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October Discobs 2019


George BM
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This is a pretty classic high shear, low cape severe event. Anything that develops later this afternoon (should be around 0z in the Blue Ridge, 2z near I-95) should quickly congeal into QLCS. The biggest concern is definitely the straight-line wind threat, although a brief spin up is not out of the question, with very strong speed and directional shear near the surface and a cranking LLJ. Question is, can we overcome that mid-level capping inversion and mix down to the surface as shown in the 0z DCA sounding? 

dca sounding.png

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 Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1112 AM CDT Thu Oct 31 2019

   Valid 311630Z - 011200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS PARTS OF
   NC...VA...WV...MD...DC...PA...

   ...SUMMARY...
   An extensive squall line of strong to severe thunderstorms is
   expected to develop during the late afternoon into this evening,
   near and east of the Appalachians from parts of New York to the
   Carolinas. At least a few tornadoes and scattered to widespread
   damaging winds are possible.

   ...Northeast to the Carolinas...
   Have upgraded peak probabilities for both wind and tornado hazards
   with an active period of severe weather anticipated later this
   afternoon into the evening.

   An amplifying shortwave trough now over IL will accelerate
   east-northeast toward the Saint Lawrence Valley through tonight
   toward PA/NY, while taking on a more neutral-to-negative tilt with
   peak 500 mb wind speeds exceeding 120 kt. Strong surface
   cyclogenesis is expected in advance of the amplifying trough, with a
   cyclone moving from OH to far southern QC by early Friday. South of
   the deepening cyclone, a strong cold front will sweep eastward
   across the Appalachians this afternoon, reaching the Atlantic coast
   overnight. The cold front will be the primary focus for severe
   thunderstorms through this evening.

   Northward advection of low-level moisture will continue with mid 60s
   boundary-layer dew points up the OH Valley west of the Appalachians
   and across southeast PA to coastal southern New England. Poor low to
   mid-level lapse rates amid widespread clouds will hinder
   destabilization with northern extent in the Northeast. Richer
   low-level moisture (upper 60s to low 70s boundary-layer dew points)
   and some surface heating in cloud breaks will support a confined
   plume of MLCAPE between 750-1500 J/kg across parts of VA into the
   Carolinas at peak heating. 

   Within the warm sector, vertical shear will increase in response to
   the approach and deepening of the mid-level trough/surface cyclone.
   Long hodographs with effective bulk shear of 50-60 kt are expected,
   and there will also be enlarged low-level hodograph curvature for
   numerous rotating storms amid effective SRH of 300-500 m2/s2.

   Strong linear forcing for ascent along the cold front will be the
   primary focus for storm development, mainly east of the Appalachians
   later this afternoon/evening. The strong linear ascent will favor
   upscale growth into a squall line, though strong deep-layer shear
   and enough east component to storm motions should allow embedded
   supercell structures to persist. The modest buoyancy and very strong
   850-700 mb flow will favor damaging winds as the primary threat.
   These could be potentially widespread within longer-tracked bowing
   segments. At least a few tornadoes are also anticipated with
   embedded supercell and mesovortex structures, particularly from
   central NC to central MD. 

   The severe threat will diminish during the late evening to overnight
   given an increasing dearth of surface-based instability with
   east/northeast extent.

   ..Grams/Wendt.. 10/31/2019
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URGENT - IMMEDIATE BROADCAST REQUESTED
   Tornado Watch Number 685
   NWS Storm Prediction Center Norman OK
   1245 PM EDT Thu Oct 31 2019

   The NWS Storm Prediction Center has issued a

   * Tornado Watch for portions of 
     Far east Kentucky
     Far northwest North Carolina
     Far northeast Tennessee
     Southwest Virginia
     West Virginia

   * Effective this Thursday afternoon and evening from 1245 PM
     until 700 PM EDT.

   * Primary threats include...
     A couple tornadoes possible
     Scattered damaging winds likely with isolated significant gusts
       to 75 mph possible

   SUMMARY...A thin line of low-topped convection should spread rapidly
   northeast this afternoon. Damaging winds from strong to locally
   severe wind gusts will be the primary hazard. Some intensification
   to the line is anticipated towards late afternoon, which should
   support a risk for a couple tornadoes.

   The tornado watch area is approximately along and 60 statute miles
   east and west of a line from 20 miles southeast of Bristol TN to 50
   miles northwest of Elkins WV. For a complete depiction of the watch
   see the associated watch outline update (WOUS64 KWNS WOU5).

   PRECAUTIONARY/PREPAREDNESS ACTIONS...

   REMEMBER...A Tornado Watch means conditions are favorable for
   tornadoes and severe thunderstorms in and close to the watch
   area. Persons in these areas should be on the lookout for
   threatening weather conditions and listen for later statements
   and possible warnings.

   &&

   AVIATION...Tornadoes and a few severe thunderstorms with hail
   surface and aloft to 0.5 inches. Extreme turbulence and surface wind
   gusts to 65 knots. A few cumulonimbi with maximum tops to 350. Mean
   storm motion vector 21045.

ww0685_overview_wou.gif

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Satellite shows some pockets of clearing ahead of the line associated with the tornado watch. Question will be whether that becomes clouded over more with stuff popping ahead of the line...or will the clearing sustain and move over our area later on. In my mind, any clearing is just bonus instability. 

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5 minutes ago, Eskimo Joe said:

Expected TOR too... and they mention potential discrete sups as well

mcd2129.gif.0dc361d5091674701d2a7d9d2e7d9e37.gif

 

Mesoscale Discussion 2129
   NWS Storm Prediction Center Norman OK
   1211 PM CDT Thu Oct 31 2019

   Areas affected...Parts of northern North Carolina...much of
   Virginia...western and central Maryland...south central Pennsylvania

   Concerning...Severe potential...Watch likely 

   Valid 311711Z - 311945Z

   Probability of Watch Issuance...95 percent

   SUMMARY...One or more severe weather watches will likely be needed
   east of the Allegheny and Blue Ridge Mountains by 4-5 PM EDT. 
   However, isolated severe storm development, including a risk for
   supercells, could require an earlier watch (likely tornado)
   issuance.

   DISCUSSION...Low-level and deep-layer shear are already strong to
   extreme across and east of the Allegheny and Blue Ridge Mountains,
   within deepening lee surface troughing, to the southeast of the
   intensifying surface cyclone now approaching Lake Erie.  Despite
   weak mid/upper lapse rates, the northward return of seasonably moist
   boundary layer air (including surface dew points near 70F), coupled
   with boundary layer warming, is also contributing to modest
   destabilization across this region.  Mixed-layer CAPE probably will
   continue to increase through the 500-1000+ J/kg range into mid to
   late afternoon, perhaps as far north as south central Pennsylvania.

   The primary increase in severe weather potential to the east of the
   Allegheny Mountains still appears likely to occur in association
   with an intensifying squall line near/just ahead of an approaching
   strengthening cold front, mainly during and after the 21-23Z time
   frame.  However, it might not be out of the question that the
   ongoing destabilization, weakening inhibition, and perhaps forcing
   for ascent associated with weak warm advection, may become
   sufficient to support isolated to widely scattered discrete
   convective development in advance of the front.  If this occurs, the
   environment will probably be conducive to supercells, which may be
   accompanied by the risk for tornadoes, in addition to localized
   strong surface gusts.

   ..Kerr/Grams.. 10/31/2019

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

   ATTN...WFO...AKQ...CTP...LWX...RAH...RNK...GSP...
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3 minutes ago, yoda said:

Expected TOR too... and they mention potential discrete sups as well

 

       The HRRR does initiate a few supercells out ahead of the main forcing,  so I understand SPC's thinking.   But they form in western VA and move north-northeast into PA, consistent with where the box will be issued.     Still think that the chances of discrete supercells in the DC metro area is really low.

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Special Weather Statement
National Weather Service Baltimore MD/Washington DC
1240 PM EDT Thu Oct 31 2019

ANZ530>543-DCZ001-MDZ003>006-011-013-014-016>018-501>508-
VAZ025>031-036>040-050>057-501>508-WVZ050>053-055-501>506-010000-
Chesapeake Bay north of Pooles Island MD-
Chesapeake Bay from Pooles Island to Sandy Point MD-
Chesapeake Bay from Sandy Point to North Beach MD-
Chesapeake Bay from North Beach to Drum Point MD-
Chesapeake Bay from Drum Point MD to Smith Point VA-
Tidal Potomac from Key Bridge to Indian Head MD-
Tidal Potomac from Indian Head to Cobb Island MD-
Tidal Potomac from Cobb Island MD to Smith Point VA-
Patapsco River including Baltimore Harbor-
Chester River to Queenstown MD-Eastern Bay-
Choptank River to Cambridge MD and the Little Choptank River-
Patuxent River to Broomes Island MD-
Tangier Sound and the inland waters surrounding Bloodsworth
Island-District of Columbia-Washington-Frederick MD-Carroll-
Northern Baltimore-Southern Baltimore-Prince Georges-Anne Arundel-
Charles-St. Marys-Calvert-Extreme Western Allegany-
Central and Eastern Allegany-Northwest Montgomery-
Central and Southeast Montgomery-Northwest Howard-
Central and Southeast Howard-Northwest Harford-Southeast Harford-
Augusta-Rockingham-Shenandoah-Frederick VA-Page-Warren-Clarke-
Nelson-Albemarle-Greene-Madison-Rappahannock-Orange-Culpeper-
Prince William/Manassas/Manassas Park-Fairfax-
Arlington/Falls Church/Alexandria-Stafford-Spotsylvania-
King George-Northern Fauquier-Southern Fauquier-Western Highland-
Eastern Highland-Western Loudoun-Eastern Loudoun-
Northern Virginia Blue Ridge-Central Virginia Blue Ridge-
Hampshire-Morgan-Berkeley-Jefferson-Hardy-Western Grant-
Eastern Grant-Western Mineral-Eastern Mineral-Western Pendleton-
Eastern Pendleton-
Including the cities of Washington, Hagerstown, Frederick,
Ballenger Creek, Eldersburg, Westminster, Reisterstown,
Cockeysville, Baltimore, Bowie, Suitland-Silver Hill, Clinton,
College Park, Greenbelt, Laurel, Camp Springs, Glen Burnie,
Annapolis, Severn, South Gate, Severna Park, Arnold, Odenton,
St. Charles, Waldorf, Lexington Park, California,
Chesapeake Beach, Huntingtown, Dunkirk, North Beach, Lusby,
Prince Frederick, Frostburg, Cumberland, Germantown, Damascus,
Bethesda, Rockville, Gaithersburg, Silver Spring, Lisbon,
Columbia, Ellicott City, Jarrettsville, Aberdeen, Staunton,
Waynesboro, Stuarts Draft, Harrisonburg, Strasburg, Woodstock,
Mount Jackson, New Market, Winchester, Luray, Shenandoah,
Stanley, Front Royal, Berryville, Lovingston, Charlottesville,
Stanardsville, Madison, Orange, Gordonsville, Culpeper,
Dale City, Manassas, Woodbridge, Lake Ridge, Montclair, Reston,
Herndon, Annandale, Centreville, Chantilly, McLean, Franconia,
Arlington, Alexandria, Falls Church, Falmouth, Fredericksburg,
Dahlgren, Warrenton, Turnbull, Hightown, Monterey, Purcellville,
Leesburg, Ashburn, Sterling, Big Meadows, Wintergreen, Romney,
Paw Paw, Martinsburg, Charles Town, Shepherdstown, Moorefield,
Bayard, Mount Storm, Petersburg, Elk Garden, Antioch, Keyser,
New Creek, Ridgeville, Russelldale, Headsville, Fort Ashby,
Riverton, Brandywine, Franklin, Oak Flat, Ruddle, and Sugar Grove
1240 PM EDT Thu Oct 31 2019

...DAMAGING LINE OF THUNDERSTORMS LIKELY TO MOVE EAST ACROSS THE
AREA BETWEEN MID AFTERNOON AND MIDNIGHT...

A line of thunderstorms is expected to move east across the
region. These storms will bring the possibility of damaging wind
gusts and isolated tornadoes. Please be sheltered when these
thunderstorms pass through your area.

The most likely timing of the line ranges from mid to late
afternoon for the Potomac Highlands, to the evening for the
Baltimore/Washington metros, to between 9pm and midnight for
areas east of I-95 to the Chesapeake Bay. The storms will likely
last an hour or less in most areas.

Stay informed. For more exact timing for your location, consult
the forecast for your community at our website weather.gov or our
Facebook or Twitter feeds for National Weather Service
Baltimore/Washington.

$$
STRONG
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17 minutes ago, Kmlwx said:

Satellite shows some pockets of clearing ahead of the line associated with the tornado watch. Question will be whether that becomes clouded over more with stuff popping ahead of the line...or will the clearing sustain and move over our area later on. In my mind, any clearing is just bonus instability. 

There were some peaks of sun in Western Albemarle right after the line of showers but quickly clouded over again. 

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