Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Nov 19 2023 20:05:44 FOUS30 KWBC 192005 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 304 PM EST Sun Nov 19 2023 Day 1 Valid 16Z Sun Nov 19 2023 - 12Z Mon Nov 20 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. ....Southern Kansas/Northern Oklahoma... Model QPF still suggests that a non-zero threat for excessive rainfall exists across south-central to southeast Kansas and north-central to northeast Oklahoma as strengthening mid to upper level dynamics are trying to draw favorable moisture/instability ingredients into the region ahead of a sharpening mid-level trough later this evening into the early morning hours on Monday. Precipitable water values only approach 1.3 inches and fairly quick steering flow/cell motion vectors, rain-rates and totals are more likely than not to reach even the naturally lower FFG values of the Flint Hills of SE Kansas into Osage county, OK (<2"/hr). This area of the country has also been very dry the last few weeks, which should help soils retain most rainfall. More specifically, 12z HREF neighborhood probabilities for greater than 2" of rain in 6-hours max out at 20-25% in north-central Oklahoma tonight, but even these amounts shouldn't create a notable flash flood risk. The risk of excessive rainfall and isolated flash flooding is non-zero, but less than 5 percent. Bann/Snell Day 2 Valid 12Z Mon Nov 20 2023 - 12Z Tue Nov 21 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE LOWER MISSISSIPPI VALLEY AND GULF COAST STATES... 20z Update... Only minor changes were made to the previous outlook in order to confine the MRGL to an area most likely to see isolated flash flood concerns. New hires guidance, including the 12z HREF, now encompasses the entire D2 period and indicates the potential for localized 2-3" totals. The best chances for these amounts are forecast between northern Louisiana and central Mississippi, where more organized supercells could track and overlap. Here, 12z HREF neighborhood probabilities reach 30-40% for at least 3" in 6-hours. Rainfall amounts up to 2" are possible elsewhere in the MRGL risk and could lead to isolated runoff concerns. In general, the flash flood risk is greatly reduced due to the very dry ground conditions in place. Limiting the flash flood risk to a mostly urban threat. Snell Previous Discussion... Once again...there was very little change to the overall synoptic set-up/evolution on Monday into early Tuesday. The only exception is that the solutions continued to trend faster/more progressive by the latter part of the outlook period and the Marginal Risk area was nudged a bit more to the east. The axis of strongest forcing is still expected to intersect the return moisture flow off the western and central Gulf of Mexico. This flattens the overall flow with increased westerly steering, allowing for a tad increased axis of training/repeating for stronger updrafts along/ahead of the cold front...which when combined with very strong moisture flux...should allow for very intense rainfall rates in the range of 2-2.5"/hr. The big question remains the duration of such rates any place and whether there is repeat convection or training. While the area of concern has been in a longer term drought and top 40cm soil conditions are at or below 30% moisture content (generally below 10th percentile historically), the sheer rates over dry soils may result in limited infiltration initially with increased run-off potential.=20 While an incident or two of flash flooding remains possible, but nearly all parameters continue to fall below thresholds for an upgrade to a Slight Risk. Bann Day 3 Valid 12Z Tue Nov 21 2023 - 12Z Wed Nov 22 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM PORTIONS OF THE MID-ATLANTIC TO THE SOUTHEAST... 20z Update... Similar to the D2 update, minor changes to the D3 MRGL risk were made to confine the area somewhat. Instability will be very limited, but elevated atmospheric moisture content (PWAT of 1.0-1.5") and terrain enhancement along the central/southern Appalachians may allow for maximum rainfall amounts of 2-3" from western NC to northern VA. Much of the region is experiencing very dry ground conditions and drought, so much of the rainfall is expected to be beneficial. However, isolated urban flooding is possible where drainage is impaired. If the trend is for lower amounts once hires guidance is available, the MRGL risk could be further confined or removed. Snell Previous Discussion... The center of low pressure taking shape on Monday is expected to make its way into the Ohio Valley Tuesday morning and reach the eastern Great Lakes late Tuesday/early Wednesday. A cold front associated with the low will be the primary focus for convection capable of producing heavy to excessive rainfall. There should be plenty of moisture already in place ahead of the front given a period of strong moisture flux. By the latter part of the period...divergence aloft from northern Georgia across parts of the Carolinas and into the Mid-Atlantic region increases as the axis of a 130 to 150 kt upper level jet approaches. This should help supply the upper support while backing low level winds continue to feed low level moisture inland and result in a maximum of low level moisture flux convergence along and east of the front. As was the case on Day 2 farther south...while the area of concern has been in a longer term drought and top 40cm soil conditions are at or below 30% moisture content (generally below 10th percentile historically), the sheer rates over dry soils may result in limited infiltration initially with increased run-off potential. As such, an incident or two of flash flooding remains possible, but nearly all parameters continue to fall below thresholds for an upgrade to a Slight.=20 Closer to the Gulf coast...trimmed the southern portion Marginal Risk a bit but there were still enough signals that locally heavy rainfall could be on-going at the start of the Day 3 period to preclude removing the Marginal risk area all together at this time. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5VYtjSd22Bkwp-MfqjcnP4BqDq00-lhjr9KqR9yDKFV5= 66nPPfR7IHvFPNrAwjzb2eC1b2a60vrXCR63l3QwRB8zNng$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5VYtjSd22Bkwp-MfqjcnP4BqDq00-lhjr9KqR9yDKFV5= 66nPPfR7IHvFPNrAwjzb2eC1b2a60vrXCR63l3QwS_pae1E$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5VYtjSd22Bkwp-MfqjcnP4BqDq00-lhjr9KqR9yDKFV5= 66nPPfR7IHvFPNrAwjzb2eC1b2a60vrXCR63l3QwAncfmvM$=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) .