Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Jun 16 2023 20:08:18 FOUS30 KWBC 162008 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 407 PM EDT Fri Jun 16 2023 Day 1 Valid 16Z Fri Jun 16 2023 - 12Z Sat Jun 17 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR THE EASTERN GULF COAST AND FOR PORTIONS OF THE CENTRAL HIGH PLAINS... 16Z Update... Adjustments were made based on current satellite and radar trends, as well as the latest hi-res guidance, including the 12Z HREF and recent runs of the HRRR. For most areas, these were small-scale changes. Along the eastern Gulf Coast, the leading edge of convection the developed overnight continues to progress through the Florida Panhandle and is not expected to pose a widespread flash flooding threat as it moves across northern Florida. SPC mesoanalysis and current imagery show plenty of stable air in its wake, further suggesting a limited threat for flash flooding in the near term. However, westerly flow into a lingering boundary may set the stage for convective redevelopment later today, with additional rounds of training storms possible. Upstream energy approaching from the northwest is expected to help support this redevelopment. A Slight Risk was maintained along the Gulf Coast, including the western Florida Panhandle and southern Alabama and Mississippi, and extended northwestward to include more of central Mississippi. HREF neighborhood probabilities indicate additional rainfall amounts of 2-3 inches are likely within this area through 12Z Sat. Given the sensitivity from recent heavy rain, will continue to monitor this area for a possible upgrade to a Moderate Risk, especially if a signal for heavy rain development over areas impacted begins to present itself. Previous Discussion... ....High Plains... A positively tilted longwave trough over the Intermountain West will gradually drift eastward between the polar jet over Canada and the subtropical jet over NM and extending eastward to LA/AR. In the subtropical jet, a southwesterly jet streak will be in place from the northern Baja California peninsula northeastward to the OK/TX Panhandles. The left exit region of this jet streak in close proximity to the aforementioned longwave trough will create and area of strong upper level diffluence across CO. Highly anomalous moisture will be in place along the CO/KS border, with PWATs approaching 1.25 inches. This is above the 95th percentile in the climatology, or over 2.5 sigma above normal. This will favor strong and moisture-laden thunderstorms across the area. CAMs guidance suggests multiple rounds of moderately fast moving storms will develop along the Front Range in central CO and move ENE into eastern CO and western KS. There is decent agreement that the storms will train over these same areas, generally from Denver and the I-70 corridor south, with much less rainfall expected further north. Antecedent conditions are quite favorable, with portions of CO at over 600% of climatological normal for rainfall for the past 2 weeks. Thus, the chances of flash flooding are considerable, with widely scattered flash flooding impacts possible. This is considered a higher end Slight risk area, particularly from the Palmer Divide east. The storms will continue northeastward into eastern NE while weakening as they track into far eastern SD. The Marginal Risk area covers these somewhat lower risk areas due to weaker storms and less favorable antecedent conditions. In the mountains, there will be a potent shortwave trough that may result in more scattered storms across portions of WY, so the Marginal Risk was expanded. Finally, there is a nearly stationary front that extends westward from CO into southeastern CA. A few flash flooding reports have been reported in southeast CA near Death Valley, and the drifting rainband may impact the Las Vegas area. Meanwhile, the convection may move over the slot canyons of southern UT, which are particularly vulnerable to localized flash flooding. ....MS River Valley and into FL... The inherited Slight Risk area along the eastern Gulf Coast was expanded southeastward into portions of north central FL with this morning's update. A stationary front with access to ample Gulf moisture is causing extreme flash flooding in the Pensacola area this morning. The front will continue to act as a "train track" of sorts for developing thunderstorms to train along. While FFGs in these areas are very high, as the stationary training thunderstorms near Pensacola have shown, these can be overcome. Since the same front will still be in the same area, with the Gulf moisture not going anywhere, and Corfidi vectors will remain parallel to the northwest to southeast oriented front, it remains probable that renewed rounds of storms will develop again this afternoon. PWATs will be steady around 1.75 inches, which while not unusual for this time of year, the addition of the extreme heat ongoing across Texas will lead to extreme instability. The extreme instability will support the continued thunderstorms over this region. ....Northeast... A strong upper level shortwave will slowly track eastward from western PA to off the Jersey Shore through the day today. Ahead of this forcing, a bit of additional Atlantic moisture will track northward up the coast, increasing PWATs to around 1.25 inches. This will be enough moisture for showers with a few embedded thunderstorms to develop across much of New England and the northern Mid-Atlantic. Some of these areas have seen enough rain the past couple weeks that the antecedent soil conditions may support isolated flash flooding, particularly where any slow-moving or training storms can move over the same areas. There is a local maximum of rainfall forecast for southeastern PA through southern NJ, then curving northward into coastal New England. The inherited Marginal Risk area was expanded quite a bit to now include much of New England and the northern Mid-Atlantic. Areas at greatest risk for isolated flash flooding are urbanized and poor drainage areas. Pereira/Wegman 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 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!4ATwgt4hz4KdoWvCx6qem9Zzp93bAKwF0ceeUXRouYZI= ncgOPnOgjzUhhIt-K-2CmFu14ciCazmOI1rscY5kKT1Wde4$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4ATwgt4hz4KdoWvCx6qem9Zzp93bAKwF0ceeUXRouYZI= ncgOPnOgjzUhhIt-K-2CmFu14ciCazmOI1rscY5kR5a7rJA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4ATwgt4hz4KdoWvCx6qem9Zzp93bAKwF0ceeUXRouYZI= ncgOPnOgjzUhhIt-K-2CmFu14ciCazmOI1rscY5ky3x4wTQ$=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) .