Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Apr 13 2024 20:16:44 FOUS30 KWBC 132016 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 416 PM EDT Sat Apr 13 2024 Day 1 Valid 16Z Sat Apr 13 2024 - 12Z Sun Apr 14 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ALONG THE CENTRAL CALIFORNIA COAST AND INTO THE UPSLOPE OF THE NORTHERN TO CENTRAL SIERRA... ....16Z Update... Only very minor tweaks were needed to the inherited Marginal Risk areas in California. The upslope of the Sierras Marginal was adjusted west off the higher terrain where at least a good portion of the forecast precipitation will be in the form of snow. Conversely, added a bit more of the adjacent Sacramento Valley to the Marginal due to antecedent wet soils and expected runoff from the mountains. Otherwise, very little has changed about the meteorological setup described in the previous discussion below. Wegman ....Previous Discussion... No changes made to the marginal risk areas along the central California coast or into the upslope regions of the northern to central Sierra. There continues is good model agreement on the slow eastward inland movement of height falls from the strong closed low off the northern California coast day 1. Simulated hi res radars continue to depict a well defined area of precip moving inland along the associated front in the first half of day 1 across the central to southern California coastal regions and inland into the upslope regions of the northern to central Sierra. Anomalous PW and 850-700 mb moisture flux values, 1 to 2+ and 1 to 3+ standard deviations above the mean respectively, expected along the slowly southeastward moving frontal boundary. Hourly rainfall rates with the lead area of precip expected to max out in the ..25"-50"+ range. HREF 1 hour neighborhood probabilities for .50"+ amounts show relatively small areas moving along the central CA Coast Range and Transverse Range with the frontal precip band. There are no HREF probabilities for hourly amounts greater than 1". This initial precip area will then be followed by a secondary precip area developing along the central CA coastal range from San Francisco south to just north of Point Conception as the main upper vort swings southward along the central California coast. HREF neighborhood probabilities for .50"+ hourly amounts are lower with the second precip area compared to the lead area. Most of the hi res runs show hourly totals in the .10-.25" range along the central CA Coast Range with the second precip area. Given the relatively low probabilities for .50"+ hourly amounts, the risk was kept at marginal with flash flooding remaining isolated. Oravec Day 2 Valid 12Z Sun Apr 14 2024 - 12Z Mon Apr 15 2024 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF COASTAL SOUTHERN CALIFORNIA AND THE CENTRAL APPALACHIANS... ....2030Z Update... ....Southern California Coast Ranges... No changes were needed to the inherited Marginal Risk area for the southern Santa Lucia and Transverse Ranges. Onshore flow supported by a vertically stacked low off the coast of central CA will transport abundant Pacific moisture into the Marginal Risk area Sunday. Amounts will not be overly impressive as the rainfall pushes east and weakens with time, but saturated soils in this area and 1-2 inches of rain should be enough to cause isolated flash flooding concerns. ....Central Appalachians... Introduced a Marginal Risk area from northern WV northeast to the Catskills of NY with this update. A strong cold front will push southward on Sunday, supported with plenty of moisture from the saturated soils across much of the Northeast due to recent heavy rainfall. The front will push south and east with time, so portions of central NY and northeast PA will see the strong storms first as MUCAPE values of 1k to 2k are drawn northward ahead of the front and marginal (PWATs to 1.25 inches) atmospheric moisture advects=20 into the area. As the front moves southward, storms will form more to the west into WV and southwest PA during the afternoon and evening. Soils here are even more sensitive than areas further northeast, and while the storms will largely move southeast, perpendicular to the axis of the front, ideal instability and enough moisture will=20 allow for the storms to become strong enough to pose a flash=20 flooding threat. CAMS guidance suggests that for most areas, a=20 single round of showers and storms is likely. However, given that=20 nearly all of the rainfall will convert to runoff, and upsloping=20 may allow some of the storms to "hang up" on the ridges, a higher-=20 end Marginal threat exists. Any slowing of the storms or increase=20 in moisture would quickly lift southwest PA into the Slight risk=20 category, so the area will continue to be closely monitored. Wegman ....Previous Discussion... The secondary area of precip dropping southeast along the central California coast late day 1 will continue to push eastward across coastal southern California day 2. No significant changes to the previous small marginal risk area through the Transverse Range of southern California. Hourly rainfall rates will mostly be in the .10-.25" range with this secondary area of precip, which is expected to be fairly progressive. These hourly rates and the progressive nature of the precip area should keep any runoff issues as isolated. Oravec Day 3 Valid 12Z Mon Apr 15 2024 - 12Z Tue Apr 16 2024 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF THE PLAINS... ....2030Z Update... No major changes were made to the inherited Marginal Risk over portions of the Plains. The parent low in the guidance is moving a bit faster, so the Marginal was trimmed a couple rows of counties from the west. On the southern end of the line across OK and KS, instability greater than 2,000 J/kg and decent moisture transport along an LLJ will support severe thunderstorms that will track across those states. The instability will favor the strongest storms carrying hail, which will diminish the flooding threat. Further, soils in OK and KS are a bit drier than normal. Finally, the storms will be moving eastward at a decent speed, largely perpendicular to the frontal axis. These factors will all work against any more than an isolated threat for localized flash flooding where any heavier rains persist longest and away from any hail cores.=20 To the north across the Dakotas, a few variables are a bit more favorable for the development of flash flooding, including a bit lower instability that will diminish the hail threat in favor of heavy rain, and the northern comma-head of the developing low largely staying in place/moving slowly. This will favor a larger precipitation shield that will have embedded convection as the instability from the south advects north with the dry slot above the lower atmospheric moisture. Thus, forecasted rainfall through 12Z Tuesday is higher than further south. The primary factor working against flash flooding are the dry antecedent conditions and overall flood resistance of this area. Greenup has yet to begin to any significant degree in this region, so that will allow more rainfall to convert to runoff. Nonetheless, with the parent low to the south still developing, it will take a while into Monday night for the low to get its act together. This will generally=20 diminish the overall forecast precipitation footprint across the=20 northern Plains. Wegman ....Previous Discussion... The closed low that is expected to move inland from central California into the Great Basin day 2 will be ejecting eastward through the Central Rockies into the Central High Plains day 3. We continue to depict a broad marginal risk area through portions of the Plains in a region of strengthening low level southerly flow and increasing PW values. PW values expected to increase to 2 to 3 standard deviations above the mean along and ahead of the strengthening north to south oriented frontal boundary expected to push eastward into the Plains on Monday. This and and a broad region of instability along and ahead of this front will support increasing convection, especially late Monday afternoon into the early hours of Tuesday. There continues to be a large spread with qpf details among the models at the day 3 period. We kept a fairly broad marginal risk area to cover this spread, although we did narrow the marginal risk area to better fit the latest spread. Two areas of concern are for a convective line forming late Monday afternoon into the early hours of Tuesday along and ahead of the front over the Southern to Central High Plains. The other area of concern would be with a slow moving comma head/deformation precip area farther to the north to the north of the closed low track. There is somewhat better qpf agreement with the comma head/deformation precip area compared to the convective frontal max farther to the south. With precip totals generally below average over the past few weeks across large portions of the Plains, and current model qpf spread, the risk level was kept at marginal. Oravec Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8Ocn1HGC3IN4HwTEG0L_jMJIW_iK6DR5bu0zLeFwqlX0= GJPJWagdh3dGr_zey38bcFeXZx58jqDAYCmNfPlYFoZWg4k$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8Ocn1HGC3IN4HwTEG0L_jMJIW_iK6DR5bu0zLeFwqlX0= GJPJWagdh3dGr_zey38bcFeXZx58jqDAYCmNfPlYjnMcybo$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8Ocn1HGC3IN4HwTEG0L_jMJIW_iK6DR5bu0zLeFwqlX0= GJPJWagdh3dGr_zey38bcFeXZx58jqDAYCmNfPlY0RMI2rI$=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. 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