Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat May 20 2023 15:58:36 FOUS30 KWBC 201558 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1157 AM 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 ID and far western MT. Any storms that form over burn scars, canyons, or poor drainage areas are at the greatest risk for flash flooding. ....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 25 kt LLJ will develop in the evening, which will bring additional moisture for any thunderstorms. MUCAPE Values between 1,000 and 1,500 J/kg along with weak storm motion between 5 and 10 kt will support slow moving thunderstorms. With recent rainfall, soils in this area are nearly saturated, which in turn may result in isolated flash flooding. ....Coastal Southeast... A stalled out front will be parked along the Gulf Coast, with the front providing the forcing, and plenty of instability and atmospheric moisture along and ahead of the front. Weak and highly variable steering flow will favor slow-moving and training thunderstorms, which is being pointed out 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 will be unlikely to be able to handle heavy rain from the scattered storms. CAMS guidance suggests multiple rounds of training thunderstorms are possible during the day Sunday across southeastern GA. Wegman Day 3 Valid 12Z Mon May 22 2023 - 12Z Tue May 23 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR WESTERN OKLAHOMA AND THE EASTERN TEXAS PANHANDLE AND FOR PORTIONS OF THE SOUTHEAST... ....OK and TX Panhandles... A second round of showers and thunderstorms will develop Monday evening across the Panhandles. An upper level disturbance will very slowly track eastward over the area. The flow over the area will be very weak, so any storms that develop will be 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. ....Southeast... The stalled out front from Sunday will remain stuck over the Southeast on Monday as well. Ample moisture is expected with PWATs exceeding 1.75 inches. The front will allow for additional forcing, in addition to any sea breezes over south Florida to add forcing 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 GA and FL. Instability to 2,000 J/kg will allow the storms to grow strong enough to support flash flooding. Despite the moist soils, FFGs are high in this region so flooding should be isolated, and is most likely in urban areas such as Jacksonville and Orlando. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_OqdxHu4buw20XaCfHD-UtbCYgVL8DYzw0UjJ1qTeMLm= Zv8h7WPcjrnSZWrjAI-HE604B5xUs2gp-R87Tf9ROl87UQw$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_OqdxHu4buw20XaCfHD-UtbCYgVL8DYzw0UjJ1qTeMLm= Zv8h7WPcjrnSZWrjAI-HE604B5xUs2gp-R87Tf9RIN-2tSg$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_OqdxHu4buw20XaCfHD-UtbCYgVL8DYzw0UjJ1qTeMLm= Zv8h7WPcjrnSZWrjAI-HE604B5xUs2gp-R87Tf9RkyU6Ak8$=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) .