Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Dec 27 2023 00:58:46 FOUS30 KWBC 270056 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 755 PM EST Tue Dec 26 2023 Day 1 Valid 16Z Tue Dec 26 2023 - 12Z Wed Dec 27 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE BLUE RIDGE MOUNTAINS OF VIRGINIA, NORTH CAROLINA AND SOUTH CAROLINA... ....01Z Update... The Slight Risk in effect for western North Carolina and surrounding areas has been removed for this update. The bulk of the moderate to heavy rainfall has shifted east of the axis of higher precip totals (3-5 inches - extending from Greenville, SC to Marion, NC). Light rainfall continues across most of the discussion area, however, and strong low-level southeasterly flow (around 35 knots) continues to aid in orographic lift against the southern Appalachian terrain to support continued rainfall through the night. The risk of flash flooding continues for the region, but the expectation is that somewhat lighter rain rates (<0.25 inch/hr) can be expected for the remainder of the night except for on an isolated basis. Cook ....16Z Update... The overall synoptic evolution and expected zone of heaviest rainfall remains steadfast from the previous forecast issuance. Currently have a slog of heavy rain across the SC Piedmont up into western NC where low-level convergence signatures from prevailing eastern flow allow for upslope flow and focused ascent within the eastern slopes of the Blue Ridge/Appalachians over the interior Southern Mid-Atlantic and Southeast. Totals of 2-3" are already a commonality across extreme northeast GA into the SC Piedmont where Flood Advisories are in effect for the rise of water along creeks and rivers located upstream of Greenville. With the anticipated heavy rainfall the next 2-4 hrs over the same areas, have brought the SLGT risk down to encompassed the area that's been hit the hardest and still has multiple hours left of heavy rain before the overall footprint shifts northeast with the surface low evolution. Removed the MRGL risk from GA as the precip has lightened or moved out of the area and conditions improve for the remainder of the period. The other addition to the forecast was the expansion of the MRGL risk to the east to include the Midlands of SC up through central NC and south-central VA. The reasoning is two fold as the convective probability over those areas will be highest later this afternoon and evening thanks to a tongue of low-level theta-E advection wrapping north on the eastern flank of the surface low over the southeast leading to better instability axis centered from I-20 in SC up through Raleigh and points west. The prospects for heavy rainfall are highest within the SC Midland's into southern NC where ensemble neighborhood probabilities have jumped for both 1" and 2" totals for the period compared to past runs. This correlates well with the latest deterministic indicating some convective swaths in the QPF field that even when blended into the mean can be seen on the latest 12Z HREF. The HRRR is the most aggressive with this stance, but other CAMs are hinting at local maximums within the Columbia to Charlotte axis, both areas of which are more prone to flash flooding due to urbanization factors. The threat is still not as prolific compared to areas northwest where heavy rain is occurring, but the ingredients are there for isolated FF concerns with convective parameters meeting the criteria, along with plentiful moisture for the time of year. Latest CIPS analogs also peg 12/25/2009 as best 500mb and PWAT comparison for the area highlighted which produced some flash flooding across portions of SC, so there's some historical precedence, but that setup was better overall, so we are not anticipating a repeat, but it shows some premise to the threat. Kleebauer ....Previous Discussion... Scattered to widespread rain will continue across the Appalachians and surrounding area as another surface low will develop along the tightening low-level baroclinic boundary near the Gulf Stream off the coast. The steady rainfall will increase in intensity thus the long duration of the steadiest rain should allow for widely scattered instances of flash flooding on the south/east facing slopes and immediately adjacent foothills. The latest guidance shows an uptick in coverage and amounts expected for parts of the Southeast up through the northern Mid-Atlantic region. The Slight Risk was broadened to include more of southern Virginia, western North Carolina and just over the border of South Carolina where amounts of 1 to 2+ inches with isolated higher amounts are expected to focus. The Marginal remains in place for the tail-end of the rainfall that is largely forecast to occur overnight tonight in the Day 1 period. Finally, the forcing and instability will be weaker further north into VA, so the Marginal remains unchanged up to the mountains adjacent to the Shenandoah Valley. Campbell Day 2 Valid 12Z Wed Dec 27 2023 - 12Z Thu Dec 28 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR NORTHERN CALIFORNIA AND SOUTHWEST OREGON, AS WELL AS FOR THE CENTRAL AND NORTHERN MID-ATLANTIC... ....20Z Update... Minor changes with regards to the MRGL risk over the Mid Atlantic with no changes from the previous issuance across northern CA into southwest OR. For the Mid Atlantic area, the forecast is persistent with multiple factors such as the increasing low to mid-level moisture advection overhead through Wednesday evening before slowly drying as our surface low scoots northeastward. 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. Regional instability will be fairly mundane, but some weak instability signatures along and east of I-95 from DC/Balt up into NYC will be plausible as some deterministic depict a small area of MUCAPE centered over the Delmarva up into NJ before dissipating the back end of the period. The main driver for flash flooding will be the strong mid-level vorticity advection propagating overhead tomorrow afternoon and evening allowing waves of moderate to heavy rainfall to impact the urban centers of the Mid Atlantic megalopolis. Recent rainfall and standard urbanization factors have brought area FFG's down significantly over the past few weeks allowing for lower thresholds to be accesses for flash flooding concerns. 12Z HREF neighborhood probability for 1"/hr or greater is running around 15-20% during the afternoon and evening hours with up to 40% over NYC overnight into Thursday morning. The signature over NYC is more of a product of two pieces of guidance (12Z HRRR and ARW) indicating a, "bullseye" of convection over the southern boroughs of NYC, so while a bit on the higher end, there will need to be a little more consistency in guidance to think about an upgrade. ECENS and GEPS continue to lead the global suite in ensemble probabilities for widespread 1-1.5" QPF during the period with both ensembles indicating 70% or better for areas along and northeast of I-95 from Baltimore up through Philadelphia into Northern NJ. There was some consideration for a SLGT over the aforementioned area, but the lack of higher rates and some guidance still limiting the threat gave pause, so will allow for the current evolution to play out a bit more and factor in any adjustments in the later suite of guidance. The forecast for the low-end AR pattern remains on track across northern CA and southwest OR with very little deviation in precip totals across ensemble guidance and associated deterministic. Main shifts were just noise in QPF maximums, but within the bounds of the MRGL risk area outlined from prior forecast. Kleebauer ....Mid Atlantic... As the systems progresses up the coast it will somewhat weaken and slow while spreading rains over the Mid-Atlantic through the day and reaching southern portions of New England by Wednesday evening/overnight. Despite a developing low moving up the New England coast, the moisture and instability should keep most areas from seeing rainfall amounts before 12Z Thursday at or below an inch, which should preclude flash flooding in this area despite the nearly saturated soils in northern New England. There's good consensus that the heavier rainfall will occur over the Mid-Atlantic from the Catskills/Lower Hudson Valley/NYC south and west. While some uncertainty remains, there's growing consensus 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. Very minimal adjustments were made to the Marginal Risk that was in effect. ....West Coast... Onshore flow paired with an approaching Pacific cold front will usher in scattered to widespread precipitation to the Pacific Northwest and northern California. Recent rains has increased the soil sensitivity thus reducing the amounts required to lead to local flooding concerns. The latest guidance keeps areal averages of 1 to 3 inches for this period. The Marginal Risk was maintained, albeit with a few adjustments to reflect the latest trends. Campbell/Wegman Day 3 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 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6UGPKI568Frm71zRbyuQEj0vHGbGRlzKuH5TIP5fFBNW= 5_LvQvHwO8yysaO3osdCXRTD-zDVvYuPI6TYiriLdeXKvYM$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6UGPKI568Frm71zRbyuQEj0vHGbGRlzKuH5TIP5fFBNW= 5_LvQvHwO8yysaO3osdCXRTD-zDVvYuPI6TYiriLNA8Kw8s$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6UGPKI568Frm71zRbyuQEj0vHGbGRlzKuH5TIP5fFBNW= 5_LvQvHwO8yysaO3osdCXRTD-zDVvYuPI6TYiriLj687eQg$=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) .