Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
810 PM EDT Sat Jul 04 2020
Valid 01Z Sun Jul 05 2020 - 12Z Sun Jul 05 2020
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE EASTERN
DAKOTAS INTO WESTERN MINNESOTA...
...Northern Plains / Upper Mississippi Valley...
The progression of shortwave troughs through west to southwesterly
upper flow should produce clusters of strong thunderstorms,
including heavy rain potential, from the eastern Dakotas into
Minnesota. There are plenty of ingredients in place,with a surface
trough in southeast ND and moist low level inflow with dewpoints
in the upper 60s to lower 70s to fuel cell development. A 700 mb
cyclonic vorticity lobe should spur more storms in northern ND
tonight. Rains combined with longer term wetness/antecedent soil
moisture lead to a continued broad Slight Risk area.
Additional clusters of storms moving east from Montana should move
into North Dakota later tonight. 12Z CAM consensus took the
moderate instability coupled with PW values around 1.5 standard
deviations from climatology will continue to support local 1.50
inch per hour rain rates.
...Eastern Colorado to Oklahoma/Texas panhandles and adjacent
northeast New Mexico...
Storms will once again form in the south-central Rockies and
spread onto the adjacent High Plains, drifting east and south. The
environment is favorable for heavy rain with PW values over 1.25
inches into eastern Colorado (2 standard deviations above normal),
and predicted cell motions of only 5 to 10 knots. Although lacking
large scale organization, the typical cell mergers and mesoscale
accidents could lead to flash flooding, so the Marginal Risk was
maintained with small expansions southwest in NM and over the TX
Panhandle based on 12Z CAM consensus.
...Eastern Oklahoma to western Arkansas and southern Mississippi
to southeast Louisiana...
Marginal Risks remain for areas of greater expected
shower/thunderstorms activity associated with lobe of 700 mb
cyclonic vorticity in the Southern Plains and along the Gulf
Coast. The low level convergence occurring along the central Gulf
Coast with the focus on southern MS and southeast Louisiana this
evening will allow a few more storms to develop before subsiding
later tonight. With generally weak inflow, however, storms should
exhaust the instability supply or propagate toward nearby areas,
thus limiting the overall rainfall totals.
Valid 12Z Sun Jul 05 2020 - 12Z Mon Jul 06 2020
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL PERSISTING OVER
THE EASTERN DAKOTAS AND FAR NORTHWEST MINNESOTA ON SUNDAY...
...Northern Plains/Upper Mississippi Valley...
The alignment of upper-level flow/lift and low-mid level moisture
over the Dakotas and Minnesota continue to favor organized
convection. Several of the models are hinting that the heaviest
amounts will occur near the border of Minnesota and eastern
North/South Dakota- a couple of the hi-res models show a a few
hours where the hourly rates may reach upwards of 1 to 1.75
inches/hour across this area. The Slight Risk that was in place is
still warranted and very minimal adjustments were made.
Previous Discussion... The area will remain located between an
upper level ridge over the central part of the country and low
pressure over parts of Manitoba and Saskatchewan. This leaves
broad west to southwest flow aloft across the region that has
multiple weak and difficult to time shortwaves embedded within
that flow. Those items, combined with a quasi-stationary front at
the surface and residual outflow boundaries from ongoing/previous
convection will play a role in where convection will continue to
develop. There will be plenty of moisture across the region with
precipitable water values between 1.6 inches and 2 inches by
Sunday afternoon...with the maximum values approximately 2
standardized anomalies above climatology focused over a region
with lowest flash flood guidance. Considering the on-going
chances for scattered convection impacting the same areas before
the Day 2 outlook period begins on Sunday morning, the Slight Risk
area remains in place with only modest adjustments needed based on
the 04/00Z suite of numerical guidance.
20Z update... PW values near 2 inches will be pooled over much of
the Gulf coast and areas north during this time; which will aid in
developing/sustaining widespread convection through this period.
Most of the guidance has the bulk of the QPF aligned from
Louisiana to Georgia with the highest amounts focused near the
Mississippi River delta and across the Florida panhandle. The EC
continued to be outlier with its much heavier qpf bullseye over
West/Northwest Louisiana as indicted from the previous discussion,
therefore minimal use of this solution was use for this forecast.
The Marginal Risk area that was inherited only requirement minimal
adjustment to reflect the latest WPC QPF.
