Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Aug 29 2023 20:27:36 FOUS30 KWBC 292027 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 426 PM EDT Tue Aug 29 2023 Day 1 Valid 16Z Tue Aug 29 2023 - 12Z Wed Aug 30 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE FLORIDA PANHANDLE AROUND THE BIG BEND REGION... ....16Z Update... ....Florida... Hi-Res guidance from this morning has shifted the axis of heaviest rainfall westward, favoring the track of Idalia and points immediately west for the heaviest rainfall through 12Z. Much of the guidance shows the eye of what is forecast to be a major hurricane approaching the coast between Apalachicola and Steinhatchee, with the heaviest rain north and west of the center at 12Z. This will be the primary limiting factor for Day 1, as almost all of the heaviest rain except at the immediate coast will occur on Day 2/Wednesday. NASA Sport soil moisture imagery shows much of northwest FL with extremely dry antecedent conditions, with 0-40 cm soil moisture at under 2 percent, largely east of Steinhatchee, and a little bit closer to normal in the Day 1 Moderate Risk area. With much of the Gulf coast in this area sparsely populated and largely swamp, think the initial hits of heavy rain associated with Idalia will soak well into the soils/swamp with few impacts from excessive rainfall likely before 12Z. Thus, the Moderate Risk was shifted westward based on the latest guidance, and shrunk to the immediate coast, which is the only area that may see 5 inches or more of rain before 12Z, which is the amount of rain needed to exceed flash flood guidance. The surrounding Slight risk area was shrunk largely out of Georgia as the steady heaviest rain is unlikely to encroach that far north prior to 12Z, and shifted a little bit to the west. It's worth noting however that current radar trends are starting to bring heavy rain associated with the easternmost rain bands of Idalia into the Gulf coast from Tampa southward as of this writing. Any heavy rain from those rain bands should generally be short-lived, as much lighter rain so far is in between the bands. The westward shift, even small, in the track should result in significantly less rainfall along the Gulf coast from Tampa south, but that area will continue to be monitored. The Slight Risk along much of the Gulf coast of FL remains in place and largely unchanged. ....Appalachians... A tropical moisture plume, the same one that Idalia is part of, is expanding northeastward up the East Coast ahead of a strong front moving in from the Midwest. A digging upper level trough will move in from the Great Lakes overnight tonight, which will greatly enhance the forcing for storms as the moisture streams north up the Appalachians. HREF FFG exceedance probabilities have increased to over 50% across portions of central Pennsylvania, and low FFGs continue further north into the Catskills. Thus, the Slight Risk was expanded northeast along the Appalachians into south-central NY in coordination with CTP/State College, PA and BGM/Binghamton, NY forecast offices. The Marginal risk was also expanded northward to include much of southern New England to account for the impressive moisture advection moving into New England ahead of the main front. Just like further south with Idalia, it's likely most of the impacts will hold off until the Day 2/Wednesday period for New England. ....Pacific Northwest... A strong, negatively tilted trough digging across the Northwest will enhance ongoing rainfall across the region through tonight. The heaviest rain is likely to develop late this afternoon and persist into the overnight, tracking northward with time. A Slight Risk area was introduced for northern Washington State east of the Cascades, much of northern ID and far northwestern MT in coordination with MSO/Missoula, MT forecast office. Burn scars and other flood sensitive areas will be at highest risk for localized flash flooding. The storms should be fast enough moving that more widespread flash flooding concerns are not likely. Wegman ....Previous Discussion... ...Florida into Georgia... Little change made to the placement of the Moderate Risk as Idalia approaches the Florida coast late Tuesday night/early Wednesday morning. Took advantage of the Excessive Rainfall Outlook being probabilistic and nudged the western boundary of the Marginal and Slight risk areas a bit westward to account for some of the track/timing uncertainty still being shown by the models...while maintaining the Moderate risk area along the path of the NHC forecast track. Considering that the NHC forecast has Idalia as a major hurricane at landfall...intense rainfall rates are expected near the core of the storm and from rain bands ahead (and generally east) of the storm which pose a threat of flash flooding. Since minor forecast changes can make notable rainfall threat changes, check with the National Hurricane Center and local offices for latest information on Idalia. ....Southeast through Mid-Atlantic into New England... A surge of deep moisture will spread north and east that overspreads the Southeast U.S. through the upcoming night while existing moisture over the Carolinas/southern Virginia spreads into parts of New England. The potential for heavy to excessive rainfall develops as a frontal boundary initially from the central Gulf Coast through the Carolinas lifts northward into the Mid-Atlantic today and into New England later tonight.=20 Convergence of tropical moisture along the front may lead to a predecessor type rainfall event per conceptual model. The 00Z suite of convective allowing models still struggled to resolve any particular focus, but there is decent agreement on heavy rain from northern Georgia through western North Carolina where the Slight Risk is maintained/expanded into southwest VA which is where heavy rain is occurring this afternoon/tonight. Farther north, the surge of higher moisture up the Mid-Atlantic in time for the Tuesday diurnal cycle ahead of the cold front expected to move into/through the area tonight will be one focus for storms capable of producing high rainfall rates. Expanded the Slight risk to cover areas where flash flood guidance lowered considerably by storms that resulted in flash flooding on Monday...and where frontogenesis is forecast by the models to be co-located with anomalously high precipitable water values over/near the terrain of the Appalachians. By late this evening...moisture stream northward into New England ahead of a shortwave trough approaching from the west. The Marginal Risk is further expanded inland over the northern Mid-Atlantic and southern New England. There could even be a risk of excessive rainfall farther up the coast of Maine as precipitable waters surge to 2 inches or more and sets the stage for some warm rain processes. ....Pacific Northwest.... A strong, negatively tilted upper trough pushes into the Pacific Northwest Tuesday with a progressive shortwave maxima rounding the base of the trough from OR to ID. Enhanced mid-level forcing generated from strong positive vorticity advection across the Cascades into northern Idaho will generate a rapidly developing area of precipitation within a corridor of modest instability. Despite fast storm motion expected within enhanced mean flow, heavy rainfall is over this area which features notably steep terrain and low flash flood thresholds. The Marginal Risk is maintained. Bann Day 2 Valid 12Z Wed Aug 30 2023 - 12Z Thu Aug 31 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FROM THE EASTERN FLORIDA PANHANDLE NORTHEASTWARD INTO SOUTHERN NORTH CAROLINA AS IDALIA TRACKS ACROSS THE REGION... ....2030Z Update... ....Florida to the Carolinas... The latest HiRes guidance has shifted the axis of heaviest rain westward from inherited for Idalia, and as such the Moderate Risk has been expanded westward to include more of the eastern FL Panhandle, and also into Macon, GA, Columbia, SC, and Fayetteville, NC. Conversely, on the eastern end, portions of the southern GA coast and the Jacksonville area have been double-downgraded from Moderate to Marginal, with no ERO risk highlighted as far north as the southern suburbs of Jacksonville. Idalia is expected to begin wrapping drier continental air around its south and east side near and after landfall, which should result in a "dry slot" developing east of the center. Thus, the heaviest rain is expected from north of center around to the west. This has helped narrow down the corridor where the greatest rainfall threat will be. As a reminder, much of this area has been bone dry lately, with soil moisture values in some areas below 2 percent of climatology. Thus, the first couple inches of rain, especially that which falls on sandy soils and forested areas will likely be absorbed with minimal impact. Expect the heaviest rain to be associated with the eyewall and remnants thereof, again particularly on the north and west side of the center of circulation. The corridor within the Moderate Risk at greatest threat for flash flooding, with locally significant flash flooding includes a roughly 50 mile wide corridor which includes Tallahassee, FL, Valdosta, GA on the east side of the greatest threat area, Augusta, GA on the western side of the greatest threat area, and Columbia, SC on the western end of the greatest threat area. Inland flash flooding is historically the aspect of a landfalling hurricane that is deadliest...and while in the Big Bend area the storm surge will be by far the most impactful aspect of the storm, further inland into GA and the Carolinas, the greatest threat will quickly transition to flash flooding. There was consideration for an upgrade to a High Risk, which remains possible, particularly from east central GA through central SC. The antecedent dry conditions and fast forward movement of the storm (in the 75th percentile climatologically) both should work to lessen the impacts a little bit, but should 2+ inch per hour rainfall rates manifest in this area as the remnant eyewall passes, then locally considerable impacts are quite possible despite the dry conditions. Even with the driest soils, there comes a point that the rainfall rates are so heavy that they can overwhelm most soils. While rainfall amounts have lessened considerably along the coast, particularly from GA south, coastal flooding from high tides and onshore flow could still be significant. The rainfall continues into NC overnight Wednesday night, which will continue into the day Thursday. The further west you go in NC, the greater the likelihood for flooding impacts in the predawn hours Thursday, with most of eastern NC seeing impacts more into the day Thursday. ....New England... In coordination with BOX/Norton, MA forecast office, the Marginal risk was expanded southward to include much of CT, RI and eastern MA. Locally heavy rain will be ongoing Wednesday morning, which will persist into the afternoon into Maine. Fortunately the front should be moving eastward quickly by this point, which will limit the total rainfall, but much of Maine may see 1 to 2 inches of rain starting early Wednesday morning and continuing into the afternoon. ....Northern ID and Northwest MT... In coordination with OTX/Spokane, WA and MSO/Missoula, MT forecast offices, a Slight Risk area was introduced with this update. Local amounts due to orographic uplift could result in local rainfall totals above 2 inches. With localized burn scars and rapid runoff, the resulting flash flooding could be more widespread, especially when adding this rainfall to tonight's rain, particularly across the northern ID Panhandle. Wegman ....Previous Discussion... ....Florida to the Carolinas... Heavy rainfall will spread northward and eastward in association with Idalia. Given its strength at the beginning of the period, a rather traditional rainfall distribution is expected with intense rainfall rates near the core of the storm and from rain bands feeding into the system and any trough interaction to the left of the track. The Moderate Risk area was largely left in place along the latest NHC track...though there was some expansion of the Marginal and and Slight Risk areas on the north and west side to account for some model guidance a bit to the left of the current forecast track. The HAFS still had decent agreement with the NHC track and the evolution on the QPF relative to the storm center.=20 WPC QPF generally remained in the 4 to 8 inches along the track. Placement of the Slight and Moderate Risks is based on the official NHC track forecast, so please refer to latest statements from the National Hurricane Center for updated Idalia information. ....Pacific Northwest.... A potent, leading shortwave low on a negatively tilted upper trough pushes northeast through western Montana Wednesday. Locally heavy rain from Tuesday night continues Wednesday until the wave lifts in to Canada. Locally moderate to heavy rainfall during the day may still result in run off problems, especially in areas of steep terrain and low flash flood thresholds, so the Marginal Risk is maintained. Bann Day 3 The Day 3 outlook will be updated by 2030Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_Qdo3y6S1n6DXr6jrlO5uStY4sIA_oM3ecm6lFMicS_R= El6OvQDrXRwt4Zla8wA_l2HSxBSRD-7Rp2tzvju2-rW37oM$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_Qdo3y6S1n6DXr6jrlO5uStY4sIA_oM3ecm6lFMicS_R= El6OvQDrXRwt4Zla8wA_l2HSxBSRD-7Rp2tzvju2ze5Zi-U$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_Qdo3y6S1n6DXr6jrlO5uStY4sIA_oM3ecm6lFMicS_R= El6OvQDrXRwt4Zla8wA_l2HSxBSRD-7Rp2tzvju2T87njpk$=20 $$ =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .