Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Sep 11 2023 08:30:48 FOUS30 KWBC 110830 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 430 AM EDT Mon Sep 11 2023 Day 1 Valid 12Z Mon Sep 11 2023 - 12Z Tue Sep 12 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE NORTHEAST AND WEST-CENTRAL TEXAS ... ....Southern Plains into the Southern-Central Rockies... A sharp cold front moving through the southern plains will intersect a developing LLJ and stout theta-e ridge extending up into the TX Caprock. The CAM guidance continues to be in agreement on the initiation of a cluster of thunderstorms across portions of west TX, propagating southeast around the eastern fringes of a mid-level ridge centered over the southwestern U.S. The convergence of the frontal boundary and axis of instability created from the LLJ will lead to an uptick in areal convective coverage with enhancement from a mid-level vorticity maxima sliding southeast out of NM. 00Z HREF guidance continues to show a strong signal for locally heavy rainfall across portions of the Rolling Plains of TX into the Concho Valley and northwest portion of the Hill Country. 00Z HREF neighborhood probabilities 2-3"/3 hrs peak between 60-70% within the Slight Risk area tonight (between 03-09Z). The heavy rain axis being depicted aligns well with a 1-1.5 standard deviation above normal PWAT anomaly that is likely the result of the convergence signature along the cold front pressing south within the LLJ axis being forecasted across all guidance, including global deterministic. Per collaboration with WFOs LUB, MAF, and SJT, have expanded the Slight Risk area from yesterday's Day 2 ERO to encompass more of the Rolling Plains and Permian Basin, again given the guidance trends. Deterministic QPFs continue to indicate a heavy rainfall footprint in this area, perhaps with a little less spread compared to previous runs. The CAMs show the potential for localized totals between 3-6". ....Parts of the Northern Mid Atlantic into New England.... SSW to SW flow within the low-mid layers persists today as shortwave energy pivoting into the Upper Midwest amplifies the longwave trough. This will keep the surface boundary relatively stationary along the Mid Atlantic-Northeastern Seaboard. While a gradual reduction of TPW is expected during the period, sufficient daytime heating should boost mixed-layer CAPEs into at least the 1000-1500 J/Kg range, which given the favorable thermodynamic profile, should allow for more numerous showers/tstms to fire along the surface front during the peak heating hours and continue into at least early evening. Latest (00Z) HREF neighborhood probabilities of at least 1-2"/hr rainfall rates peak between 60-80% between 20-03Z. Given the recent heavy rainfall over the past 24-72hrs, per collaboration with WFOs OKX, PHI, ALY, and GYX have hoisted a Slight Risk area, coinciding with the uptick in deterministic and probabilistic QPFs (especially among the CAMs). This aligns with the lower FFG, and higher 0-40cm depth soil moisture percentiles from NASA SPoRT (80-90+ percentiles). Within the Slight Risk area, the latest HREF probabilities of 3 hourly QPF exceeding 3hr FFGs tops out between 40-60% between 21-03Z. CSU UFVS verified first guess fields also support a Slight Risk within this region. ....Midwest... Strong forcing from a digging longwave trough over MN/WI will approach lower MI on Monday. The northeastern-most extension of the southern Plains LLJ will move into lower MI in advance of a cold front associated with the upper level trough. The LLJ will advect some moisture into MI, which will be lifted by the approach of the trough. The guidance shows an extensive area of light to moderate rain with embedded convective elements will move over Lower MI. Isolated flash flooding is possible, particularly in any urban areas and other low FFG portions of lower MI as a result. A small portion of northwestern Lower MI near Traverse City was hit hard by recent rainfall, and the additional rain expected Monday may allow for additional flash flooding, so the Marginal Risk continues to incorporate this area. ....Florida and Southeast Georgia... Weak upper level forcing on the tail end of the upstream mid-upper trough will provide sufficient deep layer bulk shear (~25kts between 0-6km) for more widespread/organized, clustered convection during the peak heating hours this afternoon, then continuing into the evening. The activity will no doubt be aided by the convergence of Atlantic/Gulf breezes. 00Z HREF probabilities of >2"/hr rainfall rates peak between 40-60% from 20-03Z across (mainly) interior portions of the northern-central FL peninsula. Anticipate isolated or localized instances of flash or urban flooding with these slow-moving cells before quickly becoming downdraft-dominated and weakening. Therefore, have included a Marginal Risk in the Day 1 ERO. Hurley/Kleebauer Day 2 Valid 12Z Tue Sep 12 2023 - 12Z Wed Sep 13 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF NEW MEXICO INTO EASTERN AND CENTRAL PORTIONS OF ARIZONA... ....Four Corners Region... In coordination with ABQ, EPZ, FGZ, PSR, AND TWC, the Slight Risk area was expanded south and westward to include southern NM and eastern-central parts of AZ. Southeasterly low level flow will advect Gulf moisture northwestward out of TX and into NM on Tuesday. Meanwhile a passing longwave trough will move eastward across the Plains, with a weak jet streak developing along the base of the trough. This will contribute broad-scale lift over NM. Further, monsoonal moisture from the Gulf of California will track northeastward into the desert Southwest. The clash of these air masses as well as a stalling out cold front over the Plains will result in widespread showers and thunderstorms across much of the Four Corners region. Further west over UT/AZ, fast storm motions and somewhat drier antecedent conditions should keep any flash flooding isolated. Meanwhile NM is both a bit wetter, and expected to pick up additional rainfall on Day 2/Monday, such that by Tuesday, the rainfall resulting from the aforementioned forcings should result in more widely scattered flash flooding. Flooding is most likely over any burn scars, slot canyons, and in the Sangre de Cristo Mountains of north-central NM. ....Mid Atlantic into the Northeast... Moisture recovery along the coast ahead of another strong cold front at the leading edge of energetic forcing associated with an advancing longwave trough/upper low over the Great Lakes will result in another round of showers and thunderstorms over much of the Mid-Atlantic and Northeast Tuesday. Despite the moisture recovery, it will not reach levels currently present over the area, and with some recovery time from Monday, the chances of flash flooding in the Northeast will be a little bit lower than today and previous days. As a result, the Marginal Risk from yesterday's Day 3 ERO was trimmed considerably. Hurley Day 3 The Day 3 outlook will be updated by 0830Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5vvY0qkamd7M6CIA7yGFyo8EmYVFXMqoEXuxzeB-Sg6A= 8ic_l6l8QnLdG_wy9X8myPoNx_MLJneYX_lMf_V_QsTuz3E$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5vvY0qkamd7M6CIA7yGFyo8EmYVFXMqoEXuxzeB-Sg6A= 8ic_l6l8QnLdG_wy9X8myPoNx_MLJneYX_lMf_V_1lB0NKU$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5vvY0qkamd7M6CIA7yGFyo8EmYVFXMqoEXuxzeB-Sg6A= 8ic_l6l8QnLdG_wy9X8myPoNx_MLJneYX_lMf_V_4FFFgR0$=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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