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.
&&

Posted in Hydrology | Leave a comment

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

Posted in Winter Weather | Leave a comment

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

Posted in CSTAR | Leave a comment

Orographic Cirrus Event on January 7th, 2015

On January 7th, 2015, an unrelenting moisture plume combined with the right synoptic setup to produce an orographic cirrus event in the lee of the Appalachians. The cirrus shield featured a stationary back edge spanning from Delaware southward through North Carolina.

IR Animation

GOES infrared imagery showing an orographic cirrus event on January 7, 2013.

To preface, we reference an orographic cirrus forecasting guide made for the staff here at WFO Raleigh based on local office research as a primer as well as an example of what a typical atmospheric sounding looks like during an orographic cirrus event.

Cirrus guide

Environmental conditions ideal for the development of orographic cirrus.

cirrus sounding

An example of what a typical atmospheric sounding looks like during an orographic cirrus event.

The 500 mb analysis for 1200 UTC on January 7th, courtesy of the Storm Prediction Center, shows a nearly classic pattern supportive of orographic cirrus consisting of low pressure to the northeast and a ridge to the west. While the location of both the low and the ridge are not exactly ideal, (the low is further north and the ridge is further west than desired) the resulting westerly flow over the spine of the Appalachians is good enough to support cirrus.

500 mb Analysis

SPC 500 mb analysis showing low pressure over northeastern North America and a ridge over the western CONUS.

In addition to the synoptic pattern, the analysis shows us strong winds aloft (>100 kts in most places) which is usually enough to satisfy the criteria of wind speed increasing with height. In order to know for sure we look at the 1200 UTC observed sounding from RNK and see that not only is speed increasing with height from the inversion up to the tropopause but, it is fairly unidirectional as well with some slight backing with height which is a common occurrence in most orographic cirrus cases.

RNK Sounding

Atmospheric Sounding from 1200 UTC 7 January 2015 from KRNK.

In addition to the winds, the sounding also shows us a strong temperature inversion near 800 mb extending up to 700 mb which is in the target height for the inversion near mountain top level that is necessary to create the standing wave pattern needed to produce orographically induced cirrus. Finally, the sounding shows sufficient moisture aloft to produce cirrus clouds. A quick check of the IAD sounding shows similar features to those described on the RNK sounding, thus confirming the proper environmental conditions needed to produce cirrus.

IAD Sounding

Atmospheric Sounding from 1200 UTC 7 January 2015 from KIAD.

With the synoptics and environmental conditions in place, an animation of the GOES infrared satellite imagery (top of the page) from the morning of January 7th does indeed show an orographic cirrus event setting up around 0700 UTC and lasting through the morning and into the early afternoon before the stationary back edge of the cirrus begins to break down. An animation of the GOES water vapor imagery shows clearly that there is a steady plume of moisture west of the mountains that is gradually progressing southward through the event, which resulted in the southern edge of the cirrus eventually  ending up in  North Carolina instead of just being confined to the Mid-Atlantic region. The cirrus overspread most of the Delmarva Peninsula, a good portion of VA east of the mountains including Roanoke, and the Piedmont Triad Region of North Carolina.

WV animation

Animation of GOES water vapor imagery from 7 January 2015 showing orographic cirrus over the Mid-Atlantic.

So how did the cirrus affect the forecast? From the map below we can see that Greensboro, NC (GSO) was the only one of four major forecasting points (GSO, RDU, RWI, and FAY) in the WFO Raleigh county warning area to be completely under the cirrus shield.

Stations

GOES Infrared Satellite Imagery from 1145 UTC on 7 January 2015 showing the relative locations of KGSO, KRDU, KRWI, and KFAY.

Temperature verification statistics from 1200 UTC 7 January 2015 for the Point Forecast Matrix (RAH forecast), the MET (NAM MOS), the MEX (Extended GFS Guidance), and MAV (GFS MOS) guidance for the four forecasting sites mentioned above shows that GSO was the only site to experience a cold forecast bias in four runs prior to verification. In the case of the MEX and the MAV it was 4 degrees too cold 24 hours out. This makes sense with the unforeseen extra cloud cover. The MET guidance seemed to handle the situation the best and this is logical because of the better resolution in space and time.  Data from RDU and RWI, which were on the edge of the cirrus, show that there is still a cold bias in the GFS guidance products, but not as pronounced. FAY, which was the only station clearly removed from the cirrus, did not have any cold bias in the guidance whatsoever and in fact, the MEX and MAV had a perfect forecast 24 hours out.

Verification

Verification biases for the WFO RAH PFM, MET, MEX, and MAV for KGSO, KRDU, KRWI, and KFAY for 1200 UTC 7 January 2015.

