Update on HSLC event-relative composites

Note: For ease of viewing, figures referenced herein are provided in the attached powerpoint (at bottom).

As discussed during the February 2015 CSTAR conference call, Dr. Parker and I are in the process of developing event-relative composite maps for HSLC environments associated with significant severe reports (i.e., EF2+ tornadoes, 65+ kt wind gusts, and 2”+ hail) for comparison to those associated with unverified warnings (i.e., nulls, as defined in previous work). These composites are created with North American Regional Reanalysis (NARR) data, which are available every 3 h at a 32-km horizontal resolution with 29 vertical levels. Data are temporally averaged over a 20° latitude by 20° longitude domain centered on each respective report or null. The 3D nature of the NARR offers the opportunity to calculate nearly any variable desired and represents continuity throughout the time period of our report and null datasets. Archived SPC mesoanalysis data were used in this work only to determine which reports were HSLC using our previous criteria of SBCAPE ≤ 500 J kg-1, MUCAPE ≤ 1000 J kg-1, and 0-6 km bulk wind difference ≥ 18 m s-1.

The distribution of HSLC significant severe reports (2006-2011) and nulls (October 2006-April 2011) used in this study is shown in Figure 1. Nearly every state had at least one significant severe report occur within an HSLC environment over the eight-year period of record. However, the significant severe report distribution is notably skewed towards the Ohio, Tennessee, and Mississippi Valleys (Figure 2), particularly when considering only significant tornado reports (Figure 3). Nulls, despite having a distinct maximum in the Lower Mississippi Valley, are much more evenly distributed across the CONUS (Figure 4).

The differences of typical synoptic and mesoscale features between events and nulls are striking, as shown in Figure 5. Events tend to be associated with deeper troughs and surface lows than nulls, leading to correspondingly stronger differential divergence, mid-level negative omega, low-level temperature advection, and a more pronounced upstream vorticity maximum. Buoyancy differences are relatively modest, with a mean difference in MLCAPE of 100-200 J kg-1 centered to the south of the composite report/null location.

Seasonal subsets reveal similarities to the entire dataset; however, the key feature appears to vary by season. For example, wintertime differences appear to be dominated by differential divergence near the composite center, likely influenced by a coupled-jet feature aloft and a stronger low-level front (Figure 6), while summertime differences are primarily found in low-level warm air advection and buoyancy southeast of the composite center (Figure 7). Variability in the importance of certain features is likely attributable to the annual cycle in report location (cf. Figures 8-11) and general climatology.

To assess regional variability in HSLC features, the dataset was split into three subsections (northeast, southeast, and west) based upon the latitude and longitude of each respective report and null (Figure 12). The southeast region composites, which encompass the majority of reports within our collaborating CWAs, reveal similar difference fields to the nationwide composites (Figure 13). The northeast composites, not shown, represent the weakest differences in upper-level and lower-level divergence fields and suggest reports tend to occur just south of a warm front, based upon positioning of the MSLP and MLCAPE differences. Composites in the western region (Figure 14) again suggest more of a warm front (or perhaps triple point) structure, though other fields depict differences of similar magnitude (or larger, such as low-level warm air advection) compared with the southeast composites.

As discussed during the conference call, nationwide SHERBS3 comparisons revealed that the 0-3 km shear vector magnitude was the primary constituent in discriminating between events and nulls, while the 700-500 hPa lapse rate was similar or even lower in the report composite. This is consistent with differences in the west, as shown in Figure 15 and, to a lesser extent, with those in the southeast (Figure 16). Similar differences are noted when using the 3-6 km lapse rate rather than the 700-500 hPa lapse rate (not shown). The effective shear term within the SHERBE is comparable in apparent importance to the 0-3 km shear vector magnitude in the SHERBS3. These findings suggest that the SHERBS3 and SHERBE can likely be improved upon in the future through modifications of existing terms and/or an inclusion of other terms.

