Convection Returns to the southern Plains

April 23rd, 2010

Spring convection and associated severe weather returned to the southern Plains on April 22nd. Did predictors of convection do a good job in locating the severe cells?

CIMSS has recently started to produce synthetic satellite imagery from the Weather Research and Forecasting (WRF) model run at the National Severe Storms Laboratory (NSSL). Output from daily runs at 00 UTC is produced for 9 infrared bands that correspond to those of the Advanced Baseline Imager (ABI) that will fly on GOES-R. The hourly loop of the 11.2-micrometer channel, above, for the period between 1800 UTC 22 April and 00 UTC 23 April, shows convection initially forming along the dryline in the Texas Panhandle between 1900 and 2000 UTC before progessing northeastward into Oklahoma and Kansas. Synthetic imagery of the middle of 3 ABI water vapor channels (6.95 microns), show a similar story. Model predictions give clues on where to look for convective development. How did real-time predictors of convective development perform?

The UW Convective Initiation algorithm combines observations of 10.8-micron cooling (from GOES-13) with cloud phase changes. When cooling occurs as cloud phase is changing (suggesting growing cumulus towers that are glaciating), GOES-13 pixels are flagged as showing convective initiation. Depending on the cloud phase — all water, mixed water and ice, or all ice, the initiation is flagged in the screengrabs from N-AWIPS above as pre-CI cloud growth (blue), CI likely (green), or CI occurring (yellow). Once glaciation has occurred, CI detection turns off. A previous blog entry on this method is here.

UWCI does flag individual cells that subsequently develop, ignoring adjacent towering cumulus. Thus, it can draw forecaster attention to the updrafts that, for whatever reason, are the most vigorous. For example, the image at 1701 UTC show convective initiation indicated in one spot along the dryline in west Texas. By 1745 UTC, convection has developed. Shortly after 1800 UTC, UWCI identifies individual cells along a line from the extreme western portion of the Oklahoma panhandle northward into east central Colorado. These cells subsequently spawn severe weather. UWCI also flags nascent convective development for cells that eventually develop into an arc of convection over central Kansas at 2301 UTC. Note also that UWCI flags specific convective towers within a large cumulus field over the southern Panhandle. (Consider the three images at 2131 UTC and 2145 UTC and at 2231 UTC; convection initiation flagged at the earlier two images develops most vigorously as shown in the final image). This can focus forecaster attention to the clouds that are growing most rapidly.

The two images above show where convective initiation was diagnosed to be ongoing at some time on 22 April, as well as a preliminary Storm Report from the Storm Prediction Center. Note the good general overlap of UWCI points over the High Plains and storm reports. That more Storm reports exist than UWCI points reflects the UWCI philosophy of keeping the false alarm rate low, perhaps at the expense of detection.

There are several UWCI hits over the northeast on 22 April as well. There, cold air at upper levels promoted self-destructing sunshine and shower and thunderstorm development. Clear skies early in the day (1431 UTC) gave way to cumuliform development. The strongest updrafts likely yield the strongest cloud-top-cooling signal (as shown in this loop) and evolve into the most vigorous shower or thundershower. Even though severe weather was not reported with these cells, lightning was produced, starting around 1900 UTC as shown here. Cloud-top cooling can give a forecaster a head’s up that a particular cell might become vigorous enough to electrify.

(Note: this post has been corrected to remove images from before 1645 UTC on 22 April that may have included mis-navigated regions of convective initiation).

Yet another East Coast winter storm

February 6th, 2010
GOES-11/GOES-12 water vapor composite images

GOES-11/GOES-12 water vapor composite images

The Winter of 2009/2010 has brought a number of significant snowfall events to parts of the US East Coast — and another powerful storm affected that region on 05 February06 February 2010. The highest storm total snowfall reported was 40.0 inches at Colesville in Maryland. Washington Dulles International Airport received 32.4 inches of snow (their largest 2-day snowfall on record), and Baltimore-Washington International Airport received 24.8 inches of snow (their second-largest 2-day snowfall on record). So far, this is Philadelphia’s 2nd-snowiest winter on record (56.3 inches) and Washington DC’s 3rd-snowiest winter on record (44.9 inches).

AWIPS images of 3-hourly composites of the GOES-11 and GOES-12 water vapor channel data (above) showed a strong disturbance originating over the Pacific Ocean that was progressing eastward across the southwestern US and northern Mexico during the days leading up to the storm. There was also evidence of a plume of subtropical moisture seen on the water vapor imagery.

The presence of this moisture plume was confirmed on MIMIC Total Precipitable Water (TPW) images (below), which revealed a clear linkage to the rich moisture source within the Inter-Tropical Convergence Zone (ITCZ) over the eastern equatorial Pacific Ocean. MIMIC TPW values were in the 50-60 mm (2.0 – 2.4 inch) range within this moisture plume as it was being drawn northeastward across the Gulf of Mexico — and the Blended Total Precipitable Water product showed a large area of TPW values exceeding 200% of normal from the Gulf of Mexico to the mid-Atlantic states.

MIMIC Total Precipitable Water

MIMIC Total Precipitable Water

4-km resolution GOES-12 water vapor images with an overlay of cloud-to-ground lightning strikes (below) showed 3 important phases of the storm: (1) a expansive area of cold cloud tops associated with the initial round of heavy snowfall later in the day on 05 February; (2) the penetration of a broad dry slot, which helped to release convective instability along it’s leading edge that led to periods of thunder and lightning (especially during the 08-10 UTC time period), and (3) a well-defined deformation zone where additional snowfall banding developed during the final hours of the storm.

GOES-12 6.5 µm water vapor images + cloud-to-ground ligtning strikes

GOES-12 6.5 µm water vapor images + cloud-to-ground ligtning strikes

A series of 1-km resolution AVHRR Cloud Top Temperature product images and MODIS 11.0 µm IR images (below) showed greater detail of some of the banding structures during different phases of the storm.

AVHRR Cloud Top Temperature and MODIS 11.0 µm IR images

AVHRR Cloud Top Temperature and MODIS 11.0 µm IR images

1-km resolution AVHRR visible images (below) displayed the cloud features as the surface low was rapidly deepening just offshore during the day on 06 February.

AVHRR 0.86 µm visible images + surface analyses

AVHRR 0.86 µm visible images + surface analyses

MetOp ASCAT scatterometer winds at 14:05 UTC on 06 February (below) indicated that surface winds were generally in the 30-40 knot range, which were in agreement with offshore buoys which were reporting wind gusts 31-43 knots at 15 UTC. The highest reported gust was 61 mph at Lewes, Delaware during the pre-dawn hours on 06 February.

AVHRR 0.86 µm visible image + ASCAT scatterometer winds

AVHRR 0.86 µm visible image + ASCAT scatterometer winds

===== 07 FEBRUARY UPDATE =====

A comparison of a 1-km resolution MODIS visible channel image and a false-color Red/Green/Blue (RGB) image (below) shows the extent of the snow cover on the morning of 07 February. On the RGB image, snow appears as varying shades of red, in contrast to supercooled water droplet clouds (which appear as brighter features). Even after compaction of the heavy snowfall, there were still a number of sites reporting snow depths in excess of 30 inches that morning. Additional MODIS true color and false color imagery — at resolutions up to 250 meters — can be seen at the SSEC MODIS Today and the SSEC MODIS Direct Broadcast web sites.

MODIS visible + MODIS fasle-color Red/Green/Blue (RGB) image

MODIS visible + MODIS fasle-color Red/Green/Blue (RGB) image

MODIS true color images from the SSEC MODIS Today site can also be displayed using Google Earth (below). The location of 40.0 inch snowfall report (at Colesville, Maryland) is also noted on the image.

MODIS true color image (displayed using Google Earth)

MODIS true color image (displayed using Google Earth)

Long “atmospheric rivers” of moisture

May 5th, 2009
MIMIC Total Precipitable Water

MIMIC Total Precipitable Water

AWIPS images of the MIMIC Total Precipitable Water (TPW) product (above) showed the presence of  long, narrow  filaments of moisture (often described as “atmospheric rivers“) that were moving across the North Pacific Ocean and the North Atlantic Ocean during the 04 May – 05 May 2009 period. Studies by Newell and others suggest that these atmospheric rivers can persist for more than 10 days, and are capable of transporting as much water as the Amazon River!
Composite geostationary satellite water vapor imagery (below) showed a similar signature of enhanced clouds and moisture along the axis these two atmospheric rivers — however, the presentation on the water vapor imagery was a bit different in terms of width and location.

