Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Sep 15 2023 19:06:26 FOUS30 KWBC 151906 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 305 PM EDT Fri Sep 15 2023 Day 1 Valid 16Z Fri Sep 15 2023 - 12Z Sat Sep 16 2023 ....A SLIGHT RISK OF EXCESSIVE RAINFALL IS IN EFFECT OVER PORTIONS OF WEST-CENTRAL TEXAS INTO THE CENTRAL RIO GRANDE VALLEY... ....Southern Plains... After an initial broken line of thunderstorms moving through central TX this morning, the second round of precipitation will come later this afternoon as diurnal heating kicks in and an increase in instability arises across the southern Plains. A stalled front across central TX will linger, becoming a focal point for afternoon convection, especially over the central Rio Grande. A robust signature for heavy rainfall has shown up across all major guidance with the hi-res ensemble forecast indicating a swath of heavy precipitation from convective clustering developing late in the afternoon into the evening time frame. 12z HREF probabilities for 2-3+"/hr rates and totals exceeding 3" are high on a line from Del Rio down into southern Maverick County where some guidance has totals breaching 5" over the span from 00-06z in the period. A lot of this potential stems from not only convective initiation in TX, but some boundary convergence from expected outflows off the Serranias del Burro with the stalled frontal boundary in place. Deep moisture field presence along the front encapsulates what could be an elevated flash flood threat for somewhere along the river valley between the above areas. The previous Slight Risk was modified to shrink the northern portions where less support for intense rainfall rates exist. Additionally, the Marginal Risk was expanded to the western Texas Gulf Coast where storms are expected along a stalled frontal boundary. At least 2" of PW and 4000 J/kg of SBCAPE near Corpus Christi per SPC's 15z mesoanalysis will allow for efficient rainfall within thunderstorms and a localized flash flood threat.. More showers are anticipated across the central Rockies down into the Panhandle of TX as thunderstorms develop across the terrain and advance eastward within the mean steering flow tonight in response to a longwave trough stretching into the central Great Basin. Some guidance indicates another convective cluster developing within eastern NM the following evening, advancing southeast once again within the forecast theta-E tongue and incoming height falls over the southern High Plains. However, models are not in full agreement on the magnitude and timing of any upstream complex development, but the threat is non-zero for a pseudo repeat of a repeat of last evening, maybe a bit further south with this period. It'll be something to monitor in subsequent forecast cycles. The previous Marginal Risk was confined farther south across parts of Colorado and Kansas for this update. ....Portions of Florida and the Southeast... A stationary boundary bisecting the northern portion of FL will get a push on the eastern edge of the front with increasing convective risks along and within 30 miles of either side of the boundary. 12z HREF blended mean, along with bias-corrected ensemble forecast indicate a period of heavy rainfall for northeast and north-central FL within the confines of Jacksonville down towards St. Augustine during the late-morning and afternoon timeframe before spreading inland within the axis of a surface trough juxtaposed over the Peninsula. Highest potential rainfall rates will accompany any convective development today where convergence is maximized due to sea breeze(s) intersecting the frontal boundary during peak diurnal instability. 2-3"/hr rainfall rates will be a common occurrence where the convergence occurs, as well as within the surface trough located inland. PWATs over 2" per SPC's mesoanalysis and 06z GEFS are over the 75th climatological percentile and aid in the localized heavy rainfall threat. 12z HREF neighborhood probabilities of >45% for at least 3"/6-hr potential scatter northern FL. This is where a MRGL risk is now located to account for the heavy rain concerns in the area, with urbanized regions most at risk for rapid water rises and flash flooding as high FFGs in the area limit the risk somewhat. Slow-moving storms within an area of deep instability are also possible to the north of the aforementioned frontal boundary into central GA and eastern AL. Activity over eastern GA is ending this morning, but new development associated with daytime heating is expected just to the west, where a pocket of 1000 J/kg of MUCAPE exists. Localized 2"+ amounts could create flash flooding concerns if storms develop over urban regions. Kleebauer/Snell Day 2 Valid 12Z Sat Sep 16 2023 - 12Z Sun Sep 17 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER EASTERN MAINE... ....Eastern Maine... Hurricane Lee will continue to move on a northward trajectory with landfall forecast within the general confines of western Nova Scotia leading to the western half of the storm impacting Maine and extreme coastal Massachusetts near the Cape and Nantucket. Dry air wrapping around Lee and acceleration its extra-topical transition will lead to a broader northern precipitation shield, but with less convective elements. There was little change in the model guidance this afternoon. A general 1" across central ME is forecast with up to 4-5" inches across Downeast ME, where the core of precipitation will fall on Saturday afternoon and evening. The progressive motion of the storm will also limit the potential for more widespread significant flooding, although some areas from Bangor eastward will see the best potential for scattered flood concerns as guidance indicates 1-1.5"/hr rates for a short time during the peak of impact as the storm approaches the southern Bay of Fundy. 12z HREF does indicate far eastern sections of Downeast Maine as having a 30-40% chance of exceeding 5" of total rainfall. A sharp gradient along the western edge of the precipitation shield continue to come into better focus, which led to the small eastward shift in the MRGL and SLGT risk areas. ....Tennessee Valley... A digging mid-level trough will enhance a difluent axis present over portions of the Tennessee and Ohio Valleys later Saturday into early Sunday morning. A rather solid agreement on the potential for a developing axis of convection within strengthening jet max across the above area correlating with locally heavy rainfall within a zone of terrain and lower FFG's. Global deterministic along with bias-corrected ensemble mean QPF for the period beginning 00/17z through the end of the period shows swath of 0.75-1.5" totals within the 12 hr period, mainly within a corridor extending from Knoxville up through Charleston, WV which would favor at least a marginal concern for flash flooding in an area that has been impacted over the past week. PWs largely under 1.5" and limited instability may inhibit the flash flooding threat, but 12z HREF neighborhood probabilities indicate a low-end potential for exceeding 3". A MRGL risk was maintained and extended southward into Alabama, where higher PWs and increasing instability will be present for any developing thunderstorms on the far southern periphery of the upper trough. ....Florida Atlantic Coast... A sagging stationary front is expected to sink to the Space Coast on Saturday and orient from southeast to northwest as it stretches from the western Atlantic to northern Florida. This will provide an additional focus for developing thunderstorms by midday after sufficient surface heating, along with typical sea breeze interactions. As the upper trough crosses into the Ohio Valley, mid-to-upper flow will become more southwesterly as opposed to southerly/stagnant over southern FL, allowing for storms to remain mostly stationary near the urbanized corridor of southeast FL as it opposes flow at the surface. PWs are also expected to remain near or slightly above 2" and +1.5 standard deviations above the climatological mean. High probabilities of 5" of rain from the 12z HREF sit just off the Space Coast, but worth monitoring should it trend westward. The MRGL risk was introduced to mainly account for potential urban flood impacts. ....South Texas... A lingering frontal boundary, elevated PWs, and expected early morning convective activity over the Rio Grande is expected to lead to an isolated flash flooding threat across South Texas on Saturday. 12z HREF neighborhood probabilities for 5" approach 15-30% across this region and are depicted in most hires guidance. Sea breeze interaction may enhance activity as it pushes inland, with urban areas most at risk for flooding concerns. Kleebauer/Snell Day 3 The Day 3 outlook will be updated by 2030Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9JRjtZUOkbGtv6FESqqSCcRGzcrE6ADJN5-aLdMF0PI3= abeiDTWGby52Df4RePrOOSqHLVZRqlnnr881x3ZybPVk9J8$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9JRjtZUOkbGtv6FESqqSCcRGzcrE6ADJN5-aLdMF0PI3= abeiDTWGby52Df4RePrOOSqHLVZRqlnnr881x3ZyyQjCsbQ$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9JRjtZUOkbGtv6FESqqSCcRGzcrE6ADJN5-aLdMF0PI3= abeiDTWGby52Df4RePrOOSqHLVZRqlnnr881x3Zyh8cDW0M$=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) .