FOUS30 KWBC 231603
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1202 PM EDT Wed Sep 23 2020

Day 1
Valid 16Z Wed Sep 23 2020 - 12Z Thu Sep 24 2020

...A MODERATE RISK OF EXCESSIVE RAINFALL IS IN PLACE FOR PORTIONS
OF THE LOWER MISSISSIPPI VALLEY...

16Z Update: Only minor changes to the Moderate risk area over
portions of LA into west central MS. Still on track for a swath of
3-5" across this corridor today into tonight...with localized
totals of 5-8" a possibility. This will result in numerous to
widespread flooding concerns...some of which could be locally
significant in nature. Currently seeing a pretty strong convective
cluster over western LA northeast of the center of Beta. As we
head into late this afternoon through tonight moisture convergence
ahead of Beta is forecast to increase in magnitude and become more
well defined. We should also begin to see an increasing
interaction with the upper jet to the north. Meanwhile upstream
instability over 1000 J/kg will be feeding into the area. Thus
where we see this overlap of strong lower level moisture
convergence and ~500 j/kg of MLCAPE...we should see an
increasingly robust convective cluster move northeast across the
Moderate risk area. This environmental setup seems to support the
QPF coming out of the 12z CAMs and recent HRRR runs (although the
maxes in these runs could be a tad too high). Some flash flood
threat will also continue this afternoon from southeast AR into
northern MS closer to the stronger mid/upper level forcing ahead
of the trough and south of the upper jet. Rainfall rates will not
be as robust here...nonetheless some brief heavier rates and
rainfall totals of 2-4" supports some continued flood risk through
the afternoon hours.

Chenard

...Previous Discussion...

...Middle/Upper Texas Coast into southern Louisiana...
Post-Tropical Cyclone Beta is expected to pick up forward speed
during the day on Wednesday and track northeastward from the Upper
Texas Coast into central/northern Louisiana, reaching western
Mississippi Thursday morning. Precipitable water values near and
just ahead of Beta's surface circulation were 2.2 to 2.5 inches as
of 06Z per GPS data, and this relatively small pocket of high
moisture is expected to more-or-less maintain into Louisiana
Wednesday afternoon. A SW to NE low level convergence axis
(925-850 mb) is forecast to set up to the northeast of Beta's
circulation with roughly 20-30 kt of winds converging from central
Louisiana into western Mississippi. Mean deep-layer flow will
parallel the orientation of the convergence axis, supporting the
potential for multiple rounds of heavy rain across portions of the
Lower Mississippi Valley.

A significant limiting factor for high rainfall rates and flash
flooding is weaker forecast instability to the north of the
surface low track, with a 00Z model consensus showing less than
250 or 500 J/kg MUCAPE. South of the low level convergence
boundary, however, instability up to 1500 J/kg is expected with
southerly transport of moisture/instability into the Lower
Mississippi Valley. Rainfall rates of 1-2 in/hr are expected but
locally up to 3 in/hr cannot be ruled out. Rainfall intensity is
expected to pick up later Wednesday afternoon and overnight,
continuing into Thursday morning. The track of Beta showed
relatively good agreement in the models, but some solutions took
the storm farther south. WPC followed the tracks near the 00Z
NAM_nest, 00Z ARW and 00Z NSSL...but feel the 24 hour rainfall
total point maxima of 8-10 inches are too high. Areal average
rainfall totals of 2-4 inches seems more reasonable with local
maxima of 4-6 inches with 3-hour rainfall rates in excess of 3
in/hr. A Moderate Risk was coordinated with SHV/LCH/LIX/JAN for
the Day 1 period.

...Upper Mississippi Valley...
Ahead of a shortwave trough moving into the Dakotas Wednesday
evening, thunderstorms are expected to develop shortly after 00Z
across north-central Minnesota just north of a surface front
oriented from WSW to ENE. Precipitable water values of 1.0 to 1.3
inches in the pre-convective environment and MUCAPE values in the
1000 to 2000 J/kg range are forecast across south-central to
north-central Minnesota. As southwesterly 850 mb winds of 30+ kt,
ahead of a surface trough extending into the Central Plains,
ascend over the front, mean storm motions are expected to parallel
the frontal boundary supporting training. There is a relatively
brief window between 00Z and 09Z where flash flood potential will
be greatest, before instability weakens to the north and forward
propagation increases toward the south, ahead of the surface low
and in the wake of the surface trough axis. Rainfall rates of 1 to
1.5 in/hr are expected which may cause some flash flooding. The
00Z models were fairly tightly clustered with their axis of QPF
and the Marginal Risk encompasses all of the latest 00Z HREF
members and the 00Z NAM/GFS/ECMWF.

...Pacific Northwest...
A rather strong atmospheric river is forecast to impact the
Pacific Northwest today with IVT values in excess of 750 kg/m/s,
ahead of a cold front which was analyzed roughly 200 miles west of
Washington at 06Z. Southwesterly 850 mb winds of 40-60 kt will
reach the Coastal Ranges late Wednesday morning and be embedded
within precipitable water values of 1.25 and 1.5 inches near the
Olympic Peninsula via the 00Z GFS. Orographic lift will be
augmented by a powerful 150 kt upper level jet streak and
diffluent flow which will approach the Pacific Northwest Coast
among weak instability, forecast by the 00Z hi-res model
consensus. Rainfall rates of 0.5 to just over 0.8 in/hr are
expected to impact the Olympics, Coastal Ranges and Washington
Cascades, with slow eastward translation of the moisture plume
until roughly 00Z Thursday. Beyond 00Z Thursday, the cold front
and associated moisture axis will push inland and south at a
steady pace, limiting the potential for higher rainfall
rates/duration into the Oregon Cascades. The 00Z HREF supports 0.5
in/hr rainfall rates in excess of 90 percent for the Washington
Olympics, Coastal Ranges and Cascades, with probabilities closer
to 50-60 percent farther south into Oregon and of a shorter
duration compared to farther north. 24 hour totals of 3-5 inches
are most likely for the Olympics and northern Washington Cascades,
but locally higher values cannot be ruled out.

Otto


Day 2
Valid 12Z Thu Sep 24 2020 - 12Z Fri Sep 25 2020

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS THE
TENNESSEE VALLEY INTO THE SOUTHEAST...

...Lower Mississippi Valley/Tennessee Valley/Southeast...
As remnants of Beta get picked up by a positively tilted long wave
trough extending from the OH Valley into the Lower MS Valley
during Day 2, deep moisture and instability are transported across
the TN Valley and Southeast states. There is a fair amount of
spread concerning just how far north the remnants are drawn
northward, and how much instability follows, particularly into the
TN Valley. The 00z NAM/ECMWF look too far north with the weakening
surface system, apparently allowing too much instability to reach
northern MS/AL. The 00z GFS looks too fast with the system, as it
lifts out the surface low into the eastern TN Valley. The 00z
UKMET looks like a fair proxy for the expected rainfall
distribution, so it was used as a template for the WPC QPF and
Excessive Rainfall Outlook.

With the mid level trough lifting to the northeast from the OH
Valley into the Lower MS Valley, the remnants of Beta gets lifted
northeast. However, as mentioned earlier, there is quite a bit of
spread with respect to how quickly this occurs, and how far north
the surface system gets. Since the model guidance has been too
quick to lift Beta with the trough over the past couple of days,
it seems as though the track of the system will be further south
than much of the guidance. With a track further south with the
surface system, the northern edge of the instability gradient is
expected to lie from central MS into central and possibly northern
AL during the afternoon and evening hours (when instability
peaks). During this time, a 50 knot jet streak passing just to the
north across the OH Valley provides synoptic scale ascent for
banded rainfall to extend from central and northern MS across
northern AL into far south central TN.

Much of the 00z high resolution guidance (including the 00z NAM
CONUS Nest and the 00z WRF ARW) showed a band of 4.00/6.00 inches
of rainfall along the northern edge of the instability gradient.
While placement looks reasonable, the amounts seem too high, as
these models appears to be bringing too much instability into
northern MS/northern AL. The moisture plume with Beta is
transported along the isentrope toward an elevated boundary over
this area, so heavy rainfall is expected, but lacking deeper
instability, the high resolution values look too high. Hourly
rainfall rates could top out near 1.50 inches, with total rainfall
amounts of 3.00/4.00 inches possible in spots. This area was
spared the highest rainfall associated with Sally, and three hour
flash flood guidance values are generally above 2.50 inches.

(cont'd)
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