Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Apr 11 2023 00:08:41 FOUS30 KWBC 110008 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 808 PM EDT Mon Apr 10 2023 Day 1 Valid 16Z 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 steady, long duration rainfall is expected through the rest of the period with the axis of greatest rainfall expected from coastal Oregon through south-central Washington with a secondary area across northeast Washington into northern Idaho and extreme northwest Montana. The most recent guidance and also radar/MRMS observations suggest hourly totals of 0.1-0.2" will be most common with just an isolated risk of hourly totals approaching 0.5", perhaps best chance being across northeast WA/northern ID. Through 12Z, additional amounts less than 1" are likely except for 1-2" totals across northern ID. The combination of rainfall and snow melt may result in excessive runoff and flooding, particularly of the more vulnerable creeks, streams, and low-lying areas. ....Florida Peninsula... Post-frontal convection continues across the eastern Florida Peninsula this evening, primarily focused in two separate areas, the first being across northeast Florida between the Space Coast and Jacksonville, situated along an inverted trough providing slightly enhanced low level convergence overlapped with greater instability and moisture. The persistent northeast flow will keep the threat of repeating showers and low topped thunderstorms through the night, with a slight risk of hourly totals in excess of 1" and additional rainfall through 12Z exceeding 2". The other risk area is for southeast Florida, along the highly urbanized corridor. Here, there is higher confidence in seeing more intense rain rates approaching and exceeding 1" with the longer duration more likely to bring 2-3" totals through 12Z based on the 18Z HREF neighborhood probabilities leading for mainly urban flash flooding. Taylor Day 2 Valid 12Z Tue Apr 11 2023 - 12Z Wed Apr 12 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF SOUTHERN LOUISIANA, SOUTH FLORIDA AND THE KEYS, AND THE NORTHERN INTERMOUNTAIN WEST... ....Areas along the Washington-Idaho border and northeast Oregon into northern Idaho and northwest Montana... Have included a Marginal Risk over this region, essentially a continuation (migration) of the current Day 1 ERO Marginal Risk. Along with the continued snowmelt, which is having an impact on the current FFG (generally 1" or less within 6 hours) isolated areas of heavy rainfall -- between 1-2+" per the higher-res guidance -- could certainly lead to localized flash flooding. ....Southern Louisiana... 2000 UTC Update -- Based on the latest guidance trends, have curtailed the Marginal Risk somewhat, particularly across south-central LA. Aloft the 12Z models remain in good agreement with the overall pattern through the Day 2 outlook period (12Z Wed), i.e. with the timing and location of the eventual closed mid-upper level low along the southwest LA-Upper TX Gulf Coast. However, we're still seeing quite a bit of spread with respect to low- level features, including the location of weak surface circulations and resultant QPF. Given the evolving mid-upper level pattern, would certainly expect mesoscale features (thermodynamic profile along/near the Gulf) to play a big role in the eventual forming and location of any weak surface lows. While the bulk of the guidance, including the NBM and overall multi-model mean QPF, have trended a bit drier farther N-NW of the southeast LA coast, for now given the spread in low-level progs have opted to make only a minor change to the Marginal Risk area (removing a small portion of the Marginal in south-central LA). Previous discussion below.. 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... 2000 UTC Update -- Opted to expand the Marginal Risk area to include more of South FL, including the Keys. This given the guidance trends with the stalled surface front and persistent difluent upper flow across South FL east of the closing mid-upper low. Persistent easterly low-level flow (increasing to 25-30 kts at 850mb) will maintain/prolong moisture transport over this area during Day 2, where PW values are expected to climb between 1.5-1.75". A couple of available CAMs during the Day 2 Period (FV3 and NAM-CONUS-Nest) are quite bullish with the QPF within the Marginal area, i.e. areas of 3-5" with localized amounts of 6+ inches. The differences though are with the location of heaviest rainfall (NAM CONUS-Nest farther northeast compared to the FV3), and the spatial spread in the non-CAM guidance is not much better. Supporting the expansion of the Marginal Risk included the 12Z HREF probabilistic guidance. The 12Z HREF 40km neighborhood probs of 24hr QPF exceeding 3" (exceeding 5") are 70-80 percent (40-60 percent) from Miami and Homestead south to the coast, and 80-90+ percent (60-70+ percent) across the Keys. Previous discussion below.. 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. Hurley/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 AND NORTHERN KEYS... 2000 UTC Update -- Only minor modifications were made to the Day 3 outlook areas. Still quite a bit of spread in the guidance, at least with respect to surface/low-level features and QPF, and as such felt it was prudent to 'cast a larger net' metaphorically speaking with respect to both Marginal Risk areas. Hurley Previous discussion.. ....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!784Js39a54py2BpT-_YD5LIhjYIc9DdQAw3MCtFXj9uy= 6uGKtxbzUrMd0mCD19GbPDukVnEyQ9vvLS9o4J04ILcBatk$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!784Js39a54py2BpT-_YD5LIhjYIc9DdQAw3MCtFXj9uy= 6uGKtxbzUrMd0mCD19GbPDukVnEyQ9vvLS9o4J04n8Osjbg$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!784Js39a54py2BpT-_YD5LIhjYIc9DdQAw3MCtFXj9uy= 6uGKtxbzUrMd0mCD19GbPDukVnEyQ9vvLS9o4J04qKo3lXo$=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) .