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.

Dual Jet Captured in Upper Level Simulated ABI Water Vapor Imagery

A forecast of SSEC-CIMSS Simulated ABI Band 08 Water Vapor imagery valid at 12z Wed (above) showed two strongly sheared areas, indicative of two upper level jets moving across the upper Midwest. The southern strong water vapor gradient over southern Minnesota is properly positioned when compared to the 12Z TUE GFS 300-200mb wind speed forecast valid at 12Z Wed (below). There is another strong water vapor gradient apparent at the base of the upper level trough, which is also well-placed when comparing it to model data. The simulated water vapor imagery serves as a proxy for identifying and tracking the evolution of upper level jets without having to rely solely on model data.

Submitted by Marcia Cronce (NWS) and Dan Hartung (CIMSS).

Marine Fog Near California

Marine fog was clearly apparent along the California coastline with the GOES-W 11-3.9 micron satellite channel difference imagery (above) on the morning of Tuesday, August 16, 2011. In fact, we could see fog or low clouds that spread inland near Los Angeles and became trapped against the San Gabriel Mountains.

The SSEC CIMSS GOES-W MVFR Probability product (above) showed 70-75% probability of MVFR ceilings occurring in this region, with higher probabilities further north, off the coast of San Francisco . The SSEC CIMSS GOES-W IFR Probability product (below) showed 40 to 50% probability of IFR ceilings near Los Angeles, with up to 75% near San Francisco. These products both captured the fog that was trapped up against the San Gabriel mountains very well.

The surface observations (overlaid on the satellite products) showed dense fog (1/4 statute mile visibility and vertical visibility of 100 feet) at Catalina Airport on the island, and fog (2 1/2 statute mile visibility and vertical visibility of 500 feet) at Los Angeles Airport (KLAX). Other observations just inland of LAX had MVFR visibility and 500 to 1000 foot ceilings. San Francisco had 200 foot ceilings, lower than those at LAX.

The MVFR and IFR Probability products captured the marine fog very well, spatially. Quantitatively, it tended to be underestimated near the Los Angeles area. Due to the lack of observations offshore, it was difficult to discern between low stratus and fog that we could see in the satellite imagery. Therefore, we could not fully evaluate how well the MVFR vs IFR Probability products captured the event, but we could still infer their validity.

From a short-term forecasting standpoint, we already know we have low ceilings, but it would be very useful to know how deep the fog or low cloud area actually is so that we can determine how long it will be before it can burn off/mix out. We can use NWS soundings, data from aircraft, or other data that may be available at various points that are typically far apart. However, the SSEC CIMSS group has this Fog Depth product that can show from a satellite point of view how thick the fog/low stratus is (see image below). In this image, the fog depth was about 1115 to 1175 feet thick near Los Angeles.

This NWS sounding from KNKX San Diego (above) shows the top of the marine layer was at about 1500 feet AGL, very similar to what the Fog Depth product showed.

I know I will use this product when dealing with fog in the Great Lakes Region when I'm on the short-term forecast desk.

Submitted by Marcia Cronce (NWS) and Dan Hartung (SSEC CIMSS)

Notes From 8/9/11 Session

The above image is a visible satellite image from GOES at 2031UTC on 8/9/11. It shows a line of thunderstorms from southern Lake Huron to southeast lower Michigan into far northwest Ohio and far northeast Indiana. This convection was observed along a cold front advancing east through the area. Now, compare the above image to the image below:

The above image is the Theta-E Nearcast product at 2030UTC on 8/9/11. The green areas indicated unstable conditions (low level theta-E minus mid-level theta-E), and the blue areas indicated stable conditions. Note how this product clearly shows the unstable conditions along and ahead of the cold front, and this area corresponds to the convection shown in the 2031UTC visible satellite image. This also showed the stable airmass behind the cold front. Given that this product can be viewed several hours into the future, this product could have given a forecaster more confidence to diagnose the best areas for convection to initiate and grow.

The above image is the visible satellite image from GOES at 1445UTC on 8/9/11. Note the large area of VFR to MVFR ceilings over nothern and central Minnesota into far northwest Wisconsin. Also of note is a smaller area of MVFR to IFR ceilings over southern Wisconsin. Now, compare the visible image above to the image below:

This image is the MODIS Probability of MVFR Ceilings product, at 1445UTC on 8/9/11. Two things of note here are 1) the high probabilities of MVFR ceilings across northern Minnesota into far northwest Wisconsin, and 2) the strip of low probabilities of MVFR ceilings across southern Wisconsin. This product had a very good depiction of the MVFR ceilings over central/northern Minnesota into far northwest Wisconsin. Some VFR ceilings were also mixed in with the MVFR ceilings.

Another interesting note was this product showed low probabilities of MVFR ceilings over southern Wisconsin, where MVFR to IFR ceilings were observed. These low probabilities could have helped give an aviation forecaster some confidence that low ceilings were possible across TAF sites in this area. These clouds formed quickly after sunrise, as a cold front was moving southeast through the area.

J. J. Wood

National Weather Service

Milwaukee/Sullivan, Wisconsin

8/9/11

Waiting for cirrus to dissipate

Considerable overnight and ongoing convection over the upper midwest on August 2nd has left a large cirrus shield over Wisconsin. The UW Convective Initiation (UWCI) product does not detect convective development in regions where significant ice clouds already exist. The 'ice mask' -- where UWCI cannot be computed, can be displayed in AWIPS, and it shows the extent of the ice cloudiness over Wisconsin. A small gap has developed over the extreme southwest part of the state. If this region expands with time, then UWCI might be able to detect incipient convection over the upper midwest later this afternoon.






A MODIS Cirrus Channel image (1.38 micrometers) from 1710 UTC on 2 August shows that cirrus persists over the upper midwest.














Submitted by Scott Lindstrom (UW CIMSS) and Denny VanCleve (NWS-MKX)

Using NearCast vertical theta-e differences on a Warm Humid morning

Note the outflow boundary that is moving southward (in the 1512 UTC radar composite to the left) into the MKX WFO, arcing from northern Sauk County (south of the Dells) through central Columbia County. (The convection that has spawned this outflow is in the northern strip of counties of the WFO to the west of Fond du Lac) What are the chances that convection will fire along this outflow boundary? Are there products that could help you decide if the convection will continue, or will redevelop as the outflow boundary moves south?






The NearCast forecast of Theta-e differences valid at 1530 UTC shows a region of stronger stability over the MKX WFO -- suggesting that any convection that moves into southeast Wisconsin will struggle to develop. Convection might pop -- and it did over northeast Dane County at 1627 UTC (see below) -- but that convection was short-lived.










NearCast output suggests the convection over Dane County will not persist. The highest tops were to 20-25000 feet, and no lightning occurred. (Radar image at left from 1627 UTC)
















Posted by: Scott Lindstrom (CIMSS) and Denny VanCleve (MKX)