Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 857 PM EDT Mon Aug 08 2022 Day 1 Valid 01Z Tue Aug 09 2022 - 12Z Tue Aug 09 2022 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHERN ROCKIES...MID-MISSISSIPPI VALLEY...AND NORTHERN NEW ENGLAND... ...Desert Southwest through the West... An anomalous plume of tropical Pacific moisture rides north from the Desert SW, through the western Great Basin and through central Oregon today/tonight as the upper high center settles over the Four Corners and an upper low off northern CA drifts east. Recent thunderstorm development in the Lower CO Valley appears to be instigated by an MCV moving by the the north. Skies across central and southeast AZ topography is partly cloudy -- debris clouds don't appear to be a hindrance to the usual convective cycle. Diurnal scattered convection should break out this afternoon along the Mogollon Rim, SE AZ, and southern NV terrain and over the peninsular ranges of southern CA and the Sierra Nevada, with convection in the terrain of AZ moving into deserts as low-level inflow is too weak to contain/pin them in the mountains. Ongoing thunderstorms in the deserts could migrate over to the CA Peninsular Ranges with time, moving along with the MCV. ML CAPEs are already 500-1000 J/kg across southwest AZ and southeast CA -- they should peak at over above 2000 J/kg this afternoon. Along with the increase in PW standardized anomalies (+ 2 to +2.5 per the 00Z GEFS), the environment supports peak sub-hourly rainfall rates of 0.50"+ and hourly totals up to 2" underneath any slow moving and merging cells. The monsoonal moisture plume arrives into OR late this afternoon with potential for early evening activity similar to areas farther south along the OR Cascades and western High Desert. ...Southern Rockies... Enough confidence remains in the observational trends and recent CAMs to maintain a low-end Slight Risk across portions of eastern NM. Initially terrain-based diurnal activity propagates slowly under a very light steering flow on the near-east side of the upper high settling near the Four Corners this afternoon. 00Z HREF probs are still likely for 2" -- which could fall as quickly as in an hour -- and since much of this area has 3hr FFG around 1.5", there is a risk for scattered instances of flash flooding this afternoon into this evening. ...Mid-South to Midwest... 0100 UTC Update -- Tweaked the Slight Risk a bit based on the latest guidance trend, including the 18Z HREF exceedance probabilities (including where HREF probs of QPF >3" between 00-12Z are over 40%, along with probs of >2"/hr rates between 25-50%). Previous discussion... The 12z mesoscale guidance has also honed in on the southern extent of a cold front, near a current cloud edge on satellite imagery, moving across the mid-MS Valley where there is risk for a narrow swath of repeating heavy rain to set up this evening. Upper level divergence couldn't be much better as the Westerlies to the north and an upper low slowly retreated over northern AR provide nearly pure divergence aloft. PWs of 2 to 2.25" ahead of the slowing front along with CAPE values of 3000+ J/kg should help with the maintenance and organization of convection and pose a scattered flash flood risk. The St Louis metro area through southern IL - areas that have received excess rainfall over the past week or two, remain in the Slight Risk area. Hourly rain totals to 3" are possible wherever cells move slowly, merge, or train. There has been uncertainty with the position of the cold front in peak heating, so further adjustments are possible. Generally speaking across the Mid-South through the Midwest the frontal activity will be heavy with fairly light steering flow becoming more oriented with the front raising a widespread heavy rain threat. Looped the Marginal Risk area south to near the southwest MS/LA border where it's been quite wet this past week. ...Northeast/Northern New England... Northern portions of the Northeast will greater forcing than the Midwest, with a closer proximity to the mid-level wave and upper jet. PW values of 2 to 2.25" ahead of the front are generally 2.5 to 3 sigma above normal and will be near-record for portions of the Northeast tonight. In addition to ample Gulf-moisture, the source of this plume can be traced back to the Monsoon surge that has been over the Southwest for several days being pulled up and over the ridge and then eastward across the northern tier of the country. The system is fairly progressive, but a corridor of enhanced 850mb moisture transport will be strung out enough to allow for an elongated corridor of increased convergence. This could allow for some west to east training of cells as the system as a whole progresses eastward. A longer duration rainfall with 2-3" totals is expected across portions of northern Maine down to far northern VT/NH. There is limited instability, so rates should generally not exceed 0.5"/hr, but the duration still warrants a Slight Risk which has been honed a bit based on good agreement among the 00Z CAM and global consensus. ...Mid-Atlantic to southern Appalachians... 0100 UTC Update -- Curtailed quite a bit of the Marginal Risk area across the eastern OH and TN Valleys, along with the Appalachians and Mid Atlantic, based on the diminishing deep-layer instability following sunset (negative MUCAPE trends with weak mid level lapse rates). Previous Discussion... Abundant moisture in the airmass ahead of the northern-tier cold front along with light steering flow and moderate instability warrants an isolated flash flood risk from the Atlanta Metro, up the Appalachian chain. While terrain effects look to be a better factor for development than bay/river breeze boundaries, precipitable water values of 1.75"+ being concentrated into the convergence within the lee trough where a cumulus field is already developing. The forecast 850 hPa inflow of 20-25 kts is about double the mean 850-400 hPa wind, which would lead to rainfall efficiency for thunderstorms that form. The 700 hPa temperatures at or below 9C imply minimal capping aloft, and SPC mesoanalyses show that overnight CIN has eroded. Went ahead and shifted the eastern edge of the Marginal Risk area to the I-95 corridor to cover any isolated flash flood potential in the lee trough. Roth/Jackson Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt