Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Aug 30 2023 20:51:33 FOUS30 KWBC 302051 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 450 PM EDT Wed Aug 30 2023 Day 1 Valid 2048Z Wed Aug 30 2023 - 12Z Thu Aug 31 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FROM EASTERN GEORGIA INTO EASTERN NORTH CAROLINA... 21Z Update... Trimmed risks behind Idalia as appropriate Removed the Slight Risk over the northern Rockies where precip is more stratiform now. Jackson 1600Z Update... In the Southeast -- with Idalia now moving progressively across southeastern Georgia, began to trim away areas in northern Florida that are now in the wake of system and within a notable dry slot.=20 However, training convection along an inflow band farther to the south is expected to support a continued heavy rain/flash flooding threat across central Florida. Farther to the north -- tightened the western extent of the gradient and made a small southeastward adjustment. Consensus of the HREF continued to show a narrow axis of heavy amounts north of the storm track. The 12Z HREF continues to show very high neighborhood probabilities (90 percent or greater) for accumulations of 5 inches or more with much of the Moderate Risk area and embedded probabilities of greater than 50 percent for accumulations of 8 inches or more extending from the GA/SC border to the Outer Banks. Pereira=20 Previous Discussion... ....Florida to the Carolinas... Hurricane Idalia continues to press towards the FL Big Bend coastline this morning with increasing forward speed as of the last succession of updates from the National Hurricane Center. Despite the forward propagation speed, very heavy rainfall will occur across the northern and western periphery of the circulation with continued banding on the eastern quadrant of the storm as it moves through the FL Panhandle, southern GA, and eventually hooking east near the SC/NC borders by the end of D1. Consensus on the 00z guidance was for the main precip shield to be slightly east of the previous forecast which aligns well with the trends in propagation of the storms center of circulation. Deep moist conveyor of tropical air has funneled up through the southeast, mainly along and east of a stalled frontal boundary bisecting the interior portion of the Southeast CONUS. PWATs will run a general 2-4 standard deviations above normal across much of the southeast U.S with a typical enhancement near the storms core. One of the main components that has been well forecast is the primary shield of heaviest precip with Idalia continuing to lie on the north and northwest quadrants of the cyclone which will define the precip max associated with the storm itself. The latest 00z HREF and hurricane models all shifted about 20-30 miles to the east with the primary axis of heavy precipitation leading to some changes in the regional maximum(s) being depicted a bit east of the previous forecast with a stripe of 6-9" focused just east of a line from Tallahassee-Augusta-Columbia up into southeastern NC. HREF probabilities of greater than 6" over 24 hrs spanning the period indicate a solid 30-45% chance of exceedance of the 100 year ARI with a bullseye centered along the SC/GA line southeast of Augusta, nearing 60%. The saving grace to mitigate the worst possible impacts will be the forward speed of Idalia as she gets steered northeast, then east around the western periphery of the ridge to the west, then shoved eastward thanks to a strong confluent area to the north from a digging longwave trough across New England and adjacent Quebec. Regardless, significant impacts across the southeast U.S with regards to flash flooding will be prominent and will likely cover a long stretch of land that has seen significant rainfall already in the past few days. Antecedent soil moisture coupled with intense rainfall over a short time will lend credence to a continuation of the MOD risk ERO with a broad SLGT extending the bounds of the moderate with a southern reach down the west coast of FL as feeder bands plague the area through at least the first half of Wednesday. The heaviest rainfall the end of the D1 period will be co-located within the zone of the deepest 850-700mb RH layer which will be generally across eastern NC down towards Wilmington. ....New England... Locally heavy rainfall is forecast over New England on Wednesday thanks to a potent upper trough traversing the aforementioned area. Above normal moisture field in place across the region in conjunction with strong ascent will generate periods of heavy rain capable of flash flooding concerns. Forecast 1-2" is in place across western and northern ME, which a lot will occur over a shorter period of time leading to a continuation of the MRGL risk in place. ....Northern ID and Northwest MT... Sharp mid-level trough cutting across the Pacific Northwest will close off and intensify over the northern neck of ID and western MT later this morning and afternoon. Strong ascent focused under the closed low will create plenty of forcing within the terrain to induce a period of scattered to widespread showers and thunderstorms capable of producing some heavier rainfall rates as shown by the latest 00z HREF indicating a fairly stout signal for 24 hr 100 year ARI exceedance within the 60-70th percentile across Northwest MT. 700mb vertical velocity signatures are also quite impressive for the region which gives a good indication of the type of upper environment we're contending with overall. Given the steep terrain and concerns of burn scars and debris flow potential within the axis of heaviest QPF, have maintained the previous SLGT risk inheritance with little changes to the previous bounds as they align with forecast QPF. Kleebauer=20 Day 2 Valid 12Z Thu Aug 31 2023 - 12Z Fri Sep 01 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER A PORTION OF EASTERN NORTH CAROLINA BEFORE IDALIA HEADS OUT TO SEA... 2030Z Update... Given the progressive nature of Idalia and with support from the HREF, trimmed away some of the western extent of the previous outlook areas over eastern NC. Farther south, expanded the Marginal Risk to include northern Florida. The 12Z guidance, including the HREF shows persistent westerly flow supporting deep moisture (PWs at or above 2 inches) and the potential for additional convection producing locally heavy amounts. The HREF indicates locally heavy amounts of 2-3 inches are likely, which may cause runoff concerns, especially for areas that have already been impacted with heavy rainfall amounts. Pereira Previous Discussion... ....Eastern North Carolina... Introduced a Moderate Risk of excessive rainfall as Idalia makes its turn towards the east over North Carolina and makes its way out to sea. WPC QPF was still had localized rainfall maxima in of excess of 5 inches over the eastern part of North Carolina during the Day 3 period. Because the rainfall pattern is expected to be asymmetric by the start of the Day 2 period...the decision was made to introduce a Moderate risk area where the WPC combined rainfall from Day 1 and Day 2 was approaching 7 or 8 inches of rain by the time the rain tapers off. The 00Z HREF 40 km neighborhood probabilities still exceed 20 percent for 5 inch amounts in the first 6 hours of Day 2. No other major changes were necessary given the proposed track from the National Hurricane Center with bounded regions supported by ensemble and deterministic output. ....Western 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 forecast 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 Farther north...the amplification of that longwave trough along the west coast should result in a growing area of rainfall mainly over Oregon. Amounts tended to be on the marginal side for an outlook area but the concern about flooding or run off from burn scars warranted keeping the previously issued Marginal risk area in place with only minor adjustments.=20 Bann Day 3 Valid 12Z Fri Sep 01 2023 - 12Z Sat Sep 02 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ALONG THE GULF COAST AND OVER THE SOUTHWEST UNITED STATES... 2030Z Update... Made only minor adjustments based on the 12Z guidance. Pereira Previous Discussion... ....Gulf Coast... The models draw a mid- and upper-level impulse northward from the Gulf...accompanied by a surge of moisture and areas of rain.=20 Given the magnitude of the precipitable water and moisture transport...locally heavy rainfall is expected. The models offer a range of solution in terms of placement and amounts...with the NAM being more aggressive with highest rainfall amounts well east of other guidance and the GFS was a northern outlier. For now...tended to favor a consensus approach in placement.=20 Introduced a Slight risk where there could be some sensitivity about run off where model QPF overlapped rainfall from Idalia.=20 Confidence is below average given the spread and how much overlap there actually will be.=20 ....Southwest United States... Introduced a Slight Risk area along and near the higher terrain from Arizona to Utah....where moisture gets channeled northward efficiently between an anomalously deep trough along the west coast of North America and a pronounced ridge axis extending from New Mexico into parts of Colorado. Precipitable water values over southwest Arizona early in be period in excess of 1.5 inches get drawn towards the higher terrain over central Arizona by 850 mb flow of 15 to 20 kts by late afternoon...with values in excess of three-quarters of an inch along and near the terrain into Utah by late afternoon. The moisture and resulting instability sets the stage for some of the storms to be locally heavy rainfall producers...especially from southwest Utah or northwest Arizona given the approach of the surface front and mid-level height falls on Friday. WPC QPF was comparable with guidance showing areal averaged rainfall amounts around 1.5 inches. Of particular concern would be for flash flooding in sensitive slot canyons and dry washes. The placement of the WPC Slight Risk covers the corridor of highest model QPF and along the axis of the particularly strong moisture vapor transport shown by the NAEFS and EC ensembles. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9xThbnSnMGC_EcLviJEsARXGDG0sEKFllJA3nAKKI-cR= cOcHTqaK3-DI3ejircQSHdOXWCr0Mkc1ltxqtVakb-uWBVQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9xThbnSnMGC_EcLviJEsARXGDG0sEKFllJA3nAKKI-cR= cOcHTqaK3-DI3ejircQSHdOXWCr0Mkc1ltxqtVakqz81gMo$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9xThbnSnMGC_EcLviJEsARXGDG0sEKFllJA3nAKKI-cR= cOcHTqaK3-DI3ejircQSHdOXWCr0Mkc1ltxqtVakErWvCbo$=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) .