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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 2253Z Apr 06, 2020)
 
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Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White


Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
653 PM EDT Mon Apr 06 2020

Day 1
Valid 01Z Tue Apr 07 2020 - 12Z Tue Apr 07 2020

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE UPPER MIDWEST & SOUTHERN GREAT LAKES...


...Ohio Valley...
Increasing moisture focused on a frontal boundary extending from
the Upper MS Valley across the OH Valley could be sufficient to
feed elevated convection capable of producing heavy to locally
excessive rainfall from northern IL across northern IN and OH,
mainly after 07/00z.

Surface low pressure develops across the northern High Plains in
response to approaching from the Northern Rockies. As the low
tracks north, the front is dragged slowly north across the Upper
MS Valley into the OH Valley, reaching a position extending from
northern IL across northern IN into northern OH before 07/12z. In
the broad warm air advection pattern associated with the front, a
30 to 40 knot low level west southwest flow transports 1.00/1.25
inch precipitable water air (which is about two standard
deviations above the mean) along the front.  The northern edge of
an instability axis reaches the frontal boundary, with model
soundings showing 250-500 J/KG of MUCAPE in the column, mainly
after 07/06z.

The combination of moisture and instability could support elevated
convection that potentially trains along the front (and the
northern edge of the instability gradient). Weak short waves in
the flow could allow convection to form small clusters. Where
training clusters occurs, hourly rainfall rates could touch or
briefly exceed 0.50", especially across northern IN into western
OH after 07/04z. Three hour flash flood guidance values here are
as low as 1.50", and these numbers are attainable where training
occurs. Since there remains some latitudinal spread on the
placement of the highest rainfall totals, the Marginal Risk was
broadened farther northeast into southernmost MI and north-central
OH.

Roth/Hurley/Hayes



Day 2
Valid 12Z Tue Apr 07 2020 - 12Z Wed Apr 08 2020

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE
SOUTHWESTERN UNITED STATES...

...20Z Excessive Rainfall Discussion...

...Southern California to far Southern Nevada...
Introduced a Slight Risk area over portions of southern California
into southern Nevada as a deep closed low makes its way slowly
inland.  With anomalously deep moisture having already been drawn
into the region, the upper-level system and an associated low and
surface cold front will be able to focus convection capable of
producing locally heavy rainfall, especially from late Monday
evening into the early hours of Tuesday morning.  Spaghetti plots
of model QPF suggests that the northern portion of the Slight Risk
area has the best chance for enhanced rainfall rates, while areas
farther south may not have as much rainfall but will be falling on
areas that received more rainfall in the previous days than areas
to the north.  Remainder of the Marginal Risk issued earlier this
morning was unchanged.

...Upper Ohio Valley to the Central Appalachians...
Model guidance has come into reasonably good agreement among
themselves and with their previous runs.  Made only minor
adjustments to the on-going Marginal Risk area.

Bann


...Previous Discussion...

..Southern to Southeast California into far Southern Nevada...
There are only some minor timing differences with the strong
closed low...h5 anomalies 2.5 to 3.5 standard deviations below the
mean..as it moves southeast off the Southern California coast day
2.  This closed low will become somewhat more negatively tilted
during the day 2 period.   This will allow for the low level flow
to back to a more southerly direction on the eastern side of the
closed low, helping to transport anomalous pw values ...2.5 to 3+
standard deviations above the mean..northward across southeast
California into far southern Nevada.  Across these areas...the
marginal risk was expanded from the previous issuance to account
for locally heavy rains in this anomalous pw axis.   Showers
should remain active on the north and northeast side of the strong
closed low across much of southern California day 2 with
convective elements likely as instability increases with lower mid
level heights and temps.  In addition there may be periods of
training of cells both in a south to north direction on the east
side of the closed low and in an east to west direction on the
north side.  Moderate to locally heavy areal average precip totals
likely this period with runoff issues possible in areas of
training.  Mudslide and debris flow issues also possible where
heavy rains coincide with recent burn areas.

...Upper Ohio Valley into the Central Appalachians..
No significant changes made to the marginal risk area over the
Upper Ohio Valley into the Central Appalachians.  The latest model
guidance continues to show potential for moderate to heavy precip
totals occurring in two separate events day 2.  The initial round
of precip possible early day 2 from early morning overrunning
convection that may form upstream over the mid west/lower Great
Lakes and push east southeast along the warm frontal boundary
forecast to stretch from the Mid West into the Central
Appalachians and Mid-Atlantic.  This will be followed by a second
round of activity late Tuesday into early Wednesday ahead of a
cold front pushing southeast from the Great Lakes into the Ohio
Valley.  While stream flow anomalies as per the National Water
Model are not high, the potential for two rounds of locally heavy
rains warrants the continuation of the marginal risk area for day
2.

Oravec

Day 3
Valid 12Z Wed Apr 08 2020 - 12Z Thu Apr 09 2020

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER SOUTHERN TO
SOUTHEAST CALIFORNIA...WESTERN ARIZONA...SOUTHERN NEVADA INTO
SOUTHWEST UTAH...

...20Z Update...
The Slight Risk area introduced was a continuation of the risk
area from Day 2 as a deep closed system moves slowly eastward. 
The 12Z suite of guidance still supports the idea of training
cells from south to north early in the Day 3 period on the east
side of the closed low with the cells in the comma head
deformation band also becoming slow moving. Current thinking is
that the rainfall rates...and associated risk of excessive
rainfall...will be diminishing during the latter part of the
period as the system moves north and east.

Bann

...Previous Discussion...

...Southern California into the Southwest...
The strong closed low moving southeast off the Southern California
coast day 2 will begin to push more easterly into the Southwest
day 3.  Timing differences develop with the GFS on the faster side
of the latest guidance.  Given the strength of the closed low and
how separate it is in the southern stream, we favor the slower
ECMWF type solutions.  This is especially so given the GFS
tendencies to be too progressive.  Anomalous pw values 2+ standard
deviations above the mean will persist to the north and northeast
of the closed low across much of the marginal risk area. 
Additional moderate to locally heavy precip totals likely with the
continue risk of isolated runoff issues.  Training of cells in a
south to northerly direction possible early day 3 on the east side
of the closed low over Southeast California into western Arizona. 
Slow moving cells in the comma head deformation band on the north
and northeast of the closed low possible over much of Southern
California.  Precip may also be enhanced in the upslope of the
southern Sierra by low level easterly flow.


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