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WPC Day 2 Excessive Rainfall Outlook
Risk of 1 to 6 hour rainfall exceeding flash flood guidance at a point
Updated: 2008 UTC Sat Jul 4, 2020
Valid: 12 UTC Jul 05, 2020 - 12 UTC Jul 06, 2020
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
412 PM EDT Sat Jul 04 2020
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.
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt