Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
446 PM EDT Sat Aug 17 2019

Day 1
Valid 16Z Sat Aug 17 2019 - 12Z Sun Aug 18 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE MID MS VALLEY AND THE SOUTHEAST COAST...

...Central and Northern Plains into the Mid and Upper MS Valley...
Early morning convection which prompted a few flash flood warnings
in Missouri - had become progressive and will no longer pose a
concentrated threat downstream over the Ohio Valley. Back to the
west, additional convective development is likely by later this
afternoon into the overnight hours as stronger forcing moves into
the Plains. As is always the case with summertime convection,
uncertainty exists with exact convective evolution and placement
of heaviest rainfall amounts. It appears likely, however, that
storms will be concentrated in three corridors, one being along a
cold front from the eastern Dakotas into Minnesota, another being
an MCS expected to grow out of the Dakotas activity and cross
Iowa, and a third corridor (more weakly forced but better low
level jet support) from the central High Plains across through
northeast Kansas into Missouri.

Relatively progressive, westerly, deep layer flow and progressive
storm modes may cap the upper magnitude of rainfall totals. With
that said, if convection is able to develop this afternoon ahead
of the stronger forcing to the north...the approach of the forcing
could develop enough upstream convection to enhance cell merger
potential. Also, if convection organizes into a well defined MCS
over SD/MN/northern IA and/or in the low level jet cycle over
eastern KS into MO, then a slightly greater flash flood risk could
also evolve. The 12Z hi-res models are indicative of all three
scenarios coming to fruition. In particular, the trend has been
toward greater coverage of locally heavy rainfall expected across
KS/MO overnight, which would overlap with saturated ground, a high
percentage of surface runoff, and resultant flash flooding. Thus,
we expanded the Slight Risk area southward out of Iowa.


...Florida into the coastal Southeast...
With a trough stretched across northern Florida and straddling the
Carolina coast, most convergence will persist just offshore, but
there will be periods of slow moving convection inland. This is
true especially of northern Florida through early afternoon, and
in the wake of a small meso-vortex over GA/SC through mid
afternoon. Downstream forcing will increase through the day and
overnight period into SC/NC.

Expect we will see another wave move northeast along the coast
today, which should help focus/organize convection. While the
heaviest totals may remain offshore, recent trends suggest that at
least some heavier convection should make it along at least the
immediate coastal areas. Looks like a pretty widespread 1-3" will
be common along the coast, with localized totals approaching 6" a
possibility. This is pretty similar in nature to what occurred
yesterday, although the coverage of showers/storms could be a bit
better today. Given the rain that fell yesterday combined with the
forecast rain today...think the flood threat will nudge upward a
tad today. Will probably still not be that widespread given the
nature of the soil across most of this region...but any more
susceptible urban locations could see some issues arise.

...PA/NY into northern New England...
Convection is likely this afternoon and evening from portions of
eastern PA into NY and central/northern New England. Will have a
surface trough across the region, along with persistent
southwesterly low level inflow/moisture transport. Synoptic
forcing is pretty good as well, with a mid level wave and a rather
impressive diffluent upper jet signature moving overhead as well.
Thus assuming we can destabilize enough, should have no problem
getting at least scattered convective development. The
aforementioned low level trough and persistent southwesterly
inflow may support some repeat upstream convective
development...which could result in a few instances of flash
flooding.

Chenard/Burke 


Day 2
Valid 12Z Sun Aug 18 2019 - 12Z Mon Aug 19 2019

...THERE IS A MARGINAL RISK ACROSS PORTIONS OF THE DEEP
SOUTH...THE EASTERN KANSAS AND MISSOURI...AS WELL AS THE OHIO
VALLEY INTO THE NORTHEAST...


...Central and Eastern Gulf Coast to Southern Mid-Atlantic Coast...
Tropical moisture will remain near and east of a weakening frontal
boundary extending across the Southeast into the Gulf Coast states
with precipitable water values approaching and in excess of two
inches. Aloft, a low-mid level shear axis will extend southwest of
an exiting mid-level vorticity max off of the East Coat into the
Carolinas to the Florida Peninsula. By Monday morning, there is
overall model agreement for a low-mid level low to set up from
Georgia to the Florida Panhandle with southerly flow focused into
northwestern Florida. Aloft, flow will be diffluent across the
Gulf Coast into the Southeast but this pattern relaxes into Monday
morning.

Portions of the western and northwestern Florida Peninsula have
experienced 200 to 600 percent of normal rainfall over the past 2
weeks (via AHPS), with some above average rainfall scattered
northeastward along the Southeast Coast. Onshore 850 mb flow will
be in excess of the relatively weak deeper layer mean wind
supporting the potential for slow moving and training of storms.

The 12Z hi-res guidance has 4-8 inch maxima over the 24 hour
period ending Monday morning along the Florida Panhandle to the
east of the low to mid-level low that forms Sunday night. While
there remains a bit of disagreement with where this circulation
sets up, confidence in the heaviest rain is highest along coastal
locations and just inland to the east of the low to mid-level
center.

Farther north along the coast into the eastern Carolinas, heavy
rain should be ongoing early Sunday morning near an exiting low to
mid-level vorticity max. This is where the greatest threat of
flash flooding will be focused, but high moisture will remain
along the remainder of the Southeast coast with daytime heating
supporting scattered convection from afternoon into evening which
may pose localized flash flood concerns.

...Eastern Kansas/Oklahoma into the Northeast...
A fast moving shortwave to the south of a closed low in
south-central Canada will be preceded by a cold front at the
surface moving through the Great Lakes but stalling into the
central Plains. Precipitable water values along this boundary will
remain high (roughly 1.5 to 2.0 inches). Remnant MCS activity is
forecast to be ongoing across portions of IA/WI/IL at the start of
the period (12Z Sunday) with potentially a second complex farther
south into MO/KS. Remnant outflows from these potential complexes
leads to considerable uncertainty with convective evolution later
in the period. Forecast Corfidi Vectors show backward propagation
from portions of Missouri into the Ohio Valley where CAPE will be
in excess of 1000 J/kg, sufficient to achieve rainfall rates of
1-2 in/hr when coupled with the moisture that will be present.

Shortly after 00Z, the passage of a shortwave trough aloft over MO
will lead to northwest flow aloft and lowering precipitable water
values with westerly low level flow. However, moisture and
instability will remain great enough to support potential for
heavy rain with a model signal for a convective complex in the
vicinity of the MO/KS/AR/OK borders. Farther south, convective
inhibition appears to limit convective potential near and beyond
00Z Monday into southern Oklahoma into Arkansas.

Model QPF spread was fairly high across the region which lowers
confidence given what happens Sunday afternoon/evening hinges upon
the placement of previous convection and mesoscale boundaries. The
Marginal Risk was extended across the Midwest to account for this
model uncertainty.

Otto

Day 3
Valid 12Z Mon Aug 19 2019 - 12Z Tue Aug 20 2019

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE CENTRAL GULF COAST TO THE SOUTHEASTERN U.S...

A low to mid-level circulation looks to remain near FL/GA/AL from
Monday into Tuesday morning with a weak shear axis extending
northeastward 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 northwestern Florida area has
seen well above average rainfall over the past 1-2 weeks, along
with pockets along the central to eastern Gulf Coast.

The signal for heavy rain is present in the models with the NAM
and the global guidance support of 3 to 5 inches, but with the
average location just along the coast from Louisiana to
northwestern Florida and offshore. There remains uncertainty with
where the heaviest rainfall will occur and the threat appears
slightly lower for the Monday into Tuesday time frame compared to
the previous 24 hours, especially as upper level jet flow weakens.
However, given the wet antecedent conditions and deep moisture
remaining across the region, a Marginal Risk remains warranted.

Otto


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