Verifying yesterdays forecast and comparing to NSSL-WRF lightning threat

1800 UTC 24 August - 1200 UTC 25 August 2011 dry thunderstorm probability forecast with NLDN lightning detections from 0030 (top left), 0450 (top right) and 1150 UTC (bottom) on 25 August 2011.
At the beginning of the experiment today we 'verified' our previous day's forecast for dry thunder over much of the NW US. We had probabilities of dry thunderstorms reaching 40% over much of the area (see images above). When compared to the NLDN observed lightning activity, we see that our higher threat areas (30-40%) matched up fairly well with what occurred. Most of the storms over OR were classified as dry thunder, most likely due to their rapid storm motions. We could have extended our area a little further east to cover central ID (see last image above), which was suggested by the NSSL-WRF total lightning threat. When examining the GOES fire / hotspot detection product to our forecast, we did see a few new starts in the area, with at least one large fire confirmed by observers.

When we compare the observed lightning to that which was forecast by the NSSL-WRF total lightning product, we see that overall the NSSL-WRF tended to slightly downplay some of the lightning activity, but the spatial locations and timing were fairly well forecast (see yesterday's post). This product was developed and validated over the SE US, so the values in the west have yet to be compared directly. Part of this experiment is to get a general idea of how well this product could work over the western US, with the potential for use in operational fire weather forecasts.

Dry thunder over southern Oregon

7-day observed surface dryness (top) and dryness anomaly (bottom) from the GOES-West satellite from 23 August 2011.
Today has the potential to be a fairly significant day for fire weather threat over much of the NW US. GOES-West observed surface dryness and dryness anomaly over the past 7 days (images above) indicates significant drying of potential fuels over much of the western US, with a relative maximum over much of eastern OR and into ID, CA and NV. This agrees well with the observed Predictive Service Area dryness product.

A relatively strong vorticity max for the area is expected to move through this afternoon and bring with it some moderate instability with low PW and surface RH. Storms are expected to track fairly quick, so even if there was a chance of wetting occurring at the surface, the duration would be limited, so the potential for dry thunder is fairly high. Given the good amount of instability, the amount of lightning strikes will be relatively high, which increases the potential for new fire starts. The NSSL-WRF experimental lightning threat output shows this to some degree during the 2300-0200 UTC time periods of the 24th and the 25th of August (see images below).

NSSL-WRF experimental lightning threat for 24-25 August 2011 at 2300 (top left), 0000 (top right), 0100 (bottom left), and 0200 UTC (bottom right).

Fire Weather Experiment... Day 2

HWT Fire Weather Experiment participants during domain selection map discussion.
Today we started our second day of the Fire Weather Experiment, building off of what we did yesterday by first examining the forecast we made against base reflectivity, NLDN lightning strikes and the GOES fire rating product (FRP) from UW-CIMSS. We did see the occurrence of multiple lightning strikes and relatively low base reflectivity over the area we forecast a possibility of dry thunderstorms, suggesting that there might have been some. While examining the FRP, we did see a few new start-ups over the area where lightning occurred (see figure below). However, as some participants noted, the fire rating product seemed to have only two colors for rating the fires, pale yellow or red. They were expecting to see a wider variety of fire 'ratings', so after the experiment was finished we made a few modifications to the color tables to help distinguish the ratings better.

Experimental probabilistic forecast for dry thunderstorms from 22 August 2011 with FRP detections overlaid.

NDVI and satellite-based dryness observations versus operational data

PSADryness product for 22 August 2011. This product is routinely available within SPC operations. Areas of yellow and red indicate significantly dry surface measurements
SPC forecasters routinely use a product originally developed by the NWS Salt Lake City and the Eastern Great Basin Predictive Service Office called the Predictive Service Area Dryness (psadryness) product (see above) to help make their day 1-8 fire weather outlooks. This product provides the forecasters with an idea of the dryness of burnable fuels near the surface. In addition, the forecasters use a high-resolution 'land-use' product that attempts to simulate the NDVI product, but is rarely, if ever, updated. One of the goals we wanted to accomplish from this experiment is to see how the higher resolution datasets of observed NDVI and NDVI change (below), as well as the GOES surface dryness and dryness anomaly products (also below), compare to products currently in operations, such as the psadryness product (above).

14-day composite NDVI (top) and 28-day NDVI change (bottom) from 15 August 2011. Areas of green indicate regions where increased 'greenness' is observed.
During our first day, 5 SPC fire weather forecasters participated and examined these products to make an experimental "update" for their day-1 fire weather outlook, or out to 12 UTC the next day. In particular, forecasters were asked to make a forecast graphic depicting the areas of high threat for burnable fuels. When comparing the psadryness product and the satellite-based products, we noticed that there was a noticeable discrepancy over some areas, specifically over central ID (see above). While the psadryness product said that the area was extremely dry, the NDVI and NDVI change depicted areas of increasing 'greenness'. In addition, the GOES surface dryness and dryness anomaly products indicated no significant drying over the area.

GOES 14-day composite surface dryness (top) and 5-year average dryness anomaly (bottom) for 22 August 2011. Areas of yellow and red indicate increased surface dryness.
So what gives? Well we explained to the forecasters that the satellite-based products are limited to sensing the canopy of the location they are observing. this means that if there is any forest in the area, we cannot see the undergrowth, which could be dry and only measurable from surface instruments or observers. Unfortunately, this is a limitation we have to deal with, particularly from geostationary satellite-based instruments. However, the forecasters were impressed by the spatial resolution and relative rapid updates of the products, which is not provided to them from the psadryness or land-use products. It may be useful to combine these datasets to get a more detailed picture of what may actually be going on at the surface when it comes to burnable fuels.

Fire Weather Experiment Begins Today

Example HWT display, showing satellite-based surface dryness anomaly on the left and the online survey worksheet on the right.

Today marks the beginning of the first ever fire weather experiment at the HWT. This year will be a very informal year with only a few local participants. Our goal is to establish a strategy and framework for which future fire weather experiments can occur at the HWT. We plan on examining some satellite based vegetation and surface measurements such as NDVI, NDVI change, surface dryness, and dryness anomaly. We will also be examining some experimental model data including the simulated satellite imagery and lightning threat from the NSSL-WRF.

We hope that we can get some valuable feedback on these products, as well as how to best demonstrate them in future experiments. I will be posting regularly about our progress.