FOUS30 KWBC 200828
QPFERD

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

Day 1
Valid 12Z 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, albeit shifted a bit to the south of
inherited. 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.

...West Texas into southeast New Mexico...

The northern portion of the remnant moisture plume of the 
dissipating T.S. 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, the Slight remains in
place...albeit lower confidence as the somewhat southward track of
Alberto and it's slowness with getting its act together in the form
of more organized convection has resulted in a drying trend with
expected rainfall. This area could very possibly be downgraded with
future updates should the rain underperform.

In South Texas, somewhat underperforming rainfall totals with T.S.
Alberto and a faster 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. In coordination with CRP/Corpus
Christi, TX and BRO/Brownsville, TX forecast offices, the inherited
Slight Risk was downgraded to a Marginal with this update. 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. As on previous days...hot temperatures and
abundance of moisture combined with being on the northern side of
the upper ridge resulting in periodic upper level disturbances
passing through will result in another afternoon and evening of
widely scattered convection. The storms are likely to be slow
moving, and therefore isolated flash flooding is possible under the
stronger convection.

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
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