GOES-15 and GOES-13 6.5 µm water vapor images (click image to play animation)

Heavy rain fell over much of the Front Range of Colorado overnight, with Boulder receiving an all-time 24-hour total of 9.08 inches (Weather Underground blog). Numerous CoCoRAHS locations in Boulder County reported 24-hour rainfall amounts in excess of 6″, in a region where monthly precipitation records are close to 6″. This event could be classified as a 500- to 1000-year event (as shown at NOAA’s Precipitation Data Frequency Server).

A composite of GOES-15 and GOES-13 6.5 µm water vapor imagery (above) showed the large-scale environment that supported heavy rains over Colorado. A quasi-stationary upper-level cyclonic circulation was over the western United States; that circulation was drawing moisture northward from off the Pacific coast of Mexico (some of this moisture may have been associated with tropical cyclone Lorena, which dissipated off Baja California last week). In addition, a lower-tropospheric anticyclonic circulation pushing into the Midwest was shunting moisture westward towards the Front Range (03 UTC surface analysis). Several circulations, therefore, were helping to move moisture-rich air towards eastern Colorado.

Percent of Normal Total Precipitable Water

Moisture content of the air can be defined by total precipitable water (TPW). For example, the percent of normal as determined from the blended TPW product, above, showed a thin ribbon of values approaching 200% of normal banked up against the Front Range. An animation of GOES Sounder Derived Product Imagery (DPI) TPW (here) showed values (in between small breaks in the persistent cloudiness) exceeding 30 mm or 1.2 inches — in fact, the 1.33 inches of TPW derived from the 12 UTC Denver rawinsonde data set a new record for the month of September at that site (Weather Underground blog). The sounder data also demonstrates how the Front Range capably prevented moisture from moving from eastern to western Colorado: TPW values are smaller over central and western Colorado.

GOES-15 and GOES-13 10.7 µm IR images (click image to play animation)

GOES-15 (GOES-West) and GOES-13 (GOES-East) Imager 10.7 µm IR channel data, above, show that the convection that produced the flooding rains was relatively unimpressive on satellite imagery: cloud tops were not particularly cold. This suggests that warm-cloud processes were very efficient in producing the extraordinary rains in this event.One notable convective burst was seen to develop over the Boulder area (station identifier KBJC) after about 03 UTC on 12 September (10 PM local time on 11 September).

MODIS and GOES-13 Longwave IR (~10.7 µm) images at ~0440 UTC

MODIS and GOES-13 Longwave IR (~10.7 µm) images at ~0900 UTC

The relatively small-scale nature of this event suggests that high-resolution polar-orbiter data might be useful in describing the environment surrounding the storms. MODIS, for example, provided high-resolution imagery twice overnight, and comparisons between the MODIS and concomitant GOES imagery is shown above. MODIS data does have somewhat cooler cloud-top IR brightness temperatures (-54 to -55 C on MODIS, vs -48 to -50 C on GOES). The strongest convection is over the region that reported very heavy rains. Note that the MODIS imagery does not suffer from the parallax shift inherent in GOES imagery.

Suomi/NPP VIIRS Day/Night Band imagery at 0826 UTC

Suomi/NPP VIIRS data was also available over Colorado during the overnight hours. The Day/Night Band imagery at 0826 UTC or 2:26 AM local time, above, showed a region of clouds over the Front Range of Colorado (city lights nevertheless do shine through the clouds). There is little indication of widespread intense lightning (characterized in Day/Night Band imagery as a smear of light along a scan line) — in fact, only a single negative cloud-to-ground strike was seen in the Denver area at that time.

Heavy rains over the Front Range of Colorado are typically accompanied by easterly (upslope) flow. The image below shows METAR data plotted over the Suomi/NPP VIIRS 11.45 µm IR data. Easterly flow is common at many of the reporting sites.

Suomi/NPP VIIRS 11.45 µm IR imagery at 0826 UTC

Two 12-day animations from GOES-13 (immediately below) and GOES-15 (below GOES-13) show the large-scale environment before, during and after the heavy rains.

GOES-13 6.5 µm water vapor images, 5-16 September 2013 (click image to play animation)

GOES-15 6.5 µm water vapor images, 5-16 September 2013 (click image to play animation)

Weekly Precipitation ending 16 September 2013

Weekly storm totals from this event (from this NOAA site) show widespread heavy rains over both Colorado and New Mexico. A large area exceeded 600% of normal for the week!

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Suomi NPP VIIRS IR brightness temperature difference “fog/stratus product”

An AWIPS image of the Suomi NPP VIIRS 11.45 – 3.74 µm IR brightness temperature difference “fog/stratus product” at 10:26 UTC or 2:26 AM local time on 11 September 2013 (above) revealed a relatively narrow tongue of stratus cloud (darker orange to red color enhancement) which was beginning to move northward along the nearshore coastal waters of Oregon. The northward stratus advection was being driven by the presence of an elongated trough of low pressure off the West Coast of the US, with some embedded closed low circulations along the trough axis.

