Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Sun Dec 17 2023 10:18:00 FOUS30 KWBC 170830 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 329 AM EST Sun Dec 17 2023 Day 1 Valid 12Z 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... ....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. 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... 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 2/3rds of Maine. ....Northern California... 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 shwoing 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... 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 $$ --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .