Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Apr 06 2023 08:14:22 FOUS30 KWBC 060814 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 413 AM EDT Thu Apr 06 2023 Day 1 Valid 12Z Thu Apr 06 2023 - 12Z Fri Apr 07 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FROM SOUTHEAST TEXAS TO WESTERN LOUISIANA... ....Southeast TX to Lower MS Valley... A frontal boundary off the Texas coast and a conveyor belt of 500mb shortwaves tracking northeast into the Mid-Atlantic will lead to an active day of showers and thunderstorms from Texas on north and east into the Mid-South. In addition, the 250-500mb trough axis is residing over Arizona on south through the Gulf of California. This setup is not only positioning the right-entrance region of the 150+ knot 250mb jet streak to reside over the Mid-South, but a weaker (albeit, still diffluent) left-exit region of a 100 knot jet streak over the tropical East Pacific sets up near the Rio Grande. This jet coupling will foster plenty of vertical ascent in the upper levels of the atmosphere. Strong ridging off the East Coast is responsible for an expansive area of southeasterly 850mb winds stemming from the western Caribbean and extending through the Gulf of Mexico. This provides no shortage of atmospheric moisture over southeast Texas with PWs >1.75" expected along the Texas coast and as far north as southwest Louisiana. Mean wind speeds within the 850-300mb layer are also a little slower at 20-25 knots out of the SW, while also oriented parallel to the front off the coast. With persistent inflow of 850mb moisture flux and a stalled front and upper level pattern, it will be an active day of widespread showers and thunderstorms across the southern tier of Texas and into the Mid-South. Guidance has continued to key in on southeast TX to western LA with the best odds for heavy rainfall totals >3" on the day according to the 00Z HREF. In addition, this setup is dependent upon where heavy rainfall from Wednesday occurs, and how much instability is available in these regions. The 500-1,000 MUCAPE values and PWs that according to NAEFS would top the 99th climatological percentile from Corpus Christi and San Antonio to areas southeast of Houston, giving this event a good floor as a Slight Risk. Soils have gradually grown more sensitive after rounds of storms today, most notably just west of the TX coast. That said, much of southeast TX came into today's event with as low as 10% of normal rainfall, and despite the most saturated atmospheric profiles tomorrow (GFS shows 1000-500mb RH values >90% throughout southeast TX) instability will also be harder to come by farther inland. There remain enough question marks for identifying significant rainfall amounts to not introduce a Moderate Risk, but given the dearth of moisture present and weak but sufficient MUCAPE, there does remain a good opportunity for heavy rainfall and potential flash flooding in Southeast TX and western LA. Maintained the Marginal Risk areas into the Mid-South could see PWs approach 1.5", MUCAPE near 500 J/kg, and over soils that are more sensitive and saturated compared to locations in western LA and southeast TX. It is worth mentioning that while there is no Marginal Risk in place, portions of the Mid-Atlantic could witness strong thunderstorms during the mid-late afternoon hours. PWs are likely to be at or above 1.5" with up to 1,500 MUCAPE available, which would support hourly rainfall rates close to 2"/hr in the most intense cells. The primary reasons for no Marginal Risk this forecast cycle are because cell propagation will be to the east with mean 850-300mb winds not running parallel to a more progressive frontal boundary. Northern VA to eastern MD is also sporting as low as 25% of normal rainfall over the last 7 days according to AHPS, indicating the soils in the area could use the rain. Still, a more urbanized corridor from central VA to the I-95 corridor of northern MD could be at risk for 1-3" worth of rain close to the afternoon rush hour. Should confidence in those higher end totals increase, a Marginal Risk may be necessary for the 16Z update. ...Pacific Northwest Coast... The Marginal Risk along the coast of the Pacific Northwest remains in place as guidance is in good agreement on rich 850-700mb moisture flux being oriented along the coast Thursday morning and into Thursday afternoon. A sharp 250-500mb trough axis approaching the coast Thursday morning is set to provide excellent diffluence atop the atmospheric column. This is a warmer setup aloft, causing rain to be the primary precipitation type in mountainous terrain that has a deep snowpack. Rainfall amounts as high as 2-3" (locally higher amounts in the Olympics possible) are expected from the coastal range of California and Oregon to the Olympics, where 0-40cm soil moisture percentiles courtesy of NASA SPoRT-LIS are up to 70% saturated in some locations. The combination of heavy rainfall rates topping out close to 0.5"/hr and some snow melt in higher elevations could result in rapid runoff along complex terrain, as well as rapid rises in creeks and streams. Mullinax Day 2 Valid 12Z Fri Apr 07 2023 - 12Z Sat Apr 08 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FROM CENTRAL TO SOUTHERN LOUISIANA AND SOUTHERN MISSISSIPPI... The Mid-South and as far south and west as east Texas remains under the influence of the diffluent right-entrance region of a roaring 160 knot jet streak over the Northeast. In addition, the mean 250-500mb trough over northern Mexico is also producing its own diffluent flow aloft over southeast Texas. The frontal boundary that has acted as the trigger for rounds of heavy rainfall will continue to stick around off the coast while southerly 850mb flow out of the Gulf of Mexico continues to deliver anomalous moisture to the region. The atmosphere looks to remain quite saturated (1000-500mb RH values >90% for most of the Slight Risk and Marginal Risk areas) and soils in these areas will have continued to have grown more sensitive from multiple days worth of rainfall. One limiting factor is that MUCAPE remains below 1,000 J/kg and mean 850-300mb mean winds remain around 20 knots, allowing for storms to remain on the progressive side. That said, the added soil sensitivity, anomalous moisture content, and just enough instability aloft to allow some storms to produce rainfall rates up to 2"/hr is enough to keep the Slight Risk in place over the Lower MS River Valley. The Lower MS Valley will reside directly in front of an approaching 500mb vort max, while the region also features better odds for seeing more modest instability levels. The Marginal Risk extends as far south and west as the Houston metro area to as far east as the southern Appalachians. The latter could see MUCAPE range between 500-750 J/kg Friday afternoon and PWs as high as 1.5". Revisited the Marginal Risk potential all the way to the Carolinas, but the cold front sinking further south into GA made it seem less likely sufficient instability and high enough PWs would be present to promote a more sufficient flash flood setup. Should there be a trend for more instability and moisture to become available in future forecast cycles, an extension of the Marginal Risk into portions of the central Carolinas may be necessary. Mullinax Day 3 Valid 12Z Fri Apr 07 2023 - 12Z Sat Apr 08 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FROM THE FLORIDA PANHANDLE TO THE CENTRAL CAROLINAS... Saturday features another favorable setup for a very wet day for much of the Southeast with a pesky frontal boundary still positioned from the Gulf Coast to the Carolina coast. A strong dome of high pressure over the Northeast will work with lower pressure near the Bahamas to produce strong onshore winds, providing the region with some Atlantic moisture. Aloft, the Southeast remains beneath the diffluent right-entrance region of a 250mb jet streak located over the Northeast. There is also a 250-500mb trough traversing the Mid-South that will enhance vertical motion atop the atmosphere too. This trough will tap into a stream of 850mb moisture flux and direct it at the front, providing additional moisture into the region and aid in the development of numerous showers and thunderstorms. PWs will generally hover between 1.25-1.50" while MUCAPE generally struggles to eclipse 500 J/kg. These values don't necessarily scream flash flood potential, but 1000-500mb RH values will be >90% for much of the area and soils will have grown more saturated from rainfall that transpired on Friday. Given these factors, the inherited Marginal Risk still appears to be on track, but did take out portions of the Carolina beaches where, barring a bigger increase in instability and moisture content, have the soil type to be able to better manage the steady periods of rain throughout the day. Mullinax Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_PTQP0eMQUkacNDgw_OzVWV-saTKRdxk67EfbkyqI5Q3= FH_UVr0qT2ChMw38KspQi75cK7I1jh3JT70Rl_Qy5jKlWic$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_PTQP0eMQUkacNDgw_OzVWV-saTKRdxk67EfbkyqI5Q3= FH_UVr0qT2ChMw38KspQi75cK7I1jh3JT70Rl_QyGJcUO_E$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_PTQP0eMQUkacNDgw_OzVWV-saTKRdxk67EfbkyqI5Q3= FH_UVr0qT2ChMw38KspQi75cK7I1jh3JT70Rl_QyBT8VlmM$=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) .