Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Aug 30 2023 15:06:27 AWUS01 KWNH 301506 FFGMPD SCZ000-FLZ000-GAZ000-302105- Mesoscale Precipitation Discussion 1011 NWS Weather Prediction Center College Park MD 1105 AM EDT Wed Aug 30 2023 Areas affected...Southern Georgia...Southern South Carolina... Concerning...Heavy rainfall...Flash flooding likely Valid 301505Z - 302105Z SUMMARY...Dangerous Hurricane Idalia is moving inland across south-central Georgia. Localized rainfall rates to 3 inches per hour are occurring. Numerous instances of flash flooding, some locally significant, are expected through the afternoon for much of southern Georgia and southern South Carolina. DISCUSSION...Hurricane Idalia moved ashore in the Big Bend region of the Florida Panhandle a couple hours ago. Despite rapid weakening of the wind field as the circulation moves over land, the most significant impacts with the storm are already translating towards inland flooding. As the eyewall pushes across the border from Florida into Georgia, localized radar estimated rainfall rates are approaching 3.5 inches per hour in the northern and western eyewall. Despite dry antecedent conditions, these rainfall rates are more than sufficient to overcome those conditions, resulting in localized flash flooding. The northwestern quadrant of Idalia remains the area of greatest concern, with rainfall rates of 1.5 to 2.5 inches per hour likely for a period of several hours as Idalia moves through. Fortunately, Idalia's fast movement towards the northeast will spare most areas any catastrophic impacts, as rainfall totals are generally limited to under 9 inches for storm totals. Nevertheless, the expected swath of 5 to 7 inches for storm total rainfall expected from south central GA into south-central SC is still likely to produce a swath of flash flooding which may be locally significant in flood-sensitive areas. There continues to be excellent agreement in the model guidance that the greatest rainfall rates will remain with the northern and western eyewall (or remnant thereof) and so confidence is much higher than normal for this scenario to unfold. GOES-E WV suite and Atmospheric Motion Vectors denote increasing influence of mid to upper-level divergence in the broadening anticyclonic outflow channel. This is strengthening lower level frontogenetic features with the deformation zone forming along and downstream of the core of the circulation of Idalia. Strong moisture flux within the eastern hemisphere will converge and ascend along the steep isentropic boundary maintaining evacuated vertical ascent and efficient rainfall production along this axis. So while rates will be diminishing with time in intensity, the increasing length of the deformation zone toward the northeast into the Coastal Plain of South Carolina will prolong duration for the solid but diminishing rain rates to continue into SC through the evening; maintaining the expected 5"+ totals. So the track of the center will determine which localized areas see the most extreme storm total rainfall amounts. Moving to the northeast to the Coastal Plains of South Carolina, bands of heavy rain tracking inland from off the Gulf Stream have resulted in locally heavy rainfall rates, particularly along the coast from Charleston south this morning, where MRMS totals are already over 1.5 inches. Cells within these east-west oriented bands will continue to moving north while the band shift eastward. Access to higher instability and moisture off the Gulf should allow for these storms to be the most intense with 1-2" in sub-hourly time periods, but should be transient enough.=20 Antecedent dry conditions and on-and-off heavy rains should preclude widespread flooding in SC initially this afternoon, though rainfall will fall in proximity to the immediate coast where tidal flooding and storm surge will be the primary drivers of the flooding. While transient, there remains solid potential for two or more of these bands to progress through any given location, resulting in very spotty but perhaps higher localized totals (in comparison to near the deformation zone near the storm center). Wegman ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!7FLc_VvDSnTnt4T9Qktdl0JyM_xoIQY3rB9VMzHd56XgxqNtNqC9_wtuadUX3Hi2GiR9= 8N_cN_aj1WPvEd1SeBVgbj4$ for graphic product... ATTN...WFO...CAE...CHS...FFC...ILM...JAX...TAE... ATTN...RFC...SERFC...NWC... 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