Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1143 AM EDT Fri Apr 03 2020

Day 1
Valid 16Z Fri Apr 03 2020 - 12Z Sat Apr 04 2020

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER SOUTHERN
TEXAS...

16z Update: A Marginal risk will be maintained for southern TX
ahead of the southward shifting cold front. As the corridor of
strong low level convergence near the front pushes southward, it
enters into a higher PW/CAPE environment, and also begins to
interact more with the exit region of the subtropical Pacific jet.
This should allow for an expansion of convective activity by early
this afternoon. Mean flow is parallel to the front (west to east),
which should allow for some periodic training. Although the
southward progression of the front should shift things southward
and erode instability, which should limit the duration of heaviest
rain. Thus while a few flash flood issues appear probable given
the environment in place, the coverage of the heaviest rains (3"+)
should stay isolated enough to keep flash flooding localized in
nature. Thus think the Marginal risk should suffice. By this
evening into the overnight the front should slow over far southern
TX, which may set up the potential for training there and possibly
an uptick in the flood risk. Although this part of the forecast
remains a bit more uncertain and FFG is higher here...thus a
Marginal should cover it for now.

Chenard

...Previous Discussion...

Convection is expected to become increasingly active day 1 across
eastern and South Texas along and ahead of the strong cold front
pressing southeast across the Southern Plains.  Widespread
instability, mu-capes 1000-2000+ j/kg, LI's -4 to -8 expected
along and ahead of this boundary in an axis of PW values 1.5-2.5
standard deviations above the mean.  This will be supportive of
locally very heavy rainfall amounts and localized flash flood
potential.  Precipitation has been slightly below average across
these regions over the past week, resulting in stream flows at or
below normal as per the National Water Model.  While there are a
lot of details differences among the latest models, there is
strong signal for the potential of locally very heavy rainfall
amounts.  Hourly rainfall totals from the hi res guidance are
showing isolated 1-2"+ totals and isolated storm totals of 3-5", 
but again with large differences in details of placement.  With
respect to changes from the previous issuance, the marginal risk
area was expanded slightly northeastward into southwest Arkansas
for activity across this area early in the day 1 period.  The
marginal risk area was also extended to the central Texas Gulf
coast and farther southward into South Texas to cover the spread
in the latest model solutions.

Oravec


Day 2
Valid 12Z Sat Apr 04 2020 - 12Z Sun Apr 05 2020

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF CENTRAL AND EASTERN TEXAS...

...Texas and nearby Louisiana...
Short waves in the quasi zonal mid level flow over TX during Day 2
interact with deep moisture and instability to produce convection
capable of heavy to locally excessive rainfall over portions of
central and east TX (as well as far southwest LA). The main threat
appears to be along the mid and upper TX coast, where the best
instability is expected to lie (with MLCAPE values between
1000/1500 J/KG). There is some spread in the placement of the
surface front, which should be the line of delineation for higher
instability.

Along the coast, deep moisture (with precipitable water values
near 1.75 inches, which is between two and three standard
deviations above the mean) is focused by the low level convergence
of the dropping front, which is expected to result in the highest
rainfall amounts (possibly 2.50+ inches) over the mid TX coast.
While moisture in place is expected to be deep, the low level
inflow should be less than 15 knots, which would suggest that the
convection may remain near or just off the TX coast (as indicated
by the 00z ECMWF). The moisture plume could support rainfall rates
between 1.50/2.00 inches (as indicated by the 00z NAM CONUS Nest),
though the placement of the highest rainfall rates is not yet
clear. Based on this, a Marginal Risk was placed over much of the
TX coast. Should there be better model agreement concerning the
placement of the highest rainfall near the TX coast, a Slight Risk
could be needed here in later forecasts.

Further inland across central TX, there has been a general
increase in QPF (led by the 00z ECMWF/NAM), as the short wave
energy interacts with the deep moisture. Instability here is not
quite as robust as it is closer to the surface boundary dropping
south. Elevated instability in the low level flow could allow for
higher rainfall amounts across the Hill Country and southern
Edwards Plateau (where hourly rainfall amounts could exceed an
inch), Seven day rainfall amounts have been below normal across
this area and three hour flash flood guidance values are generally
2.50 inches or higher. However, longer term wetness could result
in a flash flood threat, especially where training or cell mergers
occur. Because of this, the Marginal Risk was extended back
westward from the coast to over the threat.

...Northern California Coast,,,
Two short waves associated with a developing closed low
approaching the northern CA coast interact with moisture in an
increasing low level upslope flow, resulting in locally heavy
rainfall during the first half of Day 2. A 25 knot low level
southwest flow ahead of the initial short wave transports 0.75
inch precipitable water air (which is about two standard
deviations above the mean) to the upslope areas of Del Norte and
northern Humboldt counties, mainly before 05/00z. The combination
of upslope flow and moisture could support hourly rainfall rates
approaching 0.50 inches in these locations in this time period.

There has been an overall increase in QPF in this area, led by
some of the 00z high resolution guidance, mainly before 05/00z.
This is plausible given the depth of the moisture in the flow, but
as heights fall in the wake of the first short wave, it is not
clear how much of the QPF falls as snow. Conditions have been
relatively dry here recently, and after collaborating with WFO
EKA, we decided to forgo issuing a Marginal Risk with this
forecast. Should the threat become clearer before any
precipitation phase change, a Marginal Risk could be needed in
later forecasts.

Hayes



Day 3
Valid 12Z Sun Apr 05 2020 - 12Z Mon Apr 06 2020

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF SOUTHWEST CALIFORNIA...

...Southwest California...
A deepening closed mid level low approaching the north central CA
coast during Day 3 interacts with a deepening moisture plume to
produce heavy to locally excessive rainfall across upslope region
of southwest CA. A 20/30 knot low level southwest flow focuses
0.75 inch precipitable water air (which is between two and three
standard deviations above the mean) on the upslope areas of Santa
Barbara, Ventura and far western Los Angeles counties ahead of
surface cold front in the 06/06z to 06/12z time frame. This timing
is nearly coincident with the best synoptic scale lift in advance
of the closing mid level low.

At this point, model soundings are not showing much in the way of
instability (with 00z GFS model soundings indicating 100 J/KG of
less of MUCAPE) ahead of the front, which is not unusual this far
out. Steepening mid level lapse rates could allow for instability
rooted above the surface to develop just ahead of the front. Model
soundings did show a moist column through at least 400 mb, which
may support more efficient rain makers in the cells that do
develop ahead of the front.

This could support hourly rainfall rates approaching 0.50 inches,
especially toward 06/12z. Rainfall rates this high could result in
flood issues over burn scars in Santa Barbara and western Ventura
counties. After collaborating with WFO LOX, a Marginal Risk was
placed over the upslope region of the abovementioned counties for
Day 3.

Hayes


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