Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat May 20 2023 20:16:11 FOUS30 KWBC 202016 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 411 PM EDT Sat May 20 2023 Day 1 Valid 16Z Sat May 20 2023 - 12Z Sun May 21 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF SOUTHERN NEW ENGLAND... ....New England... A low near the Delmarva as of midday will be tracking NNE up the Eastern Seaboard as a front over the Great Lakes gradually moves east towards New England. The front will work in tandem to both guide the low up the coast, while also enhancing the rainfall both with the front and the low. This will lead to plentiful Atlantic moisture resulting in light to moderate rain due to warm-rain processes from central NJ northeast to Maine. The result will be several hours of moderate to heavy showers along the I-95 corridor through this afternoon in an area that has been relatively dry recently. Strong model support for widespread 1-3 inch rainfall amounts exist from the greater NYC area and points northeast, and although most of this rain is likely to fall at sub-impressive rates, there may be some instances of nuisance level flooding where convective training develops. PWs along coastal New England will range between 1.25 to 1.5 inches in most cases, with an 850 mb low level jet increasing to 40-50 knots. These factors will support continued moderate to locally heavy rainfall rates, which will be enhanced tonight as a strong upper level shortwave trough swings eastward and helps drive the front and its associated moisture off the coast into the Canadian Maritimes. The flash flooding threat will be largely confined to urban/poor drainage areas, and a coordinated Slight Risk area has been introduced from northern NJ to much of central/eastern MA. The cities likely to be most impacted are Jersey City, NJ; NYC; New Haven, CT; and Boston, MA. ....Mid-South... A developing jet streak and associated upper level disturbances will move across the Mid-South today. Closer to the surface, the same Midwestern front that will guide the Atlantic low into New England will also push south across this region. Ample moisture will be in place ahead of the front as PWATs approach 1.75 inches. Instability will also be prevalent as MUCAPE values increase to around 2,000 J/kg. For northern areas across TN, the front will be the driving factor, which could result in a few hours of moderate to heavy rain, which will be embedded with lighter rain associated with the front. Of course, training convection will be possible here as well. Further south across AL, expect more scattered thunderstorms, which individually are expected to be stronger than any storms in TN, but there will be dry times in between the storms. Nevertheless some areas could see 2 or 3 rounds of thunderstorms, which could result in isolated flash flooding, especially if those storms move over some of the bigger cities such as Birmingham, Montgomery, or Mobile. Of course, the further south and closer to the Gulf you get, the more available moisture and the higher the possibility of heavy rain, though the further north you go the better forcing is expected with proximity to the front. Central AL will likely be the happy middle ground between these extremes, but enough forcing may be available near the Gulf Coast near Mobile, so the Marginal Risk was expanded south to the coast. ....Rio Grande Valley... A strengthening upper level shortwave trough tracking northeastward is likely to spark additional showers and thunderstorms along the Rio Grande Valley today. The storms currently over central Texas are expected to continue turning towards the south, perhaps backbuilding somewhat west of south by the time the storms make it to the river. Much of the convection is likely to stay in the mountains on the Mexican side after about noon. Thus, after noon, much of the rain on the Texas side will be leftover weakening storms that drift east off the mountains. Thus, much of the expected rainfall today is likely to come this morning as the storms backbuild over the area. The Marginal Risk was unchanged with this update. Since much of the rain for the day is likely to stay in Mexico, this Marginal Risk is on the lower end of the category. ....Four Corners Region... The Marginal Risk over this region was expanded southeast to cover much of western NM with this update. However, the overall weather pattern for this area remains the same. Diurnal heating will increase instability to between 500 and 1,000 J/kg. Atmospheric moisture levels remain highly anomalous in this region, with PWATs between 0.5 and 0.75 inches, or 2.5 sigma above normal. The convection will be largely tied to the mountains, but a stationary upper level low over the UT/AZ border may help that convection dislodge from the mountains. Soils remain wet over the area as well, not including the storms that formed in this same region Friday. As usual, any storms that form over sensitive areas, including burn scars, slot canyons, dry washes, and any locally urbanized towns are the most likely to see isolated flash flooding. ....Pacific Northwest... Weak southerly flow will develop at all levels across eastern WA and OR today, which will support somewhat anomalous atmospheric moisture to 2 sigma above normal in this region. Widely scattered showers and thunderstorms are likely to develop this afternoon with daytime heating. An approaching strong wave tonight may help to keep the showers ongoing into the night, but the heaviest rain is likely to occur in the evening. With recent rains in the area and thus, nearly saturated soils, isolated flash flooding is possible. ....Florida Peninsula... Enhanced sea breeze convergence is expected later this afternoon across mainly western interior portions of the central and southern Florida Peninsula, and this will likely result in some slow moving cells with rainfall rates perhaps reaching 2 inches in 30 minutes in the strongest cores. There is strong high-res model support for a broken axis of 2-4 inch totals, with much of this falling within a two hour time period in any given location. The most intense convection is likely to occur within the 4-9 pm EDT period.=20 Wegman/Hamrick Day 2 Valid 12Z Sun May 21 2023 - 12Z Mon May 22 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR THE SOUTH-CENTRAL ROCKIES & HIGH PLAINS, THE NORTHERN ROCKIES, AND IN THE SOUTHEAST... ....Northern Rockies... Little has changed with the Marginal Risk in the Northern Rockies. A strong trough will approach the area, which in turn will induce large scale lift in and around northern Idaho. Scattered heavy showers and thunderstorms are likely to develop over this region, where soils are also nearly saturated from recent rains. The storms will be focused along the mountains, primarily in northern Idaho and far western Montana. Any storms that form over burn scars, canyons, or poor drainage areas are at the greatest risk for flash flooding. There has been a slight westward adjustment in the Marginal Risk area across Montana with the 20Z update, with the greatest coverage of convection expected across northern Idaho and extending to the Canadian border. ....South-Central Rockies and High Plains... An upper level disturbance crossing southern CO into the OK and TX Panhandles will force scattered showers and thunderstorms to form across the region with diurnal heating. A 20-30 kt low level jet is forecast to develop in the evening, which will advect additional moisture for thunderstorms. MUCAPE Values between 1,000 and 1,500 J/kg, along with weak storm motions between 5 and 10 kt, will support slow moving storms. Soils in this area are nearly saturated owing to recent heavy rainfall, which in turn may result in isolated flash flooding. The 20Z update includes some more of western Kansas and also extending farther south across far western Texas with a stronger signal in the 12Z CAM guidance. The high rainfall rates with the slower moving storms that develop could be enough to cause some localized flooding concerns. ....Coastal Southeast... A stalled out front will be parked along the Gulf Coast, with the front providing the forcing, and plenty of instability and deep moisture pooling along and ahead of the front. Weak and highly variable steering flow will favor slow-moving and potentially training thunderstorms, which is evident in much of the guidance by a local maximum of rainfall in the Marginal Risk area. This area was particularly hard hit by recent rains, and so soils in this area are quite saturated and this is an aggravating factor for future heavy rain from the scattered storms. The latest CAM guidance suite indicates less in the way of convection across eastern North Carolina and northeast South Carolina, so the Marginal Risk area was trimmed back to the south to account for these trends.=20 Wegman/Hamrick Day 3 Valid 12Z Mon May 22 2023 - 12Z Tue May 23 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE CENTRAL/SOUTHERN PLAINS AND FOR PORTIONS OF THE SOUTHEAST... ....Northwest Texas to South-Central Kansas... A second round of showers and thunderstorms will develop Monday evening across the Panhandles of Texas and Oklahoma. An upper level disturbance will very slowly track eastward over the area. The main steering flow over the area will be very weak, so any storms that develop will be rather slow-moving. PWATs around 1.25 inches with instability between 1,000 and 1,500 J/kg will support the slow-moving storms. Corfidi vectors at 5 kt also support the slow moving storms. No major changes were necessary with the 20Z update. ....Southeast... The stalled out front from Sunday will remain in place over the Southeast on Monday as well. Deep moisture is expected with PWATs exceeding 1.75 inches in this region. The front will allow for additional forcing, in addition to any sea breeze convergence over the Florida Peninsula to add lift for the storms. Expect greater storm coverage over this area on Monday as compared with Sunday, and with saturated soils in this region, a Marginal Risk was introduced with this update. Corfidi vectors will be westerly at 10 kt, parallel to the front, which will support training thunderstorms in the moisture-rich environment over Georgia and Florida. MUCAPE values to 2,000 J/kg will allow the storms to grow strong enough to support flash flooding. Despite the moist soils, flash flood guidance is generally high in this region so flooding should be mainly localized, and is most likely in urban locations and areas with poor drainage. The forecast remains on track for the 20Z update. Wegman/Hamrick Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4nGXIXNV70krrZbiXG-eVUlqXBMxCwInkUcrVaFAPo09= xKFodT3zg4sINrPEyGxXZoil7XAr0MtKdiOj5pMvK9TC9fA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4nGXIXNV70krrZbiXG-eVUlqXBMxCwInkUcrVaFAPo09= xKFodT3zg4sINrPEyGxXZoil7XAr0MtKdiOj5pMvpoz_ues$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4nGXIXNV70krrZbiXG-eVUlqXBMxCwInkUcrVaFAPo09= xKFodT3zg4sINrPEyGxXZoil7XAr0MtKdiOj5pMvMN0uBW4$=20 $$ =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .