FOUS30 KWBC 182014 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 414 PM EDT Sun Oct 18 2026 Day 1 Valid 16Z Sun Oct 18 2026 - 12Z Mon Oct 19 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR A PORTION OF SOUTH FLORIDA... Unseasonably high moisture lingers for South Florida as a cold front has become quasi-stationary just to the south. Daytime heating will lead to sufficient instability for thunderstorms capable of locally heavy downpours. Similar to yesterday, post- frontal onshore flow will lead to convergence along the coast to support repeated development and the potential for longer duration periods of rainfall. The latest 12Z hi-res guidance supports the possibility for totals of 5"+ which could cause at least some isolated instances of flooding. Putnam Day 2 Valid 12Z Mon Oct 19 2026 - 12Z Tue Oct 20 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHWEST... Introduced a Marginal Risk for portions of the southwest (SE AZ/SW NM). Remnant energy from a lingering upper-trough just to the west will spread eastward over the region later tonight and into the day Monday helping to trigger scattered storms. An influx of modestly anomalous moisture from the Pacific will lead to sufficient instability (1000 J/KG CAPE) by Monday afternoon to support locally heavy downpours. The now available hi-res guidance, particularly the HRRR, is rather bullish on the potential for rain rates of 1-2" per hour, more than enough to lead to isolated flash flooding. Both the HREF and RRFS ensembles also have moderate to high probabilities of rainfall exceeding 1" and low- end probabilities for 2". Putnam 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... Modestly anomalous moisture will remain in place Tuesday ahead of the more potent upper-trough beginning to dig southward along the West Coast. However, overall instability remains low and with the bulk of upper-energy still remaining to the west of the region, the risk for heavy rainfall and flash flooding should remain very low. Putnam Day 4 and Day 5 Valid 12Z Wed Oct 21 2026 - 12Z Fri Oct 23 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHWEST WEDNESDAY INTO THURSDAY... 20z Update: Introduced a broad Slight risk across portions of the Southwest on both days 4 and 5. This is a favorable setup for heavy rainfall and at least scattered flash flooding with anomalous moisture and a deep mid/upper level low pushing across the region. Deep southerly flow ahead of the approaching trough will advect tropical moisture northward, with a PW plume of near record values for mid/late October. As is typical with these events, instability will be the key factor in determining the magnitude and extent of flash flooding...as these broad moisture plumes can often bring abundant cloud cover limiting destabilization. However, both the ECMWF and AIFS depict CAPE over 1000 J/kg on both Wednesday and Thursday across portions of AZ/NM and over 500 J/kg even into portions of NV/UT/CO. There is an initial moisture/instability surge on Wednesday over AZ well ahead of the upper low, and then the steepening lapse rates with the upper low on Thursday should also produce some destabilization. So while confidence on the exact details remain low given we are 4 to 5 days out, there is growing confidence in a heavy rain and flash flood threat over the Southwest region Wednesday into Thursday. The ingredients in place also suggest that at least locally significant impacts are a possibility. Trends will continue to be monitored and can not rule out the need for embedded MDT risk area(s) if/where enough instability materializes. Chenard ...Previous Discussion... ...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 $$