Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
357 PM EDT Wed Jun 19 2019
Valid 1944Z Wed Jun 19 2019 - 12Z Thu Jun 20 2019
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE NORTHERN OHIO RIVER VALLEY...
20z Update: Made some tweaks to the outlook based on latest
radar/model trends. Biggest adjustment was to expand the Slight
Risk across much of PA, where an expanding coverage of efficient
rainfall producing cells should result in at least localized flash
flood issues through the afternoon hours.
...Mid Mississippi and Ohio Valleys...
Shortwave trough over central KS with an embedded MCV crossing
from IA into IL combine to provide larger scale forcing in
combination with right entrance region of a 100 knot jet across
the northern Great Lakes. Low- to mid-level confluence zone of
return warm conveyor belt out of the Gulf will continue to supply
ample moisture and instability across the the Mid-Mississippi into
the Northern Ohio River Valley today and tonight. Global scale
deterministic guidance suggests greater QPF across this axis given
stronger synoptic forcing. Yet, southerly upglide through the
warm conveyor along with remnant boundaries from ongoing
convection and sufficient heating in the warm sector likely will
lead to greater convective coverage farther south of this
axis...more in line with Hi-Res CAMs. Convection total across KS
was shifted a bit south than originally forecast, and so the
resultant trend continues to shift southward a shade with the
00Z/06Z guidance. This places the large area of flooded/saturated
ground conditions across S IL/S IND/Central and Southern OH where
AHPS 7 day totals are running 300%+ above average with numerous
rivers/stream running above average as well per National Water
Model. 850mb FGEN zone remains a bit north of these saturated
soils, in coordination with local forecast offices. As a result,
there was little need to make significant changes to the Moderate
Risk area or the surrounding Slight Risk area.
...Mid to Lower Mississippi River Valley to Red River Valley...
A Marginal Risk area and a Slight Risk area of Excessive Rainfall
was mantained over the MS/OH/TN River Confluence into NE Texas to
account for the next round of convection that is anticipated later
today, this evening and the overnight hours due to considering
that lingering moisture/instability. An initial convective
complex will weaken as it progress east into the MS/Mid-TN valley
through the early afternoon. Strong redevelopment is expected
along the western boundary of the deeper moisture/instability
gradient across E OK/NE TX, likely intersecting with rain foot of
this morning's thunderstorms. The latest CAMs from this morning
largely maintained continuity from the 19/00Z CAMs in terms of QPF
amounts and areal placement. It roughly coincides with the axis
of greatest instability shown by the globals models...LIs of -8 to
-12 shown by the 19/06Z GFS...so confidence has nudged up
considering that a weak shortwave/inflection in mid-level flow at
the left exit of strongly curved anti-cyclonic subtropical jet
appears to be the culprit for slower cell motions and training
orientation. QPF signals of 4-6" are starting to come into
agreement, but precise latitude differences persists. Made some
minor modifications to the Slight Risk and Marginal Risk areas but
the previous forecast reasoning still stands.
A subtle piece of shortwave energy ejected from broad closed low
in SW Canada will slide across eastern MT and the western Dakotas
into a shear axis of an of Omega Block along the northern
tier/south-central Prairies of Canada. Lingering moisture will be
reinforced by weak 20-30 kt LLJ this morning and draws 1-1.2"
Total PWats into the shear axis. Ongoing convection will be slow
moving, along southeast interface of the shortwave center, with
persistent inflow to support regeneration/mergers. Lower FFG
values/more susceptible ground conditions from Morton to Surry
county intersects the highest QPF signal. WPC maintained a small
Slight Risk area of Excessive Rainfall along this axis surrounded
by a Marginal Risk area extending back west along the MT/WY border
where additional convergence may spark convection along the
shortwave trough axis throughout the afternoon.
...Delaware River Valley...
Another day with potential for thunderstorms capable of excessive
rainfall, however, forcing is a bit less well defined as the area
is between larger scale shortwave features. Still the connecting
boundary remains in the vicinity and with lingering moisture and
sufficient day time heating, another round seems likely. Cells
that do develop with have weaker inflow and steering currents to
allow for slow cell motions. Saw little reason to change the
previously-issued Slight Risk area in the Delaware River Valley.
Valid 12Z Thu Jun 20 2019 - 12Z Fri Jun 21 2019
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE OHIO VALLEY, MID ATLANTIC AND NORTHEAST...
...Southern Appalachians, Tennessee and Ohio Valleys into the
A surface low will lift north and east along a frontal boundary
from the Tennessee and Ohio Valleys into the Northeast.
An additional low will spin up along the frontal boundary across
the Ohio Valley as shortwave energy tracks eastward, then lift the
frontal boundary through the Northeast. Low level convergence
along the front will focus moisture and instability that supports
convection producing excessive rainfall. The last couple of model
cycles have shown variability latitudinally with the QPF axis from
eastern Ohio Valley to New England. A compromise was found by
utilizing a blend of the 12Z ECWMF/00Z GFS and the in-house
bias-corrected ensemble mean; which resulted in areal averages of
1 to 2.50 inches from northeast Ohio to western Maine.
With moisture and lift expected to be in place, instability
becomes the crucial factor for the placement of the heaviest
rainfall along and ahead of the front. Cell motions could drop
below 10 knots just ahead of the surface low, which could lead to
cell mergers or short term training, amplifying the flash flood
threat. The FFG are the lowest from northeast Ohio to
south-central New York with values as low as 0.50 inch. With
forecast values of 1 to 2.5 inches possible the threat for
flooding will remain elevated across this region through day 3. A
Slight Risk area is in effect eastern Ohio/West Virginia to
...Upper Mississippi Valley into the Central Plains...
A frontal boundary extending from the Upper Mississippi Valley to
the Central Plains will have synoptic scale lift as ejected
shortwave energy, ejected from the deepening mid/upper level low
over the Intermountain West, crosses the Northern Plains.
Precipitable water, with values of 1 to 1.25 inches that is
roughly +2 deviations above the mean, will be streaming northward
prior to, and during the day 2 period. This moisture will pool
over the north-central states ahead of the approaching system.
Model guidance is depicting a swath of 1000-3000 J/kg CAPE
extending from Central Plains into Minnesota and the eastern
Dakotas along the same axis of the moisture stream early in the
period; with 2000-5000 J/kg focusing over Nebraska/Iowa and areas
south during the latter portions of day 2.
Dry air in the mid levels as the convection initiates suggests
that it should be hail/outflow driven at first. However, as the
850-300 mb flow becomes better aligned with the propagation
vectors, the threat of training of the convective clusters
increases, especially over eastern Nebraska into western Iowa. It
has been fairly dry recently in these areas, so three hour flash
flood guidance values are fairly high. Across the eastern Dakotas
into western/northern Minnesota, instability is less than further
south, which could impact the extent of convection here. Three
hour flash flood guidance values here are as low as 1.50 inches,
so training convection here could pose a flash flood threat.
Although, the extent of the convection is still unclear.
Maintained the Marginal Risk with some refinement for adjustments
in the QPF.
Valid 12Z Fri Jun 21 2019 - 12Z Sat Jun 22 2019
...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL AND FLASH
FLOODING OVER PORTIONS OF THE UPPER MISSISSIPPI VALLEY AND WESTERN
...Upper Mississippi Valley, Central Plains and Midwest...
Another shortwave will eject out of the Rockies sending a cold
front through the Northern Plains and into the Mississippi Valley.
This system looks to be stronger and more organized than the
previous shortwave, with a more robust moisture flux into the
central U.S. ahead of the front. Southerly winds of 20 to 35 knots
will transport precipitable water values of 1.5 to 1.75 inches
into the Plains and Mississippi Valley. Strong moisture
convergence and instability (2000-5000 J/kg CAPE) along the
frontal boundary will support widespread convection that will be
capable of producing efficient rainfall and high rainfall rates.
Again, the models are struggling on the timing and placement for
the higher QPF; however, they seem to focus the highest amounts
over much of Iowa, southern Minnesota and northern Illinois. WPC
QPF has areal averages of 1 to 1.75 inches over Upper/Middle
Mississippi Valley. Given the spread, confidence of the amounts is
fair at best. Given that this area remains highly sensitive to
additional rainfall with many rivers remaining above flood stage
and FFG of 1 to 2+ inches, a Slight Risk was introduced for
southern Minnesota, northern Missouri/Illinois and western
Indiana. A broad Marginal Risk spans from the Dakotas/Minnesota to
Kentucky to account for some uncertainty.
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