376 FOUS30 KWBC 262043 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 442 PM EDT Mon Jul 26 2021 Day 1 Valid 16Z Mon Jul 26 2021 - 12Z Tue Jul 27 2021 ...THERE A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF SOUTHWEST AND SOUTHEAST... ...Southwest/Central Rockies/Southern Rockies... A weakening mid level feature will continue to track west and eventually northwest with its energy becoming somewhat sheared. However, given the anomalously high moisture within the column and thermal profile, it will not take much lift to produce convection and thus heavy rain across portions of Southeastern CA, Southern NV into far Southwestern UT as seen in CA this morning. Observed precipitable water values are as high as 2.20+ inches in western AZ and anticipate this to advect to the west through tonight. This is over 3 standard deviations above the mean and with enough mid-level ascent and building instability today, expect convective coverage to blossom again in the afternoon with rain rates ranging between 1-2 inches/hour. Areal average precipitation will range between 1-2.5+ with locally higher amounts possible. This type of rain over pockets of wet antecedent conditions, complex terrain (including slot canyons and washes), and residual burn areas could cause more scattered flash flooding, some of which could result in significant impacts. The Slight Risk area was maintained. Farther to the east, anticipate convection to develop along the terrain with differential heating as breaks in the clouds occur through the morning hours. More scattered precipitation activity is anticipate under lower precipitable water values. Thus, expect rain rates to also be less, around 0.5-1+ inches/hour. Therefore, isolated flash flooding is possible across more vulnerable soils. ...Lower MS Valley/TN Valley/Southeast... The residual surface boundary will slowly sink south toward the Southeast and Deep South helping to promote convection along and south of the front, especially focused across portions of NC/VA where better 700mb lift resides associated with the sharpened trough axis. Precipitable water values will surge to over 2 inches aided by moist southerly flow. In addition, instability will also climb above 2000 J/kg through the afternoon, aiding in the production of scattered to widespread convection. With better dynamical forcing (also noted by enhanced divergence within the right entrance region of the upper jet), anticipate more precipitation coverage across the Southern Appalachians northeast into the Piedmont of NC/VA and southeast VA. Given the warm cloud depths within the entire risk area (both Marginal/Slight), efficient rain rates are expected with hourly totals surpassing 2 inches in some locations. Storm motions will be quite nebulous across the Lower MS/TN Valleys. However, slightly higher storm motions of 10+ knots with the potential for cell mergers should occur across NC/VA and may lead to higher rainfall totals overall. 12Z HREF continues to show moderate to high neighborhood probabilities for accumulations of 3-inches or more within the Slight Risk area, including the the Richmond and Hampton Roads regions. While antecedent soil conditions vary quite a bit within this region, the Slight Risk area captures the location where more organized convection will likely occur over soils that may be more susceptible to flash flooding. Meanwhile, very saturated soils exist across much of GA/AL/MS, though convection may be more scattered/less organized. There is a modest signal for more organized convection across southern TN/northern AL through the afternoon given a weak impulse aloft, but confidence on this remains too low to hoist a Slight Risk. Bumped the Marginal Risk north a bit over the Delmarva Peninsula due to increasing instability this evening with lingering moisture. ...Upper Great Lakes... A mid-level impulse currently over southern Saskatchewan will shift southeast along the northern periphery of the ridge and across the Upper Great Lakes tonight. Ahead of this wave, southerly flow will increase allowing precipitable water values to surge to over 1.50 inches. Instability will rise above 1000 J/kg through tonight which should support thunderstorms capable of dropping 2 inches per hour rainfall. Anticipate scattered convection to develop in the late afternoon over northern MN to become better organized as shear improves. With the mean wind aligning with the cold front, expect convection to potentially train across some locations, especially as the nocturnal low level jet strengthens on the nose of the 850mb front around northern Lake Michigan. Therefore, expanded the Marginal Risk east from northern WI and the UP of MI to across the northern LP of MI. ...Coastal Southeast... Convection associated with a low moving inland along the FL/GA line will allow local rainfall of 2-4+ inches across northeast FL toward coastal GA. More isolated to scattered convection will surge inland with onshore moist flow into far southern SC through the afternoon. Rain rates associated with this system will range between 1-2+ inches. Maintained the Marginal Risk for isolated flash flooding potential. ...Florida Peninsula... A surface wave along the FL/GA line this morning, supplies a tropical airmass across much of the region through the forecast period. In addition, a ridge to the Southeast will continue to feed moist southerly flow into the FL Peninsula. With precipitable water values above 2.25 inches, diurnal instability, and sea breeze convergence, expected scattered to widespread convection to develop this afternoon and linger through the evening hours. Given weak steering flow and the potential for converging sea breezes, conditions look favorable for heavy rain to develop inland or across the western peninsula (given a strong Atlantic sea breeze could pin against the GOM sea breeze). Hourly rainfall totals will range from 1.5-2.5+ inches. This could become problematic, especially over the urban corridors.Therefore, a Marginal Risk area was maintained for localized flash flooding across much of the peninsula. Pagano/Jackson Day 2 Valid 12Z Tue Jul 27 2021 - 12Z Wed Jul 28 2021 ...THERE IS A SLIGHT RISK OVER A PORTION OF THE SOUTHEAST... ...Southeast... With a surface low tracking along a surface boundary that stretches from central GA up the Carolina Coast, raised a Slight Risk for southern GA and southern SC (up to Charleston). Widespread convection is expected with precipitable water values exceeding 2.25 inches, which is 2.5 standard deviations above the mean. With low level convergence from a front coming from the north and light westerly deep layer mean flow, expect heavy activity to be slow moving and potentially repeating. With hourly rainfall locally exceeding 2 to 3 inches in the enhanced diurnal convection, decided a Slight Risk was warranted. There is an isolated flash flood risk this convergent axis, from southeast VA to east TX, so the Marginal Risk was expanded as well. ...Great Basin through the Southern Rockies... Monsoonal moisture will bring the next round of showers and thunderstorms across the Intermountain West. Areas west of the ridge axis will continue to see well above normal moisture, particularly over the Great Basin and over far north CA into central and eastern OR where precipitable water values of 1" to 1.25" are 2 to 3 standard deviations above normal. Largely maintained the expansive Marginal Risk over terrain from central OR to southern AZ by way of UT in accordance with the 12Z consensus. ...Northeast... A northern stream trough will pivot through the region Tuesday night with an associated cold front becoming oriented northeast to southwest opening the window for training convection. Ahead of the trough axis, southerly flow will increase ushering in higher precipitable water values which will exceed 1.75 inches aided by 35+ knot 850mb west-southwesterly winds. This low level flow will become aligned with the frontal zone allowing for multiple rounds and/or training convection over any given location. With instability climbing above 1000 J/kg and enough mid/upper level lift, convective coverage should be widespread. With the available moisture, instability, and large scale lift, expect hourly rain rates to exceed 1.5 inches in some locations. Due the progressive nature of the activity, only areas of repeating convection should have excessive rain, so the flash flood risk should be isolated. The Marginal Risk area was maintained and expanded a bit to include all of the eastern Great Lakes. Pagano/Jackson Day 3 Valid 12Z Wed Jul 28 2021 - 12Z Thu Jul 29 2021 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE GREAT BASIN INTO THE INTERMOUNTAIN WEST AS WELL AS THE UPPER GREAT LAKES... ...Great Basin/Intermountain West... Monsoonal moisture continues to surge north across the region with weak mid-level impulses helping to focus convection farther north than has been observed in weeks. This becomes no more evident than by the precipitable water values remaining above 1.25 inches in some locations under broad southerly low level flow. With plenty of diurnal heating, anticipate MUCAPE to climb above 1000 J/kg allowing for differential heating across the terrain. Therefore, afternoon convection will largely be tied to the mountains and eventually shift off into the valleys/deserts through the late afternoon/evening. Hourly rain rates could reach between 0.5-1 inches, though isolated higher amounts are possible, especially with slow moving convection. Given the complex terrain, wet antecedent conditions across the Southwest/Central-Southern Rockies, and burn scars, localized flash flooding could occur. Therefore, the Marginal Risk maintained. ...Upper Midwest/Great Lakes... The next shortwave trough will cross the region through the forecast period with enough synoptic scale lift, moisture, and instability to promote widespread organized convection capable of producing localized flash flooding. Ahead of this trough, deep layer moisture will surge north on strong return flow. In addition, instability will climb well above 3000 J/kg through the afternoon hours across the Midwest into the Upper Great Lakes region. This will prime the region ahead of the strong upper level forcing for ascent and surface cold front. Anticipate scattered convection in the afternoon across MN/WI (and perhaps even as far west as the Dakotas) to become better organized overnight as the nocturnal low level jet strengthens. Most of the uncertainty is related to the existing boundaries from mesoscale convection that will occur over the region tonight and Tuesday. Given the northern half of WI has observed 200% of normal precipitation over the past week with another shortwave expected to move through today/tonight, the soils within this region will likely be more sensitive to additional rainfall. Therefore, the Marginal Risk was maintained. ...Southeast... A boundary should stall near the FL/GA line extending west to the central Gulf Coast. Convergence of tropical air along this boundary should allow heavy enough rainfall to locally exceed the currently high flash flood guidance. Therefore, a Marginal Risk was raised at this time. Pagano/Jackson Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt $$
