Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
300 PM EDT Sat Apr 26 2025
Day 1
Valid 16Z Sat Apr 26 2025 - 12Z Sun Apr 27 2025
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
NORTH AND NORTHWEST TEXAS INTO SOUTHERN OKLAHOMA...
...16Z Update...
The Slight Risk area was adjusted downstream of the ongoing
convection occurring across far north Texas and central and
southern Oklahoma. A well established Mesoscale Convective Vortex
is approaching the Oklahoma City area as of the time of this
writing, with training thunderstorms actively causing flash
flooding south of the city. For that reason, a higher-end Slight
is in effect roughly from the Oklahoma City south to the Texas
state line along the I-35 corridor, extending west to about
Wichita Falls, TX.
Guidance is in understandably reasonable agreement that this MCS
will persist over the next few hours, as the embedded MCV tracks
along I-44. For now, the slow movement and lack of instability
suggests metro Tulsa only has an isolated (and therefore, Marginal)
flash flooding threat.
The Slight and surrounding Marginal were trimmed on the
northwestern side behind the MCS, as none of the guidance suggests
that there will be afternoon convection in this area with all the
instability having been used up. The Marginal is largely unchanged
owing to poor agreement in the CAMs as to how new convection will
emerge this afternoon across portions of central and northern
Texas. The behavior of the MCS as a whole will play a big role in
how any new convection develops this afternoon.
Wegman
...Previous Discussion...
...Summary...
Based on the latest observational and guidance trends, have
expanded the Slight Risk from yesterday's Day 2 ERO a little
farther west-southwest to include a bit more of the Rolling Plains
in Northwest Texas. Otherwise, more minor adjustments were made to
the surrounding Marginal Risk area.
...Southern Plains...
MCS and resultant MCV currently traversing the Texas Panhandle
early this morning will gradually track ENE along the quasi-
stationary surface frontal boundary across much of the TX-OK Red
River Valley today. By late morning, the weakening LLJ will allow
the MCS to diminish, while later in the day (afternoon) a narrow broken
line of convection develops farther upstream along the dryline
across Northwest TX during peak heating. This second area of
convection is expected to take on a more ESE trek during the
afternoon and evening, thus the southwest 'bulge' to both the
Slight and Marginal Risk areas across the Rolling Plains and (for
the Marginal Risk) into northern portions of the Hill Country.
Overnight, the surface stationary front will eventually lift
northeast as a warm front Saturday night, taking the more
organized/widespread convection with it.
As with yesterday's Day 2 ERO, the Day 1 Slight Risk area was
adjusted to capture areas where 1-2+ in/hr rainfall rate probabilities
from the HREF/RRFS ensembles are highest, while also overlapping
areas where heavy rainfall has fallen over the past couple of days
(wetter antecedent interviews).
Hurley
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
300 PM EDT Sat Apr 26 2025
Day 2
Valid 12Z Sun Apr 27 2025 - 12Z Mon Apr 28 2025
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTHERN AND SOUTHEASTERN MONTANA...
...1930Z Update...
In coordination with BYZ/Billings, MT forecast office, a Slight
Risk upgrade was introduced with this update. Increasing forecast
rainfall (areal averages of 2-3 inches southwest of Billings) will
combine with high snow levels and burn scars to locally enhance the
flash flooding potential in this area of Montana. The combination
of surface troughing ahead of a northward moving leeside low out of
Colorado, an advancing upper level low and trough out of the
Rockies, and the northern tongue of an LLJ that originates straight
out of the Gulf will all combine their respective forcings to
support the area of heavy rain across southern Montana Sunday and
Sunday night. High snow levels will support rain falling on snow,
locally increasing snowmelt. Burn scars from various large fires
last year will also locally increase the flash flooding potential
due to locally enhanced runoff. FFGs in portions of the Slight
Risk area are under an inch per hour, which are rates that are
probable to be exceeded given the convergence of all the
aforementioned ingredients favoring heavy rainfall. Widely
scattered instances of flash flooding are likely.
Wegman
...Previous Discussion...
