Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Dec 18 2023 16:01:29 FOUS30 KWBC 181601 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1101 AM EST Mon Dec 18 2023 Day 1 Valid 16Z Mon Dec 18 2023 - 12Z Tue Dec 19 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE NORTHEAST... ....16 UTC Update... Upgraded portions of western Massachusetts, southwest-central New Hampshire and central-northern 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 stronger closed mid/upper level will approach the northern California during the period, putting much of the northern California area in a favored region for forcing for ascent and result in an unsettled/active period of widespread precipitation. 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 higher, which may result in some excessive runoff/flooding concerns there as well. Otherwise, general poor drainage and nuisance flooding is expected and overall not a lot of changes were made to the Marginal Risk this cycle. Taylor Day 3 Valid 12Z Wed Dec 20 2023 - 12Z Thu Dec 21 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF COASTAL CENTRAL AND SOUTHERN CALIFORNIA... The closed mid/upper level low is expected to slide south/southeast just offshore the California coast during the period. While the 00Z guidance still shows timing and placement differences/spread with this feature, this is 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 (500-700 kg/m/s) per CW3E, particularly for the southern areas of the central CA coast where the orientation of the IVT plume are most perpendicular and would lead to the greatest orographic ascent. The 00Z guidance QPF trended upwards and show broad 2-3" 24-hr totals across those favored areas and 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. This may result in some areas of flooding, particularly for any sensitive/vulnerable location and urban areas. Taylor Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7_e7osgqTB3ORUMeP1oLWmC5pp-9IyOfnE8FksHYEywL= MlT7hj6V-k2ugzxXXl0a7Katj4bS4N1tjpWbCrppHnCZI5c$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7_e7osgqTB3ORUMeP1oLWmC5pp-9IyOfnE8FksHYEywL= MlT7hj6V-k2ugzxXXl0a7Katj4bS4N1tjpWbCrppEzeF06c$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7_e7osgqTB3ORUMeP1oLWmC5pp-9IyOfnE8FksHYEywL= MlT7hj6V-k2ugzxXXl0a7Katj4bS4N1tjpWbCrppcCoWA4c$=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) .