210
FOUS30 KWBC 150055
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
855 PM EDT Fri Jun 14 2024

Day 1
Valid 01Z Sat Jun 15 2024 - 12Z Sat Jun 15 2024

...THERE ARE MARGINAL RISK OF EXCESSIVE RAINFALL AREAS ACROSS
PORTIONS OF THE EASTERN U.S. AND THE PLAINS...

...South Florida.
Removed the Marginal Risk area from most of the state with the
expectation that convection across the storms over the north-
central portion of the peninsula continue to weaken with the loss
of daytime heating. Maintained the Marginal risk across the far
southern portion of the peninsula for the low-end potential for
excessive problems as rain begins to shift back northward from the
Florida Strait in the pre-dawn hours. Expected rainfall rates and
amounts overnight would not normally rise to a Marginal risk
level...even if mesoscale forcing like frontogenesis enhanced
rainfall rates. Rather concern remains driven by antecedent
conditions.

...Northern Mid-Atlantic Region to Northeast U.S....
Scattered showers and thunderstorms developed earlier in the day
ahead of a cold front progressing through the Northeast U.S. and
northern Mid-Atlantic region. The airmass was sufficiently moist
and unstable enough to support locally heavy downpours that have
led to problems. Some convection has already taken on linear
orientation as far west as northern West Virginia into far
southern/western Pennsylvania...setting up the potential for a
period of cell training into the late evening. Given precipitable
water values approaching 1.5 inches and instability lingering into
the evening hours...a few cells will continue to be able to produce
1.5 inch per hour rates and spotty storm totals in the 2 to 3 inch
range before rates weaken in response to low-level stabilization
late this evening.

...Portions of the Central/South Plains Including the Front Range...
On-going convection should persist into late evening as a well-
defined shortwave trough and associated close low ejects out of the
Southwest US and towards the Central Rockies. The synoptic scale
forcing will encounter low-levels being fed moisture by an
increasing low-level southerly flow and plenty of static
instability and moisture flux convergence. With this
combination...rainfall rates with the strongest storms are expected
to reach 1 to 2 inches per hours with localized storm totals
exceeding 2 inches in the event of any cell-mergers and/or repeat
convection during the evening hours.

Bann

Day 2
Valid 12Z Sat Jun 15 2024 - 12Z Sun Jun 16 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
UPPER MIDWEST...

...20Z Update...

A couple changes of note were made to the previous risk areas
across the Midwest/Plains, as well as Florida.

Across the Midwest, the SLGT risk is well represented within the
latest CAMs on both a deterministic and ensemble output, as well as
a favorable probabilistic signal. Shortwave progression is fairly
steady with regards to timing and magnitude with a significant
convective signal focused in-of the corn belt up through the Upper
Mississippi Valley. Recent ensemble trends pinpoint QPF maxima of
2-3" in multiple locations, including further south over the NE/IA
border where morning convection will move overhead from the initial
wave out of the plains, then spawn near the warm front and motion
to the northeast later in the period. Probability fields within the
general confines of the NE/IA border actually jumped significantly
and became the main focus for the highest neighborhood probs for at
least 3" the period (Areal coverage of 25-40%). Areas to the north
where the SLGT was already in place still have reasonable
outcomes with the max potential still positioned across the Upper
Mississippi Valley, including the MSP metro area. Neighborhood
probabilities are hovering between 10-20% for local totals
exceeding 5", something that cannot be said for areas south into
the corn belt. This aligns well within the previous forecast and is
well representative for the SLGT continuation. Further south into
the southern High Plains, the MRGL was extended to include
convection initiating over the Panhandle along and just east of the
dryline positioned over eastern NM. This threat is more isolated,
but given the supercellular nature of the mode in question and
sufficient buoyancy, a low-end flash flood risk is forecast for
the late-afternoon and early-evening hours Saturday.

Across Florida, the trend downward in the convective pattern will
continue as the previous pattern shifts away from the Mainland
with a general seabreeze and diurnally driven convective focus
returning for the region. The morning may see some convection off
the FL Straits and adjacent Gulf push north through the FL Keys,
but the threat is not expected to produce any significant flood
threat and will dissipate through the mornings hours. A MRGL risk
was maintained where the seabreeze convection has the best chance
for isolated flash flood concerns as it propagates inland. This is
trending towards a low-end threat and the MRGL may be further
adjusted, or removed pending the future model output and observed
environment.

Kleebauer


...Previous Discussion...

...Upper Midwest...

As a strong upper level shortwave tracks northeast out of the
central Plains and into the Upper Midwest, cyclogenesis occurring
over the Plains will work to slow the eastward movement of the
storms, while Iowa and Minnesota are on the nose of an LLJ that
will be tapping increasingly deep moisture. The result will be the
potential for training storms along a slow-moving warm frontal
boundary over Iowa and Minnesota, with cold-frontal storms moving
through overnight Saturday night having the potential for heavy
rain as the nocturnal jet strengthens and PWATs associated with the
LLJ increase to over 1.75 inches ahead of the cold front. Thus,
there is potential for backbuilding storms as the LLJ shifts east,
but Corfidi vectors will be perpendicular (westerly) to the
southerly flow within the LLJ.

Soils across much of Minnesota and northwestern Wisconsin are more
saturated than normal for this time of year, so they will be a bit
more flood prone than normal. Further, the potential for heavy rain
with backbuilding convection being heightened necessitated the
continuation of the inherited Slight Risk. The area was shifted a
bit to the north with the latest guidance, but only nominally. This
is likely to be the first of several consecutive days with threats
for heavy rain for the northern Plains and upper Midwest.

...Florida...

Plenty of lingering moisture and weak steering flow should allow
the typical afternoon convection to develop across portions of the
Florida Peninsula Saturday afternoon. Guidance is in fair agreement
that the center of the Peninsula, generally north of Lake
Okeechobee will be favored for the heaviest rainfall on Saturday,
but it will be significantly diminished in both coverage and
intensity from the rain over the Peninsula over the past few days.
At this point, the heaviest rain should be sufficiently offset from
the heaviest hit areas of the past few days, so flash flooding
should be generally isolated.

Wegman

Day 3
Valid 12Z Sun Jun 16 2024 - 12Z Mon Jun 17 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
NORTHERN PLAINS AS WELL AS FOR PORTIONS OF SOUTHERN LOUISIANA...

...20Z Update...

The previous SLGT risks over the Gulf Coast and Northern Plains
were maintained with little change in the overall risk area(s). The
best chance for flash flooding will likely occur over the Northern
Plains with a focus along a stalled cold front across the NE/SD
border up through southern MN. A fairly strong signal at lead for
exceptional 850mb frontogenesis located over northern NE extending
east-northeast into the northern Midwest. This is textbook for
primed 925-850mb convergence within the confines of the front,
tapping into suitable, moist airmass as PWATs settle within 1-2
deviations above normal. The signal has been most pronounced across
northern NE within the Sand Hills area which generally limits
widespread flash flooding concerns due to the efficiency of the
soils to prevent runoff. The same cannot be said for points to the
east, mainly across northeast NE into northwest IA and southwestern
MN, both areas that are more prone for flash flood concerns, and
will have seen a decent area of QPF in the periods prior. Any
persistent convergence pattern in those areas will be subject to
higher potential impact, so the SLGT risk from prior forecast was
well within reason to maintain continuity.

Across the Gulf Coast, signals for increasing tropical moisture
advection from the Gulf have remained steady from previous
forecasts with some discrepancy in the timing and location of the
primary plume ejecting north around the western periphery of a
broad ridge pattern to the east. In any case, the proposed
advection regime would be more than conducive for heavy rain
prospects, especially considering this airmass will be similar to
the one that plagued FL this past week. The persistence of this
pattern is less of a threat comparatively, but the chance for a
slug of heavy precip is still within the ensemble QPF footprint.
The SLGT risk remains given the persistence in guidance and
expected pattern evolution.

Kleebauer


...Previous Discussion...

...Northern Plains...

The cold front following the heavy rainfall over the northern
Plains on Saturday will stall out across the Slight Risk area
around the SD/NE/MN/IA border region. Highly diffluent upper level
low will increase lift and as the typical nocturnal jet kicks in
Sunday night, convection is expected to develop over northeastern
Nebraska through the evening, then track northeastward into
southwestern Minnesota through the overnight. Slower moving storms
and higher potential for training is expected across northeastern
Nebraska, meanwhile southern Minnesota will have the benefit of a
couple previous days of heavy rain over the region. The stalled out
front will serve as a focus for convection, increasing the
likelihood of training as the storm complex tracks northeast. With
plenty of moisture available for heavy rain, a Slight Risk upgrade
was introduced with this update to account for both higher
potential for training over this region and favorable antecedent
conditions into Minnesota.

...Southern Louisiana...

A portion of the same deep tropical air mass that has haunted south
Florida over the past few days will have trekked from south Florida
to the central Gulf Coast by Sunday. Flow straight out of the
Caribbean will increase PWATs to near 2 inches by Sunday afternoon.
While convection is not expected to be too widespread, a nearly
unlimited supply of moisture will allow any storms that form to be
capable of very heavy rainfall rates. Both Baton Rouge and New
Orleans will be threatened by storms that may move over those
respective cities. Soil moisture levels are about normal for this
time of year, so there has been quite a bit of recovery from the
last several weeks of occasional heavy rain. Thus, this is a
lower-end Slight, as more widespread convection is likely in coming
days. Flood sensitive urban areas are at greatest risk of flash
flooding Sunday afternoon.

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