Increasing Threat for HSLC Severe Weather on Tuesday Morning across Portions of Virginia and North Carolina

A quick look at some of the forecast products on the NC State HLSC web page suggests an increasing threat for severe weather on Tuesday morning. While the overall thermodynamic environment appears marginal, recent forecast trends suggest boundary layer moisture and instability will be a little greater than previously expected. Atop the increasing moisture, low and mid-level flow at 50-70kts will result in a strong warm advection pattern and shear.

SPC HREF surface based CAPE forecast postage stamps from the 01/22 12 UTC cycle valid at 15 UTC on Tuesday 01/23 shown below indicate only weak instability across eastern NC and eastern VA.

sbcape_ps.ma.f02700

At the same time, the 01/22 18 UTC cycle of the NAM valid at 15 UTC on Tuesday 01/23 shown below highlights northern and northeast NC and especially central and eastern VA for potential severe weather. At 15 UTC, the experimental SHERB and Modified SHERB composite parameters exceed 1 in this region suggestion the potential for HSLC significant severe reports. An introduction to the SHERB is available in this PDF: https://cimmse.files.wordpress.com/2016/09/sherb.pdf

20180122.18z.f21

As a reminder, forecasts of the SHERB/SHERBE and the MOSH/MOSHE products are available for GFS, NAM, and RAP:

GFS
NAM
RAP

In addition, SPC mesoanalysis products for the Mid-Atlantic/Ohio Valley are available for the:
SHERBS3
SHERBE
MOSHE

Finally, as a part of our ongoing efforts to assess and improve the SHERB indices, we ask operational forecasters to submit feedback on the performance of the SHERB and modified SHERB, for both HSLC events and null cases: https://www.meas.ncsu.edu/mdparker/sherb/feedback

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HSLC Products and Feedback Form

Given that Friday has at least some low-end HSLC potential in the Southeastern U.S., I took the opportunity to update the HSLC products feedback form so that it now includes questions about the so-called Modified SHERB (MOSH) and SHERBE (MOSHE).  The link to the survey is:

https://www.meas.ncsu.edu/mdparker/sherb/feedback

We sincerely appreciate your comments and assessments!

 

Also, as a reminder, we now have the MOSH/MOSHE products available for GFS, NAM, and RAP:

http://storms.meas.ncsu.edu/users/mdparker/gfs/hslc.html

http://storms.meas.ncsu.edu/users/mdparker/nam/hslc.html

http://storms.meas.ncsu.edu/users/mdparker/rap/hslc.html

Finally, in case you haven’t spotted it, SPC now plots the MOSHE as a part of their mesoanalysis:

http://www.spc.noaa.gov/exper/mesoanalysis/s17/moshe/moshe.gif

Please note that I am not sure whether the possible issues with their calculation have been corrected yet.  But, it may still be useful for a qualitative assessment.

Good luck to all with the coming winter/spring HSLC window, and please let us know if we can help!

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Winter 2017 Mid-Atlantic and Southeast Sub-regional SOO and Northwest Flow Project Virtual Workshop

On December 13, a winter weather oriented sub-regional SOO and Northwest Flow Project virtual workshop was held. The workshop was organized following the Spring 2017 CSTAR Workshop and Mid-Atlantic and Southeast Sub-regional SOO meeting held in Raleigh where participants suggested getting together later and sharing information related to the cool season. More than 2 dozen folks participated from 17 different locations including 10 different WFOs and 3 different universities.

The workshop featured 3 different talks covering NCEP models and winter precipitation, winter weather NWP and automated detection of mesoscale snowbands and finally Dual-Pol radar in winter weather operations. Slide decks and recorded videos of the presentations from the workshop are available via Google Drive.

Links to the specific presentation materials and videos are also available in the agenda listing below…

  • Geoff Manikin, EMC Precip Type, Snow Accumulation, and Upslope Winter Precip in NCEP Models (slides | recording)
  • Gary Lackmann and Jacob Radford, NC State – Winter Weather NWP, and Automated Detection of Mesoscale Snowbands (NWP slides | Banding slides | recording)
  • Matthew Elliott, SPC – Dual-Pol Radar in Winter Weather Operations (slides | recording)

In addition, a couple of web sites were highlighted during the presentations including…

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Late Spring HSLC Tornadoes across the Carolinas and Virginia: 4-5 May 2017

In association with a high amplitude closed upper low over the Mississsippi Valley, and a retreating wedge front at the surface, a mainly late night outbreak of wind damage and tornadoes occurred in a classic high shear, low CAPE (HSLC) environment, beginning late evening on May 4 and lasting beyond 8am on May 5.  There were a few tornado reports earlier in the day farther south that may not have occurred technically in low enough shear to meet the original CSTAR-defined thresholds for SBCAPE and MLCAPE, but most if not all in northern SC, and all of NC and VA did. This review will just focus on a couple of these tornadoes that were rated EF1 and not on the EF0s or the numerous reports determined to be straight line winds.

Synoptic U/A maps:

 

Fig1

Fig 1. 500 hPa Hgt/Temp  0000 UTC 05 May 2017.

Fig2

Fig 2. 850 hPa Hgt/Temp/DewPt  0000 05 May 2017

 

Surface frontal and large scale radar evolution:

Fig3

Fig 3. WPC surface analysis and radar mosaic, 0000 UTC 05 May 2017.

 

Fig4

Fig 4. Same as Fig 3 but for 0900 UTC.

SPC summary of national severe reports:

Fig5a Fig5b

Fig. 5.  SPC reports from 1200 UTC 04 May 2017 – 1200 05 May 2017 (left); and same for 05 May – 06 May (right).

 

Tornadoes that occurred between 8pm (May 4) and 8am (May 5) almost all appeared to occur in HSLC environments based on the Sherbun and Parker (2014) definition (SBCAPE of 500 J/kg or less, MUCAPE of 1000 J/kg or less, and 0-6km bulk wind difference of 18 ms-1 or more), when eyeballing the SPC mesoanalysis regional images.  Maps of the tornadoes (with all other reports removed) between 8pm – 8am are shown below.

Fig6

Fig. 6. All tornado reports between 0000 UTC (8pm) 04 May 2017 and 1200 UTC (8am) 05 May 2017.

 

Fig7

Fig. 7. Same as Fig 6 but zoomed in on northern NC and southeastern VA, and with the EF1 tornadoes highlighted along with time of touchdown and approx. path length labelled.

 

Following are some SPC mesoanalysis fields and radar images associated with the Rockingham Co NC EF1 at around 3am (the far SW tornado in Fig. 7 above):

Fig8

Fig. 8. MLCAPE at 0700 UTC 05 May 2017. Approximate location of Rockingham Co NC EF1 indicated by purple star.

 

Fig9

Fig. 9. Same as above but with 0-6km Bulk Shear Vector and magnitude.

 

Fig10

Fig. 10. Same as above but for 0-1km Storm Relative Helicity (SRH).

 

Fig11

Fig. 11. Same as above but for Sig Tor Parameter (STP).

 

Fig12

Fig. 12. Same as above but for SHERBE parameter.

 

Fig13

Fig. 13. Same as above but for Modified SHERBE (MOSHE).

 

Summarizing the above for the Rockingham Co NC EF1, the STP and SHERBE, while both showing underwhelming values in the location where the tornado occurred, at least indicated this was near the nose of a ridge for these parameters, but the MOSHE more clearly showed a maximum with values of at least 2.5.

Radar images for Rockingham Co NC EF1:

 

Fig14

Fig. 14. KFCX Z/SRM images at 0703 UTC (about 10 min before tornado touchdown) near Eden. Top images are the 0.5 deg slice, bottom ones are the 1.3 deg slice. Storm is about 45 nm from radar, and radar beam at 0.5 is about 6,000ft AGL. Radar is to the northwest.

 

Fig15

Fig. 15. Same as above but for 0711 UTC (about the time of touchdown near Eden).

 

Following are similar mesoanalysis fields as shown above but for 6am (was not able to capture 7am for most of these) and with the locations of the two Dinwiddie Co VA EF1s (a little south of Richmond) and the Orange Co VA EF1 (northeast of Charlottesville).

 

Fig16

Fig 16. MLCAPE at 1000 UTC (6am) May 05 2017. Approximate location of the two Dinwiddie Co EF1s (southern-most purple star) and of the Orange Co VA EF1 (northern-most purple star).

 

Fig17

Fig. 17. Same as above but for 0-1km SRH.

 

Fig18

Fig. 18. Same as above but for STP.

