Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Aug 03 2023 00:25:53 FOUS30 KWBC 030025 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 824 PM EDT Wed Aug 02 2023 Day 1 Valid 01Z Thu Aug 03 2023 - 12Z Thu Aug 03 2023 ....THERE IS A HIGH RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF MISSOURI AND SOUTHERN ILLINOIS... ....Mid Mississippi to Lower Ohio Valleys... Changes based on the recent radar reflectivity and mesoscale guidance led to some southwest nudging to the risk areas across IA, IL, KY, and MO, unfortunately into areas which have seen significant rainfall as of late and where flash flood guidance values are modest to low. The surface boundary showers and thunderstorms are attempting to focus along is drifting northward across southern KS and southern MO. The guidance suggests that with the warm 700 hPa temperatures near the surface boundary (12C+), and the cooler/more west bulbed temperatures where rain is currently trying to fall, that convection should truly get going once the boundary shifts somewhat northward. The increase in 850 hPa inflow is notable across western MO when compared to eastern KS, so made some extension of the Marginal, Slight, and Moderate Risk areas in that direction/into western MO. Since the heaviest rain is expected to overlap parts of MO, southern IL, and western KY, which have been hit with heavy rainfall recently, the general risk across the region has been raised to High for a small portion of southern IL and areas west and south of St. Louis MO with some slight extensions and southwest shifts to the risk areas. Hourly rain totals should reach, if not exceed, 3" which would be near double the three hour flash flood guidance values. Within the High Risk area, numerous occurrences of flash flooding are expected. The Marginal Risk area associated with this system has been merged with the previous existing risk area in the southern Appalachians. This was coordinated with the Kansas City MO, St. Louis MO, Lincoln IL, and Paducah KY forecast offices as well as NWS Central Region, a few of the Regional River Forecast Centers, and the National Water Center. ....Great Basin to North/Central Rockies and High Plains... Thunderstorms forward propagated across much of UT, which led to a reduction in the Slight Risk area. A broad Marginal Risk remains around the Slight. Thunderstorms with heavy rainfall are forecast to concentrate in and near Great Salt Lake as an upper level disturbance moves into the region, where anomalously high PWATs remains. The meso-low will track NNE through the day, keeping heavy rainfall ongoing to the west of the track over eastern NV with more scattered storms likely across western UT. Due to the anomalously high moisture in place across the Great Basin, and rainfall totals potentially exceeding an inch an hour, the Slight Risk remains in place, though over a smaller area than continuity due to 18z HREF and radar reflectivity trends. Roth Day 2 Valid 12Z Thu Aug 03 2023 - 12Z Fri Aug 04 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE LOWER OHIO AND TENNESSEE VALLEYS TO THE SOUTHERN APPALACHIANS AS WELL AS THE NORTH/CENTRAL HIGH PLAINS AND PORTIONS OF THE INTERMOUNTAIN WEST... ....2030Z Update... ....Portions of the Southeast... The Slight risk area across this region was expanded to include all of southwestern VA south into central GA and northeastern AL. The latest developments are that unlike the past 2 nights, the rainfall expected to begin tonight in the Mid-Mississippi River Valley will persist through the day across TN, and continue into the southern Appalachians through the overnight hours. Rain will likely be ongoing at the start of the period in central/western KY and into eastern TN, which will fill in with additional convection across central TN through the morning. There will be short break behind the wave in KY/TN as the rain continues into the Appalachians and the Piedmont of the Carolinas and GA. Overnight, renewed spotty convection will redevelop across TN and persist across northeast AL and into GA. This scenario where multiple different areas may see repeating storms, with potential impacts to Nashville, Chattanooga, Atlanta, and Asheville could result in locally considerable impacts. For now this remains a high-end Slight Risk, but with better model agreement, an localized upgrade to Moderate cannot be ruled out. ....Central Plains... Convection is expected to break out rather late into the day Thursday from the Dakotas south through Kansas. Much of this will congeal into an MCS across southern NE and northern KS. Behind the line, weaker convection is expected to persist and train, resulting in more substantial and local impacts. The Slight Risk was expanded south to include more of northern and central KS with this update. ....Wyoming and Surrounding Areas... The Slight Risk across northeast WY was expanded west in coordination with PIH/Pocatello, ID and RIW/Riverton, WY forecast offices. Rainfall has been ongoing for much of the day across eastern ID, and a strong mesolow causing widespread convection across UT and northern AZ as of the time of this writing will push north across the Salt Lake Valley overnight and into ID/WY. Thus, expect widespread shower and thunderstorm activity across the region Thursday morning, which will be aided not only by the strong mesolow but the highly anomalous moisture to 1.5 inches PWATs which is closing in on 4 sigma above normal for this time of year in this part of the country. The unusually widespread nature of the rainfall following behind a wet day today is likely to exacerbate flooding concerns. As the day goes on, stronger convection will develop across much of WY, which will persist along the WY/MT border as it tracks east into the evening. As some of the upper level energy shears off to the east, that will support the northern end of the aforementioned squall line, resulting in heavy rain from thunderstorms across western SD, which has been hard-hit in recent days. The rain is likely to persist right through the overnight all across the region, though rates should gradually decline with time as the instability is used up. ....Northern NY & VT... No significant changes were made to the Marginal Risk area for this region. It is worth noting however that the amount of rainfall seen Thursday night will impact the flash flooding threat into New England on Friday. Wegman ....Previous Discussion... ....Lower Ohio Valley, Tennessee Valley into Southern Appalachians... The setup remains favorable for heavy rainfall and flash flooding both Thursday morning and again late Thursday night into Friday morning as shortwave energy moves through the region within the northwest flow regime. At the surface, an area of low pressure slide southeastward, advancing a warm front into the region. Strong southwesterly low level flow will bring a surge of moisture into the region, characterized by precipitable water values increasing from around 1.5" to over 2" from southern Kentucky through Tennessee and western NC. A complex of storms is expected to be moving through portions of southern Kentucky and middle Tennessee early in the period followed by another round of convection igniting late in evening and overnight into Friday across middle/eastern Tennessee into western North Carolina. The repeating rounds could bring several inches of rainfall for the 24-hr period (2-4"+) and given the forcing, highly anomalous moisture, and favorable instability setup, intense rain rates between 1-2"/hr will be possible. There remains still some spatial uncertainty in where the axis of heaviest rainfall falls (global models further north/northeast compared to the southern CAMs) and the axis of heavy rainfall is likely to be rather narrow but could be locally significant, especially for the more susceptible locations and complex terrain areas. ....Great Basin to Northern High Plains... Shortwave energy lifting through the region combined with favorable right entrance jet dynamics will support a broad area of forcing for ascent from portions of the Great Basin to the Northern High Plains. Moisture anomalies will remain above normal, nearly 2-2.5 standard deviations above normal, particularly across portions of northeast WY, southeast MT and western SD. Convection is expected to develop initially over portions of the Great Basin before moving eastward through WY/MT and pushing into the Plains later in the period where it'll interact with the greater instability/moisture. Localized rain rates 0.5-1"/hr will be possible across ID while further east rain rates up to 1.5"/hr will be possible across portions of northeast WY, southeast MT, and western SD. 00Z deterministic and ensemble guidance point toward localized 2"+ totals possible in that Slight Risk. Further south into northeast CO, northwest Kansas, and southern Nebraska, while rain rates and totals may not be as intense, the much wetter antecedent conditions combined isolated/scattered convection producing upwards of 1"/hr rain rates and localized 1-2" totals may produce more scattered instances of flash flooding and the inherited Slight Risk was maintained.=20 ....Interior Northeast... Broad forcing for ascent associated with an approaching shortwave energy embedded within the larger troughing will overspread the region while at the surface, a cold front begins to advance from the west. Ahead of that, southerly flow will bring modest moisture characterized by precipitable water values between 1-1.25". With daytime heating producing favorable MLCAPE values, scattered to numerous showers and thunderstorms are expected which could produce localized intense rain rates in excess of 1"/hr and may lead to isolated instances of flash flooding across portions of the interior Northeast. Taylor Day 3 Valid 12Z Fri Aug 04 2023 - 12Z Sat Aug 05 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE NORTHERN HIGH PLAINS AS WELL AS MUCH OF NEW ENGLAND... ....2030Z Update... ....Portions of the Northern High Plains... As the upper level forcing continues to shear off but more disturbances are added, the area of potential excessive rainfall will persist along the MT/WY border across much of western SD and north central NE. The Slight Risk was expanded west along the MT/WY border and also southeast into north central NE with this update. The primary area expected to be hardest hit remains western SD, which is considered a high-end Slight risk area. Widespread 2-3 inch totals are likely across the area with locally higher amounts. This will largely be possible due to a stubbornly persistent southeasterly LLJ that holds across eastern NE and into western SD. This will advect PWATs over 1.75 inches into the area, which due to the anomalously high moisture and aforementioned forcing, will allow convection to persist over this area throughout the Day 3/Friday period with little movement in the larger scale rain shield. As always there will be localized heavier amounts, but even the amount of rain forecasted is unusual enough that it's quite possible a targeted Moderate Risk may be needed across western SD, particularly along the east facing slopes of the Black Hills into Rapid City where upslope may further locally enhance totals. An analysis of the CAMs for the day, assuming reasonable agreement, may increase confidence enough for the potential upgrade. ....New England... An upgrade to a Slight Risk was introduced across much of New England with this update. A strong southwesterly flow of...you guessed it...anomalously high moisture/PWATs will continue through the day across New England ahead of a strong cold front. Any likely convection that forms ahead of the front will prime the already wet soils for the main line of storms likely to move through during the afternoon. Since this area is already sensitive from recent rainfall, and the expectation of widely scattered rainfall amounts exceeding 2 inches, the upgraded Slight Risk was hoisted in coordination with BOX/Norton, MA; BTV/Burlington, VT; and ALY/Albany, NY forecast offices. There is some uncertainty as to how much rain will fall in western portions of the Marginal and Slight areas, with changes possible with future updates. ....Southeast... Very few changes were needed with this update, but it's worth noting that wetter antecedent conditions from Day 2/Thu and the expectation of locally heavy rainfall across the Carolinas again on Friday may result in a future Slight upgrade should there be better agreement on where that possibility is more certain. Wegman ....Previous Discussion... ....Northern Rockies to Central Plains... Fairly potent shortwave energy lifting through the Great Basin is expected to become closed off at 500 mb across eastern Montana and the western Dakotas. This will wrap very anomalous moisture into the region, especially across southeast Montana into western South Dakota where PWs > 1.5" are expected, which is over 3 standard deviations above normal. As low pressure develops and moves from WY to the Dakotas, a very favorable frontogenetical band of precipitation is expected which could produce a longer duration moderate to heavy rainfall event. The signal for 1-2"+ looks pretty strong such that the Slight Risk was maintained across western SD but also expanded into southeast MT. There remains some uncertainty in how quickly convection progresses southeast into the Plains but the Marginal Risk was generally maintained through eastern Nebraska and western Iowa. ....Southeast... Anomalous moisture (PWs > 2") combined with favorable forcing for ascent associated with shortwave energy moving through the region will keep an active/unsettled pattern in place and the threat of heavy rainfall and flash flooding. There remains uncertainties in the placement and evolution of the convection during the period but the environmental ingredients from the 00Z guidance all point toward some threat of flash flooding so the Marginal Risk was broadly placed from the Southern Appalachians toward the coastal areas of North Carolina and South Carolina. ....Northeast... Longwave troughing becomes increasingly neutrally tilted during the period with the trough axis centered through Upstate New York late Friday afternoon and evening. This will allow for a longer period of southerly flow to bring anomalous moisture northward across the Northeast, particularly across the coastal regions of southern New England through coastal Maine with the latest guidance pointing toward a narrow axis of 1.5"+ PW setting up. Depending on the timing of the advancing cold front, enough destabilization will be possible during the afternoon/evening to produce scattered to numerous showers and thunderstorms across the region before quickly moving east. The modest moisture and instability and favorable forcing will support localized intense rain rates (1-1.5"/hr) with some localized totals near 2" possible. A Marginal Risk was maintained but as mesoscale details become clearer, an upgrade could be needed across portions of southern New England where antecedent conditions are much wetter (14-day rainfall departures are running 200-400 percent of normal). Taylor Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4sKlH9dZPEmgMLWDm9DjJGUbCfSASbRVUl6ZeR5MbB2m= bJcv-GVHNgi8ojqHCzMfFtr_OeR9QeUo5GPXG8YvB33uB0M$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4sKlH9dZPEmgMLWDm9DjJGUbCfSASbRVUl6ZeR5MbB2m= bJcv-GVHNgi8ojqHCzMfFtr_OeR9QeUo5GPXG8YvxoxsIGo$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4sKlH9dZPEmgMLWDm9DjJGUbCfSASbRVUl6ZeR5MbB2m= bJcv-GVHNgi8ojqHCzMfFtr_OeR9QeUo5GPXG8YvLRUWqDw$=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) .