Debris Signature with Isolated Tornadic Supercell adjacent to KRAX on March 29, 2014

All,

The severe weather event that unfolded in central NC during the evening hours of March 29, 2014 was notable for several reasons.  Although diurnal destabilization in central NC was severely limited by widespread cloud cover, intermittent showers, and weak mid-level lapse rates during peak heating, strong synoptic ascent /layer-lifting/ in the presence of a convectively unstable airmass (associated with a drier mid-level airmass advecting atop an unseasonably moist boundary layer) resulted in an atypical thermodynamic setup in which marginal to moderate destabilization (500-1000 J/kg mlcape) occurred after peak heating during the early evening hours. This in of itself is worth another post, as I think it highlights some of the inherent limitations in convective parameters and the necessity of performing a thorough environmental analysis using data from a variety of remote sensing platforms, observations, and short-term model guidance.

The animated GIF below shows a 4-panel loop of REF, SRM, VEL and CC from 0.5 to 2.4 degrees of the 0111Z volume scan from KRAX.  This is an excellent example of a subtle tornadic debris signature associated with a supercell observed within several miles of an 88D.  Note that the ‘hole’ in CC (values 0.60 to 0.90) is co-located precisely with both the velocity couplet and the reflectivity ‘ball’ within the hook.  Vertical continuity (including a down-shear tilt) of this feature from 0.5-0.9-1.3-2.4 degrees and the lack of such a feature prior to when the supercell arrived or after it departed provide compelling evidence for a tornadic debris signature.  A weak (EF-0) tornado was indeed confirmed at this precise location/time in southeastern Wake county (see damage survey PNS below).

0.5,0.9,1.3,2.4 degree 4-Panel Loop of REF,SRM,VEL,CC at 0.5,0.9,1.3,1.8 degrees on the 0111Z March 30, 2014 Volume Scan at KRAX

0.5,0.9,1.3,2.4 degree 4-Panel Loop of REF,SRM,VEL,CC at 0.5,0.9,1.3,1.8 degrees on the 0111Z March 30, 2014 Volume Scan at KRAX

In case the aforementioned radar loop did not attach properly, you can also find it at:
http://www.redteamwx.com/2014_03_30_0111_to_0113Z_0.5_to_2.4.gif

March 30, 2014 0111Z 1.3 deg 4-Panel from KRAX with subtle tornadic debris signature

March 30, 2014 0111Z 1.3 deg 4-Panel from KRAX with subtle tornadic debris signature

...TORNADO CONFIRMED NEAR GARNER IN WAKE COUNTY NORTH CAROLINA...

LOCATION...GARNER IN WAKE COUNTY NORTH CAROLINA
DATE...03/29/2014
ESTIMATED TIME...0908 PM EDT
MAXIMUM EF-SCALE RATING...EF0
ESTIMATED MAXIMUM WIND SPEED...80 MPH
MAXIMUM PATH WIDTH...75 YDS
PATH LENGTH...1/8TH OF A MILE
* FATALITIES...0
* INJURIES...0

* THE INFORMATION IN THIS STATEMENT IS PRELIMINARY AND SUBJECT TO
CHANGE PENDING FINAL REVIEW OF THE EVENT(S) AND PUBLICATION IN
NWS STORM DATA.

...SUMMARY...
THE NATIONAL WEATHER SERVICE IN RALEIGH NC HAS CONFIRMED A
TORNADO NEAR GARNER IN WAKE COUNTY NORTH CAROLINA ON 03/29/2014.

PER EYEWITNESS AND A DAMAGE SURVEY...A WEAK EF0 TORNADO OCCURRED
SHORTLY AFTER 9 PM AT THE INTERSECTION OF TREBOR DRIVE AND CROSS
POINTE LANE. A RESIDENCE SUSTAINED PARTIAL ROOF DAMAGE WITH A
COUPLE OF WINDOWS BROKEN AND A COUPLE OF TREES DOWN.

 

Posted in CIMMSE | 5 Comments

NC State Real-time HSLC SHERB Plots Updated and Expanded

Based on feedback, Dr Parker at NC State updated and expanded the online model plots of SHERB recently.

Some of the improvements include:
1) SHERBS3 is now only shaded where MUCAPE>0 J/kg and/or MULI < +6 K
2) The SHERBE is now available
3) A new “compare” link allows you to loop through images with SHERBS3 and SHERBE side by side
4) RAP data has been extended to 12 hours. GFS/NAM imagery is available at 3 hour intervals through 36 hours, then 6 hour intervals through 84 hours.
5) The color scheme for SHERB has been improved to emphasize resolution in the 0.5-1.5 range
6) There is now a full archive of all images available online (the link is available by request)

In case you’ve misplaced them, the relevant links are:

Real-time RAP – http://storms.meas.ncsu.edu/users/mdparker/rap
Real-time NAM – http://storms.meas.ncsu.edu/users/mdparker/nam
Real-time GFS – http://storms.meas.ncsu.edu/users/mdparker/gfs

sherb.server.updates

Posted in High Shear Low Cape Severe Wx | Leave a comment

Interesting test for the SHERB Sunday in Southern GA.

We have a  pretty dynamic set up coming into the southern part of the study area on Sunday. Looking at 18Z Saturday run of the 12km NAM, the model is hinting at a comma-shaped MCS moving through southern Georgia. These images show surface pressure, winds, and model simulated surface reflectivity starting at 18Z Sunday every 3 hours. NAM12_500MB_Height_20140316_2100F027NAM12_500MB_Height_20140317_0000F030NAM12_500MB_Height_20140317_0300F033NAM12_500MB_Height_20140317_0600F036   NAM12_500MB_Height_20140316_1800F024

 

SHERB values are quite high, but instability is poor over the northern half of the region of interest. These images show the SHERB along with the surface-based CAPE during the same time frame.

NAM12_500MB_Height_20140316_1800F024NAM12_500MB_Height_20140316_2100F027NAM12_500MB_Height_20140317_0000F030NAM12_500MB_Height_20140317_0300F033 NAM12_500MB_Height_20140317_0600F036

The darker green shading indicates SHERB values greater than 1.

It seems as though the strongest “bullseye” of SHERB values stays just north of the surface-based CAPE (in this model, of course) tomorrow afternoon and evening, at least in relation to the CHS CWA. It’s going to be close, though, and makes for an interesting forecast. We could see some pretty good rotations to the west of the area on radar with those kind of values, making the radar operator a little nervous, even though the instability will be minimal.

 

Posted in Uncategorized | Leave a comment

N.C. State – NWS CSTAR Project to Examine Low-Topped Severe Convection in the Southeast Selected for Funding

cstar.v.logo.alternate.smallThe project entitled “Improving Understanding and Prediction of High Impact Weather Associated with Low-Topped Severe Convection in the Southeastern U.S.” will be led by Drs. Matthew D. Parker, Gary M. Lackmann, and Lian Xie of N.C. State University in collaboration with nearly a dozen WFOs in the Southeast along with the Storm Prediction Center. The three-year project is being funded as a part of the NOAA/NWS Collaborative Science, Technology, and Applied Research (CSTAR) Program. This project will build off of previous collaborative research between N.C. State and  the NWS which have had very successful research to operations results along with the integration of students into NOAA and the NWS.

Severe convective storms in environments with large vertical wind shear and marginal instability (so-called “high-shear low-CAPE”, or “HSLC” events) represent a significant short-term, high-impact forecasting and warning challenge, particularly in the Southeastern and Mid-Atlantic states of the U.S. Such environments account for a substantial fraction of severe wind and tornado reports in the region, and they are present for many hours each year. The long-range goal of the research is to improve predictions and warnings for hazardous weather in HSLC environments.

The research will be conducted through a set of complementary collaborative research studies including:
(i) The project intends to advance the understanding and interpretation of HSLC radar imagery by performing idealized simulations of HSLC convective storms, within which we will study the dynamical processes at work and compare them to pseudo-radar measurements of the simulated storms.
(ii) The project also intends to improve short range prediction and situational awareness of HSLC scenarios by evaluating a suite of convection-allowing hindcasts of notable HSLC events and nulls and testing the sensitivity of these hindcasts to grid spacing and model configuration.
(iii) The project hopes to improve short-to-medium range prediction and situational awareness of HSLC scenarios by applying dynamically-based statistical downscaling techniques in order to exploit the information available from operational model ensembles.
(iv) Finally, the project intends to improve operational “best practices” in HSLC environments by coordinating an assessment of a number of experimental HSLC diagnosis and forecast products within NOAA.

Posted in CSTAR, High Shear Low Cape Severe Wx | Leave a comment

Severe Weather Possible in a Borderline High-Shear Low CAPE Environment Today

The threat of severe weather across the Mid-Atlantic and Southeast has been well advertised with the initial threat noted in the “Day 5″ portion of the SPC 4-8 Day Severe Weather Outlook issued on 17 February.  Today’s potential for severe weather is focused on damaging wind gusts while brief QLCS events could result in short episodes of an increased tornado threat.

For multiple days the SHERB parameter has highlighted the potential for an enhanced severe weather threat associated with a High-Shear Low CAPE (HSLC) environment  on the northern periphery of region of more favorable surface-based instability located across southeastern NC and SC. The SHERB parameter was developed as a part of an NC State-NWS CSTAR project. You can learn more about it in a PDF version of a presentation entitled Improving Forecasting of High Shear, Low CAPE Severe Weather Environments.

SHERB guidance from multiple runs of the NAM and other NWP sources have shown an enhanced HSLC threat across northeastern NC and southeastern NC this afternoon, centered around 18 UTC, with SHERB values greater than 1 (see the first image below). These locations are forecast to have an air mass characterized by SBCAPE values of around or possibly a little more than 500 J/Kg and MUCAPE values of close to 1000 J/Kg, meaning they are a borderline HSLC case (see 03Z SREF forecast for KRDU valid 18z in the second image below). Despite the more limited surface based instability in these regions than locations further south, the SHERB highlights this region as especially susceptible to  severe weather which is consistent with the 12 UTC SPC Severe Weather Outlook.

Realtime NAM – http://storms.meas.ncsu.edu/users/mdparker/nam/
Realtime RAP – http://storms.meas.ncsu.edu/users/mdparker/rap/

cimmse.shrb.06z.nam.v18z

sref.03z.krdu.v18z

Posted in High Shear Low Cape Severe Wx | Leave a comment

1″/hr Snowfall Rates and Transition to Sleet as Observed by Time Lapse Photography and WSR-88D Data

Now that the much-awaited winter storm is unfolding across the region I thought I’d share a time lapse of today’s snow accumulation ending with a sudden changeover to sleet in Cary, NC (central Wake county).  The attached animated GIF contains 17 frames at 15-minute intervals between 12:15 pm and 4:15 pm EST (1715-2115Z) today (February 12, 2014).  Snowfall rates were around 1″/hr for roughly 4 hours.  The last 2 frames of the time lapse show a dramatic improvement in background visibility, an indication of the quick changeover to sleet that occurred between 3:45 pm and 4:15 pm EST (2045-2115Z).  You can also see my (relatively futile) attempts at shoveling the driveway.

I’ve also attached an animated GIF of KRAX 0.5 degree REF/CC from 3:45 to 4:15 pm EST (2045-2115Z).  My precise location is marked by the ‘red dot’ at the center of the range rings in central Wake county. You can clearly see the SN/PL transition occurring along a SW-NE line of reduced correlation coefficient values across Chatham/Lee/Wake/Franklin/Nash counties.  In this case, reduced CC values can be used as a reasonable proxy with regard to identifying the leading edge of the warm nose (~2+ C) advancing northwest across the Highway 1 corridor.

***Below Update at 4:00 pm (2100Z) February 13, 2014***
I’ve posted a more complete time lapse that captures virtually the entire event in central NC, from 1715Z on February 12 through 2045Z February 13.  The time lapse shows:

1) the initial round of heavy snow between 1715-2045Z (Feb 12)
2) the transition to sleet between 2045-2115Z (Feb 12)
3) the gradual transition from sleet to freezing rain between 0000-0300Z (Feb 13)
4)
the accumulation of ice as noted by the increasing ‘sag’ of trees between 0300-1000Z
5) the
melting of ice (as indicated by ‘rebounding trees’) associated with strengthening
insolation and above freezing temperatures after ~1600Z
6)
further melting and a transition from rain to snow after 1900Z in
association with the deformation band

Brandon Vincent
Meteorologist
WFO Raleigh, NC

Time Lapse of the February 12-13, 2014 Winter Storm (1715Z February 12 through 2045Z February 13)

Time Lapse of the February 12-13, 2014 Winter Storm (1715Z February 12 through 2045Z February 13)

 

ImageImage

Posted in Winter Weather | Tagged | 2 Comments

An Examination of Lake Enhanced Snow across Central NC on 22 January 2014

by Michael Strickler

A band of moderate snow developed over the far northeast Piedmont and northern and central Coastal Plain of central NC on the morning of 1/22/2014. The intensity of the snow band increased such that the visibility at KRWI was reduced to between 1SM and 2SM between 13Z and 16Z (Table 1). The snow band was particularly interesting in that it developed after the parent cyclone and associated moisture and forcing for ascent had moved well to the northeast of NC (Fig. 1), and consequently was a surprising and rather unexpected development for forecasters. In fact, the accumulations between 12Z and 18Z ranged from around a half inch to as much as one inch of snow, locally satisfying winter weather advisory criteria.

1Figure 1. 1500Z WPC surface analysis.

It appeared that the band may have been influenced by Kerr Lake, located near the VA/NC border, since there was a plume-like feature in both satellite and radar data, which seemed to emanate from the lake (Fig. 2).

2Figure 2. METAR and dual-imaged visibile satellite and regional 0.5 degree radar imagery, valid at 1345Z – 1400Z.

We remembered at least a couple of similar events in the past, where it appeared that the lakes in southern VA and northern NC were contributing factors to the development of small snow bands downwind of the lakes, well after the parent cyclone had passed.  See the following links for details for those cases: March 2, 2009 and January 23, 2003. An additional horizontal convective roll cloud band originated over, and developed downwind of, Falls Lake in northern Wake County, evident in visible satellite imagery in Fig. 3 and the photo in Fig. 4.

fig.3Figure 3. METAR and dual-imaged visibile satellite and regional 0.5 degree radar imagery, valid at 1515Z – 1518Z.

photo 2Figure 4. Photograph of the lake band looking north-northeast from NWS Raleigh, located in central Wake County.

The development of the bands seemed to be the result of a unique junction of processes and properties on a range of scales in both time and space, such that without any one of them, the bands likely would not have existed. The first of these processes, and perhaps the most important one, was the meso-gamma scale sensible and latent surface heat and moisture flux from the respective lakes, whose temperatures were in the middle 40s, into the overlying arctic air above.  The temperature differential between the lakes and the overlying 850 hPa temperatures was 18-20 C, which contributed to significant low level destabilization, on the order of several hundred J/kg of CAPE, which can be seen in the RAP BUFR soundings shown in Fig 5 and 6. Since the lake band development preceded any additional cloud street HCR development that occurred with diabatic diurnal heating of the boundary layer during the late morning to midday hours (Fig. 7), it is apparent that the presence of the lakes and their associated influence were vital to the band development.

lake11ZFigure 5. KRWI RAP BUFR sounding, 00 hour forecast valid at 1100Z.

lake16ZFigure 6. KRWI RAP BUFR sounding, 05 hour forecast valid at 1100Z.

streets2Figure 7. METAR and visible satellite imagery, valid at 1500Z – 1515Z.

Another contributing influence for the band development was residual low level, meso-alpha to meso-gamma-scale moisture on the southwest flank of the departing parent cyclone, as seen in Fig 8. This nearly saturated air at 925 hPa, possibly a combination of upstream Great Lakes moisture and an orphaned portion of the deformation head attendant to the departing cyclone, produced a suitable environment/primed the atmosphere for condensation necessary to produce the cloud/snow bands.

low_clouds_RHFigure 8. RAP-analyzed 925 hPa RH and 11-3.9 micron satellite overlay, valid at 12Z.

Processes on the progressively larger scale also favored vertical motion necessary for the development of the bands. A series of low amplitude impulses/initially shear vorticity-dominated shortwave troughs on the back/west side of the mean trough axis aloft (Fig. 9), migrated through and briefly amplified into the trough base/across the Southern-Central Appalachians (Fig. 10), before the mean trough axis lifted up and away from the Middle Atlantic coast. Associated QG forcing for ascent/differential cyclonic vorticity advection shown in Fig. 11, was the larger scale process that acted upon and deepened the underlying moisture/instability, and which consequently resulted in the broader area of snow that developed to the south of the lake-induced band, shown in Fig. 12.

fig9Figure 9. Water vapor satellite imagery and RAP-analyzed 400-200 hPa vorticity, valid at 0900Z – 0915Z.

fig10Figure 10. Water vapor satellite imagery and RAP-analyzed 400-200 hPa vorticity, valid at 14Z.

vortFigure 11. 500 hPa height and vorticity, 700-500 hPa differential vorticity advection, and radar, valid at 14Z.

12Figure 12. METAR, 850 hPa VWP data, and RAP-analyzed 850 hPa temperatures, valid at 13Z.

Table 1. KRWI METAR data valid 1153-1853Z. 

METAR KRWI 221153Z AUTO 01006KT 9SM -SN OVC025 M06/M08 A2995 RMK AO2
     SLP143 P0000 60000 70012 T10561083 11039 21056 53024 $
METAR KRWI 221253Z AUTO 35005KT 8SM -SN BKN028 OVC033 M06/M08 A2998   RMK AO2 SLP152 P0000 T10561083
SPECI KRWI 221310Z AUTO 34005KT 5SM -SN SCT026 OVC032 M06/M08 A2998 RMK AO2 P0000 T10561083
SPECI KRWI 221337Z AUTO 35007KT 2SM -SN OVC032 M06/M08 A2999 RMK AO2
     P0000 T10561083
SPECI KRWI 221339Z AUTO 34007KT 1 3/4SM -SN BKN028 OVC034 M06/M08 A2999
     RMK AO2 P0000 T10561083
METAR KRWI 221353Z AUTO 34007KT 1 1/4SM -SN BKN024 OVC031 M06/M08 A3000 RMK AO2 SLP159 P0000 T10561083
METAR KRWI 221453Z AUTO 34005KT 1 3/4SM -SN FEW013 SCT017 OVC027 M06/M08 A3003 RMK AO2 SLP171 P0000 60000 T10561083 53018
SPECI KRWI 221505Z AUTO 01008KT 1 3/4SM -SN BKN011 OVC030 M06/M08 A3003
     RMK AO2 P0000 T10561083
SPECI KRWI 221516Z AUTO 01007KT 2SM -SN BKN013 OVC021 M05/M08 A3004 RMK
     AO2 P0000 T10501083
SPECI KRWI 221524Z AUTO 35006KT 1 3/4SM -SN SCT013 OVC021 M05/M08 A3004
     RMK AO2 P0000 T10501078
METAR KRWI 221553Z AUTO 34007KT 1SM -SN FEW013 OVC019 M06/M08 A3004 RMK AO2 SLP176 P0001 T10561083
SPECI KRWI 221612Z AUTO 33005KT 1 1/4SM -SN FEW009 BKN013 OVC019 M06/M08 A3005 RMK AO2 P0000 T10561083
SPECI KRWI 221617Z AUTO 33006KT 2 1/2SM -SN SCT013 BKN019 M05/M08 A3004
     RMK AO2 P0000 T10501078
SPECI KRWI 221619Z AUTO 33005KT 4SM -SN FEW007 SCT015 BKN022 M05/M08
     A3004 RMK AO2 P0000 T10501083
SPECI KRWI 221628Z AUTO 33004KT 9SM -SN SCT015 SCT022 M05/M08 A3004 RMK
     AO2 P0000 T10501078
METAR KRWI 221653Z AUTO 29005KT 9SM -SN FEW025 BKN036 M04/M09 A3003 RMK
     AO2 SLP172 P0000 T10441094
METAR KRWI 221753Z AUTO 33010KT 9SM -SN SCT040 M03/M12 A3002 RMK AO2
     SLP166 P0000 60001 T10331117 11033 21056 58004
METAR KRWI 221853Z AUTO VRB06KT 10SM CLR M03/M12 A3000 RMK AO2 SNE03
     SLP159 P0000 T10281117

Posted in Winter Weather | 5 Comments