Atlantic Tropical Depression 15

October 11th, 2007 |

GOES-12 visible image + ASCAT winds

The National Hurricane Center began issuing advisories on Tropical Depression 15 (located in the central Atlantic, east of Bermuda) on 11 October 2007. A GOES-12 visible image with ASCAT wind data sourced from the CIMSS Tropical Cyclones website (above) suggested that maximum winds were only around 30 knots.

There was strong southwesterly flow aloft over the developing cyclone with increasing deep layer wind shear, so the bulk of the deep convection was confined to the northeastern quadrant of TD #15 — an animation of GOES-12 visible images (below) shows that the low-level center was close to becoming exposed in that strong wind shear environment.

GOES-12 visible images (Animated GIF)

Lee waves downwind of the Baraboo Bluffs

October 10th, 2007 |

GOES-12 visible images (Animated GIF)

“Lee waves” are sometimes seen just downwind of significant topography (such as high mountain ranges) when strong winds are perpendicular to the ridge lines — but even relatively subtle terrain features can produce lee wave clouds that are evident on satellite imagery (one such example of lee wave clouds in a region of subtle terrain was seen over the Bay of Fundy in Canada in August 2007). Brisk northwesterly winds were prevalent across much of southern Wisconsin during the day on 10 October 2007, and an animation of GOES-12 visible channel imagery (above) revealed a packet of quasi-stationary lee waves immediately downwind of the Baraboo Bluffs.

An AWIPS high resolution topography image (below) shows that the narrow Baraboo Bluffs terrain feature (located just to the south-southeast of Baraboo/Wisconsin Dells airport, station identifier KDLL) only has a maximum altitude of about 1503 feet above sea level (brown enhancement), but that was high enough to act as a barrier to the boundary layer winds and induce the lower tropospheric wave cloud features. Aircraft flying in the vicinity of such lee waves can encounter turbulence at times — however, in this case there was only one isolated pilot report of turbulence at 3500 feet above ground level over far southwestern Wisconsin (well to the west of the lee wave cloud features).

AWIPS topography image

Swaths of wet ground in the Plains

October 8th, 2007 |

AWIPS MODIS 4-panel images (Animated GIF)

Clearing skies in the wake of a cold frontal passage on 08 October 2007 allowed AWIPS 4-panel images of the MODIS visible, snow/ice, 3.7µm, and 11.0µm IR window channels (above) to reveal extensive swaths of slightly darker (and slightly cooler) wet ground which were oriented southwest-to-northeast across parts of Texas, Oklahoma, Kansas, Nebraska, and Iowa. One-day precipitation amounts within these swaths were as high as 1.0-2.5 inches in Kansas, with similar amounts falling across western Nebraska a day earlier.

AWIPS MODIS 3.7µm IR image

A closer view of western and central Kansas using the MODIS 3.7µm shortwave IR channel (above) shows that the IR brightness temperatures were several degrees cooler within the rain swaths (23-26º C or 73-79ºF, yellow enhancement), versus 28-33ºC or 82-91ºF (red enhancement) outside of the rain swaths — surface METAR data also depicted early afternoon (2:00 PM local time) surface temperatures a few degrees F cooler in the swaths where significant rain had fallen. The dew point temperatures were also generally a few degrees F higher at stations located within the rain swaths.

The GOES sounder Skin Temperature derived product image (below) showed a similar temperature difference between the areas that received heavy rain (skin temperature values of 25-27º C, or 77-81º F, green enhancement) and those areas that received little to no rain (skin temperature values of 31-37º C, or 88-99º F, yellow to orange enhancement).

AWIPS GOES sounder skin temperature image

UPDATE: From an operational forecasting standpoint, there was some concern that the areas in Kansas having higher soil moisture might be more prone to fog formation during the upcoming nighttime hours. An AWIPS image of the 1-km resolution MODIS fog/stratus prodcut (below) did not exhibit a fog signal (yellow enhancement) in the Kansas rain swaths at 08:39 UTC (03:39 AM local time) on 09 October. However, it was interesting to note that a few of the stations located within the primary rain swaths most notably, Hayes (KHYS) and Dodge City (KDDC) in western Kansas, as well as Salina (KSLN) and Pratt (KPTT) in central Kansas — apparently remained a few degrees warmer than adjacent sites just outside of the rain swaths, thereby not cooling close enough to their dew points for widespread radiation fog to form.

AWIPS MODIS fog/stratus product

GOES water vapor imagery: present, and “future”

October 2nd, 2007 |

AWIPS GOES water vapor imagery (Animated GIF)

AWIPS composite images of the 6.7µm GOES-11 and 6.5µm GOES-12 “water vapor” channels (above) show the eastward progression of synoptic-scale frontal systems across the US and Canada — as well as the westward motion of what could evolve into a subtropical disturbance in the eastern Gulf of Mexico — on 02 October 2007. Regions of greater middle-tropospheric moisture (blue to white enhancement) and clouds (white to green enhancement) are evident along and ahead of many of these fronts, while areas of subsidence and dry air (yellow enhancement) are seen in the wake of some of the frontal boundaries.

The CIMSS Regional Assimilation System (CRAS) numerical weather prediction model has the ability to output synthetic satellite imagery — a “forecast” of what GOES imagery might look like in the near future. AWIPS images of CRAS predictions of the GOES water vapor channel imagery (below) show the forecast covering the 02-05 October 2007 period. The CRAS imagery indicated that the pattern of alternating moist and dry features (associated with subsequent frontal passages) would continue for the next 2-3 days, while the moisture associated with the Gulf of Mexico disturbance would eventually move inland across the western and central Gulf Coast states during the forecast period.

AWIPS CRAS synthetic water vapor image forecast

CIMSS has been generating CRAS water vapor and IR forecast imagery since the mid-1990s, and this forecast imagery has been made available to AWIPS (via LDM subscription) since August 2006. The CRAS model forecast imagery in AWIPS has been well-received so far, being mentioned in several Area Forecast Discussions issued by the National Weather Service office at Milwaukee/Sullivan, Wisconsin. For more information, see the CRAS Forecast Imagery in AWIPS VISIT lesson.