Subj : HVYRAIN: Excessive Rainfall Discussion
To   : wx-storm@lists.illinois.edu
From : COD Weather Processor
Date : Wed Apr 05 2023 08:17:49

FOUS30 KWBC 050817
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
416 AM EDT Wed Apr 05 2023

Day 1
Valid 12Z Wed Apr 05 2023 - 12Z Thu Apr 06 2023

....THERE ARE SLIGHT RISKS FOR EXCESSIVE RAINFALL FROM SOUTHEAST
TEXAS TO THE OHIO RIVER VALLEY & IN PORTIONS OF NORTHERN NEW
YORK...

....Southeast Texas to the Ohio River Valley...

The upper trough over the North-Central U.S. and strong ridging
along the Southeast coast will work in tandem to funnel rich Gulf
of Mexico moisture north and east from southeast TX into the Great
Lakes. The region from east TX to the Mid-South remains favorably
positioned beneath the diffluent right-entrance region of a 150
knot jet streak. PWs up to 1.5" are expected as far north as the
Ohio Valley with PWs above 1.75" likely in the Lower MS Valley and
along the Upper Coast of TX. The 00Z HREF still shows up to 1,000
J/kg of MUCAPE as far north as the central KY, with upwards of
2,500 J/kg in central LA. One of the biggest supporting factors
for higher chances for flash flooding lies with a much more
saturated profile. The NAEFS showed mean specific humidity values
at the 925-850-700-500mb height levels that were all above the
90th climatological percentile through Wednesday afternoon and
into Wednesday night, indicating the atmospheric column through
much of the lower-mid levels of the atmosphere is forecast to
contain an exceptional amount of moisture. One supportive
proponent for flash flooding is that as the intense cyclone over
the Midwest crosses into Canada Wednesday evening, the trailing
cold front over the Mid-South and the Upper TX coast slows down to
becoming stationary. Mean 850-300mb winds and surface-500mb wind
shear will become oriented quasi-parallel to the front while
southerly 850mb winds continue to pump in Gulf of Mexico moisture
into the frontal boundary. This setup supports training
convection, and with warm cloud layers as deep as 10k feet in some
cases, efficient warm rain processes within these training
segments of convection have better odds to cause flash flooding.

FFGs are lowest in portions of the OH and TN Valleys where 1-hr
FFGs are as low as 1.5". Given the anomalous moisture and
sufficient instability, hourly rainfall rates topping 1.5"/hr are
doable Wednesday afternoon and into Wednesday night. 1-hr FFGs are
higher from southeast TX to the Lower MS Valley (lowest values are
2-3"), but greater instability and PWs do give a chance for hourly
rainfall rates to reach 2"/hr. Then add in the slowing progression
of the front with possible training convection, and 1-3hr FFGs
could be surpassed in these locations late Wednesday night and
into early Thursday morning. Some portions of southern AR,
northern LA, and central MS are more vulnerable due to 0-40cm soil
moisture percentiles as high as 70-80% according to NASA
SPoRT-LIS. These areas with more sensitive soils, as well as in
poor drainage areas and urbanized communities, are most prone to
potential flash flooding. The Slight Risk remains in place with
just minor tweaks to account for the latest QPF forecast. Farther
north, the Marginal Risk still appears to be in good shape for
portions of the eastern Great Lakes and interior Northeast.
Instability is a little harder to come by, so with hourly rates on
the low side compared to those in the South (topping out around
1"/hr), potential flooding will be more confined to poor drainage
areas and nearby creeks/streams. There is one exception to this
premise that is listed below.

....Northern New York...

The warm front lifting across the Great Lakes Wednesday morning
will progressively push north through the Northeast during the
daytime hours. The atmospheric moisture content is exceptional for
the time of year across the Great Lakes and Northeast. NAEFS are
showing PWs (1.25-1.5" PWs) that are greater than all 18Z, 00Z,
and 06Z in the CFSR climatology for the time of year. The
integrated vapor transport (IVT) are also an impressive 750
kg/m/s, which is close to values that would also be above the CFSR
climatology for the region. With the warm front also so far north,
there will be some instability to be had with the 00Z HREF
suggesting 250-500 J/kg of MUCAPE may be present. The other factor
prompting a Slight Risk upgrade here is snow melt. The Tug Hill
Plateau still contains anywhere from 1 to as much as 8 inches of
liquid equivalent snowpack. Most guidance suggests dew points
could climb to 50 degrees and breezy southerly winds, which are
snow eaters in their own right, let along periods of rain. The
concern is for the combination of hourly rainfall rates up to
1"/hr in an area with a combination of thawing soils and lingering
snow pack. The 00Z HREF does show 30-40% probabilities for >3" of
rainfall in 24-hours between Pulaski and Watertown, suggesting
there could even be a higher ceiling for possible rainfall totals.
Given the near record PWs for the date, sufficient instability,
and expected snowmelt, opted to upgrade a Slight Risk just east of
Lake Ontario and into the Tug Hill Plateau. The heavy rainfall
rates and increasing snow melt may lead to rapid runoff and
resulting flash flooding, particularly along complex terrain and
in poor drainage areas.

....

Mullinax




Day 2
Valid 12Z Thu Apr 06 2023 - 12Z Fri Apr 07 2023

....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FROM SOUTHEAST
TEXAS TO WESTERN LOUISIANA...

A stationary front just off the Texas coast and a conveyor belt of
500mb shortwaves tracking northeast into the Mid-Atlantic will
lead to an active day of showers and thunderstorms from Texas on
north and east into the Mid-South. In addition, the 250-500mb
trough axis is residing over Arizona on south through the Gulf of
California. This setup is not only positioning the right-entrance
region of the 150+ knot 250mb jet streak to reside over the
Mid-South, but a weaker (albeit, still diffluent) left-exit region
of a 100 knot jet streak over the tropical East Pacific sets up
near the Rio Grande. This jet coupling will foster plenty of
vertical ascent in the upper levels of the atmosphere. Strong
ridging off the East Coast is responsible for an expansive area of
southeasterly 850mb winds stemming from the western Caribbean and
extending through the Gulf of Mexico. This provides no shortage of
atmospheric moisture over southeast Texas with PWs >1.75" expected
along the Texas coast and as far north as southwest Louisiana.
Mean wind speeds within the 850-300mb layer are also a little
slower at 20-25 knots out of the SW, while also oriented parallel
to the front off the coast. With a persistent inflow of 850mb
moisture flux and a stalled front and upper level pattern, it will
be an active day of widespread showers and thunderstorms across
the southern tier of Texas and into the Mid-South.

Guidance is on board with heavy rainfall totals somewhere from
South Texas to as far north as east-central Texas and as far east
as the Lower Mississippi Valley. The keyword though is
"somewhere". Guidance is not in better agreement yet as to where
the axis of heaviest rainfall occurs. In addition, this setup is
dependent upon where heavy rainfall from Wednesday occurs, and how
much instability is available in these regions. The 500-1,000
MUCAPE values and PWs that, according to NAEFS, could top the 99th
climatological percentile give this event a good floor as a Slight
Risk, but would like to see how sensitive soils are in the region,
if more instability becomes available, and better agreement on
where the axis of heavy precipitation aligns before a Moderate
Risk can be issued. Should confidence grow in any combination of
these key factors playing a bigger role in flash flooding
potential over the next 24-36 hours, a Moderate Risk upgrade may
be needed.

...Pacific Northwest Coast...

The Marginal Risk along the coast of the Pacific Northwest remains
in place as guidance remains in good agreement on rich 850-700mb
moisture flux being oriented along the coast Thursday morning and
into Thursday afternoon. A sharp 250-500mb trough axis approaching
the coast Thursday morning is set to provide excellent diffluence
atop the atmospheric column. This is a warmer setup aloft, causing
rain to be the primary precipitation type in mountainous terrain
that has a deep snowpack. Over 2" worth of rainfall is expected
from the coastal range of California and Oregon to the Olympics,
where 0-40cm soil moisture percentiles courtesy of NASA SPoRT-LIS
are up to 70% saturated in some areas. The combination of heavy
rainfall rates up to a 0.5"/hr and some snow melt in higher
elevations could result in rapid runoff along complex terrain, as
well as rapid rises in creeks and streams.

Mullinax



Day 3
Valid 12Z Fri Apr 07 2023 - 12Z Sat Apr 08 2023

....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FROM EASTERN
TEXAS TO THE MID-SOUTH...

The Mid-South and as far south and west as eastern Texas remains
under the influence of the diffluent right-entrance region of a
roaring 160 knot jet streak over the Northeast. In addition, the
mean 250-500mb trough over northern Mexico is also producing its
own diffluent flow aloft over southeast Texas. The frontal
boundary that has acted as the trigger for rounds of heavy
rainfall will continue to stick around off the coast while
southerly 850mb flow out of the Gulf of Mexico continues to
deliver anomalous moisture to the region. The atmosphere looks to
remain quite saturated (1000-500mb RH values >90% for most of the
Slight Risk and Marginal Risk areas) and soils in these areas will
have continued to have grown more sensitive from multiple days
worth of rainfall. One limiting factor is that MUCAPE continues to
top 1,000 J/kg, and mean 850-300mb mean winds remain around 20
knots, allowing for storms to remain on the progressive side. That
said, the added soil sensitivity, anomalous moisture content, and
just enough instability aloft to allow some storms to produce
rainfall rates up to 2"/hr is enough to hoist a Slight Risk from
eastern Texas to the Mid-South. The Marginal Risk extends as far
south and west as the Corpus Christi area to as far est as
northern Georgia. The latter could see MUCAPE approach 1,000 J/kg
Friday afternoon and PWs could top 1.5". Contemplated extending
the Marginal Risk all the way to the Carolinas, but there was not
enough in the way of instability that led to concern for Excessive
Rainfall rates at this time. Should there be a trend for more
instability to become available in furture forecast cycles, an
extension of the Marginal Risk into portions of the central
Carolinas may be necessary.

Mullinax


Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/94epoints.txt__;!!DZ3fjg!9ggCsWxH4U_gUKkqyHX3wOQcfWrqEOaHU0hbiVS48bSq=
WsCEmWX-0vatjeyGLBS0zVR6vyBRhz0mBwqCR2SHGqlLH0M$=20
Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/98epoints.txt__;!!DZ3fjg!9ggCsWxH4U_gUKkqyHX3wOQcfWrqEOaHU0hbiVS48bSq=
WsCEmWX-0vatjeyGLBS0zVR6vyBRhz0mBwqCR2SHVFHG9_Q$=20
Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/99epoints.txt__;!!DZ3fjg!9ggCsWxH4U_gUKkqyHX3wOQcfWrqEOaHU0hbiVS48bSq=
WsCEmWX-0vatjeyGLBS0zVR6vyBRhz0mBwqCR2SHQ2tLsLY$=20


$$



=3D =3D =3D
To unsubscribe from WX-STORM and you already have a login, go to
https://lists.illinois.edu and use the "Unsubscribe" link.  Otherwise email
Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM.

--- SBBSecho 3.14-Linux
 * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105)

.