Excessive Rainfall Discussion

[Abbreviations and acronyms used in this product]
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
336 PM EDT Sat May 28 2022

Day 1
Valid 16Z Sat May 28 2022 - 12Z Sun May 29 2022


15Z Update...

A targeted slight risk has been added to far eastern PA, northern
NJ, and southern Upstate NY after coordination with the affected
WFOs. Rapidly developing thunderstorms ahead of a trough of low
pressure will continue to expand into the aftn as MLCape rises to
1000 J/kg and PWs remain around 1.2", above the 75th percentile
for the date. Mid-level divergence downstream of an upper low
combined with the RRQ of a departing upper jet streak will drive
deep layer ascent into these favorable thermodynamics, and the
available 12Z high-res guidance suggests convection will become
scattered to widespread from PA into New England. Most of this
area has been dry recently, but MRMS observations indicate 1-2.5"
of rain has fallen across parts of eastern PA, NJ, and the Hudson
Valley of NY in the past 24 hours leading to saturated soils and
compromised FFG as low as 1"/3hrs. With weak mid-level winds
driving slow storm motions, this FFG could be exceeded in a few
areas as noted by the 06Z HREF 3-hr FFG exceedance probabilities.
While the coverage of flash flooding instances will likely be
limited, the slow storm motions and favorable antecedent
hydrologic conditions support an upgrade to a SLGT risk for this
targeted area.

Otherwise, the inherited MRGL risk was expanded only slightly to
the north to account for coverage of thunderstorms this aftn/eve
with 1-2"/hr rain rates. Rainfall outside of the SLGT risk area
has been much less, so soils should mostly be able to absorb these
rain rates. Additionally, storm motions will likely be faster as
the upper trough and associated deep layer ascent begin to weaken
and merge into the westerlies. This is likely to limit the
duration of any heavy rainfall across New England, leading to only
an isolated flash flood risk, so the MRGL risk was maintained.

...South Florida...
A washed out frontal boundary draped across the Peninsula
interacting with daytime heating sea breeze convection should
allow for the development of a few clusters of strong
thunderstorms this afternoon. These cells will be capable of
producing intense rain rates over the highly urbanized south
Florida corridor. HREF probabilities indicate that some 2" an hour
rains are probable, with daily totals locally exceeding 3" in
spots. If these higher totals end up over an urban location then
some flash flood issues could arise. Thus we will maintain the
Marginal risk.

...Northern Plains...
A mid level impulse out ahead of the large scale troughing over
the western U.S. will lead to convective development over the High
Plains this afternoon. There is still a signal in most guidance
for an axis of 1-2" of rain from eastern MT into western ND in the
afternoon/evening hours. The lack of deeper instability and drier
soils/streamflows should keep any flash flood risk fairly isolated
at best. However there is some signal for amounts locally as high
as 3" near the comma head of the developing low where increased
convergence should support a longer rainfall duration. If amounts
of 3" were to occur then some runoff issues seem possible, thus we
will maintain the Marginal risk over the region.


Day 2
Valid 12Z Sun May 29 2022 - 12Z Mon May 30 2022


An amplifying mid-level closed low will drop into the
Intermountain West Sunday, leaving divergent downstream flow
across the Plains with embedded shortwaves lifting northeast
around the periphery of this trough. As this low deepens, it will
also drive a strengthening jet streak into the Plains, with
attended LFQ diffluence aiding to enhance deep layer ascent. To
the east of the closed low, a cold front will be driven slowly
eastward via height falls, moving from the Northern High Plains
into the Upper Mississippi Valley through the day and into the
night. 850mb flow gradually backing to the S and increasing to
20-40 kts ahead of this front will drive impressive moist
advection noted by PWs progged to reach +2 sigma coincident with
MUCape climbing towards 2000 J/kg. The overlap of the strong lift
and favorable thermodynamics will allow for scattered to
widespread thunderstorms to develop on Sunday.

While there remains some uncertainty into the coverage of
convection, there is likely to be at least a few rounds of storms
moving SW to NE ahead of, and along, the cold front as multiple
impulses embedded within the flow surge northward. The mean 0-6km
mean wind is forecast to be quick at 30-40 kts from the SW, but
unidirectional flow and propagation vectors aligned to the front
suggest an enhanced training potential. With rain rates likely
exceeding 1"/hr within the stronger convection, some places could
receive as much as 3" of rainfall. While antecedent conditions are
somewhat inhibiting to flash flooding due to drier than normal
soils from a lack of recent rainfall, the inherited MRGL risk will
remain with only cosmetic changes to account for recent guidance,
since any repeating storms could produce a few instances of flash


Day 3
Valid 12Z Mon May 30 2022 - 12Z Tue May 31 2022


A deep mid-level trough centered over the Intermountain West will
shed a closed low northeastward on Monday, with this feature
shearing out into an open wave across the Northern Plains by the
end of D3. At the same time, a secondary shortwave will intensify
as it digs across the Pacific Northwest, while downstream across
the eastern CONUS a large mid-level ridge amplifies. Together,
these will keep the primary trough axis anchored across the High
Plains during Memorial Day.

The evolution of this trough will allow an upper jet streak to
round the base and intensify towards 110 kts Monday, placing
favorable LFQ diffluence for ascent into ND/MN. At the same time,
a surface low beneath the upper trough will swing a cold front
eastward through the day. Ahead of this front, impressive
thermodynamic advection will occur on 850mb flow reaching as high
as 50kts from the south, driving PWs to 1.5" and MUCape of nearly
2000 J/kg. The associated theta-e ridging within this warm sector
will likely pivot north/northwest into a strengthening deformation
axis north of the surface low, driving locally enhanced ascent,
and there is likely to be widespread showers and thunderstorms
across the Northern Plains and Upper Mississippi Valley on Monday.

The guidance continues to feature some longitudinal spread in the
placement of heaviest rain due to uncertainty in the speed and
placement of the low and associated forcing. However, this spread
has decreased, and both the GEFS and ECENS ensemble means indicate
a threat for more than 3" of rainfall near the ND/MN border and
arcing into eastern SD. This area seems favorable as storm motions
may be a bit slower along the deformation axis, producing the
greatest threat for training. The inherited SLGT risk still looks
good for the area of heaviest rainfall, with the threat for flash
flooding also aided by antecedent rainfall expected on D2. Only
minor adjustments were needed to account for latest guidance.

A secondary area of enhanced rainfall may develop late Monday
night from eastern KS into western IA as the LLJ ramps up and
impinges into the eastward moving cold front, and this is
reflected by an increase in GEFS/ECENS probabilities. At this time
there is too much uncertainty in amounts and placement for a
targeted SLGT risk addition, but it is possible one will be needed
for this area with later updates.


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

Last Updated: 336 PM EDT SAT MAY 28 2022