Additional composites and variables continue to be investigated. For example, potential instability fields and 0-3 km CAPE do show some clear differences between events and nulls (Figure 17) in the vicinity of the composite center. Further, the 0-3 km CAPE differences line up quite well with the 1000-850 hPa theta-E differences, suggesting a conversion from potential instability to CAPE in the lowest 1-3 km. Finally, composite soundings are being generated at the composite report and null locations. These soundings have noted shear magnitude and CIN differences in all regions, the latter of which potentially for opposing reasons (e.g., lapse rates in southeast and low-level relative humidity in west; cf. Figures 18-19). Moreover, respectable differences in low-level relative humidity and hodograph shape were noted when comparing the significant tornado and significant wind composites (Figure 20), though given the spatial climatology of HSLC significant tornado and wind reports, regional analyses of these findings are necessary before any conclusions can be reached.

So far, we are confident in the following:

  • The composite environment associated with an HSLC significant severe report is characterized by considerably stronger forcing aloft and in low levels than the composite environment associated with an unverified warning.
  • On the mean, CAPE differences between events and nulls are modest, with primary dissimilarities noted to the south of the composite report/null location.
  • Similar findings were noted in seasonal and regional subsets, though the degree to which each variable is important fluctuates.
  • The SHERBS3 exhibits skill in all regions, but its signal may be swamped by the 0-3 km shear vector magnitude, particularly in the west. The 700-500 hPa lapse rate shows weak discrimination, suggesting an adjusted parameter may yield more skill in discriminating between events and nulls.

Since last month’s conference call, I have been in the process of re-running the composites to ensure accuracy and allow for the calculation of other variables. Initial steps are underway to develop these findings into a journal article to be submitted later in the year. Future work will assess additional composite fields and subsets along with the comparison of high-shear, high-CAPE environments to HSLC. Furthermore, skill tests utilizing the NARR composites will allow us to determine the utility of previously uninvestigated variables at discriminating between HSLC events and nulls, potentially improving existing forecasting parameters.

Figures

Posted in CIMMSE, Convection, CSTAR, High Shear Low Cape Severe Wx | 3 Comments

New Weather and Forecasting Article from CSTAR Supported Research

The latest addition of Weather and Forecasting features an article highlighting the results of recent CSTAR supported research. The article highlights decay over land, gust factors, and sustained wind speed forecasts for recent tropical cyclones impacting the study region. The final product was the result of great interactions between collaborators at North Carolina State University, the National Hurricane Center, and various National Weather Service Forecast Offices in the region.

Posted in TC Inland and Marine Winds, Uncategorized | Leave a comment

Investigation of Low Level Wind Shear in Central North Carolina

The blog post below was a combined effort of Ryan Ellis, Barrett Smith and Katie Dedeaux.

The National Weather Service describes wind shear as “…a change in horizontal wind speed and/or direction, and/or vertical speed with distance, measured in a horizontal and/or vertical direction. Wind shear is a vector difference, composed of wind direction and wind speed, between two wind velocities. A sufficient difference in wind speed, wind direction, or both, can severely impact airplanes, especially within 2,000 feet of ground level because of limited vertical airspace for recovery.”  NWS forecasters must include wind shear within the aviation forecast if non-convective vertical wind shear of 10 knots or more per 100 feet in a layer more than 200 feet thick is expected within 2,000 feet of the surface at, or in the vicinity of the airport. This stringent criterion is difficult to attain and therefore, going explicitly by this criteria, very few, if any forecasts would include non-convective low level wind shear. Feedback obtained from many users has indicated that they would like to be alerted to possible low level wind shear more frequently, even if conditions do not explicitly meet the criteria.

Using a definition of low level wind shear (LLWS) of 10 kts or more per 100 feet in a layer more than  200 feet thick within 2000 feet of the surface is a forecasting problem in the sense that it is very hard to obtain data at this resolution whether in an observational platform or a numerical model.  Radiosondes have good vertical resolution (still not enough to resolve 100 foot layers) but vary widely in space and have very poor temporal resolution.  High resolution numerical models such as the Rapid Refresh Model (RAP) and the High Resolution Rapid Refresh model (HRRR) use sigma levels that pack more datapoints into the boundary layer but even in doing so, there are only five levels below 2000 feet, the lowest two of which are roughly 90 feet apart and the second and third levels being 160 feet apart at Greensboro,NC (KGSO). This is a start for forecasting low level wind shear, but if we can’t observe it, how can we verify it? Aircraft soundings are helpful but once again few and far between, as are pilot reports, especially specific to LLWS.

During the day on 7 February, 2015, an upper level trough was just beginning to move into the Northern Plains while the associated surface moved into the Midwest (Fig. 1).  In response, the tightening pressure between the low and a high off the Southeast coast (Fig. 1), led to an increase in winds across central North Carolina during the afternoon hours.  The strongest winds occurred over the eastern half of North Carolina, where winds were sustained 15-20kts and gusting to 25kts at times.  A 22Z RAP forecast sounding (18Z run) shows these winds in a relatively shallow mixed layer depth around 3000 ft with a momentum transport value (a good approximation of gust potential) of 21kts (Fig 2).  After sunset, forecast soundings showed a rapidly developing surface based inversion despite the relatively unchanged pressure gradient.  Most model guidance indicated a 5 to 10kt wind at times overnight.  Above the surface, the RAP showed the already modest flow of 20 to 25kts at 2000 feet strengthening with an intense nocturnal low-level jet (LLJ) of around 40kt at the top of the inversion.  By 07Z, RAP soundings showed a LLJ sharply increasing to 39kts and 43kts at 1100 feet at KFAY (Fayetteville) and KRWI (Rocky Mount), respectively (Figure 3).  Surface winds throughout the night were mostly 6 to 9kt and slowly increased.  Due to the previously mentioned limitations in RAP forecast data made it difficult to determine if the technical definition of LLWS would be met since the two lowest data points (below 1000 ft) showed 21kt and 9kt, or 12kt of shear over roughly 500 ft.  However, based on the magnitude of the LLJ and sharpness of the inversion, forecasters felt confident enough to add LLWS to the 00 UTC TAF issuance,

TAF
KFAY 072338Z 0800/0824 21006KT P6SM SCT250 WS012/22035KT
FM081300 23008KT P6SM SCT250
FM081600 23014G28KT P6SM SCT250=

While a PIREP report from near KFAY did not explicitly mention LLWS, it did described severe turbulence within 1 to 3 miles of the terminal.

FAY UUA /OV FAY240001-FAY240003 /TM 1455 /FL020 /TP SR22 /TB SEV 020-BLO /RM SMOOTHED OUT AT 022-023 DURC

One observational platform that can help tremendously to observe and verify LLWS is the wind profiler. The Earth System Research Laboratory (ESRL) Hydrometeorology Testbed – Southeast project operates wind profilers in the state of North Carolina in Charlotte, Clayton, Research Triangle Park, and New Bern. The website with data for these sites and others around the country can be found at http://www.esrl.noaa.gov/psd/data/obs/datadisplay/. The data for this case was provided by the Environmental Protection Agency.

On February 8th there were a number of pilot reports for moderate to severe turbulence. Profiler data from Research Triangle Park shows winds near 40 kts on both February 8th and February 9th as low as 0.5km (~1600 ft) with lighter winds below (Fig. 4).  A great advantage of this data is that depending on the scanning mode the profiler is in, there can be as many as 11 levels below the 2000 foot threshold to use, which gives us increments every 160 feet or so. The temporal resolution is also good because data is collected every hour.  There are some disadvantages to using the data for LLWS because the presentation as wind barbs still leaves the forecaster the task of calculating both the speed shear and the directional shear on their own.

In order to better visualize this data, we have written a python script to calculate the total wind shear between each layer of the profiler data, which then outputs a total wind shear value on a graph with time on the x-axis and height on the y-axis. Data is color coded for better visualization. Shear over 5 kts is colored yellow and shear values over 10 kts are colored red.  Looking first at the data collected in the 2800 scanning mode (which directly corresponds to the wind barb plot shown) on February 8th, we see increased shear values in the lower levels before 1200 UTC and then relaxing after that time (Fig. 5). What may be surprising is that despite having values of 40 kts in the lower levels of the wind barb plot, the change in speed and direction from layer to layer is relatively small. As a result we can see that despite the obvious presence of a low level jet resulting in pilot reports of moderate to severe turbulence we still barely observe low level wind shear criteria per the definition .  Looking quickly at the data from the 400 scanning mode (more data below 2000 feet) for the same days, we see similar trends and numbers that are not all that different from those obtained in the 2800 scanning mode. What we see here is that the numbers may be a little higher in the 2800 scanning mode because the vertical distance between data points is greater.  The numbers are a little smaller in the 400 scanning mode because the vertical distance is less, but because there are more data points, the signal seems to be more consistent than in the 2800 mode.

This project is in its infancy and questions that we hope to address include an evaluation of the definition of LLWS as it currently stands and how that definition corresponds to observed impacts we are seeing in aviation. We hope to be able to provide better operational guidance for forecasters regarding LLWS by applying the same technology to high resolution numerical model output. Finally, this example did not fit our traditional conceptual model for LLWS, which typically involves a cold air damming airmass.  Thus we hope to better understand the synoptic and mesoscale situations that lend themselves to observed LLWS in North Carolina and across the mid-Atlantic.

Figure 1. 500mb (top) and surface (bottom) analysis for 00Z 8 February, 2015.

Figure 1. 500mb (top) and surface (bottom) analysis for 00Z 8 February, 2015.

Figure 2.  18Z RAP run forecast valid at 22Z at KFAY.

Figure 2. 18Z RAP run forecast valid at 22Z at KFAY.

Figure 3. 00z RAP forecast soundings valid at 07Z for KRWI (top) and KFAY (bottom)

Figure 3. 00z RAP forecast soundings valid at 07Z for KRWI (top) and KFAY (bottom)

Figure 4. Wind profiler data from RTPNC.

Figure 4. Wind profiler data from RTPNC.

Figure 5. Wind shear calculated from wind profiler data at RTPNC (2800 scan mode) on 8 February 2015.   Time increases left to right.

Figure 5. Wind shear calculated from wind profiler data at RTPNC (2800 scan mode) on 8 February 2015. Time increases left to right.

Figure 6. Wind shear calculated from wind profiler data at RTPNC (400 scan mode) on 8 February 2015.   Time increases left to right.

Figure 6. Wind shear calculated from wind profiler data at RTPNC (400 scan mode) on 8 February 2015. Time increases left to right.

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Recent Experiences with NASA SPoRT Soil Temperature Products

NWS Raleigh along with WFOs Houston and Huntsville have been participating in an assessment of several NASA SPoRT Land Information System (LIS) soil moisture products during the past year. During the winter, the same WFOs have been receiving two LIS soil temperature products as a part of an initial testbed.  These soil temperature products are the 3-km average surface skin temperature and 0-10 cm soil temperature.

Past experience has shown that the temperature of the soil and ground surface can have an impact on snow and ice accumulation. It was hoped that the availability of the SPoRT 0-10 cm soil temperature and the skin temperature products would provide useful information to forecasters.  In addition, the SPoRT analysis products would complement a network of mesonet point observations managed by the NC State Climate Office that provide 10cm soil temperatures.

The numerous winter weather precipitation events in central NC this year provided an opportunity to view and evaluate the SPoRT products. At WFO Raleigh, the SPoRT soil temperature data was informally viewed and consulted to evaluate its utility.

Fig 1. AWIPS 2 display of surface METAR data and composite regional radar reflectivity from 1000 UTC on 09 January, 2015.

Fig 1. AWIPS 2 display of surface METAR data and composite regional radar reflectivity from 1000 UTC on 09 January, 2015.

On Friday, January 9th, 2015, areas of freezing rain fell across the southern and central Coastal Plain of North Carolina during the pre-dawn hours. Surface air temperatures ranged in the mid to upper 20s at 10 UTC or 5 AM EST (Fig. 1). The precipitation was driven by convergent low level flow that resulted in a small region of ascent and saturation across portions of southeastern North Carolina. The precipitation followed a period of 24 to 36 consecutive hours of sub-freezing air temperatures.

Fig 2. AWIPS 2 display of NASA SPoRT 3-km average surface skin temperature (top) and the NASA SPoRT 0-10 cm soil temperature (bottom) valid at 0900 UTC on 09 January, 2015.

Fig 2. AWIPS 2 display of NASA SPoRT 3-km average surface skin temperature (top) and the NASA SPoRT 0-10 cm soil temperature (bottom) valid at 0900 UTC on 09 January, 2015.

Given the cold antecedent conditions, the ground across the area was very cold. The SPoRT 3-km average surface skin temperature as displayed in AWIPS 2 is shown in the top of Fig. 2. Skin temperatures in the region of freezing rain ranged between 20 and 25 degrees.  In addition, the SPoRT 0-10 cm soil average temperature (bottom of Fig. 2) indicated that the average soil temperature in the layer from the surface to 10cm or 4 inches below the ground averaged between 25 and 30 degrees. This indicated that not only was the top of the soil column well below freezing but the soil immediately below the ground was below freezing and wouldn’t be providing significant heat to warm the ground near the surface.

Fig 3. Snapshot of a WECT-TV news story on the car accidents resulting from the freezing rain on 09 January, 2015.

Fig 3. Snapshot of a WECT-TV news story on the car accidents resulting from the freezing rain on 09 January, 2015.

Given this environment, the precipitation fell as an area of light freezing rain across the Coastal Plain which resulted in icy conditions on many roadways with numerous car accidents across Sampson, Bladen, Duplin and nearby counties during the morning (Fig. 3).  While the amount and extent of precipitation was in question just hours prior to the event, the soil temperature data provided increased confidence that any precipitation that fell would have a significant impact.

Fig 4. NC State Climate Office CRONOS display of surface temperatures and composite regional radar reflectivity from 1000 UTC on 27 January, 2015.

Fig 4. NC State Climate Office CRONOS display of surface temperatures and composite regional radar reflectivity from 1000 UTC on 27 January, 2015.

In another event, during the early morning hours on Tuesday, January 27th, 2015, a short wave trough and the associated surface trough produced an area of precipitation that moved across central NC. The precipitation was in the form of snow and snow showers near the Virginia border across the northern Piedmont and northern Coastal Plain of North Carolina with air temperatures hovering near freezing (Fig. 4). Temperatures were warmer to the south where the precipitation fell as rain or mixed rain and snow.

Fig 5. Web page display of the NASA SPoRT 3-km average surface skin temperature (top) and the NASA SPoRT 0-10 cm soil temperature (bottom) valid at 1000 UTC on 27 January, 2015.

Fig 5. Web page display of the NASA SPoRT 3-km average surface skin temperature (top) and the NASA SPoRT 0-10 cm soil temperature (bottom) valid at 1000 UTC on 27 January, 2015.

The ground across northern NC was cool that morning but the soil was generally above freezing. The SPoRT 3-km average surface skin temperature as displayed via the web is shown in the top of Fig 5 with the skin temperatures in the region where precipitation was falling ranging around or a few degrees above freezing. Skin temperatures cooled to the west behind the surface trough and the precipitation, as cooler and drier air moved into the area. In addition, the SPoRT 0-10 cm average soil temperature (bottom of Fig. 5) indicated that the average soil temperature in the layer from the surface down to 10cm below the ground averaged in the upper 30s to lower 40s. These values were consistent with the soil temperature observations from the NC State Climate Office’s ECNONET network (not shown) which reported 10 cm soil temperatures in the upper 30s to lower 40s.

Fig 6. Snapshot of a WRAL-TV photo taken near Henderson, NC at around 0630 UTC on 27 January, 2015.

Fig 6. Snapshot of a WRAL-TV photo taken near Henderson, NC at around 0630 UTC on 27 January, 2015.

In locations where snow was falling, air temperatures fell to around 32 degrees, the surface skin temperatures were near or just above freezing, and soil temperatures deeper in the ground at 10 cm were in the upper 30s to lower 40s. Given these conditions, it’s possible to conclude that the snow would not accumulate efficiently or much at all.  However, the snow fell steadily for at least short periods of time with radar reflectivities in the 25-35 dBZ range. The snow rates were significant enough to produce accumulations of a quarter to a half inch with a few locations reporting an inch or more of snow with some accumulations noted on roadways (Fig. 6).

Snow accumulation forecasting, especially in the South, can be very problematic. In our region, snow accumulation events require so many variables to come together just right and even the slightest accumulation can have a significant impact. Forecasters have many variables to consider including the amount of precipitation expected to fall, the rate, the air temperature, the moisture profile, the soil and skin temperatures, solar impact, and more.  In this case, the precipitation rate overcame the other marginal factors for accumulating snow, and more snow accumulated than was anticipated (Fig. 7).

Fig 8. Subjective analysis of accumulated snow in inches from the 26-27 January, 2015 winter storm.

Fig 7. Subjective analysis of accumulated snow in inches from the 26-27 January, 2015 winter storm.

The NASA SPoRT 0-10 cm soil temperature and skin temperature products were examined this winter at WFO Raleigh. The initial results were generally favorable and the additional information was fairly well received. It is important however, that the soil temperature information is used in good context of other observational or forecast information and is used as part of the process.  In short, the soil temperature data is one piece of the puzzle, its importance should not be overvalued. There is also some feedback on some technical and delivery issues we noted at Raleigh that we have shared with the developers.

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Use of SPoRT LIS Products to Evaluate Flooding Potential during a December 2014 Event

Use of SPoRT LIS Products to Evaluate Flooding Potential during a December 2014 Event in Central NC

NASA SPoRT has developed a real-time application of the NASA Land Information System (LIS) that runs over much of the central and eastern United States.  The LIS produces several products, including a suite of soil moisture products that can be used to help assess drought and flooding potential. There are four LIS soil moisture products that are being assessed by WFO Raleigh forecasters in AWIPS-2. The products are also available online at http://weather.msfc.nasa.gov/sport/case_studies/lis_SEUS.html for the Southeast and http://weather.msfc.nasa.gov/sport/case_studies/lis_NC.html for North Carolina.

Fig. 1. WPC 72-hour QPF forecast valid 12 UTC 22 December through 12 UTC 25 December, 2014.

Fig. 1. WPC 72-hour QPF forecast valid 12 UTC 22 December through 12 UTC 25 December, 2014.

After a dry start to the month, multiple rainfall events occurred across central NC during the middle and latter portion of December 2014. During the afternoon of 22 December, forecasters were analyzing rainfall from the previous 24 hours which ranged from a tenth of an inch in the Northwest Piedmont to an inch or more across the eastern Piedmont and Coastal Plain regions. This event was the 3rd fairly significant rainfall event (>0.5 inches) over the central NC since 10 December. Forecasters became concerned as another storm system would impact the Carolinas during the next few days and was expected to produce between 1.25 to 2.0 inches of rain from 22 to 25 December (see Fig. 1). This additional rain had the potential to produce some flooding on main stem rivers, especially across the Coastal Plain.

Fig. 2. The SPoRT LIS one-week change in total column relative soil moisture valid at 15 UTC on 22 December 2014 with the WFO CWAs outlined in yellow.

Fig. 2. The SPoRT LIS one-week change in total column relative soil moisture valid at 15 UTC on 22 December 2014 with the WFO CWAs outlined in yellow.

A SPoRT-LIS field that forecasters have found useful is the one-week change in total column relative soil moisture (RSM, 0-2 m).  The RSM is the ratio of the current volumetric soil moisture between the wilting and saturation points for a given soil type, with values scaling between 0% (wilting) and 100% (saturation). The one-week change product valid at 15 UTC on 22 December, just prior to the rain event, is shown in Fig. 2 with the NWS CWAs outlined in yellow. Note that multiple significant rainfall events occurred across central NC during the previous two weeks. Not surprisingly, this product indicated that much of central NC had experienced a relative soil moisture increase from the previous week.

Fig. 3. The SPoRT LIS 0-200 cm relative soil moisture (%) analysis valid at 15 UTC on 22 December 2014 with the WFO CWAs outlined in yellow.

Fig. 3. The SPoRT LIS 0-200 cm relative soil moisture (%) analysis valid at 15 UTC on 22 December 2014 with the WFO CWAs outlined in yellow.

Another SPoRT-LIS field that forecasters found useful is the SPoRT LIS 0-200 cm Relative Soil Moisture (%) analysis product. The LIS 0-200 cm Relative Soil Moisture (RSOIM) analysis from 15 UTC on 22 December is shown in Fig. 3. The RSOM values in the area outlined by the red box across the northern and central portions of the Coastal Plain are highlighted in the deeper and darker green shading and generally exceed 55% and in many locations exceed 60%.  Subjective analysis of the RSOIM product by previously by WFO Huntsville AL during several synoptic rainfall events suggests that when the 0-200 cm RSOIM values exceed 55%, the risk of flooding on larger rivers increases substantially.

Fig. 4. An analysis of precipitation across central NC from 23 to 25 December indicates a large area of 2.0 to 2.5 inches of rain across the Coastal Plain of NC with lesser amounts in the 1.0 to 2.0 range across the western and northern Piedmont of NC.

Fig. 4. An analysis of precipitation across central NC from 23 to 25 December indicates a large area of 2.0 to 2.5 inches of rain across the Coastal Plain of NC with lesser amounts in the 1.0 to 2.0 range across the western and northern Piedmont of NC.

Significant rain did fall across central NC during the days leading up to Christmas. An analysis of precipitation across central NC from 23 to 25 December shown in Fig. 4 indicates a large area of 2.0 to 2.5 inches of rain fell across the Coastal Plain of NC with an average of around 2.0 to 2.25 inches across the Tar and Neuse River basins. These same locations were noted in Fig. 2 with RSOIM values that exceeded 55%.

The significant rain combined with the wet antecedent conditions did result in flooding at several forecast points across central NC with a few locations in the Coastal Plain reaching moderate flooding.  The observed hydrograph and multiple forecast traces for Smithfield NC (SMFN7) on the Neuse River is shown in Fig. 5. The observed stage is noted by the nearly continuous red dots surrounded by blue circles that exceed the orange horizontal line (flood stage) and the red horizontal line (moderate flood stage).  The river exceeded flood stage at Smithfield during the afternoon of 24 December and reached moderate flooding less than 24 hours later.

Fig. 5. The observed hydrograph and multiple forecast traces for Smithfield NC (SMFN7) on the Neuse River from 12 UTC on 23 December through 12 UTC on 27 December.  The observed stage is noted by the nearly continuous red dots surrounded by blue circles that exceed the orange horizontal line (flood stage) and the red horizontal line (moderate flood stage) while the forecast traces are noted by the narrower lines with dots every 6 hours.

Fig. 5. The observed hydrograph and multiple forecast traces for Smithfield NC (SMFN7) on the Neuse River from 12 UTC on 23 December through 12 UTC on 27 December. The observed stage is noted by the nearly continuous red dots surrounded by blue circles that exceed the orange horizontal line (flood stage) and the red horizontal line (moderate flood stage) while the forecast traces are noted by the narrower lines with dots every 6 hours.

