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8/7 Soaking rain, more flooding?


tombo82685
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My thoughts from what I posted on twitter. We should start to see rain break out to our west/sw tomorrow morning between 5-7am. This is associated with a warm front. You can see tomorrow around 12z how we start with west winds, then the warm front comes through and shifts them to sw. NAMMA_700_rhum_024.png

With this warm front we should start to see an enhancement of precip with building vertical velocities. This should lead to a period of moderate to possibly heavy rainfall tomorrow morning.Untitled4.png

With the warm front passing us to the north, this will open the flood gates for some tropical air to flow around the area of high pressure to our east. You can see in your skew-t how saturated the whole column is. PWATS will exceed 2" around and south of the storm track. NAMMA_prec_pwat_036.png

The main show looks to come with the low pressure Monday afternoon into the evening. Another area of very heavy rainfall will occur with some strong vertical velocities again and some potential area of frontogenesis near the low pressure track. This could cause an area of excessive/training storms. NAMMA_700_fronto_033.png

To the south of the area of low pressure there will be a warm sector with a high shear environment and modest instability for some potential severe storms. If some peaks of sun come out, that will up the ante. As of right now, this would be my guess for low track and area of heaviest rain and best shot of severe weather. In general a .75"-1.5" rainfall across the area. The black circled area has best chance of higher amounts of 2-3". Untd.png

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Current Day 1, days in which there is a flash flood watch with an actual synoptic low, warm frontal boundary moving through the area is typically a summer day in which a tornado or two also occur (in the Mount Holly CWA).

day1otlk_1200.thumb.gif.bfb5778ff150dc70045cd083b95b6370.gif

 ...Mid-Atlantic states into the southern Appalachians...
   A large shield of clouds/rain will likely be ongoing around daybreak
   over the VA/NC Piedmont in association with the early-day
   disturbance traversing east across portions of this region. 
   West-southwesterly flow in the mid levels around 40-kt will
   conditionally support organized storms.  The poleward advancing of a
   more moisture rich airmass (dewpoints rising into the lower 70s
   degrees F) into southeast VA will act to gradually destabilize the
   boundary layer despite the presence of poor 700-500 mb lapse rates
   (around 5 degrees C/km).  There is uncertainty if strong
   thunderstorm development begins during the morning on an isolated
   basis or is delayed until the afternoon during which airmass
   recovery can occur in wake of early-day convection/clouds.  Models
   show the development of a 40-kt southwesterly 850-mb jet across
   southeast VA as the surface low develops east-northeast.  The 07/00Z
   NAM is an outlier (i.e., too strong) compared to other deterministic
   models and appears to be perhaps less realistic in the strength of
   the low-level flow/evolution of the surface cyclone.  Regardless,
   isolated to scattered strong to severe thunderstorms capable of
   primarily wind damage are forecast.  A tornado is possible with any
   longer-lived/more organized supercell structure.
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10 minutes ago, cbelke said:

Did this system slide South again like the last one?

I dont know if there has been as much of a southward slide as the last one.  This one seems steadier (slight southward drift if you average all models).  The NAM IMO has not lost its propensity to track lows too far northwest into the "cold air".  The rule of thumb with flash flooding, heavy rain, is that you never can go far enough south.  This is still a forecast, so we will see.  Another good rule of thumb is to never verify a forecast with a forecast.

index.thumb.gif.aa9284dfaeb3699146295a5bd580367c.gif

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2 minutes ago, Rainshadow said:

I dont know if there has been as much of a southward slide as the last one.  This one seems steadier (slight southward drift if you average all models).  The rule of thumb with flash flooding, heavy rain is that you never can go far enough south.  This is still a forecast, so we will see.  Another good rule of thumb is to never verify a forecast with a forecast.

index.thumb.gif.aa9284dfaeb3699146295a5bd580367c.gif

Good points Tony. Question for you. Are you taking the average of all the models like you mentioned or are there one or two models that verify better than all the others in these scenario's? 

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33 minutes ago, cbelke said:

Good points Tony. Question for you. Are you taking the average of all the models like you mentioned or are there one or two models that verify better than all the others in these scenario's? 

I kind of look at all the models and lean toward consensus at this point.  While statistically the Euro scores the best, inside of 72 hours with convection its no better (some would say its worse) than any other model.  I am intrigued by the German model, might pay more attention to it this cold season.

Capture.thumb.JPG.2158c282ebbf5bf4eaa8b1880d6728c4.JPG

Right off the bat, its not that good in PA, precip is farther northwest.

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Graphic for MPD #0643

MESOSCALE PRECIPITATION DISCUSSION 0643
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
858 AM EDT MON AUG 07 2017

AREAS AFFECTED...N VA...DC...MD...DE...S NJ...EXT SE PA

CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE

VALID 071300Z - 071900Z

SUMMARY...TRAINING EMBEDDED CONVECTIVE ELEMENTS WITHIN DEFORMATION
ZONE POSES LONGER DURATION FLASH FLOOD/FLOODING RISK.

DISCUSSION...GOES-E WV AND RADAR MOSAIC DEPICT MULTIPLE SHORTWAVES
ACROSS THE MID-ATLANTIC THAT ARE FAVORABLY ORIENTED TO ALLOW FOR
NARROW CORRIDOR OF TRAINING SHOWERS WITH POTENTIAL FOR EMBEDDED
SHALLOW CONVECTIVE ELEMENTS. A SHORTWAVE LIFTING OUT OF EASTERN PA
TOWARD N NJ ANGLES A TRAILING BOUNDARY/DEFORMATION ZONE ACROSS N
DE/CENTRAL MD TOWARD THE APEX OF THE DEEPER SHORTWAVE CURRENTLY
OVER E CENTRAL WV.  THIS ORIENTS AN IDEAL CORRIDOR FOR PERSISTENT
MOISTURE CONVERGENCE AND CONFLUENCE/MERGERS FOR CELLS TO GRAVITATE
TO BEFORE TRAINING ALONG THE CORRIDOR. STRONG DPVA WILL MAINTAIN
SUFFICIENT LARGE SCALE ASCENT ACROSS THE MID-ATLANTIC.

LIMITING FACTOR FOR CONVECTIVE GROWTH IS IN AVAILABLE INSTABILITY.
RAP ANALYSIS SUGGESTS A NOSE OF CAPE OVER SE VA POINTED TOWARD MD
FROM THE CHESAPEAKE BAY...WITH 12Z RAOBS FROM IAD/RNK/WAL SUGGEST
VERY SKINNY SATURATED PROFILES WITH CAPES OF 250-750 J/KG
AVAILABLE.  AS SUCH RECENT DIX/DOX/AKQ/LWX RADAR TREND SHOWS
STRONG CONVERGENCE NEAR/ALONG THE DEFORMATION ZONE FROM JUST SOUTH
OF DC METRO ACROSS ANNAPOLIS TOWARD N DE WHERE SLANTWISE/SHALLOW
CONVECTIVE ELEMENTS HAVE FORMED.  GIVEN SATURATED PROFILES TO
1.75" TPWS PER RAOBS...RAINFALL RATES UP TO 1-1.5"/HR ARE POSSIBLE
THOUGH HIGHLY TRANSIENT...NECESSITATING THE UPSTREAM
REDEVELOPMENT/TRAINING FOR COMPOUNDED TOTAL. 

ANY STRONGER/VERTICALLY ORIENTED CONVECTION THAT DEVELOPS FURTHER
SOUTH WITHIN THE WEDGE OF INSTABILITY OVER CENTRAL VA/SE
VA...SHOULD TRACK TOWARD THE DEFORMATION ZONE AND INCREASE THE
FLASH FLOODING RISK GIVEN RATES OVER 2" (ESPECIALLY ALONG THE
EASTERN SHORE OF THE CHESAPEAKE GIVEN INCREASED SFC CONVERGENCE ON
ACCELERATED LL FLOW...QUEEN ANNE/TALBOT COUNTIES) AS SUGGESTED BY
MODELS WITH THE GREATEST  INSTABILITY SUCH AS THE 09Z HRRRX/06Z
NAM-CONEST.  TRENDS WOULD SUGGEST THESE MODELS ARE TOO AGGRESSIVE
BUT THE RECENT HRRR (WHILE IDEALLY ORIENTED TO CURRENT
OBSERVATIONAL TRENDS) APPEARS TOO LIGHT.

THIS WOULD SUGGEST 1-2" ACROSS N VA/DC... WITH 2-4" ACROSS SE
CENTRAL MD INTO THE CENTRAL EASTERN SHORE OF MD/DE THROUGH
19/20Z...WITH MORE TO COME UPSTREAM.  AS SUCH FLASH FLOODING IS
POSSIBLE BUT WITH TIME LONGER DURATION FLOODING IS BECOMING MORE
LIKELY ESPECIALLY FURTHER EAST (GIVEN LONGER DURATION).     
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