2019 Mid-Atlantic Severe Wx - General Discussion

[mappy]

5 hours ago, C.A.P.E. said:

That's pretty far south of me. I am in the northern part of Caroline county, which is pretty expansive north to south.

This was the same event that produced the stronger tornadoes in southern DE I believe, and could have been associated with the same localized cell(s)- which would make sense as that location is pretty close (just NW) of the Seaford/Laurel area.

ah. for some reason i thought federalsburg was up in Caroline county too. alas, my geography failed me this time. 

but yes, same event that produced the two in DE

[high risk]

Tonight's NAM3 still shows a strongly-forced squall line along the front later Friday.    Instability and deep-layer shear are modest, but it's probably sufficient as currently progged to warrant a MRGL risk when the new Day 2 SPC outlook is issued in a few hours.        I still think that we could eventually end up as a SLGT, depending on how much heating occurs.

[yoda]

1 hour ago, high risk said:

Tonight's NAM3 still shows a strongly-forced squall line along the front later Friday.    Instability and deep-layer shear are modest, but it's probably sufficient as currently progged to warrant a MRGL risk when the new Day 2 SPC outlook is issued in a few hours.        I still think that we could eventually end up as a SLGT, depending on how much heating occurs.

Sounds good... what did you think of the Euro and does the HRDPS have any value for severe weather?  @losetoa6 mentioned both in the main disco thread 

ETA:  Good call on the MRGL risk 

[high risk]

8 hours ago, yoda said:

Sounds good... what did you think of the Euro and does the HRDPS have any value for severe weather?  @losetoa6 mentioned both in the main disco thread 

ETA:  Good call on the MRGL risk 

          I don't have any feel for the HRDPS, but all of the hi-res guidance I've seen so far this morning points to storms here tomorrow mid-late afternoon.   Some of the guidance breaks out some initial waves of storms that cut down on the instability and the potential for a more organized and intense late-day line;  the NAM3 puts all of its eggs into a single basket with one strong line.       Whether there is a SVR threat remains to be be seen, but "good" storms seem fairly likely at this point.

[yoda]

SLGT risk issued for DC metro... MRGL pushed back west to around i81 corridor on updated Day 2... damaging winds main threat but also chance for a few tornadoes 

[yoda]

NWS Storm Prediction Center Norman OK
   1230 PM CDT Thu Apr 25 2019

   Valid 261200Z - 271200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM NORTH
   CAROLINAS INTO THE MIDDLE ATLANTIC REGION...

   ...SUMMARY...
   Thunderstorms capable of producing damaging wind and isolated
   tornadoes are expected from North Carolina into the Middle Atlantic
   Friday. Isolated damaging wind gusts are also possible over northern
   Utah, southeast Idaho, southwest Wyoming and northwest Colorado.
   Other storms may produce a few strong wind gusts over a portion of
   the Florida Peninsula.

   ...Carolinas to the Middle Atlantic region...

   Southern-stream shortwave trough will deamplify as it becomes
   absorbed within the base of an amplifying northern-stream trough and
   move off the Atlantic Seaboard early Friday. The northern-stream
   trough will continue through the Carolinas and Middle Atlantic
   region during the day accompanied by a cold front. Modest low-level
   moisture with dewpoints in the upper 50s to lower 60s F will advect
   through the warm sector along a 50 kt low-level jet. This in
   conjunction with some cloud breaks will destabilize the boundary
   layer with MLCAPE from 500-1000 J/kg possible. Some thunderstorms
   may be ongoing along the NC coast early Friday in association with
   the leading shortwave trough. Farther upstream, storms are expected
   to intensify along and ahead of the cold front as the boundary layer
   destabilizes from VA into NC and possibly as far south as SC.
   Strengthening mid-upper wind profiles accompanying the
   northern-stream shortwave trough will support 40-45 kt effective
   bulk shear supportive of embedded organized structures with storms
   developing within the evolving squall line and some supercells with
   any storms developing ahead of the line. Damaging wind appears to be
   the main threat,  but isolated tornadoes are also possible. Severe
   threat should end by early evening as the cold front moves offshore.


   ...MAXIMUM RISK BY HAZARD...
   Tornado:   5%     - Slight
   Wind:     15%     - Slight
   Hail:      5%     - Marginal

   ..Dial.. 04/25/2019

[Rtd208]

 

[cyclogenesis]

April 26, 2019

this early Pre-Dawn Friday morning

149 AM EDT

 

* * *   Risk of Severe Storms across Eastern Virginia today on Friday, April 26, 2019  * * * 

* * *   Few severe T-storm warnings, with a watch box quite possible around late morning through the late afternoon * * * 

* * *  High wind gusts & hail to be the main perils starting late morning through late afternoon * * *

I.  Introduction.

It's appearing that the best timing, in Richmond, VA, for a few severe storms will run between 10 AM and 6 PM, with the peak in the early afternoon to mid-afternoon.  The appearance on radar should be multiple clusters of cellular activing both before a thin-banded squall line and shortly afterwards.  A pre-frontal trough is scheduled to sweep through the Richmond, VA area by late Friday afternoon about 5 PM, with wind shift to the West.  Then the cold front should pour in colder air to Richmond, later Friday night starting 10 PM to 11 PM.  

II.  Upper levels.

An upper level trough extending from Eastern Michigan South through Central Kentucky & Tennessee at 11 AM on Friday morning will dig Southeastward to a position extending from Western Pennsylvania to Western Virginia & Central North Carolina / South Carolina by 5 PM Friday.  

During this time of late morning through afternoon, a SW 850 mb ~5K ft low-level jet will be in place across Virginia, at speeds of 35-45 kts.  This, atop, a mid-level jet at 500 mb, ~19K ft, 45kts - 55 kts will work together to create 0-6 km shear between 40-50 kts, with a surface wind from the South-Southwest at 15-20 kts, just prior to the pre-frontal surface trough passage in the late afternoon.  Surface dewpoints will rise to the upper 50's during the early afternoon, with a high temp. in Richmond reaching 77°.   Precipitable water, PW, amounts increase upwards to 1.4" by early afternoon, with the best pooling of moisture at that time, before falling backward by later afternoon, with the pre-frontal trough wind shift to the West.

III.  Severe Indices.

Most of these indices do not look that impressive on areal forecast Skew-T soundings.  Helicity looks low & flat, generally 200 - 250 m2/s2 on account of the straight-lined hodographs overhead Richmond at peak timing of severe.  With uni-directional wind profiles from late morning to mid-afternoon, coming from the Southwest, this suggests a better threat for a few damaging straight-line thunderstorm wind gusts, vice a smaller tornado threat.  

Conventional forecast soundings from both models show a peak wind gust from Gfs to be 48 kts and from Nam to be 51 kts with storms between late morning through late afternoon.  Amidst other models, WRF higher resolution model doesn't indicate any peak wind gusts even to 40 kts during its one-hour peak wind gusts through the convective period.

One area that did catch my eye was the forecast temperatures at 700 mb & 500 mb running between 1°C at 700 mb at 2 PM and about -14 to -15°C  at 500 mb at 2 PM.  These 500 mb temps cool further to -20 to -21C by 8 PM.  With this cold-air advection spreading in ALOFT in the upper levels during the afternoon, this could help promote a HAIL threat with SB-Cape running between 750 J/kg to about 1,200 J/kg at the height of the event.  Wet-bulb zero heights by both forecasting models reach the favorable 8000-9000 ft level for hail production.  The trickiest part about forecasting hail is that the thermal profile aloft comes typically most favorable *after* the best upper level ascent, (upward dynamic forcing), has already passed.   

While CVA / PVA is evident at 500 mb, this supports the lightning-thunder threat as cyclonic vorticity advection promotes this aspect, the trouble I'm seeing is that H-Rap doesn't show much lightning density strikes showing up, even though spotty convective cellular activity reaches 50 dbz in spots across Eastern Virginia.  Watch to see if later hourly updates includes some increase.  Hail storms are typically associated with plenty of lightning occuring within the cumulonimbus cloud.

