Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Dec 18 2023 20:25:06 FOUS30 KWBC 182025 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 324 PM EST Mon Dec 18 2023 Day 1 Valid 2014Z Mon Dec 18 2023 - 12Z Tue Dec 19 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE NORTHEAST... ....20 UTC Update... Just a cosmetic update to remove the southern and western portions of the Marginal and Slight Risk areas where heavy rainfall is no longer a threat based on the latest observational trends. Hurley Previous discussion below... ....16 UTC Update... Upgraded portions of New Hampshire and Maine to a moderate risk with ongoing rainfall, flash flood reports, and an additional 3-5 inches of rain (with locally higher amounts) before the rain comes to an end later this afternoon/evening. Elsewhere, the southern edges of the current slight and marginal risks were trimmed where rainfall has already ended or with little expected additional rainfall. Out West, there were no major changes made to the current marginal risk across portions of northern California and the Central-Southern coastline. See the previous discussion below for additional details on the setup. Santorelli ....Previous Discussion... ....Northeast... A negatively tilted upper trough over the Mid-Atlantic will phase with an anomalously deep and strengthening shortwave over the Upper Midwest this morning, resulting in the already strengthening surface low currently over eastern North Carolina to rapidly deepen and quickly lift northward through the Northeast today. A very impressive plume of moisture, characterized by PW values approaching 1.8" or between 3-4 standard deviations above normal, will brush the coastal Mid-Atlantic early this morning and then advance through the Northeast, particularly the coastal areas through this evening. Supporting this impressive surge of moisture is very strong low level flow, with 850 mb winds 60-70+ kts at times along the coast. Widespread moderate to heavy rainfall will continue to stream northward, overrunning a warm front currently draped along the coastal areas from eastern VA through southern ME. As the primary surface low lifts northward, a narrow axis of marginal but sufficient instability is expected to develop along the coastal areas of the northern Mid-Atlantic, southern NY, and southern New England, helping to enhance rain rates for a period of time early in the period across portions of NJ through southern NY, CT, and western MA and this is where the highest probabilities of seeing intense rain rates approaching/exceeding 1"/hr exists. The 00Z HREF neighborhood probabilities reach 20-25 percent for hourly totals greater than 1" this morning and the probability of 6-hr totals greater than 3" tops out at 60-70 percent this morning as well. The combination of the prolonged rainfall and narrow window of higher rain rates is driving the flooding concern over the very urbanized corridor from northern NJ, southern NY, and portions of CT/western MA where event totals may exceed 5" locally (15-30 percent chance per 00Z HREF) and the Slight Risk area was expanded southward to include these areas for the possibility for flooding and is being messaged toward the higher end of the Slight Risk category range (25-40%). Some of the flooding may be locally significant due to the already saturated ground in and around urban areas. Further north into New England, while the lack of instability will keep rain rates in check, the widespread/prolonged rainfall along with the very strong/impressive forcing/lift will drive higher rainfall totals for south/southeast facing terrain in NH/ME along with the coastal areas in Maine. The Slight Risk area here was largely unchanged from previous cycles for the possibility of scattered instances of flooding. ....Coastal Central and Northern California... A closed upper low currently analyzed well offshore of California will approach the western U.S. today, opening up as it nears the California coast by this evening. A modest plume of moisture associated with it will be directed to and brush the coastal areas through the period, with 850 mb flow out of the south-southwest to southwest between 30-40 kts and PW values generally between 1-1.5". Enough forcing for ascent and low level convergence along advancing frontal boundary will support unsettled and numerous showers through the period. HREF neighborhood probabilities for 0.5" in 1-hr peak this morning at 30-40 percent along the coastal areas then generally fall to 10 percent or less through the rest of the period as the system weakens/opens up as it moves onshore. Expect areal average totals in the period of 1-2" along the coastal areas and northern Sierra while the lack of any strong signal for intense rain rates should keep flooding concerns mainly toward sensitive/vulnerable areas and poor drainage urban locations. Taylor Day 2 Valid 12Z Tue Dec 19 2023 - 12Z Wed Dec 20 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL MAINLY FOCUSED OVER NORTHERN CALIFORNIA... A second and stronger closed mid/upper level low will approach northern California during the period, bringing an unsettled/active period of widespread precipitation this this area. As the low approaches, fairly modest values of IVT will mainly brush the coastal areas where precip rates on the favored western facing terrain may approach 0.3-0.5" hourly totals. In the northern Sierra, snow levels will be relatively high, which may result in some excessive runoff/flooding concerns there as well due to snowmelt. Otherwise, general poor drainage and nuisance flooding is expected and overall and a Marginal Risk remains on today's update across parts of northern California as well as southward down the central California coast. Santorelli/Taylor Day 3 Valid 12Z Wed Dec 20 2023 - 12Z Thu Dec 21 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF SOUTHERN CALIFORNIA... The closed mid/upper level low is expected to slide southward just offshore the California coast during the period. Despite some uncertainty still in the details of timing and placement of this feature, there is a growing signal that its proximity to the coast and an increasing plume of higher moisture transport will nose toward the central/southern coastal areas of California. The latest IVT values show modest values (~500 kg/m/s) per CW3E, with moisture anomalies nearing the 90th percentile and some instability. The 12Z guidance QPF continues to suggest broad 2-3" 24-hr totals across parts of the central and southern California coast, with locally higher amounts near the Transverse Ranges where the orientation of the IV plume are most perpendicular and would lead to the greatest orographic ascent. Given the synoptic setup and expected ingredients, hourly totals may approach or exceed 0.5", particularly the latter half of the period into Thursday morning, resulting in some areas of flooding, particularly for any sensitive/vulnerable locations and urban areas. Per collaboration with WFO LOX and consistent with an increase in this cycle of the WPC QPF, opted to include a Slight Risk on this afternoon's ERO update for the Transverse Ranges, just north of Los Angeles. A marginal risk continues northward up the coast to San Francisco and southward toward San Diego. Santorelli/Taylor Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!65cfhQbVeVroAfkJGe_95foQWZT47Kq-_TCVel7f19xp= z1d8noW6jSwk4AXuKOt1WYCOwORFimscITBpJlKp99GDM8o$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!65cfhQbVeVroAfkJGe_95foQWZT47Kq-_TCVel7f19xp= z1d8noW6jSwk4AXuKOt1WYCOwORFimscITBpJlKpYdThLmA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!65cfhQbVeVroAfkJGe_95foQWZT47Kq-_TCVel7f19xp= z1d8noW6jSwk4AXuKOt1WYCOwORFimscITBpJlKp-Lfek_g$=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) .