Composite of geostationary satellite water vapor images

MIMIC TPW + surface analysis

MIMIC TPW + surface analysis

Note that the surface frontal structure was more closely aligned with the atmospheric rivers seen on the TPW imagery (above), but there was more of a mismatch with the corresponding water vapor image features (below). This is due to the fact that the water vapor imagery is generally sensing a signal from moisture located within a fairly deep layer aloft in the middle to upper troposphere, at a level above which the bulk of the total column precipitable water is located.

Composite water vapor imagery + surface analysis

Composite water vapor imagery + surface analysis

A 4-panel comparison of the MIMIC TPW, the Blended TPW, GOES Imager water vapor channel, and the GOES Sounder TPW products (below) shows that there is good agreement to the general magnitude of the TPW values between the various products. An animation shows the various strengths and weaknesses of each in terms of their utility for tracking atmospheric rivers. The MIMIC and Blended TPW products (top 2 panels) had better  temporal continuity, while the GOES water vapor imagery and the GOES Sounder TPW product (bottom 2 panels) suffered from gaps in coverage due to either Spring eclipse or the variable GOES Sounder scanning strategy.

Comparison of MIMIC TPW, Blended TPW, GOES Sounder TPW, and water vapor imagery

Comparison of MIMIC TPW, Blended TPW, GOES Imager water vapor.  and GOES Sounder TPW imagery

Of pineapples and fire hoses

January 8th, 2009
GOES-11 6.7 µm water vapor channel images

GOES-11 6.7 µm water vapor channel images

AWIPS images of the GOES-11 6.7 µm water vapor channel (above) showed a very long and well-defined plume of moisture streaming northeastward from just north of the Hawaiian Islands to the Pacific Northwest on 07 January08 January 2009. The National Weather Service forecast office in Seattle, Washington said it best:

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
340 PM PST WED JAN 7 2009

.SHORT TERM…HEAVY RAIN AND FLOODING EVENT IS UNDERWAY. A LONG PLUME OF MOISTURE EMANATING FROM WEST OF HAWAII IS CURRENTLY POINTED AT SW WA/NW ORE. THE AXIS OF THE MOISTURE PLUME HAS BEEN SLOWLY SAGGING SOUTH TODAY…THOUGH EVERYWHERE HAS GOTTEN AT LEAST SOME AMOUNT OF RAIN. SOME PEOPLE REFER TO THIS PATTERN AS THE PINEAPPLE EXPRESS…WHILE OTHERS CALL IT A METEOROLOGICAL FIRE HOSE. BOTH TERMS APPLY HERE. 850-700 MB WIND FLOW IS MORE WESTERLY THAN WITH MOST HEAVY RAIN EVENTS (AS OPPOSED TO SOUTHWESTERLY)…SO RAIN IS FOCUSING A BIT HARDER THAN USUAL ON RIVERS WITH HEADWATERS IN THE CASCADES. THIS FLOODING EVENT COULD HAVE MANY PARALLELS TO THE NOVEMBER 2006 EVENT.


Blended Total Precipitable Water product

Blended Total Precipitable Water product

AWIPS images of both the CIRA Blended Total Precipitable Water product (above) and the CIMSS  MIMIC Total Trecipitable Water product (below) showed that TPW values were generally in the 30-40 mm (1.2-1.6 inches) range within this moisture plume. With the larger areal coverage of the CIMSS MIMIC TPW product display, you can get a better feel for the fact that this moisture plume had connections to the rich moisture contained within the Inter-Tropical Convergence Zone (ITCZ) as it began to surge northeastward on 06 January.

MIMIC Total Precipitable Water product

MIMIC Total Precipitable Water product

Heavy rainfall amounts included 9.30 inches at June Lake in Oregon and 9.05 inches at Wickersham in Washington — and as a result, there were widespread reports of flooding, mudslides, and avalanches as this plume of moisture moved inland and interacted with the topography of the region (shown below). In addition, strong winds were reported in parts of the region (with a wind gust of 130 mph at the top of Magic Mile Ski Lift in Timberline, Oregon).

AWIPS-2 topography image

AWIPS-2 topography image