Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Sep 15 2023 15:56:46 FOUS30 KWBC 151556 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1156 AM 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 to the north 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. There was very little change in the model guidance this evening with even the ensemble means only increasing/decreasing within the noise of variability on a run-to-run basis. A general 1" on the far western reaches of ME is forecast with up to 4-5" inches across Down-East ME where the core of precipitation will fall on Saturday afternoon and evening. The progressive motion of the storm will limit the potential for more widespread significant flooding, although some areas from Bangor eastward will see the best potential for significant flood concerns as guidance indicates 2-3"/hr rates for a short time during the peak of impact as the storm approaches the southern Bay of Fundy. Considering the solid agreement in guidance on track, forward speed, and QPF placement/maximum, have elected to only make minor adjustments to the positioning of both the SLGT and MRGL risk. ....Eastern Tennessee up into West Virginia... 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. A MRGL risk was added within the above zone given the favorable synoptic pattern and flash flood concerns within an area of terrain influences. Kleebauer Day 3 Valid 12Z Sun Sep 17 2023 - 12Z Mon Sep 18 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS EASTERN NORTH CAROLINA INTO THE MARYLAND/VIRGINIA TIDEWATER... ....Southern Mid-Atlantic Coastal Plain... Advancing cold front from the west along with a strengthening upper jet max over the Mid-Atlantic will bring an opportunity for locally heavy rainfall across the southern Mid-Atlantic coastal plain, especially the VA tidewater into eastern NC. Models are generally strengthening an upper jet across the east as a mid-level shortwave trough digs and tilts neutral once over the Ohio Valley. A jet streak of 95-100 kts will pivot around the base of the mean trough aligning itself from southwest to northeast over the central Mid-Atlantic. 00z GFS/ECMWF/CMC blend would place the best axis of QPF from eastern NC up through the VA tidewater with maximum precip potential of 1.5-2.5" within the core of heaviest precipitation. Given the trend in the upper level evolution and the environment well within a 1 standard deviation above normal PWAT axis, have introduced a MRGL risk for the aforementioned area. Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!94ZXRg4Bo7VVfoYZXI-pSSS0GrLkblrm9jJz-fUqlN1z= NxoEQz8DxU2h3Q-106P1NW2oL6k7kz_2rODs9yYFlbmgFe8$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!94ZXRg4Bo7VVfoYZXI-pSSS0GrLkblrm9jJz-fUqlN1z= NxoEQz8DxU2h3Q-106P1NW2oL6k7kz_2rODs9yYF2HiC1X4$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!94ZXRg4Bo7VVfoYZXI-pSSS0GrLkblrm9jJz-fUqlN1z= NxoEQz8DxU2h3Q-106P1NW2oL6k7kz_2rODs9yYFYSXNtgw$=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) .