Quantitative Precipitation Forecast
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FOUS30 KWBC 190118

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
909 PM EDT Thu Apr 18 2019

Day 1
Valid 01Z Fri Apr 19 2019 - 12Z Fri Apr 19 2019


...OH/TN Valleys to the Central Gulf Coast and Southern
The threat of heavy to excessive rainfall continues across
especially areas of the lower TN Valley, the central Gulf Coast
states and up toward the southern Appalachians overnight and into
the early morning hours of Friday. The latest synoptic analysis
shows a well-defined southern stream closed low and associated
upper trough pivoting east across the lower MS Valley along with
an attendant cold front. Ahead of these height falls is an axis of
robust deep layer southerly flow which is transporting an
abundance of subtropical moisture northward with a PWAT axis of
1.75+ inches nosing up across central and southern areas of MS/AL.
A southerly low level jet of 40 to 50+ kts is in place ahead of
the cold front, and this is also facilitating the transport of a
moderately unstable boundary layer with MLCAPE values of 1000 to
2000 j/kg across these areas. The latest radar imagery shows a
well-defined, but forward-propagating MCS out ahead of the cold
front impacting a large area of AL, with a strong MCV and
concentrated ball of convection seen over northern AL and a
southward-extending squall-line that goes down to the Gulf Coast
with impacts through southern AL and now beginning to edge into
the far western FL panhandle. Meanwhile, in the wake of this MCS,
there is a separate band of rather organized convection that is
focused along the immediate cold front across southern areas of MS
and some portions of eastern LA to the north of New Orleans.

Overnight, the low-level jet ahead of the cold front will remain
quite strong, and this will facilitate sufficient
moisture/instability transport across the central Gulf Coast
states for the ongoing axis of convection to tend to maintain
itself. The latest RAP guidance does favor a bit of a
strengthening of the low level forcing after 06Z as convection
moves farther east across the FL panhandle, western/northern GA
and up toward the southern Appalachians including far western SC
and southwest NC. The 18Z HREF and latest HRRR guidance going
through 12Z favors as much as 2 to 3 inches of rain, with some
locally heavier amounts to perhaps around 4 inches. The main
factor mitigating the threat of flash flooding overnight will be
the relatively progressive nature of the current MCS, and loss of
daytime heating which will allow the boundary layer to stabilize
somewhat. Nevertheless, the strength of the low-level jet and
proximity of strong mid/upper level forcing/divergence aloft ahead
of the southern stream upper trough should help to compensate.
Still expecting there to be at least some potential for flash
flooding, and especially if additional banded convection back
toward the cold front can maintain itself and advance east over
areas that have seen the initial heavy rain with the MCS.

Farther north toward the OH Valley, there continues to be a fair
degree of moisture surging north up along and ahead of a cold
front advancing gradually off to the east. However, the 12Z/18Z
hires model guidance has tended to be significantly overdone
through the afternoon and evening hours as there has been notable
lack of instability advancing north and becoming established
across this region. Much of the rainfall across this region has
tended to be stratiform in nature, with only some modest
convective elements embedded within the rainfall shield. Given the
lack of instability, and especially with the organized nature of
convection across the Gulf Coast states intercepting the stronger
low-level jet energy, the rainfall rates farther north are not
likely to be sufficient to drive much of a flash flood threat.

Therefore, based on the latest radar imagery, observational trends
and HRRR output, the Moderate Risk area was removed for this
update across the South, with the Marginal and Slight Risk areas
greatly trimmed in size to account for where heavier rains have
ended from west to east, and with the reduced heavy rainfall
threat across the OH Valley.


Day 2
Valid 12Z Fri Apr 19 2019 - 12Z Sat Apr 20 2019


A line of organized convection will move eastward across the
Southeast and southern Appalachians Friday morning associated with
a potent low pressure system moving from the TN Valley into the
Ohio Valley. The cold front draped from the surface low will allow
anonymously high precipitable water values (nearly 4 standard
deviations above the mean) and instability (2000+ J/kg MUCAPE) to
surge east of the boundary.  This will help maintain and expand
the convective line north into the central Appalachians and
mid-Atlantic region as the front moves quickly eastward across the
south throughout the day. Rain rates associated with this line
will be upwards of 1-1.5"+/hour. In addition, some training of
convection is possible farther north into Pennsylvania as the
corfidi vectors become better aligned with the mean 300-850mb
flow. The Slight Risk was maintained with some minor refinements
made to account for the highest rainfall amounts/rates expected,
mainly along the mountains to the piedmont of the Carolinas north
into Pennsylvania.  Also, the urban coordinator from DC toward NY,
anticipate the threat for flash flooding to be elevated as FFG is
reduced.  It should be noted that model spread it quite large in
terms of QPF, likely due to the convective nature of this system.
A lot hinges on the moisture flux convergence and instability for
the maintenance of the convective line with the largest
variability in model solutions noted across the mid-Atlantic.

Meanwhile, a couple rounds of moderate rain are expected along an
inverted trough/slow moving cold front from the eastern Great
Lakes up the interior northeast corridor across far northern
Maine. Increasingly difluent/divergent flow aloft juxtaposed with
deep moisture streaming northward ahead the aforementioned surface
low will promote brief periods of heavy rain. With snowpack across
the northern tier of New England resulting in snow liquid
equivalent values nearing 8 inches, expect snow melt to
exacerbate the flood potential.  Therefore the Slight Risk was
expanded from northern Maine west across northern New York,
Vermont and New Hampshire.


Day 3
Valid 12Z Sat Apr 20 2019 - 12Z Sun Apr 21 2019


A plume of deep moisture will still be oriented from the waters of
the Atlantic Ocean into parts of southern New England early in the
period on Saturday. The risk of excessive rainfall will linger
until the deep moisture, strong moisture transport in the
pre-frontal environment, and the moisture flux convergence along
the front all get shunted out to sea Saturday evening or even
during the early morning hours on Sunday. Additional rainfall of
around a half an inch to an inch is possible with locally higher
amounts along the upslope of the terrain.  While the hourly rates
may not be excessive, with this prolonged storm system and
expected snowmelt across the north, the region will likely become
sensitive to additional precipitation accumulations.  Therefore
the Slight Risk covers the more vulnerable regions of Maine and
southern New Hampshire.


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


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