Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Sep 14 2023 20:05:58 FOUS30 KWBC 142005 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 405 PM EDT Thu Sep 14 2023 Day 1 Valid 16Z Thu Sep 14 2023 - 12Z Fri Sep 15 2023 ....THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL OVER PORTIONS OF NORTH CENTRAL COLORADO AND NORTH CENTRAL TEXAS... ....16Z Update... The main adjustment to the Slight Risk areas in place was the one in North Texas where CAMs show continued surface theta-e advection into the southern periphery of the Dallas/Fort Worth metro area into the early afternoon hours. Expanded the Slight Risk a little farther east where the ongoing cluster of storms may support a flash flood threat into the mid-afternoon hours. New 12Z HREF shows as high as 60-70% probabilities for >3" of rainfall today south of the Dallas/Fort Worth metroplex. A similar situation is anticipated over southeast Texas where surface-850mb theta-e advection will intersect a stalled frontal boundary through this afternoon and early evening. Given how dry the region has been in recent weeks, however, opted to maintain a Marginal Risk since most rainfall will be beneficial. Adjusted the Marginal Risk area in the central High Plains to account for latest CAMs guidance, but no significant adjustments to the Slight Risk in central CO as the overnight shift's forecast rational and latest 12Z CAM guidance supports the need for the Slight Risk. To add on to the HREF probabilistic guidance from the previous shift, the 12Z HREF showed similar to slightly higher probabilities for not only the >1" and >2" totals, but also included a 20-30% area for >3" totals around the Denver metroplex. The only addition to the forecast cycle is a Marginal Risk in southern GA and far southern SC. The 12Z sounding out of Jacksonville, FL showed PWs near 2" with not much in the way of capping present. While RAP mesoanalysis shows a little stronger capping to the north, this should be eroded rather quickly this afternoon while at the same time, the region lies at the nose of a weak 250mb jet streak positioned over the Mid-South. With MLCAPE forecast to top 1,000 J/kg, PWs near 2", and 20-25 knots worth of effective bulk shear aloft, storms will have some marginally organized characteristics to go along with 2"/hr rainfall rates within the heaviest t-storm activity. MRMS FLASH soil moisture ranged between 50-70% across parts of southern GA and new 12Z HREF showed neighborhood probabilities of 40-60% for rainfall totals >3". Given these reasons, chose to add a Marginal Risk to southern GA and far southern SC this forecast cycle. Mullinax ---Previous Discussion--- ....Great Basin into the Central Rockies and Central High Plains... Height falls expected to move west to east day 1 from the Great Basin into the Central Rockies and Central High Plains region.=20 This will support a fairly broad region of scattered convection day 1 in what will be a region of above average PW values, 1 to 2+ standard deviations above the mean. A small slight risk area was added across north central Colorado from approximately Colorado Springs, northward through Denver, Boulder and into Ft Collins.=20 Convection may maximize in this region late Thursday afternoon into Thursday evening as front drops south into northeast Colorado and the above mentioned height falls push east across northern Colorado and enhance upper difluence. The slight risk area fits where HREF neighborhood probabilities are high, 70-90%+ for 1"+ amounts, high also for 2"+ amounts 40-60%, albeit over a smaller region closer to Denver. This region has seen above average precip over the past week, resulting in lower ffg values through the slight risk area. The previous marginal risk area that extended westward into western Colorado and southern Utah was expanded farther west into northern Arizona and southern Nevada where locally heavy rainfall amounts are possible in the above mentioned high PW axis/upper trof axis with embedded shortwaves.=20 HREF neighborhood probabilities, while very splotchy in appearance, do show fairly high probabilities in spots for 1"+ amounts for the marginal risk area from western Colorado into southern Utah, southern Nevada and northern Arizona. ....Southern Plains... A surface frontal boundary expected to remain stationary day 1 from Southwest Texas, eastward into south central Texas and along the Gulf Coast. Strengthening southerly inflow into this front should support increasing convection to the north of this boundary prior to 1200 UTC Thursday, with this potential organized convection then pushing downstream early day 1 from across portions of central to eastern Texas. Much of this region has been dry over the past few weeks, resulting into fairly high ffg values. The one exception is over portions of south central Texas in the vicinity of WFO SJT. A slight risk area was denoted over northern WFO SJT and southwest WFO FWD where heavy rains early day 1 may overlap the lower ffg values. In the slight risk area, HREF neighborhood probabilities are high for 1 and 2"+ amounts, 50-80 and 40-60% respectively, with hourly rainfall totals of 1-2" possible.=20 Oravec Day 2 Valid 12Z Fri Sep 15 2023 - 12Z Sat Sep 16 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF WEST CENTRAL TEXAS... ....2030Z Update... Little changes made this forecast cycle to the overnight forecast guidance based on the latest QPF and where the most flood prone areas are likely to take shape. Following today's heavy rainfall across parts of the region, the heart of Texas remains most at risk as southerly 850mb moisture transport directs added moisture flux into the stationary front located over the Edwards Plateau. Convection will likely be highly scattered, but 1-hr FFGs will likely struggle to get above 2" and between the instability (1,000-1,500 J/kg MLCAPE), and the PWs mentioned from overnight, thunderstorms will be capable of producing excessive rainfall rates in excess of 2"/hr. The Slight Risk remains in place there while the Marginal Risk remains on track across the central Rockies and central High Plains. The thinking for coastal New England also remains on track as the bulk of the heaviest rainfall looks to come ashore later Saturday morning. Mullinax ---Previous Discussion--- The overall large scale pattern is not expected to change appreciably during the upcoming day 2 period from the Central High Plains, southeastward across the Southern Plains. An axis of PW values 1.50-1.75" expected to persist along and to the north of the west to east oriented stationary front that will not move much from the day 1 period, continuing to stretch from Southwest Texas into south central Texas. Additional overrunning convection possible along and north of the stationary front with locally heavy rains from northwest Texas southeastward into southeastern Texas. There is a fairly large spread with qpf details, leading to overall low confidence with max areas. A small slight risk area was added to the previous Excessive Rainfall Outlook for portions of west central Texas that may see an overlap of day 1 and day 2 precip across regions of relatively low ffg values. Not a lot of changes made to the previous broad marginal risk area from the Central High Plains, southeastward into Texas given the above mentioned model qpf spread. ....Coastal New England... Rain bands on the west and northwest side of Lee will begin to impact coastal New England Friday night into early Saturday morning. At the moment, the heaviest Lee rains should remain offshore through the day 2 time period, with the amounts that do fall along coastal New England, not resulting in any runoff issues. Oravec 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!86SpSsvKD8KfojlCnMEZ8978pE2jZ2RND6LgzLiPRPcG= J5E7bCqAgwPqESurxyhtuC93A2hIyAH11PGNfm2IP3RVCi8$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!86SpSsvKD8KfojlCnMEZ8978pE2jZ2RND6LgzLiPRPcG= J5E7bCqAgwPqESurxyhtuC93A2hIyAH11PGNfm2I5hXhDug$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!86SpSsvKD8KfojlCnMEZ8978pE2jZ2RND6LgzLiPRPcG= J5E7bCqAgwPqESurxyhtuC93A2hIyAH11PGNfm2IJwxp4hc$=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) .