2019 Mid-Atlantic Severe Wx - General Discussion

[PDIII]

24 minutes ago, yoda said:

@Kmlwx

This afternoon's LWX AFD seems to think otherwise re instability... I guess ample would be like around 1000 J/KG:

The last GFS looked to be a fairly major rain/ flooding event.  Looks to be over 3 inches of rain in a twelve hour period.. obviously there could be significantly higher totals where training sets up. This stuff is on my radar these days seeing as though I live in the Patapsco River valley...

[Kmlwx]

Yoda - right now I'm more sold on the flooding threat. Models are in great agreement of 1-2 inches of rain - and the Euro drops even more than that in a strip running (thin) from Stafford up towards mappy-ville. 2-3 inches of rain is going to cause some problems if that verifies. GFS bullseyes Westminster area with over 3 inches. 

That being said - even sub-severe winds could then do some damage given the saturated ground. 

[Eskimo Joe]

Friday screams flooding to me.  Could be an interesting day.

[Kmlwx]

1 minute ago, Eskimo Joe said:

Friday screams flooding to me.  Could be an interesting day.

Flooding (even minor) is "interesting" around here (or anywhere) because there's always the idiots who decide they can make it through feet of water. If 1-3 inches verifies I'm sure there will be water rescue calls...probably some idiots with kids in the car too. 

"I have four wheel drive so I can make it through this washed out road"

"This appointment I'm driving to is very important...therefore the flooding won't stop me"

[Kmlwx]

The 18z NAM at range gets DC to like 78 degrees on Friday. That's quite a bit different than the GFS twins. 

[Eskimo Joe]

The track of the main low and dynamics would lead to a 'meh' to 'okay' setup.  We'd need to see a few hours of at least partly sunny skies to realize the better potential.

[Kmlwx]

Just now, Eskimo Joe said:

The track of the main low and dynamics would lead to a 'meh' to 'okay' setup.  We'd need to see a few hours of at least partly sunny skies to realize the better potential.

Going to be kind of funny if this past Sun night ends up being the better of the two and it was in the middle of the night. This one is during the afternoon/evening and might be the dud. Weather is fun(ny). 

[jewell2188]

1 hour ago, Kmlwx said:

The 18z NAM at range gets DC to like 78 degrees on Friday. That's quite a bit different than the GFS twins. 

Seems interesting bc you seem to be hugging the GFS 

[Kmlwx]

5 minutes ago, jewell2188 said:

Seems interesting bc you seem to be hugging the GFS 

I don't necessarily favor either one at this point. In my anecdotal experience - GFS does tend to downplay severe parameters around here at range. NAM maybe goes a bit too gung-ho as the event closes in but then comes back to reality as it hits. 

[purduewx80]

2 hours ago, Kmlwx said:

They also say "dewpoints well into the 60s" 

Deep layered southerly flow out of the tropics should easily produce mid-60s+ dew points, similar to Sunday PM. The ECMWF shows a peak of 69F at DCA Friday afternoon.

[CAPE]

Most recent take from Mount Holly-

For Friday and Saturday...A rather active time continues to look more likely as an upper-level trough closes off as it arrives into the East. The amplification of this system results in deep southerly flow increasing substantially ahead of the cold front Friday and Friday night, and this will result in ample warm air advection and ascent. Southerly surface winds could gust 30-40 mph Friday ahead of any showers/thunder as boundary layer warming will increase the vertical mixing. In addition, large scale ascent ahead of the closed low and strong short wave will team up with frontal forcing and potentially drive an embedded squall line eastward. The severe weather potential will depend on the amount of instability, however convection looks to arrive after peak heating on Friday. There looks to be plenty of shear in place given the robust wind fields, therefore if the parameters come together there is the potential for some severe thunderstorms with locally damaging winds especially across the southern areas. The guidance indicating a narrow but intense plume of moisture from the subtropics ahead of this closed low and with the flow increasingly more meridional, heavy rain and local flooding is possible.

[southmdwatcher]

Day 3 Slight with Enhanced off to the south.

[North Balti Zen]

If that area of excessive rain wants to bust, that would be fine by me.

[smokeybandit]

My back yard still thinks anything over 1/4" is excessive.

[Kmlwx]

12z NAM looks better for severe prospects. Temperatures nearing 80 in spots and good dews. 

[Kmlwx]

16 minutes ago, Kmlwx said:

12z NAM looks better for severe prospects. Temperatures nearing 80 in spots and good dews. 

Ditto for the 3km NAM as well. 

[Kmlwx]

Old GFS on the 12z run is less aggressive with warm temps than the NAM - consequently has much more meager looking instability and thus a much lower severe risk implied. 

[Eskimo Joe]

Interesting snippet from the afternoon SWODY3:

Quote

 ...Synopsis and Discussion...
   Deep upper trough extending along the MS Valley early Friday morning
   is expected to continue eastward, maturing into a well-formed
   mid-latitude cyclone centered over the southern Appalachians by the
   end of the period. Very strong flow aloft will exist throughout the
   base of the upper trough, with the strongest flow developing within
   its eastern periphery Friday afternoon and continuing into Saturday
   morning. 500-mb flow could exceed 100 kt, which is near record
   values across the Carolinas and into the mid-Atlantic based on SPC
   sounding climatology. In contrast to these impressive dynamics,
   overall thermodynamic environment will be marginal. Given that ample
   low-level moisture will be in place (i.e. dewpoints in the mid 60s
   to low 70s and 100-mb mean mixing ratios over 12 g/kg), the modest
   instability is largely a result of warm temperatures aloft and
   consequent poor lapse rates. 

   Given the strong forcing for ascent and little to no convective
   inhibition, additional thunderstorm development is possible across
   the Carolinas and much of the mid-Atlantic as early as the late
   morning. Widespread thunderstorm coverage is anticipated with a
   generally messy convective mode. Bowing line segments capable of
   damaging wind gusts appear to be the primary severe threat although
   tornadoes will be possible as well, particularly in areas of
   slightly better low-level moisture and backed surface winds. Highest
   coverage of severe is currently expected from the northeast SC coast
   into southeast VA where the best overlap between favorable low-level
   moisture, southeasterly surface winds, and a strong low-level jet
   exists.

Really hope we can manage some sunshine on Friday because the flood threat is looking more meager than yesterday.  To waste 2 good events in under a week would really be a dagger through the heart.

[cyclogenesis]

April 17, 2019

this Wednesday afternoon

502 PM EDT

 * * * *  Severe Weather & Heavy Rainfall event for Eastern Virginia  * * * * 

 * * * *  Coming for Friday afternoon & Friday night, 4/19/2019  * * * *

 * * * *  Tornado Watches, warnings, & urban-small stream flood advisories likely * * * *

I.  Introduction.

So the way I see this upcoming Friday afternoon, 4/19, & night event playing out is this:  I see 2 rounds of inclement weather moving in to the Richmond, VA & Eastern Virginia region starting-up early Friday afternoon.  This 1st phase on Friday afternoon, I think will bring the attendant severe weather risk of a wind damage threat & some tornadoes, too.

The 2nd phase of this event I believe will evolve on Friday night, & take shape as a heavy rainfall event, through the late night hours.

II.  Friday afternoon Severe Weather.

I'm going to quantify the severe weather threat in a numbers sense, by touching on the calculated indices.

Here's a look at the severe weather parameters, advertised on Friday afternoon, CENTERED on Richmond, VA, at 2 PM Friday:

#1.)  SB-Cape:  2 PM-5 PM Friday -- Averaging about 1,000 J/kg, varying 750 J/kg from Gfs; 1250 J/kg Nam  This average amount of 1,000 J/kg will be sufficient to sustain convective severe storms on Friday afternoon, with no doubts.

#2.)  SWEAT index -- 2 PM Friday:  321 - 349.  This assesses the tornado risk.  I pay close attention when this value reaches 350+.   To get large tornadoes a high degree of instability with high wind shear gets involved.  I also watch for a BACKED surface wind, and arched hodographs.  Straight-lined hodographs are indicative of a squall line, or line echo wave patterns, lewps.  I don't see much directional shear here, with winds coming from a Southerly direction through the depth of the troposphere.  This value upwards to 349 is marginally close enough to continue the watch for a few or several tornado warnings, owing to the cape & shear values present.

#3.)  TT's:  of 46-48 for Friday afternoon.  This is a non-player for Friday afternoon & night, as this parameter is focused on assessing the hail threat, with its examination of the thermal profile comparisons aloft at different layers.  Values at and above 50 indicate severe storms, and in the mid 50's, scattered severe storms.  But values now are shown to be mid to upper 40's.  

#4.)  Calculated PEAK WIND GUSTS:  52 kts, Gfs; 54 kts, Nam between 11 AM to 5 PM Friday 52 kts = 60 mph; 54 kts = 62 mph.

The 54-hr Nam forecast Skew-T sounding shows the presence of an inverted-V sounding at 2 PM Friday.  This along with the NOSE of an approaching jet max at 850 mb, 5K ft, level at speeds of 40 kts increase to 55 kts, will be sufficient to produce and enhance microbursts & downdrafts.  The leading edge of an accelerating jet max in the lower levels as this has well been known to de-stabilize the atmosphere by instigating the formation of warm-air advective showers & thunderstorms, at the gradient of the change, as it punches Northbound.  This will be in play on Friday afternoon over Eastern Virginia.  Watch for attendant severe weather in the form of wind damage by fast-racing showers & storms.

#5.)  0-6 km shear, (deep shear) -- Values appearing between 47 kts, Nam to 56 kts, Gfs for Friday afternoon in Richmond, VA.  Minimum threshold desired for severe weather is 40 kts; this is already forecast to be exceeded.  

#6.)  500 mb, ~18K-19K ft winds:  Forecast at 55 kts to 70 kts between 2 PM and 8 PM Friday afternoon.  I look for a minimum value of 40 kts to be reached for severe weather, so this amount forecasted well exceeds this for the promulgation of severe storms, at the mid-level altitudes.   At 250 mb, in the highest altitudes, ~35K ft, only 60 kts is forecast.  While not a particularly necessary condition, it's great when winds at this level exceed 100 kts, as a dynamic suction vacuum aloft, as it helps increase UVV's, as what transpired in your late Sunday night even.  But more importantly, the lower levels are where high winds are needed to produce the damaging wind gusts, which will be present at 850 mb.

#7.)  PW, preciptable water, moisture values escalate quickly between Friday morning to Friday afternoon, between 0.97" 8 AM Friday to 1.68" Friday at 2 PM, Nam; and 1.2" 8 AM Friday to 1.7" 2 PM Friday by Gfs.  All the right ingredients at all the right timing together should quickly inflate this event to severe levels on Friday.  Sudden increase in moisture; sudden increase in low & mid-level jets, sudden increase in SB-cape, and rising of instability with daytime heating, although will be preceded by cloud cover.  

III.  Heavy Rainfall Event, Friday night.

Whenever I spot this pattern of a closed upper 500 mb low, slowly approaching with a LENGTHY, fetch of meridional 500 mb flow from South-Southwest at all levels of the troposphere, at terrific speeds of 40-60 kts from boundary layer to mid-levels, then this *screams* th words heavy convective rainfall all over it, for Friday night.  Having been a long-time resident of New Orleans for 17 years of my life, I've come to recognize these pattern settings down South.

Of particuarly importance that I've also spotted is your 12Z Gfs rainfall grid that depicts what I call a RATHER SLOW progression of a thick, heavy rain band which only moves from Western Virginia at 2 PM Friday over to Eastern Virginia 12 hours later, at 2 AM Saturday.  Gang, that's a SLOW 12-hour progression, only going from Western Virginia to Eastern Virginia in a 12-hour span of time.  And this is coming from a model who's notoriously biased in being overly progressive in moving out rain quickly.  

Additionally, I see a sharp uprising in PW's from 1.20" 8 AM Friday up quickly to 1.70" 6 hours later, at 2 PM Friday afternoon, coincident with the nose of an 850 mb low-level jet increasing during peak heating time & instability time as well.

Whenever I see 3-hour rain rates of 3/4" to 1" being advertised 54-60 hours in advance over a progressive 12 hour period across the state of Virginia in the coarser grid domain of the GFS, I can only be thinking that once this hits the finer-mesh resolution grids of H-Rap & WRF, that these rainfall rates will be AMPLIFIED SO MUCH GREATER.  PLUS, I also have to be thinking there will be numerous heavy bands of 50+ dbz rates.  And when that happens amidst a strengthening powerful low-level & mid-level jet structures in the presence of modest, but ample levels of SB-Cape, then this spells both a severe risk & heavy rainfall potential that looks pretty certain to unfold on Friday afternoon & Friday night.  I think 1" to 3" of rainfall will be commonplace for rainfall totals between Friday afternoon & Friday night.  

Any stalling of this heavy rain banding that sets up meridionally, from South to North will accentuate rainfall totals beyond 4" for any temporal stalling of this heavy rainfall in areas as it slowly crawls East-bound.

Last Sunday night's event escaped everyone's attention because of the inopportune timing of 1 AM and 4 AM, while nearly most were sleeping.  Not this time.  This time many of you will be up to watch it on radar & feel the experience for yourselves.  

 

--  cyclogenesis

Richmond, VA

[CAPE]

AFD from Mount Holly on the potential threats-

Quote

Southerly winds will strengthen in the warm sector Friday as an area of low pressure passes to our west. By Friday afternoon this increasingly strong (and deep) southerly flow will draw subtropical moisture into the area with PWATs climbing to over 1.5 inches (at least over parts of the area) which would be rather anomalous relative to April climatology. Additionally Friday will be warm with high temperatures around 10 degrees above climo. As the surface low/ejecting shortwave energy pass to our west, expect showers and thunderstorms to develop Friday afternoon with western parts of the forecast area favored (particularly in upslope areas favored in S/SE flow). Due to the aforementioned anomalously high moisture combined with increasing warm cloud depths, heavy rain will be a threat with these storms. Additionally given that the mean flow will be largely meridional there is the potential for training storms potentially exacerbating the heavy rain threat. Although instability will be marginal there is also the potential for some isolated severe storms during this time frame as shear increases late Friday afternoon into Friday evening as the low- level jet intensifies. Generally expect the axis of heavier precipitation to slowly progress eastward Friday night into Saturday morning as the secondary low lifts north. That being said there are still some discrepancies among guidance over where exactly the heaviest axis of precipitation sets up, and also the general timing of precipitation (not surprising given how meridional the flow becomes). That being said the chance for hydro and/or severe impacts does exist and hopefully future cycles (which can take advantage of CAMs) can shed more light on which specific areas are under the greatest risk of each hazard.

Clearly the best chance of legit severe is south of the greater DC-BWI-PHI regions, into eastern VA and NE NC. Some isolated severe is possible locally, but the biggest threat would seem to be excessive rains in some areas with the potential training of cells.

[Stormfly]

Haven't had strong winds out of the south for a while.  By strong I mean 55-60 mph 3 seconds.  My front door faces south and has a weather stripping under the bottom that sings loud like someone playing a kazoo very badly.  It's quite hilarious.  I was getting excited Sun night when it was getting close to 40 but no cigar.

[AfewUniversesBelowNormal]

Looks good for tomorrow.

[BaltimoreWxGuy]

Seems like models are having trouble and going back and forth with how potent the main line of showers and storms is between 8pm and 2am. The NAMs don’t even show .5 or more of precipitation whereas 6z GFS was 2-3” area wide and now 12z is around 1-1.75”. I guess we’ll just have to see how that main line performs to verify or bust rain totals/severe weather. But it seems like that strong of a southerly jet would argue heavy rain but ?

[Eskimo Joe]

43 minutes ago, AfewUniversesBelowNormal said:

Looks good for tomorrow.

Not really.  We're just on the outside looking in...as usual.

[midatlanticweather]

Hit or miss interesting. This one has less a look in our back yards, but slight adjustments in many differing factors could have a larger impact. We will have to see how it evolves.

 

[Kmlwx]

23 minutes ago, Eskimo Joe said:

Not really.  We're just on the outside looking in...as usual.

The severe risk is non-zero but I'd put local odds of it being better than Sunday evening pretty low. This looks like a bigger event in the Carolinas. 

[Stormfly]

Myrtle Beach looks decent.

[Eskimo Joe]

1 minute ago, Kmlwx said:

The severe risk is non-zero but I'd put local odds of it being better than Sunday evening pretty low. This looks like a bigger event in the Carolinas. 

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

[Kmlwx]

4 minutes ago, Eskimo Joe said:

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

Clarifying my above post - I still think there's a severe risk - but best will be south of the Potomac and especially closer to SEVA and such. 

[yoda]

SPC moves both SLGT risk and ENH risk northward on the 1730 OTLK