Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Feb 02 2024 19:46:39 FOUS30 KWBC 021946 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 245 PM EST Fri Feb 02 2024 Day 1 Valid 16Z Fri Feb 02 2024 - 12Z Sat Feb 03 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS EASTERN PORTIONS OF THE SOUTHERN PLAINS TO THE MIDDLE AND UPPER TEXAS COAST... 16Z Update... The 12Z HREF guidance and recent runs of the HRRR remain generally in good agreement with the overnight guidance concerning the expectation for at least some broken bands of convection to develop across portions of the southern Plains as shortwave energy/height falls advance east out of the Southwest. Some localized swaths of 1 to 2+ inch rainfall totals seem likely given the level of dynamical forcing that will be in place along with at least some modest elevated instability. The strongest instability will most likely be focused down across area of south-central to southeast TX and this is where generally the heaviest rainfall rates should be focused. Based on all of this, the previous Marginal Risk area was maintained with just some subtle tweaks made to account for the new HREF guidance. Orrison Previous discussion... The latest model guidance remains consistent with the increasing shower/thunderstorm activity across the Southern Plains Friday afternoon/evening as strong height falls push through the Southwest and into the Southern High Plains. PW values expected to continue to increase day 1 ahead of these height falls as low level southerly flow strengthens off the western Gulf and spreads northward into the Southern Plains. The well defined upper difluence ahead of these height falls and the axis of increasing PW values will support potential for areas of heavy rains. There continues to be a lot of spread with where the heaviest totals will occur. This is illustrated well with the HREF neighborhood probabilities showing a large area of high probabilities for 1"+ totals across central to eastern Texas, northward into portions of southeast to western Oklahoma and south central Kansas. However, the EAS probabilities for 1"+ amounts day 1 are much lower indicating poor overlap in the latest hi res models. The previous marginal risk area still fits the geographical spread in the latest guidance, leading to no changes at the moment in the marginal risk area. No changes also to the risk level given the current spread in qpf details and consensus that any precip will be generally to the northwest of where the heaviest amounts have occurred over the past few weeks. Oravec Day 2 Valid 12Z Sat Feb 03 2024 - 12Z Sun Feb 04 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE CENTRAL CALIFORNIA COASTAL RANGES AND FROM SOUTHEAST TEXAS INTO PORTIONS OF THE LOWER MISSISSIPPI VALLEY/CENTRAL GULF COAST... 2030Z Update... The 12Z HREF guidance and the global model consensus overall suggest a bit of a quicker arrival of stronger IVT magnitudes/forcing into the coastal ranges of central CA by 12Z/Sunday. There should be a warm front arriving along the coast by that time period, and the 12Z NAM/GFS solutions show an impressive 50 to 60+ kt south-southwest low-level nosing toward the Bay Area. Enhanced warm air advection/isentropic ascent coupled with strong orthographic ascent over the coastal terrain should promote a marked increase in rainfall rates late Saturday night and especially by 12Z/Sunday. Locally as much as 2 to 3 inches of rain over the coastal ranges will be possible by then. WPC has introduced a Slight Risk as a result of the expectation of heavier rainfall rates arriving earlier with this next atmospheric river. The Marginal Risk area has also been locally expanded as well to account for this. Elsewhere, only modest tweaks were made to the Marginal and Slight Risk areas across the eastern portions of the southern Plains and also the Lower MS Valley where there will continue to be a good setup for strong warm advection/forcing and moisture transport ahead of the ejecting southern stream shortwave energy. The area of heaviest rainfall is expected to be over southeast LA where the guidance shows a robust southerly low-level jet of 50+ kts off the Gulf of Mexico ahead of a deepening low center. Thus there will be rather strong poleward transport of subtropical moisture, and at least an elevated pool of instability for enhanced rainfall rates. There is support for some training convection to set up over southeast LA and this could greatly enhance the rainfall potential. Locally several inches of rain cannot be ruled out which is supported by multiple members of the 12Z HREF guidance. This will likely drive an urban flash flood threat across the greater New Orleans metropolitan area. Orrison Previous discussion... ....Central Gulf Coast into the Lower Mississippi Valley... The expanding convection across eastern Texas at the end of day 1, will continue to push eastward day 2 across the Central Gulf coast and toward the Lower Mississippi Valley. Overall favorable conditions continue for heavy rains day 2 with well defined upper difluence spreading east in an axis of strong southerly onshore flow off the Gulf and PW values of 1 to 1.5". There is better model agreement, however, that the heaviest qpf axis will be closer to the Gulf coast, where instability will be better.=20 Subsequently, we have trimmed the northern end of the previous slight risk area by about 150-175 miles. In the area where the slight risk was removed, rainfall has been less over the past week compared to previous weeks. This is evident in many of the USGS sites that show river gauge heights lowering over the past week.=20=20 Closer to the Gulf coast where the heaviest qpf is expected, the convection will still be fairly progressive. There still is the potential for the convection to be offshore of the central Gulf coast, but with still enough models showing heavy precip across southern LA into far southern Mississippi, did not want to remove the slight risk entirely. The marginal risk area extending northwestward into Arkansas, eastern Oklahoma, eastern Kansas and far southwest Missouri, was narrowed from the previous issuance to better match the narrow axis of moderate to locally heavy precip totals. ....Central California Coast... The previous marginal risk area was expanded to the north by approximately 80 miles to cover the spread of qpf that falls by the end of day 2 associated with the next atmospheric river event. All models are in agreement that the heaviest rainfall amounts will be during the day 3 period. However, with recent heavy rains across this area and potential for additional heavy rains to start by the end of the day 2 period, a marginal risk of runoff issues is warranted. Oravec 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!_8cOXIOmGgyQidCBuOAEPCOCGGdvCyMWJwCqBvjK6dD3= RcfYovPjZUyNN43xGE9t8zou0voEHL5WxfNyZtqn1mqDhwU$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_8cOXIOmGgyQidCBuOAEPCOCGGdvCyMWJwCqBvjK6dD3= RcfYovPjZUyNN43xGE9t8zou0voEHL5WxfNyZtqnaUKwtNE$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_8cOXIOmGgyQidCBuOAEPCOCGGdvCyMWJwCqBvjK6dD3= RcfYovPjZUyNN43xGE9t8zou0voEHL5WxfNyZtqnKNR4DJc$=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) .