Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Tue Aug 15 2023 07:35:00 FOUS30 KWBC 150801 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 400 AM EDT Tue Aug 15 2023 Day 1 Valid 12Z Tue Aug 15 2023 - 12Z Wed Aug 16 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM SOUTHEAST VIRGINIA INTO PORTIONS OF EAST CENTRAL NORTH CAROLINA... ....Southeast Virginia into east central North Carolina... A surface cold front expected to move slowly east southeastward across the Mid-Atlantic, into the Southeast and Central Gulf coastal region during day 1. Well defined frontal convergence along and ahead of this front will coincide with favorable right entrance region jet dynamics in an axis of above average PW values, 1 to 2+ standard deviations above the mean. This will support potential for a line of frontal or pre-frontal convection to push eastward from the Mid-Atlantic, south through the Southeast and into the Central Gulf coastal region,. Model consensus continues to point to areas from southeast Virginia into east central North Carolina as the region of most organized convection and potentially heaviest amounts. A slight risk was maintained from previous outlooks and coincides where HREF neighborhood probabilities for 1, 2 and 3"+ amounts are well defined and high and match with where HREF EAS probabilities are high for 1"+ totals. Only some slight changes made to the western portion of the marginal risk area over the central Gulf coast after collaboration with WFO LIX and MOB, with the marginal risk removed from these areas. We also nudged the marginal risk area a bit south into North Florida to match up with the southern extent of the HREF 3"+ neighborhood probabilities max. Otherwise, thinking remain the same from previous outlooks for only a marginal risk given higher ffg values and less overall model consensus for max areas, although locally heavy totals are likely. Across southern New England...we removed the previous marginal risk area except for far southeast Massachusetts, including Martha's Vineyard and Nantucket. These areas may still see some heavy rains in the first few hours of the day 1 period (post 1200 UTC) from organized overrunning rains moving along and north of the west to east front expected to remain south of Long Island and southern New England. Across far southeast L.P. of Michigan...trimmed the small marginal risk area on the western side from the previous issuance. Latest hi res runs are suggesting the current well defined comma head/deformation band will reform over far southeast L.P. of Michigan early this morning and possibly bring a few hour of heavy rains across the Detroit metro area. HREF neighborhood probabilities are fairly high for 1 and 2"+ amounts, 70-90% and 40-80% across this region. This may lead to isolated runoff issues, especially in the more urbanized regions of the Detroit metro area. ....Southwest into the Southern Great Basin... The latest models continue to show weakening of the stationary frontal boundary across western New Mexico that helped focus convection on Monday, with less activity expected in this region for the upcoming day 1 period. However, broadly difluent upper flow to the east of the upper trof moving slowly northward off the central California coast will again support potential for widespread scattered convection from the Southwest into the Southern Great Basin and Northern Sierra. With PW values expected to remain above average across these areas, 1 to 2+ standard deviations above the mean, isolated heavy rains and localized runoff issues are possible, although confidence is very low in where it may occur, supporting the maintenance of a broad marginal risk area. Oravec Day 2 Valid 12Z Wed Aug 16 2023 - 12Z Thu Aug 17 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ALONG THE COASTAL SECTIONS OF THE CAROLINAS, SOUTHEAST GEORGIA INTO MUCH OF FLORIDA AND ACROSS PORTIONS OF THE SOUTHWEST, GREAT BASIN AND NORTHERN SIERRA... ....Carolina, Georgia Coasts into Florida... The surface frontal boundary pressing slowly eastward along the east coast day 1, will become stationary day 2 as the upper trof weakens. PW values will remain above average along the immediate NC,SC and GA coasts and across FL, with values 2+ standard deviations above the mean. While the upper trof is weakening day 2, uvvs may still be enhanced by right entrance region jet dynamics that linger along the immediate coast. Model consensus is for heavy rainfall potential in the vicinity of this front and south across much of Florida. Isolated runoff issues possible, especially if heavy totals fall across urbanized areas. ....Southwest into the Southern Great Basin... Similar to the day 1 period, a broad marginal risk area depicted across the Southwest into the Southern Great Basin. The upper low that was moving slowly northward day 1 off the central California coast is expected to become stationary day 2. This will help to maintain a broadly upper flow pattern to its east. PW values will remain above average across the marginal risk area, supporting isolated heavy rainfall totals from what may again be an afternoon of widely scattered convection. Oravec Day 3 Valid 12Z Thu Aug 17 2023 - 12Z Fri Aug 18 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS THE SOUTHWEST, SOUTHERN GREAT BASIN INTO THE NORTHERN SIERRA AND ACROSS MUCH OF FLORIDA... ....Southwest, Southern Great Basin, Northern Sierra... The closed low off the central California coast that becomes stationary during the day 2 period will begin to progress back toward the east southeast and toward the south central California coast day 3. A broadly upper difluent flow pattern will continue to the east of this closed low and to the west of the upper high centered from the Rockies into the Plains. PW values in this broadly difluent upper flow pattern will remain above average, ranging from 1 to 3 standard deviations above the mean. This will support another day of widely scattered convection, isolated heavy totals and isolated runoff issues, especially across any burn areas or slot canyons. Still low confidence in where heavy totals will occur, with models showing a lot qpf detail spread. A broad marginal risk was drawn to encompass where models are showing some precip potential. ....Florida... An axis of tropical PW values, 1.75 to 2"+ will remain across much of Florida for the upcoming day 3 period. At the moment there does not appear to be any strong surface to upper level features to focus convection, but a general model consensus for widespread scattered convection in the tropical PW axis, with slow cell motion possibly resulting in runoff issues, especially across urban areas. Oravec $$ --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .