On 5 December 2016, a significant storm was approaching the mountains of Colorado with various NWS winter weather watches/warnings posted for 6 December. This snow event on 5 December occurred ahead of the storm in what looked like drying conditions behind a fast moving shortwave. The GOES satellite imagery seemed to support the idea of drying on 5 December in all 3 available GOES Sounder water vapor channels. However, enough low-level moisture remained in the presence of low/mid level steep lapse rates to produce this storm before the storm on 5 December (generally around 6 inches at most ski resorts in the central/northern Colorado Rockies) that almost matched the amount of snow that occurred with the “main” storm on 6 December. In this blog entry, we’ll show how the low-level moisture, while not obvious in the GOES sounder/imager water vapor bands, was observed with the CIRA Layered Precipitable Water Product.
The GOES Sounder provides water vapor imagery that includes the GOES-R/16 three water vapor bands. Of course the resolution of the GOES Sounder imagery is considerably coarser (nominally hourly at 10 km) relative to GOES-R/16. In order to get accustomed to looking at three water vapor channels of GOES-R/16, we can make use of current GOES Sounder bands (at least for the western CONUS since GOES-East Sounder has failed).
Next, we bring up a loop of the GOES-West Sounder/Imager satellite imagery between 0602 UTC 5 December to 0602 UTC 6 December:
The water vapor imagery that forecasters are accustomed to viewing in AWIPS is the GOES Imager water vapor band at 6.5 microns as shown in the upper right panel. Keep in mind that there are notes at the bottom of each frame in the loop. To summarize what we see, an initial shortwave moves across northern Colorado between 0600 – 0900 UTC 5 Dec. followed by another shortwave thereafter with a fairly distinct signal in all water vapor bands, all within a strong westerly jet. The dominant signal after 1800 UTC 5 Dec. is an elongated region of warmer brightness temperatures associated with the westerly jet slowly drifting southward, most readily observed in the mid- (6.5 micron imager and 6.95 micron sounder) and upper-level (6.2 micron sounder) bands. One thing to note is that amidst the general warming signal we see an area of cooler brightness temperatures lingering over the central mountains of Colorado; what do you think this feature is? As will be shown in other imagery later, what we are seeing are the tops of a lower cloud layer that persists.
Next, we bring up a loop of multiple products, including the CIRA Layered Precipitable Water (LPW) product, along with GOES IR, visible as well as mosaic radar at night:
Forecasters are likely familiar with a satellite derived Total Precipitable Water (TPW) product, such as the NESDIS blended TPW product on AWIPS. The LPW product is derived from microwave instruments onboard multiple polar-orbiting satellites. The unique aspect of the LPW product is that the moisture is shown for 4 separate layers as indicated on the looping imagery. Keep in mind, the resolution of the instruments is considerably less than GOES and varies between instruments which is why the products appears more “blocky” at some times compared to others. Also, the satellite passes occur at irregular times, therefore post-processing displays what data is available at 3 hour intervals. The product time shows the latest available satellite pass, however keep in mind the most recent pass at a point may be several hours old due to irregular satellite pass intervals. For example, look at central Nevada between 0918 and 1218 UTC 5 December, the same data is displayed since a new pass does not occur until after 1218 UTC, which shows up in the 1517 UTC image. This product is still experimental, however it is available from CIRA in real-time, as well as in AWIPS as a satellite proving ground product (contact us at Edward.J.Szoke@noaa.gov). Additionally, CIRA is currently developing an advected version of the LPW product that will display at higher resolution and appear less blocky.
One of the main points to emphasize in this particular case is that we do not see a sharp north-south gradient over central/western Colorado in the 700-500 mb layer of the LPW (this is the most applicable layer for moisture flowing over the elevations of the Colorado mountains). Let’s focus on the 0018 UTC 6 Dec. LPW imagery when we had a higher resolution pass over Colorado. We may have concluded from the GOES water vapor imagery shown earlier that the region of warmer brightness temperatures was associated with “drying” through a deep layer upstream of the mountains (i.e., western Colorado). The LPW product clearly shows that there is moisture present in the 700-500 mb layer at 0018 UTC across western Colorado that would still be available to later move over the mountains in the westerly flow. We don’t see this moisture in the various water vapor bands since the weighting function profile is not low enough (even at 7.35 microns). If moisture is your primary forecast issue, the LPW product can clarify moisture at different levels in a more definitive way than implying it from GOES water vapor imagery alone. In this case, the moisture that remained produced a period of snow that lasted past 0600 UTC 6 Dec. in the mountains which was not predicted, perhaps owing to what looked like a drying signal in the water vapor imagery. This shallow moisture was also way underdone by even the high resolution models which predicted almost no snowfall and as seen in some of the radar mosaic images, was not resolved by radar either (due to overshooting beam). Here, the lower level moisture was sufficient to produce moderate snowfall because of the steep lapse rates that were present behind the initial shortwave, as seen in the Grand Junction sounding:
Also note the moisture between 600 and 700 mb which would be reflected in the LPW 700-500 mb layer imagery.
GOES IR imagery from 0000 to 0600 UTC 6 Dec. displayed above with the LPW product, shows the low clouds over the central mountains of Colorado, however they are subtle due to insufficient contrast between the low clouds and cold ground. The clouds are less subtle in the loop below:
This is the CIRA GeoColor product which is an experimental GOES-R Proving Ground product. The low clouds at night are peach colored because this product utilizes the 10.7 minus 3.9 micron product at night to highlight lower clouds / fog. Note that the higher level clouds are white in this product.