I reviewed 250 mb upper divergence dynamics, and don't see anything heralding; Gfs 700 mb strongest UVV's, (Upward Vertical Velocities), come in to the RVA area around 2 PM, and it appears most models are anticipating a broken line of storms that comes together as it advances in through Eastern Virginia, perhaps firming-up the strongest once passing EAST of Richmond.

IV.  Rainfall Amounts.

-- Centered on Richmond, VA --

A.)  Gfs -->>  0.49" between 8 AM and 8 PM Friday

B.)  Nam -->> 0.35" between 8 AM and 8 PM Friday

C.) ECMWF -->> 0.81 , 8 AM - 8 PM Friday

D.)  UKM -->>  0.32" , 8 AM - 8 PM Friday

E.)  WRF -->>  ~~ 0.50", approx. 8 AM - 8 PM Friday

Other non-decorated, non-celebrated showers will occur prior to 8 AM Friday morning, and this is already light rain showers appearing on radar at 145 AM as I write this.

V.)  Conclusion

While I don't see this shower & thunderstorm event being as grand as the previous 2 events, in the preceding 1-2 weeks prior, there is enough instability & speed shear present on Friday which should sustain several waves / clusters of showers and a few thunderstorms, throughout the day, with a few storms having the eligbility to reach severe levels, (high winds/hail), as the approaching upper short-wave trough sharpens & approaches the Mid-Atlantic states by early this evening.  

-- cyclogenesis

     

[yoda]

SPC DAY 1 OUTLOOK has 5% hail, 15% wind, and 5% tornado for DC and Baltimore metro regions as well as all of the Eastern Shore and Delaware into SE PA and S NJ

[yoda]

Darrow/Gleason were the writers of the 0600 Day 1 OTLK

@C.A.P.E. @high risk @Kmlwx

Quote

Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1249 AM CDT Fri Apr 26 2019

   Valid 261200Z - 271200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM NC TO THE
   MIDDLE ATLANTIC...

   ...SUMMARY...
   Thunderstorms capable of producing damaging wind and a few tornadoes
   are expected from North Carolina into the Middle Atlantic Friday.
   Isolated damaging wind gusts are also possible over northern Utah,
   southeast Idaho, southwest Wyoming and northwest Colorado. Other
   storms may produce a few strong wind gusts over a portion of the
   Florida Peninsula.

   ...NC to the Middle Atlantic...

   Strong 500mb jet is forecast to dig southeast across MO into the TN
   Valley region early in the period before shifting into the lee of
   the southern Appalachians Friday afternoon. In response, strong 12hr
   500mb height falls will develop across the Middle Atlantic region
   and a pronounced surface front will surge east of the higher terrain
   shortly after 18z. While low-level moisture is not particularly high
   across this area this morning, lower 60s surface dew points are
   expected to advance north across the Delmarva into southeast PA
   ahead of the wind shift. Latest model guidance suggests
   surface-based buoyancy will develop by 16-17z as temperatures warm
   through the upper 60s to near 70F. Additionally, strongly diffluent
   flow aloft suggests favorable venting for convection within
   aforementioned region of large-scale forcing for ascent. 

   Current thinking is scattered thunderstorms will develop ahead of
   the approaching short wave during the morning then convection should
   intensify as forcing encourages upscale growth. Strongly sheared
   profiles favor rotating storms and there is some concern for a few
   tornadic supercells, especially across the Delmarva into southeast
   PA where low-level flow will be more backed. Latest CAMs support
   this scenario and severe probs will be nudged north a bit to reflect
   this threat farther north across the Middle Atlantic.

[CAPE]

5 hours ago, yoda said:

Darrow/Gleason were the writers of the 0600 Day 1 OTLK

@C.A.P.E. @high risk @Kmlwx

Interesting. I have not paid much attention to this admittedly, with a lot of other stuff going on. Just read the AM AFD from Mount Holly...

