Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
931 AM EDT Mon May 20 2024
Day 1
Valid 1315Z Mon May 20 2024 - 12Z Tue May 21 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR THE FLORIDA
KEYS, FAR NORTHEAST COLORADO, ACROSS SOUTHERN AND EASTERN NEBRASKA
INTO NORTHWEST IOWA AND ACROSS SOUTHEAST WISCONSIN INTO
NORTHEASTERN ILLINOIS...

It has been an active morning in the Florida Keys where rainfall
gauges have been filling up to as much as 4-7 inches thanks to
training convection aloft. Surface convergence oriented WSW-ENE and
healthy upper level divergence is working in tandem with roughly
1,500-2,000 J/kg of MLCAPE nearly 40 knots of surface-6km vertical
wind shear according to the 12Z KEY sounding. The 1km and 6km wind
barbs are out of the WSW and W respectively, which will keep the
orientation of line of heavy thunderstorms over the Keys for
several more hours. Hourly rainfall rates of 2-3"/hr will continue
through late morning but eventually, instability will be exhausted
and storms should dissipate later this afternoon. While the Keys
typically drain excessive rainfall well, high tide is approaching
and can affect drainage. Given these factors, a Marginal Risk was
issued for the Florida Keys. Look for the usual 16Z update in a
couple of hours.

Mullinax

---Previous Discussion---

...Central Plains into the Mid Missouri Valley...
The amplifying mid to upper level trof pushing east across the
Central to Northern Rockies will ignite the next round of active
Central Plains convection late Monday afternoon, continuing into
the early hours of Tuesday. Overall favorable conditions for active
convection with strengthening of the low level flow, strengthening
boundary layer convergence and favorable upper difluence in the
vicinity of the frontal boundary forecast to stretch from the lee
of the Central Rockies, east northeastward into the Upper
Mississippi Valley. There is spread among the latest hi res
guidance as to whether this next round of convection will
consolidate into one distinct line as per the ARW, or have more
than one round of convection moving along the frontal zone as per
the NAMNEST, FV3LAM and ARW2. However, there is consensus that
either way, the convection will be fairly progressive to the east-
northeast, and continue to be a detriment to very heavy rainfall
amounts. We have kept the risk level at marginal given the expected
quick movement. This is reflected in HREF neighborhood
probabilities for 1"/hr amounts that are in the 20-45% range in the
marginal risk area, but much lower, mostly less than 15% and more
geographically spotty for 2"/hr amounts.

...Southeast Wisconsin into northeast Illinois...
The well defined MCV moving into the Lower Missouri Valley early
this morning is expected to remain intact as it pushes
northeastward into the Upper Mississippi Valley and Upper Lakes
region Monday into Monday night. While there may be weakening to
the current organized convection associated with this MCV early day
1, there is the possibility for additional convective enhancement
ahead of the MCV late Monday morning into afternoon across
southeast Wisconsin into northeast Illinois. No significant changes
made to the previous marginal risk area with concerns primarily
for urbanized runoff issues from Chicago, north through Milwaukee
and Green Bay.


Oravec