Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Dec 07 2022 20:26:51 FOUS30 KWBC 072026 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 325 PM EST Wed Dec 07 2022 Day 1 Valid 16Z Wed Dec 07 2022 - 12Z Thu Dec 08 2022 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM PORTIONS OF THE SOUTHERN PLAINS EAST TO THE TENNESSEE VALLEY AND SOUTHERN APPALACHIANS... ....16Z Excessive Rainfall Outlook Discussion... Preliminary look at the 12Z guidance and the ensemble guidance that arrived after the initial issuance of Day 1 outlook suggests that the on-going outlook was in good shape and only minor adjustments were needed. The 09Z SREF showed a couple of members with 2+ inch amounts in the deformation zone straddling the Oklahoma/Kansas border...an idea supported more so by the GEFS.=20 However, higher resolution guidance like the HRRR was focusing the axis of heaviest QPF farther south. As a result, opted to expand the northern boundary of the MRGL just a bit northward. Farther east...expanded the Marginal Risk area a bit more northward given on-going rainfall over eastern TN and the signal from the operational guidance for upwards of an addition inch where terrain and lower FFG may be met. Given the modest deep-layer instability...still think any problems should be isolated enough to maintain the on-going Marginal Risk area. Bann ....0830Z Excessive Rainfall Outlook Discussion... ....North Texas and Eastern Oklahoma eastward into the western Tennessee Valley... Models still suggest the potential for locally heavy to excessive rainfall from parts of North TX into central-eastern Oklahoma and across portions of the Lower MS Valley and TN Valley, though the signal for anything other than an isolated (marginal) threat has diminished considerably over the past couple of model cycles. Aloft, a southern stream trough pivots across the Southwest U.S. on Wednesday and across the High Plains Wednesday night. This initial shortwave will be moving rather swiftly, given the increased confluence (de-amplification of the shortwave with time) by late Wednesday night. Given the flattening wave and more west-east oriented front, the the degree and depth of south-southwest low-mid layer flow will be somewhat restricted (warm conveyor belt or WCB more compact). Moreover, limited deep-layer instability (MUCAPEs 500-1000 J/Kg early dropping below 500 J/Kg later this afternoon and overnight) will also limit the short-term rainfall rates. So despite a relatively quick bout of robust low-mid layer moisture transport, with PW values topping out between 1.5-1.7", the lack of instability along with progressive nature of the favorable forcing (forward-propagating Corfidi or MBE vectors) will lead to a more limited (isolated) flash flood threat. In addition, per the latest NASA SPoRT analysis, 0-100cm soil moisture percentiles are running between 10-30% of normal across much of the outlook area. As a result, the Slight Risk area that was present in yesterday's Day 2 ERO has been dropped to a Marginal. Hurley Day 2 Valid 12Z Thu Dec 08 2022 - 12Z Fri Dec 09 2022 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Bann Day 3 Valid 12Z Fri Dec 09 2022 - 12Z Sat Dec 10 2022 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ALONG THE COAST AND COASTAL RANGES IN NORTHWEST CALIFORNIA... 2030Z Excessive Rainfall Discussion... The only change made to the on-going outlook was to remove the Oregon portion of the Marginal Risk area and keep it in northern California where the models show better on-shore flow into the terrain as shortwave energy embedded aids in the re-amplification of a longwave trough along the west coast. The strength of the onshore flow allows for anomalous low- to mid-level moisture flux with modest elevated CAPE...so previous thinking that there could be isolated rainfall rates that results in flooding (with burn-scars being the most vulnerable) remains on track. Bann 0830Z Excessive Rainfall Discussion... Another vigorous upper level jet streak and associated shortwave energy dropping off the Pacific NW coast will allow the longwave trough to re-amplify toward the end of day 3 (Friday night), with robust exit region upper level forcing (DPVA/upper level difluence and strengthening low-mid level moisture transport) working into the outlook area. While not a classic AR setup given the broad central Pacific ridge and mid/upper level trajectories advecting down from the northern Pacific instead of the subtropical regions (limiting the TPW potential), the strength of the onshore flow (sw/ssw 45-55 kts at 850mb along the coast after 00Z Sat) will average between 4 and 5 standard deviations above normal per the GEFS and SREF. The anomalous degree of the low-mid layer moisture flux, with PWs between 0.75-1.00" and marginal elevated CAPEs of ~200-400 J/Kg, will likely result in isolated areas of 0.50+ inch/hr and/or 1.5+ inch/3hr rainfall rates west of the more elevated terrain where the precip will fall as snow. These rates may lead to isolated areas of rapid inundation, especially over burn scars. As a result, a Marginal Risk was included in the Day 3 ERO across these areas in southwest OR and northwest CA. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!51_baBqOakgCnqGDLYT1UfRHX82XawrIJkSXfQu2dTcr= uMQO9f5yJtlaBPWNO-RmQvOFbTdc-hbjgY7Hn_kh3puvYKA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!51_baBqOakgCnqGDLYT1UfRHX82XawrIJkSXfQu2dTcr= uMQO9f5yJtlaBPWNO-RmQvOFbTdc-hbjgY7Hn_kha9bLKSU$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!51_baBqOakgCnqGDLYT1UfRHX82XawrIJkSXfQu2dTcr= uMQO9f5yJtlaBPWNO-RmQvOFbTdc-hbjgY7Hn_khWWSSUG4$=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) .