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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: 2016 UTC Sun Sep 20, 2020
Valid: 12 UTC Sep 21, 2020 - 12 UTC Sep 22, 2020
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
423 PM EDT Sun Sep 20 2020
Valid 12Z Mon Sep 21 2020 - 12Z Tue Sep 22 2020
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS THE
MIDDLE AND UPPER TEXAS COAST...
...2130 UTC Excessive Rainfall Outlook Discussion...
Given a fairly consistent track for Beta from NHC, the
modifications needed to the QPF were fairly minor early in the Day
2 Excessive Rainfall Outlook as the storm slowly approaches the
coast. By 22/00Z, the storm was forecast to be right along the
Middle Texas Coast when it slows it westward motion and begins a
turn towards the northeast. The resulting slow storm motion
leaves the Upper Texas coast in peristent onshore flow and
rainbands. At this point, still think a Moderate Risk is
appropriate given the combined rainfall forecast for Day 1 and Day
2. One big concern is a burst of convection on the west side of
Beta during Sunday afternoon...which may ultimately result in a
discontinuious jump in the center of Beta and what implications
this has on the track/timing of the system. It may require
significant shift in QPF with a corresponding shift in the concern
for excessive rainfall.
...0830 UTC Excessive Rainfall Outlook Discussion...
The main flash flood threat remains tied directly to Beta, and the
WPC QPF and Excessive Rainfall Forecast were based on the most
recent NHC track, which is close to the 00z model suite consensus.
There is better consensus on a landfall for the system along the
Middle TX coast later in the period. Along and ahead of the
system, banded convection affects the Mid and Upper TX coast,
producing hourly rainfall rates of 2.00/3.00 inches possible
(within 2.25/2.50 inch precipitable water moisture plume). Banding
comes ashore across Upper TX coast into coastal southwest LA
later, as the low level inflow lags the center to some degree.
Much of the 00z model guidance (including high resolution
guidance) indicated a general 4.00/8.00 inch rainfall near the
landfall location, with the 00z NAM CONUS Nest showing local 10.00
inch amounts. These higher end amounts are possible where training
occurs as Beta begins to slow.
There was a near unanimous signal from the 00z guidance suite that
the moisture from Beta gets inland, and begins to slowly erode, as
drier air starts to become involved with Beta from the north and
west, particularly after 22/00z. Beta is expected to feel the
influence of a mid and upper level trough crossing the Southern
Plains, which is expected to turn Beta to the northeast and east
during Day 2. This turn could limit the northern extent of the
higher rainfall amounts.
With much of the guidance showing more of an easterly landfall
position, the highest rainfall amounts with Beta were spread
further along the Mid and Upper TX coast. Based on that, and the
expected turn with Beta, a Moderate Risk was extended across much
of this area (which was collaborated with WFOs CRP/HGX). Should
there be track changes further up the TX coast, the Moderate Risk
could move a bit to accommodate.
Further north across north TX, there could be an enhanced area of
rainfall as moisture in the deep easterly flow north of Beta
interacts with lift in the right entrance region of a 50 knot jet
streak tracking across the Southern Plains. Model soundings show
almost no instability in the column, and it appears as though
frontogenetic forcing in the absence of elevated instability
becomes the main driver of banded heavy rainfall. A Slight Risk
was considered for this area, as some of the high resolution
guidance showed 4.00+ inch rainfall amounts. Since instability is
expected to be lacking, the area was included in a larger Marginal
Risk (which was collaborated with WFO FWD).
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt