Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Sep 15 2023 08:00:45 FOUS30 KWBC 150800 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 359 AM EDT Fri Sep 15 2023 Day 1 Valid 12Z 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... Convection that formed upstream over the Panhandle is forecast to continue on a southeast progression riding along the northern edge of the theta-E gradient in place across the southern plains of TX. 00z HREF blended mean shows an axis of heavy precip potential extending from the western rolling plains down into portions of central TX before dissipating once into the eastern half of the state. Rainfall rates of 1-2"/hr will be common at peak potential, mainly between 12-18z Friday along the northern extent of the SLGT risk forecast. 00z neighborhood probabilities of 1 and 2"/hr are fairly robust for morning convection with an accompanying stretch of 35-45% for 1"/3 hrs and 15-25% for 2"/3 hrs over the same areas. Regional FFG's across north-central TX are the lowest in the state thanks to previous days of precipitation priming the grounds for future flood risks. This is the first round of precipitation that encompasses the areal SLGT risk for D1. 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 now 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. 00z 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. Given the signature in the latest guidance, have opted to extend the SLGT risk from the previous forecast package south to focus for the threat along the Rio Grande. More storms 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. Some guidance indicates another convective cluster developing within eastern NM the following evening, advancing southeast once again within the forecast theta-E tongue situated over the southern high plains. Models are not in full agreement, however on the magnitude and timing of any upstream complex development, but the threat is non-zero for a pseudo repeat of what is happening this evening, maybe a bit further south with this period. It'll be something to monitor in subsequent forecast cycles. ....Florida... 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. 00z HREF blended mean, along with bias-corrected ensemble forecast indicate a period of heavy rainfall for northeast 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. Even for FL standards, a fairly stark uptick in potential rainfall rates will accompany any convective development today, especially where convergence is maximized where sea breeze(s) can intersect 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. Neighborhood probabilities of >50 dbz and 3"/hr potential cover a majority of the northern half of FL from Lake Okeechobee up into the FL Panhandle with the highest probs located between TBW up to JAX. This is where a MRGL risk is now located to account for the heavy rain concerns in the area. Kleebauer =20 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 The Day 3 outlook will be updated by 0830Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!87nb7m85OVKRIaUTVGBQHcC6tTSHdKkCBHlu6Ugicc1n= FmvKx0yLCNpWNsNiTyKXg0Mie2N-qDT2F3C94IhB8HwwhRM$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!87nb7m85OVKRIaUTVGBQHcC6tTSHdKkCBHlu6Ugicc1n= FmvKx0yLCNpWNsNiTyKXg0Mie2N-qDT2F3C94IhBdfUViMI$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!87nb7m85OVKRIaUTVGBQHcC6tTSHdKkCBHlu6Ugicc1n= FmvKx0yLCNpWNsNiTyKXg0Mie2N-qDT2F3C94IhBA09olHM$=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) .