Several days prior to flooding, the hydrologic situation was discussed in the NWS Raleigh Area Forecast Discussion (AFD) issued at 230 PM EST on Monday, 22 December (text shown below). In the AFD, the antecedent conditions were discussed with multiple SPoRT LIS products referenced. While the rainfall during the following few days exceeded the initial forecast and the anticipated impacts, the availability of the SPoRT LIS products lead to increased awareness of the flooding potential which proved especially helpful as the observed rainfall amounts increased and river levels rose.

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RALEIGH NC

.HYDROLOGY…
AS OF 230 PM MONDAY…

ANTECEDENT RAINFALL OVER THE PAST 24 HOURS RANGED FROM A MINIMA OF ABOUT A TENTH OF AN INCH IN THE NORTHWEST PIEDMONT (THE UPPER YADKIN/PEE DEE AND UPPER HAW RIVER BASINS) TO A STRIPE OF 1 INCH PLUS ACROSS THE SOUTHEAST (THE CENTRAL NEUSE AND CENTRAL CAPE FEAR BASINS). WILL SEE ONLY MINOR RISES ON THE MAINSTEM RIVERS IN RESPONSE…BUT THIS IS THE 3RD FAIRLY SIGNIFICANT RAINFALL EVENT (>0.5 INCHES) OVER THE AREA SINCE 12/10. RELATIVE SOIL MOISTURE PERCENTAGES IN THE 0-200 CM COLUMN HAVE BEEN INCREASING…WITH HIGHER PERCENTILES NEARER THE SURFACE…SO QUICKER RUNOFF IS EXPECTED FROM OUR UPCOMING RAIN EVENT.

CURRENT QUANTITATIVE PRECIP FORECASTS FROM THE GEFS AND NAEFS ENSEMBLES ARE IN LOCKSTEP AGREEMENT AT PRESENT…WITH HEAVIEST RAIN (~1.3-1.5 INCHES) FROM TOMORROW NIGHT THROUGH CHRISTMAS MORNING. RAINFALL AMOUNTS IN THIS RANGE COULD POTENTIALLY CAUSE SOME MINOR FLOODING ON THE NEUSE RIVER AND TAR RIVER LATE ON CHRISTMAS DAY…BUT IT WOULD BE LOW IMPACT WITH THOSE RIVERS BARELY REACHING MINOR FLOOD STAGE. THE UPSHOT…RIVER FLOODING WILL ONLY BE AN ISSUE IF RAINFALL FORECASTS BEGIN TRENDING HIGHER…INTO THE 2 INCH PLUS RANGE.
&&

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Nice Time Lagged Ensemble Example During Cold Advection

We have found single model time lagged ensembles to be helpful in forecast activities at NWS Raleigh to ascertain model trends, gauge model consistency or volatility and to consider in determining forecast confidence. We access this data through Bufkit via locally constructed files of several previous model runs of a single model. The scripts used to create these files were generously provided by WFO Wilmington OH. Currently we have time lagged ensembles for the HRRR, RAP, NAM, and GFS.

An example of a GFS time lagged ensemble showing the guidance trend toward a more rapid arrival of cold air on 03 UTC Sunday 15 February, 2015 at Greensboro is shown
below. The model initialization time for each cycle is noted with all of the forecasts valid at 03 UTC Sunday. The more rapid arrival of the cold and even dry air is noted with the 850 hPa temperature becoming 12C colder during the evolution of the model cycles. This trend can also be seen in the partial thickness nomogram.  It’s important to note that the forecasting axiom of “The trend is your friend” is true much of the time but not always so forecasters must use this data intelligently.

example

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New NC State CSTAR List Server Available

Based on feedback at the fall CSTAR workshop, we created a list server to support CSTAR activities with NC State University. We will be using the Lyris list server in which most NWS folks have previously used. The list server can be accessed at http://infolist.nws.noaa.gov/read/?forum=cstar_nc_state. As a private mailing list, it is hoped that this resource will provide a mechanism for more organic, real-time, and frank discussions relating to the project. Our CIMMSE blog will still be used for more polished science sharing.

list.server

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