Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Apr 10 2023 07:38:00 FOUS30 KWBC 100737 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 336 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 The Day 3 outlook will be updated by 0830Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7smwYpJJb3b6ZPYGUHtvPXNkQwEwB4J1ErcuELgaEtZ4= 3gTr7g1geC-o16qpxXGmlEQr89bAs7Khz73_OZ1TDF5hpaM$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7smwYpJJb3b6ZPYGUHtvPXNkQwEwB4J1ErcuELgaEtZ4= 3gTr7g1geC-o16qpxXGmlEQr89bAs7Khz73_OZ1T1Bzqb2k$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7smwYpJJb3b6ZPYGUHtvPXNkQwEwB4J1ErcuELgaEtZ4= 3gTr7g1geC-o16qpxXGmlEQr89bAs7Khz73_OZ1TgpjdQXA$=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) .