Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Oct 14 2023 00:56:44 FOUS30 KWBC 140056 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 856 PM EDT Fri Oct 13 2023 Day 1 Valid 01Z Sat Oct 14 2023 - 12Z Sat Oct 14 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF NORTHERN ILLINOIS... 01Z Update... Removed the Slight risk area from the Upper Midwest as convection has been progressive with only modest rainfall rates at the time of greatest instability. The 13/12Z HREF still suggested there will be a 5 percent neighborhood probability of 3-hour rainfall exceeding the 3-hour flash flood guidance into the early morning hours over northern Illinois. This area still has the best overlap of moisture and low level dynamics...and the HREF seems to be keying on the pivot of a boundary near the border of Illinois and Wisconsin as a focus for the low-end risk of excessive rainfall. As a result...maintained the Marginal there.=20 ....Coastal South Carolina & Coastal Georgia... Once again...opted to not hoist a Marginal risk area over coastal South Carolina/Georgia. Still agree that localized downpours are possible that cause some ponding on roads during the overnight hours due to abundant moisture getting in place and enough low level dynamics to potentially support weak mesoscale circulations...but the CAPE values are modest and probably not sufficient to produce rates to overcome high flash flood guidance. Bann Day 2 Valid 12Z Sat Oct 14 2023 - 12Z Sun Oct 15 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. 20Z Update... The reasoning for holding off on a Marginal Risk from overnight remains sound. While there will be some moderate periods of rainfall in parts of the Mid-Atlantic, progressive showers and storms in the southern Mid-Atlantic and the lack of instability throughout the northern Mid-Atlantic should keep rainfall rates and amounts to sub-Excessive levels. Thus, no Marginal Risk has been issued as of this forecast cycle. Mullinax --- ....Eastern Ohio Valley into the Mid Atlantic Region... On Saturday-Saturday night, shortwave energy associated with the Midwest to OH Valley system will become absorbed within the amplifying longwave eastern CONUS trough. At the surface, the guidance shows a Miller Type B scenario as the Ohio Valley low on Saturday eventually gives way a developing low off the mid Atlantic coast Sat night. Compared to Friday and especially Thursday, the deep-layer forcing and (especially) moisture flux/transport aren't nearly as anomalous, which is typical during these transitional Miller B events. Across northern portions of the Mid Atlantic region, DPVA and left-exit region upper jet forcing (100-120kt jet streak at 250 mb) will lead to favorable deep-layer isentropic ascent over-top a more stable, CAD profile in the lower layers. Over these areas expect a steady moderate to occasional heavy rainfall, averaging between 0.50-1.5" in a 24hr period with little variation outside of that range. Over sout .