Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
541 PM EDT Mon May 25 2020
Day 1
Valid 2141Z Mon May 25 2020 - 12Z Tue May 26 2020
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER MIAMI AND
SLIGHT RISK IN THE PLAINS...
...Florida...
A tropical air mass and some injection of instability from daytime
heating was supporting high-end rain rates over Miami and more
generally southeast Florida. Three to six inches of rain had
fallen in and round Miami today, and some unofficial 48-hour
totals exceeded eight inches. Moreover, the 2-week precipitation
anomaly here was running 3 to 6 times climatology, so the area had
become saturated, leading to deep standing water. Several local
storm reports confirmed flash flooding this afternoon, and the
event was not quite over yet. At 21Z an outage of the Miami (KAMX)
WSR-88D was complicating matters, but surrounding radars and
visible satellite imagery depicted deep convection stretching from
the Everglades in western Miami-Dade County to near Pompano Beach.
Other deep convection is possible along a convergence zone from
just north of Homestead to near Fort Myers this evening. Continued
partial heating and sustained southwesterly inflow impinging on
the rain-cooled air mass farther east may continue to support
development of very heavy rain until the loss of daytime heating.
This is seen in the 18Z HRRR and NAM CONUS Nest. With precipitable
water values over 2.30 inches, and yet lapse rates still
supporting MLCAPE of 1000-2000 J/kg, we may continue to see
extreme rain rates. Earlier WFO Miami received one observation of
3.95 inches of rain in one hour.
All of this supports continuation of the Moderate Risk area. We
did remove, however, Broward and Indian River Counties,
downgrading them to Slight Risk, as the instability had been
unable to develop that far north. We expanded Marginal Risk to
cover some merging thunderstorms trying to drift toward Tampa.
...Central and South Texas...
The atmosphere has been overturned a couple of times here in
recent days, including widespread overturning last night. This is
reflected in the observations, as instability struggled to reach
the strong to extreme levels it had been at over the weekend. At
21Z heftier CAPE values of 3000-4000 J/kg were confined to far
south Texas west of Brownsville, with a gradient toward values
around 1500 J/kg from San Antonio to Midland. As large scale
troughing and an associated cold front sinks into west Texas, it
is very likely there will be another round of thunderstorms,
growing upscale into an MCS that will track along the CAPE
gradient once again tonight. The column had dried somewhat, but PW
of 1.25 to 1.70 inches will support at least 1 to 2 inch per hour
rates. These rates likely will not persist for too long at any
given location. Passage of the initial shortwaves the past two
days had led to a weakness of 850-700mb flow today, and this
should promote quick forward progression of the mature MCS
activity. This is one reason to remain in the Slight Risk
category... but there is some concern that the antecedent wetness
around San Antonio and the Hill Country could lead to some
impactful flash flooding overnight.
...Northeast Texas up through Iowa and into the Ohio Valley...
We split the Slight Risk from south Texas and created a second
Slight Risk area that follows along and north of a well defined
shortwave trough that was lifting up the east side of the larger
longwave in the central states. This shortwave will promote a
heavy concentration of thunderstorms over the ARKLATEX region up
into eastern Oklahoma this evening. Other lead waves and a
generally moist/unstable southerly flow regime will support
thunderstorms up into the central Plains and Midwest, with any
persistent cell mergers or training episodes leading to flash
flooding, given wet soils over much of this region.
Farther east, expect little organization, but as we saw Sunday
evening, seasonably steep lapse rates and rich low level moisture
have been sufficient to generate clusters of storms capable of
producing flash flooding wherever they merge or local effects
cause them to briefly stall... anywhere from eastern Missouri to
Tennessee and up to southern Ohio.
Burke
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
426 PM EDT Mon May 25 2020
Day 2
Valid 12Z Tue May 26 2020 - 12Z Wed May 27 2020
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHERN PLAINS INTO THE LOWER MISSISSIPPI VALLEY...
2030 UTC Update -- Expanded the the Slight Risk a little farther
eastward to include more areas in the lower MS Valley, based on
the latest (12Z) trends with the high-res CAMs. Still looking at
considerable longitudinal spread among the guidance as the
amplified, deepening southern stream trough evolves into a closed
mid-upper low by Wednesday. The areal uncertainty is not unlike
what we've been seeing in the short term across the southern
Plains.
0900 UTC Discussion...
...Southern Plains into the Lower MS Valley...
A mid-level closed low will continue to spin across the Southern
Plains with little forward motion thanks in large part to the
subtropical ridge in the east holding firm. As such, mid-level
shortwave energy rounding the closed circulation will interact
with moisture and unstable air below promoting shower/thunderstorm
activity capable of producing isolated flash flooding.
At this update, it appears that a convective complex should be
transitioning off the TX coast early Tuesday to start the Day 2
period. As a result, this activity will likely stabilize the
southeast coast of TX where the atmosphere will struggle to
recover. Therefore, expect limited convective coverage through the
rest of the day. However, there will be plenty of available
moisture farther north and east across the ArkLaTex region.
Precipitable water values in this region will surge north ranging
from 1.75 to 2 inches aided by 25 knot low level flow out of the
Gulf of Mexico. Instability will also be available with MUCAPE
values climbing above 1500 J/kg. While there are hints of upper
level support, though not impressive, the mid-level shortwaves
should provide an aid for a bit better organization. Also, the
850-300mb winds become better aligned with the propagation vectors
showing a sign for convective training.
Areal average precipitation across this region ranges from
0.5-1.5+ inches. It appears the main region where the
aforementioned ingredients overlap is across portions of northeast
TX, northwest LA, southeast OK and southwest AR. Antecedent
conditions vary across this region. However, with rain rates
exceeding 1.5 inches per hour, it could pose problems, especially
if training convection is realized. Outside of this corridor,
convection could be dominated by mesoscale processes, which at
this distance are difficult to identify. With this in mind, the
Marginal Risk was adjusted based on the latest WPC QPF forecast
and model trends.
...Southeast Coast...
A mid-level disturbance that is progged to move across southern FL
on Day 1 will shift quickly offshore positioning a trough/surface
low off the coast. With high pressure to the north, this will
funnel very strong moisture convergence from the tropical airmass
inland along the Southeast coast. The heaviest rain should remain
offshore as the best ingredients and upper level support will be
over the maritime region. However, with precipitable water values
well over 2 inches and the potential for an uptick in instability
(MUCAPE values climbing over 1000 J/kg), there will likely be
periods of moderate to heavy rain. Though much of this region has
received over 200% of normal precipitation, the sandy soils should
be able to withstand rain rates of just over 1 inch per hour.
However, if storm total rainfall and/or rain rates trend upward,
then there could be a need for a Slight Risk along the immediate
coastline. For now, will stick with a Marginal Risk area as
models have been less consistent with this storm system and lots
of uncertainty remains.
Hurley/Pagano
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
456 PM EDT Mon May 25 2020
Day 3
Valid 12Z Wed May 27 2020 - 12Z Thu May 28 2020
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE LOWER MISSISSIPPI VALLEY, CAROLINA COAST AND SOUTHERN
APPALACHIANS...
2100 UTC Update -- Expanded the Marginal Risk area a bit across
the lower mid Atlantic (Carolinas into southern VA), based on the
latest trends with the 12Z guidance. May need to hoist a Slight
Risk over the southern Appalachians over the next 1-2 forecast
cycles, depending on whether such trends continue. Have opted not
do for this ERO, as there is still some question as to the
progression of the closed mid-upper low over the southern
Plains-lower MS Valley and the ridge along the mid Atlantic coast.
A much slower progression will likely retard the return of heavy
rainfall bands from the Gulf Stream into the lower Mid Atlantic
region.
...LOWER MS VALLEY...
A closed low that situated over the Southern Plains will slowly
edge eastward Wednesday into Thursday, thanks in part to the ridge
to the east eroding. Mid-level energy rounding the low will
interact with moist and unstable air below resulting in widely
scattered showers and thunderstorms. At the surface a weak low
will become stacked/aligned with the mid-level low and thus be
slow move. With limited upper level forcing and uncertain
mid-level shortwave activity, it is hard to discern where
organized convection may form. However, model guidance has been
hinting at a potential convective complex forming/traversing
southern LA. Regardless, the ingredients are present for heavy
rain to accompany thunderstorms. Precipitable water values will
climb above 1.75 inches; possibly surpassing 2 inches in far
southeast LA, aided by weak southerly low level flow out of the
Gulf of Mexico. This in conjunction with available MUCAPE in
excess of 2000 J/kg should support efficient rain rates. In
addition, there could be training convection as mean wind becomes
aligned with the propagation vectors by the afternoon hours.
Though admittedly, it may be too premature to try to delineate any
mesoscale features this far out. Again, a lot depends on the
shortwave position and any residual surface boundaries that may
exist.
Areal average precipitation of 0.5-1+ inches is expected with the
highest amounts centered over southern LA where the best mid-level
and low level ingredients support more organized convection.
Although this region, on average, has seen near normal
precipitation for this time of year, convection may move into this
area by Day 2 that would help prime the soil. However, due to the
level of uncertainty that still remains with respect to convection
both on Day 2 and Day 3, a Marginal Risk seemed warranted. If in
time there is a stronger signal for organized convection and
training, a Slight Risk may be needed at a future issuance.
...Carolina Coast...
A disturbance approaching the Carolina Coast Wednesday into
Thursday will funnel strong moisture transport into the region.
With a strong high well offshore to the northeast and this
approaching low to the south, moisture will feed inland from a
tropical airmass offshore. This is evident from very high
precipitable water values which should surge above 2.25 inches
aided by low level flow off the Atlantic. Some instability will
also be present, though the extent depends on the placement of the
surface low. Regardless, expect bands of moderate to heavy rain
to move into the coast that could produce isolated flash flooding.
There is a lot of uncertainty that remains with this system.
Model guidance varies quite a bit in terms of the position of the
surface low which dictates how, where and when the moisture
advects into the region. Given this, went with a general model
blend approach which put the position of the surface low along or
just off the SC coast which would edge the associated coastal
trough inland. While much of this region has received over 400% of
normal precipitation as of late, with the level of uncertainty
that remains, felt a Marginal Risk should suffice at this forecast
update. However, it will not take much upward trend of QPF
amounts for there to be a need for a Slight Risk across a portion
of this area.
...Southern Appalachians...
As the aforementioned disturbance approaches the Carolina coast,
strong moisture transport will advect inland. The foothills and
windward terrain will help to promote strong upslope and thus
enhance rain rates and overall rainfall totals across this region.
For right now, areal average precipitation is around 0.5-1+
inches, though these amounts could go up much higher, especially
if the moisture transport becomes orthogonal to the terrain
resulting in backbuilding precipitation.
Given this area has experienced well above normal precipitation
over the past few weeks, soils are sensitive and any additional
rainfall will lead to runoff and the potential for flash flooding.
While a Slight Risk was considered given the severity of the soil
saturation, models have been very inconsistent with respect to
this system as a whole. So, it would be advantageous to see how
the models progress over the next couple of runs before upgrading.
However, if QPF trends up like some of the models suggest, then
this could result in widespread flash flooding concerns where an
upgrade to a Moderate Risk would not be out of the question.
Hurley/Pagano
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
