Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Jun 16 2023 23:03:23 FOUS30 KWBC 162303 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 702 PM EDT Fri Jun 16 2023 Day 1 Valid 2258Z Fri Jun 16 2023 - 12Z Sat Jun 17 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHEAST...WYOMING/NEBRASKA AND THE NORTHERN PLAINS... ....01Z Excessive Rainfall Outlook North Dakota/South Dakota... Maintained a Slight Risk that was introduced earlier. Some high-resolution guidance still focuses 2 to 5+ inches of rainfall overnight in convection that it focuses along and of a cold front moving across the region. Agree with the previous forecaster that the upper end of this range seems too high given the pattern, but the amount of moisture being drawn in place...a focusing mechanism and dynamics could lead of at least some increase in potential for excessive rainfall. Storm motions of ~10-15 kt quasi-parallel to an approaching cold front with southeasterly to easterly low level flow converging into the region will support convection that persists into the early overnight although the loss of daytime heating should result in rainfall becoming more stratiform in nature. Wyoming... Maintained the Slight Risk area in Wyoming along and ahead of an amplifying upper-level shortwave extending from Wyoming into Utah. Instability remains meager ahead of the wave...and rainfall rates have generally remained under 1+ in/hr but the added dynamical support that could result in locally higher rainfall rates over some areas where rainfall over the past 7 days has been 300 to 600 percent of average. With late afternoon/early evening satellite imagery still showing cooling cloud tops over portions of southeastern Wyoming and the Nebraska panhandle. Moisture is deeper across portions of southeast Wyoming and the western Nebraska into parts of South Dakota...so maintained that portion of the outlook area with little change. ....Lower Mississippi Valley into Southeast... Previous forecast reasoning remains on track with only minor adjustments made to the risk areas based on radar trends and 12Z hires output. There Lower Mississippi Valley through the Southeast... A series of impulses that have ridden the westerly subtropical jet and provided lift a persistent west to east oriented low level frontal zone over the Southeast continued throughout the day with the only a brief respite occurring in response to shortwave ridging. Another shortwave coming across Oklahoma this evening is poised to trigger organized activity that progresses over the lower MS Valley late tonight that guidance takes south and east later. Southern Plains... Mid-level ridging this morning primes OK into northeast TX for heavy and progressive convective activity this evening into the overnight. A marginal risk is warranted to be maintained over these areas with expansion farther south over the Red River where several inches have fallen over the past three days. Ohio... Added a focused Marginal Risk area where slow-moving convection has been producing locally intense rainfall. See WPC Mesoscale Precipitation Discussion 0493 for more details. Bann Day 2 Valid 12Z Sat Jun 17 2023 - 12Z Sun Jun 18 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL OVER PORTIONS OF NORTHERN AND CENRTAL FLORIDA... ....The Plains... A digging shortwave will track eastward across the Plains on Saturday as the positively tilted longwave trough speeds up and becomes more open with time. Meanwhile a strengthening LLJ will develop across the upper MS River Valley by Saturday evening, drawing PWATs to 1.5 inches north through MN. The combination of the upper level forcing and Gulf moisture drawn north will result in a renewed round of storms from western MO and eastern KS north into central MN. Since these areas have been relatively dry recently, the chances of flash flooding are lesser, but since the eastward progression of the storms will be slow, the threat for training is higher across this area, particularly from western MO, western IA, and into central MN. Corfidi vectors are 5 kts or less over much of the area, which will also favor training thunderstorms. Strong thunderstorms are likely to develop over much of OK through AR Saturday evening. These storms will form along the same front that remains stuck over this area. A 30-40 kt southwesterly LLJ will transport ample Gulf moisture, in some areas with PWATs exceeding 2 inches. Corfidi Vectors in this area will be more favorable for fast moving storms as they approach 20 kt out of the northwest. Given the plentiful moisture, it's likely that training storms will develop over portions of this area, even if the fast moving storms work against the flash flooding threat. ....Gulf Coast through much of FL... 2030Z Update... With the 12Z models showing an increasing signal for heavy to excessive rainfall amounts, a Slight Risk was introduced for portions of northern and central Florida, extending from the Nature Coast and Tampa Bay area inland. Guidance continues to show a series of shortwaves moving through a broad upper trough, producing periods of convection across the region Saturday into early Sunday. This energy along with deep moisture (PWs at or above 1.75 inches) is expected to support areas of heavy rainfall. While there remains differences in the details, the growing concensus of the hi-res models shows some of the heaviest amounts occurring as storms train along a boundary trailing a wave moving across the Southeast Saturday night into early Sunday. Within the Slight Risk area, the HREF Mean shows areal average amounts of 2-4 inches, with greater than 40 percent probabilities for exceeding the 10 year ARI. Previous Discussion... As a shortwave trough moves across the Southeast, renewed training shower and thunderstorm activity will develop across the FL Panhandle and northern FL Peninsula. Once again the storms will have access to plentiful Gulf moisture, and the multiple rounds of storms will cause a larger footprint of at least 2 inches of rain over much of the northern FL Peninsula. .... New England... The Marginal Risk area remains largely unchanged over almost the entirety of New England. A slow moving surface low will drift northeastward up the coast, bringing plentiful Atlantic moisture westward to the north of the center into New England. While instability will be limited, the slow-moving weather pattern will favor a long duration of light to moderate rain. Since some of these areas have seen rain in recent days, along with expected rain on Day 1/Friday, which may worsen any flash flooding from Saturday's rainfall. It's difficult to pinpoint where the heaviest rain will fall and where the greatest flash flooding threat will be, but a widespread 1-1.5 inches of rain over much of New England is likely. ....Northern Idaho and western Montana... Ascent is expected to increase ahead of an anomalously deep upper low dropping south from the Gulf of Alaska along the coast of British Columbia. In addition to increasing ascent, deep southwesterly flow ahea of the low will support deepening moisture with positive PW anomalies of 1-2 standard deviations. While instability will be marginal, the strong forcing, anomalous moisture, and the potential redeveloping storms along the associated low level front may produce some localized heavy amounts and isolated runoff concerns. Given that concern, a Marginal Risk was added to the 2030Z update. Pereira/Wegman Day 3 Valid 12Z Sun Jun 18 2023 - 12Z Mon Jun 19 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL OVER PORTIONS OF THE LOWER MISSISSIPPI VALLEY AND SOUTHEAST... ....Lower MS Valley into the Southeast... 2030Z Update... While model spread and run-to-run continuities have lowered confidence in the details, the setup for heavy rain remains much the same. Therefore, made only minor adjustments with this update. Did nudge the Slight Risk a little farther north into southern Missouri where some of the deterministic runs and the Euro Ensemble Mean show heavy rain associated with a well-defined shortwave developing. Previous Discussion... A flash flood risk likely continues into Sunday and Sunday night across portions of AR into MS and AL. It has been a wet pattern of late, and that will continue over the next several days as well...so ground conditions are likely to be more susceptible to flash flooding than normal across a good portions of the area. West to northwesterly flow aloft will continue to support rounds of northwest to southeast moving convection with plentiful instability persisting. PW anomalies don't jump off the screen, but we are in mid June in the South, so even modestly anomalous PWs are plenty high enough to support heavy efficient rainfall. Large scale forcing look pretty good Sunday, with a well defined shortwave moving north of the region, bringing with it broader scale mid level troughing. Meanwhile the southern stream upper jet will periodically enhance upper level divergence within it's left exit region. Thus seems likely that another round(s) of convective activity will impact the region...focused along what should still be a decently sharp instability gradient/stationary boundary. Once you get off the surface winds are pretty unidirectional...resulting in generally weak Corfidi Vectors. Thus while it's usually a losing battle trying to predict convective specifics this far out...the general environment and pattern would support additional convection with some training/backbuilding potential. Model QPF solutions are pretty aggressive for this time frame, with both the 00z GFS and UKMET depicting a swath of 3"+ amounts and the EC and CMC also generally higher than preceding days. The 90th percentile of PQPF is also a step up from previous days with a broad region over 1.5". This all adds confidence to the depiction of a Slight risk on this day. Overall there has been a southern trend in the QPF in most guidance...with the 00z GFS and UKMET overlapping fairly well across portions of southern AR into central/southern MS and AL...and the WPC deterministic QPF did generally favor this axis. However there is still some spread in the guidance and we are 3 days out so some adjustments are likely. Thus the Slight risk was left a little broader...generally encompassing where the 90th percentile of PQPF is ~1.5" and higher. ....Northern Rockies... A Marginal risk covers portions of ID into MT and northwestern WY on Sunday. Stronger mid/upper forcing will likely trigger shower and embedded thunderstorm development. Instability looks rather marginal in nature and cells that develop should be quick moving. Thus generally not looking at any extreme rainfall rates or totals, but PWs do increase enough to suggest some heavy rainfall potential. Absent the fact that much of this area has seen well above normal rainfall of late this setup probably would not warrant any risk area. However given the wet antecedent conditions, think the large scale forcing and PWs are enough to warrant a Marginal risk across the region. Pereira/Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6-kEuPUyh1-_3gyPOj4DFoeT3O6Se-B_RNW9lFiEkaAP= O5Ll5m-RW1u9Q1wsHPgW4GRqBnTOIk1FLhD5ZNuGaHbWH8E$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6-kEuPUyh1-_3gyPOj4DFoeT3O6Se-B_RNW9lFiEkaAP= O5Ll5m-RW1u9Q1wsHPgW4GRqBnTOIk1FLhD5ZNuGuQY8g7Y$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6-kEuPUyh1-_3gyPOj4DFoeT3O6Se-B_RNW9lFiEkaAP= O5Ll5m-RW1u9Q1wsHPgW4GRqBnTOIk1FLhD5ZNuGzJdgkXw$=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) .