041 FOUS30 KWBC 232251 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 550 PM EST Wed Jan 23 2019 Day 1 Valid 01Z Thu Jan 24 2019 - 12Z Thu Jan 24 2019 ...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF THE CENTRAL & EASTERN GULF COAST... A deep-layered trough and surface cold front will push across the East today, bringing showers and embedded thunderstorms to the region. As the trough and upper jet approaches, low level southerly flow has picked up, increasing both moisture transport, precipitable water values (PWs) near 1.75", and importing instability from the Gulf of Mexico ahead of the eastward moving front. MU CAPE of 500-1500 J/KG has been available thus far but has confined to portions of southern AL into the FL Panhandle. The highest rainfall magnitudes/rates could occur here, with hourly totals up to 2" and local amounts to 4" possible, which have been realized across southernmost MS and southwest AL where flash flood warnings were issued earlier. While the trough and cold front are progressive, the degree of effective bulk shear available across portions of AL and FL could allow for mesocyclone formation along the convective line, which would periodically enhance totals until the storms fully outran/exhausted the available instability later tonight. Given the very strong moisture transport and flow parallel to the front, would expect some short duration training potential as cells move northward up the front. Urban locations appear to be the most threatened. Further north and east from GA into the southern Appalachians, instability becomes negligible, but good frontogenesis is expected in the 650-850 hPa layer. The farther north you go across this area, the colder/harder the ground is and the wetter antecedent conditions have been, both suggesting flooding will be easier to come by. The lack of instability should limit hourly totals to ~0.25", which suggests that any flood threat is long duration and thus Marginal from a flash flood perspective. Enhancement is expected in the upslope terrain from north GA into the western Carolinas and far southwest VA given the strong southerly flow ahead of the front. Thus amounts should exceed 2" locally in the southern Appalachians, which could lead to a slightly higher flood threat, especially if any convectively enhanced rates materialize as has occurred occasionally across northern AL and TN. Roth/Chenard Day 2 Valid 12Z Thu Jan 24 2019 - 12Z Fri Jan 25 2019 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE NORTHEAST U.S. THURSDAY... ...Coastal Mid-Atlantic to Southeast New England... The 23/12Z suite of numerical models continue to show a well defined plume of deep moisture getting drawn northward ahead of a dynamic, deep layered trough approaching from the west. Precipitable waters which are barely an inch in the Mid-Atlantic region at the start of the forecast period surge northward during the day on Thursday by very strong low level winds. The GFS and ECMWF both show southerly winds on the order of 55 to 95 kts at 850 mb just east of the Mid-Atlantic coast pointed in the general direction of southern New England at 24/18Z. Aloft, the entrance of a 130 to 155 kt upper level jet will provide strong divergence fields. As previously mentioned, the system will be progressive which should normally limit the potential for excessive rainfall. Areal average precipitation across this region is around 0.75"-1.5". The Slight Risk area was drawn to cover low Flash Flood Guidance values over New Jersey and portions of the urbanized corridor. This area coincides with the higher potential for instability surging north leading to rain rates climbing above 0.5" at times. The risk area was then extended eastward into portions of Connecticut and Rhode Island near the model QPF maxima; though the Flash Flood Guidance is higher here, the strength of the dynamics poised to move across the area coupled with some weak instability has the potential to produce rainfall rates intense enough to result in short-fused flooding issues. These problems could be made worse in areas of frozen ground or with snow pack resulting in poor drainage. Surrounding the Slight Risk...a Marginal Risk area was maintained and refined. ...Maine... While there is no excessive rainfall risk designated for this region, it should be noted that the updated precipitation forecast with the aforementioned system and frozen ground/residual snow pack could lead to runoff and drainage issues. With strong surface convergence and moisture flux ahead of the low approaching from the south combined with strong unidirectional flow, expect a period of moderate (to at times heavy) rain from 24/18Z-25/00Z. However, a limiting factor being the frontogenesis hanging back to the west. Areal precipitation totals average around 1.5 to 2 inches. We will continue to monitor the QPF and determine if a risk area is warranted. With lower rain rates due to lack of instability, feel a mere mention of isolated flooding issues should suffice. Bann/Pagano Day 3 Valid 12Z Fri Jan 25 2019 - 12Z Sat Jan 26 2019 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Pagano Day 1 threat area: http://www.wpc.ncep.noaa.gov/qpf/94epoints.txt Day 2 threat area: http://www.wpc.ncep.noaa.gov/qpf/98epoints.txt Day 3 threat area: http://www.wpc.ncep.noaa.gov/qpf/99epoints.txt $$