A surface low in the Midwest will continue northeastward today in advance of a southern-stream vort max that is beginning to phase with a digging northern-stream shortwave trough. Amplification and negative tilting will continue through the morning as the large- scale trough approaches the Appalachians by midday. The Mid-Atlantic will be at the nose of a potent midlevel jet streak/vort max this afternoon, with strongly difluent flow in the upper levels favorable for substantial large-scale ascent along and in advance of a surging cold front. A warm front will lift north through the northern Mid- Atlantic today, with the warm sector likely making it through about the southern two-thirds of the CWA or so by early afternoon. Latest convection-allowing model guidance is suggestive of an environment favorable for organized severe storms this afternoon. Forecast soundings indicate backing surface flow as the large-scale system and associated surface low to our northwest deepens within the warm sector this afternoon. Hodographs are strongly indicative of potential for rotating storms. Low-level storm-relative helicity of 200-300 J/kg is not out of the question in advance of the cold front late this afternoon and this evening. In addition, several of the hi-res models develop decent mixed-layer based instability (with mitigating effects near the colder waters, as usual). The resulting CAPE/shear parameter space is adequate for tornadic supercells, though the strong large-scale ascent may favor more of a quasi- linear convective system (QLCS) with embedded transient/rotating storms.

[Kmlwx]

Sounds like Sterling may do a 16z balloon launch per the 13z SPC outlook text. 

[JakkelWx]

Loud thunder for the past 5 minutes and heavy rain. Those lapse rates are pretty steep already.

[CAPE]

3 minutes ago, JakkelWx said:

Loud thunder for the past 5 minutes and heavy rain. Those lapse rates are pretty steep already.

 

I am in Easton but saw that cell on radar. Looked like it went right over my yard. Stuff is popping already.

The hodos on some of the meso models show some decent veering and curvature. Later today should be interesting.

[JakkelWx]

19 minutes ago, C.A.P.E. said:

I am in Easton but saw that cell on radar. Looked like it went right over my yard. Stuff is popping already.

The hodos on some of the meso models show some decent veering and curvature. Later today should be interesting.

 I got nice views after it passed to my north-east. Looks like a couple more cells forming south of your location near Cambridge.

[CAPE]

Just now, JakkelWx said:

 I got nice views after it passed to my north-east. Looks like a couple more cells forming south of your location near Cambridge.

Lots of thunder here. I am west of Easton and its staying mostly just to the east of here.

[yoda]

Updated Morning AFD from LWX for this afternoon's severe risk:

Quote

.NEAR TERM /UNTIL 7 PM THIS EVENING/...
An upper level trough east of the MS River this morning will
deepen and take on a negative tilt as it closes off over the
eastern Great Lks Sat morning. Very impressive 12-hr 500 mb
height falls (120-150 meters) are fcst over the area today as
trough axis crosses the area during the next 24 hrs.

Ongoing elevated convection continues across portions of eastern
VA into southern and eastern MD this morning, but should stay
below severe limits. In addition, showers and isolated elevated
convection is also still moving through portions of eastern WV
and western/central MD. The main catalyst for today`s strong to
severe thunderstorms can be seen just east of Charleston WV this
morning in the form of a forced convective line. A gust of 44
mph was observed at KCRW as it passed. This will progress
eastward and towards the Allegheny Front by 12 noon, likely
reaching the metro areas between 2 and 4 PM, and then exiting
east of the Chesapeake Bay between 5 and 6 PM. Some additional
individual cells may also develop ahead of the line.

Very little surface or mixed layer instability currently exists
across the region but this is progged to change over the next
several hours as breaks in the cloud shield develop and
temperatures rise through the 60s into the low 70s. Latest
HREF/RAP forecasts indicate near 1000 J/KG of SBCAPE likely to
develop east of the Blue Ridge mountains this afternoon, and
this will be enough to both maintain/increase the intensity of
the current convective line which is currently in an atmosphere
characterized by 500 J/KG of SBCAPE. Effective shear values of
40-50 knots are also more than enough to support a convective
wind threat. Low level turning in the forecast wind profiles and
HREF/RAP forecast STP/SCP/helicity fields indicate that an
isolated tornado is also possible, either in any discrete
activity ahead of the line or in the form of a QLCS tornado.

Activity will become fast moving, so flood threat is relatively
low, however any training convection could lead to an isolated
incident.

[yoda]

seeing some breaks in the clouds here with some filtered sunshine... looks like maybe an hour or 2 of sun?  Everything is moving in a NE direction

[Kmlwx]

Empire State Building sized hail right now in Potomac. 

[yoda]

Sat loop in motion for today that i posted at the bottom of the last page can be gotten from here -- http://rammb.cira.colostate.edu/ramsdis/online/goes-16.asp

[Kmlwx]

Temps vaulting to 70+ in areas under the sunshine. 

[H2O]

Sun really starting to come out now.  We will see if this jumps up storm chances

[Chase]

MD says Enhanced is coming... and a watch

[Kmlwx]

https://www.spc.noaa.gov/products/md/md0451.html

[Kmlwx]

That frontal analysis on the meso discussion map indicates warmfront nearby. That will definitely enhance tornado risk. 

[yoda]

2 minutes ago, Kmlwx said:

https://www.spc.noaa.gov/products/md/md0451.html

I like this part that I bolded... wonder if it will be 30 wind or 10 tor... probably 30 wind for ENH

Mesoscale Discussion 0451
   NWS Storm Prediction Center Norman OK
   1037 AM CDT Fri Apr 26 2019

   Areas affected...from northern North Carolina into southeast
   Pennsylvania and parts of New Jersey

   Concerning...Severe potential...Watch likely 

   Valid 261537Z - 261730Z

   Probability of Watch Issuance...80 percent

   SUMMARY...Conditions will become increasingly favorable for severe
   storms today, with the greatest threat across eastern Virginia,
   southeast Pennsylvania, and parts of New Jersey and Delaware.
   Damaging winds appear likely, with a tornado or two possible
   especially northern areas. The 1630Z outlook will reflect an upgrade
   to ENH for parts of the MCD area, and one or more watches are
   expected over the next few hours.

   DISCUSSION...A cold front is currently surging eastward across
   western PA and VA, with a warm front from far northern VA across MD
   and DE. Satellite imagery indicates clearing, and strong heating is
   occurring across the warm sector with CIN being rapidly eroded. In
   addition, widespread pressure falls exceeding 1.5 mb/hr were noted.

   Storms are already forming near the moist axis, from northeast NC
   across eastern VA and DE where dewpoints were solidly in the 60s F.
   These storms may produce locally damaging winds as they move quickly
   northeastward with favorable low-level lapse rates and deep-layer
   shear.

   Later today, storms along the cold front will increase in intensity
   as it encounters the heating air mass. A line of storms will result,
   possibly a broken line of cells or QLCS, with damaging winds likely.
   The strongest low-level shear will remain near the warm front, which
   will lift northward into PA and NJ. Here, forecast soundings show
   larger looping hodographs which may favor tornadoes, either with
   supercells just ahead of or embedded within the line. In addition,
   the relatively cool temperatures aloft near 700 mb will maximize
   low-level instability which will also aid rotation in storms.

   ..Jewell/Hart.. 04/26/2019

   ...Please see www.spc.noaa.gov for graphic product...

   ATTN...WFO...PHI...AKQ...CTP...LWX...RAH...

[yoda]

Sat suggests a few hours of sunshine looks like through early this afternoon

[frd]

2 minutes ago, yoda said:

I like this part that I bolded... wonder if it will be 30 wind or 10 tor... probably 30 wind for ENH

That last enhanced risk for my area was like a mini derecho. Where as that line hit at 2:45 AM this looks like maybe 6 PM here. I am hoping for no downed trees, finally got the yard looking nice. 

Will be an interesting late afternoon.    

Also, just like the last time, the winds will be stong tomorrow around the area, however, doubt we get the high wind advisory, but you never know. The last advisory was posted rather late.  Has been an active April for sure .   

[MN Transplant]

Jumped almost 10 degrees here in the last 90 minutes and we still don't have much sun.  72.2

[Eskimo Joe]

Warm front looks pretty potent...leaning 60/40 on tornado watch especially if we manage more sun.