FOUS30 KWBC 281525 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1123 AM EDT Thu Oct 28 2021 Day 1 Valid 16Z Thu Oct 28 2021 - 12Z Fri Oct 29 2021 ...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF WESTERN WASHINGTON... 16Z Update... ...Pacific Northwest... Precipitation will continue to fall over the region as a strong AR event unfolds. Saturated soils only exacerbate the situation even more. Latest Hi-Res indicates 2 to 3 inches with pockets of 4 to 5 inches possible for the higher elevations in the Cascades and Olympics. Left the Slight Risk and Marginal Risk footprints as is with radar trends and model guidance aligning well with previous thoughts. ...Blue Ridge Mountains of NC/VA... Amounts through the mountain range for the period are still relatively low, but given upslope flow and overall flooding potential for the region, have opted to keep the Marginal Risk area as is. ...Florida Panhandle... The frontal boundary continues its progression eastward this morning with locally heavy rains. The Marginal footprint was adjusted further eastward and northward to account for latest radar trends and model guidance. Chiari Previous Discussion... ...Pacific Northwest... A strong atmospheric river will impact western WA today bringing some impressive rainfall totals to the Olympics and northern Cascades. Not much in the way of instability, but the strong onshore flow and anomalous PWs will still support some periodic higher rainfall rates. Just about all the higher res guidance indicate hourly totals locally exceeding 0.5" at times through 00z. The higher res model consensus is for ~3-5" of rain over the Olympics and as much as 4-6" over the northern Cascades. For the Olympics, amounts of this magnitude are not atypical...however soil conditions are quite saturated, and streamflows are already running above average before this rain falls. Thus some flooding issues can be expected. Over the northern Cascades, the forecast rainfall totals are a bit more rare...with our 24hr forecast QPF exceeding the 5 year ARI over a rather broad swath, and even locally exceeding the 50 year. Combine this with the high snow levels (meaning most of the QPF will fall as rain), and this expected rain is certainly less common for these areas. That combined with streamflows already running above average should result in some hydrologic impacts across the region. ...Blue Ridge Mountains of NC/VA... Strong upslope flow into the terrain will result in moderate rainfall amounts this evening into tonight across the area. Generally looking at 1-2", although localized amounts over 3" seem possible along the most favored terrain within a southeast wind regime. Instability is forecast to be minimal, and thus most high res guidance peak hourly rainfall at around 0.75". Given these moderate rainfall amounts, and antecedent streamflows only around normal, nothing really jumps out indicating a pronounced flood risk. However, 850mb easterly wind anomalies are close to max values for the time of year, suggesting the likelihood of strong orographics in play. So even if some weak instability is able to produce some embedded convective elements into the terrain, then some localized flood issues could develop. Autumn leaf fall could cause clogging drainage as well making some ponding of water a bit more common. Thus while a relatively low risk, thought the prudent thing was to maintain the Marginal risk that has been in place. ...Florida Panhandle... Added in a Marginal risk across portions of the FL panhandle into northwest FL. Locally heavy convection will continue to push eastward across the region through the morning hours. Some of these areas could see multiple rounds of convection within a broad swath of convective activity ahead of the front. This should be enough to push some areas over 3"...and antecedent soil and streamflow conditions are above average over this corridor. Thus can not rule out some localized flash flood issues through the morning hours until the cold front clears the area. Chenard Day 2 Valid 12Z Fri Oct 29 2021 - 12Z Sat Oct 30 2021 ...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF THE MID-ATLANTIC STATES... ...Mid-Atlantic States and Southwest New England... No changes were made to the risk areas with this issuance. A Slight risk was maintained from northern VA into eastern WV, south central PA and much of MD. As the deep layered closed low ejects eastward guidance indicates an uptick in low level convergence associated with the cyclone centered over this region. While still rather low, there is also a subtle increase in CAPE forecast. These factors should result in an uptick in rainfall amounts over the region, with a broad 1-2" currently forecast...and localized amounts over 3". For the most part, antecedent soil conditions and streamflows are around average (or even a bit below average) over this region. Thus in general, given the progressive nature of the system and limited instability, the forecast rainfall magnitudes and rates are probably not enough to result in any widespread flooding concerns. With that said, the strengthening low level convergence and uptick in marginal CAPE values does suggest at least some potential for a couple localized linear training convective segments that could bump hourly totals into the 1-2" range. This could locally exceed FFG across the DC/Baltimore urban corridor where recent rainfall has lowered FFG. Across the terrain of northern VA/WV/western MD/south central PA...anomalous easterly flow should result in pretty strong orographic enhancement to the rainfall, which combined with any embedded convective elements, could push rainfall magnitudes up over 3" in spots...potentially resulting in some flooding concerns. It is for these reasons that the Slight risk will be maintained from the urban corridor and points west. Leaf fall could also hamper drainage in spots resulting in more ponding of water than we would typically see with this amount of rainfall. The Marginal risk extends north into PA/NJ and southern NY/CT. Rainfall totals here are forecast to be lower...and little to no instability should hamper rainfall rates. However the northern 1/3rd or so of this Marginal risk has saturated soil conditions and well above average streamflows...thus even the 1-2" forecast warrants a Marginal risk. Chenard Day 3 Valid 12Z Sat Oct 30 2021 - 12Z Sun Oct 31 2021 ...A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF EASTERN NEW ENGLAND... As the closed deep layered cyclone continues to push eastward, there are some indications it may try to take on a bit more of a negative tilt on Saturday, which would result in an uptick in moisture transport and forcing ahead of the low over portions of coastal New England. Thus we may end up with another localized maximum in rainfall with the system during this time frame. At the moment the model consensus is for this max to occur over portions of southeast NH into southern ME...where localized rainfall of 2-4" is within reason. Instability is weak...but potentially non-zero as southerly flow increases off the Atlantic. Thus some 1"+ hourly magnitudes seem plausible over eastern New England. At the moment there is a bit of a displacement between the more susceptible areas and the greatest risk of heaviest rainfall totals. Portions of southern new England have the wetter soils and above average streamflows, while southern NH/ME (where the heaviest rain is currently forecast) have been drier. For this reason think the risk level is best characterized by a Marginal risk at the moment. But will need to continue to watch trends, as can not rule out a small scale Slight risk on future updates...especially if guidance trends wetter and/or there is a southward shift into the more susceptible areas. Chenard 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 $$
