Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
429 AM EDT Thu Jun 20 2019
Valid 12Z Thu Jun 20 2019 - 12Z Fri Jun 21 2019
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE NORTHERN OHIO RIVER VALLEY...
...OH Valley into the northern Mid Atlantic...
Areas of convection are moving eastward across portions of KY and
OH this evening. This activity should continue to shift eastward,
and while some decrease in intensity is possible with the loss of
daytime heating, still expect some locally heavy rains to survive
into portions of WV and western PA with this activity. Pockets of
flash flooding will remain possible, especially considering the
soil saturation levels and lower flash flood guidance. The
greatest focus of flash flooding over the next few hours should
continue to be portions of eastern OH into far northwest WV and
far southwest PA...where the eastward moving convective line is
interacting with slow moving cells along a west to east oriented
Likely to see another area of showers and embedded heavier
convective cores closer to the low...which will move from central
IL into northern IN and southern MI overnight. A flash flood
threat likely continues here into the overnight hours as
well...especially where any heavier convective cores are able to
persist for an hour or two.
Further east, isolated to scattered convection continues over
portions of PA, NJ and southern NY. These cells continue to be
efficient rainfall producers, and are exhibiting slow enough cell
motions to result in pockets of flash flooding. This activity
should generally be on a downward trend as we begin to lose
instability...but some flash flood threat will continue at least
through the evening hours.
...Northeast TX into AR...
Convective development late this afternoon has been explosive in
nature over northeast TX, far southeast OK and southwest AR. This
is not surprising given the extreme CAPE in place. Individual cell
motions have been rather quick...however will continue to see some
backbuilding of activity on the southwestern flank of the complex.
A nearly stationary axis of increasing 850mb moisture convergence,
and persistent synoptic support in the form of a mid level wave
and upper level jet streak...will support the continued
backbuilding. Eventually this complex will forward propagate off
to the southeast, but should continue to exhibit some backbuilding
component through around 06z. Thus there should be enough of a
period of backbuilding/training for an area of rainfall magnitudes
potentially exceeding 5" on a localized scale. Would expect some
flash flood threat to develop over northeast TX, extending into
far southwest AR and far northwest LA. By the time convection
moves east of these areas it should become progressive enough that
the flash flood threat should be on the decline.
Given the progression of the aforementioned mid/upper
forcing...seems plausible that this activity expands quickly
northeastward overnight, potentially moving back into portions of
southern KY. Given antecedent rainfall here, possible this results
in another flash flood threat by later tonight across these areas.
Convection continues to form along a stationary front across
eastern MT. Slow cell motions are resulting in locally heavy
rainfall, and thus a localized flash flood risk likely exists
through the evening hours over northeast MT. As the better
mid/upper level forcing moves east, convection should also expand
into portions of western ND/SD. Locally heavy rain is also likely
here, with amounts locally exceeding 2" likely.
Valid 12Z Fri Jun 21 2019 - 12Z Sat Jun 22 2019
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE UPPER MISSISSIPPI VALLEY INTO THE WESTERN OHIO VALLEY...
...Northern Plains/Upper Mississippi Valley into the western Ohio
Moisture and instability will be increasing across the central
U.S. ahead of an approaching shortwave and frontal boundary.
Southerly winds of 20 to 35 knots will transport precipitable
water values of 1.25 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 heavy rainfall and high rainfall rates.
The highest QPF will be focused over from southern Minnesota to
western Indiana, where part of the front will be slow, or
quasi-stationary. Convection training along the front could result
in local rainfall amounts exceeding 2.50 inches. Three hour flash
flood guidance values are as low as 1.50 inches over portions of
far southern Minnesota into Iowa and portions of Illinois. These
amounts may be exceeded with training occurs. A Slight Risk was
refined over this corridor. A broad Marginal Risk spans from
Montana to northern Tennessee to account for some uncertainty.
A low pressure system will be exiting New England early in the
period. Prior to pulling offshore, deep moisture and marginal
instability will be aligned with the boundary which will be
conducive for heavy rainfall, possibly locally excessive. Heavy
rainfall during day 1 across the state may pre-condition the soils
to being near saturation with reduced FFG. Areal averages upwards
of 1.50 inches in a short time may be problematic and result in
flooding across Downeast Maine, therefore the Marginal Risk was
maintained to cover the threat.
A -2 standard deviation closed mid-level low will track across the
Northern Rockies during day 2. A low pressure system will spin up
and send a cold front through the Northern and Central plains.
Steepening lapse rates associated with the strong mid level system
could result in marginal instability here, and the combination of
moisture and instability could support at least scattered
convection with heavy rainfall. With the mid level system passing
overhead, storm motions could drop to less than 10 knots, and
become chaotic over portions of western MT. Slow cell motions
could foster cell mergers or short term training where organized
convection develops. This could support a low end flash flood
threat over portions of western and central MT, where three hour
flash flood guidance values are as low as 1.00 inches. It should
be noted that snow levels drop to between 6000/7000 feet, so some
of the QPF could fall in the form of snow. Based on this, a
Marginal Risk was placed over much of western and central MT for
Day 3, mainly for the lower terrain.
The Day 3 outlook will be updated by 0830Z.
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