Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Jan 05 2025 00:00:32 FOUS30 KWBC 050000 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 700 PM EST Sat Jan 4 2025 Day 1 Valid 01Z Sun Jan 05 2025 - 12Z Sun Jan 05 2025 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Bann Day 2 Valid 12Z Sun Jan 05 2025 - 12Z Mon Jan 06 2025 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHEAST AND CENTRAL GULF COAST... ....20z update... The overall setup and environmental conditions remain on track as noted in discussion below and this update will focus mainly on guidance trends and further assessment of the Hi-Res CAM/HREF suite as the entire D2 period falls within the model run times. The environmental conditions are very favorable for strong thunderstorms and high moisture flux...as the CSU-ERO first guess fields are highlighting a potential well into the Slight Risk Range for but given forward progress/speed of convection... overall totals barely reach 1.5-2.5" totals which barely reach 10-15% coverage exceedance of 1-2 year ARI. So contingency for flash flooding conditions will be more likely due to storm mode and sub- hourly rates and totals of 1-2", favored by increased localized moisture flux with any rotating updrafts embedded along the quick moving boundary. This will likely occur earlier than later in the development cycle across E TX into central AR between 21-03z. Still, HREF probability of 2"/3hr remains maximized at 30-40% which is no particularly impressive given the environmental setup. Hydrologically, the earlier and further west, would also rise the risk given recent 7-10 precipitation anomalies and soil saturation values are more at risk points west of the Mississippi River Valley. Additionally, closer to stronger forcing/pre-frontal moisture flux convergence and naturally lower FFG values across the TN River Valley will see reduced rainfall rates/totals but still may nose to those FFG to continue a justification of the Marginal across the area into the Southern Appalachians, but did try to pull back the risk area through the southern Cumberland Plateau. The only expansion was to include portions of SE LA into coastal MS and into the western portions of Mobile, AL; this is where convergence along the western edge of the synoptic sub-tropical ridge over FL/Eastern Gulf and along the moisture/higher theta-E gradient will provide an axis of enhanced thunderstorm activity in the morning into early afternoon. In fact, these cells will have the greatest potential for some training cells and totals over 3". Both the ARW (00z and 12z) and HRRR solutions remain most aggressive and persistent with timing/placement...other Hi-Res CAMs are not as confidence. Combine this potential with recent heavy rainfall and increased soil saturation, and there was sufficient signal to press the eastern portion of the Marginal Risk to cover it. Gallina ---Prior Discussion--- An impressive deep layer cyclone will develop over the central US into Day 2, originating from a potent shortwave trough currently digging into the Four Corners region. The trough is expected to cut-off into an upper-low as it emerges into the central/southern Plains with strong DPVA interacts with the right-entrance region of a broad phased jet structure (~130 kt jet streak @ 250 mb over the Mid-Atlantic, and a smaller ~100 kt jet streak near the Ozarks). A substantial low-level jet (850 mb of 40-60 kts) is expected to form as a result of deep cyclogenesis, ushering in highly anomalous tropospheric moisture (for this time of year) with precipitable water values nearing 1.5" (near the 90th percentile to max moving average, per JAN/BNA/BMX sounding climatology). While downscaled global guidance (GFS/ECMWF) continues to suggest only localized 1-2" totals, the impressive dynamics of this system (along with the anomalous moisture) suggest the potential for this guidance to locally be exceeded (as the 00z FV3 suggests, but additional CAMs should better capture the even with the upcoming 12z runs). That said, both the progressive nature of the system and the limited window of time in the afternoon to take advantage of destabilization (with SB CAPE from east TX and southern LA to the Mid-South reaches 500-2000 J/kg) are expected limit localized totals to the 2-3" range (with as much as 1-2" in an hour with the main line of convection). Have maintained the inherited Marginal risk area for much of the Southeast (with some additional expansion towards the southwest, where instability will be most abundant) generally encompassing the expected warm sector of the system (while excluding areas farther north where more impressive broad QPF exists, as this precipitation will mostly occur in the form of ice and snow). Churchill Day 3 Valid 12Z Mon Jan 06 2025 - 12Z Tue Jan 07 2025 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Gallina Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8SVud7-Uy_2VLd87yxnVAx3JNO8opuGAlXEsTPoPWX4l= UphOghD4kyX6uMhY6XRn85AugFbOIcJ-anpXTe5WC2SP0_o$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8SVud7-Uy_2VLd87yxnVAx3JNO8opuGAlXEsTPoPWX4l= UphOghD4kyX6uMhY6XRn85AugFbOIcJ-anpXTe5WmUPeHrQ$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8SVud7-Uy_2VLd87yxnVAx3JNO8opuGAlXEsTPoPWX4l= UphOghD4kyX6uMhY6XRn85AugFbOIcJ-anpXTe5WO9iBmP0$=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 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .