Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Dec 27 2023 20:04:47 FOUS30 KWBC 272002 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 302 PM EST Wed Dec 27 2023 Day 1 Valid 16Z Wed Dec 27 2023 - 12Z Thu Dec 28 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE MID ATLANTIC... ....16Z Update... A Slight Risk was added to portions of the Mid Atlantic encompassing the northern Delmarva, far northeastern MD, Southeast PA, most of NJ, and New York City based of the latest signals from the 12z CAMs and associated HREF. The main driver was the combination of lower FFGs and anticipated 3 and 6 hr rainfall totals in conjunction with FFG exceedance probabilities of over 30% to as high as 70% within the aforementioned corridor. Current radar and satellite composite continue to show a decent moisture plume advecting north along the east coast on the eastern edge of a broad trough centered over the central US. Latest CAMs are in agreement on both the timing and proposed magnitude of expected rainfall through the period with the time frame of 22-09Z being the best potential for organized heavy rainfall across the central and northern Mid Atlantic, especially along and east of the I-95 corridor. Surface low off the east coast will continue to propagate north-northeast from the Carolinas to off the NJ coast by tomorrow morning. Some low-end MUCAPE signatures from the latest hi-res (250-500 J/kg) are just enough of a signal to aid in locally enhanced rainfall across the urban corridor in the outlined area. The organized rainfall stems from a strong PVA pattern moving through the area on the eastern flank of our upper trough. A well-positioned jet max to the south of the region will provide great upper support within the left exit region of the jet which guidance has keyed on for that above time frame later this afternoon and evening. Considering all of the variables both in a synoptic and mesoscale sense, plus the antecedent wet grounds and subsequent lower FFG's focused over the area forecast for 1-2" and locally higher, have went with a SLGT risk to cover for the potential. This is also in agreement with the local WFO's after collaboration. Kleebauer ....Mid Atlantic... The latest guidance continues to depict increasing moisture advection within the lower levels through Wednesday evening before slowly drying as our surface low scoots northeastward. PW values up to 1 inch will be transporting onshore across the Mid-Atlantic and Northeast; which will be +2 standard deviations for late December. There's also plenty of ascent within the left exit region of a powerful jet off to the southwest thanks to a broad, closed upper low centered over the Midwest. Strong mid-level vorticity advection is expected to track overhead during the afternoon and evening allowing waves of moderate to heavy rainfall to impact the urban corridor of the Mid-Atlantic. Much of this region has increased soil saturation from recent rains that are more sensitive to additional rainfall, thus having lower thresholds for flash flooding concerns. Consensus persists with a swath of 1 to 2 inches for the highest amounts to focus over eastern Pennsylvania and parts of New Jersey. Most of the guidance suggests 1 to 2 inches for this part of the country. The latest guidance and WPC QPF showed increases in amounts over the Del-Mar-VA region. Locations along and northeast of I-95 from Baltimore up through Philadelphia into Northern NJ still are favored for the highest accumulations. The Marginal Risk was maintained, albeit with a few adjustments to reflect the latest trends. As such, The Marginal Risk area was extended southward into east-central Virginia while minor reshaping of the western and northern bounds were made to reflect the latest trends. Campbell Day 2 Valid 12Z Thu Dec 28 2023 - 12Z Fri Dec 29 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Campbell/Kleebauer Day 3 Valid 12Z Fri Dec 29 2023 - 12Z Sat Dec 30 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR NORTHERN CALIFORNIA AND SOUTHWEST OREGON... ....20Z Update... Overall pattern remains steadfast from the previous forecast issuance across northern CA and southwest OR. Atmospheric river will transpire on Friday afternoon into Saturday morning before waning in intensity and shifting focus southward. NBM neighborhood probability of at least 2" is fairly robust across the northern CA coast (70+%), with the highest totals generally between Eureka and San Francisco. IVT signature is more robust with this next wave of precip impacting the area, as well as a further penetration inland, so totals this juncture should be more conducive to flooding concerns within the area outlined, but still a lower-end risk in the grand scheme. Will keep an eye on the trends for areas further south towards Monterey as some guidance has increased totals within the coastal plain south of San Jose which may necessitate an extension of the MRGL further south. Kleebauer ....Previous Discussion... The next front and surge of Pacific moisture is expected to approach the coast during this period spreading another round of rainfall over northern California and southwest Oregon. Most of the guidance depict areal averages of 1 to 3 inches along the coast, with the higher totals primarily between Eureka and San Francisco. The threat for excessive rainfall will continue to be elevated from recent rains therefore the Marginal Risk area was maintained. Minor expansions were made to the north and south. Campbell Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8jPLlLnqH4i9VQAZYjYMXbYHOcz3YA3n_75il69LT61n= Ze-sCuX263M0fsCoZWJfX6SNwH1vNSOiWFlphNwqtJFlhuc$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8jPLlLnqH4i9VQAZYjYMXbYHOcz3YA3n_75il69LT61n= Ze-sCuX263M0fsCoZWJfX6SNwH1vNSOiWFlphNwqEA--QwI$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8jPLlLnqH4i9VQAZYjYMXbYHOcz3YA3n_75il69LT61n= Ze-sCuX263M0fsCoZWJfX6SNwH1vNSOiWFlphNwqgzXWTiM$=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) .