Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon May 08 2023 07:37:13 FOUS30 KWBC 080737 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 336 AM EDT Mon May 08 2023 Day 1 Valid 12Z Mon May 08 2023 - 12Z Tue May 09 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN AND AROUND SOUTHERN ILLINOIS... 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... TX... 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... 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 Valid 12Z Tue May 09 2023 - 12Z Wed May 10 2023 ....A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS IN A BROAD AREA AROUND THE ARKLATEX... Plains... The combination of a mid-level low moving out of TX into the Midwest and an incoming closed cyclone moving through the West should set the stage for good moisture transport through the Plains, divergent flow aloft, strengthening low-level inflow/effective bulk shear, and sufficient instability for organized convection. Where those ingredients exist the most prolonged period of time is in and near the ArkLaTex, where a Slight Risk area remains. Five pieces of global guidance have a signal for local 5-8" amounts somewhere across the eastern half of TX within or near the Slight Risk area. The better signal in general heavy rainfall was where ingredients suggest it should be across eastern OK and western AR, which remain in the Slight.=20 But, it is possible that portions of coastal TX could be receiving heavy rainfall at the beginning of the period in an area where a CAPE gradient should have developed due to recent heavy rainfall per conceptual models and to some degree the 21z SREF surface-based CAPE...if the heavy rainfall signal seen on the 00z Canadian Regional and 00z UKMET becomes embraced by additional guidance, the Slight Risk could be extended down there in later cycles. Across the Central Plains, northeast CO, eastern WY, and southeast MT, a combination of upslope flow and some low- to mid-level frontogensis acting in concert with sufficient instability should lead to localized heavy rainfall. Southern NE has been wet lately, and the 00z Canadian Regional has local 6" amounts in that area, which keeps them in the Marginal Risk area. Precipitable water values near or above 1" in the High Plains are a big deal from a heavy rainfall perspective, so stretched the Marginal northwest to loop in the heavy rain signal seen across the available guidance towards and around the expected position of the 850 hPa low. Roth Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6RfJEqCUCuFts7k-3nXZtRp7Hkf_5vpDLDhiwAE3dWSO= 1HEmxFDRsitfAYcw3BZUUZ7jsppqONlRuWDkOKzviEWJtRo$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6RfJEqCUCuFts7k-3nXZtRp7Hkf_5vpDLDhiwAE3dWSO= 1HEmxFDRsitfAYcw3BZUUZ7jsppqONlRuWDkOKzv70TpE-Y$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6RfJEqCUCuFts7k-3nXZtRp7Hkf_5vpDLDhiwAE3dWSO= 1HEmxFDRsitfAYcw3BZUUZ7jsppqONlRuWDkOKzv2JI266Y$=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) .