Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Apr 12 2024 20:03:33 FOUS30 KWBC 122003 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 403 PM EDT Fri Apr 12 2024 Day 1 Valid 16Z Fri Apr 12 2024 - 12Z Sat Apr 13 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF NORTHERN NEW ENGLAND... Mid-morning Update: Two main areas of precipitation ongoing this morning, one on the leading edge of the warm front advancing northward across New England. This activity will continue to stream northward through New Hampshire and Maine, bringing widespread moderate to locally heavy rainfall. The combination of higher rain rates over sensitive ground conditions (melting snowpack, higher soil moisture) will lead to some instances of rapid runoff and flash flooding, particularly over the higher terrain areas. The Slight Risk area was largely unchanged from the previous cycle and looks to line up well with current radar and early 12Z guidance. Another area of precipitation is advancing east/northeast along the sweeping cold front across eastern PA and central NY. Based on the recent HRRR runs and other early 12Z guidance, this activity should lift east/northeast into western New England (western MA/VT/NH) over the course of the morning/afternoon hours. While more progressive, there are indications of localized higher rain rates due to a ribbon of instability developing just ahead of it (upwards of 500 J/kg MUCAPE). While the ground conditions are not as sensitive as areas to the east, the soil moisture in the 0-40cm layer is still fairly high and this additional rainfall could result in some flooding concerns. The Marginal Risk was adjusted to cover more of VT for this threat. Taylor ---previous discussion--- Strong southerly low level flow in an axis of much above average PW values...3+ standard deviations above the mean...expected along and ahead of the lead frontal boundary pushing across eastern New England early Friday. A period of heavy rains likely to push fairly quickly eastward Friday morning across New Hampshire, eastern Massachusetts and Maine. There is pretty good agreement on a stripe of heavy precip in the area of strong upslope flow from northern New Hampshire into central Maine. Not a lot of changes made to the previous slight risk area, which continues to correspond well where the HREF neighborhood probabilities are high for 1 and 2"+ amounts and where the HREF EAS for 1"+ amounts are also high. This heavy rain, along with snow melt will continue the risk of flash flooding, especially during the morning hours of Friday. A second line of convection may form along the secondary frontal boundary pushing eastward into far eastern Pennsylvania, northern New Jersey and eastern New York. The previous marginal risk area across northern NY State was extended southward to cover locally heavy rains with this second band of convection moving across this area Friday morning into early afternoon. The marginal risk fits in well where the HREF neighborhood probabilities are high for 1"+ amounts. Oravec Day 2 Valid 12Z 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... 21Z Update: No significant changes needed for the Marginal Risk area with relatively good model agreement and run to run consistency for widespread precipitation beginning late tonight/early Saturday morning. An axis of higher PWs / modest IVT values (400-500 kg/m/s) will bring some locally heavier rain to mainly the coastal central California areas but also extending into the foothills of the Sierra of the central/northern Sierra.=20 Taylor=20 ---previous discussion--- There is good model agreement of the southeastward movement of the strong closed low off the Pac NW coast to a position off the northern California coast by the end of the day 1 period and then an eastward inland movement day 2 across central California. A well defined area of precip expected along the slowly southeastward moving frontal boundary. PW values along and ahead of this slow moving front will rise to 1 to 2 standard deviations above the mean along with 850-700 mb moisture flux anomalies of 2 to 4+ standard deviations above the mean. The anomalous moisture flux values will impact the central California coast range from Santa Cruz southward to the western portions of the Transverse Range and the upslope regions of the northern to central Sierra. Across these areas max rainfall totals of 1 to 1.5"+ are possible. No significant changes made to the marginal risk along the central California coast, with a marginal risk introduced for the upslope area of the northern to central Sierra. Oravec Day 3 Valid 12Z Sun Apr 14 2024 - 12Z Mon Apr 15 2024 ....THERE IS A MARGINAL RISK FOR PORTIONS OF SOUTHERN CALIFORNIA... The closed upper level low is expected to slowly move south/east across coastal California during the period with a secondary/embedded vort max pivoting through southern California during the late morning and afternoon hours Sunday. With the lower heights aloft and potential for an axis of instability to develop=20 across this region, some intense rain showers will be possible=20 across the Transverse Range. QPF signal is for an additional 1-2"=20 in spots and this could lead to localized/isolated instances of=20 flash flooding, especially for the terrain areas, so a Marginal=20 Risk was introduced for this area. Taylor=20 Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8PyQJfpWxVKdJoXuQW906Dzt6K1EsGoUm8XdVXuf36Oi= OQWzumaRdVS403rTNAKhIB_KgWFZDq6yqlhlF6ohSrjYpCM$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8PyQJfpWxVKdJoXuQW906Dzt6K1EsGoUm8XdVXuf36Oi= OQWzumaRdVS403rTNAKhIB_KgWFZDq6yqlhlF6ohxbA7J-U$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8PyQJfpWxVKdJoXuQW906Dzt6K1EsGoUm8XdVXuf36Oi= OQWzumaRdVS403rTNAKhIB_KgWFZDq6yqlhlF6ohYJbuV4Y$=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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