Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
420 PM EDT Thu May 20 2021
Day 1
Valid 2019Z Thu May 20 2021 - 12Z Fri May 21 2021
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER
SOUTH-CENTRAL LOUISIANA...
20z Update: Main adjustment with this update was to expand the
Marginal risk northward across central IA, into southeast MN and
northern WI. A wave currently over eastern KS will track north
northeast across this area into tonight bringing with it an area
of showers and embedded heavier convective cells. Instability is
pretty weak which will likely inhibit organized deep convection.
However enough instability and deep moisture profiles to support
shallower convective elements that will likely be pretty efficient
rainfall producers given PWs approaching late May max levels and
warm rain processes. Most areas will peak in the 1-2" range...but
localized swaths of 2-4" are possible. Soil saturation levels are
below average across this region...likely an inhibiting factor for
flash flooding...nonetheless, some localized issues are possible
across any more susceptible urban or low lying areas.
Chenard
...Previous Discussion...
...Western Gulf Coast into Eastern Kansas/Western Missouri...
Large scale flow continues to show anomalous ridging over the
eastern U.S. and a low amplitude trough axis over the southern
Plains. A narrow corridor with precipitable water values of 1.5-2"
will continue to be drawn northward from the Gulf of Mexico,
across Louisiana and into the Midwest. Recent surface
observations show a surface low offshore Galveston with what
appears to be a baroclinic zone somewhat offshore the western Gulf
Coast. The magnitude of the low level flow is greater than the
deep-layer mean wind which would allow for training and repeating
of storms from south to north and high rainfall efficiency.
Short term trends in satellite and radar imagery continue to show
the potential for development of an axis of training heavy rain
over southeast Louisiana, but expectations are that a new
thunderstorm band could fill in to its west in an area of
insolation/cloud thinning and sunshine from Marsh Island northwest
across western LA within an area that should be destabilizing.
Hourly rain totals of 2-3" would be possible where training
develops. Southeasterly 850 mb wind speeds of 30-40 kt are
expected to continue through the day today and into Friday
morning, but forecasts of instability show decreasing values into
early Friday morning. Nonetheless, the potential for locally heavy
rain will be in place for the whole 24 hour period, but with
greater probabilities through sunset. The west-central Gulf Coast
has been extremely wet over the past 4, 7, 30, and 60 days which
has left many areas with saturated soils, increasing
susceptibility to flash flooding. The 12Z HREF supports a 50%+
percent chance of 5+ inches of rain in and near the Atchafalaya
ending 12Z Friday.
Farther north, unidirectional southerly flow will be in place from
the Texas/Louisiana border into eastern Kansas and western
Missouri, with potential for south-north axes of training.
Portions of these areas have been wetter than average over the
past 1-2 weeks which has lowered flash flood guidance values to 2
inches or less in 3 hours. The best timing for flash flooding will
be with the diurnal cycle and increase of available instability,
from the early afternoon to early overnight hours. 2-4 inches will
be possible for locations within the Slight and Marginal Risk
contours.
...Eastern North Dakota into northwestern Minnesota...
A quasi-stationary front is expected to remain situated across
eastern North Dakota into northern Minnesota through Thursday
night. While higher moisture will reside off toward the east into
eastern Minnesota and Wisconsin, Precipitable water anomalies near
the Red River are forecast to be near +2, or near 1.25 inches.
Thunderstorms are expected to redevelop with afternoon heating
between 18-00Z near and just north of the stationary front.
Beyond 00Z, 850 mb flow is forecast by the 00Z model consensus to
strengthen to near 20 kt, which is near or in excess of the
deeper-layer mean wind, supportive of training. Given locally
heavy rain across eastern North Dakota into northwestern Minnesota
from late Wednesday, a Marginal Risk remains to cover the
localized potential for flash flooding.
Roth/Otto
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
355 PM EDT Thu May 20 2021
Day 2
Valid 12Z Fri May 21 2021 - 12Z Sat May 22 2021
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
EXTREME SOUTHEASTERN TEXAS AND SOUTHWESTERN LOUISIANA...
...21Z Excessive Rainfall Discussion...
Few changes needed to the on-going ERD. Did expand the Slight
Risk area a bit across parts of Texas and Louisiana to account for
some of the east/west possibilities still suggested by the
guidance...combined with the antecedent conditions. Overall,
though, the changes did not reflect a significant shift in the
forecast reasoning.
Bann
...08Z Excessive Rainfall Discussion...
With the upper-level high continuing to build over the Northeast
and Appalachian Mountains, the plume of moisture over the Gulf
Coast will drift to the west during this period. As such, the axis
of convective activity will also move westward to an area that has
reduced upper-level support. However, showers may still move
toward the Texas and Louisiana coast and advance inland over an
area that has been water logged for the better part of this week.
Potential remains for moderate to heavy rain during this period,
especially with precipitable water values holding steady near the
2 inch mark. With the
Maintained the inherited small Slight Risk area straddling the
Texas and Louisiana border with minor eastward expansion further
into Louisiana. A similar adjustment was also made to the eastern
bound of the Marginal Risk area to reflect the latest WPC QPF
trend.
Campbell
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
358 PM EDT Thu May 20 2021
Day 3
Valid 12Z Sat May 22 2021 - 12Z Sun May 23 2021
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTHEASTERN TEXAS...
...21Z Excessive Rainfall Discussion...
Spread remains in the model QPF across parts of the Texas coast.
Expanded the Marginal Risk area a bit based on coherent plume of
anomalously high precipitable water values being drawn inland
shown by the ensembles might imply better coverage than shown by
the deterministic models or that there could be higher rainfall
amounts brought farther inland. Given the lack of support from
the operational runs, it was decided to maintain the Marginal Risk
area with only minor adjustments to the boundary.
Bann
...08Z Excessive Rainfall Discussion...
The moisture plume and axis of precipitation will continue to
drift westward through the day 3 period, with showers and
thunderstorms moving inland from the coast of Texas. There is some
spread with where the maximums fall, with some of the guidance
keeping it just offshore and others bringing it in the vicinity of
the Houston metro. Although amounts are largely less than 1 inch,
it will fall over an area very sensitive to additional
accumulations. A Marginal Risk area was hoisted for parts of the
Southeast Coast and points west.
Campbell
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
