Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Sun Mar 31 2024 08:46:00 FOUS30 KWBC 310800 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 400 AM EDT Sun Mar 31 2024 Day 1 Valid 12Z Sun Mar 31 2024 - 12Z Mon Apr 01 2024 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF SOUTHERN CALIFORNIA, SOUTHERN ARIZONA, AND FROM CENTRAL ILLINOIS INTO THE OHIO VALLEY... ....SOUTHERN CALIFORNIA AND SOUTHERN ARIZONA... Overall, not much change from the previous forecast, with a deep upper low now settling south along the coast of Southern California. This system will begin to fill, transitioning to an open wave/positively-tilted upper trough that will drift east into the lower Colorado Basin this period. Cold air aloft and energy dropping into the backside of the trough will continue to support scattered shower and isolated thunderstorm activity along the coast into the Transverse and Peninsular ranges. Further east, anomalous PWs (1-2 standard deviations above normal), supported by deep southwesterly flow accompanying the system's associated frontal band may fuel occasional heavy rainfall, resulting in isolated flash flooding concerns across portions of southern Arizona. ....Central Illinois to southwestern Pennsylvania and western Maryland... A Marginal Risk was introduced across this area where models are showing a modest signal for locally heavy rainfall amounts supported by training storms. As the previously described trough in the West moves further east, downstream height rises will encourage a slow-moving frontal boundary further north from the Ohio River into the I-70 corridor. Meanwhile, increasing low-to-mid level inflow will support deepening moisture along the front with PWs climbing up to around 1.25 inches (1-2 standard deviations above normal) by this evening from the mid Mississippi into the Ohio Valley. This moisture interacting with a series of weak shortwaves moving over the top of the ridge is expected to support developing storms, training west-to-east along the boundary. While individual storm motions will likely be progressive, the potential for training storms may produce isolated heavy amounts and flash flooding concerns. HREF neighborhood probabilities indicate that amounts of an inch or more are likely, with some potential for amounts exceeding 2 inches. The HREF also shows some low-end probabilities for amounts exceeding 3-hr FFGs. Pereira Day 2 Valid 12Z Mon Apr 01 2024 - 12Z Tue Apr 02 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL EXTENDING FROM THE OZARK REGION NORTHEASTWARD ACROSS THE MID MISSISSIPPI VALLEY INTO THE OHIO VALLEY... A well-defined shortwave moving through the base of the broader- scale trough in the Southwest will lift northeast, reaching the central High Plains by Monday evening. Amplifying southwesterly flow ahead of the wave will support increasing moisture advection into a region a strong ascent afforded by favorable upper jet forcing. The western portion of the outlook remains much the same, with training storms expected to develop initially along an axis of strong southwesterly inflow centered from eastern Oklahoma into Missouri and then further east along a slow-moving west-to-east boundary that will remain centered near the I-70 corridor. As the evening progresses, guidance indicates that convection will become more widespread along the front, with storms training through the Ohio Valley into the central Appalachians overnight. Deep southwesterly flow ahead of the wave will support increasing moisture through the evening, with PWs climbing to near 1.5 inches from the mid Mississippi into the Ohio Valley during the evening into the overnight hours. The models continue to move into better agreement, with several deterministic members showing a solid stripe of 1-2 inch areal average amounts extending from Missouri to northern West Virginia. Therefore, with confidence increasing, continued to bring the Slight Risk further east with this package, extending it along and north of the Ohio River into West Virginia. Pereira Day 3 Valid 12Z Tue Apr 02 2024 - 12Z Wed Apr 03 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE MID ATLANTIC AND ALONG THE CENTRAL AND SOUTHERN APPALACHIANS... Models show the last of the convection developing over the Midwest on Day 2, moving east of the Ohio Valley and across the central Appalachians into the Mid Atlantic Tuesday morning into the afternoon. As these storms move offshore, a sharp trough will remain to the west, with strong, deep southwesterly flow supporting PWs increasing upward of 1.5 inches along a cold front trailing a rapidly deepening low that will be lifting into the Great Lakes. This will fuel the potential for heavy rainfall rates as storms develop and advance east from the Tennessee Valley into the Appalachians Tuesday afternoon into the evening. While heavy rainfall rates are expected, the progressive nature of the front will limit the threat for widespread heavy accumulations and flash flooding concerns. Pereira = = = --- SBBSecho 3.20-Linux * Origin: ILink: CCO - capitolcityonline.net (454:3/105) .