FOUS30 KWBC 060751
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
350 AM EDT Thu Jul 06 2023
Day 1
Valid 12Z Thu Jul 06 2023 - 12Z Fri Jul 07 2023
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE CENTRAL/SOUTHERN PLAINS, AS WELL AS, NORTHERN LOUISIANA INTO
MISSISSIPPI...
......North/Central High Plains into the Central/Southern Plains...
Current RADAR, Satellite and observational trends suggest a few
clusters or larger complex should be ongoing across central OK
with some embedded training thunderstorms capable of producing
2-4" totals and flash flooding. Though late morning diurnal
minimum should result in a reduction in rainfall rates and flash
flooding potential, especially in crossing areas of recent dryness
of SE OK. As such, the inherited Slight Risk area was adjusted
slightly eastward to account for some of this timing.=20
By afternoon/evening, strong upslope flow will return with new
clusters of thunderstorms developing toward nightfall across E CO
with strong southerly to southeasterly LLJ increasing to 35-40kts
after 00z. Moisture flux values become very anomalous given the
45 to 60 degrees of directional convergence and increase of 1.25
to 1.75" total Pwats as the shortwave height-falls reach the
Rockies. 00z Hi-Res CAMs (and ongoing convection) suggest an
expansion of the Slight Risk toward the south and west. There did
remain some guidance suggesting of a secondary enhanced band of
mid-level convection along the northern edge of a developing MCS
toward the end of the Day 1 period. Guidance did also shift
slight west and south and so have adjusted the Slight Risk area
into the OK/northeast TX panhandle to account for this trend.=20
A second cluster further north is expected to evolve out of N
WY/SE MT with strong thunderstorms and potential training profiles
crossing into southwest SD and western Nebraska. Greatest QPF
signal appears to align with the Sand Hills, and so a broad
Marginal Risk encompasses the region with the exclusion of the
core of the Sand Hills.
...Southern Arkansas/Northern Louisiana...
Evolving convective clusters/complex mentioned in section above
will be down-trending by late morning crossing E OK/AR, but the
MCV will continue to remain providing some structure for enhanced
convergence and perhaps some weak DPVA for ascent as instability
once again builds downstream. Banded convection along the leading
edge may support short-term training as well as producing some
outflow boundaries for additional development through the
evening/early overnight hours. This is important, as a surge of
western Gulf moisture is forecast to arrive across E TX/N LA by
evening increased deep moisture availability as well as potential
orthogonal moisture flux convergence to outflow boundaries or
ongoing convection in proximity to the MCV. Strong collision/cell
mergers may allow for short-term increase in rainfall rates up to
2.5-3"/hr (as 00z HREF probability for 2"/hr rates is about
40-60%) with 60-70% probability of 3" across N LA/SW MS. As such,
in coordination with local forecast offices, have placed a small
Slight Risk across the area of enhanced probabilities and Hi-Res
CAM QPF signal.
...Texas Gulf Coast...
GOES-E SWIR shows a fairly compact MCV/low level tropical wave
south southeast of Brownsville with the start of a north-south
line of convection along/just west of the circulation axis.=20
Strong south-southeasterly low level flow is expected to be
generally convergent along the eastern side of the circulation
aligned with early morning instability axis within the Western
Gulf. There is ample uncertainty to its further evolution which
is contingent on the next cycle of this convergence/deep
convection is just starting to manifest with the north-south line
of convection. There is Hi-Res CAM signals to suggest best
convergence and convection will remain along and just offshore the
south Texas coast, though a wave of thunderstorms is highly
plausible to slide northward toward the Upper Texas coast by
mid-morning into early afternoon with spotty rates of 2-2.5"/hr;
eventually sliding north-northeast into LA. Additionally,
depending on how close early morning convection develops, much of
Southern Texas may have increased low-level Theta-E, while
remaining cloud free increasing overall instability along the
Coastal Plain. Aggressive models such as the HRRR suggest bands
of thunderstorms rotating onshore, while other solutions suggest
perhaps one band. Still, trends are increasing for enhanced
rainfall across S TX, with fairly high consensus for 30-50% of 3"
or greater from Corpus Christi and south. However, the placement
and timing remain uncertain enough to expand the Marginal Risk of
Excessive Rainfall to the Mexico boarder without delineating a
Slight Risk area at this time. The potential exists for a future
upgrade to a Slight Risk if trends continue to increase in
coverage/magnitude of rainfall in the area.
...Central Great Lakes into Ohio and Tennessee Valleys...
At the base of broad synoptic scale trough, that currently
dominates northern and central Canada, is a shortwave sliding
through the central Great Lakes with an attendant cold front that
extends into the mid to lower Mississippi Valley. Deep layer
moisture plume resides along/ahead of the front with 1.5-1.75" of
total PWats; and ample day-time heating should support stronger
thunderstorms with the capability of increased rain-rate potential
(2"/hr). A weak surface wave should exist in the LP of MI just
ahead of the core of the shortwave, this should allow for the best
enhanced potential for repeating thunderstorms and highest
localized rainfall totals throughout the length of the front with
isolated 2-3" totals possible. HREF probability fields denote a
slightly higher potential in the thumb and toward Detroit.=20
Currently, the signal appears to fall just below level to justify
an upgrade to Slight Risk at this time, but observational and 12z
guidance trends will be monitored for any upgrade later today.
...Coastal North Carolina/Southeastern Virginia...
At the start of the forecast period, a few thunderstorms may be
ongoing in the vicinity of the Outer Banks in proximity to the
core and southeastern quadrant of an exiting shortwave trof. Very
high moisture exists with Total PWats AoA 2.0-2.25" with slightly
lower values but still at or above normal through the
Mid-Atlantic. As the shortwave presses eastward, increased
insolation from clear skies along the western edge will allow for
the return of increased instability. Weak but deeper layer
northeasterly flow should provide some enhanced convergence to
break out additional round or two of thunderstorms from the Necks
of eastern VA southward across E North Carolina (generally east of
I-95). Weak steering flow should allow for some increased
duration and isolated spots of 2"/hr rates; 00z HREF depicts
2"/3hr of 40-50% across these portions of VA/NC to support a
Marginal Risk. To add to the potential, AHPS 2-week precipitation
anomalies are 200-300% of normal in spots across the area,
suggesting above normal soil saturation for reduced infiltration
and increased runoff in those intense rainfall rates.
...Southeastern Peninsular Florida...
South Florida resides in the trailing end of an exiting weak
mid-level trof centers off the Outer Banks of NC, while only
Surface to boundary layer southeasterly trades will resist
eastward progression. Overall moisture is at or slightly above
normal with 2.25" total PWats within the trof. Activation of the
western sea-breeze should be the focus for early thunderstorms,
while the eastern/southeastern sea-breeze will be weaker and
likely later to develop by mid-day. As such, increased
instability and slow eastward progression of the thunderstorms
will start to merge/collide across the eastern quarter of the
peninsula toward late afternoon. 00z HREF signals driven by ARW
members, suggest strong 3"+/3hrs probability signals between
21-00z across the highlighted Marginal Risk with 25-35% potential
of exceeding 5" in the vicinity of the I-95 corridor from Cape
Canaveral southward.
Gallina
Day 2
Valid 12Z Thu Jul 06 2023 - 12Z Fri Jul 07 2023
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE CENTRAL/SOUTHERN PLAINS AND NORTHEAST...
...Northern/Central High Plains into the Central/Southern Plains...
Broad zonal flow across the central US will likely have two
ongoing convective complexes by the start of the day 2 period
(07.12z) a northern more associated with a weak
inflection/shortwave crossing the Sand Hills of Nebraska starting
to spread across the mid-Missouri Valley and the second and much
stronger complex across SW KS/N OK. The former will have ongoing
slowing thunderstorm activity with localized 1-3" totals, though
Nam and Nam-Nest continue to depict a much higher qpf signal, this
seems less and less likely given other 00z Hi-Res CAMs and global
guidance...it still warrants a Marginal particularly as a northern
stream cold front drops out of Canada, allowing for any new
development to occur along its tail, where moisture/instability
may pool.=20=20
Further south, a very strong MCS with 90-95th percentile moisture
flux convergence values will be ongoing through S KS/N OK. While,
morning convection will wane by midday, floating outflow
boundaries and returning upslope flow and some clearing skies
should allow for a recharge of the local environment. 00z
GFS/ECMWF/UKMET and CMC all continue to highlight a broad area of
3-6"+ QPF mainly across SE KS, though trends continue southward
and a tad westward. Additionally, the ECMWF has a bit more WNW to
ESE orientation compared to other guidance but still shows fairly
good timing agreement. With apex of the ridge just west the apex
of the Rockies, upslope flow is likely to redevelop thunderstorms
across portions of central WY through central CO toward 00z,
stronger severe cells are likely to again grow upscale across SE
CO crossing areas affected over the prior few days, enhancing the
risk for flash flooding. As such, small southward adjustments
were made to the Slight Risk as a whole...narrowing the axis,
while also expanding a small sliver to the west to account for the
likely repeat event over SE CO.=20
...Northeast, Mid-Atlantic into Carolinas...
A cold front on the leading edge of the base of the large scale
closed low in Canada, will continue to slowly advance into N NY
and the Appalachians. Enhanced moisture pooled along the front,
in combination with increasing instability from morning heating,
should allow for a break out of thunderstorms along the length of
the front or pre-frontal environment as far south as the
Carolinas. Slightly stronger low to mid-level convergence further
north across N NY/PA will exist along/ahead of a subtle shortwave
crossing the region. Slightly backed flow should enhance moisture
convergence and with ample 1.75" total PWats, will allow for slow
moving but efficient rainfall production. 00z HREF shows 2"/3hr
signals across much of central NY to N VA, with highest 3"
probability of 40-60% across N NY to N PA. A Slight Risk of
Excessive Rainfall was introduced where this signal best
overlapped lowest FFG values across S NY into N PA. Some
consideration was made across N NY/NW VT as well as central
Western VA where recent heavy rainfall has fallen as well, with
out HREF/00z Hi-Res CAMs extending past 08/00z, confidence
afterward on better placement was not enough and will be
reassessed with 12z HREF/CAM cycle for further upgrades/expansion.
Further south in the Carolinas...a Marginal Risk of Excessive
Rainfall was considered across the central Piedmont/Fall line
across the Carolinas, but again, 00z Hi-Res CAMs and HREF
probability signals were not available and global guidance was
less reassuring over locally reduced (2-3"/hr) but regionally
higher FFG values along this axis. An upgrade to Marginal Risk
may be needed with subsequent updates.
Gallina
Day 3
Valid 12Z Fri Jul 07 2023 - 12Z Sat Jul 08 2023
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE CENTRAL/SOUTHERN PLAINS INTO THE MIDDLE MISSISSIPPI VALLEY...
...Central High Plains through the Ohio River Valley...
An expansive mid-level ridge centered over the Desert
Southwest/Northern Mexico and an amplifying trough digging out of
Canada will create squeezed northwest flow from the High Plains of
WY/CO through the Mid-MS Valley on Saturday. Within this pattern,
a 300mb jet streak arcing southeast from the Pacific Northwest
will intensify over the Central Plains placing its diffluent LFQ
across the region, which will combine with at least subtle height
falls to drive deep layer ascent Saturday into Sunday. This will
combine with subtle embedded shortwave impulses and local
vorticity maxima dropping through the mean flow, and while timing
and intensity of these features is uncertain at this time range,
each one will likely result in enhanced local ascent to drive
showers and thunderstorms, especially as they overlap the modest
low-level baroclinic zone associated with a wavering frontal
boundary.
This setup will support waves of convection throughout D3,
possibly manifesting as multiple convective clusters rotating over
the ridge to the south. Again, timing of these features remains in
question, but it is most probable these will occur early Saturday
morning and again Saturday night. While these clusters could pose
the most significant flash flood risk, additional diurnal
convection driven by the ample ascent into robust thermodynamics
characterized by PWs approaching +2 standard deviations according
to the NAEFS ensemble tables overlapping MUCAPE of 1000-2000 J/kg
will support rainfall rates in excess of 1"/hr, likely much higher
at times. While storm motions D3 may be progressive noted by 0-6km
mean winds around 20 kts, these are progged to be generally
aligned to the surface front indicating the potential for
training, especially during periods of enhanced 850mb LLJ which
will impinge into the boundary from the SW.
Despite the uncertainty inherent by D3, and noted by a large
spread in the greatest axis of ECENS/GEFS/SREF probabilities for
1" and 3" of rainfall, the inherited SLGT risk remains warranted,
although some modest adjustments were made. These adjustments were
to account for the most recent ensemble probabilities for 24-hr
rainfall, especially where they overlap the highest PW anomalies
and regions of greatest 14-day rainfall according to AHPS. Further
refinements will likely be needed as the event approaches, but the
continued favorable setup for heavy rainfall in some areas that
are also forecast to receive heavy rainfall D1-2 suggest a higher
threat for instances of flash flooding, and it is possible that a
higher threshold outlook may be needed if D3 rainfall can overlap
perfectly heavy rain events from D1-2.
Additionally, while the SLGT risk was tailored to focus the
highest risk areas, the MRGL risk was also adjusted northward a
bit into the High Plains where recent rainfall has been excessive
producing more sensitive soils, but also trimmed out of the
Mid-Atlantic States due to a forecast dearth of instability and
recent downward trends in model QPF.
...New England...
A MRGL risk remains for much of Northern New England and parts of
Massachusetts Saturday and Saturday night as a favorable setup for
heavy rainfall develops atop saturated soils. A surge of moisture
noted by PWs reaching 1.5 inches (nearly +2 standard deviations
according to NAEFS) will combine with 500-750 J/kg to produce an
environment that can support rainfall rates in excess of 1"/hr.
Forcing for ascent will be provided by an approaching front from
the west, driven by subtle height falls as a shortwave embedded
within the larger scale trough centered over Manitoba ejects
northeast towards New England. This deep layer ascent impinging
into these robust thermodynamics should create scattered to
widespread showers and thunderstorms during the aftn/eve, with
storm motions of 10-15 kts aligned to the approaching front
suggesting the potential for training.
While coverage of thunderstorms D3 may be somewhat scattered as
indicated by relatively modest 1"/24 hr ensemble probabilities and
within the available simulated reflectivity, this is likely at
least somewhat underdone due to a lack of higher res models.
Regardless of coverage however, the possibility of training 1"/hr
rain rates atop sensitive soils driven by 14-day rainfall that is
200-300% of normal resulting in USGS streamflow anomalies that are
almost without exception above the 90th percentile, suggests even
scattered convection could result in rapid runoff and instances of
flash flooding.
Weiss
Day 1 threat area:
https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8skCTEgjMJO6TRme7ju6eTJwePXTbSN2Y97jMstubVJO= CfnK9IYI9swWzSunB3d6GFbNWYecj_qYRXkwswbbGkhLmqs$=20
Day 2 threat area:
https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8skCTEgjMJO6TRme7ju6eTJwePXTbSN2Y97jMstubVJO= CfnK9IYI9swWzSunB3d6GFbNWYecj_qYRXkwswbbP7BCC60$=20
Day 3 threat area:
https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8skCTEgjMJO6TRme7ju6eTJwePXTbSN2Y97jMstubVJO= CfnK9IYI9swWzSunB3d6GFbNWYecj_qYRXkwswbbhCDzqRo$=20
$$
=3D =3D =3D
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