477
FOUS30 KWBC 201903
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
303 PM EDT Thu Jun 20 2024

Day 1
Valid 1900Z Thu Jun 20 2024 - 12Z Fri Jun 21 2024

...A MODERATE RISK OF EXCESSIVE RAINFALL EXISTS IN AND NEAR
SOUTHEAST SOUTH DAKOTA...

...Upper Mississippi to Central Missouri Valley...
The Moderate Risk remains in effect for much of southeastern South
Dakota with this update, with some slight northward broadening out
of deference to the 12z HREF. Gulf moisture surging northward
across the region with the LLJ will be moving into the right
entrance region of a 100 kt southwesterly jet. A warm front will
develop across South Dakota this evening as the typical evening
strengthening of the LLJ occurs. Storms will break out along that
boundary from the Sand Hills of Nebraska northeast into South
Dakota. The storms are likely to merge and train along the front as
they`re forming, then shift off to the northeast with the best
forcing through early Friday morning.

The primary point of uncertainty remains exactly where the line of
storms initially forms relative to where it has rained a lot
recently. Should the storms form too far north and west, closer to
central SD and the western Sand Hills, then it`s likely a cold
frontal boundary serving as the back edge of the storms will sweep
across the area as the much more persistent rainfall associated
with the front falls over a drier area. This would significantly
decrease the flooding potential in South Dakota. However, the trend
on a national scale with convective forecasts for multiple inches
of rain over the past week has almost unanimously trended
south...meaning the heaviest rainfall totals occur south of where
the guidance suggests they will. In this case that would support
storms occurring over areas that have been much harder hit recently
with heavy rain, resulting in soils being much less receptive to
the heavy rainfall and therefore more widespread flash flooding.
Thus, the Moderate Risk remains relatively uncertain in its
location...though there`s been good agreement on the magnitude of
the heaviest rainfall.


...Southwestern Colorado...
Increasingly anomalous atmospheric moisture to the tune of up to 4
sigma above normal with PWATs approaching 1 inch will slam into
the south facing slopes of the San Juan Mountains today into
tonight. Upslope into the mountains will support the development of
stationary cells of heavy rain. If these occur over any burn
scars, then flash flooding will be locally significantly worsened.
The Slight Risk area inherited was left unchanged and looks solid
for that area. Hourly rain totals to 2" and local amounts to 4"
cannot be ruled out. The special update bridges the Marginal Risk
between the Rockies and the Northern Plains, based on convective
trends.


...West Texas into southeast New Mexico...
The northern portion of the remnant moisture plume of the
dissipating T.D. Alberto will move northwestward up the Rio Grande
and into west Texas through the day today and into tonight. More
widespread light to moderate rainfall than is typical for the
desert of west Texas is expected. Given the low tolerance of most
of the area to widespread rainfall -- burn scars -- the Slight
remains in place...albeit lower confidence as the guidance does not
appear to be overly wet in this region. However, upstream
instability is supposed to build to the east of the designated risk
areas, and it`s anomalously moist as inflow from the Gulf forces
very high moisture around Albero`s periphery, so left in the
Slight Risk as a course of least regret.

In South Texas, diminishing of the convection in the area behind
the main plume moving into Mexico has resulted in lessening
concerns for more widespread flash flooding in the region, in favor
of more isolated flash flooding. Isolated to very widely scattered
convection today and tonight should greatly limit any resultant
flash flooding, despite the newly saturated soils in the area.


...New York into New England...
A Marginal Risk remains in effect across much of upstate New York
and much of New England and has been nudged a little southward
based on the 12z HREF.  Lying on the northern side of the upper
ridge will result in periodic upper level disturbances passing
through and another afternoon and evening of scattered convection.
Precipitable water values are 1.75-2", and ML CAPE is expected to
build towards 2000 J/kg. Effective bulk shear is 20-30 kts, which
should allow for some convective organization. The storms are
likely to be slow moving, with some mergers expected between less
and more organized thunderstorm activity. Hourly rain totals to
2.5" should be possible within this regime, with local amounts to
5". Widely scattered to possibly episodes of flash flooding are
possible.


...Northeast Florida/Southeast Georgia...
A new Marginal Risk was introduced for a small convective low that
the National Hurricane Center is monitoring for tropical cyclone
development. Precipitable water values of 2-2.5" are expected to
move into the region ahead of the low due to easterly vertical wind
shear. Hourly totals to 3" and local amounts to 5" are possible
within this regime, but such amounts should be isolated to widely
scattered at best. Soils are dry from a recent dearth of rainfall.
The track and timing around 12z remain question marks -- whether
it will be a day 1 or day 2 concern. Considering the above, added a
Marginal Risk on this update.

Roth/Wegman


Day 2
Valid 12Z Fri Jun 21 2024 - 12Z Sat Jun 22 2024

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR MUCH OF
SOUTHERN MINNESOTA AND ADJACENT COUNTIES...

...Minnesota/Iowa border...
A Moderate Risk of excessive rainfall remains in effect here,
though it has shifted southward from continuity.  This has been a
very typical pattern with forecast convection in the guidance over
the past week, possibly due to minimal mid-level capping aloft as
hinted at by the modest 700 hPa temperature field.

Convective complexes early and late are expected in and near
southern MN, with the flow backing mid-period in preparation for
the second convective round. While the guidance shows a break in
activity, this is no guarantee as guidance tends to dissipate
overnight activity too quickly in the morning. There are signs in
the HREF for local 5-8" amounts. Hourly rain totals to 3" are
possible as PWs rise at or above 2". ML CAPE should be north of
2000 J/kg in the warm sector. Effective bulk shear should be more
than sufficient for organized convection -- mesocyclones in
particular. Cell training is anticipated.

The biggest question continues to be where does this MCS move as
compared to the track of the heaviest rains from the past week,
which were 8-10" to the west-southwest of Minneapolis and south of
St. Paul. Soils are mostly saturated here, and rivers are forecast
to be in significant flood from the anticipated heavy rainfall.
For now...think there may be just enough misalignment and just
enough uncertainty to refrain from a High Risk category, for now.
Numerous local instances of flash flooding are likely, some
significant, so the Moderate Risk remains in effect.


...Four Corners/Southwest...
Anomalous moisture will remain in place across the Four Corners,
especially Colorado and Utah. Meanwhile a shortwave trough moving
into the area out of California/Nevada will result in more
widespread showers and thunderstorms, especially during peak
heating Friday afternoon. The Slight risk area remains in place and
has seen expansion based on the available ingredients and the 12z
HREF probabilities of hourly rainfall of 0.5"+. The Marginal Risk
was equally expanded to cover much of AZ.  Widely scattered to
scattered instances of flash flooding are more likely Friday as
compared with today due to the much improved upper level support.
Temperatures at 700 hPa appear too warm -- 15-18C -- implying a
bit of capping across western UT and NV -- so an effort was made to
leave those locations out of the risk areas.


...South Texas...
Continued east to east-southeast flow across the Gulf will bring
any embedded convection inland into south Texas Friday. The
convection should remain disorganized, but due to recent wet soils
from Alberto, isolated instances of flash flooding can`t be ruled
out.


...Portions of the Northeast...
A front moving south across New England and NY will support
organized convection Friday afternoon as the hot and humid air
mass ahead of the front is uplifted by the front. Since soils
across New England remain at or below normal for moisture content,
any convection should only result in isolated to widely scattered
flash flooding.  Good continuity was kept here.


...Northeast Florida/Southeast Georgia...
A new Marginal Risk was introduced for a small convective low that
the National Hurricane Center is monitoring for tropical cyclone
development which should be slowly recurving inland of the
Southeast Coast. Precipitable water values of 2-2.5" are expected
near the low. Hourly totals to 3" and local amounts to 5" are
possible within this regime, but such amounts should be isolated to
widely scattered at best. There is a range of QPF in the guidance,
and it`s unclear which areas will see overlap with any heavy
rainfall the previous day, so left the risk Marginal for the time
being.

Roth/Wegman


Day 3
Valid 12Z Sat Jun 22 2024 - 12Z Sun Jun 23 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
UPPER GREAT LAKES AND UPPER MISSISSIPPI VALLEY...

...Upper Great Lakes and Upper Mississippi Valley...
Heavy rain may cause flash flooding from the Twin Cities east
Saturday morning when added to rainfall across Wisconsin from the
overnight period. By this point however, the MCS should be picking
up eastward speed as it moves into Wisconsin. Thus...rainfall
amounts and flash flooding risk will decrease as the heavy rain
moves into areas of WI and MI that are somewhat drier than areas to
their west. Nonetheless...the MCS will remain intact as it moves
across the area, becoming more linear in structure. Strong storms
will develop to its south across IA/IL/MO...but the more
progressive and shorter lived nature of the storms should minimize
the flash flooding threat. The Slight Risk highlights where the
warm front/MCS causes rain to be most persistent before the cold
front pushes all of it to the east. Western areas across the
Northern Plains were trimmed back, based on recent model trends.


...Southwest...
As the remains of Alberto aloft continue chugging westward through
the eastern Pacific, moisture pools across the Southwest to the
southeast of a positively tilted upper level trough. There are
distinct differences of opinion between the GFS and NAM moisture
fields. Daytime heating should foster 1000-1500 J/kg of ML CAPE,
which when combined with the moisture could lead to hourly rain
totals up to 2.5". The guidance in general appears too dry across
this region, based on the available ingredients. Issues with flash
flooding are expected to be isolated to widely scattered.


...Deep South Texas...
The potential for another tropical low to impact the Gulf Coast of
Mexico could spread heavy rain north into Deep South Texas. Unlike
Alberto, the rain with this system appears more confined, so only
Deep South Texas has potential for resultant flash flooding.


...New England and NY...
Continued passage of upper level disturbances would result in
another round of afternoon showers and storms that may cause
isolated flash flooding as tropical moisture returns to the area
from the south as the front from Friday retreats back to the
north.


..Northeast Florida/Southeast Georgia...
A new Marginal Risk was introduced for a small convective low that
the National Hurricane Center is monitoring for tropical cyclone
development which may still be slowly recurving inland of the
Southeast Coast. Precipitable water values of 2-2.5" are expected
near the low. Hourly totals to 3" and local amounts to 5" are
possible within this regime, but such amounts should be isolated to
widely scattered at best. There is a range of QPF in the guidance,
and it`s unclear which areas will see overlap with any heavy
rainfall the previous couple days, so left the risk Marginal for
the time being.

Roth/Wegman


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