Mid-upper trough across the southern Great Basin early Sunday will
lift across the central Rockies by 12Z Monday. This in-turn will
nudge the upper ridge axis east of the Northern Plains and Upper MS
Valley late Sunday-Sunday night, with a quick onset of upper level
divergence in its wake. Along with the more favorable dynamical
forcing (upper divergence and deep-layer ascent), deepening
southerly flow will tap the western Gulf and lead to an uptick in
low-mid layer moisture transport, with TPW values climbing to 1.3+
inches. Deep-layer instability meanwhile will also be ramping up,
to the tune of 1000-2000+ J/Kg MUCAPE per the 00Z models. To some
degree, the latest models indicate MCS development along the
western periphery of the upper ridge, tracking east to eventually
southeast along the 850-300 mb thickness. For the most part, the
forward-propagating Corfidi vectors would support a rather
progressive area of any organized convection that can grow
upscale, though much will depend on the strength of the LLJ. For
now, have maintained a fairly broad Marginal Risk area given the
potential of intense short-term (sub-3hr) rainfall rates.
Hurley
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
300 PM EDT Sat Apr 26 2025
Day 3
Valid 12Z Mon Apr 28 2025 - 12Z Tue Apr 29 2025
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE NORTHERN PLAINS INTO THE UPPER MIDWEST...
...1930Z Update...
No significant changes were needed for the Marginal Risk area
across the northern Plains and upper Midwest. Deep soils across the
Dakotas and prevalence of lakes in northern Minnesota should
largely preclude other than isolated flash flooding. The cold
conveyor belt of an intensifying low will run into a strong cold
front with deep cold air behind it, resulting in ample baroclinic
forcing for steady light to moderate rain across the Dakotas and
Minnesota. Meanwhile into Wisconsin the pre-cold frontal convection
will be the driver for any isolated flash flooding.
There is some potential that before snow levels crash in
southeastern Montana that lingering rainfall from Sunday night may
continue into Monday. This may resulting in a locally enhanced
flash flooding risk into far northern Wyoming with the burn scars
on the northwest facing slopes of the Bighorn Mountains. The
predominant northeasterly flow orthogonal to the range may locally
enhance upslope and heavy rainfall rates in the area.
Wegman
...Previous Discussion...
Mid-level circulation will open into a shortwave trough across the
northern Plains and Upper Midwest on Monday-Monday night. Ample
pre-frontal low-mid layer moisture transport and PW values climbing
to 1.5-1.75" will make for a favorable thermodynamic profile,
along with deep-layer instability as MUCAPEs peak between
2,000-3,000 J/Kg (even ~1,000 J/Kg wrapping westward into the
TROWAL zone). The biggest concern during this period will be with
cell training, especially late Monday-Monday night across the Upper
Midwest with the strengthening LLJ and weakening downwind Corfidi
Vectors. For now, guidance spread with the heavier QPF is still
rather high (not well consolidated to support a Slight Risk at
this point). However, per the GFS/ECMWF and especially the GEM-
Regional, localized totals of 3-5+ inches within 12 hours would
appear likely in this setup.
Hurley
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
236 PM EDT Sat Apr 26 2025
During Tuesday-Wednesday, low pressure lifting away from Lake
Superior will push a cold front across the northeastern quadrant of
the lower 48 while the trailing part over the Plains stalls and
develops a wave as a Southwest U.S. shortwave ejects northeastward.
This pattern combined with anomalous moisture and sufficient
instability will favor a heavy rainfall threat over and near the
southern half of the Plains in this time frame. The Day 4/Tuesday
Excessive Rainfall Outlook introduced a slight risk from far north
Texas into southwestern Missouri, consistent with increasing model
QPF signals and high soil moisture. A broader marginal risk extends
from the southern Plains into the lower Ohio Valley.
Models/ensembles and first- guess fields generally strengthen the
heavy rainfall signal on Day 5/Wednesday with maybe a slight shift
eastward. Thus the Day 5 ERO continues the Slight Risk area
introduced overnight encompassing parts of Texas/Oklahoma and
western Arkansas/southern Missouri. The surrounding Marginal Risk
covers a similar area as the Day 4 ERO. The Storm Prediction Center
is also monitoring for severe threats on Tuesday from the eastern
Great Lakes and vicinity southwest into northern Texas, with a
smaller Wednesday risk area for parts of northern Texas. Check the
SPC outlooks for the latest info on severe threats.
After early Wednesday, continued progress of low pressure and its
trailing cold front should push rainfall across much of the East
but with lesser totals than forecast over the central U.S. Farther
west, shortwave energy and a frontal system should spread rain and
some high elevation snow from the Northwest through the northern
and central Rockies Tuesday-Thursday. By the end of the week some
rain may develop over the southern High Plains with the help of
easterly low level flow. An upper trough/cold front reaching the
West Coast by Friday-Saturday should bring rain and high elevation
snow into the Northwest.
Expect unseasonably warm temperatures over the eastern U.S. on
Tuesday ahead of an advancing cold front. Plus 10-20F anomalies
will equate to highs reaching into the 80s over the Ohio Valley.
Morning lows will be even a little more anomalous. Northern parts
of the East will trend cooler by midweek but locations farther
south should remain somewhat above normal through Friday. A more
amplified upper trough will likely push the next cold front farther
south, bringing temperatures over the East to within a few degrees
on either side of normal by Saturday. Meanwhile an upper ridge
drifting from the Northwest into the Plains after Wednesday will
support corresponding eastward progress of an area of highs 10-20F
above normal.
Santorelli/Rausch
Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
236 PM EDT Sat Apr 26 2025
During Tuesday-Wednesday, low pressure lifting away from Lake
Superior will push a cold front across the northeastern quadrant of
the lower 48 while the trailing part over the Plains stalls and
develops a wave as a Southwest U.S. shortwave ejects northeastward.
This pattern combined with anomalous moisture and sufficient
instability will favor a heavy rainfall threat over and near the
southern half of the Plains in this time frame. The Day 4/Tuesday
Excessive Rainfall Outlook introduced a slight risk from far north
Texas into southwestern Missouri, consistent with increasing model
QPF signals and high soil moisture. A broader marginal risk extends
from the southern Plains into the lower Ohio Valley.
Models/ensembles and first- guess fields generally strengthen the
heavy rainfall signal on Day 5/Wednesday with maybe a slight shift
eastward. Thus the Day 5 ERO continues the Slight Risk area
introduced overnight encompassing parts of Texas/Oklahoma and
western Arkansas/southern Missouri. The surrounding Marginal Risk
covers a similar area as the Day 4 ERO. The Storm Prediction Center
is also monitoring for severe threats on Tuesday from the eastern
Great Lakes and vicinity southwest into northern Texas, with a
smaller Wednesday risk area for parts of northern Texas. Check the
SPC outlooks for the latest info on severe threats.
After early Wednesday, continued progress of low pressure and its
trailing cold front should push rainfall across much of the East
but with lesser totals than forecast over the central U.S. Farther
west, shortwave energy and a frontal system should spread rain and
some high elevation snow from the Northwest through the northern
and central Rockies Tuesday-Thursday. By the end of the week some
rain may develop over the southern High Plains with the help of
easterly low level flow. An upper trough/cold front reaching the
West Coast by Friday-Saturday should bring rain and high elevation
snow into the Northwest.
Expect unseasonably warm temperatures over the eastern U.S. on
Tuesday ahead of an advancing cold front. Plus 10-20F anomalies
will equate to highs reaching into the 80s over the Ohio Valley.
Morning lows will be even a little more anomalous. Northern parts
of the East will trend cooler by midweek but locations farther
south should remain somewhat above normal through Friday. A more
amplified upper trough will likely push the next cold front farther
south, bringing temperatures over the East to within a few degrees
on either side of normal by Saturday. Meanwhile an upper ridge
drifting from the Northwest into the Plains after Wednesday will
support corresponding eastward progress of an area of highs 10-20F
above normal.
Santorelli/Rausch
