Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Oct 18 2024 00:44:23 FOUS30 KWBC 180044 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 844 PM EDT Thu Oct 17 2024 Day 1 Valid 01Z Fri Oct 18 2024 - 12Z Fri Oct 18 2024 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Hurley Day 2 Valid 12Z Fri Oct 18 2024 - 12Z Sat Oct 19 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE FOUR CORNERS REGION THROUGH NORTHERN NEW MEXICO... ....2030Z Update... No changes were made to the Marginal Risk area in the forecast. With the synoptic setup described below largely unchanged, the only detail to add and one of the larger points of uncertainty will be snow levels...as lower snow levels would mean a greater amount of the total precipitation falls as snow and thus does not contribute to flooding. Overall this will be the case further north for much of UT and CO. Meanwhile for southern AZ and NM lesser amounts of total rainfall are expected...thus the narrow area of northern NM and the 4 Corners remains the corridor where there is both adequate forcing and temperatures are warm enough that when added to the preponderance of flash flood prone areas such as slot canyons, burn scars, and any more developed and low-lying areas...isolated flash flooding remains possible. Wegman ....Previous Discussion... Longwave pattern will continue evolving into a fairly prolific upper trough/closed-low over the Great Basin, slowly migrating eastward with sights on the Four Corners. Large scale ascent will become maximized downstream of the mean trough with a pronounced diffluent pattern in-of the Southern Rockies, including the Sangre de Cristo and San Juan mountain ranges where heavier precip is forecast. A cold front will slowly propagate eastward from the primary disturbance with a return flow setup pressing westward across the Eastern NM plains on the western flank of a sprawling surface high east of the Mississippi. A secondary cold front to the north will plunge southward thanks to a strong ridge of high pressure developing upstream in wake of the primary western low, along with a lee side trough developing over the Front Range. The tandem of boundaries and backed flow will converge over Northern NM with a strong isentropic ascent pattern initiating later Friday evening through Saturday AM and beyond. There has been very little deviation in the guidance to amass much of a change compared to the previous forecast, so a general continuity was maintained from the inherited MRGL risk across the Four Corners into Northern NM. Ensemble guidance is in relative agreement on the placement of two maxima during the forecast period; one located Northeast AZ into Southeast UT with the initial diffluent signature ahead of the evolving upper low over the Great Basin, and the secondary maxima across Northeastern NM where the convergence pattern aided by increasing large scale ascent will generate a period of heavier precip Friday evening into Saturday morning and beyond. Each areal maxima is similar with regards to precip magnitude with a general neighborhood 1-2" max with a small chance at upwards of 2.5" across the western maxima within the best upper ascent within the nose of the 100kt 250mb jet streak. The secondary maxima is generally within the last 12-hr window encompassing 00-12z Saturday with the heaviest QPF footprint actually highest in D3 (More on that in the D3 period below). In any case, the very dry antecedent conditions, slot canyons, and flashier river basins within the Four Corners area will offer a greater risk of flash flooding with a secondary threat over the remnant burn scars in Northern NM. This allowed for a maintenance of the previous MRGL risk. Kleebauer Day 3 Valid 12Z Sat Oct 19 2024 - 12Z Sun Oct 20 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHERN HIGH PLAINS INTO SOUTHEASTERN COLORADO... ....2030Z Update... ....ERO Changes... The Slight Risk area was nominally expanded east to include the westernmost counties in the Texas Panhandle, the far western Oklahoma Panhandle and a few miles into extreme southwestern Kansas with this update. The surrounding Marginal was expanded west to include the mountains north of Santa Fe, NM and much of the Texas and Oklahoma Panhandles, as well as far western Kansas. A new Marginal Risk area was introduced for the Olympic Peninsula as well as the far northern Cascades of northwestern Washington State. ....Southern Plains... Forecast rainfall continues to slowly increase across the southern Plains this afternoon. A nearly stationary upper low will begin to tap into increasing Gulf moisture associated with a well established LLJ across far western TX/OK/ and north up much of the Plains overnight tonight. By Day 2/Friday the moisture plume will be well established. The upper level low ever-so-slowly moving east will gradually tap into more and more of that moisture. Given the steering flow changes will be slow, the excellent forcing east of the upper level low and increasing instability will all favor the development of strong and few severe thunderstorms within the moisture and instability axis. These storms will be capable of very heavy rainfall over this mostly parched/arid area. Due to the LLJ the storms will be pretty fast-moving towards the NNE. However, the slow moving upper low and constant resupply of Gulf moisture will strongly favor training cells across the Slight Risk area, particularly NM. Thus, it's likely that training storms will develop and move over the same hard-hit areas causing widely scattered flash flooding. Finally, the 12Z HRRR suggests a line of storms will follow these individual training cells overnight Friday night. The more widespread nature of the heavy rain associated with the line is what's likely to push the more flood prone areas to begin to flood. The latest guidance suggests the area near the NM/OK border at greatest risk for the most rain, and therefore the highest threat for flooding. ....Northwestern Washington State... A cold front/atmospheric river impacting Vancouver Island and portions of British Columbia just north of Washington on Friday will push south into northwestern Washington on Saturday. 1-3 inches of rain are possible in the Olympics and in the Cascades east of Bellingham Friday, so that by the Day 3/Saturday period, the full force of the A.R. will move into WA, resulting in local rainfall totals of 3-5 inches for the day. Integrated Vapor Transport values at the peak could exceed 900 kg/m/s, which is a bit stronger than the typical A.R., but nothing major. When added to Friday's rain totals, this will likely be enough rain to result in isolated flash flooding. Even if the highest elevations see snow, the large majority of the rainfall into the mountains will be in the form of rain. The usual rain shadowing from westerly or WSW flow should keep much of Seattle and its surrounding suburbs with significantly less (generally under an inch) of rain, so it was excluded from the Marginal Risk area. Further evaluation of the urban areas may be needed with future forecast updates. Wegman ....Previous Discussion... Closed upper circulation will slowly migrate eastward with a persistent diffluent channel focused in-of the Eastern NM High Plains up into the Colorado Front Range for Saturday. A uni- directional flow located within the lowest 600mb will persistent through much, if not all the forecast period allowing for moist unstable air to be pulled poleward into the confines of the Caprock and points north. ECENS PWAT anomalies are forecast to run upwards of 3-3.5 deviations above normal with an M-Climate Percentile (Forecasts relative to the last 20 years during time of year) that is peeking into the seasonal max, a testament to the anomalous nature of the moisture advection regime. With the poleward advancement of the airmass, regional theta-E's spike with a theta-E ridge positioning located right over the NM/TX border running clear into Front Range of CO and nearby KS. This signal is consistent with a persistent band of rainfall likely to be situated within the theta-E ridge mentioned above and just out ahead of the slow-moving quasi-stationary front trailing the very slow moving to perhaps stationary low pressure across Southern CO. Precip totals of 2-4" with locally higher amounts are increasingly likely within that corridor between Clovis up through Southeastern CO with the max likely driven by repeating convection as small mid-level perturbations rotate around the general upper circulation and move overhead of the areas above. The only saving grace for a more significant flash flood potential is the antecedent conditions leading in are very dry with the 0-10cm relative soil moisture percentiles running between 5-20% across the area anticipated to see the most rainfall. Initial rains will be beneficial, but will add through the course of the period with waves of heavier rain likely exacerbating areas by later in the period into early Sunday. The previous SLGT risk was generally maintained with a MRGL surrounding. Both risks were expanded somewhat on the southern and northern peripheries to account for a trend in an expansion of precip latitudinally in either direction. The heaviest will likely encompass Eastern NM into Southeast CO with these areas likely trending towards the higher end of SLGT risk if the signal remains and/or amplifies further. Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_MooXl8b_o_mkbw_HnKwk_wHMy_Eiuk_9LS1tztI1I3Y= r9tcijcq7FycZFpi81VA1QjaGabvBPmto3Ev4jRz4n2_z5M$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_MooXl8b_o_mkbw_HnKwk_wHMy_Eiuk_9LS1tztI1I3Y= r9tcijcq7FycZFpi81VA1QjaGabvBPmto3Ev4jRz0fai3h0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_MooXl8b_o_mkbw_HnKwk_wHMy_Eiuk_9LS1tztI1I3Y= r9tcijcq7FycZFpi81VA1QjaGabvBPmto3Ev4jRzzYLjOjM$=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. 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