566
FOUS30 KWBC 142010
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
410 PM EDT Sun Jul 14 2024

Day 1
Valid 16Z Sun Jul 14 2024 - 12Z Mon Jul 15 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
CENTRAL MIDWEST...

...16Z Update Summary...

The previous SLGT risk was expanded to include a portion of Eastern
IA and points east including; Southern WI, Northern IL, and the
Southwest corner of MI around the Lake Michigan shore. MRGL risk
across the Southwest was adjusted on the northern and western flank
of the risk area to align with current QPF footprint and
neighborhood probability fields.

...Midwest...

Current MCS from this morning is pressing southeast with scattered
moderate to heavy rain signatures progressing through northwest IN.
This will continue to move east-southeast with a low threat of
flash flooding as it makes headway through the rest of IN into OH
before decaying. Upstream, a pretty robust vort max is quickly
pushing eastward out of the SD/NE border with sights downstream on
the Central Midwest corridor between Eastern IA, Southern WI, and
Northern IL. CAMs this morning were struggling mightily with the
handling of the previous complex which would have some implications
on the environment downstream of where the vorticity maxima will
move overhead. The HRRR is doing the best at handling the evolving
environment with a relatively good handle on the current MCS
propagation, and even its interpretation for later is fairly
aggressive with potential heavy rain footprint intersecting where
heavy rain has previously fallen.

12z HREF probability fields are suggestive of flash flood
prospects focused within the aforementioned areas with the
neighborhood signal for at least 2" across Southern WI and Northern
IL between 40-60%, a significant percentage considering the
antecedent conditions from previous complexes, and the urban
factors running from Milwaukee down through Chicago. 3hr FFG
exceedance probabilities are also running between 20-40% with
upwards of 50% within a small portion of Northern IL, just west of
the Chicago metro. That area was significantly impacted from last
evening, so the prospects of flash flooding are reasonably higher
within that corridor. This all culminates in a sufficient SLGT risk
threshold for impact with locally significant flash flooding
possible over Southeast WI down through much of Northern IL. The
previous SLGT risk was expanded to include those zones, as well as
the Southwest corner of MI where the forecast motion of the complex
should take the northern apex of the disturbance across Lake
Michigan into the bordering lake shore areas.

...North Dakota...

Shortwave trough over the Southern Plains of Canada will continue
pivoting to the southeast with large scale ascent pattern improving
across the northern tier of ND by nightfall. Scattered
thunderstorms will develop east of Minot and grow in coverage with
forward propagating speeds generally quick in the overall pattern.
This area has had some rain recently, but the conditions are still
on the drier side as depicted within the latest NASA SPoRT soil
moisture percentiles. The area of interest for potential flash
flood impacts will be localized to the river basin areas around
Grand Forks where the urbanization factors lend credence to the
potential for higher risks. Rates between 1-2"/hr will be plausible
in any stronger cell cores, but the relevant storm motions should
be sufficient to limit widespread impacts. With coordination
from the local Bismarck and Grand Forks WFOs, have maintained a
MRGL risk with potential for a targeted SLGT if necessary this
evening.

Kleebauer

...Southwest...

An influx of drier air at the lower levels around the 4 Corners
region prompted a reduction in the Marginal Risk area for today.
Guidance suggests the greatest moisture has pushed south and west
and will be focused into Arizona for today. Otherwise, expect
typical monsoonal coverage of thunderstorms this afternoon and
evening. There will be widely scattered thunderstorms over much of
the Intermountain West, but the prevalence of low level dry air
should keep most of the precipitation from those storms as virga
except into Arizona where the lower level moisture will remain. Of
course, particularly persistent storms especially those tied to
terrain may produce enough rainfall.

Wegman

Day 2
Valid 12Z Mon Jul 15 2024 - 12Z Tue Jul 16 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE CENTRAL MIDWEST...

...20Z Update Summary...

The primary change to the previous SLGT risk issuance was to expand
the risk area further west and south to adjust for the latest
trends in the QPF footprint across the region, and to account for
the recent short range bias of convection pushing further south and
west along the western flank of a developing complex. There was no
significant deviation in guidance to make necessary changes for the
MRGL risk areas across the Southwest U.S.

Kleebauer

...Midwest...

..20Z Update..

Recent hi- res ensemble and associated CAMs are in agreement of
another MCS developing in- of the Central Midwest with roots based
over IA where shortwave energy ejecting out of the Dakotas will
help initiate another convective cluster upstream of the mean
trough. Surface low development this evening will press northeast
with a trailing cold front migrating southeastward, getting a push
from building high pressure upstream in Canada. Front will advance
into the vicinity of Northern IL and Southern WI by early tomorrow
evening with the approaching of the aforementioned shortwave from
the west. The combination of favorable mid-level ascent and surface
convergence along the frontal boundary will aid in a development
of thunderstorms that will quickly congeal and grow upscale as they
interact with a budding LLJ.

The target remains over Northern IL as the primary location of
greatest impact, bisecting a region that will have seen three MCS`s
in a span of 48 hrs prior to the setup. The antecedent soil
moisture over the area will be pretty high leading to a heightened
risk of flash flooding. Latest deterministic output is pronounced
with areal QPF between 1.5-3" with QPF maxima around 5" possible
within the expected setup. Neighborhood probabilities for 1" and 2"
around Chicago through the suburbs and exurbs are generally
between 70-90+% respectively with 3" probs between 40-70% aligned
west to east from Davenport over into Chicago proper. This is
likely the location of the front within the means as convective
development and potential training will be conducive within the
confines of the front. The SLGT risk was expanded to include the
higher probabilities on the western flank of an expected line of
storms where low-level convergence and correlating higher theta-E`s
and instability will be co-located upwind of the future complex.

Kleebauer

...Previous Discussion...

On Monday evening, a surge of moisture associated with the typical
strengthening of the low-level jet (LLJ) will run into a cold front
supported by a strong and expansive area of Canadian high pressure
moving south across Saskatchewan and Manitoba behind a strong upper
level low over western Ontario. The clash of abnormally moist and
warm air pushing northeastward on the LLJ from the central Plains
into the Great Lakes and much cooler and drier air associated with
the Canadian high is likely to occur from the Mississippi River
east across northern Illinois and into southwestern Michigan. This
area has been hard hit with flooding rains overnight tonight, and
while there will be a good 36 hours between these events (barring
any rain tonight), saturated soils across northern Illinois will
certainly support the development of additional flash flooding
across this region. As always with stalled out front, the exact
placement of the front will determine who sees the most rain and
subsequent flash flooding. However, both saturated soils towards
Rockford and urbanization in and around Chicago are lowering FFGs,
such that the potential for multiple inches of rain in just a
couple hours could result in locally significant flash flooding.

With portions of northern Illinois having been hard hit this
morning, expecting some rain again tonight, and then following that
up with potential for multiple separate rounds of training
thunderstorms Monday night, that area has been highlighted for a
higher-end Slight, which depending on the behavior of the storms
tonight may potentially need to be upgraded further.

As the night wears on Monday night, the LLJ is expected to follow
its typical nighttime evolution turning more westerly with time.
This would align the resultant storms parallel to the front which
should move them more quickly, and with less resistance may allow
the front to continue to sag southward, but the potential for
multiple rounds of organized training thunderstorms will persist,
gradually moving south with time.

...Marginal Risks...

..20Z Update..

Probability fields continue to pinpoint the two main areas of
concern for isolated flash flood threats with the signal for at
least 1" of precip holding firm with an areal extent of 25-50%
across Southwestern CO and 25-40% over the Mogollon Rim while the
AZ/MX border stands closer to 70% for the 1" threshold. Monsoonal
push into AZ is more prominent for tomorrow compared to the
previous days with an emphasis over the southern reaches of the
state as convection initiates over Sonora with a northward
expansion occurring later in the afternoon and early evening.
Convection will be very much tied to the terrain, leading to the
localized flash flood concerns mainly driven by rates of 0.5-1"/hr
coupled within complex terrain. The signal for each area
highlighted is well within the MRGL risk threshold, but the
environment is generally conducive for the climatological flash
flood concerns and not so much the higher-end scenarios. Thus, have
maintained continuity in the respective risk areas.

...Previous Discussion(s)...

South-central Colorado: The guidance is marginally more optimistic
for heavy rain producing thunderstorms to develop across the
mountains of southern Colorado, particularly the San Juans on
Monday afternoon as compared with Sunday. A bit more westerly flow
in the upper levels may allow the storms to have a bit more upslope
support on the west facing sides of the mountains, which are the
areas most likely to see flash-flooding producing thunderstorms
Monday afternoon. That said, the chances are still on the lower-end
Marginal side, and could be downgraded should drying trends
continue. For now, have divided up the inherited Marginal to only
highlight the areas the guidance has better agreement for heavy
rains.

Southern Arizona: Pacific moisture streaming northward from the
Gulf of California will shift a bit to the east on Monday as
compared with Sunday. This should allow somewhat more organized
convection to develop off the mountains of Mexico...impacting far
southeastern Arizona the most Monday afternoon and evening. The
southerly flow of moisture will support storms drifting north out
of Mexico into the southern Arizona desert. Further north, upslope
lifting along the Mogollon Rim will create a secondary maximum of
rainfall across central Arizona, though flash flooding will be
isolated.

Wegman

Day 3
Valid 12Z Tue Jul 16 2024 - 12Z Wed Jul 17 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
CENTRAL MISSOURI INTO SOUTHERN ILLINOIS AS WELL AS FOR NORTHERN
OHIO INTO NORTHWESTERN PENNSYLVANIA...

...20Z Update...

The previously inherited SLGT risk areas were linked together to
include Southern IN through Western OH as the prospects for heavy
rainfall along the sagging frontal boundary are likely to fall
anywhere within the vicinity of the front, especially in the Mid-
Mississippi to Ohio Valley corridor where PWAT anomalies of +1.5-2
standard deviations will be forecast during the time frame of
interest. These areas have been under the influence of heavier
rainfall as of the past week with areal FFGs lower than climo,
especially across Southern IN into West and Northwest OH. Repeating
convection will be a concern due to the increasing frontal
convergence regime that will setup during the period leading to
totals likely exceeding 2-4" in some places from Northern OH all
the way back to the Missouri Valley. CSU First Guess Fields are all
aboard the SLGT risk located within Southern IN, so it made more
sense to connect the two risk areas and align the center of the
risk with the cold front, aided by the positioning of the 72F
isodrosotherm.

Kleebauer

...Previous Discussion(s)...

...Central Missouri into Southern Illinois...

A strong cold front pushing south across the Midwest and down the
Mississippi Valley is expected to stall out across central Missouri
by Tuesday night. An expansive Canadian High will advance southward
bringing welcome relief from both the heat and humidity to the
northern Plains and upper Midwest. The cold front will lead the
high on the southern side. The front should make steady southward
progress during the day Tuesday. However, come Tuesday night, the
typical diurnal strengthening of the LLJ to the south/ahead of the
cold front will cause a substantial push of hot, humid and very
unstable air towards the north into the front over Missouri. This
will cause the front to stall out overnight. Where this front stalls
out will be critical to which areas see the heaviest resultant
rainfall, as the hot and humid air rides up and over the front,
causing widespread convective development along much of the frontal
interface. Multiple ingredients favorable and necessary for the
development of flash flooding are likely to be present. In addition
to a stalled out forcing, PWATs with the hot and humid air mass are
likely to exceed 2 inches Tuesday night. The LLJ gradually turning
more southwestward with time will support multiple repeating rounds
of training thunderstorms along and south of the front. Much of
Missouri has also seen heavy rain in previous days and weeks, so
soils are wetter than normal across much of the state, and will be
saturated more quickly once steady rain starts. Finally, topography
(the north facing slopes of the Ozarks), may also help to organize
the storms. This is because the Ozarks will help to slow the
southward movement of the front simply due to the rugged terrain,
and they will also help to locally uplift the hot and humid air,
supporting storms that are tied to the terrain and at least during
formation will be slow-moving and more susceptible to training. In
coordination with LSX/St. Louis, MO; SGF/Springfield, MO;
EAX/Pleasant Hill, MO; and PAH/Paducah, KY forecast offices, a
Slight risk upgrade was introduced with this morning`s update.

There is considerably more uncertainty on the east and west sides
of the Ozarks, where the topography will not be a potentially
supporting factor in the slowing of the front. For most areas, the
topography won`t be necessary as the highly contrasting air masses
alone should support strong frontogenesis. Nonetheless with the
nose of the LLJ expected to be over Missouri Tuesday night, expect
areas both west and east of this primary area will see somewhat
less rain. Thus, the Slight was largely confined to the portions of
the area with highest certainty. It`s likely the area of the
Slight will need to be adjusted with future updates.

...Ohio/Pennsylvania...

Along the same front impacting Missouri, storms moving northeast
along the front will be highly subject to training in the
unidirectional southwesterly flow. The upper level jet will be
stronger north of NY across eastern Ontario, so the upper level
support will be greater supporting the storms across OH/PA.
However, the moisture will be tracking parallel to the front
instead of into it, so expect the front to at least make slow
progress southeastward through Tuesday night as strong and heavy-
rain producing storms track along it. The inherited Slight was
nudged northeastward with this update following guidance trends in
the strongest forcing. Otherwise the plentiful Gulf moisture will
be present all along the front, so the increasingly strong forcing
across Ohio should help to invigorate the storms more than across
Indiana. Hence, the 2 Slight risks are separated for that reason.

Wegman


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