During the subsequent daylight hours, McIDAS images of GOES-15 0.63 µm visible channel data (below; click image to play animation) showed the continued northward and northwestward spread of the plume of stratus cloud along and just off the coast of Washington, with some inland intrusions of marine stratus noted later in the day. One of the aforementioned closed cyclonic circulations (which was not well-analyzed by the GFS40 model surface winds on the VIIRS image above) could be seen within the marine boundary layer stratus off the coast of Oregon. Other features of interest included the dispersion of smoke plumes from 2 small wildfires that were burning in southwestern Oregon. Meteorological fun fact: on this day, for the Lower 48 states both the daily lowest temperature (22º F at Silver Lake) and the daily highest temperature (102º F at Medford) occurred in the state of Oregon!

GOES-15 0.63 µm visible channel images (click image to play animation)

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POES AVHRR 0.84 µm visible channel and 12.0 µm IR channel images (with overlays of surface buoy, surface analysis, and ASCAT winds)

After a four and a half day hiatus, Tropical Storm Gabrielle began to regenerate south of Bermuda on 10 September 2013. AWIPS images of 1-km resolution POES AVHRR 0.86 µm visible channel and 12.0 µm IR channel data at 13:41 UTC (above) showed overlays of surface buoys, surface analysis, and ASCAT surface scatterometer winds. A number of these ASCAT winds exhibited speed values of 45-46 knots, prompting NHC to adjust the intensity of Gabrielle upward:

TROPICAL STORM GABRIELLE SPECIAL DISCUSSION NUMBER   9
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL072013
130 PM AST TUE SEP 10 2013

A PAIR OF ASCAT PASSES FROM 1342Z AND 1436Z INDICATE THAT GABRIELLE 
IS STRONGER THAN PREVIOUSLY ESTIMATED. BOTH PASSES SHOWED SEVERAL 
45-KT WIND RETRIEVALS...AND THAT IS THE INITIAL INTENSITY FOR THIS
SPECIAL ADVISORY. THE INTENSITY FORECAST HAS BEEN ADJUSTED UPWARD
AT 12 AND 24 HOURS TO 50 KT BASED ON THE INCREASE IN THE INITIAL
INTENSITY. THE ANALYZED 34-KT WIND RADII HAVE ALSO BEEN ADJUSTED
OUTWARD BASED ON THE ASCAT DATA...AND THEIR FORECAST HAS BEEN
MODIFIED AS WELL TO REFLECT THE LARGER WIND FIELD. AN AIR FORCE
RESERVE AIRCRAFT WILL INVESTIGATE GABRIELLE LATER THIS AFTERNOON TO
PROVIDE MORE DETAILED INFORMATION ON THE STRENGTH AND STRUCTURE OF
THE CYCLONE.

850 hPa relative vorticity product (04-10 September) – click image to play animation

A sequence of 6-hour interval 850 hPa relative vorticity product images from the CIMSS Tropical Cyclones site (above; click image to play animation) showed that the lower-tropospheric vorticity feature associated with Tropical Storm Gabrielle remained fairly intact during the 06-10 September period (between the times when Gabrielle had exhibited tropical cyclone organization and intensity).

GOES-13 0.63 µm visible channel images (click image to play animation)

McIDAS images of 1-km resolution GOES-13 0.63 µm visible channel data (above; click image to play animation) revealed a well-defined central dense overcast (CDO) convective burst early in the day, followed by the emergence of the low-level circulation of Gabrielle as the middle and high-altitude cloud layers were sheared off to the northeast.

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GOES-15 3.9 µm shortwave IR images on 10 September (click image to play animation)

GOES-15, in service as GOES-West over the Equator at 135 W, has experienced navigation anomalies on each of the past three mornings, September 8, 9 and 10, 2013 (as noted here, for example). Navigation anomalies occur approximately between 0830 and 1100 UTC. GOES Engineers are investigating the source of the anomaly, which can be on the order of 40 km. The animation above is the shortwave infrared from Tuesday 10 September. Animations for 6, 7, 8 and 9 September are below. Note that even the 6 September and 7 September have small navigation issues.

(Update: From SATOPS: NOTE: GOES-15 (West) Image registration (alignment of images to earth grid coordinates) was shifted by approximately 40 kilometers on DOY251 (ed. note: 8 September) from 08:50z to 10:50z and shifted by approximately 25 kilometers on DOY252 & DOY253 (ed. note: 9 and 10 September) from 9:30z to 10:40z. The earth coordinate grid shift was due to a lack of available valid star data for predicting the proper attitude pointing profile around eclipse. The root cause is under investigation.) In other words, the Star Pointer was unable to find enough targets to produce accurate navigation. (Link)

Update, 11 September: The anomalies continued on 11 September, starting after the 0845 UTC image, peaking at the 1000 UTC image, and subsiding by 1045 UTC. Link.

Update, 17 September: Large navigation anomalies were not present overnight.

GOES-15 3.9 µm shortwave IR images on 6 September (click image to play animation)

GOES-15 3.9 µm shortwave IR images on 7 September (click image to play animation)

GOES-15 3.9 µm shortwave IR images on 8 September (click image to play animation)

GOES-15 3.9 µm shortwave IR images on 9 September (click image to play animation)

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