Day 2
Valid 12Z Sat Aug 29 2020 - 12Z Sun Aug 30 2020

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS THE
SOUTHERN AND CENTRAL APPALACHIANS AND PARTS OF THE
CENTRAL/SOUTHERN PLAINS...

...Southern and Central Appalachians/Mid Atlantic/Northeast...
Laura, or anything that remains of Laura, will be moving to the
east across the Central and Southern Appalachians and adjacent
portion of the Mid-Atlantic region as it gets caught up in fast
mid-level westerly flow across the region. In addition, rainfall
should becoming more widespread farther north into parts of the
Northeast U.S. as some of the moisture gets drawn northward in
response to the approach of a wave in the northern stream. With
precipitable water values at or above 2 inches becoming fairly
widespread from the DELMARVA northward to southern New England,
maintained the previously issued Marginal Risk area largely in
tact given the potential for some downpours in such a
moisture-laden atmosphere. However, the system should be moving
steadily due to the lack of phasing between the systems which
should work to prevent much in the way of widespread flooding
problems. The one area still worthy of a Slight Risk is in the
upslope region of the Appalachians--mainly from parts of West
Virginia southward into western Virginia and along the
Tennessee/North Carolina border where upslope flow in the terrain
has the potential to focus and locally enhance rainfall amounts
after 12Z Sunday.

...Central and Southern Plains/Lower Mississippi Valley...
There is improving agreement in the models that moisture and
instability will be focusing on a frontal boundary extending KS/OK
into AR. This should help force convection capable of producing
heavy to locally excessive rainfall from late Saturday into the
early morning hours of Sunday. The NAM is most aggressive and may
be producing too much heavy rainfall over a broader area than will
verify, but it certainly is possible that increasing low level jet
will result in cells that could produce excessive rainfall.
Expanded the Marginal Risk and introduced a Slight Risk. In
addition to the signals from the operational runs, a few of the
spaghetti plots of 2 and 3 inch amounts show enough members to
also support a Slight Risk. And with Flash Flood Guidance being on
the order of 2.5 inches per 3 hours, felt confident enough to
upgrade.

...Colorado/New Mexico...
Few changes were needed to the on-going Marginal Risk area based
on 00Z guidance. Similar to the mid-section of the country,
models are fairly well aligned with development across the higher
terrain and moving east into the Plains. As of now, have not seen
enough of QPF in the Day 1 period to think there will be enough
QPF to pre-condition the soil for an upgrade...so no upgrade was
made in this cycle.

....Arizona...
Introduced a Marginal Risk area over portions of Arizona as
moisture moves northward out of Mexico. This will result in
precipitable water values increasing to 1.5 to 1.75 inches across
the southwestern part of the state. There has been an increase in
the QPF generated by the models, which looks reasonable. Forecast
soundings in the low level do show some low RH values in the
sub-cloud layer, so not sure how much rain will actually reach the
ground...but it does open the possibility of some briefly heavy
rainfall rates which could lead to some problems (especially if
they occur over a burn-scar).

Bann

Day 3
Valid 12Z Sun Aug 30 2020 - 12Z Mon Aug 31 2020
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE CENTRAL AND NORTHERN PLAINS

......Central Plains...
Carried a Slight Risk area a bit downstream of the Slight Risk
area from Day 2. Thinking is that convection should be on-going
across parts of Arkansas due to the flow of moist and unstable air
interacting with a low level boundary draped across the area.
Precipitable waters in excess of 2 inches are shown by the NCEP
guidance and the ECMWF...with the NCEP guidance also showing CAPE
values running at or slightly above 2000 J per kg. Convection
should make its way southeastward, perhaps reaching the Tennessee
Valley before weakening at the time of convective
minimum...followed by the redevelopment of convection capable of
producing additional heavy rainfall in southern Missouri or parts
of Arkansas. Expanded the Slight Risk southward into Arkansas
despite less QPF thinking that the area has been left more prone
to flooding by the recent rainfall from Laura

...Northern Plains...
A shortwave trough rotating around a mid- and upper-level low over
Canada will sweep across the Northern Plains late Sunday or early
Monday accompanied by a cold front at the surface. Winds at 850
mb accelerate to between 30 and 40 knots immediately ahead of the
cold front which will help focus convection capable of producing
locally heavy rainfall. Precipitable water values are currently
forecast to increase to 1.5 to 1.65 inches, which is roughly 2
standardized anomalies above climatology for the time of year, so
isolated rainfall rates of 1.5 inches per hour are possible...with
lower rainfall rates elsewhere. Opted to place a Slight Risk area
in the northeast corner of North Dakota and the northwest corner
of Minnesota where Flash Flood Guidance is only about 1 inch per
hour due to very wet conditions in the previous couple of months.
The front and associated convection should be very progressive so
felt no more than a Slight Risk was necessary at this time.

...Southwest U.S....
Models show the arrive of precipitable water values of 2 inches
moving north of the international border region of southwest
Arizona by late Sunday afternoon, with 1.5 inch values extending
from the California desert to western and southwest Arizona. The
is weak mid-level ridging across the area but there should be
enough moisture and instability for convection to develop. With
late-day CAPE values exceeding 2000 J per kg in such a
moisture-rich atmosphere, some locally heavy rainfall amounts are
possible. As was the case in day 2, these rainfall amounts could
result in flash flooding...especially if they occur over
burn-scars.

Bann

Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt


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