FOUS30 KWBC 170823 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 423 AM EDT Sat Oct 17 2026 Day 1 Valid 12Z Sat Oct 17 2026 - 12Z Sun Oct 18 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ALONG PORTIONS OF THE FLORIDA EAST COAST... Similar to Friday, deep moisture as shown by precipitable water value's at or above 2 inches will be in place across portions of central/south Florida throughout the period. This combined with diurnal heating will lead to enough instability for scattered thunderstorms capable pf producing 2 to 3 inch per hour rainfall rates along and ahead of a cold front. Veering onshore flow with the passing front will also help to support convergence along the east/Atlantic coast with the potential for repeated and persistent, slow-moving thunderstorms and longer- duration rainfall. The 00Z high resolution guidance supports the Slight Risk area introduced on Friday resulting in few changes from the previous outlook.\ Bann Day 2 Valid 12Z Sun Oct 18 2026 - 12Z Mon Oct 19 2026 ..MARGINAL RISKS OF EXCESSIVE RAINFALL EXIST FOR SOUTH FLORIDA AND THE OUTER BANKS OF NORTH CAROLINA SUNDAY... North Carolina Outer Banks... There will be increasing chances for heavy rainfall along the North Carolina as a Nor'easter develops off-shore. The 00Z guidance tended more off-shore than the previous runs but the magnitude of the east-west spread in the placement of the low suggests maintaining the Marginal Risk is prudent. South Florida... A well defined cold front is expected to press southward before stalling south of Florida late tomorrow night or early Monday morning. Showers and thunderstorms capable of producing downpour will be focused in vicinity of the front...especially in the afternoon and evening where moisture flux convergence is maximized by moderate post- frontal on- shore flow. Three+ inch amounts are locally possible along the coast but there is more guidance indicating the heaviest rainfall will be off-shore. A Marginal Risk still is sufficient given the low-end confidence of rainfall placement, Bann Day 3 Valid 12Z Mon Oct 19 2026 - 12Z Tue Oct 20 2026 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ALONG THE NORTH CAROLINA OUTER BANKS MONDAY INTO TUESDAY NIGHT... Numerical guidance continues ti show an area of low pressure offshore, there has been high run to run variability in the east- to-west placement of the low and its associated rainfall. That will dictate whether heavy rain makes it onshore or not. The 17/00Z GFS inched closer to the Outer Banks while the 17/00Z ECMWF headed farther out to sea showing the predictability of this system improved little. The 17/00Z ECMWF AIFS and 17/00Z AIGFS remained supportive of their control/operational runs respectively. As a result...the Marginal Risk remained largely unchanged from continuity. Bann Day 4 and Day 5 Valid 12Z Tue Oct 20 2026 - 12Z Thu Oct 22 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHWEST WEDNESDAY/WEDNESDAY NIGHT... ...Southwest... For Tuesday/Tuesday night, the Southwest is on the eastern periphery of a deep layer cyclone/"outside slider" moving west of CA. The atmosphere still doesn't appear to be sufficiently saturated with 0.75-1" precipitable water values in an environment with mild 1000-500 hPa thickness values. MU CAPE also appears skimpy, with only small pockets of 250 J/kg forecast by the 00z GFS. Continue not to carry a Marginal Risk since guidance is maintaining serve over its past day of runs. The situation changes markedly on Wednesday/Wednesday night, as the 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 above 1000 J/kg, precipitable water values of 0.75-1.25" are anticipated, and a few pieces of deterministic guidance are definitely wet enough for a Marginal Risk with local 3-5" totals. Hourly rain amounts to 2" would be possible across portions of AZ, NM, and the Four Corners. It is possible that this area could carry a greater risk as we move closer to this weather setup. ...Eastern NC/Southeast VA... The guidance advertises a Nor'easter whose center is offshore North Carolina, with an increasing signal for interior convection as -18C air at 500 hPa moves over 28-29C sea surface temperatures during the short range period and perhaps a convective comma head/TROWAL which feeds off additional instability from the western subtropical North Atlantic and tries to brush the NC coast. The guidance has almost completely trended away from heavy rainfall with the system Tuesday/Tuesday night, so removed the Marginal Risk. Wednesday/Wednesday night is a question mark, as the 00z Canadian guidance brings heavy rain back into eastern NC at that time range. As most other guidance has left heavy rainfall offshore, left any Marginal Risks out of the EROs for this system for days 4-5. There is a non-zero chance of excessive rainfall/ flash flooding should a solution like the 00z Canadian come to pass or the guidance tries to slosh/waver back westward in later runs. Stay tuned. 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 $$