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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1559Z Jul 08, 2020)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1159 AM EDT Wed Jul 08 2020

Day 1
Valid 16Z Wed Jul 08 2020 - 12Z Thu Jul 09 2020

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE
CENTRAL U.S AND ALONG THE OUTER BANKS OF NORTH CAROLINA...

...Upper Midwest...
...16Z Update...
A decaying MCS continues to progress eastward across portions of
northeast/east-central MN and far northwestern WI this morning. 
This activity will help to briefly stabilize the region, though
most of the high resolution guidance still shows
shower/thunderstorm development through the afternoon as a cold
front nears.  There may be enough time during the late
morning/early afternoon to destabilize aiding in organized
thunderstorm development just along/ahead of the approaching cold
front.  This coincides with strengthening dynamics as an upper
level jet increases through the afternoon hours. Thus, maintained
and refined the Slight Risk ERO and also accounted for subtle
shifts in the heaviest QPF, which may be slightly farther east now
into portion of northern WI based on the progression of the
MCS/residual boundary. Given this rain is also going to fall over
saturated soils from this mornings activity, feel the Slight Risk
is justified despite the expected progression as the precipitation
shifts east. 

...Previous Discussion...
A vigorous medium wavelength system, 1.5 to 2.0 standard
deviations below climatology in the 500mb height field (fairly
impressive for summer) will continue its trip out of the northern
Rockies toward the Upper Midwest today. A fast moving MCS will
churn through Minnesota this morning, riding along a developing
warm front in advance of the upper system. Expect this to lay down
its heaviest rain near the warm front in central Minnesota, while
areas farther north and northeast must wait until afternoon for
instability to develop within the warm advection regime. A well
defined corridor of enhanced southwesterly low level inflow will
persist over central and northern MN during the afternoon,
eventually veering and giving way to lesser convergence or more
progression by late evening. Thus the greatest potential for flash
flooding will be in the afternoon and early evening as the next
round of diurnally enhanced storms develops, featuring cell
mergers and upscale growth. Overall, the cell motions could still
be a bit quick for flash flooding, given the strength of the mid
and upper level flow. But strong CAPE along with precipitable
water anomalies to +2.5 (actual PW values above 1.50 inches in the
pre-convective environment) will yield intense short term rain
rates. Expect hourly rates at least 1.5 to 2.0 inches, while
mergers and brief bouts of backbuilding/training could lead to
accumulations over 2.50 inches in an hour. The hi-res models are
unanimous in forecasting spotty 3-inch event totals. This part of
Minnesota has been somewhat dry, and flash flood guidance values
are highest in the same areas where the heaviest rain is expected,
so this tempers expectations somewhat. But the short term rates
are likely to overwhelm any sensitive hydrologic basins and/or
urban areas.

...Central Plains...
...16Z Update...
Only minor adjustments made to the Slight Risk area based on
latest high resolution model guidance with respect to the expected
development of an MCS overnight.  The NMMB and ARW are showing the
heaviest QPF riding along the capping inversion a bit farther east
as compared to the other guidance.  Though, based on alignment of
the low level jet and the boundary layer convergence, kept the
heaviest QPF mainly across central NE into eastern KS. There is
uncertainty with respect to the propagation and how long the MCS
progresses through the early morning hours.  Regardless, there is
a multi-model agreement on the general development, evolution and
placement/track.  The heaviest QPF may be across portions of
central NE where thunderstorms will initially develop ahead of the
cold front; though this region of the sand hills has much higher
FFG.  Therefore, maintained a Marginal Risk across this region. 

...Previous Discussion...
The organized wave ejecting through the northern Plains brings
with it a flat flow and organized Pacific upper jet topping the
southwest U.S. ridge, and placing the central Plains in a difluent
upper flow pattern by afternoon. Many areas remain too hot and
stable for organized convection, with 700mb temps of +12 to +19
forecast over the High Plains. Along roughly the edge of the +12C
temperatures the models are all consistent in developing
thunderstorms over central Nebraska late today. These may then
show a tendency to propagate both east-northeastward along the
trailing portion of the northern Plains front, but also
southeastward into the low level inflow. In Nebraska and Kansas
this has the look of an MCS riding along the edge of the cap, fed
by inflow from the capped region, with redevelopment favored on
the south to southwest flank of the cold pool. Flash flooding
could occur early as cell mergers lead to the MCS formation, and
also as training and redevelopment occur along the edge of the
cap. We maintained a Slight Risk area, maintaining lower Marginal
Risk probabilities in the Nebraska Sand Hills where some of the
early development is likely to take place.

...Coastal Carolinas...
...16Z Update...
Extended the Marginal Risk slightly farther inland across most of
the Coastal Plain/Sandhills region to account for multiple rounds
of thunderstorms through the afternoon. The overall coverage and
intensity will be limited by low level inflow and weak
instability, despite higher precipitable water values. The Slight
Risk over OBX appears warranted as the strongest moisture
convergence should continue to feed moderate/heavy rain into the
region, especially between Cape Lookout and Cape Hatteras over the
next several hours. 

...Previous Discussion...
The NWS will continue to monitor slow development of low pressure
associated with a weakly baroclinic trough occupying the southeast
states. Low pressure, which was featured closed contours at
surface and 850mb Tuesday evening, is expected to become better
near the Carolina coast. The airmass here is rich with tropical
moisture, many RAOBS sampling 2.15 inch PW or greater. It would
not take too much concentrated convection to convert this low to
warm core or some kind of hybrid system and NHC is monitoring. The
Day 1 rainfall, however, does not look especially concentrated.
Larger scale organization of thunderstorms has tended to favor
areas offshore, while the diurnally enhanced showers developing
inland near the developing low have suffered from a lack of
sustained inflow. The QPF consensus, in fact, trended toward
lesser coverage of heavy rain potential today. We therefore shaved
off portions of the risk area, but do maintain a small Slight Risk
for the N.C. Outer Banks where at least 2 to 4 inches of rain are
expected, and totals greater than 5 are possible. Hourly rain
rates could exceed 2 inches in spots.

...Deep South...
...16Z Update...
Minor adjustments made to the Marginal Risk across the region. 
Extended the risk area over portions of TN where slow moving
diurnally driven convection will form through the afternoon. 
Also, the activity moving across central MS will slowly wane
though the southern extent of the convection could promote
convergence (a focus for heavier precipitation) through the
afternoon across portions of south/central MS through the next
several hours.

...Previous Discussion...  
A convectively enhanced vort max over southern Arkansas / northern
Louisiana will pick up a little speed today owing to changes in
the surrounding flow pattern. This feature will likely produce
some local convective flare-ups this morning in AR/LA/MS. Activity
in the deeply mixed afternoon boundary layer will more likely be
tied to lower level convergence where the mid level trough
overlaps with the CAPE axis across northern Louisiana into
southern Mississippi. We tended to prefer the WRF-ARW2 depiction
of this process. Overall, the slowest movement and greatest threat
for flooding rainfall should be during the morning hours, as the
upper trough will accelerate later on, and deep mixing will
promote more propagation along outflows. We maintained Marginal
Risk probabilities here, but also stretched that risk into parts
of northern Alabama and eastern Tennessee, where hi-res model QPF
signal was clustered tightly enough to boost neighborhood
probabilities, and cell motions will be especially slow during the
early stages of afternoon thunderstorm development.

...Northeast...
...16 Update...
No changes needed for the ERO. Convection has already initiated
across portions of PA/southern upstate NY ahead of a cold front
with more moderate/heavy rain axis moving through central/northern
ME. Given there is weak mean flow farther south, expect storm
motions to be slow and erratic across PA/NY.  Also, this region
has seen an abundance of precipitation over the past couple of
days, lowering FFG.  A mesoscale precipitation discussion (#451)
is out for this region through mid/late afternoon as activity
could approach the urban corridor.  Expect the overall coverage of
convection to increase within the Northeast through the afternoon
as there is plenty of moisture/instability present ahead of the
front. 

...Previous Discussion...
A corridor of enhanced deep layer moisture content has been
sloshing around the Northeast and mid Atlantic the past several
days, waxing and waning with the passage of each trough within
northwesterly upper flow. Today another pronounced shortwave will
arrive out of Canada early in the day, prompting a resurgence of
greater moisture and instability up into northern PA, New York,
and New England, as far northeast as central Maine. Thunderstorm
development may tend to favor higher terrain before then moving
downhill toward the coast. Precipitable water values are pretty
respectable at +1.5 to 2.0 standard deviations. With cell motions
at least 10 to 15 knots, however, and most locations likely to see
only one round of passing storms, we maintained Marginal Risk
probabilities. Certainly the intense short term rain rates could
lead to a few higher-end impacts in sensitive basins or urban
areas, but there was not enough of a focus in the meteorological
signal to upgrade any particular area to Slight Risk.

Burke/Pagano


Day 2
Valid 12Z Thu Jul 09 2020 - 12Z Fri Jul 10 2020

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE CENTRAL PLAINS...AND MARGINAL RISK FOR PORTIONS OF THE MIDWEST
AND ACROSS PARTS OF THE MID-ATLANTIC...

...Mid Atlantic...
There has been a general trend in the model guidance over the past
24 hours for the coastal surface low, and the plume of deeper
moisture associated with it, to be a bit further east with the
highest QPF likely to occur over the offshore waters.  However,
the coastal areas of eastern North Carolina and locations east of
Interstate 95 in Virginia should have enough instability and
forcing for scattered storms that may be heavy rainfall producers,
and locally produce 1 to 3 inches of rain. The Marginal Risk area
also encompasses the greater Washington, DC metro area and
extending northward to Philadelphia, and points eastward.  There
is still a fair amount of spread concerning the northward extent
of impactful rainfall on Day 2, but given some of the impressive
rainfall totals over the past 48 hours, flash flood guidance is
reduced and it will not take as much rain to result in flooding
problems.  The UKMET is farther southeast with the low and has
little in the way of significant QPF inland, whereas the NAM and
GFS have the core of the heavier rainfall closer to the Delmarva
Coast.  Given the trend to keep the highest totals offshore, there
is no need to introduce a Slight Risk area at this time.


...Southern/Central Plains into the Upper Great Lakes...
There is decent overall model signal in the 00z regional/global
guidance concerning the track of two MCSs that are likely in mid
level northwest flow ahead of a cold front from southeast Nebraska
to northwest Arkansas, with the 00z NAM among the most robust
solutions with aerial QPF on the order of 2 to 3 inches.  The
second MCS during the overnight period Thursday night will likely
have the greatest QPF, and after coordinating with the local
forecast offices, a Slight Risk area was introduced from eastern
Kansas to extreme northwest Arkansas, where the best prospects for
potential cell training will exist. 

Farther to the northeast across the Upper Midwest and western
Great Lakes, scattered to numerous showers and storms are likely
as the cold front approaches with ample instability of 2500 to
3500 J/KG of MUCAPE ahead of it.  This should be sufficient to
support multi-cell convection with support from mid level short
waves and remnant MCVs.  A 20 mph low level inflow transports PW
values approaching two inches into the developing storms, which
could support hourly rainfall rates on the order of 1.5 inches or
greater with the strongest convection.  However, this activity
will likely not be as organized as the MCS activity farther to the
southwest, thus keeping this a Marginal Risk level for now.

Hamrick

Day 3
Valid 12Z Fri Jul 10 2020 - 12Z Sat Jul 11 2020

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE MID-ATLANTIC/NORTHEAST AND THE NORTHERN PLAINS...

...Delmarva to southeast New England...
The coastal low will continue making slow progress up the coast
Friday night and into early Saturday.  A veering low level jet
from southeasterly to southerly on the order of 30-40 mph at 850
mb will advect an anomalously moist airmass quickly northward,
with PWs progged by the ECMWF to potentially reach into the 2.1 to
2.4 inch range in the warm sector of this low across southern New
England.  These values approach daily PW records for early July,
and thus deep warm cloud layers and high precipitation efficiency.
 A corridor of enhanced 300 mb divergence will aid in deep layer
ascent, along with low level frontogenesis across much of this
region.  The NAM and CMC are most aggressive in bringing excessive
rainfall amounts inland from New Jersey to Rhode Island, where the
ensemble means suggest that the core of the heaviest rainfall
should continue to remain offshore. 


...Central Dakotas and northern Nebraska...
A cold front currently approaching the Pacific Northwest is
expected to reach the northern Plains by Friday and become an
occluded surface low by Friday evening, with the warm sector
extending from central North Dakota to northern Nebraska.  A
secondary low will likely develop along the cold front across
western Nebraska as the boundary tracks eastward.  Warm air
advection should begin in earnest by Friday night as the low level
jet intensifies and advects a corridor of 1.5 to 1.8 PWs northward
over eastern South Dakota and much of central and eastern
Nebraska.  Mixed layer CAPE is also expected to rise to at least
2000 J/kg, and combined with increasing moisture will provide fuel
for a pair of MCSs that will likely develop late Friday afternoon
and track southeastward during the overnight hours.  Some training
of convection is possible with these complexes and raises the
potential for flash flooding.  However, there is still enough
spatial variability in the models to support just a Marginal Risk
area at this time.

Hamrick




Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt