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QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
500 AM EDT Sun Jul 14 2024

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

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
NORTHERN ILLINOIS...

...Northern Plains and Midwest...

In coordination with LOT/Chicago, IL forecast office, a Slight Risk
upgrade was included with this update for portions of northern
Illinois and far northwestern Indiana.

A series of shortwave disturbances will track roughly west to east
across the northern Plains and Midwest today through tonight. The
latest wave has sparked several MCS's over the Upper Midwest this
morning. The one currently moving across MI/IN is expected to
continue weakening as it moves southeast...however there is a
second one tracking southeast down the Mississippi River between
MN and WI. This one following close behind the first will raise
additional flooding concerns across northern IL and Chicagoland
this morning before moving into MI/IN and dying out later this
morning. These MCSs have a history of producing locally significant
flash flooding, particularly in those areas where cell mergers and
the back edge of the line of storms get hung up, resulting in a
prolonged period of heavy rainfall. A few spots saw 3 inch per hour
rainfall rates. On the large scale, expect these MCSs to continue
moving quickly...but small portions of the storms are likely to get
hung up and cause locally significant flash flooding. 

CAMs guidance has been very poor with the handling of these storms,
almost across the board too light and small with coverage. Thus,
there is lower than normal confidence on how the storms likely to
develop once again tonight will behave. Given the new flooding
sensitivities across much of northern Illinois, even comparatively
disorganized and fast moving storms redeveloping tonight over these
same areas will have a high enough potential to cause flash
flooding as to justify a Slight risk upgrade. It should be noted
that HREF guidance is much lower on the potential for FFG
exceedance in this area...though it's all but certain to be using
pre-storms FFGs. Thus, it is assumed the FFGs now across northern
Illinois are much lower and will therefore be much more easily
exceeded with additional shower and thunderstorm activity expected
tonight.

...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 for long enough to overcome the
dry air, but those are expected to be isolated incidences.

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
NORTHERN ILLINOIS INTO SOUTHWESTERN MICHIGAN...

...Midwest...

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...

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
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