504 FOUS30 KWBC 192004 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 304 PM EST Tue Jan 19 2021 Day 1 Valid 16Z Tue Jan 19 2021 - 12Z Wed Jan 20 2021 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Otto Day 2 Valid 12Z Wed Jan 20 2021 - 12Z Thu Jan 21 2021 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF SOUTHERN CALIFORNIA... ...Southern California/Arizona... A cut off low off the Southwest coast will slowly migrate inland over southern California and the northern Baja region through the forecast period (Wednesday into early Thursday). In response, deep southerly flow will transport the moisture rich environment north just ahead of the aforementioned low into southern CA/AZ. This is evident by the precipitable water values climbing to over 0.75 inches, possibly reaching 1 inch in far southern AZ, which is 2 to 3 standard deviation above the mean. The moisture flux will be aided by 25 knot low level south/easterly flow helping to promote upslope along the terrain, most notable the Peninsular Ranges. As a result, the precipitation will be enhanced by the upslope component. In addition, with lapse rates are expected to steepen as cooler air aloft moves in from the west allowing the instability to increase (albeit weak). Therefore, rain rates may exceed 0.75 inches per hour at times. Areal average precipitation will range between 2-3+ inches along the Peninsular Ranges and 1-2+ across the deserts and upslope terrain in southern CA/AZ. This combined with the aforementioned rain rates and QPF on Day 1 (through Wednesday morning), there is the increased potential for flash flooding within this region. Given the orographic effects will be most robust along the Peninsular Range with multiple rounds of showers and isolated thunderstorms, a Slight Risk was issued for this location. The rest of the region will see moderate to heavy rain, though the impacts should be localized given the progressive nature of the precipitation activity and less dynamic forcing and instability. Pagano Day 3 Valid 12Z Thu Jan 21 2021 - 12Z Fri Jan 22 2021 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE LOWER MISSISSIPPI VALLEY... ...Lower Mississippi Valley... A shortwave over the Desert Southwest will transition quickly east while a weak northern stream trough moves through the Great Lakes and into the Northeast. The southern and northern stream will not phase resulting in this aforementioned shortwave energy shifting east across the Lower MS Valley and into the Southeast through the forecast period (Thursday into early Friday). While the dynamics associated with this system are not robust, there will be a period of time across the region where training of moderate to heavy rain could result in localized flash flooding. Precipitable water values will increase to around 1.5 to 1.75 inches, which is 2-3 standard deviations above the mean for this time of year. This will be aided by 30-40 knot low level southern flow with a pronounced low level nocturnal jet developing Thursday night helping to enhance rain rates along and south of the surface front. A key feature that may limit rain rates will be the weak instability. Despite this however, the propagation vectors should align with the mean wind to result in training of precipitation as activity moves from west to east along the front. This will increase hourly precipitation totals and thus whittle down the high flash flood guidance that is currently in place. Areal average precipitation will range between 1-3+ inches, with a narrow corridor of 2.5-3.5 inches likely. The exact placement of the heavier footprint of QPF will depend on the position of the surface front and progression of the surface low along this boundary. While the antecedent conditions are dry, the training as mentioned above could be enough to produce nuisance/localized flash flooding issues. Therefore, a Marginal Risk was introduced. Pagano Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt $$
