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August Discobs 2020


George BM
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47 minutes ago, poolz1 said:

Was waiting for that....

Hopefully, you have something on the stove that goes well with torrential rain!

Did not last long. But watching that cell over Leesburg. It has got to be insane there. Have friends just south of Lovettsville just outside of the flash flood warning getting dumped on. 

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Pretty bad out in Leesburg.  Worst part is that this front seems to be with us, maybe into Saturday morning. Thought it was gonna clear out by Friday afternoon.  

Seen this before here where a mass settles over an area and the sponge gets squeezed dumping water over an area.  No wind just straight vertical rain.  

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Here we go again lol

Flood Watch
National Weather Service Baltimore MD/Washington DC
346 AM EDT Fri Aug 7 2020


DCZ001-MDZ004>006-011-013-014-016>018-503>508-VAZ025-026-029-
036>040-050>057-501-502-505>508-072215-
/O.NEW.KLWX.FF.A.0019.200807T1600Z-200808T0600Z/
/00000.0.ER.000000T0000Z.000000T0000Z.000000T0000Z.OO/
District of Columbia-Frederick MD-Carroll-Northern Baltimore-
Southern Baltimore-Prince Georges-Anne Arundel-Charles-St. Marys-
Calvert-Northwest Montgomery-Central and Southeast Montgomery-
Northwest Howard-Central and Southeast Howard-Northwest Harford-
Southeast Harford-Augusta-Rockingham-Page-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 Loudoun-
Eastern Loudoun-Northern Virginia Blue Ridge-
Central Virginia Blue Ridge-
Including the cities of Washington, 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, Germantown,
Damascus, Bethesda, Rockville, Gaithersburg, Silver Spring,
Lisbon, Columbia, Ellicott City, Jarrettsville, Aberdeen,
Staunton, Waynesboro, Stuarts Draft, Harrisonburg, Luray,
Shenandoah, Stanley, 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, Purcellville, Leesburg, Ashburn, Sterling, Big Meadows,
and Wintergreen
346 AM EDT Fri Aug 7 2020

...FLASH FLOOD WATCH IN EFFECT FROM NOON EDT TODAY THROUGH LATE
TONIGHT...

The National Weather Service in Sterling Virginia has issued a

* Flash Flood Watch for portions of Maryland, The District of
  Columbia, and Virginia, including the following areas, in
  Maryland, Anne Arundel, Calvert, Carroll, Central and
  Southeast Howard, Central and Southeast Montgomery, Charles,
  Frederick, Northern Baltimore, Northwest Harford, Northwest
  Howard, Northwest Montgomery, Prince Georges, Southeast
  Harford, Southern Baltimore, and St. Marys. The District of
  Columbia. In Virginia, Albemarle, Arlington/Falls
  Church/Alexandria, Augusta, Central Virginia Blue Ridge,
  Culpeper, Eastern Loudoun, Fairfax, Greene, King George,
  Madison, Nelson, Northern Fauquier, Northern Virginia Blue
  Ridge, Orange, Page, Prince William, Rappahannock, Rockingham,
  Southern Fauquier, Spotsylvania, Stafford, and Western
  Loudoun.

* From noon EDT today through late tonight

* Scattered slow moving showers and thunderstorms are expected
  this afternoon and tonight which may pose a risk of flash
  flooding.
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From Mt Holly AFD this morning-

It looks like two primary rounds of convection remaining. The first opportunity is for the remainder of the overnight through mid morning today. This comes thanks to another convectively enhanced vorticity impulse riding up along the stalled frontal boundary. Radar overnight has been very unimpressive so far, as it was yesterday afternoon and evening. Strongly suspect that the unexpectedly vigorous MCS on Thursday morning really did a number on the atmosphere, and that we still have not recovered.

Mesoanalysis indicates at least some modest amounts of elevated instability in place early this morning, and plentiful low level moisture with PWATs from 1.5 to 1.9 inches, highest to the south. Shear is weak, and storm motions are very slow from the southwest. So heavy rain remains the greatest concern through the morning. Quite a bit of uncertainty in how widespread this round of convection will be given concerns about the instability; so far it is struggling to get underway, but will wait awhile longer before cutting PoPs for the coming hours. Where storms do develop, flash flooding remains a possibility given the slow storm motions, possibility of training storms, and saturated ground. The second opportunity for convection comes this afternoon and evening, into the early overnight. This time, the trigger will be a combination of the approaching trough from the west as well as a little more energy streaming up from the southwest.

 

The bolded part is interesting, That was intense(and unexpected) here, and it expanded/strengthened as it moved towards coastal NJ.

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Excessive Rainfall discussion for the MA from WPC-

Activity on Thursday was widespread and quasi-organized on larger scales. On Friday we may see lesser coverage and/or lesser longevity of individual storm clusters, as the upper trough continues to fill, experiencing height rises at its base over the Ohio Valley and mid Atlantic today. Sufficient heating of the still tropical airmass, however, is likely to lead to scattered initiation along sea breezes, over the mountains, and near a quasi-stationary front and inverted surface trough sitting over DE/MD/PA. Hi-res model consensus places the greatest concentration of rainfall over Maryland and southern PA to southern NJ, thus near and north of the surface front where both mid level and jet level forcing should contribute to somewhat greater lift compared to areas farther south. Models generally agree, and especially the various HRRR runs, that a flash flood threat could develop early in the morning as the tail end of a jet entrance region interacts with the CAPE gradient north of the front in Pennsylvania. Coverage of storms will then increase elsewhere by midday, with HREF odds of inch per hour rates peaking after 22Z in MD/DC/VA. The environment and model signal certainly support local rain rates nearing 2 inches per hour. Much of this region is quite saturated, to say the least, in the wake recent widespread heavy rain events. Once again, a lack of sustained forcing and the expectation of lesser coverage today is what keeps this event in the Slight Risk category, but wherever multi-inch rainfall occurs in urban areas or other sensitive basins, an isolated significant flash flood is possible.

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