247
FOUS30 KWBC 142259
QPFERD

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

Day 1
Valid 2242Z 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 WESTERN PLAINS...

...2250Z Update...
Given convective trends...expanded the Marginal Risk area southward
and westward across portions of the northern Mid Atlantic...refer
to WPC Mesoscale Precipitation Discussion 439. Similarly, the
Marginal Risk area was pulled southward...refer to MPD 438.
Finally...will remove the Slight risk area across southern Florida
with this update given trends in radar and satellite imagery.

Bann


...16Z Update...

Moderate Risk inherited for the period was downgraded to a SLGT
across South FL. More on the change within the "Florida" sub-
heading below....

Elsewhere, the two MRGL risk areas across the Northeastern U.S and
the Central Plains were generally maintained with only some minor
deviations in the outlined risk areas. The setup across the
Northeast U.S is unfolding currently with a cold front progressing
through the eastern Great Lakes with sights further east by the
second half of the forecast period. A shortwave across western PA
is holding together fairly well with a core of convection
propagating eastward into the northern Appalachians within the
state. The forward speed of the cluster is a positive for limiting
the threat for flash flooGiven radar trends and lack of ding,
although some locally heavy rain within an urban setting or complex
terrain could spell some localized threats in the next 2-4 hours
on its progression. Guidance is keying on the shortwave entering
NNJ and the NYC metro with a local enhancement of precip on its
approach. There will be a general maxima within the corridor across
NNJ through NYC into the Lower Hudson and western CT based on the
latest probability fields, but the threat is generally maxed
between 2-3", at best due to the progressive nature of the event in
general. This will limit the extent of flash flooding with
generally localized impacts relegated to the urban corridor.

The threat over the plains remains steadfast with a quick
propagation of the shortwave trough and associated convection
expected later this afternoon and evening. One area to note for a
general maximum is across the Caprock of eastern NM where a surface
trough focus will provide an anchor for anticipated supercell
development earlier this evening. This will promote slow cell
motions and a higher localized threat once it develops. Otherwise,
remainder of the plains maintains general continuity.


...Florida...

Previous forecast was downgraded to a SLGT risk across all of South
Florida with a highlighted "higher-end" SLGT located over the
southeastern coastal plain in proximity to Miami up through Fort
Lauderdale. Recent trends in guidance and observations favor less
of a risk for widespread impacts and more isolated/scattered in
terms of flash flood potential given the improvements in antecedent
conditions with stream flows and urban flooding finally able to
recede over the past 12+ hours. 12z soundings out of KEY and MFL
indicate a dry tongue located above 600mb with much of the remnant
moisture trapped beneath 700mb, maintaining PWATs within 2-2.1".
Comparatively, this is a far cry from the previous days with a
much greater deep moisture presence within the column allowing for
a better warm cloud layer presentation and PWATs bordering on the
upper 95-99th percentile climatologically. Upper pattern is in a
transition phase with the deeper tropical airmass now pressing back
to the west as we begin to see more influence of the surface ridge
over the Atlantic flex further west and southwest over the next
24+ hours.

Across the southeast Gulf, a cluster of thunderstorms will maintain
intensity and begin to press southeastward as outflow driven cold
pools are forecast to bring the convective cluster towards the Keys
and not drive inland of the Peninsula. The main convective threat
over the southern Mainland will stem from a modest LLJ located
across the southern tip of the Peninsula with destabilization over
land priming for convective development now through the early
afternoon. Recent HREF and associated CAMs continue to signal the
heaviest rain threat across the Homestead to southern Miami area
with some scattered convection impacting areas north of Miami
proper up through West Palm Beach. Given the environment in place,
more organized, widespread heavy thunderstorms are less likely than
previous periods, but the potential for locally heavy rainfall
remains with probabilistic fields noting a 20-40% chance of
exceeding 3" between Miami to Fort Lauderdale with the max
positioned further south across the Everglades into Homestead
(50-60%). Conditions will settle after 20z as the primary threat
shifts north with the best chances for heavy rainfall likely around
West Palm Beach up through central FL.

With the general improvements in the FFG`s across South
Florida, more isolated to scattered in convective coverage, and
in collaboration with WFO Miami, the MDT risk was dropped to a
SLGT risk with emphasis on the southeast urban corridor due to some
relevant sensitivities.

Kleebauer


...Previous Discussion...

...Northeast...

A potent upper level shortwave will race from the Midwest to New
England by early Saturday morning. The associated cold front will
also move across New York State and into New England. Convection is
expected to form with these forcings from Pennsylvania north and
east into New England. Atmospheric moisture will be somewhat
limited, and the progressive nature of the storms will limit most
flooding concerns. However, given the low FFGs associated with the
urban I-95 corridor and potential for cell mergers to occur as the
storms move through NYC and into southern New England, the Marginal
Risk area was expanded east to include almost all of southern New
England except the Cape, as well as all of Long Island. Isolated
instances of flash flooding are possible in the most flood-prone
areas as the storms move across the area.

...Central Plains...

A separate upper level shortwave will track northeastward from
Arizona to the central Plains today. Lee-side cyclogenesis over
eastern Colorado will tap into moisture from the LLJ with storms
developing along both the warm and cold fronts across the Marginal
Risk area. Despite the presence of an LLJ, moisture will be
somewhat limited as northeasterly flow at 850 mb over the Gulf has
largely prevented full Gulf moisture from moving into the central
Plains. Thus, moisture availability will be a limiting factor for
the storms. Nonetheless, there is some opportunity for raining
convection in the late afternoon across New Mexico and eastern
Colorado. The storms will congeal into more of an MCS overnight as
the storms move a bit more quickly eastward across Nebraska and
Kansas. With the ingredients for flash flooding largely separate
from each other, and the area not particularly flood-sensitive, the
inherited Marginal Risk remains largely unchanged with this update.

Wegman

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