 

Fig19

Fig. 19. Same as above but for SHERBE.

 

Fig20a

Fig 20a. Same as above but for MOSHE.

Fig20b

Fig 20b. Same as above but for 1100 UTC (7am). [For some reason this was available at 7am but none the other fields were.]

 

Radar from Dinwiddie Co VA storm that produced two EF1 touchdowns, these are associated with initial touchdown. Radar configuration suggests more of a bow echo with circulation near comma head of bow, which was a common storm mode/configuration for several of the other tornadoes early this morning.

 

Fig21

Fig. 21. KAKQ radar at 1045 UTC, about 6 min before EF1 showing circulation coincident with comma-head region of bow echo. This location is just under 40nm from radar. Upper left panel is 0.5 deg, upper right 0.5 SRM, lower right is 0.5 NROT, and lower left is 1.3 Z.  

 

Fig22

Fig. 22. Same as above but at 1051 UTC (right at time of tornado touchdown), and lower left image is now CC showing subtle tornado debris signature (just to the left of the “M” in McKenney).

Note that a second tornado (also EF1) touched down in northern Dinwiddie Co. less than 20 min later with the same storm, and this was just after the bow echo signature went through a “Broken-S” evolution, the circulation briefly tightened again, and another TDS was observed as well (not shown).

 

Conclusions:

This is just a sampling of a few of the tornadoes associated with a classic High Shear Low CAPE (HSLC) environment, that were part of a nighttime outbreak of tornadoes and widespread severe weather on 4-5 May, 2017, and in association with a deep amplitude nearly vertically stacked upper-level trough and  retreating wedge front at the surface. Most of the tornadoes (if not all) that occurred between 8pm May 4 and 8am May 5 appeared to form in environments that easily fit the HSLC criteria from Sherbun and Parker (2014). A closer near-storm environmental analysis using surface observations and modified RAP soundings may be needed for each of these tornadoes in order to confirm this.

A closer look at the EF1 tornadoes in NC and VA showed that while they occurred on the northern edge of ridges in the analyses of traditional composite parameters (such as STP, and even the SHERBE), the Modified SHERBE (or MOSHE) seemed to show a better signal at the location of these tornadoes with values well above 1.0 in most cases. The caveat here is that the time shown for most of the analyses (1000 UTC) is about an hour before the tornadoes that are overlaid with them, but yet the 1100 UTC MOSHE analysis (which was available) fits pretty well with the tornadoes that occurred around that time.

Radar analysis of the storms associated with these tornadoes showed very shallow reflectivity signatures (and in fact, lightning activity was generally non-existent with them). In fact, in most cases there were only subtle signatures suggestive of a tornadic threat , such as small-scale bow echoes and in one or two cases evolving through “Broken-S” signature. Velocity fields did show weak to moderate circulations with many storms before the tornadoes touched down, but including some storms that did not produce known tornadoes.  Many of the tornadic storms were within 30-40nm of the nearest radar, but even so these signatures were often subtle, especially in terms of reflectivity structures.

 

 

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Modified SHERB forecast plots now available

Forecast graphics for the modified SHERB (MOSH) and SHERBE (MOSHE) parameters are now available at the following links for the RAP, NAM, and GFS:

http://storms.meas.ncsu.edu/users/mdparker/rap/hslc.html
http://storms.meas.ncsu.edu/users/mdparker/nam/hslc.html
http://storms.meas.ncsu.edu/users/mdparker/gfs/hslc.html

shrbtiles.01.gif

Example 1-hour forecast RAP 4-panel of (top left) SHERBS3, (top right) SHERBE, (bottom left) MOSH, and (bottom right) MOSHE, valid 13Z 25 August 2017.

We have noticed that the calculated values show relative consistency between models, and the spatial footprints of enhanced values tend to be similar to those of the SPC Mesoanalysis. However, the values on the SPC Mesoanalysis are generally higher than those that we have calculated, again suggesting that there may be some discrepancies in the way SPC is calculating the parameters. I plan to touch base with SPC soon to ask about their progress on calculating individual terms of the MOSH/E parameters in order to determine where the differences arise.

Please feel free to utilize these plots as we transition into the HSLC season and share any insights you have!

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Collaborative Effort to Account for the Impact of the August 21st Solar Eclipse on Operational Forecasts in the Mid Atlantic and Southeast

Map of the percentage of sun obscuration during the eclipse shown in AWIPS.

Map of the percentage of sun obscuration during the eclipse shown in AWIPS.

Meteorologists recognize that solar eclipses in the past have had a notable impact on the sensible weather in the regions in which they occur. These impacts can include a decrease in surface temperature, reduction and changes to surface winds, lowering of surface pressure, changes in cloud cover and more. National Weather Service (NWS) meteorologist in the Southeast and mid-Atlantic have been working collaboratively to account for some of these impacts on official NWS forecasts during the eclipse on Monday, August 21, 2017.

eclipse temp training

Image highlighting the some details of the Eclipse Temperature smartTool and Procedure for GFE.

Surprisingly, most operational numerical weather prediction (NWP) systems do not account for the changes in incoming solar radiation from the sun during an eclipse and the resultant changes in the weather. This is an issue as NWS forecasters provide forecasts of temperature, winds, and other fields at hourly time steps and the eclipse impacts need to be captured by forecaster intervention over model guidance. The effort to provide details on the eclipse impact on weather in our region began with the development of hourly temperature reductions based on past eclipse events and factoring local climatology proposed by Frank Alsheimer at WFO CAE. Additional WFOs in the CIMMSE area collaborated and provided input on reductions while working within the temporal confines of the National Digital Forecast Database. Joshua Weiss at WFO ILM, examined data from the 1970 and 1984 eclipses in the Southeast and created a GFE smartTool and Procedure to provide a more scientifically sound and consistent process to edit the hourly temperature forecasts.

meteograms

Official hourly forecasts of temperature and sky cover from three locations in adjacent WFOs CAE, ILM, and RAH. The temperatures were adjusted using the Eclipse Temperature GFE procedure. Columbia, SC is in the eclipse totality while Lumberton, NC and Wadesboro, NC reach 97% obscuration. Differing temperature reductions during and after the eclipse are influenced by various factors including differing amounts of cloud cover.

The GFE Procedure incorporates the forecast temperature and diurnal range without the eclipse impact, whether a location is in the total or partial eclipse, and the amount of cloud cover. Nearly a half dozen WFOs in the Southeast will be using this tool which should lead to a more consistent, more scientifically sound, and accurate forecast. In addition, shapefiles of eclipse information were developed for use in some applications, GIS maps detailing eclipse obscuration percentages were installed in AWIPS, and finally methodologies and strategies were shared via a Google document. This effort builds on the history and relationships built across the CIMMSE domain. The event is a great example of many NWS meteorologists and WFOs working together to provide enhanced forecasts and service.

hrrx.example

Screen shot of the High-Resolution Rapid Refresh (HRRRx) experimental real-time weather forecast web page the eclipse.

It is also worth noting that some experimental and non-operational NWP systems will account for the eclipse. NOAA/ESRL/GSD made changes to the 3km experimental HRRR (HRRRx) to include code to account for the sun-obscuration from the eclipse (details… https://esrl.noaa.gov/gsd/learn/hotitems/2017/eclipse2017-hrrr.html). For real-time HRRRx experimental forecasts, including the effects of the eclipse starting Saturday night looking ahead 48 hours with the 00 UTC model run, visit https://eclipse2017.noaa.gov. Some of the selected weather fields available include downward solar radiation, cloud fields and 2-meter temperature, for the HRRRx (with eclipse effect), the operational HRRR-NCEP (without eclipse effects and some other differences) and HRRRx – HRRR-NCEP difference fields.

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A pair of HSLC-related presentations: NOAA VLab Forum and AMS Mesoscale Conference

Over the last few months, I have provided two presentations on recent HSLC-related research. In June, I was the presenter at NOAA’s VLab Forum, where I provided an overview of the ongoing HSLC CSTAR project based at NC State University. A recording of this presentation can be found here.

The next month, I traveled to the AMS Conference on Mesoscale Processes in San Diego, where I presented an update on my ongoing idealized modeling work. Of particular interest, I have identified the chain of processes appearing to result in the development of strong, low-level vortices within simulated HSLC QLCSs and determined how low-level shear vector magnitude and low-level lapse rates could affect these processes.


Simulation-based research continues, and I intend to complete and defend my dissertation later this fall. Please let me know if you have any questions or comments!

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