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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0831Z Apr 10, 2024)
 
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Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White


Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
430 AM EDT Wed Apr 10 2024

Day 1
Valid 12Z Wed Apr 10 2024 - 12Z Thu Apr 11 2024

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE
LOWER MISSISSIPPI VALLEY AND SOUTHEAST SOUTH OF I-20...

...Summary...
Based on the latest guidance trends, particularly the high-res
CAMs, have shifted the Moderate Risk area a little farther south
(closer to the Gulf Coast) compared to yesterday's Day 2 ERO.
Slight Risk area was broadened a bit to including a little more of
eastern OK-TX, as well as northward across parts of the Mid MS
Valley/Midwest.

...Synopsis...
Subtropical jet streak and associated shortwave energy entering the
base of the longwave trough will allow for further amplification of
the upper trough across the Southern Plains today, as 500 mb
height anomalies increase to 2-2.5 standard deviations below normal
per the ensembles. Toward the end of the period (by 12Z Thu), the
trough will take on a more negative tilt as it pivots through the
Lower MS Valley. QG forcing ahead of the trough, aided by
ageostrophic circulations owing to the northern and southern stream
jet streaks, will maintain robust deep-layer ascent and widespread
showers/tstms over the broad outlook regions.

...Mid-South and Lower Mississippi Valley to the Southeast...
The more intense rainfall rates will be confined south of the I-20
corridor close to the Gulf Coast, given the stronger deep-layer
instability (mixed layer CAPEs 1000-2000+ J/Kg) and more anomalous
low-level theta-e/moisture transport into the effective
warm/stationary front along and north of the Gulf Coast. Per the
ensembles, 850 mb southerly flow and moisture flux anomalies are 5+
standard deviations above normal, with mean integrated water
vapor transport (IVT) values peaking between 1000-1250 kg/ms as PW
values climb close to 2.00". Expect more numerous convective areas
producing hourly rainfall rates up to 2-3 inches within the Day 1
Moderate Risk outlook area, given current observations north of the
Upper TX Coast along with the aforementioned thermodynamic
parameters. With the latest 24hr mean QPF of 3-5+ inches, 00Z HREF
exceedance probabilities are not surprisingly highest within the
Moderate Risk area, including 40-60% probability that the Day 1
rainfall will exceed 5 inches, with spotty 20-25% probs of 8+ inch
totals.

...Mid Mississippi Valley to Western Ohio Valley...
Farther north, the degree of deep-layer instability and moisture
flux/transport is decidedly less favorable compared to areas
farther south; nevertheless, the southerly influx of deep-layer
moisture and some (albeit weaker) instability will be met with
stronger frontogenetic forcing ahead of a slower moving surface
front, as the favorable right-entrance region of the northern
stream jet axis lifts slowly northward. QPF footprint (widespread
1-2+ inch totals) and anticipated 1-3 hourly rates given the
thermodynamic parameters would support a Marginal to Slight Risk of
excessive rainfall, with the lower FFG values part of the
consideration for the Slight Risk areas.


Day 2
Valid 12Z Thu Apr 11 2024 - 12Z Fri Apr 12 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF THE
CENTRAL APPALACHIANS AND MID ATLANTIC INTO SOUTHERN NEW YORK...

...Summary...
Based on the latest guidance trends, have scaled back on both the
Slight and Marginal Risk areas inherited from yesterday's Day 3
ERO.

...Synopsis...
As the mid-upper level trough becomes more negatively tilted,
large-scale forcing and deep-layer moisture transport will remain
favorable up the Eastern Seaboard. The main differences however
include the increased forward speed of the upper trough and
surface low/associated fronts, along with weaker deep-layer
instability the atmosphere will have to work with.

...Parts of the Central Appalachians-Mid Atlantic...
Per collaboration with WFOs RNK and AKQ, have removed the Slight
Risk across much of central and southern VA. Rather, have confined
the Slight Risk where the guidance means indicate greater coverage
of 1-3 inches of rainfall in areas of decidedly lower FFG values
(widespread 1hr FFGs of inch or less, 3hr FFGs 1-1.5").

...Elsewhere across the Eastern CONUS...
Maintained the Marginal Risk area outside of the areas with lower
FFGs, as 1-3" of rainfall within 2-3 hours would more likely lead
to localized/isolated areas of flash flooding.


Day 3
Valid 12Z Fri Apr 12 2024 - 12Z Sat Apr 13 2024

...THERE IS A SLIGHT RISK OVER PORTIONS OF NORTHERN NEW ENGLAND...

...Summary...
Based on the latest guidance trends, along with hydrologic factors
(latest FFGs and relative soil moisture/soil moisture percentiles),
have expanded the Slight Risk area slightly northeast across
interior Maine. Otherwise, no other changes were made.

...Northeast...
Anticipated mainly (at most) localized instances of flash flooding
as the low lifts northward across western Quebec and the cold and
occluded surface fronts push through. A big factor to the excessive
rainfall outlook areas is the aforementioned wet antecedent soils
-- exacerbated somewhat by the ongoing snowmelt. Maintained
 the Slight Risk across eastern parts of NH into ME, where the
 areal-average QPF is a bit higher than other locations.

Hurley


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