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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1605Z Feb 13, 2025)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1104 AM EST Thu Feb 13 2025

Day 1
Valid 16Z Thu Feb 13 2025 - 12Z Fri Feb 14 2025

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF SOUTHERN CALIFORNIA...

16Z Update...

Changes to the previous outlooks were rather minor overall which
includes removal of the Marginal Risk area across the Southeast
given the downward trends in organized convection and a slackening
of the rainfall rates. While some locally heavy showers and
thunderstorms will still be likely this afternoon ahead of a cold
front, the totals should be rather modest.

Over California, a well-defined/organized deep layer cyclone and
associated atmospheric river continues to impact the region. While
the overall deeper layer axis/fetch of strong moisture transport
has settled south of northern California and the Bay Area, there
will be post-frontal convective showers arriving through the
afternoon which will be capable of producing heavy rainfall rates
of 0.50"+/hour. Therefore a Slight Risk is maintained along the
coastal range of northern California coastal range. We have opted
to remove it however for portions of the Sacramento Valley where
the additional totals for today should be rather modest.

No changes farther south down the coast into southern California
where there will be significant sensitivities around the recent
burn scar activity closer into Los Angeles Basin. The latest HREF
guidance supports 0.50" to 1.0"/hour rainfall rates this afternoon
into the evening hours as strong moisture flux and a corridor of
modest instability arrives which coupled with upslope flow into the
Transverse and Peninsular Ranges will support elevated rates
including pockets of convection. Therefore the Moderate Risk
remains in place. Please consult WPC's latest MPDs for more
details on the shsort-term trends/impacts over the next 6 to 12
hours.

Orrison

Previous discussion...

...California...
Overall, not much change to the previous forecast, with the
unfolding atmospheric river expected to peak near the start of the
period.  A long warm front currently extending along the coast is
expected to press inland this morning, with IVTs climbing to around
700-800 kg/m/s near the upstream cold front as it approaches the
coast later this morning. The strongest moisture flux is expected
to center near SF Bay at the start of the period, before steadily
shifting south along the coast today. Models continue to show IVTs
decreasing as the axis moves south, dropping to around 400-500
kg/m/s as the AR reaches Southern California later this afternoon
into the evening. Guidance has remained fairly consistent,
indicating precipitation amounts of 4-6 inches along the central to
southern coastal ranges and the Sierra Nevada, falling mostly as
rain below 6000 ft.

A Moderate Risk (level 3/4) remains in place across portions
Southern California, from Santa Barbara County southeastward into
the San Gabriel and San Bernadino mountains, where damaging and
potentially life-threatening burn scar flash flooding and debris
flows remain a primary concern. Even outside of the burn scar areas,
intense rain rates over complex terrain and urban areas may produce
scattered to numerous instances of flash flooding. Elsewhere, a
broad Slight Risk (Level 2/4) exists for much of the remaining
coastal ranges as well as parts of the Central Valley and the lower
elevations of the Sierra Nevada.

...Florida Panhandle and Southern Georgia...

Lingering heavy rainfall will continue this morning across
portions of the Florida Panhandle and southern Georgia ahead of an
approaching cold front. HREF neighborhood probabilities indicate
that additional rainfall amounts of 1-2 inches are likely within
the highlighted area, with some potential for localized rain rates
high enough to cause some urban/localized flooding concerns.

Pereira


Day 2
Valid 12Z Fri Feb 14 2025 - 12Z Sat Feb 15 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE MID-SOUTH...

...Mid-South...

Although most of the convective activity, and thus heavy rainfall
threat, in the region should be concentrated in the Day 3 period
(after 12Z Saturday), model guidance does indicate that convection
is likely to begin blossoming late Friday Night. Conditions will
become increasingly favorable after 06Z Saturday with a strong 55-65
knot low-level jet expanding east from the Plains, and significant
northward moisture transport leading to rapid moistening of the
boundary layer. MUCAPE values of around 500-1000 j/kg should support
organized convection with the potential for rainfall rates in excess
of 1 inch per hour at times. Although there is a lack of a
significant front or boundary to help organize a band of convection,
the strength of the LLJ may support backbuilding within a broad zone
of developing convective clusters. This may be enough for some brief
training and a ramp-up in flash flood potential prior to 12Z
Saturday. The 00Z runs of the NAM Nest and FV3 show more widespread
convection than other models; if this trend persists as additional
hi-res guidance becomes available, an expansion of the Marginal Risk
and/or the addition of a Slight Risk area may become warranted with
time.

...Southern California...

The Marginal Risk was removed for southern California given a more
limited QPF signal on the latest guidance, and hi-res models in
excellent agreement on a lack of organized convection. The bulk of
the threat should be in the Day 1 period.

Lamers


Day 3
Valid 12Z Sat Feb 15 2025 - 12Z Sun Feb 16 2025

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE TENNESSEE AND OHIO VALLEYS AND THE CENTRAL APPALACHIANS...

...Ohio and Tennessee Valleys...

Confidence remains high in significant rainfall-related impacts on
Saturday and Saturday Night with model guidance in excellent
agreement depicting a swath of 3-6 inches of rainfall in the 24-hour
period across the region. Although models vary slightly with the
placement, they almost unanimously depict a corridor of heavy rain
about 750mi long centered somewhere in TN/KY. This same region has
generally seen above average precipitation in recent weeks, with the
14-day precip in much of KY, N TN, WV, and W VA at least double the
normal values. The confluence of these factors should make the
region especially vulnerable to flooding from the predicted
rainfall. For additional context on the current hydrologic picture
across the region, refer to the National Hydrologic Discussion from
the National Water Center at water.noaa.gov.

Convection may be ongoing at the beginning of the forecast period
and should become increasingly focused along a developing warm front
to the east of a surface low that will be emerging out of the Plains
on Saturday. The front will be positioned on the nose of very strong
SW, or even WSW, inflow, which should favor backbuilding and
training and a corridor of very heavy rainfall. The environment will
be atypical of mid-February with PWATs reaching around 1.5 inches
and MUCAPE of 500-1000 j/kg. For example, the Nashville sounding
archive exists back to 1948 and they've only ever reached 1000 j/kg
MUCAPE once in February, and 1.5 inch PWATs a few times. The
environment will support rainfall rates in excess of 1 inch per
hour, and given the potential for training and backbuilding, these
rates may be sustained for several hours in some locations. These
would be the areas of greatest concern for flash flooding -- not
only in terms of likelihood but severity -- particularly if the
higher rain rates are sustained over urban areas or vulnerable
terrain for extended periods of time.

The inherited Moderate Risk was mostly maintained as-is with a
slight expansion to the southwest in the vicinity of Memphis, TN;
and a more significant expansion eastward into S WV and W VA. The WV-
VA border region just received some significant snowfall, and model
guidance indicates a fairly high probability of warm, moist air (dew
points in excess of 45 degrees) accompanying the rainfall into this
area, which may lead to a combination of runoff from rain and snow
melt.

It's worth noting a couple additional things. First, the amount of
rain forecast from E KY into S WV and W VA in particular is quite
unusual for the heart of the winter season. Calendar day precip
records across that area for January and February are generally in
the 2-3 inch range, and NBM probabilities indicate it is likely (60+
percent chance) we will see these types of 24-hour rainfall totals
in those areas. Second, this relatively rare winter rain event would
be delivered to an area that has been quite wet recently, and in
some areas has snow on the ground. And third, the high-end scenarios
in much of the Moderate Risk area would be significant by historical
standards. The NBM 90th percentile from N TN into S/E KY and S WV is
on par with the top several wettest days (for any season!) in the
station records in the area. This corridor is where the greatest
concern for severe impacts exists, and it's possible an upgrade to
High Risk may be needed for portions of the area in subsequent
updates.

Lamers

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