Quantitative Precipitation Forecast
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000
FOUS30 KWBC 200101
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
800 PM EST Tue Feb 19 2019

Day 1
Valid 01Z Wed Feb 20 2019 - 12Z Wed Feb 20 2019

0100 UTC update

Minor adjustments were made to the remainder of the day 1 ERO
valid period (01-12Z Wed) -- largely to pare the western periphery
of the Slight and Marginal risk areas (near the ArkLaTex) based on
the observational trends, current soil moisture analysis, and
latest model suite of QPF during the remainder of the period. No
changes made to the previous Excessive Rainfall Outlook.

Hurley

...A MODERATE RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF
THE LOWER MISSISSIPPI VALLEY, WESTERN TENNESSEE VALLEY, AND LOWER
OHIO VALLEY...

Lower MS Valley/Southeast/Mid-South/Ohio Valley...
The large scale synoptic pattern shows moisture surging north out
of the Gulf of Mexico north on an 850 mb jet of 50-70 kts, which
advects a plume of precipitable water value of 1.5-1.75" north
across LA into southeast AR and adjacent MS as a surface low moves
in from the southwest. Ascent is aided by increasing
difluence/divergence aloft as the approaching upper level trough
amplifies. The result will be increasing areal coverage and
increasing rainfall rates in this area.  Based on the 18z & 00z
GFS guidance, Integrated Vapor Transport (IVT) eclipses 1000
kg/(m*s), with its narrowness indicating that the moisture plume
qualifies as an atmospheric river after 00z Wednesday/Tuesday
evening.  The mean flow becomes increasingly unidirectional out of
the southwest, which increasingly favors cell training with time.
Effective bulk shear rises to 60-80 knots.  Mesocyclones are
expected, but the ingredients available increasingly favor a
convective mode more along the lines of a cold trough squall line
once the 850 hPa flow starts to veer towards Wednesday morning
closer to the Gulf coast within the surface low`s warm sector,
which should limit overall totals across southern LA and southern
MS, where a lesser excessive rainfall risk has been indicated.

A total of 2-4" appears to be the most likely areal average
amounts, with locally higher amounts in the 5-6" range possible
per the modest signal seen in the latest mesoscale guidance.  In
northern LA, northern MS, eastern AR, and TN, hourly totals in the
1-2" range should be possible given the moisture and instability
forecast (despite the 00z HREF probabilities indicating lower).
This, along with two week precipitation of 300-500% of average
somewhat farther north across portions of AR, TN, KY, and the OH
valley resulted in maintaining the Moderate Risk area from
continuity with cosmetic changes.  The models continue to show
spread in regards to the QPF in general with the 00z NAM the
farthest northwest -- as usual -- while the 00z Canadian Regional,
00z ECMWF to some degree, and 18/00z GEFS-based QPF reforecast
were more eastward.  When adjustments were made concerning where
the best low-level convergence and instability gradient were
expected, this broadened the heavy rainfall footprint from
continuity eastward towards the Canadian Regional and QPF
reforecast.  The instability gradient is expected to shift from
the Deep South into TN with time, which is where the highest
hourly rain totals are expected.  To the north and northeast,
rainfall should be of longer duration due to more limited
instability with hourly totals closer to the 0.5" range so did not
allow the moderate risk area to shift very far into KY.
Coordinated with the Winter Weather Forecaster to attempt to
delimit the northeast portion of the marginal and slight risk
areas.

Roth


Day 2
Valid 12Z Wed Feb 20 2019 - 12Z Thu Feb 21 2019

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHERN U.S. AND TENNESSEE VALLEY WEDNESDAY TO THURSDAY
MORNING...

....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE
TENNESSEE VALLEY INTO THE OHIO VALLEY...

