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Diagnosis of Elevated Mesoscale Ascent: The Midland TX Heavy Snow Event of 11 Dec 1998

Talking Points

PAGE 1 -- WELCOME SLIDE

* - Introduce yourself (and any other staff who might be assisting the training effort)

* - Introduce the NWS Integrated Sensor Training (IST) Professional Development Series (PDS) and Virtual Institute for Satellite Integration Training (VISIT) training concepts

* - This lesson is part of Professional Competency Unit (PCU) 9 "Using AWIPS in the Forecast Process", and is the first lesson of Instructional Component (IC) 9.1.3 "Diagnosis of Elevated Mesoscale Ascent"

PAGE 2 -- INTRODUCTION

* Self-explanatory...

PAGE 3 -- LIFTING MECHANISMS

* - The 3 ingredients for convection (whether gravitational, slantwise, elevated, etc) are Lift, Moisture, and Instability.

* - As far as lifting mechanisms are concerned, here are some processes (QG and non-QG) that are capable of forcing elevated mesoscale ascent.

PAGE 4 -- TYPES OF INSTABILITY

* - The types of instability that can be released by the aforementioned lifting mechanisms are Moist Gravitational Instability and Moist Symmetric Instability (of which 2 forms are Conditional Symmetric Instability [CSI] and Potential Symmetric Instability [PSI])

* - In the absence of instability, there can be stable ascent (for example, over a stably-statified frontal boundary such as a warm front)

PAGE 5 -- KMAF CWA

* - To familiarize ourselves with the Midland TX (KMAF) County Warning Area (CWA), here is a map of the region with CWA boundaries and WSR-88D and NOAA Wind Profiler locations overlaid on an image of topgraphy.

* - Click on the "Toggle" button to show a similar map overlay of towns and Interstate highways

* - Note that the most significant topographical variation is across the southern and southwestern portion of the KMAF CWA, where elevations as high as around 8400-8700 feet (2600-2700 m) are found in the Guadalupe and Davis Mountains; the broad Edwards Plateau and Llano Estacado features occupy most of the northern and eastern portions of the KMAF CWA, with elevations generally in the 2000-3000 feet range. The Pecos River valley occupies the central portion of the KMAF CWA.

PAGE 6 -- GOES IR + MESOETA LOOP

* - Now let's begin to look at the overall synoptic situation during the early morning of Fri 11 December 1998

* - This is a loop of GOES 10.7 micrometer (longwave) IR at 15-minute intervals, covering a 9-hour interval from 03:15 to 11:45 UTC. Overlaid on the GOES IR is 500 mb height and absolute vorticity from the 11/03 UTC run of the MesoEta model

* - A closed upper low is moving slowly eastward across northern Mexico into southwestern Texas; IR cloud tops along the Texas / New Mexico border are cooling to -30 C and colder (yellow enhancement) within the deformation zone forming northeast of the Low. Positive vorticity advection at the 500 mb level is evident along the southern edge of the colder cloud top region.

* - Convection is developing farther east, across eastern Texas.

PAGE 7 -- GOES WV + MESOETA LOOP

* - This is a similar loop of GOES 6.7 micrometer IR ("water vapor") at 15-minute intervals, from 03:15 to 11:45 UTC. Overlaid on the WV imagery are 500 mb geopetential height and absolute vorticity from the 11/03 UTC run of the MesoEta model

* - A pronounced mid-level dry slot (warm/dry counts, orange to yellow enhancement) can be seen advancing northeastward across the Big Bend region into central Texas.

* - This comparison of satellite and model data suggests that the 11/03 UTC MesoEta was well-initialized; note smaller-scale features such as the warming/drying just behind the "18" vorticity lobe across central Texas at 03 UTC, and the the entrainment of the warm/dry tongue into the circulation of the developing secondary closed low over the Texas/Oklahoma panhandle region by 12 UTC. Extensive layered cloudiness persists within the deformation zone along the Texas/New Mexico border region, just northeast of the primary closed low entering southwest Texas.

PAGE 8 - GOES WV + 300 JET

* - This page shows GOES 6.7 micrometer IR ("water vapor") images at 3 hour intervals (09, 12, and 15 UTC), with overlays of the 300 mb height and isotachs. These images show a 100-knot jet streak exiting northeastern Texas at 09 UTC, with another 90-knot jet streak moving from northeastern Mexico into extreme southern Texas.

* - 300 mb wind speeds across the KMAF CWA decrease from about 70 to 40 knots as the upper trough axis approaches the region.

PAGE 9 -- MESOETA 500+VORT LOOP

* - This is a loop of 500 mb height/vorticity from the 11 December / 03 UTC run of the MesoEta model, at 3-hour intervals, running out 24 hours to 12 December / 03 UTC.

* - Note two periods of positive vorticity advection from 11/03 UTC to 11/18 UTC, as elongated lobes of cyclonic vorticity rotate northward across the KMAF region ahead of the advancing closed low.

* - After 11/21 UTC, there is negative vorticity advection over the KMAF region as the closed low moves off to the northeast.

PAGE 10 -- MESOETA 700+OMEGA LOOP

* - This is a loop of 700 mb height and omega from the 11 December / 03 UTC run of the MesoEta model, at 3-hour intervals, again running out 24 hours to 12 December / 03 UTC.

* - Upward vertical motion is forecast over most of the CWA until 15 UTC as the 700 mb trough moves into southwestern Texas; after 18 UTC, subsidence prevails as the low moves off to the east of the CWA.

PAGE 11 -- MESOETA RH+PRECIP LOOP

* - This is a loop of mean 1000-500 mb relative humidity and 3-hour precipitation accumulation from the 11 December / 03 UTC run of the MesoEta model, at 3-hour intervals, again running out 24 hours to 12 December / 03 UTC.

* - An early area of precipitation accumulation is forecast through 09 UTC over the northern portion of the CWA (including southeastern New Mexico); the main region of precipitation is forecast to stretch southwest-northeast across the southern portions of the CWA, tapering off after 18 UTC.

* - Total accumulation at KMAF is forecast to be 0.45 inches, with more accumulation to the south and east of KMAF. (as a comparison, the RUC-2 forecast 0.21 inches over KMAF); precipitation should end after 21 UTC as drier air filters across the region behind the exiting upper low.

PAGE 12 -- MESOETA PMSL+THICKNESS

* - This is a loop of mean sea level pressure and 1000-500 mb thickness from the 11 December / 03 UTC run of the MesoEta model, at 3-hour intervals, again running out 24 hours to 12 December / 03 UTC.

* - Thickness values in the vicinity of KMAF remain in the 540 to 542 dam range during the forecast period; the typical rain/snow thickness threshhold for this region is 546 dam (source: KMAF staff).

* - Other than a broad region of high pressure north of the CWA, there is a total absence of surface features over the region.

PAGE 13 -- GOES FOG PRODUCT

* - This is a loop of GOES 11-3.9 micrometer IR difference product (otherwise known as the "Fog/Stratus Product"), at 30 minute intervals from 03:15 to 09:45 UTC

* - On this default enhancement, low stratus and/or fog show up as darker gray regions (some might prefer an enhancement that looks similar to that developed at CIRA and implemented by NWS Western Region -- using that enhancement, low stratus/fog appear as lighter gray regions; under the "Choose and enhancement" menu, select "Invert" to apply a similar enhancement to this loop; more details on the GOES Fog Product enhancement can be found at http://www.cira.colostate.edu/ramm/visit/istpds/awips/awips_12.html)

* - Note that there is extensive low stratus and/or fog across the entire CWA and most of Texas, even into northern Mexico; this low cloudiness is likely beneath an inversion above the northeasterly flow of cold air across the region (seen on the previous page of mean sea level pressure).

PAGE 14 -- GOES FOG + BL WINDS

* - This loop shows the GOES fog/stratus product at 3 hour intervals, from 03:15 to 12:15 UTC. Overlaid on the fog/stratus product are Boundary Layer wind barbs and Relative Humidity from the 11/03 UTC run of the MesoEta model. The large region of fog/stratus across Okalhoma, Texas, and northen Mexico was likely reinforced by the upslope component of the northeasterly BL flow across the region. The final images suggest a trend of drier BL air moving southward across the Texas panhandle region, as the northern edge of the fog/stratus begins to move southward.

PAGE 15 -- PROFILER PERSPCTIVE

* - This image shows a 12 UTC "perspective view" of the 3 NOAA wind profiler sites closest to the KMAF CWA; Midland is located in the bottom right corner of the image

* - On the Jayton, TX profiler (JTNT2), note the northeasterly flow in the lowest layers -- this northeasterly flow was maintaing the supply of cold air and low cloudiness across the region.

PAGE 16 -- JAYTON TX PROFILER

* - This is a time series of hourly profiler data from the Jayton TX site (northeast of KMAF); time is increasing from right to left, from 00 to 12 UTC.

* - Note that the low-level northeasterly flow begins to deepen later in the morning; a shallow layer of southeasterly flow exists around 4 km; strong southwesterly flow (50-90 knots) dominates above 7 km (23 kft).

PAGE 17 -- KMAF VAD VWP LOOP

* - Higher temporal variability of the vertical wind profile can be observed with ths WSR-88D Vertical Azimuth Display (VAD) Vertical Wind Profile (VWP) data from KMAF; this loop is from 09:24 to 12:02 UTC.

* - As seen on the Jayton TX wind profiler data, there is northeasterly flow below about 8 kft; higher aloft, winds veer to southeasterly, southerly, and then southwesterly; there are no suitable targets above 24 kft to derive wind measurements.

PAGE 18 -- 12Z KMAF RAWINSONDE

* - The rawinsonde profile from Midland TX at 12 UTC reveals several important features; hit the "Toggle" button to view a slide listing these features.

PAGE 19 -- GOES + MESOETA PRECIP

* - This first image is the GOES 6.7 micrometer IR ("water vapor") image at 12:15 UTC, with an overlay of layer precipitable water (PW) from the 11/03 UTC MesoEta. The PW value of around 0.60 inches at KMAF is somewhat high for the cold season (source: KMAF staff).

* - Hit the "Toggle" button to show the same GOES water vapor image with an overlay of 3-hour convective (red) and stable (cyan) precipitation forecasts from the MesoEta; 3-hour observed precipitation amounts are in green; note that the MesoEta convective precipitation is confined to the Texas Gulf coast -- all the precipitation over the KMAF CWA is forecast to be of a stable nature.

* - The forecast (and observed) precipitation is occurring ahead of and to the left of the advancing mid-level dry slot (evident on the GOES water vapor image); at this time, no precipitation was being reported by stations beneath the layered cloudiness in the deformation zone region along the Texas/New Mexico border region.

PAGE 20 -- MESOETA 305K LOOP

* - This is a loop of GOES 10.7 micrometer IR, every 15 minutes, from 05:45 to 15:15 UTC. Overlaid on the GOES imagery are MesoEta isentropic analyses at the 305K surface, showing Montgomery stream function (cyan), pressure (red), and wind (black); in addition, hourly observed precipitation is overlaid in green.

* - Weak isentropic upgide is evident across the western portion of the CWA from 06 to 09 UTC

* - After 12 UTC the winds along the 305K surface back to southeasterly across the eastern portion of the CWA, enhancing isentropic upglide in that region. This corresponds well with the southeastern edge of the cold (-17 C and below) cloud tops, where banding is beginning to appear (to apply a color enhancement to this IR loop, go to the "Choose an enhancement" pull-down menu and select "Cold season snow enhancement"). This weak isentropic upglide was acting to bring the ascending moist flow to saturation.

* - Hourly precipitation rates at KMAF then increase to .06 inches at 14 UTC and 0.16 inches at 15 UTC

PAGE 21 -- 700:500 MOISTURE FLUX

* - This is a loop of GOES 10.7 micrometer IR, at 3 hour intervals from 08:45 to 17:45 UTC. Overlaid on the IR imagery are contours of 700:500 mb layer Specific Humidity (green), 700:500 mb layer Moisture Flux Divergence (red), and 700:500 mb layer wind barbs (cyan) from the 11/03 UTC MesoEta.

* - Note that the region of southeasterly flow coincided with enhanced moisture flux convergence across much of the CWA.

* - Within this 700:500 layer, the source region of the southeasterly flow exhibited mean specific humidity values of about 3 g kg-1. Using the rules of the Garcia Method, with efficient forcing and instability the snowfall totals could reach 6 inches (2 inches of snow per g kg-1 of mean specific humidity).

PAGE 22 -- 700:500 FRONTOGENESIS

* - This is a loop of GOES 10.7 micrometer IR, at 3 hour intervals from 08:45 to 17:45 UTC. Overlaid on the IR imagery are contours of 700:500 mb layer QG Frontogenesis (red), 700:500 mb layer Qn Divergence (green), and 700:500 mb layer Geostrophic Wind barbs (cyan) from the 11/03 UTC MesoEta.

* - At 09 and 12 UTC, note the axis of weak (contours of 3-5) frontogenetic forcing north of the CWA; the corresponding Qn convergence (indicating upward vertical motion) was centered across the eastern portions of the CWA

* - The Frontogenetic forcing axis and corresponding Qn convergence shift norhteastward from 15-18 UTC.

PAGE 23 -- 500:300 FRONTOGENESIS

* - This is a loop of GOES 10.7 micrometer IR, at 3 hour intervals from 08:45 to 17:45 UTC. Overlaid on the IR imagery are contours of 500:300 mb layer QG Frontogenesis (red), 500:300 mb layer Qn Divergence (green), and 500:300 mb layer Geostrophic Wind barbs (cyan) from the 11/03 UTC MesoEta.

* - Within the layer of the middle to upper troposphere, there was stronger (contours of 20 and greater) frontogenetic forcing over the CWA, but the axis of the forcing was shifted southward, directly over the CWA; the resulting Qn convergence (implying upward vertical motion) was therefore also located farther south, across the far southern/southeastern portions of the CWA.

* - Although the forcing was stronger within this layer, the moisture was not as high at these upper levels.

PAGE 24 - MESOETA XSEC FRONTOGEN LOOP

* - This is a loop showing vertical cross sections (oriented SW-NE) of MesoEta fields at 3-hour intervals from 06-21 UTC. QG Frontogenesis appears as the color image, while Equivalent Potential Temperature is plotted in red (K), Relative Humidity in green (%), and Geostrophic Wind barbs in cyan (knots).

* - Note the upper-level frontogenetic forcing (red to white shading) which increases within the 600:300 mb layer during the period; deep moisture (RH > 80%) is in place beneath this upper level forcing; a layer of elevated instability (theta-e decreasing with height) is evident across the central portion of the cross section from 09-15 UTC (corresponding to the layer of southeasterly flow).

* - The upper level frontogenetic forcing persists until 21 UTC, but drier air can be seen overspreading the region from the southwest.

PAGE 25 -- MESOETA XSEC PV

* - This is a loop showing vertical cross sections (oriented SW-NE) of MesoEta fields at 3-hour intervals from 06-21 UTC (similar to the previous cross sections). Equivalent Potential Vorticity appears as the color image, while Equivalent Potential Temperature is plotted in red (K), Relative Humidity in green (%), and Geostrophic Wind barbs in cyan (knots).

* - Note the layer of elevated instability (negative EPV, blue shading) within the 700:500 layer from 06-15 UTC. This layer corresponds to the layer of southerly/southeasterly flow.

* - After 18 UTC, this layer of elevated instability dissipates. The process of slantwise convection is an adjustment process which restores the layer to neutral instability.

PAGE 26 -- MESOETA XSEC OMEGA LOOP

* - This is a loop showing vertical cross sections (oriented SW-NE) of MesoEta fields at 3-hour intervals from 06-21 UTC (similar to the previous cross section). Omega appears as the color image, while Equivalent Potential Temperature is plotted in red (K), Relative Humidity in green (%), and Geostrophic Wind barbs in cyan (knots).

* - Note that upward vertical motion (yellow to red) is forecast to increase within the 700:500 mb layer over the region from 06-12 UTC; instability (theta-e decreasing with height) and deep moisture (RH > 80%) is in place within this layer of upward motion.

* - A transition to downward vertical motion (blue) is forecast after 18 UTC across the southwestern portion of the cross section.

PAGE 27 -- MESOETA XSEC MG LOOP

* - This is a loop showing vertical cross sections (oriented SW-NE) of MesoEta fields at 3-hour intervals from 06-21 UTC (similar to the previous cross sections). Geostrophic Momentum is plotted in yellow (m/s), Equivalent Potential Temperature in red (K), Relative Humidity in green (%), and Geostrophic Wind barbs in cyan (knots).

* - Equivalent Potential Temperature (theta-e) and Geostrophic Momentum (Mg) are often employed on a cross section to diagnose regions of Potential Symmetric Instability (PSI). Regions where the Mg surfaces slope more steeply than the theta-e surfaces are unstable to slantwise convection; these conditions are met within the 700:500 mb layer between 09 and 15 UTC.

PAGE 28 -- RUC CSI DIAGNOSTICS

* - This page shows 2 examples of CSI diagnostics generated from RUC-2 model data using GEMPAK. There are some uncertainties in how AWIPS calculates similar parameters, so GEMPAK output is shown here.

* - The first image is 700-750 mb Moist Geostrophic Potential Vorticity (orange shading), along with contours of Conditional Instability within that layer. High MPVg* values can be seen across southeastern New Mexico and parts of the Big Bend region of Texas, but Conditional Instability remains well east of the KMAF CWA (across eastern Texas)

* - Click on the "Toggle" button to show 300-400 mb Potential Vorticity (orange shading) and PV Advection (solid contours), along with 850 mb Frontogenesis (dotted countours).

* - Very little 850 mb Frontogenetic forcing is evident, but upper-level PV advection is strong across much of the KMAF CWA

PAGE 29 -- RUC CSI XSEC FADER

* - This page shows S-N cross sections from the 11/11 UTC run of the RUC-2 model, along 102 W longitude (KMAF is near the center of the cross section); solid contours are Theta-E* (K), with shaded regions of Moist Geostrophic Potential Vorticity (orange)

* - Click on the "Toggle" button to show a corresponding cross section of Relative Humidity (orange shading, greater thn 80%) and Frontogenesis; note the deep layer of moist air extending upward into the region of mid to upper level frontogenesis over the southern and central portions of the cross section.

PAGE 30 -- 10.7 IR / CG LTG LOOP

* - This is a loop of GOES 10.7 micrometer IR at 30-minute intervals, covering the period 03:15 to 17:32 UTC. National Lightning Detection Network (NLDN) 15-minute Cloud-to-Ground (CG) strikes are plotted in cyan.

* - All CG strikes are over southern and eastern TX, associated with the convection developing along the nose of the dry intrusion moving over that region.

* - Note that there are no CG strikes indicated over the KMAF region; KMAF staff and METAR reports also did not indicate any thunder during this period (from Intra-Cloud or Cloud-to-Cloud strikes)

PAGE 31 -- WINTER LIGHTNING

* - Some heavy snow events are associated with lightning, but there was no electrical activity noted during this event. Some studies (P. Rhoor, CSU PhD thesis, 1999) have shown that CG activity often occurs near the *gradient* of cloud top temperatures, not in the region of coldest cloud tops; when observed, lightning can often identify regions of oncoming heavy snow bursts before there are indications on satellite or radar.

PAGE 32 -- GOES IR FADER

* - The first GOES 10.7 micrometer IR image at 14:45 UTC has the "Default IR" enhancement applied, which uses color gradations for brightness temperatures of -30 C and colder; the coldest cloud tops are located northwest of KMAF, and are in the -30 to -35 C range.

* - Often, snow situations like this require an IR ehancement that highlights warmer cloud tops in order to detect important areas of precipitation. Click on the "Toggle" button to show the corresponding GOES IR image with a special "winter precipitation" enhancement applied; this enhancement cloud tops in the -17 to -29 C range (light blue to dark blue). Using this enhancement, southwest-to-northeast bands can be seen forming near and to the southeast of KMAF; as these bands propagated northwestward into the main cold cloud top region, moderate to heavy snow was reported at KMAF (from 13:56 to 16:35 UTC).

PAGE 33 -- GOES 10.7 IR LOOP

* - This loop of GOES 10.7 micrometer IR cover the period from 11:02 to 14:45 UTC. Hourly METAR reports are overlaid on the IR imagery.

* - As mentioned on the previous page, note the southwest-to-northeast bands forming near and to the southeast of KMAF; as these bands moved northwestward and reinforced the gradient of colder cloud top temperatures, moderate to heavy snow was reported at KMAF (from 13:56 to 16:35 UTC).

PAGE 34 -- GOES VISIBLE LOOP

* - This loop of GOES visible imagery covers the period from 13:32 to 16:02 UTC. Hourly precipitation totals are averlaid on the visible imagery (in red).

* - Note the textured appearance and shadowing resulting from the convective bands feeding northwestward across the KMAF region; hourly precipitation totals at KMAF increased to 0.16 inches at 15 UTC, when S+ was reported.

PAGE 35 -- KMAF 0.5 BREF LOOP

* - This loop of KMAF 0.5 degree Base Reflectivity covers the period from 11:02 to 17:28 UTC. The radar was in clear air mode during this event.

* - Early in the animation, note the SW-NE bands of higher reflectivity over the Texas / New Mexico region, beneath the upper level deofrmation zone; as the loop progresses, SW-NE bands of higher reflectivity form between KSJT and KMAF, which grow somewhat as they feed northwestward across the KMAF CWA.

* - Another item of interest is the appearance of "gravity waves" that propagate southwestward early in the loop; the begin between KMAF and JTNT2, and move toward KMAF; it appears that the SW-NE bands of precipitation intensify somewhat as they intersect with the "gravity waves" near KMAF.

PAGE 36 -- KMAF BREF 4-PANEL LOOP

* - This page shows a 4-panel animation of the 4 lowest tilts of Base Reflectivity from the KMAF radar, covering the period 11:03 to 14:50 UTC. Click the Big Red Cursor near the red * to center each of the 4 panels near KMAF. To interactively roam within the panels, hold the SHIFT key as you also click and roam with the Right mouse button. Each of the 4 panels has a small red cursor -- these cursors all point to the same geographic location as you roam to change the viewing area.

* - Note that the precipitation banding becomes less well defined in the higher-elevation scans, indicating that these features were fairly shallow.

* - Also note that the southwestward-moving "gravity waves" (observed on the previous page) are evident on all 4 tilts, arriving over the KMAF region about halfway through the animation (between about 12 and 14 UTC, when snowfall was increasing from light to moderate at KMAF)

PAGE 37 -- KMAF VAD VWP Loop

* - This animation shows the KMAF 88D Vertical Azimuth Display (VAD) Vertical Wind Profile (VWP) during the period 09:24 to 16:38 UTC. Note that the depth of the northeasterly boundary layer flow increases from 8000 to 13000 feet in 2 distinct pulses (around 13 UTC and then again around 15 UTC). The southwestward-moving "gravity waves" noted earlier may be related to these 2 periods of deepening of the northeasterly boundary layer flow.

PAGE 38 -- KMAF 88D / GOES 4-PANEL LOOP

* - This page shows a 4-panel animation of KMAF 0.5 degree Base Preflectivity, 1-km Composite Reflectivity, and Echo Top height, along with the GOES 10.7 micrometer IR imagery with hourly METAR reports during the period 11:02 to 14:50 UTC. Click the Big Red Cursor near the red * to center each of the 4 panels near KMAF. To interactively roam within the panels, hold the SHIFT key as you also click and roam with the Right mouse button. Each of the 4 panels has a small red cursor -- these cursors all point to the same geographic location as you roam to change the viewing area.

* - Note that the precipitation features appear quite similar on the 0.5 degree and the 1-km composite base reflectivity products, indicating their shallow vertical extent.

* - The highest echo tops are only 20-25 kft (medium green enhancement), west of KMAF near the Texas / New Mexico border. Note that the coldest cloud tops on the GOES IR are generally farther to the northwest, well into southeastern New Mexico. The 88D reflectivity only shows significant returns back into extreme southeastern New Mexico, with a fairly sharp cutoff to the >20 dBZ contour.

PAGE 39 -- GOES / 88D BREF FADER

* - This is a comparison of the 14:45 UTC GOES 10.7 micrometer IR image with the 14:50 UTC 0.5 degree KMAF Base Reflectivity. Click on the "Toggle" button to alternate between each image; to fade between images, click the "Fade" button, then the "Rock" button, and increase the fading speed by moving the "Animation Speed" slider bar to the right.

* - Note that there is some horizontal displacement of the location of the forward (northwestern) edges of the precipitation bands on radar compared to satellite.

PAGE 40 -- VIS / IR FADER

* - This is a comparison of the 14:45 UTC GOES visible and 10.7 micrometer IR iamges. Click on the "Toggle" button to alternate between each image; to fade between images, click the "Fade" button, then the "Rock" button, and increase the fading speed by moving the "Animation Speed" slider bar to the right.

* - The visible channel has better resolution than the IR channels, allowing greater detail to be seen with the banding features. Note the texture and shadowing that is apparent with many of the bands.

PAGE 41 -- IR / WV + TOTAL PRECIP FADER

* - This is a comparison of GOES 10.7 micrometer IR and 6.7 micrometer IR ("water vapor") imagery and total precipitation at the end of the day (00:00 UTC on 12 December 1998). Click on the "Toggle" button to alternate between each image; to fade between images, click the "Fade" button, then the "Rock" button, and increase the fading speed by moving the "Animation Speed" slider bar to the right.

* - Note how preciptation totals across the Big Bend region of Texas were restricted to 0.10 inch or less as the mid-level dry slot moved across that area. Amounts of 0.25 to 0.50 inch occurred across the KMAF CWA. Even thought the coldest cloud top temperatures were seen over southeastern New Mexico, precipitation totals in that region were very small.

PAGE 42 -- KMAF STP LOOP

* - This loop of KMAF Storm Total Precipitation (STP) covers the period 15:19 to 18:47 UTC. Note the 2 main regions of higher estimated precipitation -- west and northwest of KMAF across the Texas / New Mexico border region (in the deformation zone region), and also east and southeast of KMAF (where the banding features were observed to form). Keep these 2 regions in mind as we take a look at the erosion of the snow cover during the following day...

PAGE 43 -- SNOW COVER ON 12 DEC LOOP

* - This loop of GOES-8 visible imagery covers the period 15:15 to 20:45 UTC on the following day (12 December 1998). As the snow cover melts during the afternoon hours, note the 2 areas with greater snow depth remain longer. The snow cover only extends into the southeasternmost county of New Mexico...not as far as suggested by the coldest cloud tops on GOES IR.

PAGE 44 -- KMAF STP / TOPO FADER

* - This is a comparison of the KMAF 88D Storm Total Precipitation (STP) and Topography. Click on the "Toggle" button to alternate between each image; to fade between images, click the "Fade" button, then the "Rock" button, and increase the fading speed by moving the "Animation Speed" slider bar to the right.

* - Although there was an upslope component with the Boundary Layer northeasterly winds, the resultant precipitation pattern was not related to the topography of the region.

PAGE 45 -- SUMMARY

* - Midland Texas received a total of 9.75 inches of snow on 11 December 1998 -- a record snow accumulation for any single event, any 24-hour period, or any month! Jal in extreme southeastern New Mexico observed 12 inches of snow.

PAGE 46 -- SUMMARY (CONT'D)

* - Self explanatory...

PAGE 47 -- SUMMARY (CONT'D)

* - Self explanatory...

PAGE 48 -- ADDITIONAL CASES

* - For comparison purposes, we can briefly examine a case of heavy snow across Mississippi and Alabama on 14 December 1997. Mesoscale banding was evident on GOES 10.7 micrometer IR and WSR-88D.

PAGE 49 -- GOES 10.7 IR - 14DEC97 LOOP

* - This is a loop of GOES-8 10.7 micrometer IR at one-hour intervals during the period 04:45 to 22:45 UTC ON 14 December 1997. Hourly precipitation type is overlaid on the IR imagery. Cloud top temperatures between -15 C and -50 C are enhanced with the light to dark blue colors.

* - Note the mesoscale banding (darker blue enhancement) across central Mississippi and Alabama. Light to occasionally moderate snow was observed at some locations beneath the banding.

PAGE 50 -- GOES VISIBLE

* - This is a GOES-8 visible image from the following day (15 December 1997), snowing the areal extent of the snow cover across central Mississippi and Alabama. 4-8 inches of snow were observed at some locations within this band.