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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1559Z May 27, 2024)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1159 AM EDT Mon May 27 2024

Day 1
Valid 16Z Mon May 27 2024 - 12Z Tue May 28 2024

...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF
THE NORTHERN AND CENTRAL MID-ATLANTIC STATES...

16Z Update...
Expanded the Slight Risk farther north into New York where area VWP
have been showing southerly 850 mb flow on the order of 35 to 45
knots. This was a bit higher than shown by earlier model
runs...which has resulted in higher precipitable water values
getting drawn northward than earlier forecast. In addition...the
12Z HREF probability values were pretty bullish for 1 inch per hour
accumulations near the border of northeast PA with NY state. Saw
little reason to make too many changes elsewhere.

Bann

...0830Z Excessive Rainfall Discussion...

...Mid Atlantic...

Surface low currently analyzed over Lake Michigan will continue to
move northeast into Ontario/Quebec with a cold front progressing
eastward through the Ohio Valley along its tail end. Pre-frontal
trough axis will also be located just ahead of the cold front with
a ridge of higher theta-E's advecting poleward along the confines
of the pre-frontal trough. The tongue of elevated theta-E's will
promote an area of generally favorable instability with higher
PWATs anomalies (2-2.5 deviations above normal) co-located within
the axis of favorable instability. The combination of
thermodynamic favor and increasing large scale ascent with the
incoming trough will help initiate a round of convection during the
midday time frame across the Mid Atlantic with locally heavy
rainfall likely within any cells that develop. 00z HREF probability
fields depict a heightened threat for 1-2"/hr rates across
portions of central MD up through much of eastern PA to the Lower
Hudson Valley. Totals generally 1-2" will be common within the
hardest hit areas, but neighborhood probabilities for at least 3"
are relatively high across parts of northeast MD up through the
Poconos with values upwards of 50-60% encompassing parts of
northeast PA. Considering the lower FFG indices in place over the
urban corridor down by Baltimore to Philadelphia, and for areas
across northeast PA where the relative forecast maximum is
expected, the SLGT risk inherited was maintained with a minor
southern expansion based on the latest probabilities and QPF
forecast within the zone encompassed. Flash flood prospects will
drop off precipitously with the passing of the cold front expected
between 00-06z which will end any further threat and push area
precip off to the northeast.

A secondary maximum is also popping up across hi-res for the area
around the VA Tidewater where a combination of strong mid-level
ascent from a vortmax swinging northeast out of NC will interact in
tandem with the approach of the surface trough to the west. A line
of heavy thunderstorms is increasingly likely late-afternoon to
early evening time frame with totals forecast between 2-4" within a
span of 2-3 hrs. The prospects are fairly consistent across most
guidance leading into the threat, so the localized maximum has
merit. Despite relatively higher FFGs for the area, the urban
corridor down by Hampton Roads and VA Beach will probably have the
greatest threat for localized flooding. The southern end of the
MRGL remained over the area for the threat.

...Southeast...

Locally heavy rainfall is possible over the Southeast through the
period due to the remnants of a complex ejecting out of the
Tennessee Valley initially, followed by cold front convective
initiation over the southern half of the MRGL risk area. Complex
out of the Tennessee Valley will continue to push southeast into
the Deep South with relative QPF maximums positioned across
northern GA through east-central AL. As the cold front progresses
into the region in wake of the complex remnants, more thunderstorms
will fire from the central Gulf coast up through southern GA into
southern SC before the front clears the region. Considering the
enhanced moisture and instability component in place over the above
areas, strong thunderstorms capable of 2-3"/hr rain rates could
cause some issues within urbanized areas in those general locales.
The areas in question have been hit recently in the past 1-2 weeks
with waves of heavy rainfall, so the FFG's are not as high as
usual, but they are high enough to limit the threat to more
localized standards with emphasis on population centers. With the
convective nature of the precip highlighting the mode for rainfall,
the threat was just high enough within the probability fields to
necessitate the continuation of the previous MRGL risk.

...Northern Michigan...

Intense deformation axis on the northwest side of a maturing
surface cyclone moving across the Great Lakes will induce a small
window for heavy rainfall across portions of the northern MI
"Mitten" up into the adjacent UP. Rates will be relatively mild
considering the lower end convective threat, although totals of
1.5-3" in a shorter period of time could very well cause some
issues locally. Considering the previous MRGL inheritance and
forecasted totals, didn't feel there was a need to remove
completely, thus maintained continuity.

Kleebauer


Day 2
Valid 12Z Tue May 28 2024 - 12Z Wed May 29 2024

...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF
TEXAS AND SOUTHERN OKLAHOMA...

...Southern Plains...

Extensive surface ridging to the north will aid in the advection of
rich Gulf air to the northwest across much of TX into southern OK
with the axis of higher theta-E's really denoting the wedge of
elevated instability over the southern plains. A weak mid-level
ridge will develop across west TX and parts of the southwest US
which will assist in the expected steering pattern of convection
across the region. Shortwave propagating across the northern
periphery of the ridge will help initiate convection across
portions of northwest TX into southern OK with a secondary wave of
development over west TX thanks to anticipated diurnal
destabilization to the east of a dryline positioned across the
Caprock down through the Texas Big Bend. Ample instability in place
given by the superfluous MUCAPE forecast over across the
aforementioned areas will lead to an enhanced convective
environment within a moisture rich environment thanks to the
poleward advancement of a Gulf- centric airmass. In fact, recent
ensemble guidance is in agreement on a broad area of 2-3 deviations
above normal axis of PWATs situated over the Concho Valley down
through central TX which will be important in the forecast when it
comes to the expected heavy rain threat.

Convective initiation will blossom and eventually congeal with
areal cold pool mergers moving southward due to the previously
mentioned storm relative steering pattern dictated by the mid-level
ridge to the west. Area thunderstorms are forecast to ride along a
tight theta-E gradient bisecting northern and central TX with
expected forward propagation to the south and southeast through the
TX Hill Country, eventually into the I-35 corridor between Waco to
San Antonio. QPF maximums of over 4" are becoming more common
within the latest guidance with the ensemble bias corrected QPF
driving upwards, closer to 5" in spots with the Hill country to the
northeast of the Edwards Plateau. Considering the environment in
place, rainfall rates exceeding 2-3"/hr is very likely within some
of the stronger cells with mature mesocyclones. A general 2-4" QPF
swath is in place over southern OK down through central TX with an
areal extent of at least 1" going back close to the dryline
positioning in west TX, and as far east as portions of the
ArklaTex.

Considering the magnitude of rainfall in place, the threat for
flash flooding, including some considerable flash threats a SLGT
risk was maintained from previous forecast with a higher-end
wording for flash flooding occurring over northern and central TX
with a relative bullseye across Hill Country. There is still some
discrepancy on the exact placement of the heaviest precip axis with
the global deterministic keying on different areas. This leads to a
slight pause on any further upgrades, however, the next run of CAMs
at 12z will be out through the full time frame of interest with
the accompanying hi-res ensemble suite adding in references of
probability and mean QPF fields. This will enhance the area of
focus more, likely leading to an upgrade of a MDT in the next
succession of updates across parts of TX, perhaps as far north as
the Red River.

...Northwest...

Surface low generation with a trailing cold front will be
positioned over the Pacific Northwest come Tuesday with attendant
height falls occurring to the west thanks to an incoming, strong
shortwave trough moving into the region. Increased upper level
ascent focused within a broad axis of diffluence on the lead side
of the mean trough will take aim over the interior Pacific
Northwest with an area of convection forming in-of the northern ID
mountains, likely along due to weak instability ahead of the
surface cold front and ample large scale ascent over the terrain.
Scattered thunderstorms capable of rainfall rates bordering
0.5-1"/hr will allow for localized flooding concerns within the
complex terrain over central ID up through the western portions of
MT. Total precip forecast is relatively modest with guidance
settling between 0.75-1.5" over the aforementioned area, but that
is just enough to cause some low-end flood concerns within the
rugged terrain and adjacent valleys north of the Snake River.
A MRGL risk was added to the above areas to account for the low-end
threat with the evolving dynamical setup.

Kleebauer


Day 3
Valid 12Z Wed May 29 2024 - 12Z Thu May 30 2024

...A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF
THE SOUTHERN AND CENTRAL PLAINS, LOWER MISSISSIPPI VALLEY, MONTANA,
AND NORTHWEST NORTH DAKOTA...

...Southern and Central Plains through the Lower Mississippi...

Mid-level ridge axis across the Southern Plains will slide
eastward over the course of Wednesday, but more convection will
spawn upstream under the broad expansion of relatively prominent
axis of theta-E's that will advect as far northwest as the Front
Range of the Rockies on the western flank of the surface ridge
encompassing much of the eastern CONUS. More shortwaves will round
the northern periphery of the mid-level ridge, helping to spawn
another round of convection Wednesday afternoon and evening, moving
south to southeast with the mean steering flow and expected cold
pool propagation. This round of convection is not currently
forecasted to be as robust due to lower bounds of instability
compared to the previous D2 offering. Still, the threat for more
flash flooding within zones expected to see impact during the
previous period, as well as an extension of the flash flood threat
further north and east has lead to a broad MRGL risk area extending
through portions of the southern and central plains, all the way
into the Lower Mississippi Valley as the steering pattern places
areas downstream into a better chance for organized convection
moving overhead. Ensemble guidance favors heavier precip further to
the east over the ArklaTex and portions of south-central TX within
the eastern flank of Hill Country through the I-35 corridor.
Considering the threat being conditional on what happens the day
prior, and QPF magnitudes less than appealing for a higher end risk
(1-2" totals), decided against any upgrade, but will mention the
likelihood of at least a SLGT is very high as we move closer and
have better definition on where the heaviest precip axis will lie.
For now, the inherited MRGL risk was maintained with some minor
modifications along the edges of the risk area.

...Montana and North Dakota...

Diffluent pattern across the Northwest will allow for a continued
threat of convection within the favored ascent pattern focused
within a formidable jet coupling situated over northern MT back to
WA state. The enhanced upper level dynamics in conjunction with
sufficient surface forcing will promote a period of scattered
strong to even potentially severe thunderstorms to propagate
northeast out of western MT and southeast ID through central and
eastern MT with the northeast expansion through northwest ND by the
end of the forecast period. QPF forecasts are sufficient for flash
flood concerns within the complex terrain of central MT, especially
with the environment capable for rainfall rates breaching 1"/hr at
times within the stronger cell cores. The previous MRGL risk was
maintained, but would not be surprised for a targeted SLGT risk
introduction in later updates as we move closer to the event time
and gain more confidence in the relative magnitude and placement of
the convective heavy rain potential.

Kleebauer


Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt