Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Aug 29 2023 15:55:35 FOUS30 KWBC 291555 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1154 AM 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 NORTH FLORIDA NORTHWARD INTO NORTH CAROLINAS AS IDALIA TRACKS ACROSS THE REGION... ....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 Valid 12Z Thu Aug 31 2023 - 12Z Fri Sep 01 2023 ....A SLIGHT RISK OF EXCESSIVE RAINFALL IS IN EFFECT FOR EASTERN NORTH CAROLINA AS IDALIA EXITS THE REGION... ....Eastern Carolinas... Idalia will continue to press eastward off the SC/NC coast by Thursday morning through the period with heaviest rainfall occurring during the beginning 12 hr time frame from 12z Thu until 00z Fri. Long range deterministic and ensemble guidance shows a sharp cutoff on the northern extent of the precip field thanks to a confluent area generated by the longwave trough traversing the northeast US out of Quebec. Deep, rich tropical moisture plume within the general confines of Idalia will bisect the eastern half of the Carolinas with enhanced rainfall bordering on 2-3"/hr during Thursday morning, scaling back as the moisture and mid-level ascent from the storm exit the eastern seaboard, but still bringing rain to the far eastern shore of NC until late in the afternoon, early evening time frame. Main adjustments for the MRGL and SLGT were based on ensemble mean QPF and probabilistic signatures for greater than 4" within 24 hr window allowing a nudge further west for the MRGL and a touch south on the southern edge of the SLGT to encompass Wilmington proper where some guidance has heavier precipitation potential during the earliest 6 hr time frame for the D3 period. No other major changes were necessary given the proposed track from the National Hurricane Center with bounded regions supported by ensemble and deterministic output. ....Southwest U.S.... Enhancement within the monsoonal pattern arises by Thursday as a deepening longwave trough over the west coast in tandem with a building upper ridge center over AZ/NM creates a funnel for increased low to mid-level moisture over the Southwest U.S, mainly AZ up through UT. Modest instability between 1000-1500 J/kg is forecasted up through the aforementioned area with several deterministic outputs placing scattered to widespread convective coverage from the southern border up through the Mogollon Rim into UT. Locally heavy rainfall within any convective development will offer a threat for flash flooding, especially near any terrain or urbanized corridors.=20=20 Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_n--WY8TL6V1znAuHbHgNWFO8A1_rVx2ArY6bCjNOM-d= rfYiTtIEq65qHwGDApFln94-yPSfu3H4uy4NtvzAXyKQScg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_n--WY8TL6V1znAuHbHgNWFO8A1_rVx2ArY6bCjNOM-d= rfYiTtIEq65qHwGDApFln94-yPSfu3H4uy4NtvzA1lqTTXc$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_n--WY8TL6V1znAuHbHgNWFO8A1_rVx2ArY6bCjNOM-d= rfYiTtIEq65qHwGDApFln94-yPSfu3H4uy4NtvzALiFDtRU$=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) .