Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Sep 18 2023 00:38:13 FOUS30 KWBC 180038 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 837 PM EDT Sun Sep 17 2023 Day 1 Valid 01Z Mon Sep 18 2023 - 12Z Mon Sep 18 2023 ....THERE ARE MARGINAL RISKS OF EXCESSIVE RAINFALL IN EFFECT ACROSS PORTIONS OF THE SOUTHWEST... ....Southeast Arizona, southern New Mexico, & Far West Texas... A weak 500mb disturbance at the nose of a 250mb jet streak was making its way inland across northwest Mexico...providing increasing upper level support over parts of the Southwest/Southern Rockies. 00Z soundings showed precipitable water values of an inch or more were hugging the international border...which was lagging where earlier models were forecasting the deepest moisture to be at this point. Even so, moisture transport into the region and was helping spawn scattered showers and storms in parts of the Southwest. Now that the daytime heating is about over...a decrease in areal coverage and rainfall intensity should set in soon...but felt it was a bit too early to remove the Marginal risk area with continued upslope enhancement along orographically favored terrain being driven by the broad 15 kt steering flow. As previously noted...antecedent soil conditions remain overly saturated given portions of the at-risk region has received as much as 1-3" of above normal rainfall over the past 7-days and any local downpours on top of sensitives soils could still lead to isolated flooding or run-off problems...especially in complex terrain and dry washes. Elsewhere...convection has either weakened/diminished or moved off-shore and those areas have been removed from the outlook. Bann Day 2 Valid 12Z Mon Sep 18 2023 - 12Z Tue Sep 19 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF NEW ENGLAND... ....New England... A deepening upper level trough over the eastern U.S. is setting the stage for a strengthening low pressure system along the Northeast coast on Monday. NAEFS shows there are several unusually strong variables at play that include: 500mb heights that are 2-3 SDs below normal in the Mid-Atlantic, PWs near the 90th percentile along the southern New England coast, and an IVT topping the 90th percentile as far north as the Maine coastline. As the the 250-500mb trough begins to take on a sharper and more neutral tilt, exceptional divergence atop the atmosphere will result in a deepening 850mb low tracking northeast along the coast. 850mb moisture flux will wrap around the low, courtesy of the aforementioned IVT, and help to create a robust deformation zone of moderate-to-heavy rainfall. The southern New England coast has the best odds for hourly rainfall rates approaching 1"/hr with MUCAPE as high as 250-500 J/kg, highlighted by the 12Z HREF showing 40-50% chances for 1-hr QPF > 1" over southeast MA Monday afternoon. Farther inland, instability will be hard to come by, but much of the Northeast (especially ME on south through southern NH and into northern MA) have received as much as 2-4" rainfall above normal over the last 7 days. While hourly rates will struggle to reach 1"/hr within the deformation axis, the region's overly saturated soils make it susceptible to flash flooding. The Marginal Risk remains in place as of this forecast update. ....Southern Florida Atlantic Coast... A cold front approaching South Florida will be a catalyst in sparking strong thunderstorms Monday afternoon. Area averaged soundings within the Marginal Risk show PWs between 1.9-2.1", MLCAPE up to 1,000 J/kg, and falling 500mb heights as a broad upper trough approaches from the northwest. Storms should form through both surface based heating and along the sea breeze front propagating inland, but they could also fire along the cold front to the north as well. This makes for a busy afternoon in South Florida, especially along the more urbanized area of southeast Florida. The 12Z HREF shows 40-60% probabilities for 1-hr QPF > 2" within the Marginal Risk, as well as 20-30% probabilities for 3-hr QPF > 3-hr FFGs. With such a high concentration of impermeable surfaces along the southeast coast of FL, there is the potential for flash flooding in poor drainage areas Monday afternoon. Storms should dissipate by Monday evening with the gradual loss of daytime heating. Mullinax Day 3 Valid 12Z Tue Sep 19 2023 - 12Z Wed Sep 20 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE CENTRAL AND SOUTHERN PLAINS... ....Central/Southern Plains... 2030Z Update... The forecast from the previous forecast discussion remains on track. Only adjustments to the inherited ERO were to expand the Marginal Risk farther south into North Texas. NAEFS shows a brisk low level jet at 850mb with wind speeds at/above the 90th climatological percentile that will supply rich 850mb moisture flux at the entrance region of the LLJ. Strong storms may form as far south as the Dallas/Fort Worth metro area Tuesday night. There was some consideration to a separate Marginal Risk in the Central Rockies, but there was not enough confidence in whether there would be sufficient moisture and instability at this time. Mullinax ---Previous Discussion--- ....Central/Southern Plains... A sharp increase in moisture will occur across the central and southern plains thanks to Gulf moisture funneling northward on the western periphery of a surface ridge over the east and surface low over the west. LLJ development over the plains will aid in significant low-level moisture transport as indicated by both standardized PWAT anomaly forecast, as well as forecast theta-E indices located over the areas of interest. Aloft, a progressive shortwave will traverse the northern periphery of a ridge in place over Mexico extending into the southern-central Rockies. The timing of the shortwave will place the best ascent from the disturbance within the best diurnal instability maximum over OK/KS on Tuesday afternoon and evening creating a threat for widespread convective coverage, including multi-cell generation. Elevated moisture field in place will create an environment favorable for heavy rainfall within any thunderstorm, especially congealing storm fields where global and ensemble outputs have targeted the area from Topeka down to the Red River for heavy rain concerns stemming from organized convection. A fairly robust signal for >1" of precipitation is being forecast by both the GEFS/EPS with probabilities ~20-25% for both at extended range. Bias-corrected ensemble forecast has a bullseye of over 2" of precipitation between OKC and Wichita which is a great indication of model guidance keying on the area mentioned above. A Marginal Risk has been continued from previous forecast, but it would not surprise if this period is escalated to a higher risk category as we gather more data and trends from hi-res ensemble guidance, as well as deterministic CAMs.=20 Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6EXBEExYQyuGVJsfgrWqxjq0NRIS0VROIEFiSC5MHvQD= 5mUPp-aLiUDZIYBsx8fLyaE-ESZLR8oCrNJKMh06IiePuBA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6EXBEExYQyuGVJsfgrWqxjq0NRIS0VROIEFiSC5MHvQD= 5mUPp-aLiUDZIYBsx8fLyaE-ESZLR8oCrNJKMh06RTbKlv4$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6EXBEExYQyuGVJsfgrWqxjq0NRIS0VROIEFiSC5MHvQD= 5mUPp-aLiUDZIYBsx8fLyaE-ESZLR8oCrNJKMh06P-BkPrk$=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. 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