Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Aug 10 2023 19:49:01 FOUS30 KWBC 101948 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 348 PM EDT Thu Aug 10 2023 Day 1 Valid 16Z Thu Aug 10 2023 - 12Z Fri Aug 11 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL PARTS OF SOUTHERN NEW ENGLAND, MISSISSIPPI AND ALABAMA, AND THE SOUTHWEST... ....Northeast... 16 UTC Update: No major changes were made to the outlook in this region, other than some small tweaks to the Marginal Risk area to clear out some of the western extent behind the advancing MCV, and include some areas of Upstate New York and northern Vermont that were previously excluded. Hi-res models do not have any sort of consistent signal on a focused area of heavy rainfall, but most do show narrow corridors with hourly rain rates peaking above 1 inch per hour. Therefore, isolated flash flooding seems possible in the region. Previous Discussion: 00Z model guidance made a sharp move away from a heavy rainfall threat that it began signaling on Wednesday...especially from parts of the Mid-Atlantic into New England. Some of the higher resolution guidance still has localized amounts in excess of half an inch embedded within a broader area of lower amounts and the HREF signal for 1 inch of rain falling in an hour were in the single digits until 21Z before beginning to grow in areal coverage over eastern New York and New England. Global models...on the other hand...showed considerably less rainfall. WPC maintained the Marginal risk area along and ahead of the path of an MCV making its way eastward from the Ohio Valley early this morning due to areas of lower flash flood guidance along part of its path...especially across far northern Pennsylvania and adjacent parts of New York that received decent rainfall in the past couple of days. Also kept the Marginal in place over New England and New York into the Mid Atlantic where...despite the lower model QPF...enough precipitable water was in place and favorable sounding profiles to support at least brief downpours. Using streamflow as a proxy for sensitivity to rainfall...maintained the Slight risk over portions of eastern Massachusetts into Connecticut and Rhode Island. Higher Flash flood guidance and the progressive nature of the rainfall/MCV should prevent more than widely scattered problems. ....Interior Southeast... 16 UTC Update: Hi-res models have not been handling ongoing convection this morning that well across the Southeast, with a convective outflow boundary (and some active convection) much further south than depicted in the models. This is somewhat common in the summer. Therefore, the outlook was modified to account for some of the ongoing trends and the typical model biases. First, risk areas were removed to the north of the HUN-ATL corridor, or the general southern Appalachians region. Early morning convection has advanced quickly into the easterly Carolinas, and a drier and more stable air mass is settling in across the region in the wake of that. Second, the Slight Risk was reduced in size and refocused into MS and AL; the centroid was positioned about 100mi WSW of the maximum 12Z HREF EAS probabilities of 2 inches of rain in the ERO period (through 12Z Friday). This was to correct for the fact that models generally had ongoing activity too far to the northeast. However, we do anticipate the area north of the outflow boundary to modify through the day as there is not consistent or overly dense cloud cover, and the 12Z soundings from BMX and FFC did not indicate a very strong or deep cold pool. The Slight Risk was generally confined to areas with lower FFG, and thus greater potential vulnerability to impacts from heavy rainfall. ....Southern Nevada, Southern Utah, Northern Arizona... The outlook was upgraded to a Slight Risk for portions of the Southwest, largely centered on the UT-AZ border region. Although there will be a broader area of typical isolated to scattered convection over a greater portion of the Southwest region, we did have concern about a concentrated area with greater potential for high rainfall rates and even some backbuilding of convective elements. This was being facilitated by a mid-level circulation from the remnants of Eugene in the East Pacific, now working into Southern California. A corridor of enhanced 700mb flow, around 20-25 knots was expected to develop up the Lower Colorado River basin and pointed into NW AZ. This low-level inflow should be roughly equivalent (or even slightly stronger) than the overall mean flow, a configuration that can favor some backbuilding. Furthermore, RAP forecasts do show a pool of 1000-2000 j/kg SBCAPE centered in NW/NC AZ this afternoon, and this upstream pool of instability would also favor backbuilding. The 12Z FGZ sounding was quite moist and unstable, with over 1000 j/kg of CAPE (above the 90th percentile) and nearly 1" PW at high elevation (near the 90th percentile), which provides some observational evidence for these concerns. Hi-res models do show a concentrated area of convection near the UT-AZ border with some backbuilding evident in the simulated reflectivity. Given this, a Slight Risk was coordinated with the WFOs in the region. The Slight Risk does overlap some vulnerable terrain with numerous slot canyons, raising concern with any convective elements that can sustain heavy rain in a particular basin for an hour or more. ....Southeast Florida... A small Marginal Risk was introduced in southeast Florida due to a fairly consistent hi-res model signal for some pockets of over 3 inches of rainfall in 3 hours, indicating potential for some persistence of high rainfall rates with slow-moving thunderstorm activity. Precipitable water values are likely to be near the 90th percentile, and the higher than normal moisture content will help support potential for localized rain rates in excess of 2 inches per hour. Lamers/Bann Day 2 Valid 12Z Fri Aug 11 2023 - 12Z Sat Aug 12 2023 ....THERE ARE MARGINAL RISKS OF EXCESSIVE RAINFALL IN THE SOUTHEAST, OZARKS, CENTRAL GREAT LAKES, DESERT SOUTHWEST, AND CENTRAL SIERRA NEVADA... ....Ozarks and Mid-Mississippi Valley into the Southeast... The broad Marginal Risk was maintained over the Southeast region, and was expanded back to the northwest to include a large portion of the Ozarks and Mid Mississippi Valley. The Marginal Risk area was placed near a slow-moving front that will persist across the region, with the front embedded in the northern portions of a plume of strong instability and deep moisture. Ensemble probabilities indicate that CAPE in excess of 2000 j/kg and PWs in excess of 1.8 inches are likely (over 60 percent chance) during the afternoon and early evening hours over most of the area outlined in the Marginal Risk. Hi-res models show a scattering of more intense rainfall with associated scattered non-zero chances of FFG exceedance. However, there is not a consistent signal on placement or convective evolution, so the risk level was kept at the Marginal level for now. ....Southern Michigan, Northern Indiana, Northwest Ohio... Models continue to show convection developing ahead of an advancing cold front in the Great Lakes region on Friday. For the most part, the convection depicted in hi-res model simulated reflectivity appears to be progressive and unlikely to linger in any one location for very long. Nevertheless, the Marginal Risk was maintained due to an increasingly focused signal for heavy rainfall on southeast Michigan, and to a lesser extent in some of the adjacent portions of the region. The HREF does show 40km neighborhood probabilities of 3 inches of rainfall (total) in the Day 2 period as high as 30 percent in southeast Michigan, which would correspond to a 5-10 year average annual recurrence interval. However, this rainfall is also likely to fall in much shorter bursts, so there does seem to be some risk of excessive rainfall and flash flooding. We did consider the possible need for a small Slight Risk, but trends will be monitored in future guidance. ....Eastern Arizona and Western New Mexico... A northward surge of moisture, increasing precipitable water values to slightly above average, and the entrance region of an anticyclonically curved, strengthening, upper level jet streak may combine to lead to an increase in convective coverage near the Arizona/New Mexico border region on Friday. Instability tends to be a factor more highly correlated to flash flood activity in the Southwest region, so overall the threat looks to be fairly isolated at this time. The instability should be limited by a fairly deep, dry sub-cloud layer. ....Central Sierra Nevada... A localized excessive rainfall and flash flooding threat continues to be forecast for the Central Sierra Nevada region, where there is expected to be a surge of deeper moisture and a lingering mid-level vorticity center from the now-decayed tropical cyclone (Eugene) in the East Pacific. A Marginal Risk was maintained, and expanded slightly into central Nevada, where hi-res models do indicate the potential for some localized heavy rainfall in the lee of the Sierra Nevada. The main question continues to be whether or not denser cloud cover associated with the deeper moisture limits heating and instability more than models are currently anticipating. Lamers Day 3 The Day 3 outlook will be updated by 2030Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8ga38utFSwYTFVg9JJCxOpm1U4fxv-LkhpKAKRdcJ0uF= wEQSmre4A1hGkwy2UPCKjf8vBNNoC4HzCwhkRiFiWvcBpRU$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8ga38utFSwYTFVg9JJCxOpm1U4fxv-LkhpKAKRdcJ0uF= wEQSmre4A1hGkwy2UPCKjf8vBNNoC4HzCwhkRiFiyEWQ3Zg$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8ga38utFSwYTFVg9JJCxOpm1U4fxv-LkhpKAKRdcJ0uF= wEQSmre4A1hGkwy2UPCKjf8vBNNoC4HzCwhkRiFi6sBRmpQ$=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) .