Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
822 PM EDT Tue May 30 2023

Day 1
Valid 01Z Wed May 31 2023 - 12Z Wed May 31 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PARTS OF
KANSAS...

01Z Update:

A Slight Risk was introduced this update for portions of Kansas.
Currently, a quasi west/east band of deep convection producing
intense rain rates (2-3"+/hr) is across portions of northwest
Kansas while across east-central KS, another cluster of deep
convection continues to organize. Last remaining images of visible
satellite imagery shows a thin boundary extending west/east across
the state with isolated convection beginning to develop in
between. The environment is characterized by weak mid-level
troughing coming out of the Central Rockies with another shortwave
noted over eastern KS. Precipitable water values range from around
1" across western KS to locally 1.5"+ across eastern KS. With the
low level southerly flow expected to increase this
evening/overnight, a gradual moistening of the environment is
expected with 850 mb flow increasing to 20+ kts. Pockets of
greater instability are expected, in areas not overturned from
earlier convection. Mean flow is out of the west/southwest and
rather weak. CAMs are handling current convection in northwest KS
poorly but also are very persistent, particularly the HRRR, that
additional convection will redevelop overnight toward sunrise
across eastern Kansas. 18Z HREF neighborhood probabilities
indicate nearly 40 percent chance for 3" in 3-hrs across
east-central KS with hourly total probs reach 70 percent for 1" in
1-hr and nearly 30 percent for 2" in 1-hr. Given the high
moisture, slow storm motions, and threat for additional convection
overnight (particularly eastern KS), a Slight Risk was introduced
for potential of scattered instances of flash flooding.

Taylor

...Northern CA northeast across ID and southwestern
MT/northwestern WY...
A Mid-upper low off the Central CA Coast this morning will slowly
push inland and wobble through SoCal this evening. Favorable
upper-level diffluence north of the compact mid-upper low, south
of the digging northern stream upper trough/height falls across
the Pacific NW, will generate favorable broad-scale forcing across
the outlook area this afternoon. PWATs are expected to climb to
between 0.75-1.00", which is ~1.5 standard deviations above normal
for the end of May (and above the 90th percentile), which is
sufficient for heavy rain concerns at elevation. Combination of
upper forcing, moisture, weak flow aloft, along with increased
daytime instability (mixed layer CAPEs peaking between 500-1000
J/Kg) will allow for scattered to more numerous showers/storms
within the outlook area -- a few of which will be capable of
hourly or sub-hourly rainfall rates of 0.50-1.00" which is around
FFG values. 12Z HREF neighborhood probs for exceeding 1"/hr rates
and/or 1/3-hr FFG values max out around 40%, supporting at least a
Marginal Risk outline. Lack of focus for convection (aside from
terrain) and dependencies on subsequent convection-induced
mesoscale boundaries precludes a focused Slight Risk contour.
Generally, a localized flash flood risk, especially over
urban/poor drainage areas and any recent burn scars into northeast
CA and southern OR, have a locally higher risk.


...Northern and Central Plains into Midwest...
A pair of mid/upper level shortwave troughs will pivot across
eastern portions of the Northern Plains and across the far
northern MS Valley today. Weak/modest upper divergence combined
with surface lift near and in advance of a frontal boundary will
support a heavy rainfall threat with afternoon and early evening
convection. This largely covers a similar area to the SPC area as
well. PWATs look to peak around 1.3" (near the 75th percentile,
per TOP sounding climatology), with SB CAPE rising to as high as
3000 J/Kg into the outlook area. Low-level inflow and effective
bulk shear appear sufficient to organize convection that forms,
especially initially along the CO/KS border and moving east.
Southern extent has potential for a more appreciable excessive
rainfall threat (OK/TX Panhandles, where FFG values are a bit
lower) depending on the forward speed and development of the
southern end of convection. 12Z HREF neighborhood exceedance
probabilities continue to suggest the potential for locally
extreme amounts (3-5"+), but it is too difficult to pinpoint a
specific area within this broad region (as indicated by a lack of
consistent HREF EAS neighborhood probabilities for 1" exceedance).
Did adjust the area slightly over southwestern MO and the TX
Panhandle to account for some 12Z CAM guidance that showed
potential for convection farther south later this
evening/overnight.


...Southeast OK into Northeast TX and Ark-La-Tex...
Precipitable water values of 1.5"+ and SB CAPE expected to
increase to 1000-3000 J/kg support the Marginal Risk area near the
Ark-La-Tex this afternoon. Low-level inflow appears weak, only on
the order of 10-15 kts, but the mean 850-300 hPa winds are
virtually null (which suggests minimal if not chaotic storm motion
and better rainfall efficiency than average from storms that do
form and maintain). Effective bulk shear may be as high as 25 kts
on the western edge of the MRGL risk area, which may help to
organize storms (and would potentially organize a complex, should
upscale growth be sufficient). Even so, FFG values are quite high
in this area so any excessive rainfall concerns should remain
isolated.


Fracasso/Churchill/Roth


Day 2
Valid 12Z Wed May 31 2023 - 12Z Thu Jun 01 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE
SOUTHERN HIGH PLAINS...

...West into the Northern Rockies and Plains...
The localized risk for excessive rainfall looks to continue across
portions of northern CA into northwest NV into Day 2 (beginning
12Z Weds), also extending farther northeastward across ID/MT and
into ND as well. This is primarily due to the aforementioned
mid-upper low over SoCal beginning to lift northeastward through
the Four Corners region (with another shortwave trough in the
northern stream digging on its approach into the PacNW). Axis of
sufficient instability, above normal PWs (as high as ~0.9" and
around the 90th percentile per the NAEFS) and generally weak flow
aloft will combine with afternoon heating to support another day
of isolated to scattered showers and storms. 12Z CAM guidance
shows the focus a bit farther northeast from CA/NV where they have
seen some locally higher QPE, where a targeted Slight Risk may be
warranted (contingent upon rainfall D1). Lower FFG values over ID
could support such an upgrade as well, but scattered/random nature
of post-initiated convection has been a hindrance to outlining a
higher threat.


...Central and Southern High Plains...
In coordination with ABQ, upgraded an area of northeastern NM into
the western TX/OK Panhandles to a Slight Risk for Wednesday. Upper
low moving through the Desert Southwest and a ~100kt subtropical
jet across northern Mexico will aide in providing lift to areas
just east of the Rockies. Initiation and focus of convection will
likely be a bit southwest of the D1 area with a moist
south-southeasterly flow promoting PWs near and over 1.25" (about
the 95th percentile) with CAPE to 1000-2500 J/kg. Organized
thunderstorms will slip eastward across northeastern NM into the
TX/OK Panhandle during the afternoon per the 12Z model consensus,
and although it is not universal, the multi-cycle trends support
the upgrade over this area as they have been relatively wet as
well in the past week (QPE 500-600%+ of normal). This generally
aligns with the SPC Slight Risk for severe (southern half) and the
CSU machine-learning first guess fields. Farther north, FFG values
are lower over northeastern CO into NE/KS, but sufficient QPF may
be focused farther south closer to the better inflow/moisture flux
and forcing.


Fracasso/Churchill

Day 3
Valid 12Z Thu Jun 01 2023 - 12Z Fri Jun 02 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER MUCH OF
EAST-CENTRAL MONTANA AND SOME NORTHERN PORTIONS OF WYOMING...

...Northern/Central Rockies and Plains...
Things start to get interesting going into Thursday across the
Northern Rockies and High Plains, as a remnant mid-upper low and
associated vorticity maxima eject northward from the Four Corners
region. This should provide broad ascent via DPVA, along with
modest diffluence aloft and sufficient instability. While we are
not quite in the range of CAM guidance at this juncture, the GEFS
does indicate non-zero probabilities (5-10%) for 2" and 3"
exceedance over central MT and the ECMWF EFI shows values over 0.7
(a fairly unusual event forecast from the ECMWF EPS). The GEFS
also indicates PWATs increasing to as high as 1.25" (+3.0 SD, or
approaching the 95th percentile climatologically). Have maintained
the inherited Slight Risk and this area will continue to be
fine-tuned with subsequent updates as more CAM guidance comes into
range through the D2 period. Farther south into the Central
Rockies and into the Great Basin, any flash flooding is expected
to be localized with much less organized convection.


...Central and Southern High Plains...
Farther south into the central and southern High Plains, another
active day of convection looks likely, with some additional lift
being provided by the northward moving aforementioned mid-upper
low and associated vort max. While convection overall should be
less organized than on Day 2, there is still some potential for
the introduction of a Slight Risk area (depending on the rainfall
footprint D2). First Guess fields do show a Slight Risk, so a
future upgrade is certainly possible. However, at this juncture
have maintained the Marginal Risk area due to the overall lack of
agreement between models with regard to the spatial placement of
2-3" amounts (varying across the Marginal Risk area, from
west-central KS to southern OK/North TX) and contingency on D2
amounts.


...Central and Southern Florida...
Also maintained the Marginal Risk for much of central/southern FL
going into Thursday, with an expansion across the FL Peninsula as
there is potential for the surface low to become more coherent and
track toward this area. Aloft, a very slow-moving upper-trough
over the Gulf will finally make its way directly overhead the
Florida Peninsula with the strength/track of its surface
reflection still uncertain (some 12Z CAM guidance was quite
aggressive). This should act to enhance typical diurnal convective
activity across the region, with strong mid-level lapse rates
allowing for more intense updrafts with tropical-like rainfall
efficiency (given PWATs ~2.0" and wet bulb zero heights of ~15k
feet). Any localized flooding impacts are most likely across
urbanized metro areas with poor drainage.


Fracasso/Churchill


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