Previous discussion... Models continue to show broad upper level
forcing over a very moist and unstable airmass along and the Gulf
Coast region...with greatest coverage of precipitation and
greatest chances for intense downpours looks to be from the
afternoon through the evening across the area. Spaghetti plots of
2 inch per 6 hour rainfall amounts from high resolution guidance
favored the area along the Gulf coast on Sunday morning which
gradually lifts north throughout the day. Other than the 04/00Z
ECMWF, the models tended to agree in principle on the northward
shift from the Gulf but certainly offered a variety of placement
and amounts. The 04/00Z ECMWF backed off its maximum QPF amounts
associated with a well-defined MCV dropping southward across
Louisiana during the day but remained about the only piece of
guidance with an emphasis on that solution.
...Western High Plains...
There continues to be a signal for heavy rain shower activity,
however the spread remains moderate/significant in regards to the
the exact location and intensity. Maintained the Marginal risk
that was in effect for this region, somewhere in this area will
likely have isolated flash flooding.
Previous discussion... Maintained the Marginal Risk area over
parts of the Western High Plains from southwest Kansas into the
Oklahoma/Texas panhandle region given a continued multi-model
signal for locally heavy rainfall to develop Sunday evening/early
Monday morning. Model QPF values have come down a bit and Flash
Flood Guidance values are at or above 2.5 inches per hour...but
the area is near the western end of a stationary front that
provides focus for rainfall along the Gulf and near a lee-side
trough. The Marginal Risk area was focused near the proximity of
the best moisture transport and the boundaries. Spaghetti plots
from the SREF show the ARW members focusing some potential for 2
inch QPF per 24 hours in southeast Colorado while the GEFS has a
few members farther south.
Valid 12Z Mon Jul 06 2020 - 12Z Tue Jul 07 2020
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PARTS OF
THE UPPER MIDWEST AND ACROSS THE SOUTHEAST U.S. ON MONDAY...
...Minnesota/Wisconsin to Central South Dakota...
20Z update... Additional rainfall is expected over an area where
recent rain has significantly lowered flash flood guidance. With
heavy rainfall possible leading up to time time period and more
over much of the same general area, the risk for flash flooding
will remain elevated. Little to no changes were made to the
Excessive Rainfall Outlook area.
Previous discussion... The best forcing for heavy rainfall in the
Upper Midwest/Upper Mississippi Valley will gradually be shifting
away from the area that has been the focus over the past few days.
A cold front initially extending from the Great Lakes Region into
Minnesota and the Dakotas will be the focus for thunderstorms
capable of producing heavy rainfall as it progresses southward on
Monday. Precipitable water values at or above 1.5 inches will be
in place ahead of the front and southerly winds of 20 kts or so at
850 mb will focus a region of moisture flux convergence along the
front. Flash Flood Guidance values are on the order of 1.5 to 2
inches per one and three hour periods with lowest values in the
urbanized areas and along the Buffalo Ridge. Rainfall rates may
approach or locally exceed 1.5 inches leading to the possibility
of flash flooding...especially if there are any cell mergers or
training where the forward motion of the front slows.
20Z update... The latest guidance continues to have a very
favorable environment strong, organized showers and thunderstorms
across the Gulf Coast and the Southeast states. Some of the models
are showing the higher amounts along/near the higher terrain of
the Southern Appalachians. Broad forcing for ascent in the
mid-to-upper levels along with deep moisture and orographic
enhancements, 1 to 2 inches of QPF may very well focus over these
areas with isolated higher amounts possible. However, the exact
placement of the highest QPF still is a bit uncertain at this
time. Minimal adjustments were made to the Marginal risk to
reflect the latest WPC QPF.
Previous discussion... The same broad area of forcing will be
lingering across the Southeastern United States during the period.
Some locally intense downpours are expected given the abundant
moisture in place and the expected 1500 to 3500 J per kg of
most-unstable CAPE that are expected by late afternoon and
evening...but the placement and timing of individual shortwaves
that will help initiate the convection and any outflow boundaries
that focus additional convection are nearly impossible to forecast
with any confidence at this point. The 04/00Z ECMWF continues to
favor some 3 to 6 inch rainfall amounts in 24 hours in Louisiana
associated with an old MCV...but it has little support from other
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