Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Mar 17 2023 23:26:09 FOUS30 KWBC 172326 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 724 PM EDT Fri Mar 17 2023 Day 1 Valid 01Z Sat Mar 18 2023 - 12Z Sat Mar 18 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ALONG AND NEAR THE FLORIDA/GEORGIA LINE... ....Previous Discussion... A cold front continues to move eastward across the northeast Gulf coast. There have been some attempts at cell training, but so far, amounts lately have stayed under 2". Showers should remain active along and ahead of this front where PW values will remain above average, 1.5 to 2 standard deviations above the mean, boundary layer convergence will be strong, effective bulk shear remains strong enough for convective organization, and upper difluence well defined. The continued progression of the front, and declining instability, will be a detriment to widespread runoff issues. The Marginal Risk area was reduced to cover the area most likely to receive 0.5"+ an hour, which led to changes on its western portion due to system progression and its northern portion as instability is in decline inland. Stream flows across the marginal risk area are at or below normal levels. This and the expected progression of the front and associated rainfall keeps the risk level at marginal. Roth Day 2 Valid 12Z Sat Mar 18 2023 - 12Z Sun Mar 19 2023 ....THERE IS A MARGINAL RISK OF RAINFALL ACROSS PORTIONS OF NORTH FLORIDA... ....18Z Update... No changes were made with this update. The cold front currently advancing across the Southeast will stall out over portions of north FL on Saturday. However, the front by then will be substantially weaker than it is currently. Therefore, while convection is likely to train over this area, it is unlikely the storms will get strong enough to result in flash flooding. This is especially considering the very high FFGs over this area. Thus, the Marginal Risk area is especially for the urbanized portions of north Florida. While HREF values described in the previous discussion below are generally the same, the total expected rainfall is unlikely to overcome the high FFGs except in those particularly sensitive areas. Wegman ....Previous Discussion... Model consensus is for a narrow axis of moderate to locally heavy rains early in the upcoming day 2 period across northern Florida along and ahead of the strong cold front that will be moving southeast from northern Florida at the beginning of the day 2 period. Similar to the day 1 period, the cold front is expected to be fairly quick moving, which should limit the time that heavy rainfall rates may occur. The heavy rainfall rates will likely occur at the beginning of the day 2 period in a narrow axis of instability along and ahead of this front. HREF neighborhood probabilities during the first half of day 2 are high for 1"+ amounts across northern Florida, 50-75%, and 20-50% for 2"+ amounts. The previous marginal risk area for this period was narrowed to better match the model qpf consensus axis, which does overlap where above average precip values have occurred over the past week or so. While soils are generally sandy in this area, the heavy rain potential area will also overlap more urbanized areas of northern Florida, where some urban runoff issues are possible.=20 Oravec Day 3 Valid 12Z Sun Mar 19 2023 - 12Z Mon Mar 20 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. However there are a couple areas of interest worth noting. ....Northern California... Another round of rain is expected to move into California on Sunday. The precipitation will accompany snow levels generally rising to between 5,000 and 6,000 feet in the northern Sacramento Valley. Rainfall amounts will be meager as compared with the previous atmospheric river events, and the comparatively low snow levels will allow much of the heaviest precipitation (1-3 inches) in the mountains to fall as snow. Thus, despite the favorable soil conditions for flash flooding, what was left over in rainfall between the low snow levels, the relatively small response from the much stronger round of rain with this past atmospheric river, and the couple more days for the rivers to continue draining, no risk areas were introduced here with this update, though potential certainly exists for a future Marginal Risk for this area. ....South Florida... The same front over north Florida Saturday will gradually drift to the southern part of the state by Sunday. Afternoon convection is likely to develop along the front, but just like in north FL on Saturday is unlikely to become particularly heavy. This area will also bear watching if forecasted rainfall increases, especially in the urbanized areas around Fort Myers and Miami/Ft Lauderdale. FFGs are also very high in this part of Florida as well, so it will take a lot of rain to cause flash flooding, which is not forecast at this time. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_7JGtGL1OCLYSzg4t6E9_OeXMOf7u4i49mTxxWu7Cz7I= OMNqOXQt8Dl7bowB5mmONV4MbXozUscrlnTBhatl1KEJF1U$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_7JGtGL1OCLYSzg4t6E9_OeXMOf7u4i49mTxxWu7Cz7I= OMNqOXQt8Dl7bowB5mmONV4MbXozUscrlnTBhatl3soxJEE$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_7JGtGL1OCLYSzg4t6E9_OeXMOf7u4i49mTxxWu7Cz7I= OMNqOXQt8Dl7bowB5mmONV4MbXozUscrlnTBhatlqv-IiCo$=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) .