Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Aug 30 2023 00:44:41 FOUS30 KWBC 300044 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 843 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 BIG BEND REGION... ....Florida into Georgia... Idalia is forecast to approach the Florida Big Bend as a major hurricane late tonight. Heaviest rain still expected immediately north and west of the eye which limits extreme rainfall to right along the Big Bend coastline where the Moderate Risk remains in place. Still expecting 5 inches or more of rain before 12Z on west side of Apalachee Bay, which is the amount of rain needed to exceed flash flood guidance. The surrounding Slight Risk area was retracted a bit toward the west coast of FL, up along the GA border and back into the FL Panhandle where inner bands will continue moving ashore from south to north overnight. Recent HRRRs are too far west with activity, but there is enough confidence to retract west a bit to hone in on the central and northern portions of the west coast where the repeating activity is bands is most likely overnight. ....Interior Southeast up the Appalachians through Mid-Atlantic into New England... The tropical moisture plume ahead of Idalia is expanding northeastward up the East Coast ahead of a strong cold front moving in from the Midwest which extends south into the interior Southeast where the front has persisted the past couple of days. Convergence of tropical moisture along the front may lead to a predecessor type rainfall event which may be unfolding over north Georgia into the Carolinas. The Slight Risk was expanded east a bit through central NC where activity should persist into the overnight per recent HRRRs. Farther north, ongoing activity over the Appalachians continues shifting northeast ahead of the cold front. The Slight and Marginal Risks are generally maintained over the north-central Appalachians and Northeast urban corridor. ....Pacific Northwest... A strong, negatively tilted upper trough with developing upper low over OR/ID this evening will lift ENE overnight. Locally heavy thunderstorms have developed over western MT ahead of this vort max which should enable further development over similar areas into Wednesday. Therefore, the Slight Risk is expanded farther east into MT where there is a risk for scattered flash flooding with 1 to 1.25" PW over northeast WA into MT which is 2.5 to 3 standard deviations above normal along with sufficient instability with MUCAPE of 1000-2000 J/kg. Jackson 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 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... ....2030Z Update... ....Eastern North Carolina... The Slight Risk across eastern NC was expanded westward to account for differences in the guidance as to how quickly Idalia's rainfall moves through eastern NC. Idalia's circulation will hold the advancing front from moving through eastern NC until the circulation passes through, and the front will add an additional source of lift which may locally enhance rainfall over the area. The potential for as much as 5 inches of rain exists from 12Z Thursday on, which when added to any rainfall from Wednesday night may cause flash flooding. Thus, it's possible additional upgrades may be needed. ....Gulf Coast of the FL Peninsula... A Marginal Risk was added for portions of the Gulf Coast from the Big Bend south through Ft. Myers. The trailing front behind the circulation of Idalia may result in heavy rain along the coast, which if that occurs over an urbanized or flood-sensitive area may result in localized flash flooding. ....Portions of the Southwest... Monsoonal flow may result in localized flash flooding from the Mexican border in AZ north through the state into southwest UT. The Marginal Risk area was trimmed a little to align with the latest forecasts of where any heavier showers and storms may develop, but no significant changes were made. ....Pacific Northwest... A Marginal Risk was added for the OR and southern WA Cascades east into portions of eastern OR with this update. Locally heavy rain is possible in this area, which over burn scars and canyons may result in localized flash flooding. Wegman ....Previous Discussion... ....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!8uLO2ZoBOwnOEOrw52ye4cOyNLm2TbxbSsk9i6gMDFYh= -9ho6kQTc5e8aUvRdnCzTTdegoroV1Q-h8qBfQMpF-pj0oA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8uLO2ZoBOwnOEOrw52ye4cOyNLm2TbxbSsk9i6gMDFYh= -9ho6kQTc5e8aUvRdnCzTTdegoroV1Q-h8qBfQMpMfe3pME$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8uLO2ZoBOwnOEOrw52ye4cOyNLm2TbxbSsk9i6gMDFYh= -9ho6kQTc5e8aUvRdnCzTTdegoroV1Q-h8qBfQMpFCvJkSk$=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. 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