21z update...
Overall, there was a signal in the 12z model suite to move the
cold front a bit faster across the TN/OH valleys into Southeast
during Day 2. Based on the slightly faster timing, the Moderate
Risk was moved a bit further south into northern AL, on the nose
of the 45/55 knot low level jet, mainly in the 20/18z to 21/06z
time frame. The same was done with the Slight Risk area, with the
area covering more of eastern KY/eastern TN into northern GA (with
the lowest three hour flash flood guidance over eastern KY/eastern
TN).

The northern end of the Slight Risk was held more or less in
place. There will be precipitation phase changes in these areas,
which will reduce the amount of QPF available for runoff.

Hayes


...South and Southeast U.S. into the TN/OH Valley...
A heavy rainfall event should be on-going at the beginning of the
Day 2 forecast period, aided by a well defined moisture plume
coming off the Gulf of Mexico...good mid-level forcing and a
strong upper level jet.  Maintained the on-going Moderate Risk
area in parts of Alabama where there looks to be the best overlap
of rainfall amounts in the Day 1 and Day 2 period.

Rainfall amounts are forecast to be less than on Day 1, with most
areas under 2 inches.
The slight to moderate risk that remains in effect reflects the
higher ground moisture content
and reduced flash flood guidance values in the central to southern
Appalachians and adjacent valley areas of eastern Kentucky and
Tennessee.

The moderate risk remained where overlap of higher QPF occurs, but
the increase in flash flood guidance values further south in both
MS/AL leads to only a slight risk despite similar rainfall totals
to northern AL. Later updates may be made as rainfall totals
adjust, but also changes in flash flood guidance due to prior
rains occurring between now and this outlook period.

Bann


Day 3
Valid 12Z Thu Feb 21 2019 - 12Z Fri Feb 22 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE
SOUTHERN U.S. ON THURSDAY INTO EARLY FRIDAY...

21z update...
There was an increasing signal in the 12z model guidance to place
the highest rainfall amounts on Day 3 across portions of southern
AR into northern MS and western KY. This placement is closest to a
12z GFS/12z UKMET placement, and this matches the 09z SREF
depiction of the best instability just south of a frontal boundary
extending from eastern TX across central AR into TN. These changes
resulted in some minor adjustments to the current Slight Risk
placement.

Since the rainfall maximum is centered further west across the
Lower MS Valley on Day 3 than on Day 2, WPC did not upgrade
portions of southeast AR into northwest MS with this package.

Hayes

...Southern U.S....
After a brief lull, more moisture with increasing coverage of
rainfall begins to move off the Gulf of Mexico into parts of
Louisiana, Arkansas and Mississippi...mainly Thursday night and
early Friday.

High pressure to the north will initially lead to decreasing
coverage of rainfall across the Southeast U.S. during the day on
Thursday.  With maximum value of Precipitable Water being 1.4 to
1.5 inches early in the event and a feed of cooler/drier/more
stable air coming in from the north, rainfall rates and coverage
of rainfall should be waning early.

By Thursday afternoon, though, the low level flow backs and begins
to accelerate off the Gulf of Mexico in response to an area of low
pressure ejecting out of the Rockies. The 00Z suite of numerical
guidance showed differing solutions as to how quickly it takes to
initiate another round of convection capable of producing heavy to
excessive rainfall...but the consensus at this point appears to be
after 22/00Z when isentropic lift in the 300K to 310K levels
increases.  Precipitable Water values over LA/AR/MS recover and
should range between 1.5 and 1.7 inches by 22/12Z. This should be
sufficient to focus and sustain rainfall rates high enough for
flash flooding.

There was some debate over the need for a Moderate Risk area...but
felt there was enough uncertainty in the guidance (both in terms
of timing and placement) to remain with a Slight Risk area despite
the antecedent conditions.  An upgrade to a Moderate can still be
made if needed.

Since the next phase of this heavy rainfall event is expected to
continue into Friday and beyond, refer to forecasts and
discussions prepared by the WPC Medium Range section for
additional information.

Bann



Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt


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