Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Dec 17 2023 20:29:46 FOUS30 KWBC 172029 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 329 PM EST Sun Dec 17 2023 Day 1 Valid 16Z Sun Dec 17 2023 - 12Z Mon Dec 18 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE EASTERN U.S. FROM THE CAROLINAS TO THE NORTHERN MID-ATLANTIC REGION AND SOUTHERN NEW YORK/NEW ENGLAND... 16Z update... Latest observations show the surface low pressure center is near the northern Florida coast with the bulk of the convection further north over the Carolinas and Mid-Atlantic region with showers over Florida and Georgia rapidly decaying. Given these trends the southern bounds of the Marginal was trimmed northward to southeast Georgia while the Slight was trimmed northward to eastern Georgia. The highest rainfall is still anticipated for the eastern Carolinas up to the VA/DC/MD area. Campbell ....Southeast US into the Mid Atlantic Region... Longitudinal differences in where the axis of heavy rainfall occurs does linger in the higher resolution guidance from the Carolinas into the Mid-Atlantic. The differences were continuing to revolve around the slightly faster solutions...and for a secondary northern shortwave into pivot into the deepening trof farther north and west. This draws higher moisture from the increasingly distant warm conveyor belt back west into the foothills of the Appalachians and western Piedmont regions of the Carolinas and dries out the Coastal Plains. The 17/00Z run of the HREF supported this idea...showing a potential for 2 inch and 3 inch per hour rainfall rate over portions of the Carolina coastal plains during the morning before rates decrease to around an inch per hour in southeast Virginia as instability tapers off. Given the nature of the coastal plains to be difficult to flood combined with 30 day rainfall amounts being so minimal...think this will be a high-end Slight Risk although some hydrologic concerns beyond urban and small stream flooding could occur in areas should some of the higher amounts from the models materializes. ....Northeast/Northern Mid-Atlantic... Moderate to occasionally heavy rainfall is expected over parts of the Northeast and northern Mid-Atlantic region given widespread synoptic scale forcing with embedded higher amounts due to mesoscale enhancements from frontogenesis/TROWAL ahead of the deepening mid-level cyclone. As was the case farther to the south...increasing distance from the core of the higher theta-E air and reduced instability relative to the core of the digging upper-low to the south should limit rates. But However, favorable orographic enhancement along terrain of eastern Pennsylvania/New Jersey as well as southern New York very late will allow for some 2.5 to 3.5 inch amounts mainly in that favored southeast exposed terrain. So little adjustments were required at this time. ....Northwest California... Removed the Marginal Risk area across northern California given an overall slowing of the system. Areal average deterministic rainfall amounts are generally at or below an inch and the latest HREF guidance shows any risk of 0.5 inch of rainfall in an hour now remaining off-shore through 12Z Monday.=20 Bann Day 2 Valid 12Z Mon Dec 18 2023 - 12Z Tue Dec 19 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE NORTHEAST USA... 21Z update... The majority of the guidance has areal averages of 1 to 4 inches spreading across New England during this period. The higher end of that range will likely focus over/ear the higher terrain of northern New England. A Slight Risk area was already in effect for this part of the region but saw a minor expansion southwest to cover more of western Massachusetts and some trimming out parts of southeast New Hampshire and far southern Maine. Additionally, less QPF over parts of northern New Jersey and eastern Pennsylvania supported the reduction of the Marginal Risk area on the southwest side. Campbell The strength of low level flow from the Western Atlantic combined with deep layer moisture remain off the scale in terms of statistical anomalies (7-9 standard anomaly compared to CFSR), with 1250+ kg/m/s IVT values. Heavy rainfall amounts will result from the combined effect of nearly orthogonal intersection that moisture plume and the northern Appalachians. Eastward propagation of the warm conveyor belt will be fairly quick with diminishing moisture values as the low's connection to the sub-tropical source region severs limiting overall rainfall totals. Overall, 3-5" of rain is forecast near the ridge line with 1-2" closer to the coasts of ME. As such, will maintain the broad Slight Risk across the White Mtns into much of the lower two-thirds of Maine. ....Northern California... 21Z update... No major changes to the QPF for this area. Very minor reshaping of the Marginal Risk area was made to account for some areas that may be snow instead of rain. Campbell An increase in coverage of rainfall is expected during the day as a deep layered low pressure system over the eastern North Pacific inches towards the north and east. Deeper moisture will overspread parts of northern California in the wake of a cold front...with precipitable water values around 1.25 inches nudges farther inland from the Bay area into the north end of the Great Valley. With the 17/00Z HREF showing increasing areal coverage of 10-15 percent probabilities of 0.5 inches of rain in an hour...introduced a Marginal risk area that largely paralleled the WPC deterministic QPF. Bann Day 3 Valid 12Z Tue Dec 19 2023 - 12Z Wed Dec 20 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL MAINLY FOCUSED OVER NORTHERN CALIFORNIA... 21Z update... Onshore flow resulting in moderate to heavy rainfall across northern California is still expected. Amounts did come down a little across southern portions of the Sierra Nevada Range and along the central California coastline. With this trend in mind the Marginal Risk area was cut back over these locations, trimmed more of the Valley out and trimmed a few snow areas out as well along the northern boundary of the Marginal Risk area. Campbell Placed a Marginal Risk area in a similar position as the Day 2 Marginal Risk area as one area of low pressure moves inland and a second area of low pressure drops southward over the eastern North Pacific later in the period...keeping a fetch of moisture over northern California. Still some uncertainty given model spread on the placement of the second low by Tuesday evening...it could remain far enough off-shore that only light amounts of rain occurs. However...there seemed to be enough consensus to account for a Marginal Risk area from the effects of both lows. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7ll-e0Ja8M3dFTVHGONTvh7K2k-Y3TdTeFwXTyHNmI49= eqh2s4oN9I2KvwmKu6HGL5fPBDurVvpRimLJplPMEjYw3-w$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7ll-e0Ja8M3dFTVHGONTvh7K2k-Y3TdTeFwXTyHNmI49= eqh2s4oN9I2KvwmKu6HGL5fPBDurVvpRimLJplPM1cIyj5I$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7ll-e0Ja8M3dFTVHGONTvh7K2k-Y3TdTeFwXTyHNmI49= eqh2s4oN9I2KvwmKu6HGL5fPBDurVvpRimLJplPMokvqRB8$=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. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .