Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Thu Apr 27 2023 17:29:00 FOUS30 KWBC 272031 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 430 PM EDT Thu Apr 27 2023 Day 1 Valid 16Z Thu Apr 27 2023 - 12Z Fri Apr 28 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE CENTRAL GULF COAST... ....Gulf Coast through Northern Florida... 16Z update... Convection across the central Gulf Coast is progressing east/northeast this morning and is expected to be diminishing over south-central Louisiana, thus lowering the risk for excessive rainfall and flooding threat. The Slight and Marginal Risk areas were trimmed to now cover far eastern Louisiana and points east. Additionally, with the frontal boundary sprawled across the southern Gulf states/Southeast convection is expected to persist along and north of this boundary today and into the overnight hours. The latest hi-res guidance depicts hourly rates of 1 to 3+ inches streaking across this part of the region, with some areas getting repeating storms. The Slight Risk was expanded north to encompass southern Mississippi, Alabama, Georgia and most of northern Florida. Campbell A squall line moving across Louisiana as of this writing will continue moving east along the Gulf Coast today. Rainfall rates are over 2 inches/hour at times, which despite the short duration of the rain could very possibly cause localized flooding, particularly in urbanized and poor drainage areas. Since these are the most likely areas to develop flash flooding, the inherited Slight Risk was trimmed to the immediate urbanized coastal areas and Baton Rouge, LA. Already this morning widely scattered showers and thunderstorms have developed ahead of the main squall line. This scenario is expected to increase today, meaning the pre squall line heavy rain is likely to be more widespread along the Gulf coast in the Slight Risk area. These storms may increase the potential in the coastal cities of a bit more scattered flash flooding. Given the very progressive nature of the squall line, it's likely the pre squall line showers and storms will be necessary to get more widespread flooding, and as such if there's little pre-frontal storms, then the flash flooding in the Slight Risk area will be more isolated. As the squall line and pre-frontal storms are moving across the central Gulf Coast, typical afternoon storms are likely to develop along the Atlantic coast of GA and northern FL, which, having similar amounts of atmospheric moisture to work with, could prime the area, especially any urbanized areas such as Jacksonville and Savannah, GA for potential flash flooding when the main squall line moves through overnight tonight. There will likely be a break in the rain, with a longer break the farther south you go, but nonetheless the one-two punch of heavy rain may cause isolated flash flooding, so the Marginal Risk was expanded to the coast in coordination with CHS/Charleston, SC and JAX/Jacksonville, FL forecast offices. For all areas, high FFGs will be the primary mitigating factor preventing more widespread flooding. ....Ohio and Tennessee Valleys... 16Z update... Much of this area remains in good order with isolated to scattered convection lifting into the Tennessee Valley this morning. More activity is expected for the middle to later part of this period when areal coverage and intensity increases further north. The Marginal Risk area was adjusted eastward a bit over western portions of the Carolinas to account for an uptick in QPF trends. Campbell A negatively tilted shortwave trough in the upper levels will race northeastward up the Ohio Valley late today into tonight. This wave will use the 1-1.25 inches PWATs to locally increase storm coverage and intensity across the Ohio and Tennessee Valleys today. Thus, the Marginal Risk remains in effect for the entire region today. There is likely to be an area across central and northern MS and AL that will be in between the squall line moving along the Gulf Coast and the heavier convection over this region, so the Marginal Risk over northern MS/AL is a lower-end Marginal than further north. Despite the extra forcing, atmospheric instability will be rather lacking, only getting up to 500 J/kg over the Tennessee Valley, with lesser amounts further north. Thus, given the expectation of lower rainfall rates, only isolated flash flooding coincident with the Marginal Risk is expected. Wegman Day 2 Valid 12Z Fri Apr 28 2023 - 12Z Sat Apr 29 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS THE MID ATLANTIC REGION AND TEXAS... ....Mid-Atlantic... 21Z update... Consensus continues to show 1 to 3 inches falling from the northern Mid-Atlantic coast to central Pennsylvania, with the highest QPF likely focused near the coastline. No changes were made to the Marginal Risk area as it currently highlights the region most likely to have an elevated threat for excessive rainfall and isolated areas of flooding. Campbell As the shortwave trough that caused the heavier rains over the Ohio and Tennessee Valleys Thursday lifts northeast, it will be able to take advantage of the warm frontal/tropical moisture from the Gulf being pumped northwestward into the Mid-Atlantic on a 40 kt southeasterly jet. PWATs will increase to over 1.5 inches into southeastern VA, and over 1 inch across much of the rest of the mid-Atlantic by Friday afternoon. This moisture will include instability to 500 J/kg which will also be advected north up the coast. Light to moderate rain will be widespread across all of the mid-Atlantic on Friday, but the instability will contribute to locally heavier embedded elements. Soils are very dry over the area, but the long-duration rain with heavier rain embedded will likely result in isolated flash flooding, especially in urbanized and poor drainage areas. The feed of Atlantic moisture will be slow to diminish and move northeast, so it will take until well into Friday night for the plume of rain to make it to the New York City area. ....Southern Plains... 21Z update... The latest guidance has trended the QPF a little heavier and further north into parts of Oklahoma; with the GFS suggesting as much as 3 inches across southeast Oklahoma. Given this uptick among the consensus, the Marginal Risk area was adjusted to reflect this trend. Campbell A vigorous positively tilted trough will dig southward across New Mexico and west Texas Friday afternoon and overnight. Ahead of this forcing, a bit of southerly return flow will bring Gulf moisture with PWATs to 1.5 inches north across eastern Texas. Further, there will be extreme instability advected and developing across central Texas, as MUCAPE values exceed 3,000 J/kg, with values diminishing to closer to 1,000 J/kg as you go north to the TX/OK border. Much cooler and drier air racing south on 20-30 kt winds will provide additional forcing along the front for strong to severe storms to develop across northern and eastern Texas Friday afternoon. Since the flow behind the front will be N to NNW, it will be moving only slowly eastward, which in turn will mean any storms tied to the frontal forcing will also be moving slowly. Given the extreme instability, moisture, and a slow-moving front, the Marginal Risk remains in place, with a small expansion to the Red River. There's some uncertainty as to how the storms will organize, but given the heavy rainfall that fell today, it's possible a small Slight may be needed with future updates when there's more certainty as to how the storm evolution will develop. Wegman Day 3 Valid 12Z Sat Apr 29 2023 - 12Z Sun Apr 30 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR THE FLORIDA PANHANDLE INTO SOUTHERN GEORGIA... ....Southeastern US... 21Z update... Recent heavy rainfall across the Gulf Coast and the Southeast will have lowered local FFG across the region. The latest guidance is depicting another round of 1 to 3+ inches for much of the same area. The inherited Slight Risk area remains in good order, minor west-east expansion was made with this update. The northern bounds of the Marginal Risk area across central South Carolina was readjusted to reflect recent QPF trends as well. Campbell A developing low over the Mississippi Valley and a pair of upper level shortwaves moving northeast across the Southeast will provide the forcing as highly anomalous Gulf moisture to 3 sigma above normal with PWATs approaching 2 inches, and a 50-60 kt LLJ by late Saturday night over the FL Panhandle will all combine over the Slight Risk area to result in multiple rounds of thunderstorms each capable of rainfall rates exceeding 2 inches/hour, which will move northeast out of the Gulf and into southeastern GA and SC. The first round of storms will be with a weaker front moving across the area Saturday afternoon, then the second will follow it with the stronger surface front late Saturday night. Instability will increase through this time as greater instability is advected northward with the Gulf moisture on the strengthening LLJ. The Slight Risk area was coordinated with the TAE/Tallahassee, FL, JAX/Jacksonville, FL, and MOB/Mobile, AL forecast offices. Depending on how much flooding is seen today/Thursday, today's round of storms may prime the soils for the greater rainfall event expected on Saturday. Each individual storm should be fast moving, which should help mitigate the flash flooding potential a bit. ....New York City Metro Area and Southern New England... 21Z update... no changes were made for this area. Campbell Remnant rainfall from the front that moved up the Mid-Atlantic Friday and Friday night will continue slowly moving northward into the Marginal Risk area Saturday morning. A steady southeasterly flow of rain will continue for much of the day, with low amounts of instability that may locally increase rainfall rates. Expect the best chances for isolated flash flooding in urbanized and poor-drainage areas. As the front causing the rainfall will be weakening throughout the day as it slowly lifts northeastward across New England, this poses an overall low flash flooding risk. Wegman $$ --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .