORECAST THETA-E RIDGES AND MID-LEVEL TRIGGERS

Check the models for theta-e ridges or for the presence of weak short wave troughs in the mid-levels of the atmosphere. Thunderstorms tend to develop in theta-e ridges since this is the location of warm, moist, unstable air. Check the models for areas of Isentropic Positive Vorticity Advection (IPVA) between 200 and 400 mb. Areas of IPVA can indicate synoptic-scale lift and cooling aloft due to upper level fronts and are often the cause of nocturnal and/or dry thunderstorms.

MOS GUIDANCE

MOS thunderstorm guidance for sites in western Washington (i.e., FWCSEA, FWCBLI, FWCSMP, etc.) can provide a first guess on the probability of thunderstorms. MOS thunderstorm probabilities are given in 6 hour increments out to 48 hours. Pay particular attention to MOS probabilities that are much above climatology for the site. Note that most of the MOS thunderstorm probabilities available for this area are for low elevation sites and may be skewed on the low side when using this guidance in mountainous terrain.

STEP #3 - OBSERVED LIGHTNING UPSTREAM FROM THE NATIONAL LIGHTNING DETECTION NETWORK

Monitor current lightning activity upstream from your fire weather district. This can be done by looking at the LDS charts available on AFOS, or by tracking the location of lightning strikes detected by the NLDN available on RAMDIS images. Use the lightning detection system to evaluate the number of lightning strikes in a particular area. Use the strike count frequency on the LAL Guide to determine the correct

STEP #4 - SATELLITE IMAGERY, WSR-88D, AND PIREPS

Monitor satellite imagery, WSR-88D data, and pireps for signs of convective instability. Satellite imagery and pireps give good information on cloud top heights; radar data provide information on echo heights. Use the LAL guide to determine the correct LAL for the give echo or cloud top height. Water vapor imagery can also be particularly helpful in determining the presence or advection of moisture even before clouds develop.