Convection that has developed along a well-defined cold front will remain ongoing, producing areas of moderate to heavy rain and possibly isolated runoff concerns early in the period from central Missouri back into east Texas. Rainfall rates are likely to wane through the late morning hours before storms re-intensify farther east from southeastern Missouri to Louisiana during the afternoon. Ample moisture (PWs 1.50-1.75+) focused along a 40-50+ kt low level jet interacting with strong upper jet forcing ahead of an amplified shortwave will contribute to organized storms capable of producing heavy rates. While the forward motion of these storms may limit the threat for widespread heavy amounts and flooding concerns, the intense rates produced by these storms are expected to raise the threat for at least isolated runoff concerns, especially in poor drainage and urbanized areas. As these storms continue east across the Mid South, several hi-res guidance members show them converging with less-organzied storms lifting out of the southeast along the western edge of a persistent ridge that extends across the Southeast. These converging storms are expected produce an axis of heavy amounts centered from northwestern Alabama through Middle Tennessee into Kentucky. A Slight Risk was maintained across portions of the lower Mississippi Valley and Mid South into the Tennessee Valley where the REFS and HREF indicate widespread amounts of 1-2 inches, with locally heavier amounts, are likely.
While the mid-level shortwave that helped organize heavy amounts across central and southern portions of the state is expected to weaken and drift west farther into the Gulf, deeper moisture will linger across southern Florida. Persistent easterly flow will maintain PWs around 2 inches across portions of Southeast Florida, supporting storms capable of producing efficient, heavy downpours. While storms that develop are expected to be less organized and the footprint of heavy rains more confined than yesterday, there is the potential for additional localized heavy amounts that may exacerbate ongoing flooding. A small Slight Risk was introduced across Southeast Florida where the HREF and REFS indicate that localized additional amounts of 3+ inches are likely today.
Storms developing late in the previous period will likely remain ongoing across parts of the Tennessee and lower Ohio valleys, carrying an isolated threat for runoff concerns into the early hours of the Day 2 period. Then by the afternoon, moist southerly flow into the southern Appalachians along with modest mid-to-upper level forcing will contribute to an increasing chance for locally heavy showers and storms. The general consensus of the 00Z guidance indicates that orographically focused amounts of 1-2 inches, with isolated heavier amounts, can be expected from northern Georgia into the southern Blue Ridge.
The threat for heavy amounts and flooding concerns is expected to continue its downward trend. However, could still see some isolated heavy amounts with flash flooding especially in the urbanized coastal areas. While drier air will continue to spread into the region, especially late in the period, PWs remaining near 2 inches could support some additional afternoon heavy downpours, resulting in additional isolated runoff concerns.
The probability of rainfall exceeding flash flood guidance is less than five percent.
A Marginal risk area was added for southern New England, NYC and Long Island with this update. There remains considerable uncertainty how fast the front over the Northeast sweeps east, especially into the Mid-Atlantic. However, there is better certainty about a corridor of heavier rain getting hung up across southern New England on Friday. Soils all up and down the Eastern Seaboard, with the notable exception of Florida, remain very dry after over 2 weeks without a drop of rain. Thus nearly all of the expected rainfall will be beneficial. However, the plume of moisture with PWATs over 1.5 inches stalling over New England, potential for some limited instability to support some convective elements, upslope, and plentiful urban areas will all support the possible development an isolated instance of flash flooding or two. This will be where the heaviest rains are most persistent. Thus, this is a low-end Marginal as considerable uncertainty remains.
On Saturday, lingering rain in eastern Maine, and a weak tropical disturbance approaching the lower Texas coast both have the potential to cause an isolated instance of flash flooding or two in those respective areas. In Maine several pieces of guidance move the heaviest rains east into New Brunswick and Nova Scotia faster. Meanwhile in Texas, a lot of the expected rain may stay offshore where there will be much more instability. These factors precluded a Marginal Risk issuance for now in both areas.
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