Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Jun 16 2023 08:32:39 FOUS30 KWBC 160832 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 431 AM EDT Fri Jun 16 2023 Day 1 Valid 12Z 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 EASTERN COLORADO INTO WESTERN KANSAS... ....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. Wegman Day 2 Valid 12Z Sat Jun 17 2023 - 12Z Sun Jun 18 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL OVER PORTIONS OF THE GREAT PLAINS TO MID-SOUTH...CENTRAL GULF COAST/FLORIDA AND NEW ENGLAND... ....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... 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. Depending on how the storms evolve on Day 1/Friday, there's potential for needing a Slight Risk upgrade for portions of this area, as the pattern is largely unchanged. .... 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. 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... 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. ....ID and MT... A Marginal risk covers portions of ID into MT 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. Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6PBVdi7NToG_rnIHqcfSDXvmdJFgXno3ua-DaynisVdA= 7x2ukDTC5_hXb2IG84H4S8ntw_2wP1f8cccE1BZTf49S-Do$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6PBVdi7NToG_rnIHqcfSDXvmdJFgXno3ua-DaynisVdA= 7x2ukDTC5_hXb2IG84H4S8ntw_2wP1f8cccE1BZT3lCkTYw$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6PBVdi7NToG_rnIHqcfSDXvmdJFgXno3ua-DaynisVdA= 7x2ukDTC5_hXb2IG84H4S8ntw_2wP1f8cccE1BZTXacCXi0$=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) .