Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
830 PM EST Fri Feb 22 2019

Day 1
Valid 01Z Sat Feb 23 2019 - 12Z Sat Feb 23 2019

...A HIGH RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF MS, AR, AL
AND TN...

0100 UTC Update: Little change in the overall axis and
meteorological setup with respect to current High/Moderate/Slight
Risk areas; however, the axis has shifted slightly based on the
recent observational data/mesoanalysis, along with the latest FFG
and most recent HRRR and NBM trends with the max QPF axis
overnight. Still expect the highest risk of QPF>FFG exceedance
along the lower MS Valley from the Arkansas-northern MS border ene
into northwest AL and southwest TN -- particularly after 03Z owing
to the surge of low-level moisture transport/flux convergence out
ahead of the upper height falls across the Southern Plains
(increasing left exit region upper forcing) and resultant
northward push of the surface warm front. The High and Moderate
risk correspond with the ensemble of most recent high-res CAM
guidance, again, especially with the past 4 HRRR runs. The High is
also depicted where the FFG remain quite low (3 hourly values
generally less than 1.00").

Hurley


---Prior Discussion---
A continued wet pattern across portions of MS/AL/TN will result in
a significant, and potentially life-threatening, flash flooding
threat through tonight. A HIGH risk of excessive rainfall remains
in effect for portions of northern MS into northwest AL and south
central TN. A MDT risk surrounds this extending across much of TN,
far southern KY and far southwest VA. Showers and embedded
thunderstorms are likely across this region almost continuously
through tonight, although two periods of heavier activity appear
probable...one this morning into early afternoon...and an
additional round later tonight into early Saturday.

Convection is increasing in intensity early this Friday morning
across MS. This is likely driven by the approach of an upper jet
streak, placing the region in the favorable right entrance region.
This is resulting in a gradual increase in diffluence aloft, and
also helping tighten the low level confluent flow. Given the
nearly stationary convergence axis, would anticipate we see some
training this morning into early afternoon across portions of
northern MS into northern AL and far southern TN. Will likely be a
pretty narrow corridor where the most intense rainfall and thus
greatest flash flood risk will persist this morning. Some
uncertainty on this exact axis, however it will likely occur over
areas that are already saturated. Thus flash flooding, some of
which could be significant, is likely with this activity through
the morning and early afternoon hours.

By this afternoon the upper jet passes by off to the east, which
should decrease diffluence aloft, with the lower level convergence
axis also forecast to become less defined. Thus, while showers and
embedded convection are still likely across the High and MDT risk
areas, the intensity and organization may take a notch downward.
Any lull will be rather short lived however, as we see additional
robust convective activity develop overnight. This activity will
have much stronger low/mid level moisture transport to work with
as southerly flow really ramps up. The region will also begin to
feel the effect of height falls associated with the strong trough
over the Plains, to go along with the increased isentropic
lift...thus would expect the coverage of heavy rains to be more
widespread than what we see earlier today. Given the strong
southerly flow, this activity may tend to move a but more
progressively off to the north compared to the earlier
convection...however would still expect some backbuilding into the
southerly flow resulting in a training threat. Even without
training, rainfall rates will likely be high enough to cause flash
flooding, some of which could be significant in nature, given the
very wet antecedent conditions over northern MS into central TN
and southern KY.

Model agreement was pretty good with the overnight guidance.
Overall a consensus of the 0z HREF members and HRRR seemed to form
a good middle ground solution for QPF through the period. Rainfall
of 2-4" appears likely across the High risk area, with localized
higher totals of 4-6" a possibility. The trend in the guidance was
to focus the heaviest rains a bit further southwest of our
previous forecast. For that reason had to expand the High risk
back to the southwest into more of MS, and was also able to cut
back a bit on the southeastern extent off the higher risk. Some
uncertainty with the northern and eastern extent of the higher
flooding risk. Was a bit liberal with the northern extent of the
Slight Risk, as think the high res models may end up being too dry
on the northern edge of the rain shield. Generally confined the
MDT risk area to those locations that have saturated soil
conditions and have the best chance of exceeding 1.5" of
additional rainfall, with the threat tapering to a Slight and
Marginal Risk outside of these areas.

Chenard



Day 2
Valid 12Z Sat Feb 23 2019 - 12Z Sun Feb 24 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE LOWER MISSISSIPPI VALLEY NORTHEASTWARD INTO THE OHIO VALLEY
AND THE WESTERN MID-ATLANTIC...

A strengthening negatively tilted mid-level low will lift
northeast from near the TX Panhandle into the Great Lakes Saturday
and Saturday night. Southeast of this feature, a surface low will
lift northeast, with warm moist advection occurring ahead of it.
Anomalously high PWAT air, forecast to reach 3 standard deviations
above the mean, will spread northeast as rich dewpoint air from
the Gulf of Mexico lifts northward on a robust LLJ of 50-60 kts.
As the upper low lifts northeast, enhanced mid-level diffluence
will spread eastward from the Mississippi Valley through the
western Mid-Atlantic states. This synoptic ascent will be aided by
a brief window of SBCape up to 1000 J/kg, supportive of intense
rain rates in convection as far north as Kentucky.

While the forcing should begin to eject northeastward quickly the
latter half of Saturday, any additional rainfall will occur over
pre-saturated grounds as recent heavy rainfall has lowered 3hr FFG
values to less than 0.25 inches from N MS into eastern TN. This
supports a southern extension of the SLGT risk despite a
relatively short window for heavy rain as dry air advects quickly
from the west. SREF probabilities for 1"/3hr are modest, but with
such low FFG values even moderate rain rates may be problematic.

Further northeast into the OH Valley and the Appalachians, rain is
expected to be more widespread under stronger synoptic lift, but
instability is more limited. This suggests more areal coverage of
rainfall than further southeast, but with lower rain rates. In
this area, FFG is still only around 1"/3hr, which combined with
the sensitive terrain of the mountains still warrants the SLGT
risk.


Weiss




Day 3
Valid 12Z Sun Feb 24 2019 - 12Z Mon Feb 25 2019

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE NORTHERN COAST OF CALIFORNIA AND THE EXTREME SOUTHWEST
PORTIONS OF THE PACIFIC NORTHWEST...

An elongated mid-level trough will dive slowly southward along the
Pacific Northwest Coast while Pacific Jet energy aloft transports
robust moisture into northern CA and southern OR. Focused moist
advection on rich W/SW 850-700mb flow will drive PWAT values into
the area of 1.5 standard deviations above the climatological mean,
favorable for periods of heavy rainfall. Synoptic ascent due to
height falls and diffluence within the Pacific Jet streak will
produce a relatively narrow but prolonged band of heavy
precipitation south of the sinking upper trough.

This long duration of moisture transport and orographic
enhancement into the terrain due to nearly orthogonal 700mb winds
has the potential to produce widespread heavy rain, and there is a
multi-model signal for 3-6 inches of rainfall during this time.
Rain rates are not expected to be extraordinarily heavy as noted
by SREF probabilities remaining low for 1" in 6 hours, but will
likely become enhanced along the upwind side of the terrain.
Additionally, snow levels rising to around 5000 feet will allow
for rain falling on top of snow-covered ground, potentially
exacerbating flooding concerns locally.

Weiss


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