FOUS30 KWBC 180816 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 416 AM EDT Sun Oct 18 2026 Day 1 Valid 12Z Sun Oct 18 2026 - 12Z Mon Oct 19 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR A PORTION OF SOUTH FLORIDA... South Florida... A secondary cold front will make its way southward across south Florida today resulting in redevelopment of thunderstorms. Some of the storms will be capable of producing localized downpours over urbanized areas. After the rainfall in the period prior to the start of the Day 1 period...the area may be more prone to flash ponding and excessive rain, Hence the Marginal Risk is maintained. Bann Day 2 Valid 12Z Mon Oct 19 2026 - 12Z Tue Oct 20 2026 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Most of the guidance from the 18/00Z cycle continues to keep the system far enough away from the east coast to keep the excessive rainfall threat off-shore. Even so...it should be acknowledged that the probability of excessive rainfall is probably not zero percent but not to the level of re-introducing a Marginal risk area. Bann Day 3 Valid 12Z Tue Oct 20 2026 - 12Z Wed Oct 21 2026 The probability of rainfall exceeding flash flood guidance is less than 5 percent. ...Southwest... The atmosphere still doesn't appear to be sufficiently saturated with 0.75 inches to 1 inch nor terribly unstable with only a few hundred J/kg of CAPE forecast by the 00Z GFS. As a result...did not outlook a Marginal Risk in this part of the country. Bann Day 4 and Day 5 Valid 12Z Wed Oct 21 2026 - 12Z Fri Oct 23 2026 ...Southwest... On Wednesday/Wednesday night, a deep layer cyclone approaches SoCal and scoops up moisture from a dying Trudy and flings it into the Southwest. Near the cold low itself, MU CAPE of 250-500 J/kg and precipitable water values of 1-1.3" with up to 30 kts of onshore flow at 850 hPa is forecast. There's enough of a heavy rain signal for a Marginal Risk for the Transverse and Peninsular Ranges, which is there to help cover burn scar concerns. In and near the Four Corners, MU CAPE is forecast to rise to 500-750 J/kg, precipitable water values of 0.75-1.25" are anticipated. The ingredients available along with deterministic guidance QPF output remain wet enough for a Marginal Risk, though the higher totals seen 24 hours ago have come down a bit, possibly due to a ~6 hour slow down/westward shift of the overall system when compared to this time yesterday. Hourly rain amounts to 2" would be possible across portions of AZ, NM, and the Four Corners in isolated to widely scattered spots. For Thursday/Thursday night, the system shifts eastward across Southwest and Great Basin/Intermountain West into portions of the Southern High Plains, with similar moisture and instability in tow. The guidance advertises local totals in the 2-3" range, and hourly rain amounts to 2" remain possible. The mountains of SoCal appear to have enough moisture/instability left and guidance QPF signal to introduce a Marginal Risk there with the Marginal Risk across the interior West shifting eastward. It is possible that the risk levels could increase as we move closer to this weather setup -- particularly if instability trends reverse/increase -- but the slowing trend in the guidance with this system introduces enough uncertainty not to do go with a risk level beyond Marginal either day at the present time. Roth Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt $$