Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Mar 16 2023 18:31:41 FOUS30 KWBC 161831 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 230 PM EDT Thu Mar 16 2023 Day 1 Valid 16Z Thu Mar 16 2023 - 12Z Fri Mar 17 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN AND NEAR THE ARKLATEX... An incoming amplifying upper level trough across the mid section of the nation pushes from the Southern Plains into the Lower Mississippi Valley. The overall pattern remains favorable for increasing convection as strengthening southerly to south southwesterly flow raise precipitable water values to 2+ standard deviations above the mid-March mean along and ahead of the associated eastward moving cold front. Strong frontal convergence and favorable upper difluence in this high moisture axis will support potential for widespread heavy precip totals day 1 from east Texas/far eastern OK, eastward across much of Arkansas, northern Louisiana, far southeast Missouri and far western Tennessee and far western Mississippi. The 12z HREF guidance (probabilities of 0.5"+ an hour) and conceptual models suggest that heavy rain may come in two waves. An initial area of convection forming late morning into early afternoon across the ArkLaTex, followed by a potentially organized convective squall line near or ahead of the southeastward moving frontal boundary late afternoon into tonight. The initial area of convection should be slower moving and have potential for a period of training in a southwest to northeasterly direction within an area of 500-1000 J/kg of MU CAPE. The second round likely will be more progressive, but HREF probabilities for .50 and 1"/hour totals are greater with the second round associated with the cold frontal passage as MU CAPE surges into the 1000-2000 J/kg range. Hourly rain totals to 1.75" with local amounts to 4" are expected within the Slight Risk area and possible within the surrounding Marginal Risk area. A Slight Risk size was increased, with this area encompassing where the 12z HREF neighborhood probabilities are highest for 2"+ totals. Some expansion was made to the Marginal Risk area to account for the most recent model guidance. Roth/Oravec =20 Day 2 Valid 12Z Fri Mar 17 2023 - 12Z Sat Mar 18 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM THE LOWER MISSISSIPPI VALLEY & SOUTHEAST... The surface cold front moving through the Lower Mississippi Valley early Friday morning is expected to remain progressive Friday as it pushes through the remainder of the Lower Mississippi Valley and into the northern Gulf of Mexico by early Saturday. While precipitable values will remain above average (mostly 1.25-1.5") along and ahead of this front, 1.5 to 2 standard deviations above the mid-March mean. MU CAPE is expected to surge to 1000 J/kg or so near the front. The expected progression of the front and its associated convection should preclude any widespread runoff issues; issues would likely be dependent on mesocyclone formation temporarily holding up the convective line and/or cell mergers with any activity that forms ahead of the main line. No significant changes made to the Marginal Risk area from the previous outlook for this period, with the marginal risk area drawn to fit the model consensus for moderate to locally heavy precip totals. Much of the Marginal Risk area has stream flows at or below average levels. Along with the expected progression, any runoff issues to isolated in nature. Hourly totals to 2" and local amounts to 4" are expected in the Marginal Risk area. Roth Day 3 Valid 12Z Sat Mar 18 2023 - 12Z Sun Mar 19 2023 ....THERE IS A MARGINAL RISK OF RAINFALL IN AND NEAR NORTHERN FLORIDA... Some of the guidance indicates that as a shortwave moves from the Southern Rockies into the Mid-South that the frontal progression across northern Florida could hold up and lead to isolated 3-4" totals, higher than what is expected in the day 2 ERO period.=20 With precipitable water values in the 1.5-1.75" range, 850 hPa inflow and effective bulk shear in the 30-40 kt range, 500+ J/kg nearby and upstream, and fairly unidirectional west-southwest flow with height, hourly rain totals to 2" and local amounts to 4" are expected within any short, training bands and any mesocyclones that form. While most of the soils are sandy in portions of this region, it has been wet over the past week with portions of the FL Big Bend and North Florida and southeast GA showing some swaths of 200-400% of their average precipitation during this time frame.=20 In these areas, NASA SPORT imagery shows pockets of 50-60% saturation down to 100 cm. Considering the above, thought it prudent to introduce a Marginal Risk area. Roth Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-aqCBVf1h0OLxpHG982wBDQ9DKvCfjdgXKF6zXDNziDI= JNfl72e0O7SK80gJqY5h-AItXTdARsvDrMVkDmu_BVvGbf8$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-aqCBVf1h0OLxpHG982wBDQ9DKvCfjdgXKF6zXDNziDI= JNfl72e0O7SK80gJqY5h-AItXTdARsvDrMVkDmu_s5XZJr8$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-aqCBVf1h0OLxpHG982wBDQ9DKvCfjdgXKF6zXDNziDI= JNfl72e0O7SK80gJqY5h-AItXTdARsvDrMVkDmu_5NTRCdo$=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) .