933
FOUS30 KWBC 201559
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1159 AM EDT Thu Jun 20 2024

Day 1
Valid 16Z Thu Jun 20 2024 - 12Z Fri Jun 21 2024

...THERE`S A MODERATE RISK OF EXCESSIVE RAINFALL FOR MUCH OF
SOUTHEASTERN 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.


...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.

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...

...Southern Minnesota...

A Moderate Risk of excessive rainfall remains in effect across
Southern Minnesota. The Moderate Risk and surrounding Slight were
shifted several counties to the south as compared to inherited.
This has been a very typical pattern with forecast convection in
the guidance over the past week. Unfortunately, this southward
trend continues to paint areas already hard hit from recent rains
with additional heavy rainfall amounts.

Meteorologically, an MCS will be ongoing across eastern portions of
the risk areas from the Day 1/Thursday period at the start of Day 2
at 7am. The end of this event will bring some rainfall...locally
heavy...to portions of the area, generally from the Twin Cities
south into southeastern Minnesota and western Wisconsin. The
convection should be on the weakening trend as typical for MCS`s as
solar heating quickly reduces the inflow into the storms and there
are many more heating sources for the storms, resulting in
increasing disorganization. Moisture and storms will still be
around across Minnesota and Wisconsin through the day, but anything
other than local organization and cell mergers is not expected
through the afternoon hours.

On Friday evening, a stronger low and associated stronger shortwave
will eject into the central Plains. The typical diurnal
strengthening of the LLJ ahead of the low will raise PWATs to close
to 2 inches. This will allow for very efficient rainfall production
as the storms that form in a rather disorganized fashion across
much of South Dakota organize into a strong MCS over the Moderate
Risk area.

As with much of the forecast convection over the past week...the
guidance continues to trend south with the track of the MCS. The
biggest question continues to be where does this MCS move as
compared to the track of the heaviest rains from tonight/Thursday
night`s rainfall/MCS. Nearly all of Minnesota has saturated soils,
including the southern part of the state, but significant overlap
between the heaviest rains tonight and the heaviest rains Friday
night would result in more widespread and significant flash
flooding. For now...think there may be just enough misalignment
between the 2 to keep any resultant flash flooding from reaching a
High Risk category, but this could very well change, especially
should future forecasts for Friday night correlate with where
tonight`s rains occur. Regardless, there should be enough flash
flooding even with enough misalignment that numerous local
instances of flash flooding are likely, and the Moderate Risk
remains in effect.

...Four Corners Region...

The already anomalous moisture in the area from Day 1 will remain
in place across the 4 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 as is...namely because more of
eastern Utah and a bit less of southwest Colorado will get the
heaviest rains. Should this shift east and overlap with the San
Juans of Colorado...assuming some flash flooding occurs
today...then it`s possible a further upgrade may be needed. Widely
scattered instances of flash flooding are more likely Friday as
compared with today due to the much improved upper level support,
but the severity of any individual flash flooding may be reduced a
bit in favor of the more widespread rains. Thus...the Slight
remains solidly in place.

...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 T.S. 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
somewhat more 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 flash
flooding. The Marginal was adjusted along and south of the expected
frontal position Friday afternoon.

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...

An MCS will be moving across Minnesota and Wisconsin to start the
day Saturday morning. 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 frontal 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.

...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 should be much 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 Day 2/Friday retreats back to the
north.

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