Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Apr 10 2023 07:39:01 FOUS30 KWBC 100738 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 337 AM EDT Mon Apr 10 2023 Day 1 Valid 12Z Mon Apr 10 2023 - 12Z Tue Apr 11 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE PACIFIC NORTHWEST AND FLORIDA... ....Pacific Northwest... A Marginal risk continues for western OR into southwest WA. A steady rainfall will continue across this region, with pretty much all the risk area picking up 1" of additional rainfall after 12z, with localized amounts upwards of 1.5". Hourly rainfall will generally peak around 0.1"-0.2", so nothing all that heavy. Nonetheless the long duration steady rainfall could result in some minor flooding impacts, especially over any more susceptible low lying areas. The second Marginal risk covers portions of northeast WA into northern ID and far northwestern MT. This is for a combination of heavy rainfall and snow melt. Rainfall amounts of 1-2" are expected, with as much as 1-2" of snow water equivalent melting as well. Thus quite a bit of water will be running off across this area today into tonight, which may result in some flooding of creeks and streams, as well as any more susceptible low lying areas. Some of the 00z HREF members show some weak instability developing between 00-06z, which could result in some uptick in rainfall rates towards or a little over 0.25" in an hour. ....Florida Peninsula... Two Marginal risk areas were maintained, one across portions of the northeast FL coast near Jacksonville, and the other over the Southeast FL coast near Miami. Some HREF members are showing pretty impressive rainfall totals with the northeast FL risk. Neighborhood probabilities of exceeding 3" in the HREF are 40-60%, and exceeding 5" 30-40%. This is all post frontal lower topped convective activity flowing in off the Gulf Stream and focusing along a low level convergence axis. PWs are not all that high, and instability is pretty weak overall. Thus tend to think some of these HREF members are probably overdone with rainfall magnitudes today, with amounts upwards of 1-2" more likely. However, the low topped nature of cells will allow for decent rainfall efficiency even in the cooler airmass, and any more intense cells that do develop would likely be slow moving/repeating. Thus can not completely rule out some localized higher amounts as shown by several HREF members, and so think maintaining the Marginal risk (5-15%) to account for this threat is the way to go. Higher confidence that we will see some heavier convective cells move into the southeast coast of FL today as a cold front and/or trough moves across the region. Better instability to work with here (upwards of 1500 J/KG) and even around 30 kts or so of effective shear to organize cells to some extent. HREF guidance shows a 60+% chance of localized 3" rainfall amounts and over 40% chance of 5". Do tend to believe these probabilities a bit more here given the frontal proximity and aforementioned instability/shear values...thus a localized urban flash flood risk could exist. Chenard Day 2 Valid 12Z Tue Apr 11 2023 - 12Z Wed Apr 12 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS SOUTHERN LOUISIANA AND SOUTHEAST FLORIDA... ....Southern Louisiana... Mid level shortwave energy diving south across the Plains on Monday will close off into a mid/upper level low near the central Gulf Coast by Tuesday. This should result in an expansion of convective activity over the Gulf of Mexico, with an eventual surface low reflection taking shape. There is still some uncertainty and model spread regarding how much rainfall makes it onshore through Tuesday night, but guidance is generally beginning to cluster much better. The heaviest rainfall totals should remain offshore this period, but it looks like some heavy rain could get into coastal portions of LA. We were able to shrink the northern extent of the Marginal risk with this update, following the converging solutions depicted by the 00z GFS/EC and their ensembles. ....Southeast Florida... A Marginal risk was introduced across the urban corridor of Southeast FL. By this time the better instability will have shifted south, but still a signal in the guidance for continued scattered convective activity within the easterly low level flow. At the moment the ingredients Tuesday do not look quite as good as Monday or Wednesday, but still enough of a signal to carry a Marginal. Chenard Day 3 Valid 12Z Wed Apr 12 2023 - 12Z Thu Apr 13 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ALONG AND NEAR THE CENTRAL GULF COAST REGION AND OVER SOUTHEAST FLORIDA... ....Central Gulf Coast... Increasingly likely that some heavy rain moves onshore by day 3 (Wed into Wed night) across portions of the central Gulf Coast. By this time the mid level closed low looks to intensify, as well as a stronger lower level trough and/or low near the coast. While some model spread still exists with regards to how far inland and exactly where the heaviest rainfall ends up, guidance is beginning to show signs of converging on a solution. The 00z EC, 00z GEFS mean and 12z EC mean show the most overlap and the Marginal risk outline generally follows the depiction of these solutions. Rainfall of 1-3" seems probable within at least some of the Marginal risk, with at least some potential for locally higher amounts. Initially most of the better instability will be offshore over the Gulf of Mexico, but it should be gradually pulled northward towards the coast into Wednesday night. However, exactly to what extent and how far north remains a question...and this will have an impact on rainfall rates and thus also the flash flood potential. Whether these higher rates end up confined offshore to just along the immediate coast, or if they make it inland will be partially dependent on this northward transport of instability. PWs with this system are not really that impressive as the developing cutoff low will not have a great tropical moisture connection. So while PWs will be high enough for heavy rain, at the moment it does not look like we are looking at very anomalous values (generally below 1.5"). The instability and PW limitations and overall uncertainty keep the risk of flash flooding at the Marginal level for now, although we will have to continue to monitor trends. Certainly some chance that a Slight risk upgrade will be needed for a portion of the area as the event gets closer and confidence in the details increases. ....Southeast Florida... This will be the fourth day in a row with a Marginal risk over southeast FL. So what transpires over the previous days will have some impact on the susceptibility to flooding by this time. Although with much of this area urban in nature, this dependence on antecedent conditions will not drive the entire risk. So either way it looks like another day where localized heavy convection and flooding will be possible along the urban corridor. Moisture and instability currently look higher Wednesday compared to Tuesday, so the higher end rainfall potential may end up a bit greater. Confidence is not overly high, but the general persistence of the pattern from previous days suggests a continuation of the Marginal risk is warranted. Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5lDSVwUB5UsRCCrzAeEbyCgCt9k9zYkZTk0fALntu_qS= ESfzbAF8XT4xZR0PiZUJFfbxLNLLjey96qobmiYJCCW4EN0$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5lDSVwUB5UsRCCrzAeEbyCgCt9k9zYkZTk0fALntu_qS= ESfzbAF8XT4xZR0PiZUJFfbxLNLLjey96qobmiYJ-partFA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5lDSVwUB5UsRCCrzAeEbyCgCt9k9zYkZTk0fALntu_qS= ESfzbAF8XT4xZR0PiZUJFfbxLNLLjey96qobmiYJVQLTBpA$=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) .