Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon May 08 2023 18:56:25 FOUS30 KWBC 081856 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 255 PM EDT Mon May 08 2023 Day 1 Valid 16Z Mon May 08 2023 - 12Z Tue May 09 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN AND AROUND SOUTHERN ILLINOIS... 16Z update... With the convection from both shortwaves and the track of the upper low potentially overlapping from the Texas to the central Gulf Gulf, the two Marginal Risks were merged. The broad Marginal across the central/east-central U.S. largely still covers the elevated threat area for isolated flash flooding concerns. Minor reshaping of the western bounds were made along with a southern expansion into northern Georgia to reflect the latest hi-res guidance. There was a signal for higher QPF to setup over southern Missouri therefore the southwest bounds of the Slight Risk was broadened to reflect this change. Campbell Midwest/Ohio Valley... A shortwave moving by northern IL early on should continue overnight convective activity past 12z and spur new activity under its base due to daytime heating this afternoon. On Monday morning, enough CAPE exists to help continue the potential for moderate to heavy rainfall from overnight, which should be entering the confluence of the Mississippi and Ohio Rivers around 12z before fading and redeveloping in the early afternoon. There is decent overlap in the model guidance for heavy rainfall in and around Southern IL in an area where moisture, low level inflow, and instability should be sufficient for heavy rain concerns into Monday night, and area which should see some heavy rainfall during the early morning hours Monday. Hourly rain totals to 2" and local amounts to 4" are expected in this area. A Slight Risk area was added here. To the north, a combination of low- to mid-level frontogensis around a retrograding mid-level system, available moisture, and some instability should allow for a longer duration but convective heavy rainfall somewhere around southeast MN and WI -- local 2" amounts are possible. Since a stripe between Minneapolis/St. Paul MN and the Door Peninsula of WI has seen much above normal rainfall this past week, they were within the Marginal Risk area.=20 To the east, flash flood guidance is low in parts of the west facing Appalachians across PA and WV where rainfall has been a bit above average this past week. Although convection is expected to be progressive as a shortwave approaches from the west, sufficient moisture, instability, and effective bulk shear could lead to organized structures -- either random mesocyclones or short training bands -- to allow for a limited flash flood risk. To the southwest in and near the eastern border of KS/OK, Monday night should see a return of low-level inflow/effective bulk shear and moisture within an area of ample instability. With guidance (particularly the 00z Canadian) showing local amounts in the 2-4" range, despite very dry conditions over this past week, thought it useful to have a Marginal Risk in this region should any storms with heavy rainfall -- up to 2" in an hour -- intersect urban areas. Central Gulf Coast... A subtle shortwave aloft combined with a weak boundary at 850 hPa with sufficient instability and just enough effective bulk shear are expected to lead to thunderstorm formation which could organize across portions of LA and MS. Hourly rain totals to 2" with local amounts to 4" are anticipated, which would be most problematic in urban areas. This led to a new Marginal Risk area. The southern portion of the Marginal Risk area (portions of southeast LA north of Lake Pontchartrain) has seen heavy rainfall during the past few days, which potentially makes them more sensitive. South-Central TX... A shortwave moving across northern Mexico should lead to enhanced divergence aloft. Ample low-level inflow/effective bulk shear, moisture, and instability out ahead of the system should be present to produce thunderstorms with heavy rainfall -- up to 2" in an hour. The signal in the guidance is quite scattered, showing local amounts in the 2-4" range. There's a limited area west of Zapata that has experienced above normal rainfall this past week and it has rained heavily over the past 24 hours inland of the Middle TX Coast. Otherwise, the heavy rain threat would be concentrated in urban areas as much of the region has been quite dry this past week. Roth Day 2 Valid 12Z Tue May 09 2023 - 12Z Wed May 10 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS SOUTHEAST TEXAS... ....Texas... 21Z update... There was an overall eastward spread of the higher QPF approaching the Texas/Louisiana border with areal averages stills favoring the 1 to 3 + inch range. A couple pieces of guidance suggest local maximums of 5 to 6 inches near the western Gulf coast and adjacent points inland. The eastern boundaries of the Marginal Risk was expanded into southwest Louisiana and the eastern bounds of the Slight Risk was expanded to the Texas-Louisiana state line. Campbell A southerly low level flow combined with a forming mid-level cyclone should lead to ample moisture, instability, and effective bulk shear to support organized convection capable of producing heavy to potentially excessive rainfall. A few pieces of guidance indicate the potential for local amounts in the 5-10" range but don't show a great deal of overlap with their heavy rainfall signal. However, that overlap does exist across southeast TX so the threat level has been raised to Slight Risk in that region.=20 There remains dispersion on placement, but given time, the instability gradient should recede to the Gulf Coast and the best onshore flow appears targeted into portions of the Middle and Upper TX coast, which fits the guidance overlap and conceptual models. Should the signal become stronger, increased risk levels are possible in succeeding cycles. ....Central Plains... 21Z update... The environment and expected setup has largely remained unchanged for this part of the country. Minor westward and southern expansions were made to reflect the latest QPF trends. Consensus has the best potential for the highest QPF to occur over southern Nebraska to central Kansas during this period. Campbell A second focus for storms should extend from Nebraska across Kansas into northwest Arkansas in proximity to the surface boundary where any mid-level capping inversion appears weak enough to be able to be overcome by daytime heating. Enough moisture, low level inflow/effective bulk shear, and instability should exist to allow for convection with hourly rain totals to 2" and local amounts to 5" per the available model guidance. Portions of southern NE have seen some saturation from heavy rainfall over the past few days. With the best signal for heavy rainfall over the area which has been driest over the past week, thought it prudent to leave the risk as Marginal for the time being.=20 Roth 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!4aWsXdrO6N3kIrofh7T4vuNaGluBXcS5V4rx8FBPf6eY= 2xolxdV5vH0UsrDGZVJcbApvfcA-NdUsJemvY1xVOXRNHFU$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4aWsXdrO6N3kIrofh7T4vuNaGluBXcS5V4rx8FBPf6eY= 2xolxdV5vH0UsrDGZVJcbApvfcA-NdUsJemvY1xVD-V9RRo$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4aWsXdrO6N3kIrofh7T4vuNaGluBXcS5V4rx8FBPf6eY= 2xolxdV5vH0UsrDGZVJcbApvfcA-NdUsJemvY1xV4yMZQHE$=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) .