Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
853 PM EDT Sun Aug 18 2019
Day 1
Valid 01Z Mon Aug 19 2019 - 12Z Mon Aug 19 2019
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
GEORGIA AND FLORIDA...
...Northern Florida and Southern Georgia...
The main focus for flash flooding over the overnight hours is
expected to be near and along the inflow associated with the
surface wave over southern GA.
The activity over northern and western FL has diminished
temporarily as earlier activity has overturned the airmass in
place. As the surface wave over the western FL Panhandle tracks
into southern GA late this evening, in the low level inflow ahead
of it continues to pump deep moisture across the Big Bend area
into southwest and south central GA. Convection is expected to
form in the instability axes over western FL into southern GA in
areas that have not been affected by earlier storms.
The most recent RAP/HRRR showed MUCAPE values recovering to
between 1000/2000 J/KG between 19/01z and 19/05z in this area,
which should be sufficient to support increasing convection. Low
level convergence in conjunction with the surface wave allow the
storms to form into southwest/northeast lines from east of KTLH
into southern GA (an idea supported by the most recent HRRR). Weak
mid level flow around the mid level low is expected to result in
short term training and cell mergers.
At this point, it is not clear just how extensive the convection
will be. The deep moisture associated with the strengthening
inflow could allow for very efficient rainfall makes (with warm
cloud depths in excess of 4000 meters), and hourly rainfall rates
between 2.00/3.00 inches are possible. The HRRR has had a multi
run signal for local 6.00+ inch rainfall amounts somewhere over
west of KTLH into southernmost GA, though the placement of the
maximum rainfall amounts has changed from run to run. Given the
potential for a flare up of the deep convection in the low level
inflow (mainly between 19/04z and 19/10z), the Slight Risk area
was left mainly intact from the previous forecast.
...Southern Plains, Mid MS Valley into the Ohio Valley and the
Northeast...
Several impulses in the fast mid level flow interacting with
instability and deep moisture will feed convection that continues
to support a low lend flash flood threat through the late evening
hours (for the most part).
The first area is across PA/NY into NJ, where short term training
along bands of convection will continue to pose a low end flash
flood threat until the instability in place is exhausted. That
process has already begun. and is expected to persist though about
18/04z. The next impulse is moving through IN/lower MI/OH, also
located in an axis fo 1500/2000
J/KG of SBCAPE (per 00z soundings). The impulse is in the southern
edge of the westerly mid level flow, and it is allowing the
convection to remain progressive as it moves east. As the
instability in place becomes elevated (or exhausted), the
convection is expected to weaken as it moves across OH headed for
western PA. Three hour flash flood guidance values are as low as
1.50 inches over portions of Ohio, so the Marginal Risk was left
in place through the end of the activity (which is expected to be
before 19/06z).
The final area is across the Southern Plains into the Lower and
Mid MS Valley. convection over northeast AR/far western is
expected to follow the instability axis present across AR into far
northern LA and western OK. there is a fair amount of spread
concerning how this occurs, with much of the high resolution
guidance sending the convection south with the propagation
vectors. The most recent HRRR, however, is not quite as bullish
bringing the activity into portion of OK and western AR, keeping
it closer to the MS River before it runs out of instability and
weakens. Since there is some spread, the Marginal Risk was
modified to account for current activity, while left in place over
OK to account for the potential shown by the 12z WRF ARW/NMMB.
Hayes
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
352 PM EDT Sun Aug 18 2019
Day 2
Valid 12Z Mon Aug 19 2019 - 12Z Tue Aug 20 2019
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE FLORIDA PANHANDLE...
...Gulf Coast...
A low to mid-level circulation remains over FL/GA/AL during Day 2
with a weak shear axis extending northeast into the Carolinas.
This circulation is the main area of concern given forecast
precipitable water values near 2 inches in the area and a
continued weak 850-300 mb mean flow which will support localized
training where locally stronger 850 mb flow develops, likely
tapping stronger instability present just offshore of the Gulf
Coast. Much of northwest Florida area has seen well above average
rainfall over the past 14 days, along with pockets along the
central to eastern Gulf Coast.
There continues to be a strong multi-model signal for heavy rain
from the FL Panhandle toward the Big Bend region in to a portion
of southern GA. After an early morning maximum of QPF with the
lower to mid level system, there could be a minimum in activity
during the afternoon, possibly followed by another flare up after
20/00z over southern GA. Rainfall amounts of 2.00/4.00 inches are
expected with the track of the low to mid level system, mainly
from east of TLH into southern GA, in areas that have received
heavy rainfall over the past two days. Given that the soil is
nearly saturated in some of these places, the Slight Risk was left
moistly intact.
Further north across central GA, there is also a multi model
signal for heavy rainfall, as the low to mid level system moves
lowly north. While rainfall amounts here could be similar to those
along the FL Panhandle, three hour flash flood values here are
generally above 3.50 inches (a result of mainly dry conditions
over the past 30 days). The Marginal Risk was extended to account
for the low end flash flood threat here.
...Ohio Valley into the Northeast,,,
Storms moving slowly through a high moisture content airmass along
a frontal boundary are expected to produce heavy to locally
excessive rainfall from the Ohio Valley into portions of the
Northeast during Day 2. Storms are expected to develop in an axis
of >3000 J/KG of MLCAPE extending from KY into western PA/western
NY state, mainly during the afternoon and evening hours. Along and
ahead of the front, the low level southwest flow transports
1.75/2.00 inch precipitable water air into these areas.
While best mid level support for convection appears to just north
of the region in the fast mid level flow (south of a closed mid
level low over western Ontario), there is some model support
(especially from the 12z NAM) for left over MCVs from activity
over the Great Lakes to cross the front. If this occurs, it could
support clusters of storms, raising the potential for short term
training or cell mergers. Overall though, storms are expected to
drop slowly southeast (less than 10 knots) away from the front.
There is high resolution model support for hourly rainfall rates
of 1.50 inches from KY across OH, as well as western PA into
western NY state, between 19/18z and 20/03z. Portions of
NY/eastern PA into northern New England received heavy rainfall
over the weekend, which could result in some locations with higher
soils moistures. For the most part, three hour flash flood
guidance values are between 2.00/3.00 inches, so training or cell
mergers would be required to initiate flash flooding. Given the
potential for training in the deeper moisture along the front over
areas that have received heavy rainfall recent, a Marginal Risk
was extended across much of the Ohio Valley into Northern New
England for Day 2.
Pagano/Hayes
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
354 PM EDT Sun Aug 18 2019
Day 3
Valid 12Z Tue Aug 20 2019 - 12Z Wed Aug 21 2019
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE CENTRAL PLAINS INTO THE MID MISSISSIPPI VALLEY......
...Central Plains into the Mid Mississippi Valley...
20z update...
There is still a fair amount of model spread concerning the
position of the surface front extending across the Central Plains
into the Mid MS Valley. Much of the difference in the placement of
the front (and the associated rainfall maximum) is latitudinal,
mainly due to the speed at which the various models drop the front
south, especially after 21/00z. The model consensus appears to
place the rainfall maximum from southeast NE into southwest IA (in
an area that has seen 300 percent of normal rainfall over the past
14 day).
Convection should be outflow dominated early (as is typical in
high instability airmasses), but there is a model signal for the
development of an MCS (supported by the 12z GFS simulated IR
images) along the instability axis. As mentioned earlier, there is
spread in where the instability axis lies, and where the MCS
tracks. As this point, given the amount of spread, there is not
enough confidence for the placement of the Slight Risk area (after
collaboration with WFO OAX). However, as models converge on a
solution (especially if it is over areas with lower flash flood
guidance values, a Slight Risk could be needed in later forecasts.
Previous discussion...
A positively tilted trough will continue to dig out of Ontario
advecting mid-level impulses through the Plains late Tuesday into
Wednesday. Meanwhile, a strengthening jet axis moving into the
Great Lakes region will position the right entrance region atop
the Central Plains into the Mid-MS Valley region. These two
factors will promote sufficient ascent within the atmospheric
column. At the surface a cold front will be progressing
southward. The low level jet axis will increase through the
evening/overnight hours helping to advect precipitable water
values of >1.5 inches from the southeast. In addition, impressive
instability is expected across this region ahead of the front with
MUCAPE and surface based CAPE values well over 4000 J/kg. All
features pointing to organized convection forming along the
frontal zone aided by mid-level impulses and the LLJ. However,
the model spread seems to have expanded at this update with the
GFS showing little to no organized convection. For this reason,
confidence is slightly below average with a general model blend
weighted away from the GFS and toward the EC/UKMET/CMC/ECens/GEFS.
Areal average precipitation is forecast between 0.5-1 inches with
locally higher amounts possible. With the corfidi vectors
aligning with the mean flow, anticipate training to occur which
may exacerbate the flash flood potential. Though there is a level
of uncertainty and modest FFG across this region, there has been a
multi-model signal for organized convection through the last
couple of forecast cycles with impressive instability and ample
moisture transport expected. Therefore a Marginal Risk was
introduced.
...New Mexico/Colorado...
Moisture returning from the western Gulf of Mexico as the mid
level ridge repositions itself across the Southern Plains into the
Southwest could be sufficient to support convection with locally
heavy rainfall across New Mexico and Colorado during Day 3.
Integrated water vapor transport vectors (IVTs) from the GFS
showed the return of mid level moisture from the western Gulf of
Mexico across west TX into NM and CO during Day 3. A 15 knot low
level southeast flow transports 1.00/1.25 inch precipitable water
air (which is approaching two standard deviations above the mean)
into eastern NM, peaking near 21/00z. Model soundings showed
MLCAPE values generally between 1000/1500 J/Km, mainly across the
terrain of southern NM, as well as the Sangre de Cristo Mountains
of northern NM into southern CO. The combination of moisture and
instability is expected to support scattered thunderstorm activity
in these location, mainly between 20/20z and 21/04z.
As the mid level ridge becomes repositioned across NM, the mid
level flow drops below 10 knots (as do propagation vectors). This
is expected to result in slow moving storms that initiate over the
higher terrain. The environment could support hourly rainfall
rates between 0.50/1.00 inches, especially before the storms
become outflow dominated moving out of the terrain. Three hour
flash flood guidance values are as low as 1.50 inches over
portions of the terrain in southern NM. and cell mergers or short
term training could produce rainfall amounts that approach these
values.
At this point, it is unclear just how extensive the flash flood
threat will be. It is possible that the moisture is not deep
enough to support more than scattered storms, which could mitigate
the threat to some degree. It is also possible that the threat may
be a day away from reaching the point that would trigger an
outlook. For now, no excessive area was assigned, but if later
model solutions show an increased threat, a Marginal Risk could be
needed for Day 3.
Pagano/Hayes
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
