2019 Mid-Atlantic Severe Wx - General Discussion

[yoda]

Afternoon AFD from LWX about tomorrow into tomorrow night:

SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/...
Potent storm system will affect the region for week`s end and
the start of the weekend. Early Friday, a very sharp upper
trough will be over the Mississippi Valley, with a surface low
over the Ohio Valley. The warm front will be up in New York
while a cold front will extend south across the southern
Appalachians. Through the day Friday and night Friday night, the
sharp trough will cut-off into a closed low to our west, futher
increasing difluence aloft as it moves towards us. The surface
low will continue to strengthen as well as it moves towards
western Virginia and eastern West Virginia. Very strong low
level jet ahead of the cold front will bring very high moisture
(PW`s near 2 inches) into the region, and limited insolation
should provide some CAPE, perhaps as much as 1000-1500 J/kg.
Shear will be very high, especially low-level shear, thanks to
the powerful low level jet reaching 60-70 knots at 850 mb. Thus,
once we destabilize during late morning, showers and
thunderstorms are expected to blossom across the region ahead of
the main cold front. There may be a lull later in the day before
another round of storms associated more closely with the cold
front itself crosses the region during the night. Timing is
still a little uncertain, but confidence has increased overall.
With such a strong windfield aloft, the main threat is damaging
winds, which could easily be mixed down. However, the low level
shear and helicity is more than strong enough to produce at
least an isolated tornado threat, limited more by the CAPE than
anything else. Hail threat seems minimal given the warm
atmosphere and relatively weak CAPE in the hail growth zone, but
some cannot be ruled out. Even outside of storms, strong
southerly flow will bring gusts of 30, perhaps even 40 mph,
especially near the open waters of the bay on Friday and Friday
night.

Highs on Friday will be limited by the clouds and storms, but
70s look likely east of the mountains. Temperatures will fall
after the primary front crosses the region late Friday night,
with 50s east of the mountains and 40s in the mountains.

[yoda]

2 hours ago, Eskimo Joe said:

The flood potential really went in the toilet over the past couple of runs too.  Pretty sad overall.

Guess LWX thinks otherwise 

Flood Watch
National Weather Service Baltimore MD/Washington DC
207 PM EDT Thu Apr 18 2019


DCZ001-MDZ003>006-011-013-014-016-503>508-VAZ031-052>055-501-502-
505-506-WVZ052-053-190215-
/O.NEW.KLWX.FF.A.0001.190419T1600Z-190420T0900Z/
/00000.0.ER.000000T0000Z.000000T0000Z.000000T0000Z.OO/
District of Columbia-Washington-Frederick MD-Carroll-
Northern Baltimore-Southern Baltimore-Prince Georges-Anne Arundel-
Charles-Northwest Montgomery-Central and Southeast Montgomery-
Northwest Howard-Central and Southeast Howard-Northwest Harford-
Southeast Harford-Clarke-Prince William/Manassas/Manassas Park-
Fairfax-Arlington/Falls Church/Alexandria-Stafford-
Northern Fauquier-Southern Fauquier-Western Loudoun-
Eastern Loudoun-Berkeley-Jefferson-
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, Germantown, Damascus, Bethesda, Rockville,
Gaithersburg, Silver Spring, Lisbon, Columbia, Ellicott City,
Jarrettsville, Aberdeen, Berryville, Dale City, Manassas,
Woodbridge, Lake Ridge, Montclair, Reston, Herndon, Annandale,
Centreville, Chantilly, McLean, Franconia, Arlington, Alexandria,
Falls Church, Falmouth, Warrenton, Turnbull, Purcellville,
Leesburg, Ashburn, Sterling, Martinsburg, Charles Town,
and Shepherdstown
207 PM EDT Thu Apr 18 2019

...FLASH FLOOD WATCH IN EFFECT FROM FRIDAY AFTERNOON THROUGH LATE
FRIDAY NIGHT...

The National Weather Service in Sterling Virginia has issued a

* Flash Flood Watch for portions of Maryland, The District of
  Columbia, Virginia, and panhandle West Virginia, including the
  following areas, in the District of Columbia, Washington DC. In
  Maryland, Anne Arundel, Carroll, Howard, Montgomery, Charles,
  Frederick, Baltimore, Harford, Prince Georges and Washington
  counties. In Virginia, Arlington, Clarke, Loudoun, Fairfax,
  Fauquier, Prince William and Stafford counties and adjacent
  independent cities. In eastern West Virginia, Berkeley and
  Jefferson counties.

* From Friday afternoon through late Friday night

* Showers and thunderstorms with heavy rain are expected to
  overspread the region by Friday afternoon and continue through
  Friday evening before departing late Friday night. Training
  storms and multiple rounds of heavy rain may result in rainfall
  amounts which could exceed 3 inches locally. Flash flooding is
  possible, particularly in the urban areas.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

A Flash Flood Watch means that conditions may develop that lead
to flash flooding. Flash flooding is a very dangerous situation.

You should monitor later forecasts and be prepared to take action
should Flash Flood Warnings be issued.

[Kmlwx]

Yoda - LWX does have grounds to issue the *watch* but the amounts certainly have been cut on some of the recent guidance. Also the strip of maximum is going to be REALLY narrow as shown on a lot of guidance. Some places might get 0.25-0.5 and a few isolated spots in the jackpot corridors may get 3. But this is not going to be a widespread 2-4 inch flood event. The watch is probably because flooding WILL occur but it's not going to be prolific area-wide flooding like EJ wants

So the watch is warranted - but it's not likely to be a major river flooding event or anything of that nature. 

[yoda]

Both 18z NAM and 18z NAM NEST look pretty good for severe in our region tomorrow afternoon into tomorrow night 

[CAPE]

This looks pretty meh for our area in general. Less impressive overall on recent model runs. Flooding threat looks extremely localized at best, and the same can be said for any severe potential. Mostly garden variety stuff, with some very isolated low-end severe. Different story for south-central VA and especially down into the Carolinas. Potential high impact for more in the way of excessive rains with some localized moderate wind damage.

[yoda]

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

This looks pretty meh for our area in general. Less impressive overall on recent model runs. Flooding threat looks extremely localized at best, and the same can be said for any severe potential. Mostly garden variety stuff, with some very isolated low-end severe. Different story for south-central VA and especially down into the Carolinas. Potential high impact for more in the way of excessive rains with some localized moderate wind damage.

Huh?  SPC moved the SLGT and ENH north a good amount... the disco from LWX sounds good as listed above... 18z NAM has a nice QLCS or squall line coming through the region... 18z NAM NEST has some sups around...

[CAPE]

4 minutes ago, yoda said:

Huh?  SPC moved the SLGT and ENH north a good amount... the disco from LWX sounds good as listed above... 18z NAM has a nice QLCS or squall line coming through the region... 18z NAM NEST has some sups around...

We shall see. Mount Holly seems less impressed. I see no indication of widespread 2" amounts on any model, but PWATs are impressive so certainly could be some localized flash flooding.

I don't see what you see with the SPC maps at all. A bit of a tick north with the Slight area, yeah. My area is still right on the line between marginal and slight. Literally no change. Enhanced risk area is still well south.

[Rtd208]

Just now, C.A.P.E. said:

We shall see. Mount Holly seems less impressed. I see no indication of widespread 2" amounts on any model, but PWATs are impressive so certainly could be some localized flash flooding.

I don't see what you see with the SPC maps at all. A bit of a tick north with the Slight area, yeah. My area is still right on the line between marginal and slight. Literally no change. Enhanced risk area is still well south.

Less impressed? And yet they issued a Flash Flood Watch for my area from late Friday night thru mid day Saturday for 1-2" of rain with locally higher amounts up to 3". Doesn't sound less impressed to me.

[CAPE]

Just now, Rtd208 said:

Less impressed? And yet they issued a Flash Flood Watch for my area from late Friday night thru mid day Saturday for 1-2" of rain with locally higher amounts up to 3". Doesn't sound less impressed to me.

You might consider reading their discussion. Then compare it to LWX. Then, go back and read Yoda's post and mine again, except this time, read for context. Maybe you will have better comprehension the second time.

Hope you get your epic flood though! 

 

[Eskimo Joe]

1 hour ago, yoda said:

Guess LWX thinks otherwise 

While the watch is justified, this is just going to be a lucky spot or two that jackpots.  It was looking much better for wider rainfall a few days ago. LWX even mentions this in the afternoon discussion about how the system has trended a bit more progressive in the past 24 hours in the guidance.

[frd]

1 hour ago, C.A.P.E. said:

You might consider reading their discussion. Then compare it to LWX. Then, go back and read Yoda's post and mine again, except this time, read for context. Maybe you will have better comprehension the second time.

Hope you get your epic flood though! 

 

You are correct,  there was a rather dramatic decrease in areal QPF in the latest WPC discussion and forecast. Not often do you see such a  reduction close to real time from them.  

What was consistent for over 8 cycles, 4 days,  has shifted the heaviest rainfall well South of us , coinciding with the higher severe risk. 

I am still expecting a decent rainfall event but not epic. 

 

 

 

 

[CAPE]

Rtd208 is a bit giggly tonight.

[BaltimoreWxGuy]

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

We shall see. Mount Holly seems less impressed. I see no indication of widespread 2" amounts on any model, but PWATs are impressive so certainly could be some localized flash flooding.

I don't see what you see with the SPC maps at all. A bit of a tick north with the Slight area, yeah. My area is still right on the line between marginal and slight. Literally no change. Enhanced risk area is still well south.

The 6z GFS was bonkers on QPF..2-3” area wide then 12z backed off. We’ll see, sometimes this close in, an older model run gets it more correct than one right before it happens. 

[frd]

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

Rtd208 is a bit giggly tonight.

To some the weekend has started early. Many folks I know are off tomorrow. 

[Kmlwx]

A lot of the severe parameters seem to have a cutoff of the Potomac (roughly) on a lot of the runs. 

[yoda]

37 minutes ago, Kmlwx said:

A lot of the severe parameters seem to have a cutoff of the Potomac (roughly) on a lot of the runs. 

Which isn't what either the 18z NAM or 18z NAM NEST show... or the 00z HRRR

[yoda]

00z HRRR crushes the region between 23z and 01z with the main line and has  a nice UD Helicity swath right over DC

[Kmlwx]

[southmdwatcher]

That was from 15Z run of SREF, this is 21Z

[yoda]

40 minutes ago, southmdwatcher said:

That was from 15Z run of SREF, this is 21Z

Haven't seen numbers that high around here in quite a while

[southmdwatcher]

3 minutes ago, yoda said:

Haven't seen numbers that high around here in quite a while

We shall see what SPC and Sterling have to say over the next few hours. The lapse rates over us now are quite strong. That will move out as we head towards morning. The sun will definitely be out for some period of time tomorrow, just a matter of how long and whether a new batch of prefrontal storms breaks out and squashes any substantial buildup of instability. The timing of the event across AL/MS is slightly quicker than was expected. That throws wrinkles into our forecast tomorrow.

[high risk]

NAM nest seems like an outlier with only a mid-evening show.   00Z HRRR and current and earlier hi-res windows all support a late afternoon round 1 and then an evening round 2.    There is still some healthy disagreement on whether the low-level shear is sufficiently "good" by late afternoon or if the better shear sets up after dark, but I would certainly expect a 5% tornado outlook in the early morning Day 1.

[cyclogenesis]

April 19, 2019

this Pre-Dawn Friday morning

115 AM EDT

 

* * *  Still a good chance for severe storms & HEAVY RAINFALL for the Richmond, VA area, Friday afternoon & Friday night, 4/19  * * *

* * *  Tornado watches likely to be issued by early Friday afternoon, for Richmond & Eastern Virginia * * *

* * *  Predominant threat still looks to be spots of wind damage & heavy rainfall, ponding of water * * * 

* * *  3 distinct rounds of storms coming  * * * 

 

I.  Timing for Richmond, VA.

 

I'm seeing 3 distinct time periods for showers & storms for Richmond, VA on Friday.  The first wave comes betweeen 3 PM and 4 PM.  The 2nd more serious severe round of weather comes in the evening hours between 5 PM and 7 PM, and then after 8 PM, comes the 3rd round which I think will be the heavy rain period spanning from 7 PM onward to 11 PM.

Viewing late this Thursday night, at timing progressions, appears this weather event will present itself starting as soon as early Friday afternoon, in the form of detached showers randomly racing Northward through the area.  Shower activity will first start out WEST of us and SOUTHWEST of us in the late morning hours.  Then, about the timing in the early afternoon, between 12 PM and 2 PM, I think the 1st tornado watch will be issued close for Richmond.  Storm warnings for detached multi-cellular activity should come between 1 PM and 5 PM.  Interestingly enough, WRF's animation shows NO pockets or LINES of 40+ or 50+ 10 meter surface wind gusts in its 1-hour grids, as I step through its 00Z, 4/19 output.  Non-important, low-ranking moisture advection showers will begin racing Northbound late Friday morning in our area, with SB-Cape increasing during the 11 AM to 2 PM time frame. 

II.  More details. 

Discrete & unattached multi-cellular showers & a few storms should develop across the area between 11 AM and 2 PM, well ahead of the parent line, since a strong morning capping inversion at 8 AM, shown at 12Z/8AM on Nam's 12 hr depiction btwn 800 mb - 750 mb withers away completely by 2 PM, with 0 CINH, (convective inhibition) remaining by 2 PM.   

It's appearing there'll be a revised 750 J/kg, to at most 1,000 J/kg of SB-Cape available for Friday afternoon's storms in the Richmond area. 

While severe weather parameters such as SWEAT, Cape, helicity, Total-totals have all de-escalated since I last wrote you, and numbers have come down a bit, it cannot be ignored that a stout 45kt-55 kt low-level 850 mb jet and 55-60 kt 500 mb jet will be overhead during peak afternoon hours, concurrent with rising Cape levels, although still a bit on the low side of 750 - 1000 J/kg.  While neither speed shear nor directional shear for long-lived supercellular storms are present, the overall wind momentum throughout the depth from boundary layer to mid-levels is impressive enough to be able to transport some of this high wind momentum downward in the heaviest rain shafts produced of the multi-cellular & linear squall line modes.  You generally see a greater occurrence of wind damage with a veered WESTERLY component, but in this case we're dealing with a SOUTHERLY direction, and a fetch of deeper moisture, which is why I believe the heavy rain & flooding threat is kept alive for Friday night across Virginia.  So, not the most idyllic directional consideration for production of severe winds, the speeds are still great enough to produce instances of wind damage.

Also, on account of the TIMING of day, during the afternoon hours, helicity is shown to run upwards to 300 m2/s2 by WRF's modeling scenario, while NAM shows a spiked pocket of 350 to 500 m2/s2 helicity at 5 PM very near the Richmond area.  WRF's maximized CAPE timing is shown to come about 6 PM to the RVA area, right along the SAME TIMING of an approaching squall line to Richmond in the LATE afternoon hours between 5 PM and 7 PM.  

This timing of late afternoon & early evening 5 PM to 7 PM looks to correlate to the highest amount of Cape nearest to Richmond, at a timing when a squall line should be traversing the scene, along with the helicity, so tornado threat & SEVERE WIND THREAT look to be MAXIMIZED, I believe, at this timing of late afternoon & early evening for Richmond. 

Beyond 7 PM, it's a matter of how much RAIN & STORMS LINGER stagnate behind the parent line, as this is where I see the heavy rainfall event unfolding from Friday evening in to Friday night, from 7 PM to 11 PM.  This is the timing for a few flood warnings to be issued.  Virginia Beach & the Hamptons looks to get their heaviest rain threat later in the night, 11 PM to 2 AM.  All of this mess scoots out the area by sunrise, Saturday.

 

-- cyclogenesis    

[yoda]

Day 1 MOD RISK issued for C VA into SC

 

[southmdwatcher]

6Z 3km NAM shows the strongest 850mb winds are running through VA later this afternoon. CAPE of 1000-1500 runs Fredericksburg and then southward. 

[George BM]

5 hours ago, yoda said:

Day 1 MOD RISK issued for C VA into SC

 

Surprised that we aren't at least in some tornado risk atm. I would think that changes later though.

[H2O]

woo storms friday!!

[Kmlwx]

Moderate for DC or I'm not interested

[Eskimo Joe]

Really hope we can manage some sun.  Satellite is not promising though.  

[high risk]

not to split hairs, but at least the 13z outlook gets the entire region into 2% tornado probabilities.     The initial outlook with <2% made no sense.      I was bullish on my tornado probs last night, but with some indication now that there will be an early afternoon wave of showers, this may hurt our better instability later in the day and lead to slightly elevated storms with less tornado potential (but certainly heavy rain potential).