Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Mar 20 2024 15:48:43 FOUS30 KWBC 201548 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1148 AM EDT Wed Mar 20 2024 Day 1 Valid 16Z Wed Mar 20 2024 - 12Z Thu Mar 21 2024 The probability of rainfall exceeding flash flood guidance is less than 5 percent.=20 Otto Day 2 Valid 12Z Thu Mar 21 2024 - 12Z Fri Mar 22 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF=20 THE WESTERN AND CENTRAL GULF COAST... A 500 mb southern stream trough is forecast to track eastward from the southern High Plains Thursday morning to the west-central Gulf Coast by Friday morning. The advancing of this feature will allow=20 a northward returning warm front in the western Gulf to lift into=20 southeastern Texas during they day on Thursday. A weak low along=20 the front is forecast to hover near or just south of the Gulf Coast into Thursday night (with the 00z GFS depicting the track=20 nearest to the shore with the highest resulting totals).=20 Instability and hence the heaviest potential for flash flooding=20 intense rainfall rates is likely to remain within about ~100 miles=20 of the coast with strong convection developing ahead of the upper=20 trough within divergent and diffluent flow aloft, with possible=20 repeating cells and training from west to east (or west-southwest=20 to east- northeast) within the plume of instability forecast to=20 reach into the 1000-2000 J/kg range along the Coastal Plain.=20 Instability farther north remains a bit more questionable, but both the FV3 and NAM-nest depict a stronger closed low at 500 mb=20 compared to the model consensus. Given the continued convective=20 uncertainties farther north, the inherited Marginal Risk was=20 expanded a bit northward into more of eastern Texas (while also=20 being shifted a bit west overall given the new models).=20 Churchill/Otto Day 3 Valid 12Z Fri Mar 22 2024 - 12Z Sat Mar 23 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHEAST AND SOUTHERN MID-ATLANTIC, AS WELL AS FOR COASTAL NORTH CALIFORNIA AND FAR SOUTHWESTERN OREGON... ....Southeast into southern Mid-Atlantic... The upper-level southern stream trough and associated surface low look to slow their progression on Friday and Friday night as ridging builds eastward over the western North Atlantic. This may result in a complicated storm evolution over the 24 hour period, as models suggest that two distinct surface lows track across=20 portions of the Southeast (the first possibly a result of a MCS=20 that develops as a result of organized convection on Day 2). Most models agree that some of the heaviest resulting rainfall totals=20 will occur over southern portions of Florida (including South=20 Florida and the Florida Keys), but there is less agreement across=20 north- central Florida (where many models are actually rather dry). Despite the better agreement farther south, decided to maintain a=20 Marginal Risk across the board given the notoriously difficult to=20 flood character of swampy South Florida, though a Slight may be=20 considered for subsequent outlook should confidence increase in=20 higher totals for more urbanized terrain (as of now, areal average=20 of 2-4" over a relatively long period appears insufficient to=20 upgrade, though the inclusion of the CAMs in future outlooks may=20 change that). Given the uncertainty farther north, the Marginal was kept in place as well.=20 The Marginal Risk was also maintained even farther north into eastern Alabama, Georgia, and the Carolinas, as better model agreement exists here for prolonged moderate to heavy rainfall on Friday and Friday night with the slowing upper-level trough. Localized totals of 2-3" appear likely across the region, though limited instability may generally keep rainfall rates capped below 1"/hr. Therefore expect any flash flooding to be rather localized with more longer duration impacts, especially given that previously wet soils have had some days to recover (a notable=20 signal in the NASA SPoRT-LIS 0-100 cm soil moisture trends back=20 towards normal). ....Coastal northern California and far southwest Oregon... A relatively weak (and narrow) atmospheric river is forecast to=20 come ashore portions of the Pacific Northwest early Friday morning, as an associated low pressure system remains nearly=20 stationary offshore Washington and Oregon. The IVTs look to be=20 concentrated into coastal portions of northern California and far=20 southwest Oregon, where localized rainfall totals of 1-3" may=20 occur. More flood prone localities (including burn scars and low=20 lying areas) may experience some flooding impacts in association=20 with the expected rainfall.=20 Churchill Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4PG2nLE-gpdn7Z7QysA9hW-om4uHpdpQz_-10Y1zeUHx= adWughIqzLQZ6nAyKMlDhx3EdCNKjNtxl3kx7FzA2PkpD3U$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4PG2nLE-gpdn7Z7QysA9hW-om4uHpdpQz_-10Y1zeUHx= adWughIqzLQZ6nAyKMlDhx3EdCNKjNtxl3kx7FzAUBulKsI$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4PG2nLE-gpdn7Z7QysA9hW-om4uHpdpQz_-10Y1zeUHx= adWughIqzLQZ6nAyKMlDhx3EdCNKjNtxl3kx7FzAV0Gtj4s$=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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