Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
352 AM EDT Sun Apr 14 2024

Day 1
Valid 12Z Sun Apr 14 2024 - 12Z Mon Apr 15 2024

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
COASTAL SOUTHERN CALIFORNIA AND FROM PORTIONS OF THE NORTHERN MID-
ATLANTIC INTO THE CENTRAL APPALACHIANS...

...Coastal Southern California...
No changes made to the previous marginal risk area across the
Transverse range and the southern portions of the central
California Coast Range. A second area of showers is developing
early Sunday morning along the central California Coast Range ahead
of the strong closed low along the central California coast. This
upper center is expected to rotate southeastward along the central
California coast and through the Transverse Range day 1. An
organized area of showers will accompany this upper low, supporting
additional moderate to locally heavy precip totals day 1. Hourly
rainfall totals from the hi res guidance suggests .10-.25"+ amounts
are possible. HREF neighborhood probabilities are rather low for
any hourly totals greater than .50". Isolated runoff issues
possible where additional moderate to locally heavy rains fall over
regions that received rain over the past 24 hours.

...Northern Mid-Atlantic into the Central Appalachians...
A surface frontal boundary expected to sink southward late Sunday
afternoon into Sunday night across the Northern Mid- Atlantic. A
narrow axis of instability, LIs 0 to -4, MUCAPE 200-500 j/kg, and
increasing PW values .75-1"+, expected along and ahead of this
front. HI res model consensus is for a line of
showers/thunderstorms to form along and ahead of this front across
the Northern Mid- Atlantic Sunday afternoon and sink southward
into early Sunday evening. Recent heavy precipitation from the
Central Appalachians into the Northern Mid- Atlantic has resulted
in saturated soils and high stream flows. Hi res model consensus
is for potential of .25-50"+ hourly amounts along this front. With
soils saturated, much of this will runoff, possibly resulting in
isolate flash flooding. No significant changes made to the previous
marginal risk across this region.

Day 2
Valid 12Z Mon Apr 15 2024 - 12Z Tue Apr 16 2024

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE PLAINS...

The latest model guidance is in good agreement on the eastward push
of the strong closed low that is expected to move inland from
central California into the Great Basin day 1, across the Central
Rockies into the Central High Plains day 2. A broad marginal risk
area maintained through portions of the Plains in a region of
strengthening low level southerly flow and increasing PW values. PW
values expected to increase to 2 to 3 standard deviations above
the mean along and ahead of the strengthening north to south
oriented frontal boundary forecast to push eastward into the
Plains on Monday. This and and a broad region of instability along
and ahead of this front will support increasing convection,
especially late Monday afternoon into the early hours of Tuesday.
There continues to be a large spread with qpf details among the
models for the upcoming day 2 period. We kept a fairly broad
marginal risk area to cover this spread. Two areas of concern
continue in this broad marginal risk area. 1: For a convective line
forming late Monday afternoon into the early hours of Tuesday
along and ahead of the front over the Southern to Central High
Plains. This line will likely be fairly progressive to the east,
limiting flash flood potential. The other area of concern would be
with a slow moving comma head/deformation precip area farther to
the north to the north of the closed low track. The 850-700 mb
moisture flux to the north of this closed low becomes very
anomalous Monday afternoon into Monday night/early Tuesday with
values 2 to 5 standard deviations above the mean in the
strengthening southeasterly flow level flow. There is general
consensus for a comma head/deformation max from far southeast
MT/far northeast WY into SD. With precip totals generally below
average over the past few weeks across large portions of the
Plains, and current model qpf spread, the risk level was kept at
marginal.

Oravec


Day 3
Valid 12Z Tue Apr 16 2024 - 12Z Wed Apr 17 2024

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS EASTERN
PORTIONS OF THE NORTHERN PLAINS SOUTHWARD THROUGH MUCH OF THE
MISSISSIPPI VALLEY...

Similar to the day 2 period, we continue to depict a broad marginal
risk area ahead of the strong closed low forecast to push from the
Central High Plains into the Upper Mississippi Valley during day 3.
An axis of anomalous PW values, 1 to 2.5+ standard deviations above
the mean, will persist day 3 along ahead of the well defined front
forecast to push east through the Central and Southern Plains and
Lower Missouri Valley. There continues to be potential for
organized convection in this anomalous PW axis ahead of the front.
However, this convection will likely be fairly progressive, again
limiting the heavy precip potential. This will keep the threat
level at marginal for the day 3 period along and ahead of this
front.

Farther to the north...model consensus is for a greater chance of
organized precip along and to the north of the west to east
oriented frontal boundary stretching from the Northern Plains
eastward into the Upper Mississippi Valley. 850-700 mb moisture
flux anomalies rise to 4 to 5 standard deviations above the mean
into this boundary where strengthening southerly to southeasterly
flow level flow will impact the front. An axis of heavy rains
likely to the north of the front, with model consensus for areal
average 1-2" amounts. This region has been relatively dry over the
past few weeks, resulting in dry soils and below average stream
flows. At the moment, we have kept the threat level also at
marginal across this area.

Oravec

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