The interesting take from this event for me personally is that prior to the enhanced cirrus development, the MAV forecast was already calling for broken to overcast skies 24 hours out. Despite that, the development of the more widespread and more opaque cirrus shield over the area contributed to the four degree cold bias despite an already cloudy forecast. It would have been interesting to see how much of a cold bias there would have been if the forecast had called for clear skies. Prior case studies have shown differences in temperature of up to ten degrees under an orographic cirrus shield with otherwise clear skies.

For more on the orographic cirrus research conducted at WFO RAH, please see the poster located here.

-Ryan Ellis

Posted in Uncategorized | Leave a comment

Dual-Pol Radar Data Used to Identify Birds During a Freezing Rain Event on January 9, 2015

Fig 1. WECT-TV news story on the car accidents resulting from the freezing rain.

Fig 1. WECT-TV news story on the car accidents resulting from the freezing rain.

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. The precipitation was driven by convergent low-level flow in a region of isentropic up-glide that resulted in a small region of ascent  and saturation across portions of southeastern North Carolina. With surface air temperatures in the mid to upper 20s, following a period of 24 to 36 consecutive hours of sub-freezing temperatures, the ground across the area was largely frozen. This resulted in icy conditions on many roadways with numerous car accidents across Sampson, Bladen, Duplin and nearby counties during the morning.

Fig 2. Regional radar reflectivity and METARS from 10 UTC (left) and 12 UTC (right).

Fig 2. Regional radar reflectivity and METARS from 10 UTC (left) and 12 UTC (right).

The precipitation developed a little after 08 UTC between Fayetteville and Wilmington NC, with the areas of freezing rain expanding and moving northeast. The coverage of precipitation peaked at around 10 UTC (left panel of the image above) with the precipitation generally weakening and exiting the area to the northeast at 12 UTC (left panel of the image above). Of some concern was the area of reflectivity that was expanding near and to the southeast of Raleigh at around 12 UTC, highlighted in the yellow circle.

Fig 3. The morning RAOB from 12 UTC on 09 January 2015 from Newport NC (MHX).

Fig 3. The morning RAOB from 12 UTC on 09 January 2015 from Newport NC (MHX).

Given the air temperatures near 30 degrees in the Raleigh area and the history of car accidents produced by the freezing rain in the Coastal Plain, forecasters were concerned that this area of expanding reflectivity may result in accumulations of freezing rain near Raleigh and the potential for more car accidents.  The morning RAOB from 12 UTC at Newport NC (MHX) shown above, highlights the environment across southeastern North Carolina with an above freezing layer centered near 925 hPa as well as a shallow, subfreezing layer near the surface with a surface temperature of 26 degrees.

Fig 4. KRAX WSR-88D four panel imagery from 1200 UTC on 09 January 2015 with reflectivity (upper left), ZDR (upper right), KDP (lower left) and CC (lower right).

Fig 4. KRAX WSR-88D four panel imagery from 1200 UTC on 09 January 2015 with reflectivity (upper left), ZDR (upper right), KDP (lower left) and CC (lower right).

Forecasters during the event noted that the character and structure of the reflectivity signature near and southeast of Raleigh,  seemed peculiar as it appeared to expand in all directions.  The forecasters used dual-pol radar data (see image to the right) to examine the radar returns near Raleigh, highlighted in the yellow circle, more closely.  In the dual-pol radar imagery below from KRAX at 1200 UTC on 09 January 2015, the reflectivity in the upper left included returns of up to 19 dBZ. The ZDR values in the upper right varied considerably but were sampled at -1.75 dB. The CC values in the lower right were very low and ranged and were sampled at 0.43 while the KDP values were removed because of the low CC values.  This information, especially the very low CC and low ZDR indicated, indicated that the these returns were from birds taking off at daybreak and not a developing or expanding area of freezing rain. The dual-pol products were a considerable resource for forecasters and they allowed forecasters to correctly avoid unnecessary forecast updates or special weather statement issuances.

Posted in Winter Weather | Leave a comment

A Brief Look Back at the SHERB Parameter During the Christmas Eve 2014 High-Shear Low CAPE Event

141224_rpts_filtered

Fig 1. SPC storm reports for 24 December, 2015.

First, I need to note that I was not on the forecast desk on December 24th, but I did help with the forecast during a few of the days preceding the event.  The blog post below is not intended to be an in-depth summary of the event but an opportunity to share a few images and comments about the event.

18_500mb

Fig 2. SPC analysis of 500 hPa heights, winds and temperatures at 1800 UTC on 24 December, 2014.

This event was characterized by a highly amplified upper trough across the Mississippi Valley during the afternoon of  12/24 with strong winds through a deep layer of the atmosphere (see image to the right). At the surface, deepening low pressure was moving across the Ohio Valley during the afternoon of  12/24 with a cold front extending south across the Tennessee Valley into the northern Gulf of Mexico while an inland penetrating warm front was moving across the Carolinas.

Fig 3. The 30 to 33 hour forecast of the SHERBS3 parameter from 12 UTC on 12/23 and 09 UTC on 12/23 valid at 18 UTC on 12/24 from the GFS, NAM, SREF, and Canadian models as depicted in AWIPS-2.

The 30 to 33 hour forecast of the SHERBS3 parameter valid at 18 UTC on 12/24 from the GFS, NAM, SREF, and Canadian models are shown to the right as depicted in AWIPS-2. The SHERB is a composite parameter composed of the surface to 3-km shear magnitude, the 0-3-km lapse rate, and the 700–500-hPa lapse rate intended to separating significant High-Shear Low CAPE Event (HSLC)  severe reports from non-severe HSLC storms using a threshold of 1(with significant severe weather more likely for SHERB above 1).

Fig 3. The 36 to 39 hour forecast of the SHERBS3 parameter from 12 UTC on 12/23 and 09 UTC on 12/23 valid at 00 UTC on 12/25 from the GFS, NAM, SREF, and Canadian models as depicted in AWIPS-2.

Fig 4. The 36 to 39 hour forecast of the SHERBS3 parameter from 12 UTC on 12/23 and 09 UTC on 12/23 valid at 00 UTC on 12/25 from the GFS, NAM, SREF, and Canadian models as depicted in AWIPS-2.

The 36 to 39 hour forecast of the SHERBS3 parameter valid at 00 UTC on 12/25 from the GFS, NAM, SREF, and Canadian models are shown to the right as depicted in AWIPS-2.

Two areas of elevated SHERB values can be seen in the forecast images, the most prominent is located across the upper Tennessee and Ohio Valleys and another located across the eastern Carolinas. From the various sets of model guidance, the SHERB was most consistently forecast to be greater than 1 across eastern Kentucky, Ohio and West Virginia. The SHERB was forecast to be near but generally below 1 across the eastern Carolinas.  The SPC Day 2 Outlook also included these areas in the marginal severe weather threat.

Regional radar imagery from 1630 UTC and 2230 UTC on 24 December, 2014.

Fig 5. Regional radar imagery from 1630 UTC (top) and 2230 UTC (bottom) on 24 December, 2014.

Regional radar imagery from 1630 UTC (top) and 2230 UTC (bottom) on 24 December, 2014 is shown to the right. The convection across the Carolinas occurred earlier in the day and was multi-celluar. The Ohio Valley convection occurred later in the day, primarily as a narrow convective line. HSLC severe weather was observed in the Ohio Valley along with a single severe weather report in southeastern NC.  There were two tornadoes reported on 12/24 including an EF1 tornado in Castle Hayne NC at 1530Z and an EF0 tornado in Lancaster, OH that occurred at 2216Z. In addition, there were more than 20 thunderstorm wind damage reports in the Ohio Valley.  A full summary of storm reports are available from the SPC web site.

SPC analyzed SBCAPE from 1600 UTC (top_ and 2200 UTC (bottom) on 24 December, 2014.

Fig 6. SPC analyzed SBCAPE from 1600 UTC (top_ and 2200 UTC (bottom) on 24 December, 2014.

The 0-6km shear values were very high during the times in which severe weather occurred with shear values ranging between 50 and 60 kts across southeastern NC and generally between 80 and 100 kts across the Ohio Valley. The SPC mesoanalysis of 0-6km shear values valid at 19 UTC on 24 December (approximately halfway between the two tornado events) indicate the strong deep layer shear across the eastern United States. Surface-based instability was very limited. The SPC mesoanalysis SBCAPE values were very low across both areas, analyzed at less than 250 J/Kg.

Plots of the SPC analyzed SHERBS3 parameter at 1600Z near the time of the Castle Hayne, North Carolina EF1 tornado at 1530Z (left side of first image), and the SHERBS3 parameter at 2200Z near the time of the Lancaster, Ohio EF0 tornado which occurred at 2216Z (left side of second image), along with all of the severe weather reports for that day (right side of both images).

SPC analyzed SHERBS3 parameter at 1600Z near the time of the Castle Hayne,  North Carolina EF1 tornado at 1530Z (left) along with all of the severe weather reports for that day (right).

Fig 7. SPC analyzed SHERBS3 parameter at 1600Z near the time of the Castle Hayne, North Carolina EF1 tornado at 1530Z (left) along with all of the severe weather reports for that day (right).

 

 

 

 

 

 

 

 

SPC analyzed SHERBS3 parameter at 2200Z near the time of the Lancaster, Ohio EF0 tornado which occurred at 2216Z (left). The other severe weather reports in Ohio generally occurred between 21Z and 00Z. All of the severe weather reports for that day are shown to the right.

Fig 8. SPC analyzed SHERBS3 parameter at 2200Z near the time of the Lancaster, Ohio EF0 tornado which occurred at 2216Z (left). The other severe weather reports in Ohio generally occurred between 21Z and 00Z. All of the severe weather reports for that day are shown to the right.

 

 

Posted in CSTAR, High Shear Low Cape Severe Wx | 1 Comment

RE: Possible HSLC Event on 4 Jan 2015

All,

This is a follow-up to Justin’s HSLC post.  I created a new post because the response box doesn’t allow (or seem to allow) multimedia content.

Thoughts: Based on anecdotal forecast experience and pattern recognition, my opinion is that environmental conditions do not appear favorable for severe weather on Sunday (very similar to Dec 24, 2014). Despite excellent low-level moisture (esp for early January) in the warm sector and favorable diurnal timing with the approaching cold front, pervasive cloud cover will limit insolation and, perhaps more importantly, destabilization via the presence/release of convective instability is unlikely given nearly saturated (moist adiabatic) mid-level thermal profiles and the absence of a sufficient release (layer-lifting) mechanism. 

Anecdotally, I strongly suspect that the presence of convective instability (and release thereof) is a crucial component of severe HSLC episodes.  In other words, a high shear low cape environment becomes a high shear moderate cape environment when convective instability is present and released.  In this manner, a high shear low cape environment east of the Appalachians can evolve into a high shear moderate cape environment more typical of warm sectors that develop in the lower MS river valley and Deep South.

A more detailed environmental analysis is presented below, along with a few images.

Brandon Vincent
Meteorologist
NWS Raleigh, NC

2015-01-01_18Z_NAM

Mid-level lapse rates Upper Left Panel Low-level Moisture/Wind in Lower Right Panel

2015-01-01_18Z_GFS_NAM

GFS H5 height, MSLP and QPF in Upper Left GFS H5 height/vorticity in Lower Left NAM H5 height, MSLP and QPF in Upper Right NAM H5 height/vorticity in Lower Right

12Z 01/01 NAM Forecast Sounding Valid 18Z Sunday Afternoon

12Z 01/01 NAM Forecast Sounding for GSP Valid 18Z Sunday Afternoon

20141224_18Z_S19_ttd

MSLP and Surface Wind/Dewpoints at 18Z on December 24, 2014

700-400 mb Vorticity Advection at 18Z on December 24, 2014

700-400 mb Vorticity Advection at 18Z on December 24, 2014

CHS raob on 12Z December 24, 2014

CHS raob on 12Z December 24, 2014

Synoptic Overview: Model guidance indicates that the parent upper trough /occluded surface low/ will be located over the Great Lakes when the attendant cold front progresses eastward into the Carolinas during the day Sunday.  Although both the GFS/NAM show additional shortwave energy approaching the southern Appalachians from the west Sunday afternoon, both models indicate the wave will provide only a glancing bout of DPVA to northwest portions of NC and lag well behind the cold front/warm sector, in addition to de-amplifying as it tracks northeast into the Mid-Atlantic.  It should be noted that the aforementioned shortwave energy will not move ashore the Pacific NW until Friday night and that the precise evolution of this feature remains rather uncertain.  Assuming the timing/track of this feature do not considerably change, upper level forcing over the Carolinas appears marginal at best, i.e. confined to small amplitude waves in southwest flow aloft.

Thermodynamic Overview: The GFS/NAM indicate weak low and mid-level lapse rates on Sunday with deeply saturated (i.e. moist adiabatic) thermal profiles during the afternoon.  Although diurnal timing appears favorable with regard to the cold frontal passage, cloud cover will significantly reduce insolation. Warm sector destabilization should largely be driven by advection, i.e. a higher theta-e airmass (sfc dewpoints 62-65F) surging inland from the Atlantic in advance of the front. Significant uncertainty persists with regard to the location of the retreating wedge front Saturday night into Sunday. Previous experience would suggest full wedge erosion /warm sector establishment/ is unlikely prior to the cold frontal passage in the far NW piedmont (i.e. winston W/NW to the blue ridge).

Kinematic Overview: Strong unidirectional (SW) deep-layer shear. Directional shear will be confined to the lowest levels in vicinity of the retreating wedge front.

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