Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Oct 09 2024 00:59:30 FOUS30 KWBC 090058 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 858 PM EDT Tue Oct 8 2024 Day 1 Valid 01Z Wed Oct 09 2024 - 12Z Wed Oct 09 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER WESTERN PORTIONS OF THE FLORIDA PENINSULA... ....0100 UTC Update... Per collaboration with the Florida WFOs, we have trimmed the Slight Risk considerably for the overnight period -- maintaining the Slight now across western portions of the peninsula along and south of the surface stationary boundary, which will continue to draw in more tropical moisture (increasing low-level moisture transport/flux convergence) ahead of Hurricane Milton. Elsewhere, the flash flood risk will be more isolated/localized, so will maintain the Marginal Risk there. Hurley Day 2 Valid 12Z Wed Oct 09 2024 - 12Z Thu Oct 10 2024 ....THERE IS A HIGH RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF CENTRAL FLORIDA... ....WIDESPREAD AND NUMEROUS INSTANCES OF FLASH FLOODING ARE EXPECTED WITH LIFE-THREATENING AND CATASTROPHIC FLASH FLOODING PROBABLE... 2030Z Update Excessive Rainfall Discussion... Rainfall amounts continued to increase across portions of the Florida peninsula in the 08/12Z model production cycle...which was consistent with the 08/15Z NHC guidance in initially being a bit slower than earlier forecast. With WPC QPF tending to follow suit with a stripe of 10 to 12 inches along the axis of heaviest amounts...there were some adjustments made in the location/orientation of the excessive rainfall outlook areas. The High risk area was extended across the entire width of the Florida peninsula along the axis of heaviest rainfall. Was reluctant to trim too much area north of the track despite the southward trend in the short- term given the possibility that trend will stop and the track starts inching northward. Always refer to latest statements and forecasts from the National Hurricane Center. Bann 0830Z Excessive Rainfall Discussion... With the ingestion of some of the longer range CAMs into the model suite, the forecasted rainfall directly associated with Hurricane Milton's crossing of the Florida Peninsula has increased markedly from previous forecasts. The latest storm total rainfall has over 11 inches of rain for Tampa, 8 inches near and north of Orlando, and 5.5 inches for Jacksonville. The corridor of the High risk following the entire length of I-4 has seen the most impactful increases in forecasted rainfall. Obviously there has been a nearly historic increase in Milton's strength in the last 24 hours, and all of this added energy and Gulf moisture will mean more rainfall to come along its track, despite its fast forward speed. The heaviest rainfall amounts along I-4 and north will generally will be immediately along and north of the most probable track of Milton's center. As mentioned in the NHC discussions, Milton is expected to be well into the process of extratropical transition by the time the storm crosses the Peninsula. This process should greatly limit the amount of rainfall expected south of the center, as dry air entrainment into the southwest quadrant of the storm is an essential aspect of the extratropical transition process. Thus, nearly all of the impressive tropical moisture associated with Milton will be focused along and north of the center, and therefore represents the greatest threats for flash flooding. The ERO risk categories drop off a bit slower on the south side since the predecessor rainfall event (PRE) of the past couple days and today was largely focused on south Florida, as the stationary front that has been there has persisted. Inflow into Milton's circulation and the eventual development of a cold front south of the center should still lead to bands of rain criss-crossing south Florida from west to east. Since the PRE saturated this area the most...the lesser amounts of rain expected here may still cause flooding impacts...so the ERO risk categories south of the track may be a bit generous. Meanwhile on the north side of the circulation, abundant dry air associated with a separate cold front and jet streak over the Panhandle and far north Florida should act as a very effective sink for the portion of Milton's rainfall that drifts too far away from the center. Since this area has been quite dry in recent days, the threat for flooding will be much more constricted, and therefore the ERO risk categories are much more compact. The additional rainfall now in the forecast will be one factor for flooding...while another one will be storm surge and tidal as Milton slows = in the short term and then takes a track more normal to the western Florida coastline per the 08/15Z NHC guidance.flooding, especially south of the center where the flow will be onshore and to a slightly lesser but still impactful extent, to the north of the center on the Atlantic side where there will also be onshore flow. At the time of high tide and the storm surge, the added water from the Gulf/Atlantic will effectively block effective drainage from the rainfall falling over the interior. The 6-12 inches (with locally higher amounts of rainfall) having nowhere to drain due to the high tide and storm surge flooding will also work to exacerbate the flooding impacts from Milton since that rain water will have nowhere to drain. Given all of the above, and despite the small to medium size of the storm and the acceleration of the storm in the forecast, a High Risk was introduced in coordination with TBW/Tampa, FL; MLB/Melbourne, FL, and JAX/Jacksonville, FL forecast offices. It's probable that the greatest flooding impacts will be with storm surge primarily, but also the portions of the nearshore rivers, streams, and creeks (especially near landfall) where the inland flood waters from heavy rainfall are unable to drain to the Gulf. Wegman Day 3 Valid 12Z Thu Oct 10 2024 - 12Z Fri Oct 11 2024 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR ORLANDO AND PORTIONS OF THE FIRST AND SPACE COASTS... ....2030Z Excessive Rainfall Discussion... Only minor nudges were made to the placement of the Slight and Moderate risk area based on the latest WPC QPF for the 24 hour period and the 48 hour rainfall amount ending at 12Z Friday morning. Rainfall should be tapering off from west to east as the system pulls away from the region. ....0830Z Excessive Rainfall Discussion... A rare double upgrade was introduced with this update, with potential for further adjustments with future updates. The latest guidance suggests that the wraparound rain to the north and northwest of the center of Milton will persist well into the day on Thursday. Thus, associated forecast rainfall has increased markedly from the previous forecasts. For the Moderate Risk area, expect an additional 2 to 4 inches of rain after the 12Z/8am Thursday start of the day 3 period. With a High Risk on Day 2, further increases in the forecast rainfall may require a High Risk in this general area to continue into Thursday. Obviously, adjustments in the speed and track of Milton as it races off the coast will result in additional big changes to this ERO forecast. Essentially, the Day 3 ERO is just a continuation of the Day 2 rainfall with Milton. The vast majority of the rainfall for the period will fall during the day Thursday, and expect trimming and downgrades through the day as the rain ends from west to east. Expect widespread and catastrophic flooding from the Day 2 period to continue through the Day 3 period as the rain ends. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!90SOM0xsZYkBeLLhCJT7weWRKCHY5LlFhpQJXAnJqUFS= HMxcd9fiMezlvbZ28cBRTtbC4jSVRup_KuSmC0_IZlb7yKI$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!90SOM0xsZYkBeLLhCJT7weWRKCHY5LlFhpQJXAnJqUFS= HMxcd9fiMezlvbZ28cBRTtbC4jSVRup_KuSmC0_IlLwc-BU$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!90SOM0xsZYkBeLLhCJT7weWRKCHY5LlFhpQJXAnJqUFS= HMxcd9fiMezlvbZ28cBRTtbC4jSVRup_KuSmC0_IxkU6fx4$=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 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .