Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Apr 14 2024 20:12:53 FOUS30 KWBC 142012 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 412 PM EDT Sun Apr 14 2024 Day 1 Valid 16Z Sun Apr 14 2024 - 12Z Mon Apr 15 2024 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF COASTAL SOUTHERN CALIFORNIA AND FROM PORTIONS OF THE NORTHERN MID- ATLANTIC INTO THE CENTRAL APPALACHIANS... ....Coastal Southern California... Only cosmetic changes were made for the 16Z Day 1 ERO update to the previous Marginal Risk area across the Transverse range and the southern portions of the central California Coast Range. A second area of showers is developing early Sunday morning along the central California Coast Range ahead of the strong closed low along the central California coast. This upper center is expected to rotate southeastward along the central California coast and through the Transverse Range day 1. An organized area of showers will accompany this upper low, supporting additional moderate to locally heavy precip totals day 1. Hourly rainfall totals from the hi res guidance suggests .10-.25"+ amounts are possible. HREF neighborhood probabilities are rather low for any hourly totals greater than .50". Isolated runoff issues possible where additional moderate to locally heavy rains fall over regions that received rain over the past 24 hours. ....Northern Mid-Atlantic into the Central Appalachians... For the 16Z update, we removed the northern portion of the Marginal Risk area that had extended into southern NY. Despite the 850-700 mb kinematic and moisture transport anomalies and steepening 700-500 mb lapse rates, lower (sub 850 mb) tropospheric dry air, which is not currently being handled well by the models (including 13Z NBM), is a big mitigating factor for the overall flash flood threat (in all aspects...real extent, duration, and intensity). Believe the drier ARW, HRRR, and RRFS are having a better handle on the evolving situation, vs. the much wetter ARW2, FV3, and NAM CONUS-Nest. Have opted to maintain a portion of the Marginal Risk area (over PA, far eastern OH, and northern WV), however from an excessive rainfall/flash flood threat, expect the risk to be on the lower end of the marginal risk scale -- i.e. closer to 5% vs. near 15%. ....Previous Discussion... A surface frontal boundary expected to sink southward late Sunday=20 afternoon into Sunday night across the Northern Mid- Atlantic. A=20 narrow axis of instability, LIs 0 to -4, MUCAPE 200-500 j/kg, and=20 increasing PW values .75-1"+, expected along and ahead of this=20 front. HI res model consensus is for a line of=20 showers/thunderstorms to form along and ahead of this front across=20 the Northern Mid- Atlantic Sunday afternoon and sink southward into early Sunday evening. Recent heavy precipitation from the Central=20 Appalachians into the Northern Mid- Atlantic has resulted in=20 saturated soils and high stream flows. Hi res model consensus is=20 for potential of .25-50"+ hourly amounts along this front. With=20 soils saturated, much of this will runoff, possibly resulting in=20 isolated flash flooding. No significant changes made to the=20 previous Marginal Risk across this region. Hurley/Oravec Day 2 Valid 12Z Mon Apr 15 2024 - 12Z Tue Apr 16 2024 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF THE PLAINS... ....2000 UTC Update... Minor changes were made to the Day 2 ERO, mainly to nudge the=20 southern flank of the Marginal Risk area a bit farther east across the eastern Plains and Lower MO Valley based on the latest guidance trends. Deepening low slowly pivoting into the Upper Midwest will begin to draw moisture from the western GOMEX later Monday afternoon and overnight, with 850 mb southerly flow and moisture flux anomalies reaching 4 standard deviations above normal per the 12Z GEFS. Pre-frontal quasi-linear convection across the eastern Plains will likely be fairly transient per the latest simulated reflectivity and 1-3 hour QPFs per the guidance. However, with mixed-layer CAPEs of at least 1000-2000 J/Kg over the southern portion of the Marginal Risk area, there will be a higher probability of more intense 1-3hr rainfall rates compared to areas farther north. Higher FFGs (lower soil moisture percentiles=20 lower) over the southern portion of the outlook area will maintain a Marginal or localized flash flood risk.=20 Meanwhile, lower FFGs/wetter antecedent soils over the northern=20 portion of the outlook area, along with a better chance for more=20 prolonged activity along/north of the surface warm front, will be=20 offset by decided lower deep-layer instability (even elevated). So even here, again the flash flood risk remains marginal.=20=20 ....Previous Discussion... The latest model guidance is in good agreement on the eastward=20 push of the strong closed low that is expected to move inland from=20 central California into the Great Basin day 1, across the Central=20 Rockies into the Central High Plains day 2. A broad Marginal Risk=20 area maintained through portions of the Plains in a region of=20 strengthening low level southerly flow and increasing PW values. PW values expected to increase to 2 to 3 standard deviations above=20 the mean along and ahead of the strengthening north to south=20 oriented frontal boundary forecast to push eastward into the Plains on Monday. This and and a broad region of instability along and=20 ahead of this front will support increasing convection, especially=20 late Monday afternoon into the early hours of Tuesday. There=20 continues to be a large spread with qpf details among the models=20 for the upcoming day 2 period. We kept a fairly broad Marginal Risk area to cover this spread. Two areas of concern continue in this=20 broad Marginal Risk area. 1: For a convective line forming late=20 Monday afternoon into the early hours of Tuesday along and ahead of the front over the Southern to Central High Plains. This line will likely be fairly progressive to the east, limiting flash flood=20 potential. The other area of concern would be with a slow moving=20 comma head/deformation precip area farther to the north to the=20 north of the closed low track. The 850-700 mb moisture flux to the=20 north of this closed low becomes very anomalous Monday afternoon=20 into Monday night/early Tuesday with values 2 to 5 standard=20 deviations above the mean in the strengthening southeasterly flow=20 level flow. There is general consensus for a comma head/deformation max from far southeast MT/far northeast WY into SD. With precip=20 totals generally below average over the past few weeks across large portions of the Plains, and current model qpf spread, the risk=20 level was kept at marginal. Hurley/Oravec Day 3 Valid 12Z Tue Apr 16 2024 - 12Z Wed Apr 17 2024 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS EASTERN PORTIONS OF THE NORTHERN PLAINS SOUTHWARD THROUGH MUCH OF THE MISSISSIPPI VALLEY... ....2000 UTC Update... Very few changes made to the Day 3 Marginal Risk area based on the latest guidance trends. Similar to the day 2 period, we continue=20 to depict a broad marginal risk area ahead of the strong closed low forecast to push from the Central High Plains into the Upper=20 Mississippi Valley during day 3. An axis of anomalous PW values, 1=20 to 2.5+ standard deviations above the mean, will persist day 3=20 along ahead of the well defined front forecast to push east through the Central and Southern Plains and Lower Missouri Valley. There=20 continues to be potential for organized convection in this=20 anomalous PW axis ahead of the front. However, this convection will likely be fairly progressive, again limiting the heavy precip=20 potential. This will keep the threat level at marginal for the day=20 3 period along and ahead of this front. Farther to the north...model consensus is for a greater chance of organized precip along and to the north of the west to east oriented frontal boundary stretching from the Northern Plains eastward into the Upper Mississippi Valley. 850-700 mb moisture flux anomalies rise to 4 to 5 standard deviations above the mean into this boundary where strengthening southerly to southeasterly flow level flow will impact the front. An axis of heavy rains likely to the north of the front, with model consensus for areal average 1-2" amounts. This region has been relatively dry over the past few weeks, resulting in dry soils and below average stream flows. At the moment, we have kept the threat level also at marginal across this area. Hurley/Oravec Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9wDrWcEtzSyFYi7maCzwOh-YP2aO5Ih6UNADo5j-4fbt= vcDJMJe7h2kL0J0q4spNCzUOvoSyJj3yIYGW8afnL9ldSlQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9wDrWcEtzSyFYi7maCzwOh-YP2aO5Ih6UNADo5j-4fbt= vcDJMJe7h2kL0J0q4spNCzUOvoSyJj3yIYGW8afnFlBNqSA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9wDrWcEtzSyFYi7maCzwOh-YP2aO5Ih6UNADo5j-4fbt= vcDJMJe7h2kL0J0q4spNCzUOvoSyJj3yIYGW8afnjRJBvik$=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 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .